Voltage converter

Abstract

A voltage converter that features: a primary branch (2) configured to generate a pulse-modulated voltage or current from an input DC voltage; a transformer arrangement m ≥ 1 primary windings (4) and n ≥ 2 secondary windings (5) inductively coupled to each other, wherein the m primary windings (4) are connected to the primary branch (2); a secondary branch (3) configured to output a pulsed DC voltage or pulsed DC current, and comprising n series-connected capacitors (15, 16) and n secondary controllable semiconductor switches (13, 14); and n soft commutating networks, each of which is connected in parallel to one of the n secondary controllable semiconductor switches (13, 14); wherein each of the n secondary windings (5) is connected via at least one of the secondary controllable semiconductor switches (13, 14) to at least one of the capacitors (15, 16); and where each of the n softly commuting networks has: a series circuit consisting of a commutation inductor (22a, 22b), a first commutation diode (23a, 23b) and a commutation capacitor (24a, 24b), wherein the series circuit is connected in parallel to the relevant secondary controllable semiconductor switch (13, 14) and the commutation inductor (22a, 22b) is connected to a node between the secondary controllable semiconductor switch (13, 14) and the relevant of the n capacitors (15, 16); and a second commutation diode (25a, 25b) is connected between the nodes between the first commutation diode (23a, 23b) and the commutation capacitor (24a, 24b) and the nodes between the relevant secondary winding (5) and the relevant of the n capacitors (15, 16).

Description

TECHNICAL AREA

[0001] The disclosure relates to a voltage converter, in particular a bidirectional voltage converter. BACKGROUND

[0002] A flyback converter is used to convert alternating current (AC) to direct current (DC) and DC to DC with galvanic isolation between input and output. This galvanic isolation and voltage conversion are achieved by a transformer whose primary winding is alternately connected to and disconnected from an input voltage source, while its secondary winding supplies an output voltage to an output capacitor via a rectifying diode. A flyback converter operates in two states. In the "on" state, energy is transferred from the input voltage source to the transformer, and the output capacitor supplies energy to an output load. In the "off" state, energy is transferred from the transformer to the output capacitor and the output load.An ordinary flyback converter, as described above, can provide reactive power because of the output diode which blocks current in one direction, i.e., positive voltage with negative current or negative voltage with positive current.

[0003] A bidirectional flyback converter can provide reactive power because the (high-voltage) diode is replaced by a (high-voltage) switch that allows current flow in both directions. Bidirectional flyback converters typically use metal-oxide-semiconductor field-effect transistors (MOSFETs) as high-voltage switches. However, high-voltage MOSFETs have higher on-resistance than low-voltage MOSFETs, and the maximum voltage for high-voltage MOSFETs is 1000 volts (V). Therefore, there is a need for an improved bidirectional high-voltage flyback converter using MOSFETs.

[0004] Document US 6,304,461 B1 discloses a DC-DC converter with a bipolar output and bidirectional reactive power transfer. The DC-DC converter comprises: a DC power source; a modulator coupled to the DC power source, the modulator comprising at least two switching means for generating a waveform pulse; a transformer primary winding capacitively coupled to the modulator; at least two transformer secondary windings magnetically coupled to the transformer primary winding; and two synchronous rectifiers coupled to a first and second secondary winding, respectively, of the at least two transformer secondary windings. The first and second synchronous rectifiers each comprise: a switching device for interrupting a current path connected in series with a respective pole of the first polarity of the secondary winding, and a capacitor coupled in series with the switching device.The second polarity pole of the first secondary winding is coupled to the first polarity pole of the second secondary winding in such a way that an output voltage of the first synchronous rectifier is electrically opposite to a second output voltage of the second synchronous rectifier.

[0005] Document US 2008 / 0205109A1 discloses a power distribution system configured to facilitate the transfer of energy between a power source and a load. The distribution system can be configured to convert direct current (DC) to alternating current (AC) when the load is driven and to convert AC to DC when the power source is regenerating.

[0006] Document US 5,278,748 A discloses a voltage-resonant DC-DC forward converter equipped with a damping circuit in its secondary circuit. The damping circuit comprises a series connection of a first capacitor and a first diode, which are connected to both ends of the secondary winding of the main transformer to form a secondary resonant circuit loop. This loop allows a secondary resonant current to flow in the forward direction of the first diode, the forward direction being such that the first diode prevents the secondary resonant current from flowing at least when the second winding of the smoothing circuit supplies a current through a second diode to rectify the secondary current. The converter is further equipped with a control circuit for regulating the converter's output voltage. The circuit includes a transistor for controlling the switching of the output current, which changes linearly with time.The transistor is controlled by the sum of the voltage signal, which indicates the output current, and the deviation signal, which indicates the deviation of the output voltage from a specified value. When the sum exceeds the base-emitter threshold voltage, the transistor is switched on to cut off the output current. OVERVIEW OF THE INVENTION

[0007] A voltage converter according to one embodiment of the invention comprises a primary branch configured to generate a pulse-modulated voltage from an input DC voltage; a transformer arrangement m≥1 primary windings and n≥2 secondary windings inductively coupled to one another, wherein the m primary windings are connected to the primary branch; a secondary branch configured to output a pulsed DC voltage, comprising n series-connected capacitors and n secondary controllable semiconductor switches; and n soft-commutating networks, each of which is connected in parallel to one of the n secondary controllable semiconductor switches; wherein each of the n secondary windings is connected to at least one of the capacitors via at least one of the secondary controllable semiconductor switches.Each of the n soft-commutating networks comprises a series connection of a commutation inductor, a first commutation diode, and a commutation capacitor, wherein the series connection is connected in parallel to the relevant secondary controllable semiconductor switch, and the commutation inductor is connected to a node between the secondary controllable semiconductor switch and the relevant of the n capacitors. Each of the n soft-commutating networks further comprises a second commutation diode, which is connected between the nodes between the first commutation diode and the commutation capacitor and the nodes between the relevant secondary winding and the relevant of the n capacitors.

[0008] Other systems, methods, features, and advantages are or will become apparent to the person skilled in the art upon consideration of the following figures and the detailed description. It is intended that all additional systems, methods, features, and advantages contained in this description are within the scope of the invention and are protected by the following claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The system can be better understood with reference to the following drawings and description. The elements in the figures are not necessarily to scale; rather, emphasis has been placed on illustrating the principles of the invention. Furthermore, in the figures, the same reference numerals in the different views consistently denote corresponding parts. Fig. Figure 1 is a circuit diagram of a bidirectional flyback switching converter with reactive power capability; the semiconductor switch is used with a lower voltage; Fig. Figure 2 is a circuit diagram of a bidirectional flyback switching converter according to Fig. 1 with additional soft commutating networks; Fig. 3 is a diagram that represents a primary current, a secondary current and a soft commutation current over time; Fig. 4 is an alternative transformer structure with two transformers whose primary windings are connected in series; and Fig. 5 is an alternative transformer structure with two transformers whose primary windings are connected in parallel. DETAILED DESCRIPTION AND EXPLANATORY EXAMPLES

[0010] A bidirectional flyback converter with reactive power capability according to an embodiment of the invention is schematically shown in Fig. 1 shown. The converter according to Fig. The device 1 contains a transformer 1 with a primary side forming part of a primary branch and a secondary side forming part of a secondary branch 3. The primary side has a primary winding 4, and the secondary side has two secondary windings 5 ​​and 6. Furthermore, the primary branch 2 contains a controllable switch 7, in this example an N-line depletion-type MOSFET (NMOSFET), a capacitor 8, and a control circuit 9 for controlling the switch 7. The switch 7 and the primary winding 4 are connected in series, and this series connection is connected in parallel with the capacitor 8. In this example, the capacitor 8 transforms a current supplied by a DC power source 10, for example a photovoltaic cell, into a DC voltage, so that the DC power source 10 and the capacitor form a DC voltage source.This DC voltage is chopped by switch 7 and fed to the primary winding 4.

[0011] The primary branch 2 further includes a switch 11, e.g., an NMOSFET controlled by a control circuit 9, and a capacitor 12, which are connected in series and in parallel to the primary winding 4 of the transformer 1. When the switch 7 is off, the switch 11 is on. The leakage energy is transferred via the switch 11 to the capacitor 12, and the voltage across the switch 12 is used to reset the transformer 1. This clamps the voltage across the switch 7.

[0012] The secondary branch 3 contains, in addition to the two secondary windings 5 ​​and 6, two switches 13 and 14, e.g., NMOSFETs controlled by the control circuit 9, each of which is connected in series with one of the secondary windings 5 ​​and 6. The secondary branch 3 also contains two capacitors 15 and 16 connected in series. Capacitor 15 is connected in parallel with the series connection of switch 13 and secondary winding 5, and capacitor 16 is connected in parallel with the series connection of switch 13 and secondary winding 6. Switch 7 is controlled to provide a sinusoidal pulse-width modulation to generate rectified sinusoidal 50 Hz (or 60 Hz) currents in the secondary windings 5 ​​and 6 and the associated switches 13 and 14. Fig. Figure 1 shows a voltage-time diagram of the rectified sinusoidal current on the secondary side above winding 5. Whenever switch 7 is turned on, switches 13 and 14 are turned off, and vice versa. Switches 13 and 14 operate as active diodes with reduced conduction losses. Capacitors 15 and 16 have a sufficiently low capacitance so that the current waveform is not distorted. The objective is to dissipate the high-frequency carrier current pulses and smooth the current to its intended frequency: 50 Hz or 60 Hz. Switch 11, together with capacitor 12 and the leakage inductance of transformer 1, forms a soft-switching network to facilitate soft switching (zero-voltage switching) of switch 7 (lossless turn-on).

[0013] To provide DC-AC conversion, the pulsed DC voltage across the series connection of capacitors 15 and 16 is unfolded by a switching H-bridge, which alternately outputs the pulses inverted and non-inverted. The H-bridge contains four switches 17-20, e.g., NMOSFETs, controlled by the control circuit 9, in a bridge configuration with one of the switches 17-20 in each leg of the H-structure. A bridge structure is a type of electrical circuit in which two parallel circuit branches are "bridged" by a third branch, e.g., a load, which is connected between the first two branches at any tap along the branches, with the tap separating two legs of each branch. Switches 17-20, which unfold the rectified current to an AC current without a DC level, are controlled so that a voltage or current is present in each direction across the respective...by the load applied, whereby the switches in a branch are never closed at the same time.

[0014] The in Fig. The bidirectional flyback converter shown in Figure 1 can form the basis for a solar microinverter with reactive power capability. For example, the secondary branch of the flyback converter was modified to absorb the negative current by replacing the rectifying diode with high-voltage MOSFETs 13 and 14. The secondary winding was split into two windings, which divide the output voltage by two. This allows, for example, the use of two 650 V voltage-rated MOSFETs as synchronous rectifiers in a 1200 V secondary branch. The switch 11 and the clamping capacitor 12, together with the leakage inductance of the transformer 1, form an active clamping network that provides zero-crossing switching (ZVS) for the switch 7 upon turn-on.

[0015] In the present example, ZVS operates as follows. Each time switch 7 is opened, energy is stored in the transformer's leakage inductance in the capacitor, which stores a voltage greater than that in winding 4. Each time before switch 7 is opened, switch 11 is first opened briefly, and the voltage in capacitor 11 plus the input voltage is applied to winding 4. The difference between this applied voltage and the reflected voltage in winding 4 is then applied to the leakage inductance. A current then flows through the leakage inductance in the direction of winding 4. Switch 11 is then opened, and the current flowing in the leakage inductance discharges the output charge (Q). oss) of switch 7, which forces the voltage between switch 7 and the primary winding 4 to move towards zero. As soon as the drain voltage is zero, switch 7 is turned on by switching at the zero crossing.

[0016] In reactive power mode, switches 13 and 14 act as controlling switches, and switch 7 acts as an active diode. Energy flows from the output to the input. The modulation scheme applied to switches 13 and 14 can also be a sine pulse-width modulation.

[0017] Reference is now made to the in Fig. 2 converters shown, in which in Fig. A circuit controlling the commutation has been added to the flyback converter shown in Figure 1 (indicated by a darkened area). The circuit includes an inductor 21 connected in series with the primary winding (e.g., the existing inductance of the primary winding) and two soft commutating networks coupled to the secondary windings. Each soft commutation network can comprise a series connection of a commutation inductor 22a or 22b, a first commutation diode 23a or 23b, and a commutation capacitor 24a or 24b, wherein the series connection is connected in parallel to the relevant secondary controllable semiconductor switch 13 or 14, and wherein the commutation inductor is connected to the nodes between the secondary controllable semiconductor switch 13 or 14 and the relevant of the n capacitors 15 and 16.A second commutation diode 25a or 25b can be connected between the nodes between the first commutation diode 23a or 23b and the commutation capacitor 24a or 24b and the node between the relevant secondary winding 5 or 6 and the relevant of the n capacitors 15 and 16.

[0018] A high-voltage MOSFET exhibits a higher reverse recovery charge (Q). rr ) and a higher initial charge (Q oss ) as, for example, a silicon carbide junction field-effect transistor (JFET). When the converter is operated in continuous current mode (CCM), Q oss and Q rr This increases the turn-on losses of switch 7. Furthermore, secondary-side high-voltage MOSFETs may be subjected to hard commutation of their body diodes each time switch 7 is turned on, which can cause the destruction of these components at high frequencies.

[0019] The foregoing in connection with Fig. The soft commutating network described above overcomes the problems mentioned above. The choke 21 serves two purposes here. It provides ZVS for the switch 7, just as in the active clamp topology with the switch 11 and the capacitor 12, and supports the soft commutation of the body diodes of the high-voltage MOSFETs. During the soft commutation of the body diodes, the additional current required to increase their Q rr to restore and the Q oss The high-voltage MOSFETs are charged by trapping the energy in the inductor 21. Diodes 23a and 23c, together with capacitors 24a and 24b, form a path or storage for the energy trapped in the inductor 21 via transformer 1. Opening switch 7 releases the energy supplied with Q. rr and Q ossThe energy stored in capacitors 24a and 24b, before the high-voltage MOSFETs conduct, is returned to the output via diodes 25a and 25b. During reactive power processing mode, chokes 22a and 22b provide zero-current turn-on for switches 13 and 14.

[0020] As explained above, the in Fig. Figure 2 shows a microinverter with a bidirectional flyback converter and a voltage clamping network (switch 11, clamping capacitor 12, and choke 21 to provide ZVS for switch 7). The secondary side of the converter can contain two (or more) identical secondary winding circuits (soft commutating networks) that can be connected in series by switches 13 and 14. These switches operate either as synchronous rectifier switches when handling active power or as control switches when handling reactive power, when switch 7 operates as a synchronous rectifier. The secondary winding circuits are connected in series so that they can, for example, match the mains high voltage while still using lower voltage MOSFETs (650 V) than would otherwise be required in the case of a single output stage, e.g., a 1200 V switch.The soft commutation networks smooth the commutation of the body diodes of the high-voltage MOSFETs and prevent their destruction. This soft commutation (associated with Q) rr and Q oss The energy associated with the high-voltage MOSFETs, which was originally trapped in the choke 21, is recovered and stored in the capacitors 24a and 24b and then, as soon as the switch 7 opens, sent to the output.

[0021] Fig. Figure 3 shows simulation results as current I over time t for the primary current Ip and the secondary current Is of the transformer, where the one with Q rr and Q oss The current associated with the high-voltage MOSFETs, i.e., the smooth commutation current Scc, is highlighted.

[0022] The Fig. 4 and Fig. Figure 5 illustrates transforming circuits that can replace transformer 1. In the case of the one in Fig. In the circuit shown in Figure 4, two transformers 26 and 27 are used, each having a primary winding and a secondary winding, with the primary windings of transformers 26 and 27 connected in series. In the circuit shown in Figure 4, the primary windings of transformers 26 and 27 are connected in series. Fig. In the circuit shown in Figure 5, the primary windings of two transformers 26 and 27 are connected in parallel. However, any other configuration with different numbers of transformers, different numbers of primary windings, different numbers of secondary windings, and different connection structures can also be used.

[0023] The converters described above can be used for both AC-DC and DC-DC power conversion. They are suitable for low-power as well as high-power applications. Each switch, inductor, diode, or capacitor component can be replaced by any number of corresponding components, which can be connected in parallel or in series. The primary and secondary control can be independent or interdependent, e.g., implemented by multiple controllers or a single controller (as shown). In the examples described above, the control circuit provides 9 control signals for all switches. Switches 7, 13, and 14 can be controlled as in conventional bidirectional flyback converters, and switches 17-20 can be controlled as in conventional H-bridge circuits.

[0024] Instead of MOSFETs based on a silicon material, transistors based on a large bandgap material such as gallium nitride, silicon carbide, zinc oxide or any other suitable material can be used.

[0025] With further reference to Fig. 1. The converter shown can simply be operated as a DC / DC converter by omitting the unfolding bridge (switches 17-20) and using the corresponding terminals of capacitors 15 and 16 to connect a DC load (or a high-voltage battery). For DC / DC boost converters, the input source 10 could be a DC current source such as a photovoltaic panel, or voltage sources such as a battery or battery banks or fuel cells.

[0026] In DC / DC converter applications, the output capacitors 15 and 16 are larger than those of the microinverter described above, so that they provide a constant voltage source with low ripple at the output. In contrast, in the microinverter application, the purpose of such capacitors is to isolate the high-frequency current from the output, which would otherwise only receive the 50 Hz or 60 Hz current. Furthermore, other conversions and alternatives are also possible. As will be obvious to those skilled in the art, other circuit elements can be added to or used as substituents in the special circuit configurations discussed above, for example, other types of switching devices, other types of control units, etc. Moreover, the invention can be used in a wide range of different applications.

[0027] By describing various embodiments of the invention, it will be obvious to those skilled in the art that many further embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be limited, except in light of the appended claims and their equivalents.

Claims

[1] A voltage converter which features: a primary branch (2) configured to generate a pulse-modulated voltage or current from an input DC voltage; a transformer arrangement m ≥ 1 primary windings (4) and n ≥ 2 secondary windings (5) inductively coupled to each other, wherein the m primary windings (4) are connected to the primary branch (2); a secondary branch (3) configured to output a pulsed DC voltage or pulsed DC current, and comprising n series-connected capacitors (15, 16) and n secondary controllable semiconductor switches (13, 14); and n soft commutating networks, each of which is connected in parallel to one of the n secondary controllable semiconductor switches (13, 14); wherein each of the n secondary windings (5) is connected via at least one of the secondary controllable semiconductor switches (13, 14) to at least one of the capacitors (15, 16); and where each of the n softly commuting networks has: a series circuit consisting of a commutation inductor (22a, 22b), a first commutation diode (23a, 23b) and a commutation capacitor (24a, 24b), wherein the series circuit is connected in parallel to the relevant secondary controllable semiconductor switch (13, 14) and the commutation inductor (22a, 22b) is connected to a node between the secondary controllable semiconductor switch (13, 14) and the relevant of the n capacitors (15, 16); and a second commutation diode (25a, 25b) is connected between the nodes between the first commutation diode (23a, 23b) and the commutation capacitor (24a, 24b) and the nodes between the relevant secondary winding (5) and the relevant of the n capacitors (15, 16). [2] Voltage transformer according to claim 1, wherein the transformer arrangement comprises a transformer (1) with m primary windings (4) and n secondary windings (5). [3] Voltage transformer according to claim 1, wherein the transformer arrangement comprises m transformers (1), each of which has a primary winding (4) and a secondary winding (5). [4] Voltage converter according to claim 1, wherein the m primary windings (4) are connected in series or in parallel or partly in series and partly in parallel. [5] Voltage converter according to claim 1, wherein at least one of the controllable semiconductor switches (13, 14) is a metal oxide semiconductor field-effect transistor (MOSFET) or a transistor using a wide band gap material. [6] Voltage converter according to claim 1, further comprising a circuit structure configured to generate a bidirectional energy flow between the secondary branch (3) and the primary branch (2). [7] Voltage converter according to claim 6, wherein the circuit structure is a flyback structure. [8] Voltage converter according to claim 1, wherein the secondary branch (3) is configured to output an alternating voltage which contains a reactive power component and an active power component. [9] Voltage converter according to claim 8, wherein the secondary branch (3) has four further controllable semiconductor switches (17, 18, 19, 20) in an H-bridge configuration to convert the pulsed DC current into an AC current and the pulsed DC voltage into an AC voltage. [10] Voltage converter according to claim 1, wherein the pulsed DC voltage has a peak voltage of at least 1000 volts. [11] Voltage converter according to claim 1, wherein the primary branch (2) has a first primary controllable semiconductor switch (7) configured to switch a primary winding (4) to the input DC voltage. [12] Voltage converter according to claim 11, in which a primary choke (21) is connected in series with the first primary controllable semiconductor switch (7). [13] Voltage converter according to claim 1, wherein the primary branch (2) has a first primary controllable semiconductor switch (7) configured to switch the primary windings (4) to the input DC voltage. [14] Voltage converter according to claim 13, wherein a primary inductance is connected in series with the first primary controllable semiconductor switch (7). [15] Voltage converter according to claim 1, wherein the primary branch (2) has a second primary controllable semiconductor switch (11) configured to connect a primary capacitor (12) in parallel to a primary winding (4). [16] Voltage converter according to claim 1, wherein the primary branch (2) has a second primary controllable semiconductor switch (11) configured to connect a primary capacitor (12) in parallel to the primary windings (4).

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