GB2436176A

GB2436176A - Power converter with a buck power factor correction stage

Info

Publication number: GB2436176A
Application number: GB0604806A
Authority: GB
Inventors: George Young
Original assignee: Commergy Technologies Ltd
Current assignee: Texas Instruments Ireland Trading Ltd
Priority date: 2006-03-10
Filing date: 2006-03-10
Publication date: 2007-09-19
Also published as: GB0604806D0

Abstract

A power converter (particularly an AC/DC converter) comprises a converter input 3, a converter output 5, a power factor correction (PFC) stage 7, and an isolation stage 9. The PFC stage is implemented by way of a buck PFC with low side drive and low side current sensing. There is provided a third stage , an intermediate buck pre-regulation stage 11, between the buck PFC stage and the isolation stage. Control of the power converter output voltage is achieved by varying the duty cycle of the intermediate buck pre-regulation stage and therefore the isolation stage may be an unregulated stage operated as a fixed DC/DC voltage converter. The isolation stage is operated as a 50%-50% duty cycle double ended stage.

Description

V

"A Power Converter"

Introduction

This invention relates to a power converter comprising a converter input, a converter output, a power factor correction (PFC) stage and an isolation stage. More particularly, this invention relates to an AC/DC converter.

Power converters and in particular AC/DC power converters are used in a wide range of applications. Generally speaking, these power converters are used to transform the incoming AC mains supply line voltage to one or more DC voltages suitable for use with the equipment that the power converter is charged with supplying voltage to.

In many cases, the power converter will transform the AC mains line voltage into several DC line voltages such as in the case of electronic equipment. When designing a power converter, there are essentially two main considerations taken into account, namely cost and efficiency. Usually, there is a cost/efficiency trade-off in most applications of power converter design.

Certain applications, such as personal computer power supplies, server power supplies and telecommunications equipment power supplies, typically require highly efficient power supplies so that the relatively sensitive equipment contained therein may operate to a sufficient standard. Heretofore, this has often resulted in the power supply for these types of equipment being relatively expensive to construct. One common approach used in the construction of power supplies for such equipment entails providing a boost pre-regulator followed by a phase-shifted full-bridge approach. There are however problems with such an approach as it tends to be relatively expensive to implement while still not being highly efficient i.e. in the range of 90%+ efficiency.

It is an object therefore of the present invention to provide a power converter that is relatively inexpensive to construct while at the same time being highly efficient in use.

It is a further object of the present invention to provide a power converter that is relatively simple to construct and manufacture.

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Statements of Invention

According to the invention there is provided a power converter comprising a converter input, a converter output, a power factor correction (PFC) stage and an isolation stage, characterised in that the PFC stage further comprises a buck PFC stage, the isolation stage comprises an unregulated self driven stage and the power converter further comprises a pre-regulation stage, the pro-regulation stage comprising an intermediate buck stage which follows the buck PFC stage.

By having such a power converter, it is no longer necessary to use a boost pre-regulator followed by a phase-shifted full-bridge approach or other similar approach. The power converter implements a three-stage approach with a pair of buck converters, one followed by the other and an isolation stage following the second buck stage. This is seen as a particularly simple configuration of power converter that will be relatively inexpensive to construct, while at the same time providing a highly efficient power converter with 90%+ efficiency. Such a power converter is capable of complying with International Standards for Harmonic Current Compliance over a wide range of input powers depending on the conduction angle and wave shape employed. Due to the fact that a buck PFC stage is used, bulk capacitor voltage must be less than typically 90 volts (V) to ensure adequate conduction angle at low line voltages. Therefore, the intermediate buck pre-regulation stage can operate at voltage levels where Schottky diodes can be used.

These Schottky diodes have virtually zero reverse recovery charge and therefore increased operating frequency of the intermediate buck stage is possible. This helps to increase the efficiency of the converter.

Furthermore, as the intermediate buck converter is operating using a quasi-fixed input voltage, typically in the range of 60V to 90V, with a relatively small double line frequency component, the efficiency of the second buck stage wilt be very high.

Furthermore, the range between the nominal bulk capacitor voltage and the voltage level going into the isolation stage is typically chosen to permit sufficient hold-up time and to handle transient conditions in the bulk capacitor. By using the intermediate buck stage before the isolation stage, it is possible to manage conditions where reverse power flow may occur as may be the case when using synchronous rectifiers.

* * * *** *** * * S * S S S S * S S * S * S S. * S S S S * S * S S * S S. S *** S S Finally, and very importantly, the isolation stage can be designed for optimum efficiency. The isolation stage may function approximately as a fixed ratio DC voltage transformer and therefore can be easily designed for zero-voltage switching to allow easy deployment of self-driven synchronous rectifier approaches in the converter outputs. All of these provide for a relatively inexpensive power converter that is highly efficient.

In one embodiment of the invention there is provided a power converter in which the buck PFC stage is implemented using low side drive. By operating the buck PFC stage with a low side drive, the drive is particularly simple and also allows for relatively simple peak current sensing which is valuable in containing inrush currents. Furthermore, this approach reduces stress on input rectification and filter components and enhances immunity of the power converter to circuit transients.

In another embodiment of the invention there is provided a power converter in which the intermediate buck stage is implemented using low side drive, with the drive taken from the voltage of the low side of a bulk capacitor, Vintermediate. In this way, the intermediate buck stage may also be provided with low side drive and low side current sensing. The isolation stage can therefore be deployed with the high side of the isolation stage referenced to the high side rail. This particular configuration will allow simple control of the intermediate buck stage.

In a further embodiment of the invention there is provided a power converter in which the isolation stage further comprises a 50%-50% duty cycle double ended stage. Preferably, the isolation stage may be implemented using self driven synchronous rectifiers. By having a 50%-50% duty cycle double ended stage, the isolation stage functions approximately as a fixed ratio DC voltage transformer.

The isolation stage can be designed for zero voltage switching and allows for easy deployment of self-driven synchronous rectifier approaches.

In one embodiment of the invention there is provided a power converter in which the power converter further comprises an input filter stage and an input rectification stage. Preferably, the input rectification stage is provided by way of a full bridge rectifier.

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* * S S S S S S S. S 55* 5 S In another embodiment of the invention there is provided a power converter in which the buck PFC stage comprises a bulk capacitor, the bulk capacitor being referenced to the high side rail. This is seen as a relatively convenient configuration of power converter buck PFC stage which will allow the drive circuitry to be referenced to the low side rail thereby enhancing control of the buck PFC stage.

In another embodiment of the invention there is provided a power converter in which there is provided means to sense the bulk capacitor voltage in a differential fashion. It is envisaged that the means to sense the bulk capacitor voltage in a differential fashion comprises a differential amplifier. Alternatively, the means to sense the bulk capacitor voltage in a differential fashion may comprise a PNP transistor operating as a low cost current source. In another alternative, the means to sense the bulk capacitor voltage in a differential fashion may comprise an optocoupler. Each of these means for sensing the bulk capacitor voltage in a differential fashion are seen as effective and useful and the final choice will depend on the cost and resolution requirements of the overall power converter.

In one embodiment of the invention there is provided a power converter in which the bulk capacitor voltage is sensed in a linear fashion. Alternatively, the bulk capacitor voltage may be sensed in a non-linear fashion.

In another embodiment of the invention there is provided a power converter in which the buck PFC comprises a buck switch and the control algorithm used for the buck switch is a clamped current approach. Alternatively, the buck PEG comprises a buck switch and the control algorithm used for the buck switch is a truncated sinusoid approach.

In a further embodiment of the invention there is provided a power converter in which the buck PEG stage is operated using a skip-mode PWM controller. By using a skip-mode PWM controller to control the PFC stage, it is possible to get excellent no-load consumption of the power converter and therefore it is possible to keep the input power factor correction stage on at all times. This allows for the * S S *** *IS * S S * S S S * * S S S * S S *S S S S S S * S * S S S S S 5 555 S S stand-by supply to be derived from the bulk capacitor. The stand-by supply efficiency can therefore by optimised as it is fed from a voltage that varies in a relatively narrow region about a DC level typically in the range of 60V to 90V. This allows usage of relatively low voltage FET devices in the stand-by supply circuitry and also contains light-load power loss and optimises active mode efficiency. This further facilitates usage of synchronous rectifiers if their use is appropriate such as when a high current stand-by supply is required.

In one embodiment of the invention there is provided a power converter in which the buck PFC comprises a buck switch and the buck PFC is driven with a signal referenced directly to the voltage at the source terminal of the buck switch.

In another embodiment of the invention there is provided a power converter in which the intermediate buck stage operates using low side current sensing. This is seen as a particularly simple way of controlling the intermediate buck stage.

In a further embodiment of the invention there is provided a power converter in which the intermediate buck stage is operated using a Schottky diode. The Schottky diode will have virtually zero reverse recovery charge, and by using the Schottky diode in the intermediate buck stage, the frequency of the intermediate buck stage may be increased significantly thereby enhancing the efficiency of the intermediate buck stage and hence the overall converter efficiency is improved.

This increased efficiency of the intermediate buck stage will have further knock-on benefits for the isolation stage.

In one embodiment of the invention there is provided a power converter in which the intermediate buck stage operates in a down conversion mode of the order of 40%. By having the intermediate buck stage operating in a down conversion mode of the order of 40%, the efficiency of the intermediate bulk converter, which is inversely proportional to the range of input voltage to output voltage, will be greatly enhanced.

In another embodiment of the invention there is provided a power converter in which the overall power converter output is controlled by controlling the duty cycle * S S *** **S * * S * S I * S * S S * S * S S. S S S S S S * * S S S S S S. S **5 I S of the intermediate buck stage. This is seen as particularly efficient way of controlling the overall power converter-output. In this way, the isolation stage may operate as an unregulated fixed DC/DC converter. The isolation stage can therefore be easily designed for zero voltage switching and to allow easy deployment of self-driven synchronous rectifier approaches on the converter output rails. A self-driven synchronous rectifier approach preceded by the buck stage is advantageous in the context of management of reverse power flow conditions and under fault conditions.

In a further embodiment of the invention the duty cycle of the intermediate buck stage is controlled using current mode control. Alternatively, the duty cycle of the intermediate buck stage is controlled using voltage mode control.

In one embodiment of the invention there is provided a power converter in which the intermediate buck stage further comprises an intermediate buck switch and the intermediate buck stage is driven with a signal referenced directly to the voltage at the source terminal of the intermediate buck switch.

In another embodiment of the invention there is provided a power converter in which the isolation stage high side is referenced to the high side rail of the power converter.

In a further embodiment of the invention there is provided a power converter in which the isolation stage is operated as a fixed ratio DC/DC voltage transformer.

This leads to a particular simple implementation of isolation stage that improves the overall efficiency of the power converter.

In one embodiment of the invention there is provided a power converter in which the isolation stage is arranged for zero-voltage switching.

In another embodiment of the invention there is provided a power converter in which the isolation stage is arranged to allow deployment of self driven synchronous rectifiers on the converter output.

* S S *S* *** * S S * S S I I * S S * I S S SS S * S * I * S S I S S S I *S * *SI I I In a further embodiment of the invention there is provided a power converter in which the isolation stage is provided with a balanced winding. By having an isolation stage with a balanced winding, it is feasible in many cases to obviate the need for a shield layer in the transformer. Preferably, the balanced winding is implemented using a full bridge approach.

In one embodiment of the invention there is provided a power converter in which the isolation stage secondary winding is arranged to ensure quiet foil terminations are adjacent to the primary winding. Preferably, a full bridge secondary winding is employed using a redundant centre tap.

In another embodiment of the invention there is provided a power converter in which the isolation stage secondary windings are wound around an internal primary winding and the quiet nodes of the redundant centre tapped secondary winding are located adjacent to the internal primary winding. Preferably, the free terminals of the secondary windings are connected to output rectifier elements. Furthermore, it is envisaged that there may be provided a dummy half turn section connected to the quiet secondary section to minimise noise in the power converter.

In a further embodiment of the invention there is provided a power converter in which the isolation stage is implemented using a cross coupled self driven approach.

In one embodiment of the invention there is provided a power converter in which there is provided a standby supply circuit, the standby supply voltage being taken from the bulk capacitor of the buck PFC stage. In this way, the stand-by supply efficiency can be optimised as it is typically fed from a voltage in the range of 60V to 90V. This allows the use of low voltage field effect transistor (FET) devices and the limited operating range contains light load power loss and optimises active mode efficiency thereby facilitating usage of synchronous rectifiers if their use is appropriate such as when high current stand-by power supplies are desired.

In a further embodiment of the invention there is provided a power converter in which there is provided means to keep the high side rail of the power converter * * S *SS *** * S S S * * S S * * S S * S S SS * S S * * * * * S S S S * *5 * *Sa S quiet relative to the system earth. Preferably, the means to keep the high side rail of the power converter quiet relative to the system earth comprises placing a differential mode filter on the low side line.

In one embodiment of the invention there is provided a power converter in which there is provided a high side heat sink connected directly to the cathode connection of one or more diodes of the power converter.

In a further embodiment of the invention there is provided a power converter in which the intermediate buck stage is replaced by using an integral cycle control algorithm at the isolation stage.

Detailed Description of the Invention

The invention will now be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which Fig. 1 is a diagrammatic, part schematic representation of a power converter according to the invention; Figs. 2(a), 2(b) and 2(c) are schematic representations of various differential sensing means used in the power converter according to the invention; Figs. 3 is a diagrammatic, part schematic representation of an alternative construction of power converter for enhanced electromagnetic compatibility performance; and Fig. 4 is a diagrammatic representation of an arrangement of transformer windings for use with the power converter according to the present invention.

Referring to the drawings and initially to Fig. 1 thereof there is shown a power converter, indicated generally by the reference numeral 1, comprising a converter * * * S.. *** * S S S S * S S * S S S S S S S. S S S * S

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S. S *SS S S input 3, a converter output 5, a power factor correction (PFC) stage 7 provided by way of buck PFC stage, an isolation stage 9 and an intermediate buck pre-regulaf ion stage 11 which follows the buck PFC stage 7. The power converter 1 further comprises an input filtering circuit 13 and an input rectification stage provided by way of a full bridge rectifier 15. The power converter further comprises a heat sink 17, a stand-by circuit 19 and an output sensing unit 21.

The buck PFC stage 7 further comprises a buck switch 23, a buck switch controller 25, a buck diode 27 and a buck inductor 29 which feed a bulk capacitor 31. The intermediate buck pre-regulation stage 11 further comprises a buck switch 33, a buck drive 35, a buck diode 37 and a buck inductor 39 which in turn feed an intermediate bulk capacitor 41. A differential sensing unit 43 is provided to monitor the voltage across intermediate bulk capacitor 41 and provide an output to the buck drive 35.

The buck drive 35 further receives an input from the output sensing unit 21. The isolation stage 9 in turn comprises a transformer 45 having a transformer primary 47 and transformer secondary 49. The isolation stage is a double ended unregulated self driven stage 51 with 50%-50% duty cycle. The output of the isolation stage is delivered to output synchronous rectifiers (not shown).

The buck PFC stage 7 is provided with low side drive and low side current sensing and the positive terminal of the bulk capacitor 31 is connected to the high side line coming out of the full bridge rectifier 15. The control algorithm for the buck switch 23 from the buck switch controller 25 is either a clamped current approach or a truncated sinusoid approach. As the bulk capacitor 31 voltage is arranged so that the positive terminal is connected to the high side of the full bridge rectifier 15 with the control circuit referenced conveniently to the low side of the full bridge rectifier output, and therefore the bulk capacitor 31 voltage needs to be sensed in a differential fashion.

This is required for normal control functions as well as over voltage protection on the capacitor. The intermediate buck pre-regulation stage 11 drive is taken from the low capacitor rail 53, termed VNTERMEDIATE. The intermediate buck pre-regulation stage 11 further has low-side current sensing. The isolation stage 9 is deployed with the high side referenced to the high side rail of the overall power converter.

The buck diode 37 may be provided by a Schottky diode which will enable the * S S *SS *** * * * * S S * * * S S S S * S S. S S S * S * S * * S * S S *5 * *5* * . frequency of operation of the intermediate buck pre-regulation stage 11 to be increased. Furthermore, due to the fact that the intermediate buck pre-regulation stage 11 operates at low voltage, in or around the range of 90V or less, the frequency of the intermediate buck pre-regulation stage 11 may also be increased. The efficiency is therefore enhanced by the fact that the input voltage to the intermediate buck pre-regulation stage 11 is quasi-fixed. Typically, the input to the intermediate buck stage is in the range 60V to 90V with a relatively small double line frequency component. The down conversion in magnitude of voltage is typically of the order of 40%, perhaps from 83V to 50V, which corresponds to high efficiency in the stage as the efficiency of the buck stage is inversely proportional to the range of input to output voltage it must handle and a buck converter designed in this context can be very efficient indeed. The range between the nominal bulk capacitor voltage and the level going into the isolation stage is typically chosen to permit sufficient hold-up time and to handle transient conditions on the bulk capacitor. Furthermore, by using a buck stage in this position it is also useful when managing conditions where reverse power flow may occur as this may be a risk particularly when using synchronous rectifiers.

Control of the overall converter output voltage may be achieved by varying the duty cycle of the intermediate buck stage using either voltage mode control or current mode control. In this way, the isolation stage may function approximately as a fixed ratio DC voltage transformer. The isolation stage may be designed for optimum efficiency and can be easily designed for zero voltage switching and to allow easy deployment of self driven synchronous rectifier approaches on the converter output 5.

The self driven synchronous rectifier approach preceded by a buck stage is advantageous in the context of management of reverse power flow conditions and under voltage fault conditions.

It is envisaged that the input buck PFC stage may be controlled using skip-mode PWM controllers which provide for excellent no-load consumption characteristics.

Furthermore, it is feasible to keep the input PEG stage on at all times and derive the stand-by supply ordinarily required in this type of power converter from the bulk capacitor. The stand-by supply 19 is connected across bulk capacitor 31. The stand-by supply efficiency can therefore be optimised as it will be fed from a voltage supply that varies in a relatively narrow region about a DC level typically in the range of 60V * * * S.. *S* * S S S S S * . * S S * S S S S* S S * S S * I I I 5 IS. I S to 90V. In this way, usage of 200V FET devices is possible and the limited operating range contains light load power loss and optimises active mode efficiency, facilitating usage of synchronous rectifiers whose usage is appropriate where high current stand-by supplies are desired.

Referring to Figs. 2(a) to 2(c) inclusive of the drawings there is shown three separate means for differential sensing which may be implemented depending on the cost and resolution required from the power converter. Fig. 2(a) shows a differential amplifier configuration with op-amp 61. Fig. 2(b) shows a PNP transistor 63 which can be configured to provide a low-cost current source whose magnitude is proportional to the capacitor voltage. Finally, in Fig. 2(c) there is shown an optocoupler configuration incorporating a zener diode 65. The type of voltage sensing is either linear, as typically required for loop control, or non-linear, as typically required for detection of overload conditions.

Referring now to Fig. 3 of the drawings there is shown an alternative configuration of power converter in which the electromagnetic compatibility (EMC) performance of the converter may be enhanced. In this embodiment the intermediate buck stage may be provided or alternatively the isolation stage may be operated using an appropriate integral cycle algorithm instead of the intermediate buck pre-regulation stage. In the embodiment shown, in which parts similar to those described already are identified by the same reference numerals, the high side rail 71 is kept relatively quiet in the context of noise voltages relative to overall system earth. This is achieved by placing differential mode filtering on the low side line 73, the heat sink 17 is directly connected to the tab, otherwise known as the cathode connection, of diodes of the power converter. In this way, the risks associated with washer based connections are obviated. If an isolation stage with a suitably balanced winding, such as a full bridge rectifier approach, is used, this will in most cases obviate the need for a shield layer in the transformer.

Referring to Fig. 4 of the drawings, there is shown a particularly suitable construction of transformer for use with a power converter with improved electromagnetic compatibility performance. The transformer comprises core material sections, 81 83 and 85 and a primary winding 87 wound around core section 83. In this approach, * * ** * S S * S S * S * S S S S S S S. S S S S S S * S S S * S I SS 5 e * the secondary winding is designed in such a fashion to ensure that the quiet foil terminations are adjacent to the primary winding 87. The secondary winding is a centre tapped full bridge winding with redundant centre tap. Therefore there are a pair of secondary windings 89 and 91. A link 93 may be provided between the pair of secondary windings 89, 91. A dummy half-turn section 95 is provided on the quiet secondary section to minimise noise from this source. By having the tapped full bridge secondary windings with redundant centre tap, it is possible to retain quiet foil in closer proximity to the primary winding. The free terminals of the windings are connected to the appropriate rectifier devices (not shown) and given appropriate output voltages, use of a cross-coupled self-driven approach is feasible.

In this specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms "include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail within the scope of the claims.

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Claims

Claims 1) A power converter comprising a converter input, a converter

output, a power factor correction (PFC) stage and an isolation stage, characterised in that the PFC stage further comprises a buck PFC stage, the isolation stage comprises an unregulated self driven stage and the power converter further comprises a pre-regulation stage, the pre-regulation stage comprising an intermediate buck stage which follows the buck PFC stage.

2) A power converter as claimed in claim 1 in which the buck PFC stage is implemented using low side drive.

3) A power converter as claimed in claim 1 or 2 in which the intermediate buck stage is implemented using low side drive, with the drive taken from the voltage of the low side of a bulk capacitor, Vintermeduate.

4) A power converter as claimed in any preceding claim in which the isolation stage further comprises a 50%-50% duty cycle double ended stage.

5) A power converter as claimed in claim 4 in which the isolation stage is implemented using self driven synchronous rectifiers.

6) A power converter as claimed in any preceding claim in which the power converter further comprises an input filter stage and an input rectification stage.

7) A power converter as claimed in claim 6 in which the input rectification stage is provided by way of a full bridge rectifier.

8) A power converter as claimed in any preceding claim in which the buck PFC stage comprises a bulk capacitor, the bulk capacitor being referenced to the high side rail.

9) A power converter as claimed in claim 8 in which there is provided means to * * * )** **S * * * * * * * * I * 4 S 4 S.. 4 I S I * a, a. ISS IS I las * p sense the bulk capacitor voltage in a differential fashion.

10) A power converter as claimed in claim 9 in which the means to sense the bulk capacitor voltage in a differential fashion comprises a differential amplifier.

11) A power converter as claimed in claim 9 in which the means to sense the bulk capacitor voltage in a differential fashion comprises a PNP transistor operating as a low cost current source.

12) A power converter as claimed in claim 9 in which the means to sense the bulk capacitor voltage in a differential fashion comprises an optocoupler.

13) A power converter as claimed in any of claims 9 to 12 in which the bulk capacitor voltage is sensed in a linear fashion.

14) A power converter as claimed in any of claims 9 to 12 in which the bulk capacitor voltage is sensed in a non-linear fashion.

15) A power converter as claimed in any preceding claim in which the buck PFC comprises a buck switch and the control algorithm used for the buck switch is a clamped current approach.

16) A power converter as claimed in any of claims I to 14 in which the buck PFC comprises a buck switch and the control algorithm used for the buck switch is a truncated sinusoid approach.

17) A power converter as claimed in any preceding claim in which the buck PFC stage is operated using a skip mode PWM controller.

18) A power converter as claimed in any preceding claim in which the buck PFC comprises a buck switch and the buck PFC is driven with a signal referenced directly to the voltage at the source terminal of the buck switch.

* S * S.. *SJ * . S S * * * S * S S * * * S SS S S * * . S * S * . I S * S. S *s* * . 19) A power converter as claimed in claim 3 in which the intermediate buck stage operates using low side current sensing.

20) A power converter as claimed in any preceding claim in which the intermediate buck stage is operated using a Schottky diode.

21) A power converter as claimed in any preceding claim in which the intermediate buck stage operates in a down conversion mode of the order of 40%.

22) A power converter as claimed in any preceding claim in which the overall power converter output is controlled by controlling the duty cycle of the intermediate buck stage.

23) A power converter as claimed in claim 22 in which the duty cycle of the intermediate buck stage is controlled using current mode control.

24) A power converter as claimed in claim 22 in which the duty cycle of the intermediate buck stage is controlled using voltage mode control.

25) A power converter as claimed in any preceding claim in which the intermediate buck stage further comprises an intermediate buck switch and the intermediate buck stage is driven with a signal referenced directly to the voltage at the source terminal of the intermediate buck switch.

26) A power converter as claimed in any preceding claim in which the isolation stage high side is referenced to the high side rail of the power converter.

27) A power converter as claimed in any preceding claim in which the isolation stage is operated as a fixed ratio DCDC voltage transformer.

28) A power converter as claimed in any preceding claim in which the isolation stage is arranged for zero-voltage switching.

* . S.. 555 * S I S S * S S * S * S S S S S. * S S S * S S S S S S S * S. S **5 S S 29) A power converter as claimed in any preceding claim in which the isolation stage is arranged to allow deployment of self driven synchronous rectifiers on the converter output.

30) A power converter as claimed in any preceding claim in which the isolation stage is provided with a balanced winding.

31) A power converter as claimed in claim 30 in which the balanced winding is implemented using a full bridge approach.

32) A power converter as claimed in claim 31 in which the isolation stage secondary winding is arranged to ensure quiet foil terminations are adjacent to the primary winding.

33) A power converter as claimed in claim 31 or 32 in which a full bridge secondary winding is employed using a redundant centre tap.

34) A power converter as claimed in claim 33 in which the secondary windings are wound around an internal primary winding and the quiet nodes of the redundant centre tapped secondary winding are located adjacent to the internal primary winding.

35) A power converter as claimed in claim 33 or 34 in which the free terminals of the secondary windings are connected to output rectifier elements.

36) A power converter as claimed in any of claims 32 to 35 in which there is provided a dummy half turn section connected to the quiet secondary section to minimise noise.

37) A power converter as claimed in any preceding claim in which the isolation stage is implemented using a cross coupled self driven approach.

38) A power converter as claimed in any preceding claim in which there is provided a standby supply circuit, the standby supply voltage being taken * * S *SS *SS * S S * S S S S S S * S * * *

SS S S S S S

* S S S S S * S ** S SsS S S from the bulk capacitor of the buck PFC stage.

39) A power converter as claimed in any preceding claim in which there is provided means to keep the high side rail of the power converter quiet relative to the system earth.

40) A power converter as claimed in claim 39 in which the means to keep the high side rail of the power converter quiet relative to the system earth comprises placing a differential mode filter on the low side line.

41) A power converter as claimed in claim 39 or 40 in which there is provided a high side heat sink connected directly to the cathode connection of one or more diodes of the power converter.

42) A power converter as claimed in any preceding claim in which the intermediate buck stage is replaced by using an integral cycle control algorithm at the isolation stage.

* * S **S *SS * S S S S S S S S S * * S * S S5 5 5 S S S * S * S S * S S S *5S S S