The field of the invention

A couple of electric devices, for example different kind of lighting devices, utilize some kind of transformer circuit electronics.

The state of the art

A transformer is a device that transfers electrical energy from one circuit, ie primary circuit, to another, ie secondary circuit, through inductively coupled conductors, which are the transformer's coils. A varying current in the primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), ie voltage in the secondary winding.

In many prior art embodiments voltage and power of the secondary circuit has been substantially low and there has not been a need for big energy storage, ie. a capacitor, on the secondary circuit. An increasing amount of electric device embodiments for example in lighting devices require that on the secondary circuit is utilized a substantially high voltage and power. This brings a need for bigger energy storage, ie. capacitor, on the secondary circuit. The problem is the situation when a load such as an optoelectronics unit is first connected off from the circuit under live conditions. When the load is connected again on the circuit, a current peak exists in the circuit as electric charge discharges from the capacitor. This current peak may cause optoelectronics unit damages, for example light emitting diode (LED) damages, thus resulting as failures in the circuit operation and other additional costs, because damaged LEDs have to be replaced with new ones. Also some prior art problematic is related to the fact that the secondary circuit is a constant current device. In the normal operation there is a need for both voltage measurement arrangement and current measurement arrangement for measuring both voltage and current of the load. Frequency response of said voltage measurement has to be fast, and frequency response of said current measurement has to be slow, because current measurement must not react to wave signals or wave signal parts caused by mains voltage in normal operation of the secondary circuit. In the state of the art said voltage measurement has been arranged by a first amplifier and said current measurement has been arranged by a second amplifier.

Anyway in power failure situations and/or in sensor applications occur current peaks. Typically the current peak exists in the secondary circuit, when the load such as an optoelectronics unit is connected again on the circuit for example in or after a power failure situation. Thus there exists also a need for fast response current measurement to prevent current peaks from causing optoelectronics unit damages, for example light emitting diode (LED) damages, thus resulting as failures in the circuit operation and other additional costs, because damaged LEDs have to be replaced with new ones. A prior art solution to this fast response current measurement is the use of an third amplifier, which, first of all, is an expensive solution, and

furthermore the third amplifier takes more place on a circuit board where the amplifiers and other components of the transformer circuitry are assembled. As an example document of the prior art is mentioned patent application WO 2006/046207 Al. During power-up of LED, current to the LED is directed through a resistor to reduce the voltage across the LED, preventing levels that would cause the LED light output to exceed the LED light output corresponding to the dim command signal. Thus, flickering of the LED is suppressed. The resistor, however, does not protect the LED from current peaks. Short description of the invention

The object of the invention is to accomplish a transformer circuit

arrangement, in which is successfully and with reasonable costs minimized the risk of optoelectronics unit damage. This is achieved by a transformer circuit arrangement comprising a transformer, a primary circuit on a primary side of the transformer, and a secondary circuit on a secondary side of the transformer, the transformer being arranged to transfer electrical energy from the primary circuit to the secondary circuit, the secondary circuit comprising at least one capacitive element for realizing a capacitive feature in the secondary side, and at least one optoelectronics unit for performing an optoelectrical function. The arrangement comprises a first protection unit integrated to the secondary circuit in serial connection to the at least one capacitive element to protect said at least one optoelectronics unit from substantially high secondary circuit current values caused in a discharge of the at least one capacitive element, the first protection unit comprising at least one resistor having a first terminal coupled to the capacitive element, a first transistor, having a first terminal coupled to a second terminal of the at least one resistor and a second terminal coupled to the optoelectronics unit, and a second transistor, having a first terminal connected to the first terminal of the at least one resistor, a second terminal coupled to a control terminal of the first transistor, and a control terminal connected to the second terminal of the at least one resistor. The invention is based on that at least one capacitive element for realizing a capacitive feature, ie. energy storage function, is located on the secondary circuit, and that also to the secondary circuit is integrated a first protection unit in serial connection to the at least one capacitive element for protecting at least one optoelectronics unit in the secondary side from substantially high secondary circuit voltage values caused in a discharge of the at least one capacitive element, said discharge existing especially in situations when the optoelectronics unit is switched on to the transformer circuit for example after some component has been replaced with a new one in the

optoelectronics circuit and/or after a power failure.

The benefit of the invention is that it provides a convenient and relatively low cost implementation for preventing optoelectronics unit damages in the secondary side of the transformer circuitry.

Short description of figures Figure 1 presents a block diagram according to a exemplary embodiment of the invention.

Figure 2 presents a detailed example of a preferred embodiment

according to the invention.

Figure 3 presents a block diagram illustrating a second protection

arrangement.

Detailed description of the invention

In figure 1 is presented a block diagram according to an exemplary embodiment of the invention. A transformer 100 is a device that transfers electrical energy from one circuit to another circuit through inductively coupled conductors Nl, N2. The transformer circuit arrangement in figure 1 comprises a primary circuit 102 on the primary side of the transformer 100, and a secondary circuit 104 on the secondary side of the transformer 100, and the transformer transfers electrical energy from the primary circuit to the secondary circuit. A block 106 in the primary circuit comprises connection means for input voltage, which is for example 230 V and 50 Hz alternating voltage. The block 106 in the primary circuit may also comprise at least one filter for performing electrical filtering operations, for example RFI (Radio Frequency Interference) filtering, and a rectifier for converting alternating current (AC) to direct current (DC), but the block 106 may also have a different content depending on the embodiment, in which the invention is utilized. The secondary circuit 104 may comprise current measuring electronics 108 for performing current monitoring operations and voltage measuring electronics 110 for performing voltage monitoring operations. At least one capacitive element 112 is located on the secondary circuit for realizing energy storage function, ie. capacitive feature on the secondary side. The capacitive element 112 is preferably at least one capacitor 112. At least one optoelectronics unit 114 on the secondary circuit 104 performs optoelectrical functions, such as lighting or signalling. The optoelectronics unit 114 comprises one light emitting diode (LED) or preferably more of them (LEDs). Also the optoelectronics unit 114 can comprise for example at least one PIN, i.e. intrinsic barrier, diode, and/or at least one laser diode in order to, for example, perform transceiver functions as optoelectrical functions. A first protection unit 116 is connected in serial connection to the at least one capacitor 112 for protecting said at least one optoelectronics unit 114 from high secondary circuit voltage and/or current values. For example a peak current value exists because of discharge of the at least one capacitor 112. A typical situation for this is when the optoelectronics unit 114 is switched on to the secondary circuit 104.

Furthermore the arrangement in figure 1 may comprise between the primary circuit 102 and the secondary circuit 104 a feedback arrangement 118 for providing information from the secondary side to the primary side. This information may include for example voltage and/or current information of the secondary circuit 104, which information is provided to the primary circuit for example through an optoelectronics coupling, which the feedback arrangement 118 may comprise. The primary circuit 102 may comprise a flyback electronics 120 for receiving said provided information from the secondary circuit 104 via the feedback arrangement 118, and for utilizing the received information in the operation of the transformer circuitry.

Above presented and related to figure 1 applies also to figure 2, but in figure 2 is presented a detailed and exemplary circuit diagram of a preferred embodiment according to the present invention. The secondary circuit 104 comprises a capacitor 112 for realizing a capacitive feature on the secondary circuit. An integrated circuit 135 comprises amplifiers, preferably operational amplifiers. The first amplifier is part of the current measuring electronics 108, and the second amplifier is part of the voltage measuring electronics 110. The optoelectronics unit 114 comprises at least three light emitting diodes (LEDs) having a need for essentially higher voltage and power values on the secondary circuit 104 than is the case with a single LED. A first protection unit 116 is connected in serial connection to the at least one capacitor 112 for protecting said at least one optoelectronics unit 114 from high secondary circuit voltage and/or current values. For example a peak current value exists because of discharge of the at least one capacitor 112. A typical situation for this is when the optoelectronics unit 114 is switched on to the secondary circuit 104.

The first protection unit 116 comprises at least one resistor 125, a first transistor 126 and a second transistor 127. The first protection unit may comprise several resistors, for example three resistors 123, 124, 125, connected in parallel. The first terminal of the resistor 125 or parallel connected resistors 123, 124, 125 is coupled to the capacitor 112, and the second terminal of the resistor 125 or parallel connected resistors 123, 124, 125 is connected to the first terminal of the first transistor 126. The second terminal of the first transistor 126 is connected to the optoelectronics unit 114. The first terminal of the second transistor 122 is connected to the first terminal of the resistor 125 or parallel connected resistors 123, 124, 125, and the second terminal of the second transistor 122 is connected to the control terminal of the first transistor 126. The control terminal of the second transistor 122 is connected to the second terminal of the resistor 125 or parallel connected resistors 123, 124, 125. Preferably, the first transistor 126 is a field-effect transistor (FET), the first terminal of the first transistor 126 being the source of the FET, the second terminal being the drain of the FET, and the control terminal being the gate of the FET. Preferably, the second transistor 122 is a bipolar transistor, the first terminal of the second transistor 122 being the emitter of the bipolar transistor, the second terminal being the collector of the bipolar transistor and the control terminal being the base of the bipolar transistor. Thus, the potential difference between the emitter and the base of the bipolar transistor 122 corresponds to the voltage over the resistor 123, 124, 125.

To the control terminal of the first transistor 126 is also connected an anode of a diode 127, preferably a Zener diode. The cathode of the diode 127 is connected to the first terminal of the first transistor 126. In case of FET, the anode of the diode 127 is connected to the gate of FET and the cathode of the diode 127 is connected to the source of FET. A capacitor 128 may be connected in parallel with the diode 127.

During normal operation, when the current to the optoelectronics unit 114 is not excessive, the voltage over the resistors 123, 124, 125 remains low. The low potential difference between the emitter and base of the bipolar transistor 122 keeps the bipolar transistor 122 in non-conductive state.

Under these conditions, there is a constant voltage over the Zener diode 127, and, thus, between the gate and source of FET 126. This constant voltage ensures that FET 126 is in conductive state during normal operation. Excessive currents may arise, for example, when a disconnected

optoelectronics unit 114 is replaced, completing the secondary circuit 104. Significant amount of charge may have accumulated at the capacitor 112, and it is discharged when the circuit is complete. Current arising from this discharge may be large enough to cause damage to the optoelectronics unit 114. Using the first protection unit 116, the current at the optoelectronics unit is limited to a maximum safe current I _max . At this maximum safe current Imax, the voltage over the resistors 123, 124, 125 is V _max . The potential difference between the emitter and base of the bipolar transistor 122 is also equal to V _max . By selecting suitable resistance values for the resistors 123, 124, 125, Vmax is large enough to switch the bipolar transistor 122 into conductive state. In other words, a discharge of the capacitor 112 produces in the first protection unit a voltage bigger than the emitter-base threshold voltage of the bipolar transistor 122. Due to the bipolar transistor being in conductive state, FET 126 adapts to linear operation (saturation region), so that essentially constant current equal to I _ma x is provided to the

optoelectronics unit 114.

The detailed embodiment presented in figure 2 also provides a solution how to avoid problems arising from that frequency response of the voltage measurement has to be fast, and frequency response of the current measurement has to be slow, because current measurement must not react to wave signals or wave signal parts caused by mains voltage in normal operation mode of the secondary circuit.

A block diagram corresponding to this solution is presented in figure 3, which comprises similar parts as already presented related to figure 1. The secondary circuit 104, as presented in figure 3, comprises current measuring electronics 108 for performing current monitoring operations. The secondary circuit also comprises voltage measuring electronics 110 for performing voltage monitoring operations for example in open circuit situations. At least one optoelectronics unit 114 on the secondary circuit 104 performs optoelectrical functions, such as lighting or signalling. The optoelectronics unit 114 comprises one light emitting diode (LED) or preferably more LEDs. A second protection unit 111 is integrated in serial connection to the at least one optoelectronics unit 114 for protecting the optoelectronics unit from substantially high secondary circuit current value signals.

The secondary circuit comprises current measuring electronics 108 for performing current monitoring operations, which current measuring electronics is arranged to react to frequencies essentially lower than 100 Hz, so that said current measuring electronics shall not react for example to mains voltage ripple frequencies. This is achieved by coupling a first terminal of a current measuring element, such as a resistor, in series with the optoelectronics unit 114 and connecting the second terminal of the current measuring element to integrated circuit 135. Due to internal delays in the integrated circuit 135, the current measuring electronics 108 measures an average current and does not react to high frequencies. Feedback from the current measuring electronics 108 is provided to fly-back electronics 120 on the primary side via feedback arrangement 118. A second protection unit 111 is integrated to the secondary circuit 104, the second protection unit 111 being in serial connection to the at least one optoelectronics unit 114 for protecting the optoelectronics unit from substantially high value secondary circuit current signals, which comprise frequencies higher than said frequencies to which the current measuring electronics 108 is arranged to react. By this kind of arrangement is achieved a convenient and relatively low cost implementation for preventing

optoelectronics unit damages on the secondary side of the transformer circuitry. The second protection unit is arranged to react to high current values without delays. The second protection unit 111 presented in figure 2 is an overcurrent protection unit comprising a third transistor 130 and at least one resistor 132, 134 to perform current limitation by said third transistor 130. The first terminal of the at least one resistor 132, 134 is coupled to the optoelectronics unit 114. The second terminal of the at least one resistor is connected to the integrated circuit 135. The control terminal of the third transistor 130 is connected to a node between the first terminal of the at least one resistor 132, 134 and the optoelectronics unit 114. The first terminal of the third transistor 130 is connected to the second terminal of the at least one resistor 132, 134. The second terminal of the third transistor 130 is coupled to the feedback arrangement 118. There may be other components between the third transistor 130 and the feedback arrangement 118, but the components must not introduce any significant delays to a signal from the third transistor 130 to the feedback arrangement 118.

Preferably, the third transistor 130 is a bipolar transistor, the first terminal being the emitter of the bipolar transistor, the second terminal being the collector and the control terminal being the base of the bipolar transistor.

The second protection unit 111 does not have an effect, i.e. it is passive, on secondary circuit 104 operation in normal operation conditions where current value signals are at lower current stage than the disruptive high current value signals. Said disruptive high secondary circuit 104 current value signals are caused for example in switch-on situations and/or in rapid load change situations. The current limit value is set by the at least one resistor 132, 134 to protect the optoelectronics unit 114 from substantially, i.e. disruptive, high current value signals. The resistance of the at least one resistor 132, 134 is selected such that the voltage over the at least one resistor is lower than the emitter- base threshold voltage of the bipolar transistor 130 when the current is below the current limit value, keeping the bipolar transistor 130 in non- conductive state. When the current exceeds the current limit value, the voltage over the at least one resistor 132, 134 is large enough to put the bipolar transistor 130 in conductive state.

When the bipolar transistor 130 is in conductive state, it provides a signal to feedback arrangement 118, which provides information to fly-back electronics 120. Based on the information, the fly-back electronics 120 controls the transformer 100 such that the current on the secondary side is reduced. There are no delays between the second protection unit 111 and the feedback arrangement 118, and, therefore, the second protection unit 111 can respond rapidly to high frequency current peaks.

Other transistor types (bipolars, FETs, etc) can also be used to perform the similar kind of operation as described with protection units 111, 116, but for example in first protection unit 116 FET transistor is the preferred choice because the first protection unit 116 circuitry comprises minimum amount of components, and furthermore, FET transistor causes minimum amount of losses in the circuitry. Also the protection unit 111, 116 circuitries can have many kind of variations to achieve objects or part of the objects of the protection units 111, 116.

Although the invention has been presented in reference to the attached figures and specification, the invention is by no means limited to those, as the invention is subject to variations within the scope allowed for by the claims.