DC-ac power converter with series connected power modules

Abstract

The present invention provides an electric DC-AC power converter based on a plurality of DC-AC power modules connected with their AC output terminals connected in series. Each DC-AC power module: a circuit board (PCB or DBC) having a top side and a bottom side, a resonant DC-AC circuit comprising at least one semiconductor switch arranged for operation at a switching frequency of more than 1 MHz, and at least one capacitor connected to the semiconductor switch via conductive parts of the circuit board. At least one damping circuit with a resistor component, preferably with a resistor and capacitor in parallel, is connected in parallel with the conductive parts of the circuit board connecting the capacitor and the semiconductor switch. The damping circuit is tuned to dampen a resonance caused by parasitic behaviour of the circuit board, wherein said resonance has a frequency higher than the swithing frequency. In this way, the high frequency resonance caused by said parasitic behaviour of the circuit board can be eliminated. This high frequency resonance in each power module has been found to be critical for stability of a power converter based on series connected power modules. With the simple damping circuit, high power DC-AC conversion is possible using GaN or SiC semiconductor technology.

Description

[0001] 84129PC01

[0002] 1

[0003] DC-AC POWER CONVERTER WITH SERIES CONNECTED POWER MODULES

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to power electronics. Especially, the invention realtes to an electric DC-AC power converter with a plurality of DC-AC power modules connected in series. Especially, the power module is suited for high power applications where an AC voltage in the MHz range is advantageous.

[0006] BACKGROUND OF THE INVENTION

[0007] Resonant DC-AC converters implemented with semiconductor material technologies such as gallium nitride (GaN) and silicon carbide (SiC) can operate at switching frequencies of several 10s MHz. This opens a whole new range of application domains such as industrial heating, radar, electric vehicle charging, and communication equipment used by maritime and airline industry.

[0008] However, the challenge with these applications is the fact that the power required is usually high in order of 10s of kW, sometimes going to 100s of kW.

[0009] The high power requirement can not be met with a single resonance DC-AC converter, and therefore a high output power can only be achieved if several resonant DC-AC converters are combined.

[0010] One way to combine several resonant DC-AC converters is to use a power combiner which requires an additional component in the system.

[0011] Another approach to increase the output power is to series connect several resonant DC-AC converters.

[0012] However, experiences have shown that a converter based on a series connection of several resonant DC-AC converter modules typically suffers from instability, and these instability problems can be detrimental to the overall function of the system and can even lead to permanent damage of the power modules. 84129PC01

[0013] 2

[0014] SUMMARY OF THE INVENTION

[0015] Following the above, it may be seen an object of the present invention to provide a DC-AC power converter based on interconnection of several DC-AC power modules to allow a stable and efficient generation of an AC power output in the MHz frequency range for high power and high voltage applications, preferably without the need for power combiner components.

[0016] In a first aspect, the invention provides an electric DC-AC power converter comprising an electric input arranged to receive a DC voltage and an electric output arranged to output an AC voltage, the DC-AC power converter comprising:

[0017] - a plurality of DC-AC power modules connected between said electric input and output, wherein the plurality of DC-AC power modules are connected with their AC output terminals in series, wherein each of the plurality of DC-AC power modules comprises:

[0018] - a circuit board having a top side and a bottom side,

[0019] - a resonant DC-AC circuit comprising at least one semiconductor switch arranged for operation at a switching frequency of more than 1 MHz, and at least one capacitor connected to the semiconductor switch via conductive parts of the circuit board, such as comprising two semiconductor switches and two capacitors,

[0020] - at least one damping circuit comprising at least a resistor component, wherein the damping circuit is connected in parallel with the conductive parts of the circuit board connecting the capacitor and the semiconductor switch, and wherein the damping circuit is tuned to dampen a resonance caused by parasitic behaviour of the circuit board, wherein said resonance has a frequency higher than the swithing frequency.

[0021] Such DC-AC power converter made up of several resonant power modules connected in series has been tested and found to be stable, even at high switching frequencies in the MHz range, and for capable of handling of power levels of 10s of kW. The switch(es) can be implemented in for example GaN or SiC technology, which allows efficient switching at high switching frequencies. 84129PC01

[0022] 3

[0023] The invention is based on the inventors' insight, that a high frequency resonance in each power module, e.g. at a frequency in the range of more than 50 MHz, at a frequency higher than the switching frequency, causes instability when the modules are connected in series. This high frequency resonance is observed in the drain-source voltage of a power module, and it has been found to propagate through the series connected power modules which thereby disturb the function of each other. This causes instability and in some cases causing break down of the converter.

[0024] This high frequency resonance in each power module has been identified by the inventors to be caused by parasitic of the circuit board (e.g. Printed Circuit Board PCB) design on which the semiconductor and other components of the power module are mounted. The expected impedance behavior is that the impedance increases linearly with frequency. However, due to the presence of a Parallel Resonance Frequency (PRF) in the circuit board due to parasitic behavior of the trace and capacitive components, a significant peak in the impedance curve can be observed at a high frequency which far exceeds the expected behavior. This causes a high frequency energy component in the drain-source voltage waveform, usually at frequency of 60 MHz or above. The actual responance frequency has been found to be determined by the practical implementation of the power module, e.g. if the semiconductor switch and capacitor components are positioned on the same side or opposite sides of the circuit board. For example, it has been found that with the semiconductor switch and capacitor on the same circuit board side, a resonance frequency of 67 MHz has been observed, while shifted to 130 MHz, if the semiconductor switch and capacitor are on opposite sides of the circuit board.

[0025] The simple damping circuit or "snubber", e.g. implemented as a resistor or a parallel connection of a resistor and capacitor, directly connected to the source and / or drain terminal of the semiconductor switche(s), it has been tested than an efficient reduction of the high frequency resonance of the power module can be obtained, thereby ensuring a stable function of the entire converter. Of course, the components (resistor, capacitor values) of the damping circuit should be selected to be tuned to dampen the actual high frequency resonance in a power module. 84129PC01

[0026] 4

[0027] Since the damping circuit is arranged for connection in parallel with the circuit board trace (i.e. a very low impedance). Thus, the voltage drop seen by the darning circuit is very low, and thereby the power loss in the damping circuit will be low. For example, in a power module operating with a drain-source voltage of 400 V, the power loss in the damping circuit can be kept below 2 W.

[0028] This means that an efficient high power DC-AC converter can be made of several power modules directly connected in series without the need for power combining elements which cause loss of power. Thus, power modules each with a limited voltage handling capability can be connected to provide a DC-AC converter with a high voltage output. In this way an efficient high power DC-AC converter can be implemented.

[0029] The inventors have specifically tested a DC-AC converter with three GaN based class E push-pull power modules in series operating at 2.43 MHz, where application of a damping circuits in the form of 2 resistors have proven to provide a stable 4 kW AC output with an efficiency of 98%. Ripple in the output voltage was successfully dampened using a simple damping circuit in the form of a resistor on each of the power modules, thereby ensuring stability.

[0030] By using a plurality of interconnected power modules, a power converter can be built which is scalable with respect to voltage and power requirements. Thus, the power module is suitable for large scale production and still it can be used in a variety of power converter configurations.

[0031] It is to be understood that the invention relates in general to high frequency switching power DC-AC modules connected in series, whether these modules are implemented as switching networks including only one controllable switch or two controllable switches or more than two controllable switches. Especially, the modules may be implemented with two or four electrically controllable bidirectional two-quadrant switches.

[0032] In the following, preferred features and embodiments will be described. 84129PC01

[0033] 5

[0034] Here and in the following "connected" means electrically connected.

[0035] By "output an AC electric output signal" is meant that the power circuit outputs a stepped voltage waveform which can approximate an AC signal.

[0036] It is to be understood that even though the invention relates to direct series connection of two or more resonant DC-AC modules without any intermediate or extra components such as a power combiner. However, it may be preferred that the DC-AC converter has a transformer at its output to provide galvanic separation, whereas the internal series connected DC-AC modules are directly connected.

[0037] The semiconductor switches may especially be bidirectional two-quadrant switches may be implemented based on SiC or GaN technology.

[0038] By 'circuit board' is to be understood implementations such as Printed Circuit Board (PCB) or Direct Bonded Copper (DBC) or the like.

[0039] In essence, the power converter is suited for a large number of high power, high frequency applications, e.g. to replace vacuum tube based electric power conversion solutions. For example, the power converter may be used to provide electric power for a heating machine or a drying machine in an industrial application.

[0040] In some embodiments, the damping circuit is implemented only by one resistor component.

[0041] In preferred embodiments, the damping circuit comprises a parallel connection of a resistor component and a capacitor component. In some embodiments, the damping circuit comprises an inductor component connected in series with said parallel connection of the resistor component and the capacitor component.

[0042] In preferred embodiments, the power converter comprises respective damping circuits connected to the drain terminal of the semiconductor switch and connected to the source terminal of the semiconductor switch. 84129PC01

[0043] 6

[0044] Preferably, the damping circuit is directly connected to traces of the top side or the bottom side of the circuit board. More preferably, the the power converter comprises respective damping circuits directly connected to traces of both of the top side and the bottom side of the circuit board.

[0045] Especially, said resonance caused by parasitic behaviour of the circuit board, is a resonance caused by parasitic behavior of traces and capacitive components on the circuit board.

[0046] The power converter preferably comprises first and second DC-AC power modules wherein an electric AC output terminal of the first DC-AC power module is directly connected to an electric AC output terminal of the second DC-AC power module. Especially, the power converter may comprise three or more DC-AC power modules with their AC electric output terminals directly connected in series. In some embodiments, the power converter comprises at least four, such as 4-10, such as 4-20, such as more than 10, DC-AC power modules with their AC electric output terminals directly connected in series.

[0047] The DC-AC power modules are preferably connected with their DC electric input teminals to receive the DC voltage.

[0048] In some embodiments, the power converter comprises a series connection of at least: an input DC-AC power module, a plurality of DC-AC power modules with their AC electric output terminals connected in series, and an output DC-AC power module. Especially, the DC-AC power converter may comprise a galvanic isolation device connected between the electric input of the DC-AC power converter and an input terminal of the input DC-AC power module. Especially, the power converter may comprise a galvanic isolation device connected between an electric ouput terminal of the DC-AC power converter and an output terminal of the output DC- AC power module.

[0049] Especially, the plurality of DC-AC power modules may comprise one or more push-pull current type resonant DC-AC power modules. Specifically, each of the one or more push-pull current type resonant DC-AC power modules comprises: 84129PC01

[0050] 7

[0051] - two semiconductor switches, such as GaN or SiC switches, with their drain terminals directly connected,

[0052] - two capacitors connected across source and drain terminals of the respective semiconductor switches, and

[0053] - one inductor connected to the source terminals of both of the semiconductor switches at one end and the opposite end forming a first DC input terminal of the DC-AC power module,

[0054] - two inductors connected with one end to the respective drain terminals of the semiconductor switches and wherein their opposite ends are connected to form a second DC input terminal of the DC-AC power module, wherein the drain terminals of the semiconductor switches form respective first and second AC output terminals of the DC-AC power module. More specifically, each of the two semiconductor switches may have at least one damping circuit connected directly to their drain or source terminals at one end and connected directly to the capacitor terminals at the opposite end. More specifically, each of the two semiconductor switches may have respective damping circuits connected directly to their drain terminals and respective damping circuits connected directly to their source terminals.

[0055] At least one semiconductor switch of the plurality of DC-AC power modules may be implemented as SiC or GaN technology. Especially, all semiconductor switches of all of the DC-AC power modules may be implemented as SiC or GaN technology, GaN High-electron-mobility-transistor (HEMT), e.g. semiconductor switches having a voltage rating of 650 V.

[0056] The plurality of DC-AC power modules may be configured to operate at a switching frequency of at least 5 MHz, such as at least 10 MHz, such as at least 20 MHz, such as at least 30 MHz, such as at least 40 MHz, such as at least 50 MHz, such as at least 100 MHz.

[0057] The DC-AC power converter may be configured to generate an AC output voltage of at least 100 V, such as at least 300 V, such as at least 500 V, such as at least 1.5 kV, such as at least 5 kV, such as at least 10 kV. Due to the series coupling, a high output voltage from the DC-AC power converter can be obtained, e.g. using 84129PC01

[0058] 8

[0059] GaN HEMT semiconductor switches with a voltage rating of 650 V, e.g. reaching above 1.5 kV if three power modules are used.

[0060] The DC-AC power converter may be configured to generate an electric output power of at least 1 kW, such as at least 10 kW, such as at least 50 kW, such as at least 100 kW.

[0061] Preferably, the DC-AC power converter comprises a control circuit configured to control switching of the semiconductor switches of the plurality of DC-AC power modules. Especially, the power modules may have at least one externally accessible terminal to allow external control of the switching cycle of the resonant DC-AC circuits. However, still the power modules may comprise a switching controller or switching circuit connected to control switching of the resonant DC- AC circuits. Alternatively, the switching controller or switching circuit may be provided outside the power module, e.g. outside a casing housing the power module components. Specifically, the switching controller may be configured to control switching so as to provide a multilevel AC electric output voltage, such as 3-10 levels of AC electric output voltages at the electric output of the power converter.

[0062] In a second aspect, the invention provides a device comprising the DC-AC power converter according to the first aspect, wherein the device comprises at least one electrically power consuming component connected to receive electric power from the electric output of the power converter.

[0063] Especially, the device may comprise a heating machine connected to receive electric power from the DC-AC power converter, or the device may comprise a drying machine connected to receive electric power from the DC-AC power converter. Other types of devices may be a radar device, an electric charger for charging electric vehicles or the like. It is to be understood that these devices are merely examples where the DC-AC power converter of the invention is advantageous.

[0064] In a third aspect, the invention provides use of the DC-AC power converter according to the first aspect. Especially, the use is for producing or performing a 84129PC01

[0065] 9 process on one or more of: 1) wood, 2) furniture, 3) fiberglass, 4) a pharmaceutical substance, 5) a chemical substance, 6) a silicon or ceramic, 7) foam, 8) a textile, 9) a dairy product, 10) a vulcanized product, and 11) food (specifically for sterilizing, heating, or cooling food).

[0066] In a fourth aspect, the invention provides use of the device according to the second aspect. Especially, the use is for producing or performing a process on one or more of: 1) wood, 2) furniture, 3) fiberglass, 4) a pharmaceutical substance, 5) a chemical substance, 6) a silicon or ceramic, 7) foam, 8) a textile, 9) a dairy product, 10) a vulcanized product, and 11) food (specifically for sterilizing, heating, or cooling food).

[0067] In a fifth aspect, the invention provides a method for converting a DC electric signal to an AC electric signal, the method comprising

[0068] - series connecting output terminals of a plurality of resonant DC-AC power modules, wherein each of said resonant DC-AC power modules is implemented with one or more semiconductor switches and one or more capacitors mounted on a circuit board,

[0069] - providing at least one damping circuit comprising a resistor component on each of the plurality of resonance DC-AC power modules, wherein each of the damping circuits is tuned to dampen a resonance caused by parasitic behaviour of the circuit board,

[0070] - applying a DC voltage to input terminals of the series connected DC-AC power modules, and

[0071] - operating the semiconductor switches of the DC-AC power modules at a switching frequency of at least 1 MHz, so as to generate the AC electric signal based on an electric output from the series connected DC-AC power modules.

[0072] Especially, the method comprises generating at least 1 kW, such as at least 10 kW, electric power for powering an electric power consuming device.

[0073] Especially, the method comprises generating at least an AC output voltage of at least 100 V, such as at least 500 V, such as at least 1 kV, such as at least 2 kV, for powering an electric power consuming device. 84129PC01

[0074] 10

[0075] The step of tuning component(s) of the damping circuit may comprise detecting a resonance frequency of the DC-AC power module and selecting component(s) to provide damping at this resonance frequency.

[0076] Features and embodiments of the mentioned aspects of the present invention may each be combined with each other. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0077] BRIEF DESCRIPTION OF THE FIGURES

[0078] The present invention and in particular preferred embodiments thereof will now be disclosed in more detail with regard to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0079] FIG. 1 illustrates a DC-AC power converter based on series connected power modules to produce an AC output voltage being the sum of output voltages generated by each of the power modules,

[0080] FIG. 2 illustrates an electric circuit example of a semiconductor switch with two damping circuits,

[0081] FIG. 3 illustrates photos of top and bottom of a power module with GaN switches implemented on a circuit board,

[0082] FIG. 4 illustrates sketches of top and bottom layer of a circuit board where damping circuits in the form of resistors are indicated, including a circuit board trace layout specifically suited for mounting of the damping resistors,

[0083] FIG. 5-7 illustrate various examples of interconnecting power modules,

[0084] FIG. 8 illustrates an example of two series connected class E push-pull power modules with damping circuits in the form of resistors and their positions indicated on a circuit board sketch,

[0085] FIG. 9 illustrates connection of a power converter embodiment with four power modules generating AC voltage to power a heating device HT, and

[0086] FIG. 10 illustrates steps of a method embodiment. 84129PC01

[0087] 11

[0088] DETAILED DESCRIPTION OF EMBODIMENTS

[0089] FIG. 1 shows a DC-AC power converter based on series connected power modules PM to produce an AC output voltage being the sum ZV of output voltages VI, V2, V3, V4 generated by each of the power modules PM. The AC output voltage ZV is here shown connected to a load ZL.

[0090] Such series connection of power modules is advantageous, since a high output voltage ZV can be obtained which exceeds the voltage rating of the single power modules PM. In this way for example GaN switches can be used with a 650 V voltage rating to produce high voltage AC outputs.

[0091] However, the inventors have experienced that directly series connection of power modules result in problems with stability. Based on the insight of the inventors, the experienced instability is caused by high frequency oscillations at a frequency clearly above the switching frequency. These oscillations in the output voltage of each power modules spreads to the other power modules in the series connection and disturbs their function which causes instability. The oscillation is caused by practical implementation on a circuit board which introduces parasitic electric components.

[0092] FIG. 2 illustrates a circuit diagram with a solution to the high frequency oscillation problem, namely mounting a damping circuit or snubber to each of the series connected power modules, here illustrated as two separate damping circuits DMP1, DMP2 mounted at the connection points between a semiconductor switch SW1 and a capacitor Cl forming the resonant DC-AC power module.

[0093] The parasitic resistance and inducatance of the circuit board trace is represented here by R_P and L_p, and the damping circuits DMP1, DMP2 are connected in parallel with these parasitic components, meaning parallel to the conductive circuit board trace connecting the switch SW1 and the capacitor Cl.

[0094] In the illustration, the damping circuits DMP1, DMP2 each has a resitor R_d, a capacitor C_d and an inductor L_d. However, in some cases the resistor R_d has proven to be sufficient to dampen the high frequency oscillations. It may even be 84129PC01

[0095] 12 enough in some cases to introduce a darning circuit DMP1, DMP2 only at the drain or source side of the switch SW1 rather than at both source and drain side, as illustrated.

[0096] The damping circuit DMP1, DMNP2 can be implemented in the form or a single resistor, a combination of a resistor and a capacitor, or a combination of a resistor, a capacitor and an inductor, rather than all three R_d, C_d, L_d as in the illustration of FIG. 2. The element or elements R_d, C_d, L_d of the damping circuit DMP1, DMP2 should be tuned to dampen the high frequency oscillations which has a frequency determined by various factors such as the practical circuit board implementation.

[0097] FIG. 3 illustrates photos of bottom and top sides of a practical circuit board implementation of a power module with a resonant circuit in the form of two GaN switches GAN1, GAN2 connected to two capacitors Cl, C2. Due to the parasitic behaviour of the conductive trace of the circuit board together with the capacitors Cl, C2, the inventors have found that a problematic high frequency oscillation is introduced, usually at 50 MHz or higher.

[0098] FIG. 4 illustrates sketches of top layer and bottom layer of a circuit board for implementation of a push-pull current source type resonant DC-AC power module. For optimal performance of the damping circuits it has been found to be advantageous to mount elements of the damping circuits, e.g. a resitor and a capacitor on both of the top layers and bottom layers of the circuit board. In the illustration, a resistor elements are indicated on the top layer Dl_t, while position for mounting of a capacitor elements Dl_b is indicated (white dotted area) on the bottom layer. Corrspondingly another damping circuit has resistor elements D2_t on the top layer while capacitor components are shown on the bottom layer.

[0099] FIG. 5-7 illustrate examples of possible ways to provide a DC-AC power converter based on series connected push-pull current type resonant DC-AC power modules, which is made possible by means of the damping circuits or snubbers as explained to suppress high frequency oscillation problems. However, for simplicity, the damping circuits are not shown in FIG. 5-7. 84129PC01

[0100] 13

[0101] FIG. 5 shows four such resonant DC-AC power modules in series connection AC output terminals are AC1 and AC2 between which the load can be connected. The DC power supply to the different cells is connected in parallel to achieve a modular power converter. As shown all the cells have their DC terminals connected to the same DC power supply.

[0102] FIG. 6 shows the same push-pull current source type resonant DC-AC power modules connected to achieve a multi-cell modular power converter is also shown where the DC side is series connected. To achieve the series connection of DC power supply the connection between individual cell has to be fitted with additional DC blocking capacitors as illustrated. The additional inter-cell capacitor allows for negative terminal of one cell and positive terminal of subsequent cell to the same point to avoid short-circuiting the DC power supply. In this configuration the AC output voltage across the AC output terminals AC1, AC2 will have a DC offset equal to the total sum of voltages on inter-cell capacitors minus one.

[0103] FIG. 7 shows a third DC connection strategy for the multi-cell configuration where a combination of FIG. 5 and 6 is applied. In this embodiment, two DC power supplies are connected to two cells in parallel method as in FIG. 5, and then they are connected in series using the DC blocking capacitor as in FIG. 6. The output AC voltage is achieved at the output terminals AC1 and AC2.

[0104] FIG. 8 illustates an embodiment with two class-E push-pull power modules connected in series thus producing respective AC output voltages VI, V2 thereby constituting the total AC output voltage ACV. Damping circuits are used on both power modules, but for simplicity here only layout details showing the damping circuits of one module only.

[0105] In this implementation, the damping circuits are formed only by resistors Rd which are shown mounted on both drain and source side of the GaN HEMT based switches Qla, Qlb, Q2a, Q2b and their connecting points to the capacitors.

[0106] As indicated by the grey area, it is seen that the circuit board traces have indentations for mounting of the damping resistors Rd in the area between the connecting points of the switches and capacitors. In this way the damping 84129PC01

[0107] 14 resistors Rd are added in parallel to the conductive trace connecting each connection point between the switches Qla, Qlb, Q2a, Q2b and the capacitors.

[0108] In the example illustrated in FIG. 8, a resistor value Rd of 2 has been found effective for suppressing a 55 MHz ripple which was otherwise observed.

[0109] FIG. 9 illustrates a device embodiment, namely a heating device HT connected to be powered by an AC voltage output ACV from a DC-AC power converter PCNV formed by a plurality, here four, series connected power modules PM1, PM2, PM3, PM4 which are applied by a DC input voltage DCV from a DC power source. Specifically, for a heating element for industrial heating, the DC-AC power converter PCNV is advantageous, since it can replace currently used vacuum-tube triode based solutions which suffer from a poor efficiency.

[0110] It is to be understood in general that the series connected DC-AC power converters of the invention can be used for powering various devices in various applications where a high power DC-AC power conversion with high efficiency is required.

[0111] FIG. 10 illustrate steps of a method embodiment, namely a method for converting a DC electric signal to an AC electric signal. The method comprises series connecting SI output terminals of a plurality of resonant DC-AC power modules, wherein each of said resonant DC-AC power modules is implemented with one or more semiconductor switches and one or more capacitors mounted on circuit a board. Next, providing S2 at least one damping circuit comprising a resistor component on each of the plurality of resonance DC-AC power modules. Each of the damping circuits is tuned to dampen a resonance caused by parasitic behaviour of the circuit board. Next, applying S3 a DC voltage to input terminals of the series connected DC-AC power modules, and operating S4 the semiconductor switches of the DC-AC power modules at a switching frequency of at least 1 MHz, so as to generate the AC electric signal based on an electric output from the series connected DC-AC power modules. 84129PC01

[0112] 15

[0113] To sum up, the invention provides an electric DC-AC power converter based on a plurality of DC-AC power modules connected with their AC output terminals connected in series. Each DC-AC power module: a circuit board (e.g. PCB or DBC) having a top side and a bottom side, a resonant DC-AC circuit comprising at least one semiconductor switch arranged for operation at a switching frequency of more than 1 MHz, and at least one capacitor connected to the semiconductor switch via conductive parts of the circuit board. At least one damping circuit with a resistor component, preferably with a resistor and capacitor in parallel, is connected in parallel with the conductive parts of the circuit board connecting the capacitor and the semiconductor switch. The damping circuit is tuned to dampen a resonance caused by parasitic behaviour of the circuit board, wherein said resonance has a frequency higher than the swithing frequency. In this way, the high frequency resonance caused by said parasitic behaviour of the circuit board can be eliminated. This high frequency resonance in each power module has been found to be critical for stability of a power converter based on series connected power modules. With the simple damping circuit, high power DC-AC conversion is possible using GaN or SiC semiconductor technology.

[0114] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

84129PC0116CLAIMS1. An electric DC-AC power converter comprising an electric input arranged to receive a DC voltage (DCV) and an electric output arranged to output an AC voltage (ACV), the DC-AC power converter comprising:- a plurality of DC-AC power modules (PM) connected between said electric input and output, wherein the plurality of DC-AC power modules (PM) are connected with their AC output terminals in series, wherein each of the plurality of DC-AC power modules (PM) comprises:- a circuit board having a top side and a bottom side,- a resonant DC-AC circuit comprising at least one semiconductor switch (SW1) arranged for operation at a switching frequency of more than 1 MHz, and at least one capacitor (Cl) connected to the semiconductor switch (SW1) via conductive parts of the circuit board,- at least one damping circuit (DMP1, DMP2) comprising at least a resistor component (R_d), wherein the damping circuit (DMP1, DMP2) is connected in parallel with the conductive parts of the circuit board connecting the capacitor (Cl) and the semiconductor switch (SW1), and wherein the damping circuit (DMP1, DMP2) is tuned to dampen a resonance caused by parasitic behaviour of the circuit board, wherein said resonance has a frequency higher than the swithing frequency.

2. The electric DC-AC power converter according to claim 1, wherein said damping circuit comprises a parallel connection of a resistor component and a capacitor component.

3. The electric DC-AC power converter according to claim 1 or 2, wherein said damping circuit comprises an inductor component connected in series with said parallel connection of the resistor component and the capacitor component.

4. The electric DC-AC power converter according to any of the preceding claims, comprising respective damping circuits connected to the drain terminal of the84129PC0117 semiconductor switch and connected to the source terminal of the semiconductor switch.

5. The electric DC-AC power converter according to any of the preceding claims, wherein the damping circuit directly connected to traces of the top side or the bottom side of the circuit board.

6. The electric DC-AC power converter according to any of the preceding claims, comprising respective damping circuits directly connected to traces of both of the top side and the bottom side of the circuit board.

7. The electric DC-AC power converter according to any of the preceding claims, wherein said resonance is caused by parasitic behavior of traces and capacitive components on the circuit board.

8. The DC-AC power converter according to any of the preceding claims, comprising first and second DC-AC power modules wherein an electric AC output terminal of the first DC-AC power module is directly connected to an electric AC output terminal of the second DC-AC power module.

9. The DC-AC power converter according to claim 8, comprising three or more DC- AC power modules with their AC electric output terminals directly connected in series.

10. The DC-AC power converter according to any of the preceding claims, comprising at least four, such as 4-10, such as 4-20, DC-AC power modules with their AC electric output terminals directly connected in series.

11. The DC-AC power converter according to any of the preceding claims, wherein each of the DC-AC power modules are connected with their DC electric input teminals to receive the DC voltage.

12. The DC-AC power converter according to any of the preceding claims, comprising a series connection of at least: an input DC-AC power module, a84129PC0118 plurality of DC-AC power modules with their AC electric output terminals connected in series, and an output DC-AC power module.

13. The DC-AC power converter according to claim 12, comprising a galvanic isolation device connected between the electric input of the DC-AC power converter and an input terminal of the input DC-AC power module.

14. The DC-AC power converter according to any of the preceding claims, wherein the plurality of DC-AC power modules comprises one or more push-pull current type resonant DC-AC power modules.

15. The DC-AC power converter according to claim 14, wherein each of the one or more push-pull current type resonant DC-AC power modules comprises:- two semiconductor switches, such as GaN or SiC switches, with their drain terminals directly connected,- two capacitors (Cl, C2) connected across source and drain terminals of the respective semiconductor switches (GAN1, SW1, GAN2, SW2), and- one inductor (LI) connected to the source terminals of both of the semiconductor switches (GAN1, SW1, GAN2, SW2) at one end and the opposite end forming a first DC input terminal of the DC-AC power module,- two inductors (L2, L3) connected with one end to the respective drain terminals of the semiconductor switches (GAN1, SW1, GAN2, SW2) and wherein their opposite ends are connected to form a second DC input terminal of the DC-AC power module, wherein the drain terminals of the semiconductor switches (GAN1, SW1, GAN2, SW2) form respective first and second AC output terminals of the DC-AC power module.

16. The DC-AC power converter according to claim 15, wherein each of the two semiconductor switches has at least one damping circuit connected directly to their drain or source terminals at one end and connected directly to the capacitor terminals at the opposite end.84129PC011917. The DC-AC power converter according to claim 16, wherein each of the two semiconductor switches has respective damping circuits connected directly to their drain terminals and respective damping circuits connected directly to their source terminals.

18. The electric DC-AC power converter according to any of the preceding claims, wherein the at least one semiconductor switch of the plurality of DC-AC power modules are implemented as SiC or GaN technology.

19. The electric DC-AC power converter according to any of the preceding claims, wherein the plurality of DC-AC power modules are configured to operate at a switching frequency of at least 5 MHz, such as at least 10 MHz, such as at least 20 MHz, such as at least 30 MHz, such as at least 40 MHz, such as at least 50 MHz, such as at least 100 MHz.

20. The electric DC-AC power converter according to any of the preceding claims, being configured to generate an AC output voltage of at least 100 V, such as at least 300 V, such as at least 500 V, such as at least 1.5 kV, such as at least 5 kV, such as at least 10 kV.

21. The electric DC-AC power converter according to any of the preceding claims, being configured to generate an electric output power of at least 1 kW, such as at least 10 kW, such as at least 50 kW, such as at least 100 kW.

22. The electric DC-AC power converter according to any of the preceding claims, comprising a control circuit configured to control switching of the semiconductor switches of the plurality of DC-AC power modules.

23. A device comprising the DC-AC power converter according to any of claims 1- 22, wherein the device comprises at least one electrically power consuming component connected to receive electric power from the electric output of the power converter.

24. The device according to claim 23, wherein the device comprises a heating machine connected to receive electric power from the DC-AC power converter.84129PC012025. The device according to claim 23 or 24, wherein the device comprises a drying machine connected to receive electric power from the DC-AC power converter.

26. Use of the DC-AC power converter according to any of claims 1-22.

27. The use according to claim 26 for producing or performing a process on one or more of: 1) wood, 2) furniture, 3) fiberglass, 4) a pharmaceutical substance, 5) a chemical substance, 6) a silicon or ceramic, 7) foam, 8) a textile, 9) a dairy product, 10) a vulcanized product, and 11) food.

28. Use of the device according to any of claims 23-25.

29. The use according to claim 28 for producing or performing a process on one or more of: 1) wood, 2) furniture, 3) fiberglass, 4) a pharmaceutical substance, 5) a chemical substance, 6) a silicon or ceramic, 7) foam, 8) a textile, 9) a dairy product, 10) a vulcanized product, and 11) food.

30. A method for converting a DC electric signal to an AC electric signal, the method comprising- series connecting (SI) output terminals of a plurality of resonant DC-AC power modules, wherein each of said resonant DC-AC power modules is implemented with one or more semiconductor switches and one or more capacitors mounted on a circuit board,- providing (S2) at least one damping circuit comprising a resistor component on each of the plurality of resonance DC-AC power modules, wherein each of the damping circuits is tuned to dampen a resonance caused by parasitic behaviour of the circuit board,- applying (S3) a DC voltage to input terminals of the series connected DC- AC power modules, and84129PC0121- operating (S4) the semiconductor switches of the DC-AC power modules at a switching frequency of at least 1 MHz, so as to generate the AC electric signal based on an electric output from the series connected DC-AC power modules.

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