WO2013085522A1

WO2013085522A1 - Secondary side cycloconverter drive circuit for resonant coverter in solar application

Info

Publication number: WO2013085522A1
Application number: PCT/US2011/063979
Authority: WO
Inventors: Bruce Modick; Madhuwanti Joshi
Original assignee: Petra Solar, Inc.
Priority date: 2011-12-08
Filing date: 2011-12-08
Publication date: 2013-06-13

Abstract

An apparatus for converting DC power to AC power may be provided. The apparatus may include a power stage (205) and a gate driver (210) to operate the power stage (205). The power stage (205) may include resonant circuit (220) connected to a solar DC source. The resonant circuit (220) may be connected to a cycloconverter (230). The cycloconverter (230) may include a plurality of switching pairs. The gate driver (210) may operate power stage (205) by operating the plurality of switching pairs.

Description

TITLE

SECONDARY SIDE CYCLOCONVERTER DRIVE CIRCUIT FOR RESONANT CONVERTER IN SOLAR APPLICATION

This application is being filed on 08 December 201 1 , as a PCT International Patent application in the name of Petra Solar, Inc., a U.S. national corporation, applicant for the designation of all countries except the U.S., and, Bruce Modick, a citizen of the U.S., and Madhuwanti Joshi, a citizen of U.S., applicants for the designation of the U.S. only.

BACKGROUND

[001] With the growing initiative for clean energy, solar energy is becoming an important power generation element. In a solar power plant, energy from the Sun is converted into a direct current (DC) voltage by photovoltaic cells and then converted to a grid compatible alternating current (AC) voltage by a DC to AC voltage converter or inverter. This inverter may either be a string inverter, which takes input from many panels or a micro-inverter, which takes input from a single panel.

SUMMARY

[002] An apparatus may be provided. The apparatus may comprise a cycloconverter and a gate driver. The cycloconverter may comprise a plurality of switching pairs and may be configured to convert a second AC voltage to a third AC voltage. The gate driver may be configured to alternately switch the plurality of switching pairs.

[003] Both the foregoing general description and the following detailed description are examples and explanatory only, and should not be considered to restrict the invention's scope, as described and claimed. Further, features and/or variations may be provided in addition to those set forth herein. For example, embodiments of the invention may be directed to various feature combinations and sub-combinations described in the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[004] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various embodiments of the present invention. In the drawings:

[005] FIG. 1 shows a conventional semiconductor driver circuit;

[006] FIG. 2 shows a driver circuit including a cycloconverter with common source connections;

[007] FIG. 3 shows a driver circuit including a cycloconverter with common drain connections; and

[008] FIG. 4 shows a gate driver circuit.

DETAILED DESCRIPTION

[009] The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar elements. While embodiments of the invention may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description does not limit the invention. Instead, the proper scope of the invention is defined by the appended claims.

[010] Growing demand in the world power consumption and shortages of conventional fossil fuels gave rise to the concept of distributed power generation using nonconventional energy sources. Solar energy in particular became very attractive because of the government initiatives and technological advancement in the solar cells and associated electronics technology. In a typical solar power plant, the energy from the Sun is converted into a DC voltage using photo-voltaic effect. The DC voltage is further converted into an AC voltage which is compatible to the electric grid voltage using a DC to AC converter or an inverter.

[01 1] Solar inverters may be faced with the challenges of minimum cost, maximum efficiency, and high operating life. With these challenges, selecting the right inverter topology may be important for the inverter. The inverter topologies based on the resonant converter may become an attractive solution. They may comprise, for example, a cascade connection of MOSFETs/IGBTs connected in either a full bridge or a half bridge configuration, a resonant circuit consisting of series/parallel/series-parallel connections of inductors and capacitors, a high frequency transformer, diode rectifier, and either an unfolder or a high frequency inverter followed by passive filters. These types of converters may have low switching losses, less high frequency harmonics, and high DC to AC conversion efficiency.

[012] While using the resonant converters in solar inverters, losses may further be reduced. To reduce losses, a technique to combine the active rectification and unfolder in a single switching stage may be used. This may have an effect of converting high frequency AC voltage into a low frequency AC voltage. This switching stage is also called as "cycloconverter." There are numerous advantages of using this technique such as a further increase in the efficiency, possibility of bidirectional power flow, high reliability, primary side storage possibility etc.

Embodiments of the present invention may comprise a gate driver circuit design for a cycloconverter.

[013] FIG. 1 shows a conventional semiconductor driver circuit. The conventional semiconductor driver circuit comprises a high frequency DC switching stage consisting of four semiconductor switching devices Ql , Q2, Q3 and Q4, a resonant tank consisting of a series connection of an inductor and a capacitor, a high frequency transformer, a diode rectifier, and an unfolder that switches at the AC voltage frequency with four semiconductor switching devices Q5, Q6, Q7 and Q8. The semiconductor devices such as MOSFETs, BJTs, IGBTs, thyristors, etc may be used in the high frequency switching stage or in the unfolder. The DC switching stage may have the half bridge configuration or full bridge configuration. Further the resonant stage may consist of either a series or parallel or a combination of series and parallel connection of one or more inductors and capacitors. The DC switching network generates a high frequency AC voltage (also referred to as first AC) and an AC current at the primary of the transformer. The transformer transforms the first AC voltage and current into a second AC voltage and AC current. The rectifier and filter converts the second AC current into a second DC voltage which has a form of rectified sine wave. The unfolder converts the second AC voltage into a low frequency AC voltage which is compatible to the grid voltage also referred to as third AC voltage.

[014] Embodiments of the present invention may provide high frequency rectification and unfolding. Consistent with embodiments of the invention, a driver for an unfolder may comprise a zero voltage detection circuit. The zero voltage detection circuit may detect a zero crossing of an AC voltage in each cycle and may generate gate control signals for semiconductor devices (e.g. Q5, Q6, Q7 and Q8.) The control signals may pass through an electrical isolation stage and then to a driver and buffering stage.

[015] FIG. 2 shows a driver circuit 200 consistent with embodiments of the invention. As shown in FIG. 2, driver circuit 200 may comprise a power stage 205 along with a gate driver 210. Power stage 205 may comprise, for example, a cascade connection of a high frequency direct current (DC) switching stage 215, a resonant circuit 220, a high frequency transformer 225, and a cycloconverter 230. Resonant circuit 220 may comprise, for example, a series connection of an inductor 235 and a capacitor 240. As shown in FIG. 2, cycloconverter 230 may comprise, but is not limited to, common source connected MOSFETs.

[016] Power stage 205 may comprise of one or multiple stages of DC switching stage 215, resonant circuit 220, high frequency transformer 225, and cycloconverter 230. Power stage 205 may take a first voltage 245 (e.g. a DC voltage) and convert it to a second alternating current (AC) voltage 250 (e.g. a high frequency AC voltage). Power stage 205 may then take second AC voltage 250 and convert it to a third AC voltage 255 (e.g. a low frequency AC voltage) with cycloconverter 230 driven by gate driver 210. Cycloconverter 230 may combine the functionality of a high frequency rectifier and an unfolder into one stage.

Accordingly, driver circuit 200 may take energy from, a solar panel 260 and efficiently inject the energy into an AC source 265 (e.g. an electric utility grid.)

[017] Cycloconverter 230 may operate by alternate switching, for example, of devices switching pairs formed by Q5 and Q7 and Q6 and Q8 at switching frequency and at the frequency of third AC voltage. During the positive voltage cycle of third AC voltage 255, one of the switches from the switching pair switching in synchronization with the zero crossing instants of third AC voltage 255 may be kept on and the other switching pair may be switched on and off in synchronization with cycloconverter current sense signal 270 (e.g. a high frequency AC current.) Opposite switching events may take place in the negative cycle of third AC voltage 255. As discussed in more detail below with respect to FIG. 4, gate driver 210 may comprise a zero current detector 275, a zero voltage detector 280, a signal conditioner 285, an isolator 290, and a driver and buffer 295.

[018] The switches Q5 and Q6 (and Q7 and Q8) may be connected in two possible ways. Either their source may be shorted to each other (i.e. common source) as shown in cycloconverter 230 of FIG. 2 or their drains could be shorted to each other (i.e. common drain) as shown in cycloconverter 230' of FIG. 3. The sequence of high frequency and low frequency switching may depend on the common source or common drain MOSFET configuration. One of the possible truth tables for the common source configuration is shown in Table 1.

Table 1

[019] From Table 1 two main control signals : i) third AC voltage 255 (e.g. the low frequency AC voltage); and ii) cycloconverter current sense signal 270 (the high frequency AC current signal) may be derived. At any point in time, the overall gate signal of the MOSFET may be a logical gate combination of these two control signals.

[020] FIG. 4 shows gate driver circuit 210 in greater detail. As shown in Fig. 4, gate driver 210 uses a current sense transducer 405 to obtain cycloconverter current sense signal 270 and a voltage sense transducer 410 to obtain third AC voltage 255. Current sense transducer 405 may comprise a current sense transformer or current sensors with hall effect, or current sense resistors with an isolation amplifier. Current sense transducer 405 may be located at the primary side in series with the primary of high frequency transformer 225 or on the secondary side in series with the secondary winding of high frequency transformer 225 as shown in Figs. 2 and 3. Primary may mean the transformer winding at the solar panel side and the secondary winding may mean the transformer winding at the third AC voltage side. The zero current detector 275 may detect the direction of cycloconverter current sense signal 270 (e.g. a high frequency AC current signal) and may generate a high frequency signal HF. Voltage sense transducer 410 may be a resistor divider network or any other transducer. Zero voltage detector circuit 280 may determine the polarity of third AC voltage 255 (e.g. a low frequency AC voltage signal) and may generate a low frequency signal LF.

[021] Consistent with embodiments of the invention, signal conditioner 285 may be configured to implement the logic designated by the truth table of Table 1. As shown in Fig. 4, signal conditioner 285 may include a first signal inverter 415, a second signal inverter 420, and a logic OR circuit 425. First signal inverter 415 inverts the polarity of signal HF and second signal inverter 420 inverts the polarity of signal LF to respectively generate the signals HFN and LFN. First signal inverter 415 and/or a second signal inverter 420 may comprise any diode, opamp logic or any digital inverters. Logic OR circuit 425 may combine signals LF, LFN, HF and HFN to create drive signals to drive switches Q5, Q6, Q7, and Q8 according to the truth table of Table 1. Isolator 290 may isolate or level shift the output signals from logic OR circuit 425 and driver and buffer 295 may prepare and condition the isolated output signals to drive respective switches Q5, Q6, Q7, and Q8.

[022] Embodiments of the invention may be practiced in an electrical circuit comprising discrete analog and digital electronic elements such as diodes, transistors, MOSFETs, packaged or integrated electronic chips containing logic gates, a circuit utilizing a microprocessor, or on a single chip containing electronic elements or microprocessors. Embodiments of the invention may also be practiced using other technologies capable of performing logical operations such as, for example, AND, OR, and NOT, including but not limited to mechanical, optical, fluidic, and quantum technologies. In addition, embodiments of the invention may be practiced within a general purpose computer or in any other circuits or systems.

[023] Embodiments of the invention, for example, may be implemented as a computer process (method), a computing system, or as an article of manufacture, such as a computer program product or computer readable media. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process. Accordingly, the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.). In other words, embodiments of the present invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. A computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

[024] The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific computer-readable medium examples (a non-exhaustive list), the computer- readable medium may include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read- only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read-only memory (CD-ROM). Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. [025] Embodiments of the present invention, for example, are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products according to embodiments of the invention. The functions/acts noted in the blocks may occur out of the order as shown in any flowchart. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functional ity/acts involved.

[026] While the specification includes examples, the invention's scope is indicated by the following claims. Furthermore, while the specification has been described in language specific to structural features and/or methodological acts, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as example for embodiments of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of driving the semiconductor switches in the

cycloconverter connected in cascade with a solar DC source, a resonant converter and a high frequency transformercomprising:

receiving a high frequency alternating current (AC) current signal receiving a low frequency AC voltage signal;

determining the direction of the received high frequency AC current signal; determining the polarity of the received low frequency AC voltage signal; and

driving a plurality of switches in a cycloconverter based upon the determined direction of the received high frequency AC current signal and the determined polarity of the received low frequency AC voltage signal to cause the cycloconverter to convert a second AC voltage to a third AC voltage.

2. The method of claim 1, wherein receiving the high frequency AC current signal comprises receiving the high frequency AC current signal from the cycloconverter.

3. The method of claim 1 , wherein determining the direction of the received high frequency AC current signal comprises determining the direction of the received high frequency AC current signal using a zero current detector.

4. The method of claim 1, wherein determining the polarity of the received low frequency AC voltage signal comprises determining the polarity of the received low frequency AC voltage signal using a zero voltage detector.

5. The method of claim 1 , wherein driving the plurality of switches in the cycloconverter comprises:

receiving the determined direction of the received high frequency AC current signal at a signal conditioner;

receiving the determined polarity of the received low frequency AC voltage signal at the signal conditioner; and creating drive signals by the signal conditioner based upon the direction of the received high frequency AC current signal and the polarity of the received low frequency AC voltage signal; and

transmitting the created drive signals to the plurality of switches in the cycloconverter to cause the cycloconverter to convert the second AC voltage to the third AC voltage.

6. The method of claim 1, wherein creating drive signals comprises generating inverse signals from the received high frequency AC current signal and the received low frequency AC voltage signal.

7. The method of claim 1, wherein receiving the determined direction of the received high frequency AC current signal at the signal conditioner comprises receiving the determined direction of the received high frequency AC current signal at the signal conditioner comprising a logical OR circuit.

8. The method of claim 1, wherein creating the drive signals comprises generating four drive signals from logical OR operation of the following:

a low frequency signal generated from the third AC voltage and high frequency signal generated from the first or second AC voltage or current;

an inverted low frequency signal generated from the third AC voltage and high frequency signal generated from the first or second AC voltage or current; the low frequency signal generated from the third AC voltage and inverted high frequency signal generated from the first or second AC voltage or current; and the inverted low frequency signal generated from the third AC voltage and inverted high frequency signal generated from the first or second AC voltage or current.

9. The method of claim 1, wherein driving the plurality of switches in the cycloconverter comprises driving the plurality of switches comprising

MOSFETs.

10. The method of claim 1 , wherein driving the plurality of switches in the cycloconverter comprises driving the plurality of switches comprising

MOSFETs with common sources.

1 1. The method of claim 1 , wherein driving the plurality of switches in the cycloconverter comprises driving the plurality of switches comprising

MOSFETs with common drains.

12. An apparatus comprising a cascade connection of solar DC source, resonant converter configured to generate first AC voltage and current, a high frequency transformer converting the first AC voltage and current in to a second AC voltage and current and a cycloconverter configured to convert the second alternating current (AC) voltage to a third AC voltage, the cycloconverter comprising a plurality of switching pairs; and

a gate driver configured to alternately switch the plurality of switching pairs.

13. The apparatus of claim 12, wherein the plurality of switching pairs comprise MOSFETs.

14. The apparatus of claim 12, wherein the plurality of switching pairs comprise MOSFETs with common sources.

15. The apparatus of claim 12, wherein the plurality of switching pairs comprise MOSFETs with common drains.

16. The apparatus of claim 12, wherein the gate driver comprises:

a zero current detector configured to determine a direction of a high frequency AC current signal received from the cycloconverter;

a zero voltage detector configured to determine a polarity of a low frequency AC voltage signal; and

a signal conditioner configured to; create drive signals based upon the determined direction of the high frequency AC current signal and the determined polarity of the low frequency AC voltage signal; and

transmit the created drive signals to the plurality of switching pairs in the cycloconverter to alternately switch the plurality of switching pairs to cause the cycloconverter to convert the second AC voltage to the third AC voltage.

17. The apparatus of claim 12, wherein the signal conditioner being configured to create drive signals comprises the signal conditioner comprising a logical OR circuit configured to alternately switch the plurality of switching pairs according to a truth table.

18. The apparatus of claim 12, comprising a signal conditioning circuit configured to generate four drive signals for the cycloconverter from logical OR operation of following:

An inverted low frequency signal generated from the third AC voltage and high frequency signal generated from the first or second AC voltage or current; the low frequency signal generated from the third AC voltage and inverted high frequency signal generated from the first or second AC voltage or current; the inverted low frequency signal generated from the third AC voltage and inverted high frequency signal generated from the first or second AC voltage or current.