WO2006043837A2

WO2006043837A2 - A polyphase ac to dc power converter

Info

Publication number: WO2006043837A2
Application number: PCT/NZ2005/000276
Authority: WO
Inventors: Michael John Harrison
Original assignee: Eaton Power Quality Company
Priority date: 2004-10-22
Filing date: 2005-10-21
Publication date: 2006-04-27
Also published as: WO2006043837A3

Abstract

A polyphase AC to DC converter consisting of a number of buck converters having their outputs connected in series. In a preferred embodiment three buck converters are used for a 3 phase AC supply. In another embodiment two buck converts are used for a 3 phase AC supply. The AC inputs may be connected in star or delta configuration. The buck converters may include power factor correction stages and/or isolation transformers.

Description

A Polyphase AC to DC power converter

Field of the invention

This invention relates to a polyphase AC to DC power converter. More particularly, although not exclusively, the invention relates to a high power (typically greater than 1kW) three phase AC to DC power converter suitable for use in telecommunications applications.

Background to the invention

In the telecommunications industry a number of special requirements must be satisfied by power converters. These include:

(a) Low noise within the speech band so as to satisfy the CCIF psophometric standard;

(b) a power factor conforming to the IEC 1000-3-2 EMC standard; and

(c) galvanic isolation as a functional and safety requirement.

Figure 1 shows a single phase energy model for a single phase converter. Figure 2 shows the energy transfer graph for such a converter. It can be seen that there can be no energy transfer from source to load at zero mains crossing, hence, in order to provide a constant transfer of energy to the load, an energy storage medium is required.

The typical solution has been to provide a power converter which converts a single phase AC input into a DC output. The converter is typically a two stage topology consisting of a pre- converter stage (typically a boost stage) providing power factor correction (PFC) and a DC-DC converter stage (typically a half bridge or full bridge converter) to reduce output ripple to conform to the psophometric standard.

Buck converter topologies are generally not used for power factor correction due to the fact that the buck topology cannot draw power from the supply when the instantaneous input voltage falls below the output voltage (unlike a boost converter). This will result in poor power factor correction and significant current distortion. To improve the power factor the output voltage can be lowered but this results in poor utilisation of the silicon due to the increase in voltage transformation taking place. A large EMI filter is also required due to the discontinuous current input. However, buck topologies can provide effective protection from output short circuit failure.

Boost topologies have been preferred as they can perform almost perfect power factor correction due to the fact that they can draw power over almost the entire mains cycle. The maximum power transfer takes place at the mains peak, as a result minimum voltage boost is required to reach the required output voltage, and hence minimal voltage transformation needs to take place. As a result a boost topology yields a good silicon utilisation factor. Due to continuous input current much less EMI filtering is required compared with a buck and a buck- boost topology. This configuration cannot provide isolation or effective protection from output short circuit failure.

The total efficiency of the power converter is dependent upon the efficiency of all stages. Assuming a 95% efficiency per stage the total efficiency would be 90% for a two-stage solution. This efficiency figure affects the amount of input energy required to produce the required output energy. It also affects the cost of climate control in dealing with heat generated due to inefficient energy conversion. Further, the use of two switches and two controllers increase circuit complexity and thus circuit cost.

Single stage solutions suffer from the drawback that the voltage on the internal energy storage capacitor varies with line voltage and load current. At high power levels this could involve expensive high voltage storage capacitors. Single stage solutions are generally only used at lower power levels (i.e. <200 W.)

A number of three phase AC to DC power converters utilising boost topologies are known.

A three phase bridge capacitor converter is shown in figure 3. This produces a smaller output ripple, a better power factor and better total harmonic distortion than a single phase bridge capacitor. However, as a result of the high current harmonics drawn, this bridge capacitor converter would not comply with the IEC 1000-3-2 standard and would require a DC to DC converter stage for further ripple reduction, isolation and voltage transformation to be suitable for use in a telecommunications application.

A three phase single switch pseudo PFC booster rectifier circuit is shown in figure 4. This topology, if used with a simple output voltage feedback into a vmode control, results in a scalloping of the current waveform and so would not comply with the IEC 1000-3-2 standard. A DC to DC converter would be required for isolation and voltage transformation to be suitable for use in a telecommunications application.

Figure 5 shows a three phase single switch power factor correction converter. This converter works in the discontinuous mode whereby switching occurs in a constant frequency and duty cycle at a rate substantially higher than the line frequency. This converter has a high power factor but a second stage is again required for ripple reduction, isolation and voltage transformation. This arrangement requires a higher filtering effort to remove the switching harmonics and has high peak current stresses on the semiconductors.

Figure 6 shows a six switch power factor correction converter. This type of converter is normally used for high power high-performance applications as it has high efficiency, good current quality and low EMI emissions. The topology is a three phase full bridge boost rectifier. A DC to DC output stage is required for isolation and voltage transformation. This arrangement also requires a high number of components.

Figure 7 shows an isolated Vienna rectifier. The Vienna rectifier is a boost derived three level pulse width modulated converter that has unity power factor, provides output voltage isolation and transformation and has minimal output ripple. This arrangement can realise a power supply that can comply with the psophometric standard and IEC 1000-3-2 EMC standard.

However, this arrangement has no protection from output short circuit, being a boost derived topology. It is also effectively a two-stage converter topology in terms of efficiency.

Figure 8 shows an arrangement consisting of three single phase power converters and three DC to DC converters providing isolation and connection to a common bus. This circuit is still a two-stage approach resulting in circuit complexity, high component costs and the resulting inefficiencies of a two-stage approach.

The above arrangements are all isolated converters and show the variety of circuit topologies currently used for high power applications. The telecommunications industry has three specific requirements for (a) low output ripple, (b) low input harmonic current, and (c) galvanic isolation. Any one of these requirements or in fact any two can be achieved with a single stage converter as shown above, but to concurrently meet all three of these requirements necessitates the need for two stages of power conversion. Attendant with two stages of power conversion comes an increase in circuit complexity, component count, and circuit losses leading to an overall bulky and costly circuit solution At present there is no single stage converter in the high power range that meets all the telecommunication installation requirements and complies with all the necessary standards and has the high bandwidth control capabilities of a two-stage converter.

Is an object of the presentation to provide a single stage power converter topology providing good power factor correction, low output ripple, galvanic isolation and high bandwidth control or to at least provide the public with a useful choice.

Summary of the invention

According to a first aspect of the invention there is provided a polyphase AC to DC power converter comprising a plurality of buck converters, each buck converter having input terminals for connection to one or more phase of a polyphase power supply and output terminals connected in series.

According to further aspect of the invention there is provided a three phase AC to DC power converter comprising three buck converters, each buck converter having input terminals for connection to one or more phase of a three phase power supply and output terminals connected in series.

According to another aspect of the invention there is provided a method of converting power from a polyphase AC power supply to a DC output comprising:

a. supplying energy from the polyphase AC power supply to each of a plurality of buck converters; and

b. collecting the outputs of the converters in series to provide a DC output.

According to restore further aspect of the invention there is provided a method of converting power from a three phase AC power supply to a DC output comprising:

a. supplying energy from the three phase power supply to each of three buck converters; and

b. collecting the outputs of the converters in series to provide a DC output. Brief description of the drawings

The invention will now be described by way of non limiting example with reference to the embodiments shown in the company in drawings in which:

Figure 1 shows a single phase energy model for a single phase converter.

Figure 2 shows the energy transfer graph for the converter shown in figure 1.

Figure 3 shows a three phase bridge capacitor converter.

Figure 4 shows a three phase single switch pseudo PFC converter.

Figure 5 shows a three phase single switch PFC converter.

Figure 6 shows a six switch PFC converter

Figure 7 shows an isolated Vienna rectifier.

Figure 8 shows a converter comprising three single phase parallel converters.

Figure 9 shows an idealised three phase power supply.

Figure 10 shows the combined power of the three phases shown in figure 9.

Figure 11 shows a model of an ideal series buck converter topology.

Figure 12 shows a series buck circuit diagram utilised for simulation.

Figure 13 shows a series buck subcircuit utilised in the circuit of figure 12.

Figure 14 shows the input voltage and current waveforms of the red phase of the series buck simulation circuit of figures 12 and 13.

Figure 15 shows the output voltage and current of the simulation circuit of figures 12 and 13. Figure 16 shows a three phase AC to DC converter consisting of three buck converters.

Figure 17 shows a three phase AC to DC converter consisting of two buck converters.

Detailed description

Single phase power converters suffer from the problem that energy storage is required for periods around mains crossings. However, as shown in figure 10 the idealised three phase power supply shown in figure 9 produces a constant energy output resulting from the combined outputs of the three phases. The use of a three phase converter can thus overcome the energy storage requirements of single phase converters.

The converter of figure 8 utilises three boost stages connected in parallel. This approach still requires two converter stages for each phase. Figure 11 shows a series buck circuit diagram in which the outputs of the buck converters for each phase are connected in series. By connecting the outputs of the buck converters in series the normal problem with buck converters (i.e. inability to draw power from the supply when the instantaneous input voltage falls below the output voltage) can be overcome.

Figure 12 shows a series buck circuit diagram and figure 13 shows a series buck subcircuit used in the circuit of figure 12 used to model a three phase series buck converter. Figure 14 shows that the converter of figures 12 and 13 achieves unity power factor and figure 15 shows there is no output ripple at all.

Figure 16 shows a circuit for a three phase AC to DC series buck converter. The phases 1 , 2,

3 of a three phase supply are connected to the input terminals 4 to 9 of buck converters 10 to 12 in a delta configuration. It will be appreciated that the converters may be connected in a star configuration also as shown in figure 12. A delta configuration has the advantage that no neutral connection is required. A star configuration can use lower voltage rated semiconductor devices as the voltage across phases is spread across two converters. A star connection may also provide better immunity against single phase failure.

Buck converters 10 to 12 consist of bridge rectifiers 13 to 15 and power factor correction stages 16 to 18. The outputs of power factor correction stages 16 to 18 are connected to isolation transformers 19 to 21. The output terminals 22 to 27 are connected in series to provide output terminals 22 and 27 connected to output stage 28 supplying load 29. By connecting the outputs of the buck converters 10 to 12 in series the problem in utilising buck converters in such applications is overcome (i.e. the buck topology cannot draw power from the supply when the instantaneous input voltage falls below the output voltage).

This arrangement allows the power factor correction stages 16 to 18 to utilise full bridge converters. It is particularly desirable to utilise full bridge converters in high power converter stages. Accordingly most telecom style power supplies (>1kW) use full bridge converters but they are much more complex solutions having two actively switched power conversion stages. Resonant switching (ZVS) may be employed to control the power factor correction stages utilising a control circuit based on a UNITRODE 3895 chip or similar (e.g. part numbers UCC

3895 or UC3879 or a digital signal processor).

Current doubler circuit 28 reduces the voltage drop across the diodes and the number of windings required on the isolation transformers. Other output circuits could be utilised in place of current doubler circuit 28 depending upon the application. The size of capacitor 30 of output stage 28 may be varied depending upon the nature of the power supply and load to provide appropriate storage. Alternatively, energy storage may be provided after rectifiers 13 to 15 or within the buck converters.

It will be appreciated that this approach can be adopted for any polyphase supply, be it two phase or more than three phase. Further, the number of converters does not have to be the same as the number of phases. As shown in figure 17 utilising a Scott connection two converters 31 and 32 may provide a power converter for a three phase power supply. In this case the input terminals 36 and 37 to buck converter 31 are connected across phases 33 and 34 whereas the input terminals 38 and 39 to buck converter 32 are connected to phase 35 and a primary centre tap 40 of output transformer 41.

The power converters shown in figures 16 and 17 have the efficiency of a single stage converter and thus have reduced power consumption, heat dissipation and cost. The converters provide good power factor correction, low output ripple, galvanic isolation and high bandwidth control. As the converter utilises buck stages the converters provide effective protection from output short circuit failure. The power converters have reduced energy storage requirements due to the three phase summing effect. The number of components is relatively low and the silicon utilisation is high. As active control is required for only a single stage, complexity and cost is reduced and resonant mode switching may be easily employed. The use of multiple converter stages provides some redundancy in case of faults. While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.

Claims

Claims:

1. A polyphase AC to DC power converter comprising a plurality of buck converters, each buck converter having input terminals for connection to one or more phase of a polyphase power supply and output terminals connected in series.

2. A power converter as claimed in claim 1 wherein the buck converters are connected to phases of the power supply in a delta configuration.

3. A power converter as claimed in claim 1 wherein the buck converters are connected to phases of the power supply in a star configuration.

4. A power converter as claimed in any preceding claim wherein the buck converter stages include full bridge power factor correction stages.

5. A power converter as claimed in claim 4 wherein the full bridge power factor correction stages are driven in resonant mode.

6. A power converter as claimed in any preceding claim wherein each buck converter includes an isolation transformer.

7. A power converter as claimed in any preceding claim including a current doubler circuit connected in series with the output terminals.

8. A power converter as claimed in any preceding claim including two buck converters connected in a Scott connection to the phases of a polyphase power supply.

9. A three phase AC to DC power converter comprising three buck converters, each buck converter having input terminals for connection to one or more phase of a three phase power supply and output terminals connected in series.

10. A power converter as claimed in claim 9 wherein the buck converters are connected to phases of the power supply in a delta configuration.

11. A power converter as claimed in claim 9 wherein the buck converters are connected to phases of the power supply in a star configuration.

12. A power converter as claimed in any one of claims 9 to 11 wherein the buck converter stages include full bridge power factor correction stages.

13. A power converter as claimed in claim 12 wherein the full bridge power factor correction stages are driven in resonant mode.

14. A power converter as claimed in any one of claims 9 to 13 wherein each buck converter includes an isolation transformer.

15. A power converter as claimed in any one of claims 9 to 14 including a current doubler circuit connected in series with the output terminals.

16. A method of converting power from a polyphase AC power supply to a DC output comprising:

b. collecting the outputs of the converters in series to provide a DC output.

17. A method as claimed in claim 16 wherein each buck converter includes a full bridge power factor correction stage switched in resonant mode.

18. A method as claimed in 16 or claimed 17 wherein the number of buck converters is less than the number of phases of the polyphase AC power supply and wherein power from the output of one of buck converter is supplied to an input terminal of another buck converter.

19. A method of converting power from a three phase AC power supply to a DC output comprising:

a. supplying energy from the three phase power supply to three buck converters; and

b. collecting the outputs of the converters in series to provide a DC output.

20. A method as claimed in claim 19 wherein each buck converter includes a full bridge power factor correction stage switched in resonant mode.