WO1999025059A1

WO1999025059A1 - Dc-dc converters

Info

Publication number: WO1999025059A1
Application number: PCT/CA1998/001048
Authority: WO
Inventors: Praveen Kumar Jain; Raymond Kenneth Orr; Yan-Fei Liu; Youhao Xi
Original assignee: Praveen Kumar Jain; Raymond Kenneth Orr; Liu Yan Fei; Youhao Xi
Priority date: 1997-11-10
Filing date: 1998-11-10
Publication date: 1999-05-20
Also published as: CA2220747A1

Abstract

A single ended forward converter comprises a primary switch, a transformer having a primary winding coupled in series with the primary switch to supply voltage terminals and a secondary winding coupled via a rectifying and smoothing circuit to output voltage terminals, and a series-connected first inductor and auxiliary switch coupled in parallel with the primary switch. Leakage inductance between the primary and secondary windings, which may be supplemented by an inductive component in series with one of the windings, resonantly couples energy to a capacitance in parallel with the primary switch, the energy being recovered, when the auxiliary switch is open, via a second inductor inductively coupled to the first inductor. Control signals for the switches are timed to provide substantially zero voltage switching of the primary switch to provide high efficiency. The converter also provides for resetting of the transformer in each switching cycle.

Description

DC-DC CONVERTERS This invention relates to DC-DC converters, and is particularly concerned with single ended forward converters providing reduced power losses. Background of the Invention

Single ended forward converters are well known, and it is also well known that in such a converter it is necessary to provide for resetting of the transformer to avoid saturation of the transformer core. Various forms of reset circuit are known, including active reset circuits in which magnetizing energy is conserved rather than being dissipated in a resistor, in order to reduce power losses in the converter, and in which the duty cycle of the main switching transistor can exceed 50%. For example, such converters and reset circuits have been described in "High Power SMPS Require Intrinsic Reliability" by Bruce Carsten, Proceedings of PCI '81, September 1981, pages 118-133 and in "Design Techniques For Transformer Active Reset Circuits At High Frequencies And Power Levels" by Bruce Carsten, High Frequency Power Conversion Proceedings, May 1990, pages 235-246.

Distributed power supplies, including low output voltage supplies, constituted by such converters are increasingly required to operate with high efficiency, and hence low power losses, and at a high switching frequency to minimize size and power dissipation. However, increased switching frequencies result in increased power losses in the form of switching losses of active devices such as switching transistors. Although single ended forward converters with active reset circuits can be intended to operate with zero voltage switching in order to reduce switching losses, this is not necessarily achieved especially with varying load conditions and with a high switching frequency which makes very precise timing necessary for controlling the switching transistors.

An object of this invention is to provide an improved single ended forward converter. Summary of the Invention

One aspect of this invention provides a method of operating a single ended forward converter comprising a primary switch in series with a primary winding of a transformer, comprising the steps of, for each switching cycle of the primary switch: coupling a resonant circuit comprising a capacitance and a first inductor to the transformer by closing an auxiliary switch whereby energy is transferred from the transformer and the capacitance to the first inductor in a resonant manner whereby a voltage of the capacitance is reduced to substantially zero; closing the primary switch when the voltage of the capacitance is substantially zero; opening the auxiliary switch to recover energy stored in the first inductor via a second inductor coupled to the first inductor; and opening the primary switch.

Another aspect of the invention provides a method of operating a single ended forward converter comprising the steps of, for each of successive switching cycles of the converter: closing an auxiliary switch to resonate a capacitor with a first inductor, the capacitor being coupled to a combination of a primary switch and a primary winding of a transformer coupled in series to a voltage source, whereby a voltage across the primary switch is reduced to substantially zero; closing the primary switch when said voltage across it is approximately zero; and opening the auxiliary switch and the primary switch. The auxiliary switch can be opened at substantially the same time as or soon, e.g. immediately, after closing of the primary switch, or it can be opened later after closing and no later than opening of the primary switch.

The time when the voltage across the primary switch is reduced to substantially zero can be modified by an inductive component coupled in series with a winding, for example the secondary winding, of the transformer.

A further aspect of the invention provides a method of operating a DC-DC converter to provide substantially zero voltage switching to close a primary switch in series with a primary winding of a transformer, comprising the steps of first closing an auxiliary switch to resonate an inductor with a capacitor coupled to a winding of the transformer whereby a voltage across the primary switch is reduced, and closing the primary switch when the voltage across it is substantially zero. Another aspect of this invention provides a method of operating a single ended forward converter including a primary switch in series with a primary winding of the transformer, comprising the steps of: providing a capacitor coupled in parallel with the primary switch or a winding of the transformer; and resonantly removing energy from the capacitor to an inductor when the primary switch is open.

This method preferably includes the step of timing closing of the primary switch to be when a voltage across the primary switch is reduced to substantially zero by the resonance of the capacitor with the inductor, to achieve zero voltage turn-on of the primary switch. This method preferable further includes the step of recovering energy from the inductor by switching current through the inductor with an auxiliary switch, and coupling energy from the inductor via a second inductor, inductively coupled to said inductor, when the auxiliary switch is open.

The invention also provides a single ended forward converter comprising: a primary switch; a transformer having a primary winding coupled in series with the primary switch to supply voltage terminals and a secondary winding coupled via a rectifying and smoothing circuit to output voltage terminals; and a first inductor and an auxiliary switch coupled in series, the first inductor and auxiliary switch being connected in parallel with the primary switch or a winding of the transformer; the converter including an inductance comprising at least a leakage inductance of the primary and secondary windings and a capacitance comprising at least a parasitic capacitance in parallel with the primary switch or a winding of the transformer.

Conveniently said capacitance comprises, in addition to parasitic capacitance, a capacitor coupled in parallel with the primary switch or a winding of the transformer, and/or said inductance comprises, in addition to the leakage inductance, an inductive component coupled in series with a winding of the transformer.

The converter preferably includes a second inductor inductively coupled to the first inductor for recovering energy from the first inductor when the auxiliary switch is open. The second inductor can be coupled via a diode either to the supply voltage terminals or to said capacitance. The invention further provides a single ended forward converter comprising: a transformer having a primary winding and a secondary winding; a primary switch connected in series with the primary winding for coupling the primary winding to a voltage source; an output circuit comprising forward and free-wheel diodes, an output inductor, and a smoothing capacitor coupling the secondary winding to output voltage terminals; and an auxiliary circuit comprising: a first inductor and an auxiliary switch controlled for each switching cycle of the converter to close before closing of the primary switch and to open not before closing of the primary switch and not after opening of the primary switch, the auxiliary switch when closed coupling the first inductor in parallel with the primary switch or a winding of the transformer.

The converter preferably includes a capacitor coupled in parallel with the primary switch or a winding of the transformer. The converter preferably further includes a second inductor inductively coupled to the first inductor, and a diode in series with the second inductor for coupling the second inductor to the capacitor or to the voltage source for returning recovered energy thereto. The converter can advantageously further include an inductive component coupled in series with the primary or secondary winding of the transformer.

Thus different aspects of the invention provide for zero voltage switching of the primary switch and/or recovery of magnetic energy in a single ended forward converter, using resonant characteristics of the converter which involve leakage inductance of the transformer and/or an inductive component in series with one of its windings, and parasitic capacitance and/or a capacitor coupled in parallel with the primary switch or one of the transformer windings.

The invention facilitates the provision of a single ended forward converter which has the advantages of relatively lossless switching independent of supply and load conditions, simple power and control circuitry, no increase in conduction losses between switching times, and ability to operate in either current or voltage mode control. Brief Description of the Drawings

The invention will be further understood from the following description with reference to the accompanying drawings, in which: Fig. 1 schematically illustrates a single ended forward converter in accordance with an embodiment of this invention; Fig. 2 illustrates operating waveforms of the converter of Fig. 1; and Fig. 3 schematically illustrates a modified single ended forward converter in accordance with another embodiment of this invention. Detailed Description Referring to Fig. 1, a single ended forward converter is illustrated for providing from an input or source voltage +V supplied at DC supply terminals 10 a desired output voltage at DC output terminals 12 for supply to a load (not shown). The magnitude of the source voltage +V is arbitrary; this may, for example, be in a range from 30 to 70 volts. An input capacitor 14 connected between the terminals 10 ensures a low impedance source for the converter. The magnitude of the output voltage is also arbitrary; this may, for example, be 5 volts. A control circuit (not shown) serves to supply a pulsed control signal Gl to the converter for maintaining this output voltage at its desired level in known manner, for example using pulse width modulation in a current mode or voltage mode feedback arrangement. The signal Gl typically has a high frequency, for example a fixed frequency of 300 kHz, to permit the converter to be implemented using components of relatively small size.

The converter includes a magnetic core transformer 16 having a primary winding 18 and a secondary winding 20 the senses of which are represented conventionally in Fig. 1 by dots adjacent to the windings. The primary winding 18 is connected in series with the drain-source path of an N-channel MOSFET 22 (each of the MOSFETs in Fig. 1 is illustrated as including its parasitic or body diode connected in parallel with the drain-source path of the MOSFET) between the terminals 10. This series circuit may also include a low impedance resistor (not shown) for current sensing. The MOSFET 22 constitutes a primary switch of the converter, and is controlled by the signal Gl supplied to its gate.

The secondary winding 20 of the transformer 16 has a first end connected via a forward diode 24 and an output inductor 26 to one of the terminals 12, and a second end connected via an inductance 32 to the other of the terminals 12, which conveniently may be grounded as illustrated in Fig. 1. An output capacitor 28 is connected between the output terminals 12, and a free-wheel or catch diode 30 is connected between the grounded one of the terminals 12 and the junction between the diode 24 and inductor 26. The inductance 32, which is discussed further below, can alternatively be connected in series with the diode 24 on the first side of the winding 20, and/or can be partly or wholly constituted by leakage inductance of the transformer 16 so that a separate inductive component is not required. Thus the inductance 32 represents the leakage inductance between the primary and secondary windings of the transformer 16, optionally with an additional inductive component.

Except in respect of the inductance 32, the single ended forward converter as described above has a known form and operates in known manner which need not be further described here. As is well known, such a single ended forward converter also requires some circuitry or mechanism for resetting the transformer core to avoid magnetic saturation and to permit use of a physically small transformer. Typical of such circuitry in the prior art is an active reset circuit as described in the Carsten articles referred to above. The single ended forward converter illustrated in Fig. 1 includes an auxiliary circuit 34, shown within a dashed-line box, which with the inductance 32 operates as described below to reset the transformer on each cycle of the control signal and to provide other advantages such as facilitating zero voltage switching of the primary switch 22 of the converter, and hence low switching losses. The circuit 34 comprises a capacitor 36, an auxiliary switch constituted by an

N-channel MOSFET 38, two coupled inductors 40 and 42, and a diode 46. The coupled inductors 40 and 42 effectively constitute primary and secondary windings respectively of a transformer 44 and have the relative senses shown in Fig. 1 by dots, and accordingly are referred to below as primary and secondary inductors. The capacitor 36 is connected in parallel with the primary switch constituted by the drain- source path of the MOSFET 22, and hence in series with the primary winding 18 between the terminals 10. The drain-source path of the MOSFET 38 is connected in series with the primary inductor 40 across, i.e. in parallel with, the capacitor 36. An auxiliary control signal G2 is supplied from the control circuit (not shown) to the gate of the MOSFET 38. The secondary inductor 42 is connected in series with the diode 46 between the terminals 10, the diode being poled to prevent a short circuit of the supply voltage by the inductor 42.

In order to avoid undesired oscillation under certain operating conditions, the circuit 34 can also optionally include a diode 48, shown in dashed outline to indicate its optional presence, connected in series with the drain-source path of the MOSFET 38 and the primary inductor 40, poled oppositely to the body diode of the MOSFET 38. The operation of the converter of Fig. 1 is described in detail below with additional reference to the waveforms illustrated in Fig. 2. These waveforms comprise, for one cycle at the switching frequency in normal operation of the converter, the control signal Gl for the primary switch constituted by the MOSFET 22, the voltage VI across the drain-source path of this MOSFET 22 (and hence across the capacitor 36) and the current II through this path, the control signal G2 for the auxiliary switch constituted by the MOSFET 38, the voltage V2 across the drain-source path of this MOSFET 38 and the current 12 through this path (and hence also through the primary inductor 40), the current 13 through the inductance 32, and the current 14 through the secondary inductor 42. The locations of these voltages and currents are illustrated for convenience in Fig. 1. Each switching cycle can be regarded as comprising sequential operating phases commencing at times Tl to T7 shown at the bottom of Fig. 2, and these phases are described below starting for convenience at the time Tl.

For convenience below, the primary switch is also designated by the reference 22 pertaining to the MOSFET constituting this switch, and the auxiliary switch is also designated by the reference 38 pertaining to the MOSFET constituting this switch. In the description below, n represents the reciprocal of the (primary to secondary) turns ratio of the transformer 16, Ls represents the magnitude of the inductance 32, combining the leakage inductance with the inductance of any additional inductive component, and it is assumed that the inductance Lo of the output inductor 26 is relatively very large. The magnitude of the inductance 32 reflected back to the primary

2 winding 18 is Ls/n , and (when the diodes 24 and 30 are both forward biased) this appears in parallel with the magnetizing inductance Lm of the transformer 16. It is also assumed that Lm is much greater than the inductance Lp of the primary inductor

2 40; for example, the inductances Lp and Ls can be related by the equation Ls = n Lp.

The control circuit determines in known manner as indicated above a duty cycle or on period of the primary switch 22 of the converter, which starts at the time T3 and ends at the time T5, as shown by the control signal Gl in Fig. 2. The control circuit also determines the control signal G2 for the auxiliary switch 38 to turn on this switch starting at the time Tl, which is in advance of the time T3 by an amount which is determined by characteristics of the converter as described below to be such that substantially zero voltage switching of the primary switch 22 takes place at the time T3. Fig. 2 illustrates the control signal G2 as turning off the auxiliary switch 38 at substantially the same time T3. In practice, it is desirable to ensure that the auxiliary switch 38 is turned off only when the primary switch 22 is substantially fully turned on. For this reason, the end of the pulse of the control signal G2 may be delayed until after the time T3, as indicated by a broken line 50 in Fig. 2. The timing of the end of this G2 pulse is not particularly critical, so that this delay can be extended potentially up to the time T5. However, during such delay period the auxiliary switch 38 is on so that there is a conduction loss, and it is preferable to keep the delay period short to reduce conduction losses and enhance the efficiency of the converter.

Immediately prior to the time Tl, the transformer has been reset and both switches are off, with the currents II, 12, 13, and 14 all being substantially zero. Consequently the voltages VI and V2 are the same and, as can be seen from the description below, are greater than the supply voltage +V. Commencing at the time Tl, the auxiliary switch 38 is turned on by the control signal G2.

Between the times Tl and T3, a resonant tank circuit is formed by the capacitor 36 in parallel with the primary inductor 40, both in series with the magnetizing inductance Lm of the transformer 16. The voltage V2 across the auxiliary switch 38 falls rapidly to substantially zero, and the current 12 through it rises in a resonant fashion. This current comprises two parts: discharge current of the capacitor 36, and current flowing via the primary winding 18. The voltage VI across the primary switch 22 and the capacitor 36 consequently falls also in a resonant fashion, until it reaches zero at the time T3.

At the time T2, when the voltage VI reaches the supply voltage +V, current in the output circuit starts to flow via the diode 24 (as well as flowing via the diode 30), so the current 13 increases in a resonant fashion from zero until the time T3. Consequently, the inductance 32 is reflected at the primary winding 18 to appear as the

2 reflected inductance Ls/n in parallel with the magnetizing inductance Lm of the transformer. The consequently decreased inductance reduces the time constant for the resonant waveforms of the voltage VI and the current 12 between the times T2 and T3, in comparison to the time constant between the times Tl and T2.

At the time T3, the voltage VI across the primary switch 22 is substantially zero, and this switch 22 is turned on by the control signal Gl. Thus there is substantially zero voltage switching of the primary switch 22, avoiding any significant switching loss, at this time. Starting at the time T3, the supply voltage +V applied by the primary switch 22 across the primary winding 18 causes the current 13 to rise linearly, until at the time T4 it reaches a current Io which flows through the output inductor 26 to the load connected to the terminals 12. At this time T4 the free-wheel diode 30 ceases conducting, and the full output inductor current Io continues to be supplied via the diode 24 for the remainder of the on period of the primary switch 22 until the time T5, in the known manner of a conventional single ended forward converter. Between the times T3 and T5 the voltage VI across the primary switch 22 is substantially zero, and the current II through the primary switch 22 similarly rises linearly between the times T3 and T4. Between the times T4 and T5, the currents II and 13 ramp up slightly. At the time T3, or subsequently as mentioned above and indicated by the broken line 50 in Fig. 2, when the pulse of the control signal G2 ends the current 12 through the auxiliary switch 38 is interrupted. The abrupt interruption in current through the primary inductor 40 reverses the voltage polarity of the coupled inductors, so that the diode 46 is forward biased and conducts, feeding the stored energy of the coupled inductors back to the supply voltage line (terminals 10 and input capacitor 14). With the diode 46 conducting, the secondary inductor 42 is coupled to the constant voltage +V, so that the current 14 flowing through it decreases linearly from an initial value to zero, as shown at 52 in Fig. 2. The voltage V2 across the auxiliary switch 38 between the times T3 and T5 is the supply voltage +V multiplied by the turns ratio of the coupled inductors 40 and 42. When the current 14 reaches zero, the diode 46 is no longer forward biased and the voltage V2 drops to zero, tracking the voltage VI.

At the time T5, the pulse of the control signal Gl ends to turn off the primary switch 22, so that its current II is interrupted. The current I3=Io in the output circuit is maintained by the inductance of the output inductor (the diode 24 can be considered as a short circuit, and the diode 30 an open circuit, at this time so that the output inductance is connected in series with the secondary winding 20 and the inductance 32) to cause the capacitor 36 to be charged with a substantially constant current. Consequently the voltage VI across the primary switch 22 and the capacitor 36 rises linearly from substantially zero, until it reaches the supply voltage +V at the time T6. Thus there is substantially zero voltage switching to turn off the primary transistor 22 at the time T5. The voltage V2 across the auxiliary transistor 38 is pulled up with the rising voltage VI.

At the time T6, the voltage across the primary winding 18 is reversing through zero due to the rising voltage VI, so that the diode 30 begins to conduct (the diodes 24 and 30 are both forward biased and hence can be considered to be short circuits, connecting the inductance 32 across the secondary winding 20). The inductance 32 is reflected via the transformer 16 to appear in parallel with the magnetizing inductance of the transformer, forming a resonant circuit with the capacitance 36 so that the energy stored in the inductance 32 is quickly transferred to the capacitor 36 between the times T6 and T7. Between these times the current 13 through the inductance 32 falls to zero, so that at the time T7 the diode 24 is no longer forward biased. The voltage VI continues to rise above the supply voltage +V until the time T7, and is followed by the voltage V2.

For the remainder of the switching cycle, until a time Tl' for the start of the next cycle, the capacitor 36 resonates with the magnetizing inductance Lm of the transformer 16, this resonant combination having a time constant that is much greater than the period of the switching cycle so that there is relatively little change in the voltage VI, and in the voltage V2 following it, between the times T7 and Tl'. Starting at the time Tl', the switching cycle is repeated. It can be appreciated that the transformer is reset during the interval from the time T6 in each switching cycle until the time T2 in the following switching cycle, when the polarity of voltage applied to the primary winding is reversed, the volt- seconds applied during this interval being equal and opposite to the volt-seconds applied in the switching cycle from the time T2 to the time T6.

As already mentioned, the inductance 32 can be constituted partly or entirely by the leakage inductance of the transformer 16. Similarly, the capacitance of the capacitor 36 can be constituted partly or entirely by parasitic capacitances of the MOSFET 22 and of the primary winding 18 of the transformer 16. In any event, these parasitic values can be taken into account in determining desired physical component values of the inductance 32 and capacitor 36. Generally, it may be desirable to provide both of these components in order to provide predictable and stable, and not too small, time constants for the resonant operating phases of the converter as described above. This is particularly the case because these determine for example the time T3 at which the control circuit is desired to provide the start of the pulse of the control signal Gl to achieve zero voltage switching for turning on the primary switch 22. The provision of an inductive component 32 additional to the leakage inductance of the primary and secondary windings of the transformer 16, and/or a capacitor 36 in addition to the parasitic capacitances of the circuit, enables time constants of the resonant circuits to be increased so that switching times in operation of the converter can be easily and accurately determined. The following parameters are provided purely by way of example for a 100 watt converter providing an output of 5 volts from a supply voltage +V of 40 to 60 volts: Magnetizing inductance Lm 150 μH

Reciprocal of turns ratio n \

Capacitor 36 10 nF

Inductance 32 Ls 0.33 μH

Primary inductor 40 Lp 3 μH

Secondary inductor 42 100 μH

Fig. 3 illustrates a modification of the single ended forward converter of Fig. 1. In the converter of Fig. 3, the blocking diode 46 is moved to the opposite, or 0V, side of the secondary inductor 42 and is still in series with it, and the other end of this inductor 42 is connected to the junction of the primary inductor 40, capacitor 36, primary switch 22 and primary winding 18. The coupled inductors 40 and 42 can thus be simplified to provide only three connections. The optional diode 48, if provided in the converter of Fig. 3, is connected in series with the primary inductor 40 for example between this inductor 40 and the auxiliary switch 48. The converter of Fig. 3 operates in a similar manner to that of Fig. 1 as described above, except for the recovery of energy from the coupled inductors. In this case, when the auxiliary switch 38 is opened at the end of the pulse of the control signal G2, at or after the time T3, current is initially circulated via the closed primary switch 22 and the diode 46 with relatively little loss. When the primary switch is opened at the time T5, this current instead serves to charge the capacitor 36, whereby the energy stored in the coupled inductors is in this case recovered in the capacitor 36 instead of in the capacitor 14.

In the converter of Fig. 3 the recovery of energy from the coupled inductors does not start until the time T5, it can be seen that it is possible to delay until the time T5 the end of the pulse of the control signal G2 as discussed above. The particular time for ending of the pulse of the control signal G2 to turn off the auxiliary switch 38 can depend on particular circumstances, such as relative conduction losses for the switches 22 and 38.

The single ended forward converter topologies described above provide particular advantages in that substantially zero voltage switching of the primary switch 22 can be achieved, for turning the switch both on and off, under diverse supply voltage and load conditions, so that switching losses can be reduced and efficiency of the converter can be increased. The auxiliary switch 38 is not switched under zero voltage conditions, but it switches a lower current than the primary switch 22 so that it can be a smaller MOSFET having less capacitance and, therefore, its switching losses are much lower than would be the case for the primary switch. Consequently, higher frequency operation of the converter, and hence smaller size and costs, are possible. In addition, there is no need for a reset winding on the transformer 16, resetting of the transformer core being achieved as described above by the capacitor 36 and inductance 32 (including leakage inductance of the transformer). As can be seen from the waveforms VI and V2 in Fig. 2, voltage stresses on the switches 22 and 38 are not high and are limited by the capacitor 36. Energy stored in each switching cycle in the inductance 32 (including leakage inductance of the transformer) is transferred to the capacitor 36 and then recovered via the coupled inductors, thereby facilitating efficient operation of the converter. Furthermore, the primary switch duty cycle of the converter is not limited to 50%, as is often the case for single ended forward converters, but can be considerably greater than this.

Although as described above the capacitor 36 is connected in parallel with the primary switch 22, it can be appreciated that a capacitor having an equivalent function can alternatively be connected in parallel with the primary winding 18 of the transformer 16. In addition, connections of the auxiliary circuit 34 can be re-arranged to suit particular desires, for example to use different polarities of MOSFETs for the switches and/or to interchange connections to the +V and 0V lines connected to the terminals 10. Furthermore, although the auxiliary circuit 34, and specifically the components 36 to 40 (and 48 if present) of this circuit, are described above as being coupled to the primary winding 18, these or their equivalent could alternatively be coupled on the secondary winding 20 side of the transformer 16, or to an auxiliary winding (not shown) of the transformer 16. Thus for example the capacitor 36 or its equivalent can be connected in parallel with the secondary winding or in parallel with an auxiliary winding of the transformer.

It can also be appreciated that any inductive component additional to the leakage inductance between the primary and secondary windings of the transformer constituting the inductance 32 can alternatively be provided in series with the primary winding 18 of the transformer.

Thus although particular embodiments of the invention have been described in detail, it can be appreciated that these and numerous other changes, variations, and adaptations may be made without departing from the scope of the invention as defined in the claims.

Claims

WHAT IS CLAIMED IS:

1. A method of operating a single ended forward converter comprising a primary switch in series with a primary winding of a transformer, comprising the steps of, for each switching cycle of the primary switch: coupling a resonant circuit comprising a capacitance and a first inductor to the transformer by closing an auxiliary switch whereby energy is transferred from the transformer and the capacitance to the first inductor in a resonant manner whereby a voltage of the capacitance is reduced to substantially zero; closing the primary switch when the voltage of the capacitance is substantially zero; opening the auxiliary switch to recover energy stored in the first inductor via a second inductor coupled to the first inductor; and opening the primary switch.

2. A method as claimed in claim 1 wherein the auxiliary switch is opened at substantially the same time as or immediately after closing of the primary switch.

3. A method as claimed in claim 1 wherein the auxiliary switch is opened after closing and no later than opening of the primary switch.

4. A method as claimed in any of claims 1 to 3 wherein energy stored in the coupled first and second inductors is recovered to a voltage source via a diode in series with the second inductor.

5. A method as claimed in any of claims 1 to 3 wherein energy stored in the coupled first and second inductors is recovered via the second inductor to said capacitance when the primary switch is open.

6. A method as claimed in any of claims 1 to 5 and including the step of inhibiting oscillation between the first inductor and the capacitance by a diode in series with the first inductor.

7. A method as claimed in any of claims 1 to 6 and including the step of increasing a time constant for reducing the voltage of the capacitance to substantially zero by an inductive component coupled in series with a winding of the transformer.

8. A method as claimed in claim 7 wherein the inductive component is coupled in series with a secondary winding of the transformer.

9. A method of operating a single ended forward converter comprising the steps of, for each of successive switching cycles of the converter: closing an auxiliary switch to resonate a capacitor with a first inductor, the capacitor being coupled to a combination of a primary switch and a primary winding of a transformer coupled in series to a voltage source, whereby a voltage across the primary switch is reduced to substantially zero; closing the primary switch when said voltage across it is approximately zero; and opening the auxiliary switch and the primary switch.

10. A method as claimed in claim 9 and including the step of recovering energy stored in the first inductor via a second inductor inductively coupled to the first inductor.

11. A method as claimed in claim 9 or 10 wherein the auxiliary switch is opened at substantially the same time as or soon after closing of the primary switch.

12. A method as claimed in claim 9 or 10 wherein the auxiliary switch is opened after closing and no later than opening of the primary switch.

13. A method as claimed in any of claims 9 to 12 wherein the time when the voltage across the primary switch is reduced to substantially zero is modified by an inductive component coupled in series with a winding of the transformer.

14. A method of operating a DC-DC converter to provide substantially zero voltage switching to close a primary switch in series with a primary winding of a transformer, comprising the steps of first closing an auxiliary switch to resonate an inductor with a capacitor coupled to a winding of the transformer whereby a voltage across the primary switch is reduced, and closing the primary switch when the voltage across it is substantially zero.

15. A method as claimed in claim 14, wherein the converter comprises a single ended forward converter.

16. A method as claimed in claim 14 or 15 and including the step of recovering energy from the inductor, via a second inductor inductively coupled thereto, when the auxiliary switch is open.

17. A method as claimed in any of claims 14 to 16 and including the step of modifying a time constant for determining a time at which the primary switch is closed by an inductive component coupled in series with a winding of the transformer.

18. A method of operating a single ended forward converter including a primary switch in series with a primary winding of the transformer, comprising the steps of: providing a capacitor coupled in parallel with the primary switch or a winding of the transformer; and resonantly removing energy from the capacitor to an inductor when the primary switch is open.

19. A method as claimed in claim 18 and including the step of timing closing of the primary switch to be when a voltage across the primary switch is reduced to substantially zero by the resonance of the capacitor with the inductor.

20. A method as claimed in claim 18 or 19 and including the step of recovering energy from the inductor by switching current through the inductor with an auxiliary switch, and coupling energy from the inductor via a second inductor, inductively coupled to said inductor, when the auxiliary switch is open.

21. A single ended forward converter comprising: a primary switch; a transformer having a primary winding coupled in series with the primary switch to supply voltage terminals and a secondary winding coupled via a rectifying and smoothing circuit to output voltage terminals; and a first inductor and an auxiliary switch coupled in series, the first inductor and auxiliary switch being connected in parallel with the primary switch or a winding of the transformer; the converter including an inductance comprising at least a leakage inductance of the primary and secondary windings and a capacitance comprising at least a parasitic capacitance in parallel with the primary switch or a winding of the transformer.

22. A converter as claimed in claim 21 wherein said capacitance comprises, in addition to parasitic capacitance, a capacitor coupled in parallel with the primary switch or a winding of the transformer.

23. A converter as claimed in claim 22 wherein the capacitor is coupled in parallel with the primary switch.

24. A converter as claimed in any of claims 21 to 23 wherein said inductance comprises, in addition to the leakage inductance, an inductive component coupled in series with a winding of the transformer.

25. A converter as claimed in claim 24 wherein the inductive component is coupled in series with the primary winding of the transformer.

26. A converter as claimed in claim 24 wherein the inductive component is coupled in series with the secondary winding of the transformer.

27. A converter as claimed in any of claims 21 to 26 and including a second inductor inductively coupled to the first inductor for recovering energy from the first inductor when the auxiliary switch is open.

28. A converter as claimed in claim 27 wherein the second inductor is coupled via a diode to the supply voltage terminals.

29. A converter as claimed in claim 27 wherein the second inductor is coupled via a diode to said capacitance.

30. A converter as claimed in any of claims 21 to 29 wherein the first inductor and auxiliary switch are coupled in parallel with the primary switch.

31. A converter as claimed in any of claims 21 to 30 and including a diode coupled in series with the first inductor and auxiliary switch.

32. A single ended forward converter comprising: a transformer having a primary winding and a secondary winding; a primary switch connected in series with the primary winding for coupling the primary winding to a voltage source; an output circuit comprising forward and free-wheel diodes, an output inductor, and a smoothing capacitor coupling the secondary winding to output voltage terminals; and an auxiliary circuit comprising: a first inductor; and an auxiliary switch controlled for each switching cycle of the converter to close before closing of the primary switch and to open not before closing of the primary switch and not after opening of the primary switch, the auxiliary switch when closed coupling the first inductor in parallel with the primary switch or a winding of the transformer.

33. A converter as claimed in claim 32 and including a capacitor coupled in parallel with the primary switch or a winding of the transformer.

34. A converter as claimed in claim 33 and including a second inductor inductively coupled to the first inductor, and a diode in series with the second inductor for coupling the second inductor to the capacitor for returning recovered energy thereto.

35. A converter as claimed in claim 32 or 33 and including a second inductor inductively coupled to the first inductor, and a diode in series with the second inductor for coupling the second inductor to the voltage source for returning recovered energy thereto.

36. A converter as claimed in any of claims 32 to 35 and including an inductive component coupled in series with the primary or secondary winding of the transformer.

37. A converter as claimed in any of claims 32 to 36 wherein the primary and auxiliary switches comprise MOSFETs.