FIELD OF THE INVENTION

This invention relates to a power converter and power conversion method, in particular in which the power converter is designed to power a load and to provide at least one auxiliary power supply output.

BACKGROUND OF THE INVENTION

Electronic systems frequently have multiple components requiring different power supplies.

By way of example, in a smart lighting system, multi-output auxiliary power supplies are needed to supply the master control unit (MCU), logic circuits, gate drivers etc. To save cost, the multi-output auxiliary power supplies are often integrated with a main power supply that powers the main load, such as the LEDs that emit the lighting of the lighting system.

The increasing popularity of smart lighting systems has led to increasingly stringent regulations about the standby power consumption of connected light sources. For example the US "Energy star" energy efficiency program requires the standby power consumption of lamps or stand-alone LED drivers to be below 500mW. The California Energy Commission is pushing ahead with a requirement of 200mW maximum standby power for connected lamps. Thus, low standby power consumption for multi-output power supplies becomes a major issue.

To provide the integrated main power supply and the auxiliary power supplies, a transformer with multiple windings is a well-known solution for providing multiple outputs, with low cost and small size. The non-ideal transformer performance means that the coupling between windings is not perfect, giving a reduced cross load regulation of each output.

In order to keep each individual output voltage in its required range, a DC/DC converter or low drop out (LDO) converter, combined with a with high voltage rating switch, is generally added to keep a stable output voltage. Furthermore, it is known to add a dummy load to each output to improve cross regulation to decrease the voltage stress of the switches used in the DC/DC or LDO converter.

However these measures increase power consumption as well as cost.

Figure 1 shows a typical power converter circuit with multiple outputs. The circuit comprises a transformer 10 with a primary side 12 and a secondary side 14.

A first primary winding 16 of the transformer is adapted for connection to a source 18 of AC input power through a bridge rectifier 20.

A first secondary winding 22 of the transformer is adapted for connection to a load 24 which is shown as a smoothing capacitor for downstream power consumption units such as ICs or microcontrollers. This is the main power converter output Vsec l, and it is associated with a feedback control loop 26, 28, 30, 32 coupled between the first secondary winding 22 and the first primary winding 16.

As shown, the feedback control loop comprises a connection 26 from the first secondary winding 22 to an opto-coupler 28, and a feedback controller 30 having a control unit 30a and a shorting transistor 30b, connected via connection 32 to the first primary winding 16. The feedback control loop is adapted to control the power transfer from the first primary winding 16 to the first secondary winding 22. More specifically, in the shown embodiment, the feedback control loop may be a voltage control loop that detects and regulates the voltage on the power converter output Vsec l .

There is at least one auxiliary power supply. For example, a first auxiliary power supply is formed by a third winding 34, which is magnetically coupled to the first primary winding 16 and the first secondary winding 22 and connects to a first auxiliary power supply output Vsec_2. This first auxiliary power supply is on the secondary side so that the third winding 34 is a second secondary winding. The first and second secondary windings share a common ground. There are rectifier diodes at the secondary side outputs.

In this example, there are two other auxiliary power supplies.

A second auxiliary power supply Vpri l is defined by a fourth winding 36 also magnetically coupled to the first primary winding 16 and the first secondary winding 22. This second auxiliary power supply Vpri l is on the primary side so that the fourth winding 36 is a second primary winding.

A third auxiliary power supply Vpri_2 is defined by a fifth winding 38 also magnetically coupled to the first primary winding 16 and the first secondary winding 22. This third auxiliary power supply Vpri_2 is on the primary side so that the fourth winding 38 is a third primary winding. The second and third auxiliary power supplies are on the primary side: they share a common ground with the first primary winding 16.

The second auxiliary power supply Vpri l has a DC/DC converter 37 and the third auxiliary power supply Vpri_2 has a LDO converter 39. There are rectifier diodes at the primary side outputs.

Figure 1 also shows that each auxiliary power supply is associated with a dummy load. The first auxiliary power supply Vsec_2 has a dummy load 40, the second auxiliary power supply Vpri l has a dummy load 42 and the third auxiliary power supply Vpri_2 has a dummy load 44.

The dummy loads decrease the voltage stress on the converters 37, 39 and improve cross regulation, but at the expense of increased power consumption.

WO 00/38304 discloses the concept of a switchable dummy load. The dummy load is switched into and out of circuit to improve circuit stability. In particular, at low output powers, a low dummy load impedance is used to provide stabilized operation, whereas at high output powers a higher impedance is used to reduce dissipation in the dummy load.

This approach still does not enable optimized power consumption, in particular at low output powers.

CA1207382A1 discloses a power converter with multiple ouput windings and an energy store cycle control part for feedback control.

US3824427A1 discloses a high voltage regulator with a regulating winding coupled to a vraible loading circuit.

SUMMARY OF THE INVENTION

In the prior art, the dummy load in one power supply and winding is used for controlling the voltage or power in that power supply only, and the voltage or power in one power supply is controlled by the dummy load in that power supply and winding only. This has a drawback of limited regulation range: in some extreme cases, for example when the power on the winding is too large or too small, the dummy load in that winding is not sufficient for providing the desired regulation.

It is advantageous to provide a power converter with wider range of regulation in the auxiliary power supply.

A basic idea of embodiments of the invention is: tuning the power in one target winding by a dummy load in another winding, given there is a power control loop that regulates the power in a main output winding and all windings are magnetically coupled. The dummy load would cause a change in the power of the main output winding, and the power control loop would change the total input power so as to bring the power of the main output winding back, this changed input power would leak to the target winding and tuning the power in the target winding.

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a power converter comprising:

a transformer with a primary side and a secondary side;

a first primary winding of the transformer at the primary side and adapted for connection to a source of input power;

a first secondary winding of the transformer at the secondary side and adapted for connection to a load;

a feedback control loop coupled between the first secondary winding and the first primary winding and adapted to regulate the power transfer to the first secondary winding;

a third winding, magnetically coupled to the first primary winding and the first secondary winding, and adapted for connection to a first auxiliary power supply output;

a fourth winding magnetically coupled to the first primary winding and the secondary winding;

a first variable load connected to the fourth winding; and

a control circuit for sensing the first auxiliary power supply output on the third winding and controlling the variable load connected to the fourth winding to regulate the first auxiliary power supply output, wherein the control circuit is for controlling the variable load to increase the load at the fourth winding in order to increase a power at the first auxiliary power supply output.

This arrangement has a variable load used for regulating an auxiliary power supply generated by a transformer-based power converter.

The load functions as a dummy load. By making the load variable, losses can be reduced because it is only used as a power dissipating element when the required regulation of the output is needed. It may be made variable by providing a control transistor in series with an impedance. By "magnetically coupled" is meant having an overlapping electromagnetic field.

The controllable dummy load in one winding is used to regulate the output voltage in another winding to realize good cross regulation, with control according to the instantaneous output load distribution. This arrangement enables a minimum dummy load to be maintained to keep the output voltage in a required range, and in this way the power consumption can also be reduced at light loads.

The first auxiliary output is thus controlled by a dummy load at a different transformer winding.

In a first set of examples, the third and fourth windings are on opposite sides of the transformer. For example, the third winding is on the secondary side and the fourth winding is on the primary side.

In this way, the auxiliary power supply and the variable load are on opposite sides of the transformer. The variable load can then be used to maintain the auxiliary power supply within a band just above a lower threshold. In particular, since they are in the opposite side of the transformer, if one variable load in one auxiliary power supply is increased, the flux leakage changes the output at the first secondary winding and the feedback control loop will increase the power transfer to the first secondary winding in response to an increase in the variable load. This this will then increase the power supply to another auxiliary power supply.

The control circuit of the power converter may comprise a first power detection circuit, connected to the first auxiliary power supply output, for detecting the power at the first auxiliary power supply output.

The power detection provides the control input to the control circuit for regulating the first auxiliary power supply output. The detection circuit can connect to the control circuit so as to provide information relating to how to control the variable load.

The third winding for example shares a ground with the one of the first primary winding and the first secondary winding on the same transformer side. Similarly, the fourth winding for example shares a ground with the one of the first primary winding and the first secondary winding on the same transformer side.

The control circuit is adapted, when regulating the first auxiliary power supply output, to control the first variable load to maintain the first auxiliary power supply output above a lower threshold.

In this way, the dummy load is used to prevent the output to the first auxiliary power supply dropping below a lower threshold. More specifically, if the output of the first auxiliary power supply cannot be regulated on its own, the variable load can be involved.

For example, the control circuit may be adapted to: increase the first variable load to increase the first auxiliary power supply output when the first auxiliary power supply output is about to drop below the lower threshold; and

decrease the first variable load to decrease the first auxiliary power supply output when the first auxiliary power supply output is about to exceed a second level above the lower threshold.

In this way, the output is maintained within a band just above the lower threshold.

In one set of examples, the power converter comprises a second variable load connected to the third winding.

In this way, there are two variable loads. Each of the two variable loads is associated with a different transformer winding so that they have different influences on the output delivered to the first auxiliary power supply output. The two variable loads are preferably on opposite sides of the transformer. They then have opposite influence on the output delivered to the first auxiliary power supply output. An increased load at the same transformer side as the first auxiliary power supply output will decrease the output whereas an increased load at the opposite transformer side will increase the output.

The third winding can draw power from the first secondary winding more easily because it is on the same (e.g. secondary) side, causing the power on the first secondary winding to drop more quickly. The feedback control loop causes the first primary winding to deliver more power, thus the fourth winding on the same (e.g. primary) side can obtain more power due to leakage flux from the first primary winding to the fourth winding.

The control circuit may then be adapted, when regulating the first auxiliary power supply output, to control the second variable load to maintain the first auxiliary power supply output below an upper threshold.

For example, the control circuit may be adapted to:

increase the second variable load to decrease the first auxiliary power supply output when the first auxiliary power supply output is about to exceed the upper threshold; and

decrease the second variable load to increase the first auxiliary power supply output when the first auxiliary power supply output is about to drop below a first level below the upper threshold.

In this way, the output is maintained within a band just below the upper threshold. By combining control near the upper and lower thresholds, the regulation enables the auxiliary power supply output to be regulated between two threshold levels.

The fourth winding may be adapted for connection to a second auxiliary power supply output such that the second variable load is connected to the second auxiliary power supply output.

The power converter then has two auxiliary power supply outputs, in addition to the main output (which has feedback control). There may be further auxiliary supply outputs, so that there may be two or more outputs on one or both sides of the transformer.

The control circuit may in this case comprise a second power detection circuit, connected to the second auxiliary power supply output for detecting the power at the second auxiliary power supply output to regulate the second auxiliary power supply output.

The second power detection circuit provides the control input to the control circuit for regulating the second auxiliary power supply output.

The two auxiliary power supply outputs may thus both be regulated.

The control circuit may be adapted, when regulating the second auxiliary power supply output, to:

control the first variable load to maintain the second auxiliary power supply output below an upper threshold; and

control the second variable load to maintain the second auxiliary power supply output above a lower threshold.

In the same way as for the first auxiliary power supply output, variable loads associated with different windings are used to regulate the second auxiliary power supply outputs. The windings are preferably on opposite sides of the transformer.

The first auxiliary power supply and/or the second auxiliary power supply output may be connected to a DC/DC converter or to a low drop out voltage regulator.

The first auxiliary power supply output may be on the secondary side and the second auxiliary power supply output may be on the primary side.

In another example, the third winding and the fourth winding are on same side of the transformer, and the control circuit is then adapted to:

increase the first variable load to decrease the first auxiliary power supply output when the first auxiliary power supply output is about to increase above an upper threshold; and decrease the first variable load to increase the first auxiliary power supply output when the first auxiliary power supply output is about to drop below a second level below the upper threshold.

In this arrangement, the dummy load is on the same transformer side as the auxiliary power supply output to be controlled. It is thus used to regulate the power output around the upper threshold. This is also novel over the prior art since the dummy load in one supply can adjust the power in another supply, although the two supplies are on the same side of the transformer.

As explained above, the use of two dummy loads enablers the power output to be regulated around the lower threshold as well.

The windings are for example coupled with leakage inductance.

The invention also provides a lighting device comprising:

a power converter as defined above;

a lighting element functioning as the load; and

a microcontroller unit coupled to the third winding and powered by the first auxiliary power supply output.

Examples in accordance with another aspect of the invention provide a power conversion method comprising:

connecting a first primary winding of a transformer to a source of input power; connecting a first secondary winding of the transformer to a load; regulating power provided to a third winding, magnetically coupled to the first primary winding and the first secondary winding, and to a first auxiliary power supply output;

providing a fourth winding magnetically coupled to the first primary winding and the first secondary winding and a first variable load connected to the fourth winding; and controlling the first variable load connected to the fourth winding to regulate the first auxiliary power supply output to regulate the first auxiliary power supply output, wherein controlling the variable load to increase the load at the fourth winding in order to increase a power at the first auxiliary power supply output.

This method makes use of a dummy load at one winding to regulate an auxiliary power supply output at another winding.

There may be a second variable load connected to the third winding. In this case, the method may make use of two variable loads (dummy loads) used for regulating the auxiliary power supply. The two dummy loads have different influences on the output delivered to the first auxiliary power supply output.

The third and fourth windings are for example on opposite sides of the transformer. The two dummy loads then have opposite influence on the output delivered to the first auxiliary power supply output so that the output can be controlled to be maintained within a desired range.

The method may further comprise:

connecting the fourth winding to a second auxiliary power supply output; and controlling the first and second variable loads to regulate the second auxiliary power supply output, by:

controlling the second variable load to maintain the second auxiliary power supply output below an upper threshold; and

controlling the first variable load to maintain the second auxiliary power supply output above a lower threshold.

The method thus provides regulation of two (or more) auxiliary power supplies.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows a typical power converter circuit with multiple outputs.

Fig. 2 shows the circuit of Figure 1 modified to provide the additional functionality in accordance with the invention;

Fig. 3 shows the control logic used to derive the gate control signals;

Fig. 4 shows the waveforms for the auxiliary power supply;

Fig. 5 shows the waveforms for the second auxiliary power supply;

Fig. 6 shows the same control approach applied to the third auxiliary power supply;

Fig. 7 shows a circuit with a variable load at the main output of the power supply; Fig. 8 shows controlling the first auxiliary power supply by the variable load in figure 7;

Fig. 9 shows a universal mains auxiliary flyback power supply with four outputs using the approach of Figs. 2 to 6; and

Fig. 10 shows the underlying power conversion control method.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a power converter which uses a transformer with main first (primary) and second (secondary) windings and having a main output and at least one auxiliary output. A third winding is for connection to a first auxiliary power supply output, and there is also a fourth winding. All windings are magnetically coupled together. A variable dummy load is connected to at least the fourth winding and is used regulate the first auxiliary power supply output on the third winding. There may be two variable loads, each associated with a different transformer winding so that they have different influences on the output delivered to the first auxiliary power supply output.

Figure 2 shows the circuit of Figure 1 modified to provide the additional functionality in accordance with the invention. The same components as in Figure 1 are given the same references. As with Figure 1, the power converter is a multi-output flyback circuit with secondary side control for providing a common auxiliary power supply.

Thus, the power converter circuit comprises a transformer 10 with a primary side 12 and a secondary side 14. A first primary winding 16 of the transformer adapted for connection to a source 18 of AC input power through a bridge rectifier 20.

A first secondary winding 22 of the transformer is adapted for connection to the main output load 24 controlled the feedback control loop 26, 28, 30, 32.

The feedback control loop controls the main switching transistor 30b of the switch mode power converter, to implement the desired power control. The remaining parts of the switch mode power converter (in particular the resonant circuit and other switching components) are not shown. Typically this feedback control loop is a voltage control loop and the operation of the switch mode power converter is conventional and will not be described in further detail. Alternatively, the LEDs are connected at the first secondary winding 22 as the load, and this feedback control loop is a current control loop such as those typically for driving LEDs.

The first auxiliary power supply Vsec_2 is formed by a third winding 34, a second auxiliary power supply Vpri l is defined by a fourth winding 36 and a third auxiliary power supply Vpri_2 is defined by a fifth winding 38. The second auxiliary power supply Vpri l has a DC/DC converter 37 and the third auxiliary power supply Vpri_2 has a LDO converter 39. It should be noted that all auxiliary windings can have either or both of the DC/DC converter and LDO converter.

Figure 2 differs from Figure 1 in that each dummy load is associated with a control switch. The first auxiliary power supply Vsec_2 has a dummy load 40 and a dummy load control switch 41, the second auxiliary power supply Vpri l has a dummy load 42 and a dummy load control switch 43 and the third auxiliary power supply Vpri_2 has a dummy load 44 and a dummy load control switch 45.

As a minimum, there is a first dummy load 42 and associated control switch

43 at a different winding to the controlled first auxiliary output. There is preferably a second dummy load 40 which is preferably on the opposite transformer side.

The control switches are for example bipolar junction transistors, and the dummy load is connected in parallel with a smoothing capacitor. Any other device with a controllable impedance, for example a MOSFET or even an electrometric rely, may replace the bipolar transistors.

The switches are controlled by gate signals Vo4i VG43 and VG45, and these control signals are generated by the control unit 30a.

Although not shown, the control unit 30a includes one or more power detection circuits. There is at least a power detection circuit for the first auxiliary power supply Vsec_2. As discussed below, there may be more than one controlled auxiliary power supply outputs, and indeed Figure 2 shows three such outputs, Vpri_l, Vrpi_2 and Vsec_2. There is a respective power detection circuit for each controlled auxiliary power supply output.

The dummy load is thus in each case provided with a series transistor, to enable a controllable dummy load. There is no dummy load for the main output Vsec l in this example because this output has feedback control, meaning the output voltage is already very stable.

The controlled dummy loads for the three auxiliary outputs are used to regulate their own winding voltage, as well as to regulate other winding voltages, with in a required range. The regulation of other winding voltages is a key difference over the prior art of Figure 1. Figure 3 shows the control logic used to derive the gate control signals of each transistor that controls different dummy loads. They may be understood further with reference Figures 4 to 6 and Figure 8 discussed below.

Each auxiliary power supply has a set lower voltage limit and a set upper voltage limit.

For the first auxiliary power supply Vsec_2, the upper limit is VH sec i and the lower limit is VL sec 2.

For the second auxiliary power supply Vpri l, the upper limit is VH_pri_i and the lower limit is VLJI .

For the third auxiliary power supply Vpri_2, the upper limit is VH_pri_2 and the lower limit is VLJII i.

A comparator with hysteresis is used to generate the gate control signals.

When the voltage exceeds the desired upper limit, the dummy load is switched on (i.e. a reduced impedance is provided) to introduce an additional load. This is the function of the hysteresis comparators 50a, 50b, 50c. In all cases, when the voltage reaches the upper threshold, the corresponding gate control signal goes high.

When the voltage drops to the lower threshold, the dummy load of a winding on the opposite side of the transformer is reduced. This is the function of the hysteresis comparators 50d, 50e, 5 Of.

For example, if the voltage at the first auxiliary power supply Vsec_2 drops below the lower threshold VL sec . , the gate voltage to both dummy loads on the primary side is increased (to reduce the dummy load). If the voltage at the second or third auxiliary power supply Vpri_l and Vpri_2 drops below the respective lower threshold VLJ _I , VLJ _II 2, the gate voltage Vo4i to the single dummy load 40 on the secondary side is increased.

This arrangement provides a required voltage range for Vpri l of VLJ _I to

VH_pri_i . The required voltage range for Vpri_2 is VLJII 2 to VH _j ri 2. The required voltage range for Vsec_2 is VL sec 2 to VH sec 2.

Waveforms to explain the operation are shown in Figures 4 to 6.

Figure 4 shows the waveforms for the second auxiliary power supply Vpri l . In order to keep safe operation, VH _j ri i-hi and VH_pri_i-h2 are used as hysteresis levels for the maximum voltage for Vpri l . Thus, the when Vpri l is being regulated at its upper limit, the hysteresis control maintains the level between VH _j ri i-hi and VH_pri_i- 2. In particular, when Vpri_l is higher than VH _j ri i-hi , the gate signal G43 is turned on, meaning the dummy load 42 is involved to consume the power on the winding 36 so as to decrease the voltage Vpri l on the winding 36.

When Vpri_l is lower than VH _j ri -hi, G43 is turned off. Thus by turning on and off the dummy load in the winding 36, the voltage Vpri l on the winding 36 is controlled in a desired range.

It may happen that due to an increase in the power consumption of the load such as a master control unit at the first auxiliary power supply, the first auxiliary power supply cannot maintain its output voltage Vpri l, and this is shown in the decrease of Vpri l from the maximum voltage. In this case, the dummy load 42, even if being turned off, can do nothing to increase the output voltage Vpri l back. The output voltage may drop to the minimum voltage that is allowed by the load such as the master control unit.

VL _J II l-hi and VL _J II -hi are used as hysteresis levels for the minimum voltage for Vpri l . Thus, the when Vpri l is being regulated at its lower limit, the hysteresis control maintains the level between VL _J II i-hi and VL _J II i-h2.

In particular, when Vpri_l is lower than VL _J II -hi, a secondary side dummy load 40 is added on by turning on G ₄₁ . When G ₄₁ is turned on, the load of converter, especially the load on the secondary side as seen by the control loop, is increased, the output sensed by the control loop drops, and the peak current or turn on duration of the main switch 30b in the main circuit will increase as controlled by the control loop to pull high the output. The leakage inductance energy of the transformer increases, namely the leakage inductance energy to the winding 36 increases to generate an increase in Vpri_l .

Extra energy will be absorbed by the load at the winding 36 and thus Vpri l will increase again.

When Vpri_l is higher than VL _J II i-hi, the gate signal G ₄₁ is turned off.

Furthermore, if the load conditions change, for example, if the load on Vpri l increases, Vpri_l may continuously increase into the maximum range ((VHjri -hi)- (VH_pri_i- hi)) and keep stable there. Alternatively, it may increase beyond (VH _j ri i-hi), in which case the upper control will operate, to turn on VG43 to lower down Vpri l with hysteresis control o f the maximum vo ltage .

Figure 4 shows a period of hysteresis control at the upper limit, followed by a change in load conditions causing the control to provide a period of hysteresis control at the lower limit. It can be seen from Figure 4 that there is control of two dummy loads in order to provide voltage regulation at both the upper and lower voltage limits. In particular, by controlling dummy loads on opposite sides of the transformer it is possible to provide voltage regulation within a range while minimizing the use of the dummy loads.

Figure 5 shows the same control approach applied to the third auxiliary power supply Vpri_2.

In the same way as for Figure 4, VH_pri_2- i and VH_pri_2-h2 are used as hysteresis levels for the maximum voltage for Vpri_2. When Vpri_2 is higher than VH_pri 2-hi , the gate signal G45 is turned on.

When Vpri_2 is lower than VH_pri_2-h2, G45 is turned off.

VL _J II 2-hi and VL _J II 2-h2 are used as hysteresis levels for the minimum voltage for Vpri_2. When Vpri_2 is lower than VL _J II 2-h2, a secondary side dummy load is added on by turning on G ₄₁ . When Vpri_2 is higher than VL _J II 2-hi , the gate signal G ₄₁ is turned off.

In Figures 4 and 5, the auxiliary power supply to be regulated is at the primary side, and the dummy load and winding involved in the regulation is at the secondary side. It may also be in an opposite configuration. Figure 6 shows the same control approach applied to the first auxiliary power supply Vsec_2 at the secondary side.

In the same way as for Figures 4 and 5, VH see 2-hi and VH see 2-h2 are used as hysteresis levels for the maximum voltage for Vsec_2. When Vsec_2 is higher than VH sec 2- hi , the gate signal G ₄₁ is turned on, meaning the dummy load 40 is involved to consume the power on the winding 34 so as to decrease the voltage Vsec l on the winding 34.

When Vsec 2 is lower than VH sec 2-h2, G41 is turned off.

VL sec 2-hi and VL sec _2-h2 are used as hysteresis levels for the minimum voltage for Vsec_2. When Vsec_2 is lower than VL sec 2-h2, a primary side dummy load is added. There are two possible primary side dummy loads, 42 and 44, and they may both be turned on by turning on both G43 and G45 as shown. Alternatively, only one may be needed. When Vsec_2 is higher than VL sec 2-hi , the gate signals G43 and G45 are turned off.

The examples above make use of variable loads at the outputs other than the main feedback-controlled output. However, that output (Vsec l) may also have a variable load as shown in Figure 7. To be noted is that the variable load may be a dedicated dummy load instead of the main load of the power converter such as LEDs of the lighting device.

A dummy load is shown, again in the form of an impedance 70 together with a series control transistor 71. Thus, in this example, there are two variable loads on each side of the transformer. This means that the third and fourth windings may be on the same side of the transformer - where the third winding is the one 34 connected to the first auxiliary power supply (i.e. Vsec_2) and the fourth winding is the one connected to the first variable load, i.e. winding 22 and the associated dummy load 70 in the example of Figure 7.

Again, the dummy load is connected to a different winding to the auxiliary power supply being controlled.

Figure 8 shows the how additionally dummy load 70,71 alters the control approach applied to the first auxiliary power supply Vsec_2. It corresponds to Figure 6, but when the dummy load 40,41 is turned on, the dummy load 70,71 is also turned on. Thus, the two dummy loads on one transformer side are used to provide control of the upper threshold, and the two dummy loads on the opposite transformer side are used to provide control of the lower threshold. For control of the other auxiliary power supply outputs, the dummy load 70,71 can again be controlled in synchronism with the control of the other dummy load 40,41 on the same transformer side. Alternatively, only one dummy load on each side can be involved.

Thus, part of the control of the circuit of Figure 7 involves increasing the first variable load (i.e. load 70 in this example) to decrease the first auxiliary power supply output (Vsec_2) when the first auxiliary power supply output Vsec_2 is about to exceed an upper threshold; and decrease the first variable load (i.e. load 70 in this example) to increase the first auxiliary power supply output Vsec_2 when the first auxiliary power supply output Vsec_2 is to drop below a second level below the upper threshold.

The invention may be used in a universal mains (120 Vac -277 Vac) 6W auxiliary flyback power supply with four outputs as shown in Figure 9.

The outputs are:

Output voltage Load

15V (Vpri l) 0~20mA

5V (Vpri_2) 0~50mA

18V (Vsec_2) 0~70mA

24V(Vsec_l) 0~170mA

In order to keep good load regulation of the primary output voltages, a 30mW dummy load is provided on the secondary side outputs. By adopting the controlled dummy load method described above, a total 30mW power consumption can be saved when the system is under light load conditions while still keeping good load regulations.

The example above has four outputs. However, the invention may be applied to a system with only one auxiliary power supply and only one dummy load. The dummy load may be on the same transformer side or the opposite transformer side as the auxiliary power supply, and it may then be used either for regulating the maximum power or the minimum power. If dummy loads are provided on both transformer sides, power regulation within bands adjacent the upper and lower thresholds become possible as explained above. All of these options are within the scope of the invention.

If there is only a single auxiliary power supply but two dummy loads, there may be a winding (e.g. the fourth winding) only provided for the purposes of adding a dummy load and not serving as a power supply output.

The invention may be extended to multiple auxiliary power supply outputs, such as two, three (as in the example given above) or indeed more.

Figure 10 shows the underlying power conversion control method.

In step 80, a first primary winding of a transformer is connected to a source of input power.

In step 82, a first secondary winding of the transformer is connected to a load. In step 84 a third winding is connected to a first auxiliary power supply output. This defines the connections to a power converter.

In step 86, the power transfer from the first primary winding to the first secondary winding is controlled using feedback control.

In step 88, a first variable load connected to a fourth winding (e.g. on the opposite side of the transformer to the third winding) is controlled, in order to regulate the first auxiliary power supply output to maintain the first auxiliary power supply output above a lower threshold.

In step 90 optionally a second variable load connected to the first auxiliary power supply output is controlled, in order to regulate the first auxiliary power supply output to maintain the first auxiliary power supply output below an upper threshold.

The method may comprise connecting the fourth winding to a second auxiliary power supply output. In this case, the first and second variable loads may be controlled to regulate the second auxiliary power supply output, by controlling the first variable load to maintain the second auxiliary power supply output below an upper threshold; and controlling the second variable load to maintain the second auxiliary power supply output above a lower threshold. The first and second auxiliary power supply outputs may be on opposite sides of the transformer, so there are both primary and secondary side auxiliary power supplies.

The effective impedance of the dummy loads may be altered in a binary manner as shown in Figures 4 to 6 and 8. However, it may instead be altered gradually in steps. Although the embodiments relate to flyback converters, the invention is also applicable to other types of switched-mode power supplies.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. 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. Any reference signs in the claims should not be construed as limiting the scope.