WO2010015974A1

WO2010015974A1 - Method for controlling a power converter

Info

Publication number: WO2010015974A1
Application number: PCT/IB2009/053334
Authority: WO
Inventors: Christoph Loef
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2008-08-08
Filing date: 2009-07-31
Publication date: 2010-02-11

Abstract

It is especially provided a method for controlling a power converter, wherein the power converter comprises a first and a second full bridge 105, 107, 705, 707, a first and a second resonant circuit and a transformer 109, 710, wherein the first resonant circuit is supplied by the first full bridge 105, 705 with a first AC voltage, wherein the second resonant circuit is supplied by the second full bridge 107, 707 with a second AC voltage, wherein a first resonant current runs in the first resonant circuit, wherein a second resonant current runs in the second resonant circuit, wherein the transformer 109, 710 comprises a first primary side 119, 719, a second primary side 119, 719, a first secondary side 120, 720, and a second secondary side 120, 720, wherein the first primary side 119, 719 is connected to the first resonant circuit and the second primary side 119, 719 is connected to the second resonant circuit, the method comprising the steps of supplying the first and the second full bridge 105, 107, 705, 707 by a DC-voltage, controlling the first and the second full bridge 105, 107, 705, 707 by a control factor such as the first full bridge is switched at or adjacent to a zero crossing of the first resonant current and such as the second full bridge is switched at or adjacent to a zero crossing of the second resonant current, wherein a first AC-current results at the first secondary side of the transformer 109, 710 and a second AC-current results at the second secondary side of the transformer 109, 710, wherein a resulting AC-current is obtained by coupling the different AC-currents. The result thereof is the possibility to control the ripple of the resulting output AC-current.

Description

METHOD FOR CONTROLLING A POWER CONVERTER

FIELD OF INVENTION:

The present invention relates to a method for controlling a power converter. Further, the invention relates to a programme element, which, when being executed by a processor, is adapted to carry out the inventive method. Furthermore, the invention relates to a computer readable medium having stored this programme.

BACKGROUND OF THE INVENTION:

From literature (refer to: Mohan, Undeland, Robbins, "Power Electronics" ) three phase thyristor rectifiers are known. These are in use to control the DC output voltage. Further, AC-AC converter, which will be fed by an AC current with a special frequency (e.g. 50 Hz.), which will be transformed into another AC-current with another frequency, wherein this output frequency can be controlled, are well- known by the person skilled in the art. It is also well-established to adapt such AC-AC converter for a three phase supply. SUMMARY OF THE INVENTION:

The result of such an AC-AC converter will be e.g. an AC-current at the output side of the power converter. This AC-current is e.g. characterized by its phase.

It would be desireable to provide a method, a programme and a computer readable medium to control this phase of the output AC-current. The invention provides a method for controlling a power converter, wherein the power converter comprises a first and a second full bridge, a first and a second resonant circuit and a transformer, wherein the first resonant circuit is supplied by the first full bridge with a first AC voltage, wherein the second resonant circuit is supplied by the second full bridge with a second AC voltage, wherein a first resonant current runs in the first resonant circuit, wherein a second resonant current runs in the second resonant circuit, wherein the transformer comprises a first primary side, a second primary side, a first secondary side, and a second secondary side, wherein the first primary side is connected to the first resonant circuit and the second primary side is connected to the second resonant circuit, the method comprising the steps of supplying the first and the second full bridge by a DC-voltage, controlling the first and the second full bridge by a control factor such as the first full bridge is switched at or adjacent to a zero crossing of the first resonant current and such as the second full bridge is switched at or adjacent to a zero crossing of the second resonant current, wherein a first AC- current results at the first secondary side of the transformer and a second AC-current results at the second secondary side of the transformer. It is advantageously that there is a resonant circuit, which leads to more sinus-shape of the input current of the transformer and thus to lower switching losses since the inverter changes his instances at zero current levels.

The invention provides also a power converter comprising a first full bridge for providing a first AC voltage, a second full bridge for providing a second AC voltage, a first resonant circuit, a second resonant circuit and a transformer, wherein the transformer is adapted for transforming the first and the second AC voltage, wherein a method according to one of claims 1 to 6 is applied.

Further, it is provided a programme element, which, when being executed by a processor, is adapted to carry out one of the methods according to the claims 1 to 6.

Furthermore, it is provided a computer readable medium having stored the programme element of claim 11.

Further embodiments are incorporated in the dependent claims. According to a further aspect of the invention it is provided a method, comprising the step of shifting the phases of the first and the second AC-current by the control factor.

According to an exemplary embodiment a method is provided, comprising the step of coupling the first and the second AC-current, which leads to one resulting output current. According to a further aspect of the invention it is provided a method, comprising the step of shifting the phase of the control factor, such as to control the ripple of the resulting output current.

According to an exemplary embodiment a method is provided, comprising the step of shifting the phase of the control factor to reduce the ripple of the resulting output current. The aim of the person skilled in the art is always to reduce the ripple in order to reduce losses of power and efficiency. There is also the aspect of cross-over talking, which can be reduced by minimizing the ripple. Reduced ripples also lead to the positive effect of a less amount of mains disturbances. Further, there is no more need for large filter arrangements to fulfil the given requirements (especially because of electro-magnetic-interference restrictions).

According to a further aspect of the invention it is provided a method, comprising the step of shifting the phase of the control factor, such as to maximize the ripple of the output current. Eventually it is the aim of the person skilled in the art to maximize the ripple. E.g. it could be useful to have a corresponding ripple to have cross-over talking. This leads e.g. to the possibilty to detect the AC-currents without galvanically conjunction.

According to a further aspect of the invention it is provided a power converter, wherein one of the group consisting of the first full bridge and the second full bridge is a full bridge module. This leads to a more compact and cheaper power converter.

According to an exemplary embodiment a power converter is provided, wherein one of the group consisting of the first resonant circuit and the second resonant circuit comprises a capacitance and an inductance. This leads to a more sinus-shape of the input AC- voltage of the transformer. Therefore, there is a higher efficiency and less losses of power.

According to an exemplary embodiment a power converter is provided, wherein the inductance comprises the leakage inductance of the transformer. In this case there is no separate inductance necessary, which leads to a more compact and cheaper power converter. It should be noted that the following described exemplary embodiments of the invention apply also for the method, the programme element and the computer readable medium.

It may be seen as a gist of the present invention to provide a method for controlling a power converter, wherein the power converter comprises a first and a second full bridge, a first and a second resonant circuit and a transformer, wherein the first resonant circuit is supplied by the first full bridge with a first AC voltage, wherein the second resonant circuit is supplied by the second full bridge with a second AC voltage, wherein a first resonant current runs in the first resonant circuit, wherein a second resonant current runs in the second resonant circuit, wherein the transformer comprises a first primary side, a second primary side, a first secondary side, and a second secondary side, wherein the first primary side is connected to the first resonant circuit and the second primary side is connected to the second resonant circuit, the method comprising the steps of supplying the first and the second full bridge by a DC- voltage, controlling the first and the second full bridge by a control factor such as the first full bridge is switched at or adjacent to a zero crossing of the first resonant current and such as the second full bridge is switched at or adjacent to a zero crossing of the second resonant current, wherein a first AC-current results at the first secondary side of the transformer and a second AC-current results at the second secondary side of the transformer, wherein a resulting AC-current is obtained by coupling the different AC- currents. The result thereof is the possibilty to control the ripple of the resulting output AC-current.

It should be noted that the above features may also be combined. The combination of the above features may also lead to synergetic effects, even if not explicitly described in detail.

These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS:

Exemplary embodiments of the present invention will be described in the following with reference to the following drawings. Fig. 1. depicts a circuit according to the invention with full bridge rectifier at the secondary side of the transformer and a coupled resulting output AC- current. Fig. 2. depicts an inverter output voltage in one phase and the resonant current therein at low power level.

Fig. 3. depicts an inverter output voltage in one phase and the resonant current therein at medium power level.

Fig. 4. depicts an inverter output voltage in one phase and the resonant current therein at high power level.

Fig. 5. depicts an inverter output voltage in one phase and the resulting AC-current at the secondary side of the transformer after rectification, wherein the different output voltages have a small phase. The resonance current shown in this picture equals the identical control of three inverter brances, which are syncronized. Fig. 6. depicts an inverter output voltage in one phase and the resulting

AC-current at the secondary side of the transformer after rectification, wherein the different resonant currents are phase-shifted against each others and thus lower output voltage ripple is achived.

Fig. 7. depicts a circuit according to the invention with rectifier at the secondary side of the transformer and uncoupled AC-currents.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS:

Fig. 1 shows the circuit 116 of a system according to the invention. This circuit 116 is connected to three phase mains 101. The mains terminal connects the three phase full wave mains rectifier 102 to the mains grid. To avoid high frequency interference at the mains grid, a filter 104 is placed between the mains grid terminals and the three phase rectifier stage. Therefore, the three phase mains 101 will be rectified by e.g. a stage comprising diodes 102, which was fed by 115, which establishes the connection between the three phase mains 101 and the rectifier 102. The stage 103 can provide possibilities to filter or smoothen the rectified voltage. This filter 103 is placed between the rectifier 102 and the smoothing capacitor 104. This rectified voltage is supplied to an array of capacitors 104 to receive a further effect of smoothing. However, the intermediate capacitor can also consist of two capacitors in series. The centre tap of the in series connected capacitors can be connected to the neutral mains terminal. Two full bridges 105, 107, e.g. two three phase IGBT full bridge modules, are connected to the positive rail voltage 106 and the negative rail voltage 114. These modules are available in different dimensions in terms of voltage and current rating. Furthermore, these modules are also available as so-called "Intelligent Power Modules". The module comprises the driver electronic and the required protection functionality. Thus, a compact and robust inverter can be established. As a further advantage, the use of these modules will reduce the costs of the inverter.

The output terminals of the in total two times three switching phases 108 are connected each to a resonant circuit. The resonant circuit consists of a resonant capacitor 117 and optionally a discrete inductance 118 and a high frequency transformer 109, wherein the windings of the transformer contributes to the resulting inductance because of the leakage inductance. The resonance frequency is determined by the sum of inductances (leakage inductance of the transformer and the discrete inductance 118) and the series resonant capacitance 117 per phase. The transformer 109 provides galvanically isolation. Thus, the secondary terminals are isolated from mains, which is relevant in aspects of safety. At the secondary winding terminal a three phase rectifier 110 is connected. The secondary rectified voltage is smoothed by means of an output capacitor 111. The different single output AC-currents are switched together with the help of the rectifier 110. This leads to a resulting output AC-current at the output terminal of the power converter. Preferably, the resonance frequency of all three phases is identical to avoid beating. In order to control the output voltage of the inverter, a controller 113 is located at the mains-connected side. The switching devices will switch only near or at a zero-current crossing.

Fig. 2 shows exemplary an output voltage 201, 202 of the full bridges 105, 107. This output voltage 201, 202 is fed to the combination of the resonant circuit 117, 118, 718, 717. As a result thereof there is an AC-current 203 (the AC-current on the primary side or the secondary side of the transformer are similar, there is only the difference because of losses of power).

Fig. 3 depicts an output voltage 301, 302, wherein the output voltage 301, 302 has more power compared with the situation of fig. 2 (the on-state of the output voltage 301, 302 is longer than the on- state of the output voltage 201, 202). The effect to the resulting AC-current 303 is corresponding in comparison with the situation of fig. 2 (The AC-current 303 has a higher amplitude than the above mentioned AC- current 203).

Fig. 4 depicts a situation, wherein it is continued with the tendency of increasing on-state duration of the output voltage. The output voltage 401, 402 is exaggerated compared with the output voltage 301, 302 and especially 201, 202. The resulting AC-current 403 has the highest amplitude. It has to be mentioned that the switching takes place at zero current instances (zero current switching).

By varying the number of active phases ( polarity of voltage and current is identical, current is delivered to the output stage) and the number of passive phases (output voltage is zero, no power delivered to the resonance circuit 709, energy stored in the resonance circuit 709 is transmitted to the output) the output values (power, voltage) can be controlled.

Besides an on-switching state of the bridges and an off- switching state of the bridges there is a third state during which all switches are open. In this state the energy stored in the resonant circuit will be transferred to the output and/or to smoothing capacitance 103, 104 of the intermediate stage.

The IGBT-inverter modules are switching in a medium frequency range, e.g. 2OkHz to 100kHz . In this frequency range the propagation delay times and the driving losses are acceptable. However if three phases are operating with identical switching instances at the same time the current ripple at the intermediate stage will be high. In this case the system operations could be assumed as one single inverter which transfers all the power. This case can be named "single Phase operation".

Fig. 5 depicts the situation when all three output voltages 501, 502 of the full bridges 105, 107, 705, 707 are working with the same phase. As a result thereof the

AC-currents have the same phase. The sum of the transformed three AC-currents at the secondary winding of the transformer 120, 720 is shown as reference sign 503. The energy transmission does not show a continuous train, the energy gathered from the inverter input is pulsating with the resonance frequency of the resonant circuit 709. Thus, there will be a high amount of emitted high frequency interference to the mains grid. Therefore a large filter will be needed.

Fig. 6 shows the situation when the full bridges 105, 107, 705, 707 are switched at different times (especially with a phase of 120 degrees). In this situation the output voltages 601, 602 lead to a sum of the single AC-currents 603, 604, 605. This sum of the single AC-currents 603, 604, 605, the resulting AC-current, has a smaller ripple in comparison with the ripple of the resulting AC-current 503 of fig. 5.

In figures 5 and 6 the waveforms for both the discussed control methods are shown for the identical power level. In figure 5 all the phases are operating at identical switching instances at the same time and thus, the output voltage ripple is high. The use of a phase-shifted control is shown in figure 6. With this type of control the currents in each the phases are shifted against each other and thus, the output voltage ripple is reduced and a more uniform power flow is achieved. As a result, smaller filter sizes are required at the mains input side and the DC output voltage. The reduction of voltage ripple and the reduction of filter sizes are two important aspects achieved within this invention described herein. A further embodiment of the invention is the phase-shifted modulation scheme of the inverters. In this mode of operation the controller provides a modulation sequence in a way that all the phases will provide a nearly identical power amount of power will be transmitted. To do so, the controller provides a dedicated phase-shift between all the resonant phases. In the described control method the voltage ripple at the output capacitance (fig. 1, 111) will be lower than the voltage ripple in the "single phase mode" of operation.

Fig. 7 shows a further embodiment of the invention, which has a different winding turn ratio and thus different voltage levels can be addressed. This embodiment of the invention is intended to provide multiple independently controllable DC output voltages. In Fig. 7 a system is shown which provides DC-voltages to three independent, galvanically isolated loads. In this arrangement each converter is controlled independently. Figure 7 shows an outline with 3 output voltages, however other number of output voltages are thinkable. Thus individual required output voltages (e.g. ul, u2, u3) can be addressed and controlled. A further embodiment of the invention is the generation of identical, galvanically isolated output voltages. In this case all the output voltages are on an identical level. This application may be used in multilevel-type of inverters to provide galvanically isolated voltages to the inverter stages.

Fig. 7 depicts the same components as fig. 1, wherein there is a three phase mains supply 701, a rectifier 702, a filter 703, which can be left out, an array of capacitors 704, the positive resulting potential 706 and the negative resulting potential 714, two three phase full bridges 705 and 707, the connection 708 to the resonant circuits 709 with the inductances 718 and the capacitors 717, the primary side of the transformer 719 and the secondary side of the transformer 720, as well as the controllers 712, 713 and the mains grid filter 715. The difference to the circuit 116 of fig. 1 is the fact that the circuit 716 of fig. 7 shows at the secondary side of the transformer 720 three different rectifiers. The result thereof is that there are three different uncoupled AC-currents with three different single voltages ul, u2 and u3.

A further embodiment of the invention is the use of a half-bridge configuration of the circuit outline. In this outline the system operates towards half the intermediate voltage level. In this case, the number of output voltage levels are reduced to three different stages. The outline mentioned herein provides a solution with less restrictions on the output voltage level.

This disclosure describes a system and method of operation to enable a high efficiency operation of the proposed converter. Within the described system standard IGBT-modules will be used, which will be operated with low losses. A further embodiment of the invention provides IEC-compliant isolation between the AC-mains input terminals and DC output.

A further embodiment of the invention provides independent controllable DC output voltages, which could be connected in series. Thus, stacked inverter topologies could be supplied with independent controllable, isolated DC-voltages. For high power applications a mains grid connected power supply is required to supply a defined value or range of DC output voltage(s). The DC-voltage is used to supply e.g. an inverter which transforms the DC voltage to a high frequency waveform. The magnitude of the DC voltage can be either constant or variable depending on the application. When a constant DC voltage is necessary fluctuations in the mains voltage must be compensated or regulated by means of the input stage. In many applications the AC - DC input stage must cope with large variations of output power ranges.

The circuit and method described herein focuses on a mains input stage which provides a controllable DC output voltage, especially in medium and high power applications. In the DC-DC converter controllable semiconductor switches (MOSFET,

IGBT) are used. The DC input voltage is converted to a high frequency AC-voltage. The AC-voltage is supplied to a primary winding of a transformer. Thus galvanic isolations can be provided. The voltage across the secondary winding of the transformer is rectified. To achieve volume- and size-reduced outlines of the converters, high switching frequencies are desired. Thus the size of the magnetic and filter components can be reduced and the control dynamic is improved since less energy is stored in the filter components As a drawback, the semiconductor devices will dissipate higher losses when switched at higher frequencies. To reduce the switching losses when switching at higher frequencies, passive snubbers are implemented in the inverter. However, these snubbers only provide switching loss reduction at certain power levels and not over the whole range of output power.

The use of load-resonant DC-DC converters (ref. Mohan, Undeland, Robbins, "Power Electronics", Chapter 7 : "Series resonant DC-DC converters" ), will avoid the switching losses when operating at the resonant frequency of the load- resonant converter. In this mode of operation the inverter switches at zero current. The output power and voltage is controlled by means of changing the operating frequency. While lower switching frequencies are applied, the amount of filter components will rise to achieve a required small ripple of output voltage. Furthermore, the cross-section and thus the size of a high frequency transformer (which is required to provide galvanic isolation) will increase while the switching frequency is reduced. Contrariwise, the switching frequency can be increased above the resonant frequency. This will lead to an operation without switching at zero current instances and thus switching losses will occur.

The circuit and method described herein focuses on the operation of a mains grid supplied resonant converter which provides an isolated, variable DC output voltage. A main feature of the system described herein is the loss-reduced operation of the switching devices. This enables the operation at higher frequencies and thus, smaller components can be used. The reduction of losses and smaller components lead to a cost- reduced outline of the converter and higher dynamic response.

The inventive power converter can be used to provide high power x-ray applications with regulated DC-voltages. However, the circuit can be used other medical- and -non-medical applications (e.g. Mains power supply for MRI- Applications ).

There are especially the advantages that compared to existing industrial solutions the circuit described herein provides less switching losses, best utilisation of electronic components. Further it can be used low-cost switches and multiphase motordrive integrated modules can be used. Especially, there are lower EMI- interferences according the methods of the inventive concept.

It should be noted that the term 'comprising' does not exclude other elements or steps and the 'a' or 'an' does not exclude a plurality. Also elements described in association with the different embodiments may be combined.

It should be noted that the reference signs in the claims shall not be construed as limiting the scope of the claims.

REFERENCE SIGNS:

101 three phase mains

102 rectifier

103 filter

104 array of capacitors

105 full bridge

106 positive potential

107 full bridge

108 connection full bridges to resonant circuits

109 transformer

110 rectifier

111 capacitor

112 control

113 control

114 negative potential

115 filter

116 circuit

117 capacitance

118 inductance

119 primary side of transformer

120 secondary side of transformer

201 output voltage

202 output voltage

203 AC-current

301 output voltage

302 output voltage

303 AC-current

401 output voltage

402 output voltage

403 AC-current

501 output voltage 502 output voltage

503 resulting AC-current

601 output voltage

602 output voltage

603 AC-current

604 AC-current

605 AC-current

701 three phase mains

702 rectifier

703 filter

704 array of capacitors

705 full bridge

706 positive potential

707 full bridge

708 connection from full bridges to the resonant circuits

709 resonant circuits

710 transformer

711 rectifier

712 control

713 control

714 negative potential

715 filter

716 circuit

717 capacitance

718 inductance

719 primary side of transformer

720 secondary side of transformer

Claims

CLAIMS:

1. A method for controlling a power converter, wherein the power converter comprises a first and a second full bridge (105, 107, 705, 707), a first and a second resonant circuit and a transformer (109, 710), wherein the first resonant circuit is supplied by the first full bridge (105, 705) with a first AC voltage, wherein the second resonant circuit is supplied by the second full bridge (107, 707) with a second AC voltage, wherein a first resonant current runs in the first resonant circuit, wherein a second resonant current runs in the second resonant circuit, wherein the transformer (109, 710) comprises a first primary side (119, 719), a second primary side (119, 719), a first secondary side (120, 720), and a second secondary side (120, 720), wherein the first primary side (119, 719) is connected to the first resonant circuit and the second primary side (119, 719) is connected to the second resonant circuit, the method comprising the steps of:

- supplying the first and the second full bridge (105, 107, 705, 707) by a DC-voltage,

- controlling the first and the second full bridge (105, 107, 705, 707) by a control factor such as the first full bridge is switched at or adjacent to a zero crossing of the first resonant current and such as the second full bridge is switched at or adjacent to a zero crossing of the second resonant current, wherein a first AC-current results at the first secondary side of the transformer (109, 710) and a second AC-current results at the second secondary side of the transformer (109, 710).

2. The method according to claim 1 , comprising the step of

- shifting the phases of the first and the second AC-current by the control factor.

3. The method according to one of the preceding claims, comprising the step of

- coupling the first and the second AC-current, which leads to one resulting output current.

4. The method according to claim 3, comprising the step of

- shifting the phase of the control factor, such as to control the ripple of the resulting output current.

5. The method according to claim 4, comprising the step of

- shifting the phase of the control factor to reduce the ripple of the resulting output current.

6. The method according to claim 4, comprising the step of - shifting the phase of the control factor, such as to maximize the ripple of the output current.

7. A power converter comprising

- a first full bridge (105, 705) for providing a first AC voltage, - a second full bridge (107, 707) for providing a second AC voltage,

- a first resonant circuit (709),

- a second resonant circuit (709) and - a transformer (109, 710), wherein the transformer is adapted for transforming the first and the second AC voltage, wherein a method according to one of preceding claims is applied.

8. The power converter according to claim 7, wherein one of the group consisting of the first full bridge (105, 705) and the second full bridge (107, 707) is a full bridge module.

9. The power converter according to one of the claims 7 or 8, wherein one of the group consisting of the first resonant circuit (116) and the second resonant circuit (709) comprises a capacitance (117, 717) and an inductance (118, 718).

10. The power converter according to one of the claims 7 to 9, wherein the inductance (118, 718) comprises the leakage inductance of the transformer (109, 710).

11. A programme element, which, when being executed by a processor, is adapted to carry out one of the methods according to the claims 1 to 6.

12. A computer readable medium having stored the programme element of claim 11.