FIELD OF THE INVENTION

The invention relates to the field of power converters.

BACKGROUND OF THE INVENTION

US7173833 discloses a power supply device for providing energy to an apparatus. The power supply device comprises a main converter and an auxiliary converter. When the apparatus is in full operational mode and consumes a significant amount of power, the power converter operates in the so-termed normal mode and both the main converter and the auxiliary converter are active. Further, the apparatus may operate in a standby mode. During such a standby mode all or most of the parts of the apparatus are switched off and the apparatus consumes a low amount of energy. In this standby mode, the main converter of the power supply device is switched off and only the auxiliary converter is operated to provide the standby power.

The auxiliary converter is a switch mode power converter comprising a primary side with primary transformer windings, and a secondary side with secondary transformer windings. The auxiliary converter comprises a feedback loop from the secondary side to the primary side in order to control the amount of power that is transferred via the transformer from the primary side to the secondary side.

In general the feedback circuits of a switch mode power supply comprise an optocoupler. The optocoupler is used to provide a signal to the controller of the primary side in order to control the amount of power that is transferred by the switched power supply. The optocoupler diode feeds information related to the output voltage at the secondary side in the form of light to primary side. The higher the output voltage at the secondary side, the more intense the light intensity of the light source of the optocoupler will be and the more the optocoupler transistor will electrically conduct at the primary side. If the transistor of the optocoupler has a higher conductivity, the controller changes the frequency and/or the duty cycle of the opening and closing of the switch or switches cooperating with the primary winding such that the power converter transfers less energy via the transformer from the primary side to the secondary side. The auxiliary converter of US7173833 is a switch mode power converter. The converter is used to deliver a low standby power only to the apparatus. In general, a switch mode converter may operate efficiently when converting a significant amount of power. Always a minimum amount of power is dissipated by the auxiliary converter, like for example the power dissipated by a controller thereof. Especially in a standby mode, the minimum amount of dissipated energy contributes heavily to the inefficiency of the converter.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a power supply that is more efficient in a standby mode. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims.

A power converter in accordance with the first aspect of the invention comprises a primary side, a mains-isolated secondary side, a mode controller and a feedback element. The primary side receives a mains voltage and comprises a primary winding of a transformer. The mains-isolated secondary side comprises an electrical charge storage element, a secondary winding of the transforming which supplies an output voltage across the electrical charge storage element and the secondary side comprises an undervoltage detector for detecting a voltage across the electrical charge storage element. The feedback element comprises a transmitter and a receiver. The undervoltage detector is configured to only supply a transmitter power to the transmitter when the voltage across the electrical charge storage element drops below a predetermined level. The mode controller controls the primary side to be in an active more or an inactive mode. In the active mode the primary side transfers output power to the secondary side and in the inactive mode the primary side does not transfer power to the secondary side. The mode controller is coupled to the receiver and is configured to detect whether the transmitter power to the transmitter is being supplied, and is configured to at least temporarily control the primary side to be in the active mode when the transmitter power is being supplied to the transmitter, and to control the primary side to be in the inactive mode when the transmitter power is not being supplied to the transmitter.

The power converter may operate in a standby mode. In this mode no power, or only a very small amount of power, is delivered to an apparatus. The power converter may detect that the power consumption is very low and enter the standby mode, or the apparatus provides information to the power converter to indicate that the apparatus entered a low power mode such that the power converter may enter the standby mode. In the standby mode the power provided by the power converter to the apparatus is less than 1 Watt.

During the standby mode and while the primary side is in the inactive mode, the power converter provides power to the apparatus from the electrical charge storage element. The electrical charge storage element may be a capacitor or a rechargeable battery. When the primary side is in the active mode the power converter provides power to the apparatus by the secondary winding of the transformer and the electrical charge storage element is charged by the power that it receives from the secondary winding of the transformer. The voltage across the electrical charge storage element is monitored by the undervoltage detector. If the voltage across the electrical charge storage element is higher than the predetermined level, the undervoltage detector switches off the transmitter power to the transmitter of the feedback element. The mode controller is coupled to the receiver of the feedback element and is capable of detecting whether the transmitter receives the transmitter power or not. If the mode controller detects that the transmitter power to the transmitter is switched off, the mode controller controls the primary side to be in the inactive mode and as such no output power is transferred from the primary side to the secondary side.

The transmitter power is supplied to the transmitter of the feedback element. The transmitter power is provided to the transmitter to operate - in other words: the power is used to transmit a signal to the receiver of the feedback element and the transmitter power is dissipated by the transmitter while transmitting the signal. Further, if no transmitter power is provided to the transmitter, the transmitter cannot operate, cannot transmit a signal to the receiver of the feedback element and does not dissipate power.

The charged electrical charge storage element is slowly discharged by the standby power delivered to the apparatus and/or by power consumed by the undervoltage detector. Consequently the voltage across the electrical charge storage element decreases. As soon as the undervoltage detector detects that the voltage across the electrical charge storage element is below the predetermined level, the undervoltage detector switches on the transmitter power to the transmitter of the feedback element. Subsequently, the mode controller detects via the receiver that the transmitter power has been switched on and controls the primary side to be in the active mode and power is transferred from the primary side to the secondary side to charge the electrical charge storage element. If the electrical charge storage element is sufficiently charged, the voltage across the electrical charge storage element is above the predetermined level and, as described above, the primary side is switched to the inactive mode. By way of example, the feedback element is an optocoupler, the transmitter is an optocoupler diode at the secondary side of the power converter and the receiver is an associated optocoupler transistor at the primary side.

The power converter according to the first aspect is, in the standby mode, more efficient than the known power converters with regard to two facets. According to a first facet, if the primary side is in the inactive mode, no power is consumed at the primary side. For example, if the primary side is inactive, no energy is lost in the transformer.

Alternatively, the primary side may comprise a primary side controller and the primary side controller does not consume energy in the inactive mode. According to a second facet, the feedback element does not receive transmitter power when the electrical charge storage element stores enough energy. The transmitter power is the power used by the transmitter for transmitting information to the receiver. If the feedback element does not receive transmitter power, it does not consume any power and does not discharge the electrical charge storage element.

Thus, in the standby mode and when sufficient energy is stored in the electrical charge storage element, no power is consumed by the feedback element and no power is dissipated at the primary side of the power converter. By well dimensioning the electrical charge storage element and the power conversion capacity of the power converter, the duration of the periods of time during which the primary side is in the active mode compared to the duration of the periods of time during which the primary side is in the inactive mode may be minimized, which results in maximum power savings. On the other hand, the undervoltage detector and the mode controller consume some energy all the time. However, known undervoltage detectors and mode controllers are very efficient and the amount of power saved by the invention according to the first aspect of the invention is much more than the power use of the undervoltage monitoring circuit.

The mode controller detects whether the transmitter power is supplied to the transmitter. In other words: the mode controller detects the presence or absence of the transmitter power at the transmitter, or the mode controller detects whether the transmitter receives the transmitter power, or the mode controller detects whether the undervoltage detector has switched on the transmitter power or has switched off the transmitter power.

The mode controller controls the primary side to be at least temporarily in the active mode. At least temporarily means that the primary side is controlled to be in the active mode for a predefined period of time, or that the primary side is in the active mode until the instant on which the mode controller detects that the transmitter does not receive transmitter power anymore. It is to be noted that the above described invention is different from the so- termed burst-mode of stabilized switch mode power converters. The stabilized switch mode power converters enter the burst mode if the output voltage is too high as the result of a limited power delivery to the apparatus to which the stabilized switch mode power converter is connected. Stabilized switch mode power converters comprise in general at the primary side a switch control circuitry and a primary switch for controlling the periodic current through the primary winding. In the burst mode the switch control circuitry controls the primary switch such that only during intervals power is transferred from the primary side to the secondary side. A feedback element which is coupled to the output voltage provides information to the switch control circuitry about the output voltage. The higher the output voltage is, the more power is provided to the feedback element and if the feedback element receives more power than a specific value the switch control circuitry enters the burst mode. Thus, the feedback element consumes all the time energy and consumes even more energy when the output voltage is higher. In contrast the present invention wherein the feedback element does not consume energy when the primary side is in the inactive mode.

A further improvement of the invention compared to the known burst mode is that, except the periods of power transfer, the primary side of the power supply according to the first aspect of the invention is inactive and does not consume energy. A lot of energy is saved which would normally be dissipated in, for example, the switch control circuitry of the primary side.

In an embodiment, the power converter provides, while the power converter is in the standby mode and the primary side is in the inactive mode, a standby power to at least a part of an apparatus from the electrical charge storage element. The electrical charge storage element stores an amount of energy and the electrical charge storage element is charged if the voltage drops below a predetermined level. The voltage over the electrical charge storage element may also be used to provide standby power to a circuitry of the apparatus. The consumed standby power by the circuitry is small to prevent a too fast discharging of the electrical charge storage element.

In an embodiment, the power converter further comprises a mains voltage switch in-between a mains voltage input and the primary side. The mode controller of the power converter is configured to control the mains voltage switch by connecting or disconnecting the mains voltage to or from the mains voltage input to control the primary side to be, respectively, in the active mode or the inactive mode. Using a mains voltage switch is a very effective and cheap solution to control the primary side to be in the active or the inactive mode. It does not require a major change in the design of the primary side and it allows the use of known power supplies which are controlled to be in the active more or the inactive mode by the mains power switch. The main power switch may be used to completely disconnect the power converter from the mains power, which guarantees no power consumption by, for example, parasitic components that are coupled to the mains voltage.

In a further embodiment, the power converter further comprises a one-shot circuit. The one-shot circuit receives the mains voltage. The one-shot circuit is configured to provide a one-shot signal to the mode controller when, in use, the power converter receives the mains voltage after a period of time during which the mains voltage was not received. The mode controller is configured to, at least temporarily, control the primary side to be in the active mode in response to the receiving of the one-shot signal. When the mains cord of the power converter, or of the apparatus which comprises the power converter, is connected to the mains power, the power converter has to start to operate for at least a short period of time, because it has to charge the electrical charge storage element. The one-shot circuit detects that the power converter receives a mains voltage and generates a one-shot signal after a period of time during which the mains power was absent. The mode controller receives the one-shot signal and controls the primary side to be in the active mode such that the power converter is able to start its operation. The electrical charge storage element will be charged, and if the voltage across the electrical charge storage element is above the predetermined level, the primary side is controlled to be in the inactive mode as the result of a switched off transmitter power to the transmitter.

In an embodiment, the power converter is further configured to operate in a normal mode to provide the output voltage to at least a part of the apparatus. The primary side is in the active mode when the power converter is in the normal mode. The output voltage is obtained from the secondary winding. The power converter of the embodiment is able to provide the apparatus power in two modes, which allows the apparatus to operate in two modes as well. The apparatus may consume a substantial amount of energy in the normal mode and as such the power converter operates under these circumstances in the normal mode. The power converter may be dimensioned for the expected power consumption in the normal mode and may be dimensioned such that it operates efficiently while providing the substantial amount of energy. The apparatus may operate in a standby mode during which most of the circuitry of the apparatus is switched off. If the apparatus is in the standby mode it consumes only a little amount of standby power. The power converter provides the standby energy while operating in the standby mode, and as discussed before, the power converter operates very efficiently in the standby mode. Thus, the power delivered by the power converter in the normal mode as well as in the standby mode is converted efficiently.

In a further embodiment, the power converter is a switched mode power converter. The primary side of the power converter comprises a primary switch and a switch controller. The primary switch obtains a periodically varying current through the primary winding. T switch controller controls the primary switch. Switched mode power converters are known as efficient power converters and the switch controller may control, by controlling the primary switch, the output voltage, the output current and/or the output power.

In an embodiment, the switch controller is configured to close the primary switch when the primary side is in the inactive mode, and the switch controller is configured to further operate in a burst mode when the primary side is in the active mode and the output voltage is above a predetermined voltage level. In the burst mode, the switch controller controls the primary switch in alternating periods of time to be open during a first period of time and to periodically open en close during a subsequent period of time. The primary side further comprises a startup circuit in-between the mains voltage and the switch controller. The startup circuit provides startup power to the switch controller for closing the primary switch after a period of time when the primary switch was open. The mode controller is configured to control the startup circuit to provide startup power, or not provide startup power, when, respectively, transmitter power is received by the transmitter or no transmitter power is received by the transmitter. When the switch mode power converter becomes connected to the mains voltage, for example, by connecting the mains cord to the mains power, the switch mode power converter has to start its operation. A known solution is the use of a startup circuit at the primary side of the switch mode power converter. The startup circuit provides startup power which is used to close the primary switch for the first time when the power converter becomes connected to the mains voltage. Further, the primary switch of the primary side may be open for a period of time, for example, when the primary side is in the inactive mode, or when the primary side is operating in the so-termed burst mode. After such a period of time during which the primary switch was open, the switch controller requires startup power to close the primary switch. If no startup power is available, the switch controller cannot close the primary switch after such a period of time.

The burst mode is an operating mode of the switch controller. The switch controller enters the burst mode when the primary side is active and when the output voltage at the secondary side increases to a level above a predetermined voltage level. The voltage at the secondary side may rise above the predetermined voltage level if more power is transferred from the primary side to the secondary side than the amount of power that is consumed at the secondary side. Especially in the standby mode not much standby power is consumed by the apparatus, and the electrical charge storage element only stores a limited amount of energy. Thus, in the standby mode, when the primary side is active, the output voltage rises fast to a level above the predetermined voltage level.

When the switch controller starts operating in the burst mode, during at least a period of time the primary switch will be open. If no startup power is received from the startup circuit, the switch controller cannot close the primary switch anymore to start an interval during which the primary switch is periodically opened and closed. As such the switch controller is not anymore able to continue operating in the burst mode and the primary switch remains open. No power can be transferred from the primary side to the secondary side an consequently enters the primary side the inactive mode. As soon as the startup power becomes available again, the switch controller is able to close the primary switch and the primary side starts to operate in the active mode. Thus, controlling the availability of the startup power is an effective solution to control the primary side to be in the active mode or to be in the inactive mode. Power converters of which the switch controller may start operating in a burst mode and of which the closing of the primary switch depends on the availability of the startup power are well known power converters. Only a minimal amount of modifications to the known power converter are required to control the primary side to be in the active mode or to be in the inactive mode.

In an embodiment, a startup circuit switch is connected in series with the startup circuit and is connected between the startup circuit and the mains voltage input or is connected between the startup circuit and a startup power input terminal of the switch controller. Using a startup circuit switch is a very simple and effective solution for activating and inactivating the startup circuit. The mode controller opens the primary switch to inactive the startup circuit and the mode controller closes the startup circuit switch to activate the startup circuit. In a further embodiment, the startup circuit may be a startup resistor connected between the mains voltage input and the startup power input terminal of the switch controller. If the startup circuit is the resistor and the startup switch opens the path through the resistor, no power can be dissipated in the resistor, and the switch controller cannot consume the startup power.

In an embodiment, the power converter further comprises an output voltage feedback element. The output voltage feedback element provides feedback from the secondary side to the primary side of the output voltage level. The switch controller is coupled to the output voltage feedback element. The switch controller controls the output power transferred from the primary side to the secondary side when the primary side is in the active mode to stabilize the output voltage. In a further embodiment, the output voltage feedback element does not receive power when the primary side is in the inactive mode and no power is transferred from the primary side to the secondary side. The output voltage feedback element is used to stabilize the output voltage. In known power converter the output voltage feedback element consumes energy in all operational modes. The output voltage feedback element of the embodiment only consumes power when the primary side is in the active mode. Especially in the standby mode of the power converter this results in a huge reduction of power consumption by the output voltage feedback element.

In an embodiment, the undervoltage detector of the power converter is configured to further monitor the output voltage when the primary side is in the active mode. The undervoltage detector varies the transmitter power in order to feed back an output voltage signal from the secondary side to the primary side. The mode controller is configured to detect the output voltage signal and provides the output voltage signal to the switch controller. The switch controller uses the output voltage signal to control the amount of output power that is transferred from the primary side to the secondary side to stabilize the output voltage. The output voltage of a switch mode power converter is in general stabilized by feeding back information about the output voltage from the secondary side to the primary side. In the embodiment this information is encoded in the transmitter power. For example, by providing an amount of transmitter power to the transmitter that depends on the output voltage. If the transmitter power provided to the transmitter of the feedback element increases or decreases, the mode controller may detect the variations via the receiver of the feedback element and generates the output voltage signal to provide the output voltage signal to the switch controller. If, for example, the feedback element is an optocoupler, the optocoupler diode may emit light at different intensities in dependence of the output voltage which results in a varying conductivity of the optocoupler transistor, which is detected by the mode controller.

The advantage is that only one feedback element is required for performing two different functions, namely, the feedback of a signal to control the primary side to be in the active mode or in the inactive mode, and the feedback of the output voltage signal.

Having only one feedback element results in a cheaper and more compact power converter. Compared to known power converters that feed back the output voltage signal, the embodiment is advantageous, because in known power converter the feedback element to feed back the output voltage signal consumes also power in the standby mode, while the feedback element of the power converter of the embodiment uses most of the time no power when the power converter is in the standby mode. Especially in the standby mode the power converter is more efficient.

In a further embodiment, the undervoltage detector is configured to decrease transmitter power to the transmitter if the output voltage increases. The switch controller is configured to control the primary switch such that less output power is transferred from the primary side to the secondary side if the transmitter receives less transmitter power. The undervoltage detector increases transmitter power to the transmitter if the output voltage decreases. The switch controller is configured to control the primary switch such that more output power is transferred from the primary side to the secondary side if the transmitter receives more transmitter power. If a low amount of power has to be transferred, the transmitter of the feedback element receives not much transmitter power. As such the transmitter power does also not contribute much to the inefficiency if a low amount of power needs to be transferred.

In an embodiment the undervoltage detector is configured such that it only switches on the transmitter power to the transmitter when the voltage across the electrical charge storage element is below another predetermined level being lower than the predetermined level. The other predetermined level introduces hysteresis. Because the voltage across the electrical charge storage element has to be lower than the predetermined level to switch on the transmitter power, more power may be stored in the electrical charge storage element during the period of time that the primary side is active. This results in a slightly longer period of time during which the primary side is in the active mode, and it results also in longer periods of time during which the primary side is in the inactive mode. Thus, the frequency of switching between the active mode and the inactive mode of the primary side decreases. It may be that during every period of time during which the primary side is in the active mode at the beginning of the period of time a little amount of additional energy is dissipated in order to initially charge electrical charge storage elements and/or inductances. By decreasing the frequency the total efficiency improves because of lower losses at the beginning of the period of time during which the primary side is in the active mode. Further, the embodiment prevents that the power converter starts to oscillate with a high frequency if the amount of standby power, which is consumed during a period of time during which the primary side is in the inactive mode, is just a little below the amount of energy that is stored in the electrical charge storage element during every period of time during which the primary side is active.

In another embodiment the feedback element may be an optocoupler, a relay or a transformer. These feedback elements provide a galvanic isolation such that the secondary side is mains-isolated. The mains-isolated secondary side is advantageous with regard to minimizing the risk of having the mains voltage on parts of the apparatus which receive power from the power converter.

According to a second aspect of the invention a display panel device is provided that comprises the power converter according to the first aspect of the invention.

According to a third aspect of the invention a method of operating a power converter in a standby mode is provided. The power converter comprises a primary side, a mains-isolated secondary side, a feedback element with transmitter and receiver, and a mode controller coupled to the receiver. The primary side comprises a primary winding of a transformer and the mains-isolated secondary side comprises an electrical charge storage element, a secondary winding of the transformer and an undervoltage detector. The method comprises a step of receiving a mains voltage, supplying an output voltage across the electrical charge storage element, detecting a voltage across the electrical charge storage element, supplying transmitter power, detecting the transmitter power, and controlling the primary side. The mains voltage is received by the primary side of the power converter. The secondary winding of the transformer supplies the output voltage across the electrical charge storage element. The undervoltage detector detects the voltage across the electrical charge storage element. The undervoltage detector only supplies the transmitter power to the transmitter when the voltage across the electrical charge storage element drops below a predetermined level. The mode controller controls the primary side to be at least temporarily in an active mode when the transmitter power is being supplied to the transmitter. The mode controller controls the primary side to be in an inactive mode when the transmitter power is not being supplied to the transmitter.

The display panel device according to the second aspect of the invention, and the method according to the third aspect of the invention, provide the same benefits as the power converter according to the first aspect of the invention.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

It will be appreciated by those skilled in the art that two or more of the above- mentioned embodiments, implementations, and/or aspects of the invention may be combined in any way deemed useful. Modifications and variations of the system, the method, and/or of the computer program product, which correspond to the described modifications and variations of the system, can be carried out by a person skilled in the art on the basis of the present description.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically shows an embodiment of a power converter in accordance with the first aspect of the invention,

Fig. 2 schematically shows an embodiment of a switch mode power converter, Fig. 3 schematically shows an embodiment of a power converter comprising a mains power switch and two feedback elements,

Fig. 4 schematically shows an embodiment of a power converter with a one- shot circuitry and wherein the transmitter power is varied,

Fig. 5 shows a circuit diagram of an embodiment of the power converter in accordance with the first aspect of the invention,

Fig. 6 schematically shows an embodiment of a flat panel display device in accordance with the second aspect of the invention, and

Fig. 7 schematically shows a flow diagram of an embodiment of the method in accordance with the third aspect of the invention.

It should be noted that items denoted by the same reference numerals in different Figures have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item have been explained, there is no necessity for repeated explanation thereof in the detailed description. DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of a power converter 100 is shown in Fig. 1. The power converter 100 comprises a primary side 102, a mains-isolated secondary side 110, a feedback element 116 comprising a transmitter 126 and receiver 118, and the power converter 100 comprises a mode controller 114 which is coupled to the receiver 118. The primary side 102 receives a mains voltage 112 and comprises a primary winding 104 of a transformer. The secondary side comprises a capacitor 108 and a secondary winding 106 of the transformer for supplying an output voltage across the capacitor 108. The output voltage may be provided to an apparatus via the output terminals 120 of the power converter 100. The secondary side further comprises an undervoltage detector 122 for detecting a voltage across the capacitor 108. The undervoltage detector 122 may provide a transmitter power 124 to the transmitter 126.

In the standby mode of the power converter 100 and when the primary side 102 is in the active mode, the secondary winding 106 provides an output current to the capacitor 108 to charge the capacitor thereby increasing the voltage across the capacitor 108. When the voltage across the capacitor 108 is larger than a predetermined level, this is detected by the undervoltage detector 122 which switches off the transmitter power 124 to the transmitter 126. The mode controller 114 is coupled to the receiver 118 of the feedback element 116 and the mode controller 114 is able to detect whether the transmitter 126 receives the transmitter power 124 from the undervoltage detector 122 or not. If the mode controller 114 detects that the transmitter power 124 has been switched off, the mode controller controls the primary side 102 to be in an inactive mode. In the inactive mode the primary side 102 does not transfer power from the primary side 102 to the secondary side 110. Thus, the capacitor 108 does not receive any output current and is not charged anymore. The voltage across the capacitor 108 does not increase anymore. The apparatus which is coupled to the output terminals 120 of the power converter 100 may consume a small amount of standby power. The standby power is provided by the capacitor 108 if the primary side is in the inactive mode. Further, the undervoltage detector 122 may consume a small amount of the energy stored in the capacitor 108. Because of the power consumption of the

undervoltage detector 122 and the standby power consumed by the apparatus, the voltage across the capacitor 108 decreases. As soon as the voltage across the capacitor 108 drops below the predetermined level, the undervoltage detector 122 switches on the transmitter power 124 to the transmitter 126. Subsequently, the mode controller 114 detects that the transmitter power 124 is received by the transmitter 126 and controls the primary side 102 to be at least temporarily in an active mode. In the active mode the primary side transfers power from the primary side to the secondary side.

In an embodiment the primary side 102 is in the active mode until the mode controller 114 controls the primary side 102 to be in the inactive mode because the transmitter 126 receives no transmitter power 124. In another embodiment the mode controller 114 controls the primary side 102 to be in the active mode for a predefined period of time which is expected to be large enough to charge the capacitor 108 to a sufficient level at which the voltage across the capacitor is above the predetermined level.

The feedback element 126 is, for example, an optocoupler with a light emitting diode being the transmitter 126 and a light sensitive transistor being the receiver 118. The transmitter power 124 provides power to the light emitting diode of the optocoupler. Only if the transmitter power is provided to the light emitting diode the feedback element 116 consumes energy. In another embodiment, the feedback element 116 may be a transformer of which one winding is the transmitter 126 and another winding is the receiver 118. The transmitter power 124 may be provided to the winding by means of an alternating current. In another embodiment the feedback element 116 may be a relay and the coil of the relay is the transmitter 126, and the electromagnetic switch is the receiver 118.

Thus, if the power converter 100 is in a standby mode, the feedback element 116 only consumes energy if the primary side 102 is in the active mode. If the primary side 102 is in the inactive mode, the feedback element does not consume power, which

contributes to an efficient operation of the power converter 100 in the standby mode. Further, the primary side 102 does not transfer all power that is consumed from the mains power line to the secondary side 110, but will dissipate also some energy in its circuitry. By controlling the primary side 102 to be in the inactive mode when it is not necessary to transfer power to the secondary side 110, energy losses at the primary side 102 are prevented as well, which and this contributes to an efficient standby operation of the power converter 100.

The power converter 100 may operate in a standby mode when the apparatus that consumes power at the output terminals 120 of the power converter 100 consumes only a little amount of power. In a typical embodiment the power consumption of the apparatus is lower than 1 watt during the standby mode of the power converter 100.

The power converter 100 may detect that the power consumption of the apparatus is low in order to switch to the standby mode. In another embodiment the apparatus may provide a signal to the power converter 100 which indicates that the apparatus has switched to a low power regime and now consumes only a little amount of energy. The received signal may trigger the power converter 100 to enter the standby mode. In another embodiment the power converter is used in a device for charging batteries. It may be detected whether the batteries are connected to the device and need to be charged, if this is not the case, the power converter 100 may receive a signal that it may enter the standby mode.

By way of abbreviation in the remainder of the detailed description "an active primary side" or "an inactive primary side" is used instead of "the primary side is in an active mode" or "the primary side is in the inactive mode", respectively. Instead of "the mode controller controls the primary side to be in the active mode" and "the mode controller controls the primary side to be in the inactive mode", "the mode controller activates the primary side" and "the mode controller inactivates the primary side" are used, respectively. Fig. 2 shows another embodiment of a switch mode power converter 200. The power converter 200 comprises a primary side 210 to receive the mains voltage 112, a mains- isolated secondary side 216, a feedback element 116 that comprises a transmitter 126 and a receiver 118, and the power converter comprises a mode controller 214 that is coupled to the receiver 118. The primary side 210 comprises a primary winding 104 of a transformer and a primary switch 204 to obtain a periodically varying current through the primary winding 104.

The primary side 210 further comprises a switch controller 206 for controlling the primary switch 204. Further, a startup circuit 212 is comprised in the primary side 210. The startup circuit 212 provides startup power via a startup power switch 202 to the switch controller 206 if the startup circuit 212 receives the mains voltage 112. The switch controller 206 uses the startup power to close primary switch 204 after a period of time during which the primary switch 204 was open. The switch controller 206 may start operating in a burst mode when the primary side 210 is in the active mode and the output voltage is above a predetermined voltage level. In the burst mode the switch controller 206 controls the primary switch 204 in alternating intervals in which during a first period of time the primary switch 204 is open en in a subsequent period of time the switch is periodically opened and closed.

The switch mode power converter 200 further comprises at the secondary side 216 a secondary winding 106 of the transformer and a rectifying circuit 208. An output voltage provided by the secondary winding 106 is rectified by the rectifying circuit 208 and the rectified output voltage is provided to a normal power output terminal 218. If the apparatus operates in a normal mode, it consumes a significant amount of energy from the normal output power terminal 218. The secondary side 216 further comprises a capacitor 108 and a diode 220 for providing the rectified output voltage to the capacitor 108. The voltage across the capacitor 108 is provided to a standby power output terminal 120. The apparatus may consume standby power from the standby power output terminal 120.

The secondary side 216 further comprises an undervoltage detector 122 for monitoring the voltage across the capacitor 108. If, in a standby mode of the power converter 200, the voltage across the capacitor 108 is below a predetermined level, the undervoltage detector 122 provides a transmitter power 124 to the transmitter 126. If, in the standby mode, the voltage across the capacitor 108 is above a predetermined level, the undervoltage detector 122 does not provided the transmitter power 124 to the transmitter 126.

The mode controller 214 detects via the receiver 118 whether the transmitter 126 receives the transmitter power 124. If, in the standby mode, the transmitter power 124 is not received by the transmitter 126, the mode controller 214 inactivates the primary side 210 by opening the startup power switch 202. If, in the standby mode, the transmitter power 124 is received by the transmitter 126, the mode controller activates the primary side 210 by closing the startup switch 202. In an embodiment, the startup circuit 212 is a resistor that provides startup power to the switch controller 206. If the mode controller 214 opens the startup power switch 202, the resistor cannot provide a startup power, which prevent power dissipation in the resistor and prevents startup power dissipation in the switch controller 206 when the primary side 210 is inactive.

The switched mode power converter 200 may operate in two modes. A first mode is the standby mode during which a small amount of power is provided to the standby power output terminal 120. A second mode is the normal mode during which at least a significant amount of power is consumed via the normal power output terminal 218 by the apparatus. The switching between the modes may be done by automatic detection of power consumption in the power converter 200, or may be done by receiving a signal that indicates to the power converter 200 how much power is consumed. The primary side 210 is active during the normal mode.

Fig. 3 presents another embodiment of a switch mode power converter 300. The power converter 300 comprises a mains voltage switch 302, a primary side 304 that comprises a primary winding 104 of a transformer, a mains-isolated secondary side 306 that comprises a secondary winding 106 of the transformer, a feedback element 116 that comprises a transmitter 126 and a receiver 118, an output voltage feedback element 308 to provide output voltage feedback to a switch controller 305, and the power converter comprises a controller 310. The primary side comprises a primary switch 204 to control the current through the primary winding 104, and comprises the switch controller 305 to control the primary switch 204. The secondary side comprises a rectifying circuit 208 to rectify an output voltage received from the secondary winding 106. The rectified output voltage is provided to a normal power output terminal 218 to provide power to an apparatus if the power converter 300 is operated in a normal mode. The secondary side 306 further comprises a capacitor 108 and a diode 220 to provide the rectified output voltage to the capacitor 108. The voltage across the capacitor 108 is provided to a standby output terminal 120 to provide standby power to the apparatus if the power converter 300 operates in the standby mode and the apparatus consumes only a small amount of standby power. The secondary side 306 further comprises an undervoltage detector 312 for detecting the voltage across the capacitor. The mode controller 310 is coupled to the receiver 118 and controls the opening or closing of the mains voltage switch 302. If the primary side 304 is active, power is transferred to the secondary side 306 resulting in the rectified output voltage provided by the rectifying circuit 208. The output voltage feedback element 308 provides feedback from the secondary side 306 to the primary side 304 about the output voltage. The feedback is provided to the switch controller 305 to control the primary switch 204 such that the output voltage is stabilized, which is especially advantageous in the normal operation mode when the apparatus may require the availability of a stabilized output voltage. The feedback loop functions as follows: if the output voltage increases, a signal is fed back to the primary side 304 indicating that the output voltage increases, and in response to this signal the switch controller 305 controls the primary switch 204 such that less power is transferred from the primary side 304 to the secondary side 306. The output voltage feedback element 308 has only to feed back information about the output voltage when the primary side 304 is active. Therefore, the output voltage feedback element 308 may be constructed such that it does not consume power when the primary side 304 is inactive. This is especially advantageous in the standby mode, because the primary side will be inactive for most of the time during the standby mode. Unnecessary power use by the output voltage feedback element 308 is prevented with such a configuration.

In the standby mode, the undervoltage detector 312 monitors the voltage across the capacitor. If the voltage across the capacitor is below a predefined level, the undervoltage detector 312 provides transmitter power 124 to the transmitter 126. The mode controller detects via the receiver 118 that the transmitter 126 receives the transmitter power 124, and closes the mains voltage switch 302 such that the primary side 304 becomes active and transfers power to the secondary side 306. During the power transfer to the secondary side 306 a current is supplied to the capacitor 108 to replenish the capacitor 108. The voltage across the capacitor 108 increases. At a specific moment the undervoltage detector 312 detects that the voltage across the capacitor is above another predetermined level, which is higher than the predetermined level, and switches off the transmitter power 124 to the transmitter 126. Subsequently, the mode controller 310 detects the switching off of the transmitter power 124 and opens the mains power switch 302 to inactive the primary side 304. Using a mains voltage switch 302 is advantageous, because the opening of the voltage switch 302 prevents any mains power consumption as a result of the absence of the mains power.

Another embodiment of a switch mode power converter 400 is presented in Fig. 4. The power converter 400 comprises a primary side 402 comprising a primary winding 104 of a transformer, a mains-isolated secondary side 404 comprising a secondary winding 106 of the transformer, a feedback element 410 comprising a transmitter 418 and a receiver 412, a mode controller 408 coupled to the receiver and the power converter comprises a one- shot circuit 406. The primary side 402 further comprises a primary switch 204 to control the current through the primary winding 104, and a switch controller 305 to control the primary switch 204. The secondary side 404 comprises a rectifying circuit 208 coupled to the secondary winding 106 to provide a rectified output voltage to the normal power output terminal 218. The secondary side 404 further comprises a capacitor 108 and a diode 220 to provide the rectified output voltage to the capacitor 108. The voltage across the capacitor 108 is provided to a standby power output terminal 120. The secondary side 404 further comprises an undervoltage detector 414 to monitor the voltage across the capacitor 108 and for monitoring the rectified output voltage to provide to the transmitter the transmitter power 416 including an output voltage signal. The one-shot circuit 406 receives the mains voltage 112 and is coupled to the mode controller 408 to provide a one-shot signal. The mode controller 408 is coupled to the receiver 412 and coupled to the one-shot circuit 406 and controls the inactive or active state of the primary side 402 and provides the output voltage signal to the switch controller 305.

The one-shot circuit 406 detects when the mains voltage 112 comes online, in other words, it detects the instant at which the mains voltage 112 is available after a period of an unavailable mains voltage 112. At the instant that the mains voltage becomes available, the one-shot circuit 406 generates a one-shot signal and provides the one-shot signal to the mode controller 408. In response to receiving the one-shot signal the mode controller 408 at least temporarily activates the primary side 402. The at least temporarily activation of the primary side 402 allows the transfer of at least some power to the secondary side 404 to start the power converter 400 and charge the capacitor 108.

The undervoltage detector 414 detects, in a standby mode of the power converter 400, whether the voltage across the capacitor 108 drops below a predetermined level to provide the transmitter power 416 to the transmitter 418. The availability of the transmitter power 416 at the transmitter 418 is detected by the mode controller 408 via the receiver 412 and in response to the availability of the transmitter power 416 the primary side 402 is activated. If, in the standby mode, the voltage across the capacitor 108 is above the predetermined level, the undervoltage detector 414 interrupts the providing of the transmitter power 416 to the transmitter 418. Consequently, the primary side 402 is inactivated.

The undervoltage detector 414 further monitors the rectified output voltage. On basis of the monitored rectified output voltage an output voltage signal is generated which is also provided to the transmitter 418. In an embodiment the undervoltage detector 414 may modulate the transmitter power on basis of the output voltage signal, for example, if the output voltage increases, the transmitter power is reduced and if the output voltage decreases, the transmitter power is increased. Other methods of modulating the transmitter power may be used as well, for example by providing an alternating current to the transmitter 418 with a frequency that depends on a value the output voltage signal.

The feedback element 410 is configured such that the transmitter is able to transmit a signal that represents the characteristics of the transmitter power 416 to the receiver and that the receiver is able to detect these characteristics. For example, the feedback element 410 may be an optocoupler comprising a light emitting diode, which is the transmitter 418, and a light sensitive transistor, which is the receiver 412. The light emitting diode transmits more light to the light sensitive transistor if it receives more transmitter power 416 and as a consequence the light sensitive transistor becomes more conductive. Or, if the feedback element 410 is a transformer, the frequency of an alternating current received at a first winding of the transformer can be detected at a second winding of the transformer.

The mode controller 408 is coupled to the receiver 412 and may detect what kind of signal is provided to the transmitter 418 of the feedback element 410. As discussed in other embodiments, the mode controller 408 detects the receiving of the transmitter power 416 at transmitter 418, and the mode controller 408 is further able to detect the output voltage signal which is for example modulated in the transmitter power 416. The output voltage signal is provided by the mode controller 408 to the switch controller 305. The switch controller 305 uses the output voltage signal to control the periodically opening and closing of the primary switch 204 such that the amount of power that is transferred from the primary side 402 to the secondary side 404 is controlled.

The power converter 400 uses only one feedback element 410 for providing the feedback from the secondary side 404 to the primary side 402. In the normal mode the feedback element 410 is used for feeding back the output voltage signal to stabilize the rectified output voltage. In the standby mode the feedback element 410 is mainly used for activating and inactivating the primary side and may be used to stabilize the rectified output voltage when the primary side is active. The transmitter 418 of feedback element 410 only receives power from the undervoltage detector 414 if the primary side 402 is active. Thus the feedback element 410 does not contribute to a possible inefficiency of the power converter 400 during periods that the primary side is inactive, which is especially advantageous in the standby mode during which the primary side is inactive for relative long periods of time. Fig. 5 shows a circuit diagram of another embodiment of the power converter 500. The mains voltage 501 is received by a mains rectifying circuit 502 and by an auxiliary rectifying circuit 517. The mains rectifying circuit 502 comprises four diodes Dl to D4 and a capacitor CI and provides a rectified mains voltage to the primary side 511. The auxiliary rectifying circuit 517 comprises capacitors C2, C3 and C4, and diodes D5 and D6 and provides power to a mode controller 504. The power converter 500 further comprises a one- shot circuit 518 formed by capacitor C5. The mode controller 504 of the power converter 500 comprises resistors R2 and R3 and MOSFETs Ml and M2. The primary side 511 comprises a startup circuit 506, a primary winding of a transformer 512, a primary switch 508 to obtain a periodically varying current through the primary winding, and a switch controller 510 to control the primary switch 508. A mains-isolated secondary side 513 comprises a secondary winding of the transformer 512, an undervoltage detector 515, diodes D7 and D8, and capacitors C6 and C7. The secondary side 513 provides operating power to the operational load 514 of an apparatus, and further provides standby power to the standby load 516 of the apparatus. The operational load is schematically presented by means of a resistor R5 and the standby load is schematically indicated by resistor R6. The power converter further comprises a feedback element 522 comprising a transmitter 526 and a receiver 524. The feedback element 522 comprises optocoupler Ql and resistor R4. The power converter further comprises an output voltage feedback element 520 comprising optocoupler Q2 and an output voltage detector 528.

If the power converter 500 is operating in a normal mode the primary side 511 is active, which means that the switch controller 510 controls the primary switch 508 such that power is transferred from the primary side 511 to the secondary side 513 via transformer 512. Diode D7 receives a voltage from the secondary winding of the transformer 512 and diode D7 cooperates with capacitor C6 to provide an output voltage to the operational load 514. The output voltage detector 528 monitors the output voltage and provides via optocoupler Q2 an output voltage feedback signal to the switch controller 510 at the primary side 511 of the power converter 500. If the output voltage increases, the light emitting diode of optocoupler Q2 receives more power from the output voltage detector 528 resulting in a more conductive light sensitive transistor at the receiving side of optocoupler Q2. This is detected by the switch controller 510 which changes the control of primary switch 508 such that less power is transferred to the secondary side 513, which results, if the operational load remains the same, in a decrease of the output voltage. If the output voltage decreases, the light emitting diode of the optocoupler Q2 receives less power from the output voltage detector 528 resulting in a less conductive light sensitive transistor at the receiving side of optocoupler Q2. The switch controller 510 detects the change in conductivity and controls the primary switch 508 such that more power is transferred from the primary side 511 to the secondary side 513. The result is a stabilized output voltage.

It should be noted that in another embodiment the light emitting diode of optocoupler Q2 receives less power if the output voltage increases and the light emitting diode receives more power if the output voltage decreases which, respectively, results in a less conductive light sensitive transistor or a more conductive light sensitive transistor. In this embodiment, the switch controller 510 controls the switch 508 such that less power is transferred to the secondary side 513 if the light sensitive transistor becomes less conductive, and controls the switch 508 such the more power is transferred to the secondary side 513 if the light sensitive transistor becomes more conductive.

It should be noted that the output voltage detector 528 receives power from the output voltage and as soon as the output voltage is absent, because of an inactive primary side 511 and an empty capacitor C6, the output voltage detector becomes inactive and does not provide power to optocoupler Q2. Thus, optocoupler Q2 will not consume energy most of the time when the primary side is inactive.

Further, it should be noted that the power converter 500 may enter the so- termed burst mode. If the output voltage is too high, the switch controller 510 enters a mode during in which the primary switch 508 is controlled in alternation periods of time during which in a period of time the primary switch 508 is controlled such that power is transferred to the secondary side via the transformer 512, and in the following period of time the primary switch 508 is permanently open. After a period during which the primary switch 508 was permanently open, the switch controller 510 uses startup power, provided by the startup circuit 506, to close the primary switch 508 for the first time at the beginning of a period in which power is transferred from the primary side 511 to the secondary side 513. Thus, when the switch controller 510 enters the burst mode, startup power is required to close the primary switch 508 and to start the transfer of power to the secondary side 513.

Further, it should be noted that during the normal mode the capacitor C7 is charged with energy via diode D8. It is expected that the voltage across the capacitor C7 is above a predetermined level during normal operation. The undervoltage detector 515 detects that the voltage across the capacitor C7 is above the predetermined level and switches off the transmitter power to the transmitter 526 of the feedback element 522. In a standby mode the operational load 514 is disconnect or does not draw any current. The standby load 516 consumes a small amount of standby power. The standby power is provided by capacitor C7 if the primary side 511 is inactive. As a result of energy consumption by the standby load and as a result of energy consumption by the undervoltage detector 515, the voltage across capacitor C7 decreases. The undervoltage detector 515 monitors the voltage across the capacitor C7 and switches on a transmitter power to the transmitter 526 if the voltage across capacitor C7 drops below the predetermined level. As a result of the receiving of the transmitter power, the light emitting diode of the optocoupler Ql emits light and the light sensitive transistor, which is the receiver 524 of the feedback element 522, becomes conductive. This is detected by the mode controller 504. If the receiver 524 becomes conductive, the gate of MOSFET M2 receives a low voltage and stops conducting, which results in a high voltage at the gate of MOSFET Ml which starts conducting. If MOSFET Ml conducts, a startup power provided by startup circuit 506 is provided to the switch controller 510. As a consequence the switch controller 510 closes the primary switch 508 and the primary side 511 becomes active and transfers power to the secondary side 513. The transferred power is rectified at the secondary side 513 and charges capacitor C7 and consequently the voltage across the capacitor increases. If the voltage across the capacitor is above the predetermined level, the undervoltage detector 515 switches off the transmitter power to the transmitter 526. The receiver 524 becomes nonconductive, and consequently MOSFET M2 starts conducting, and MOSFET Ml stops conducting. The switch controller 510 does not receive anymore a startup power and is therefore not anymore able to start up the power delivery from the primary side to the secondary side. In the meanwhile the switch controller 510 enters the burst mode, because the output voltage further increases. In the burst mode the switch controller 510 opens the switch 508 for a specific period of time. In order to close the switch 508 after this specific period, the switch controller 510 requires startup power. However, this power is not anymore available, and consequently the transfer of power cannot start again. Thus, the primary side 511 is inactivated.

The one-shot circuit 518 of the power converter 500 is used to provide a one- shot signal at the moment that the power converter 500 starts receiving the mains voltage 501. If the mains voltage is not connected, the voltage across capacitor C5 of the one-shot circuit 518 is about zero. At the instant when the mains voltage becomes connected the gate of MOSFET M2 has a voltage of zero and MOSFET M2 remains initially nonconductive. Consequently, MOSFET Ml initially conducts and MOSFET Ml provides the startup power to the switch controller 510 and the primary side 511 becomes active. After a short period of time the capacitor C5 of the one-shot circuit 518 becomes fully charged via resistor R3 and the voltage across the capacitor C5 is high. Thus, after a short period M2 starts conducting and Ml stops conducting and no startup power is provided anymore to the switch controller 510. If no operational load 516 is connected to the power converter 500 the output voltage and the voltage across the capacitor increases such that the switch controller 510 enters the burst mode in which, during a specific period of time, the switch 508 is opened. In order to close the switch 508 after this period, the switch controller 510 requires startup power. Thus, if no operational load 514 is connected to the power converter 500, or the operational load 513 does not draw any current, the primary side 511 becomes inactive and because of a voltage higher than the predetermined level across capacitor C7, the primary side cannot become active because of the absence of startup power.

Fig. 6 presents a schematically overview of a flat panel display device 600 according to second aspect of the invention. The flat panel display device 600 comprises a flat panel display 602, a power converter 606 according to the first aspect of the invention, a standby circuitry 608, and a mains cord 610 to receive a mains voltage. If the flat panel display device 600 is connected to the mains voltage via the mains cord 610, the flat panel display device 600 may operate in two modes: the normal operational mode and the standby mode. In the normal operational mode the flat panel display 602 and probably a further circuitry are active and consume a significant amount of operational power 605 which is provided by the power converter 606. If the flat panel display device 600 is operating in the standby mode, most circuits of the flat panel display device 600 are switched off or become inactive except the standby circuitry 608. The standby circuitry 608 consumes a small amount of standby power 604. The power converter 606 may provide the standby power 604 and the operational power 605. If the amount of energy consumed by the flat panel display device 600 is small, the power converter switches to the standby mode. Thus, if the flat panel display device 600 is in the standby mode, the power converter will be in the standby mode as well. If the flat panel display device 600 is in the normal mode and consumes a significant amount of energy, the power converter operates in the normal mode as well.

Fig.7 schematically presents a method 700 of operating a power converter in a standby mode. The power converter comprises a primary side that comprises a primary winding of a transformer, a mains-isolated secondary side that comprises a secondary winding of the transformer and an undervoltage detector, and the power converter comprises a feedback element with a transmitter and a receiver, and a mode controller that is coupled to the receiver. The method 700 comprises the steps of receiving 702 a means voltage at the primary side of the power converter, supplying 704 an output voltage across a capacitor by the secondary winding of the transformer, and detecting 706 a voltage across the capacitor by the undervoltage detector. The method further comprises the step 708 of switching on or off a transmitter power to the transmitter by the undervoltage detector. When the voltage across the capacitor drops below a predetermined level the transmitter power is switched on in step 708, when the voltage across the capacitor drops above the predetermined level the transmitter power is switched off in step 708. The method further comprises the steps of detecting 710, by the mode controller, whether the transmitter power to the transmitter has been switched on or off, and at least temporarily activating or inactivating 712 the primary side by the mode controller. The primary side is activated if the transmitter power to the transmitter is switched on and the primary side is inactivated if the transmitter power to the transmitter is switched off.

It should be noted that the embodiments are not limited to switch mode power converter or stabilized switch mode power converter. In other embodiments a fly-back, LLC resonant or full bridge power supply may be used. Further, the embodiments are not limited to the use of a capacitor for storing electrical charge. A rechargeable battery may be used as well.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.