Power converter

Abstract

To provide a power conversion device capable of preventing occurrence of an overcurrent at a start of operation without using a dedicated circuit.SOLUTION: An initial charging signal generation unit 122 provided with a power conversion device 1 generates monitored circuit control signals Smc21 to Smc24 for controlling a secondary side bridge circuit 113 in an initial charging period and non-monitored circuit control signals Snm11 to Snm14 for controlling a primary side bridge circuit 112 in the initial charging period. The non-monitored circuit control signals Snm11 to Snm14 are a pulse signal whose pulse width is set such that a sum of voltage-time products of a positive output voltage and a negative output voltage output from the primary side bridge circuit 112 becomes zero.SELECTED DRAWING: Figure 1

Description

Technical Field

[0005] ,

[0001] The present invention relates to a power conversion device having a switching element.

Background Art

[0002] As a power conversion device capable of supplying power bidirectionally, a bidirectional DC / DC (hereinafter sometimes abbreviated as "DC / DC") converter such as a DAB (Dual Active Bridge) converter is known (see, for example, Patent Documents 1 and 2, and Non-Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a DC / DC converter comprising a transformer, two capacitors electrically separated by the transformer's insulation, and a semiconductor element with a switching element for converting DC to AC, if there is a voltage difference between the two capacitors, an overcurrent may occur in the semiconductor element and other components at startup (i.e., when the device is in operation). Therefore, DC / DC converters have the problem that this overcurrent may occur in the semiconductor element and other components at startup, potentially causing them to fail.

[0006] Patent documents 1 and 2 disclose a technique for providing a predetermined circuit to prevent such overcurrents. However, this technique has the problem of increasing the size of the DC / DC converter. Non-patent document 1 discloses a technique for performing initial charging at the start of operation using a predetermined pulse signal. However, this technique has the problem of a DC current being superimposed on the transformer current.

[0007] The object of the present invention is to provide a power conversion device that can prevent the occurrence of overcurrent at the start of operation without using a dedicated circuit. [Means for solving the problem]

[0008] To achieve the above objective, a power conversion device according to one aspect of the present invention is: An isolated DC / DC converter having an isolation transformer, a primary bridge circuit provided on the primary side of the isolation transformer and having at least two switching elements, and a secondary bridge circuit provided on the secondary side of the isolation transformer and having at least two switching elements, A DC voltage monitoring unit that monitors the DC voltage of at least one of the primary bridge circuit and the secondary bridge circuit, An initial charging signal generation unit generates an initial charging control signal for controlling the isolated DC / DC converter during the initial charging period from the voltage at the start of operation until the DC voltage monitored by the DC voltage monitoring unit becomes higher than the reference voltage. Equipped with, The initial charging signal generation unit generates, as the initial charging control signals, a monitored circuit control signal for controlling the monitored bridge circuit whose DC voltage is monitored by the DC voltage monitoring unit among the primary and secondary bridge circuits during the initial charging period, and an unmonitored circuit control signal for controlling the unmonitored bridge circuit among the primary and secondary bridge circuits that is not the monitored bridge circuit during the initial charging period. The aforementioned non-monitoring circuit control signal is a pulse signal whose pulse width is set such that the sum of the voltage-time products of the positive and negative output voltages output by the non-monitoring bridge circuit is zero. [Effects of the Invention]

[0009] According to one aspect of this disclosure, it is possible to prevent the occurrence of overcurrent at the start of operation without using a dedicated circuit. [Brief explanation of the drawing]

[0010] [Figure 1] This is a block diagram showing an example of a schematic configuration of a power conversion device according to one embodiment of the present invention. [Figure 2] This is a timing chart showing an example of the operation of a power converter according to one embodiment of the present invention. [Figure 3] This is a timing chart illustrating an example of the operation of a conventional power converter. [Figure 4] This is a timing chart showing another example of operation of a power converter according to one embodiment of the present invention. [Figure 5] This is a timing chart showing another example of operation of a power converter according to one embodiment of the present invention. [Figure 6] A block diagram showing an example of a schematic configuration of a power conversion device according to a second embodiment of the present invention. [Figure 7] This is a timing chart showing an example of the operation of a power conversion device according to the second embodiment of the present invention. [Figure 8]A timing chart showing an example of another operation of the power conversion device according to the second embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0011] Each embodiment of the present invention illustrates an apparatus and method for embodying the technical idea of the present invention, and the technical idea of the present invention does not specify the material, shape, structure, arrangement, etc. of the components as the following. The technical idea of the present invention can be variously modified within the technical scope defined by the claims described in the claims.

[0012] The power conversion device according to an embodiment of the present invention will be described with reference to FIGS. 1 to 5. First, the schematic configuration of the power conversion device according to the present embodiment will be described with reference to FIG. 1. FIG. 1 is a block diagram showing an example of the schematic configuration of the power conversion device 1 according to the present embodiment. In FIG. 1, for ease of understanding, the primary-side device 2 and the secondary-side device 3 connected to the power conversion device 1 are shown together.

[0013] (Schematic Configuration of Power Conversion Device) The power conversion device 1 includes an isolated DC / DC converter 11 and can supply power bidirectionally between the primary-side device 2 connected to the primary side and the secondary-side device 3 connected to the secondary side.

[0014] <你提供的原文中此标签后无内容,请确认是否完整。若完整,翻译后也保留原样> As shown in FIG. 1, the primary device 2 and the secondary device 3 are devices connected to the power conversion device 1. Each of the primary device 2 and the secondary device 3 may exhibit a function of outputting power and a function of receiving power. For example, one of the primary device 2 and the secondary device 3 (as an example, the primary device 2) may be a power storage device. When the primary device 2 is a power storage device, the primary device 2 receives and charges the power transmitted from the other of the primary device 2 and the secondary device 3 (as an example, the secondary device 3) via the power conversion device 1. The primary device 2 may supply the charged power to the secondary device 3 via the power conversion device 1 as needed. Also, for example, one of the primary device 2 and the secondary device 3 (as an example, the secondary device 3) may be a driving device such as a motor. When the secondary device 3 is a driving device, the secondary device 3 is driven using the power supplied from the other of the primary device 2 and the secondary device 3 (as an example, the primary device 2). The secondary device 3 may supply the power obtained by regeneration during the regeneration operation to the primary device 2 via the power conversion device 1. Also, each of the primary device 2 and the secondary device 3 may be, for example, a system combining a solar power generation device and a power storage device, etc.

[0015] As shown in FIG. 1, the power conversion device 1 includes an isolation type DC / DC converter 11 having an isolation transformer 111, a primary bridge circuit 112 provided on the primary side of the isolation transformer 111 and having transistors Q11, Q12, Q13, Q14 (an example of at least two switching elements), and a secondary bridge circuit 113 provided on the secondary side of the isolation transformer 111 and having transistors Q21, Q22, Q23, Q24 (an example of at least two switching elements).

[0016] The isolation transformer 111 has a primary winding and a secondary winding. The primary bridge circuit 112 is electrically connected to the primary winding of the isolation transformer 111, and the secondary bridge circuit 113 is electrically connected to the secondary winding of the isolation transformer 111. The winding ratio of the primary winding and the secondary winding of the isolation transformer 111 may be determined according to the ratio of the rated voltages of the primary device 2 and the secondary device 3. In this embodiment, for convenience of explanation, it is assumed that the winding ratio of the isolation transformer 111 is 1:1.

[0017] The power converter 1 has a primary side positive electrode line P1 connected to the positive side of the primary side bridge circuit 112, and a primary side negative electrode line N1 connected to the negative side of the primary side bridge circuit 112. The primary side positive electrode line P1 is connected to the positive side terminal of the primary side device 2. The primary side negative electrode line N1 is connected to the negative side terminal of the primary side device 2. As a result, the primary side bridge circuit 112 can exchange power with the primary side device 2, and can exchange power with the primary winding of the isolation transformer 111.

[0018] As shown in Figure 1, the primary bridge circuit 112 has transistors Q11 and Q12 (an example of at least two switching elements) connected in series, and transistors Q13 and Q14 (an example of at least two switching elements) connected in series. Transistors Q11, Q12, Q13, and Q14 (hereinafter sometimes referred to as "transistors Q11 to Q14") are composed of, for example, metal-oxide-semiconductor field-effect transistors (MOSFETs). Transistors Q11 and Q12 and transistors Q13 and Q14 are connected in parallel.

[0019] The primary bridge circuit 112 has a capacitor C1 connected in parallel with transistors Q11 and Q12. Capacitor C1 is also connected in parallel with transistors Q13 and Q14. Capacitor C1 is positioned between transistors Q11 and Q12 and the primary device 2. One electrode of capacitor C1 is connected to the primary positive line P1, and the other electrode of capacitor C1 is connected to the primary negative line N1. Capacitor C1 functions as a smoothing capacitor, smoothing the voltage and current exchanged between the primary bridge circuit 112 and the primary device 2.

[0020] The primary bridge circuit 112 includes a freewheeling diode D11 connected in reverse parallel to transistor Q11, a freewheeling diode D12 connected in reverse parallel to transistor Q12, a freewheeling diode D13 connected in reverse parallel to transistor Q13, and a freewheeling diode D14 connected in reverse parallel to transistor Q14.

[0021] The drain of transistor Q11 is connected to the cathode of freewheeling diode D11, the drain of transistor Q13, and the cathode terminal of freewheeling diode D13. The source of transistor Q11 is connected to the anode of freewheeling diode D11, the drain of transistor Q12, and the cathode of freewheeling diode D12. The gate of transistor Q11 is connected to a gate drive unit (GDU) 112a (details described later) provided in the primary bridge circuit 112. As a result, the gate signal Sg11 output from the gate drive unit 112a is input to the gate of transistor Q11, and the on / off (non-conductive) state of transistor Q11 is controlled.

[0022] The source of transistor Q12 is connected to the anode of freewheeling diode D12, the source of transistor Q14, and the anode of freewheeling diode D14. The gate of transistor Q12 is connected to gate drive unit 112a. As a result, the gate signal Sg12 output from gate drive unit 112a is input to the gate of transistor Q12, controlling the on (conducting) / off (non-conducting) state of transistor Q12.

[0023] The source of transistor Q13 is connected to the anode of freewheeling diode D13, the drain of transistor Q14, and the cathode of freewheeling diode D14. The gate of transistor Q13 is connected to gate drive unit 112a. As a result, the gate signal Sg13 output from gate drive unit 112a is input to the gate of transistor Q13, controlling the on (conducting) / off (non-conducting) state of transistor Q13.

[0024] The gate of transistor Q14 is connected to the gate drive unit 112a. As a result, the gate signal Sg14 output from the gate drive unit 112a is input to the gate of transistor Q14, controlling whether transistor Q14 is on (conducting) or off (not conducting).

[0025] The drain of transistor Q11, the cathode of freewheeling diode D11, the drain of transistor Q13, and the cathode of freewheeling diode D13 are connected to the primary side positive line P1. One electrode of capacitor C1 is connected via the primary side positive line P1 to the drain of transistor Q11, the cathode of freewheeling diode D11, the drain of transistor Q13, and the cathode of freewheeling diode D13. The source of transistor Q12, the anode of freewheeling diode D12, the source of transistor Q14, and the anode of freewheeling diode D14 are connected to the primary side negative line N1. The other electrode of capacitor C1 is connected via the primary side negative line N1 to the source of transistor Q12, the anode of freewheeling diode D12, the source of transistor Q14, and the anode of freewheeling diode D14. As a result, the voltage output from the primary side device 2 and smoothed by capacitor C1 is input to the full bridge circuit formed by transistors Q11 to Q14.

[0026] The primary bridge circuit 112 has an inductor L1 which has one terminal connected to the source of transistor Q11, the anode of freewheeling diode D11, the drain of transistor Q12, and the cathode of freewheeling diode D12, and the other terminal connected to one terminal of the primary winding of isolation transformer 111. In other words, one terminal of inductor L1 is connected to the connection point a1 between the source of transistor Q11 and the anode of freewheeling diode D11, and the drain of transistor Q12 and the cathode of freewheeling diode D12.

[0027] The other terminals of the primary winding of the isolation transformer 111 are connected to the source of transistor Q13, the anode of freewheeling diode D13, the drain of transistor Q14, and the cathode of freewheeling diode D14. In other words, the other terminals of the primary winding of the isolation transformer 111 are connected to the connection point b1 between the source of transistor Q13 and the anode of freewheeling diode D13, and the drain of transistor Q14 and the cathode of freewheeling diode D14. Therefore, the inductor L1 and the primary winding of the isolation transformer 111 are connected in series between connection point a1 and connection point b1, in other words, to the full bridge circuit composed of transistors Q11 to Q14.

[0028] As shown in Figure 1, the secondary bridge circuit 113 has the same configuration as the primary bridge circuit 112. That is, the secondary bridge circuit 113 has the same configuration as the primary bridge circuit 112 when the components constituting the primary bridge circuit 112, the components connected to the primary bridge circuit 112, and the input gate signals are read from left to right as shown by the arrows below, and performs the same function.

[0029] Transistor Q11 → Transistor Q21 Transistor Q12 → Transistor Q22 Transistor Q13 → Transistor Q23 Transistor Q14 → Transistor Q24 Freewheel diode D11 → Freewheel diode D21 Freewheel diode D12 → Freewheel diode D22 Freewheel diode D13 → Freewheel diode D23 Freewheel diode D14 → Freewheel diode D24 Connection part a1 → Connection part a2 Connection point b1 → Connection point b2 Capacitor C1 → Capacitor C2 Inductor L1 → Inductor L2 Primary positive electrode line P1 → Secondary positive electrode line P2 Primary negative electrode line N1 → Secondary negative electrode line N2 Primary winding of isolation transformer 111 → Secondary winding of isolation transformer 111 Gate drive unit 112a → Gate drive unit 113a Gate signal Sg11 → Gate signal Sg21 Gate signal Sg12 → Gate signal Sg22 Gate signal Sg13 → Gate signal Sg23 Gate signal Sg14 → Gate signal Sg24 Primary side device 2 → Secondary side device 3 Transistors Q21, Q22, Q23, and Q24 (hereinafter sometimes referred to as "transistors Q21-Q24") are composed of MOSFETs, for example, similar to transistors Q11-Q14.

[0030] As shown in Figure 1, the power converter 1 includes a DC voltage monitoring unit 123 that monitors the DC voltage of at least one of the primary bridge circuit 112 and the secondary bridge circuit 113. In this embodiment, the DC voltage monitoring unit 123 is configured to monitor the DC voltage of the secondary bridge circuit 113. This DC voltage is the capacitor voltage Vc2 applied to the capacitor C2.

[0031] The DC voltage monitoring unit 123 includes a comparator 123a connected to the secondary positive electrode line P2, and a reference voltage generation unit 123b that generates a reference voltage that serves as a reference for the initial charging period of the isolated DC / DC converter 11. The inverting input terminal (-) of the comparator 123a is connected to the secondary positive electrode line P2, and the non-inverting input terminal (+) of the comparator 123a is connected to the reference voltage generation unit 123b. The output terminal of the comparator 123a is connected to the selection units 124 and 125 (details described later).

[0032] The reference voltage generation unit 123b is configured, for example, as a DC power supply. The positive terminal of the reference voltage generation unit 123b is connected to the non-inverting input terminal (+) of the comparator 123a. The negative terminal of the reference voltage generation unit 123b is connected to the reference potential terminal (for example, the ground terminal). The reference voltage Vr generated by the reference voltage generation unit 123b is set based on the rated voltage of the applied voltage applied to the secondary bridge circuit 113. In this embodiment, the reference voltage Vr is set to, for example, 90% of the rated voltage.

[0033] The DC voltage monitoring unit 123 compares the DC voltage of the secondary bridge circuit 113 (i.e., the capacitor voltage Vc2 of capacitor C2) with the reference voltage Vr via the secondary positive electrode line P2 using the comparator 123a. The comparator 123a outputs a high-level voltage when the capacitor voltage Vc2 is less than or equal to the reference voltage Vr. On the other hand, the comparator 123a outputs a low-level voltage when the capacitor voltage Vc2 is higher than the reference voltage Vr. The DC voltage monitoring unit 123 outputs the voltage output from the comparator 123a as a monitoring signal Sm to the selection units 124 and 125.

[0034] As shown in Figure 1, the power converter 1 includes an initial charging signal generation unit 122 that generates initial charging control signals Sec11, Sec12, Sec13, Sec14, Sec21, Sec22, Sec23, and Sec24 for controlling the isolated DC / DC converter 11 during the initial charging period, from when the capacitor voltage Vc2 (an example of a DC voltage) monitored by the DC voltage monitoring unit 123 rises above the reference voltage Vr. The initial charging signal generation unit 122 generates monitoring circuit control signals Smc21, Smc22, Smc23, Smc24 for controlling the secondary bridge circuit (an example of a monitored bridge circuit) 113, one of the primary bridge circuits 112 and secondary bridge circuits 113, whose capacitor voltage Vc2 is monitored by the DC voltage monitoring unit 123, during the initial charging period, and non-monitoring circuit control signals Snm11, Snm12, Snm13, Snm14 for controlling the primary bridge circuit (an example of an unmonitored bridge circuit) 112, one of the primary bridge circuits 112 and secondary bridge circuits 113, which is not a monitored bridge circuit (secondary bridge circuit 113 in this embodiment), during the initial charging period, as initial charging control signals Sec11, Sec12, Sec13, Sec14 and initial charging control signals Sec21, Sec22, Sec23, Sec24.

[0035] In this embodiment, the secondary bridge circuit 113 corresponds to an example of a monitored bridge circuit, and the primary bridge circuit 112 corresponds to an example of an unmonitored bridge circuit. Therefore, the monitored circuit control signals Smc21, Smc22, Smc23, and Smc24 input to the secondary bridge circuit 113 during the initial charging period become the initial charging control signals Sec21, Sec22, Sec23, and Sec24. On the other hand, the unmonitored circuit control signals Snm11, Snm12, Snm13, and Snm14 input to the primary bridge circuit 112 during the initial charging period become the initial charging control signals Sec11, Sec12, Sec13, and Sec14.

[0036] Hereinafter, the initial charging control signals Sec11, Sec12, Sec13, and Sec14 will be abbreviated as "Initial charging control signals Sec11-Sec14," and the initial charging control signals Sec21, Sec22, Sec23, and Sec24 will be abbreviated as "Initial charging control signals Sec21-Sec24." Additionally, the unmonitored circuit control signals Snm11, Snm12, Snm13, and Snm14 will be abbreviated as "Unmonitored circuit control signals Snm11-Snm14," and the monitored circuit control signals Smc21, Smc22, Smc23, and Smc24 will be abbreviated as "Monitored circuit control signals Smc21-Smc24."

[0037] As shown in Figure 1, the power converter 1 includes a DC voltage monitoring unit 123, an initial charge signal generation unit 122, a control signal generation unit 121 that generates control signals Sc11, Sc12, Sc13, Sc14, Sc21, Sc22, Sc23, and Sc24 for controlling the isolated DC / DC converter 11 after the initial charge period has elapsed, and a control device 12 having selection units 124 and 125 that select one of the unmonitored circuit control signals Snm11 to Snm14 and monitored circuit control signals Smc21 to Smc24 input from the initial charge signal generation unit 122 and the control signals Sc11, Sc12, Sc13, Sc14, Sc21, Sc22, Sc23, and Sc24 input from the control signal generation unit 121 and output it to the isolated DC / DC converter 11 based on the voltage level of the monitoring signal (an example of an output signal) Sm output from the DC voltage monitoring unit 123. Hereafter, control signals Sc11, Sc12, Sc13, and Sc14 will be abbreviated as "control signals Sc11-Sc14," and control signals Sc21, Sc22, Sc23, and Sc24 will be abbreviated as "control signals Sc21-Sc24."

[0038] As will be described in detail later, during the initial charging period, the power converter 1 inputs gate signals Sg11~Sg14 based on pulse-shaped unmonitored circuit control signals Snm11~Snm14 of a predetermined shape to the gates of transistors Q11~Q14, and inputs gate signals Sg21~Sg24 based on monitored circuit control signals Smc21~Smc24 with a constant low voltage level to transistors Q21~Q24 provided in the secondary bridge circuit 113. As a result, the power converter 1 can prevent overcurrent from occurring in the primary bridge circuit 112 and the secondary bridge circuit 113, even if there is a potential difference between capacitor voltages Vc1 and Vc2 when the isolated DC / DC converter 11 starts operating.

[0039] The selection unit 124 receives control signals Sc11 to Sc14 from the control signal generation unit 121 and non-monitoring circuit control signals Snm11 to Snm14 from the initial charge signal generation unit 122. The selection unit 124 selects non-monitoring circuit control signals Snm11 to Snm14 and outputs them to the gate drive unit 112a when the voltage level of the monitoring signal Sm input from the DC voltage monitoring unit 123 is high. On the other hand, the selection unit 124 selects control signals Sc11 to Sc14 and outputs them to the gate drive unit 112a when the voltage level of the monitoring signal Sm input from the DC voltage monitoring unit 123 is low.

[0040] The selection unit 125 receives control signals Sc21 to Sc24 from the control signal generation unit 121 and monitored circuit control signals Smc21 to Smc24 from the initial charge signal generation unit 122. The selection unit 125 selects monitored circuit control signals Smc21 to Smc24 and outputs them to the gate drive unit 113a when the voltage level of the monitoring signal Sm input from the DC voltage monitoring unit 123 is high. On the other hand, the selection unit 125 selects control signals Sc21 to Sc24 and outputs them to the gate drive unit 113a when the voltage level of the monitoring signal Sm input from the DC voltage monitoring unit 123 is low.

[0041] The initial charging period is the period from the start of initial charging (i.e., the start of operation of the isolated DC / DC converter 11) until the DC voltage of the secondary bridge circuit 113 (i.e., the capacitor voltage Vc2) becomes higher than the reference voltage Vr. For this reason, the DC voltage monitoring unit 123 outputs a high-level monitoring signal Sm to the selection units 124 and 125 during the initial charging period, and outputs a low-level monitoring signal Sm to the selection units 124 and 125 after the initial charging period has ended.

[0042] As a result, during the initial charging period, the selection unit 124 outputs the unmonitored circuit control signals Snm11 to Snm14 as selection signals Ss11 to Ss14 to the gate drive unit 112a, and the selection unit 125 outputs the monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a. Specifically, the selection unit 124 outputs the unmonitored circuit control signal Snm11 as selection signal Ss11, the unmonitored circuit control signal Snm12 as selection signal Ss12, the unmonitored circuit control signal Snm13 as selection signal Ss13, and the unmonitored circuit control signal Snm14 as selection signal Ss14. Furthermore, the selection unit 125 outputs the monitored circuit control signal Smc21 as a selection signal Ss21, the monitored circuit control signal Smc22 as a selection signal Ss22, the monitored circuit control signal Smc23 as a selection signal Ss23, and the monitored circuit control signal Smc24 as a selection signal Ss24.

[0043] On the other hand, after the initial charging period has elapsed, the selection unit 124 outputs control signals Sc11 to Sc14 as selection signals Ss11 to Ss14 to the gate drive unit 112a, and the selection unit 125 outputs control signals Sc21 to Sc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a. Specifically, the selection unit 124 outputs control signal Sc11 as selection signal Ss11, control signal Sc12 as selection signal Ss12, control signal Sc13 as selection signal Ss13, and control signal Sc14 as selection signal Ss14. The selection unit 125 also outputs control signal Sc21 as selection signal Ss21, control signal Sc22 as selection signal Ss22, control signal Sc23 as selection signal Ss23, and control signal Sc24 as selection signal Ss24.

[0044] The gate drive unit 112a, provided in the primary bridge circuit 112, generates a gate signal Sg11 using the selection signal Ss11 input from the selection unit 124. The gate drive unit 112a generates a gate signal Sg12 using the selection signal Ss12 input from the selection unit 124. The gate drive unit 112a generates a gate signal Sg13 using the selection signal Ss13 input from the selection unit 124. The gate drive unit 112a generates a gate signal Sg14 using the selection signal Ss14 input from the selection unit 124. In this embodiment, the gate drive unit 112a generates gate signals Sg11, Sg12, Sg13, and Sg14 that have different voltage levels and are in phase with the selection signals Ss11, Ss12, Ss13, and Ss14.

[0045] The gate drive unit 113a, provided in the secondary bridge circuit 113, generates a gate signal Sg21 using the selection signal Ss21 input from the selection unit 125. The gate drive unit 113a generates a gate signal Sg22 using the selection signal Ss22 input from the selection unit 125. The gate drive unit 113a generates a gate signal Sg23 using the selection signal Ss23 input from the selection unit 125. The gate drive unit 113a generates a gate signal Sg24 using the selection signal Ss24 input from the selection unit 125. In this embodiment, the gate drive unit 113a generates gate signals Sg21, Sg22, Sg23, and Sg24 that have different voltage levels and are in phase with the selection signals Ss21, Ss22, Ss23, and Ss24.

[0046] Therefore, during the initial charging period, the power converter 1 can be considered to control the isolated DC / DC converter 11 by the unmonitored circuit control signals Snm11~Snm14 and the monitored circuit control signals Smc21~Smc24 generated by the initial charging signal generation unit 122. On the other hand, during the period after the initial charging period has elapsed, the power converter 1 can be considered to control the isolated DC / DC converter 11 by the control signals Sc11~Sc14 and Sc21~Sc24 generated by the control signal generation unit 121.

[0047] (Operation of the power converter) The operation of the power converter 1 according to this embodiment will be explained with reference to Figure 1 and Figure 2. Figure 2 is a timing chart showing an example of the operating waveform of the power converter 1. In Figure 2, "Sg11" shows the voltage waveform of gate signal Sg11, and "Sg14" shows the voltage waveform of gate signal Sg14. In Figure 2, "Sg12" shows the voltage waveform of gate signal Sg12, and "Sg13" shows the voltage waveform of gate signal Sg13. In Figure 2, "Sg21" shows the voltage waveform of gate signal Sg21, and "Sg24" shows the voltage waveform of gate signal Sg24. In Figure 2, "Sg22" shows the voltage waveform of gate signal Sg22, and "Sg23" shows the voltage waveform of gate signal Sg23.

[0048] In Figure 2, "V111a" shows the voltage waveform of the applied voltage V111a applied to the primary winding of the isolation transformer 111. The applied voltage V111a is represented as positive when one terminal of the primary winding of the isolation transformer 111 (the terminal electrically connected to connection part a1 (see Figure 1)) has a higher potential than the other terminal of the primary winding of the isolation transformer 111 (the terminal electrically connected to connection part b1 (see Figure 1)), and as negative when that terminal has a lower potential than the other terminal.

[0049] In Figure 2, "V111b" shows the voltage waveform of the applied voltage V111b applied to the secondary winding of the isolation transformer 111. The applied voltage V111b is represented as positive when one terminal of the secondary winding of the isolation transformer 111 (the terminal electrically connected to connection part a2 (see Figure 1)) has a higher potential than the other terminal of the secondary winding of the isolation transformer 111 (the terminal electrically connected to connection part b2 (see Figure 1)), and as negative when that terminal has a lower potential than the other terminal.

[0050] In Figure 2, "I111a" shows the current waveform of the primary current I111a flowing through the primary winding of the isolation transformer 111. The primary current I111a is represented as positive when it flows from one terminal to the other terminal of the primary winding of the isolation transformer 111, and as negative when it flows from that terminal to the other terminal.

[0051] In Figure 2, "Vc2" represents the capacitor voltage Vc2 generated across capacitor C2, i.e., the DC voltage of the secondary bridge circuit 113. The capacitor voltage Vc2 is represented as positive when one electrode electrically connected to the secondary positive electrode line P2 has a higher potential than the other electrode electrically connected to the secondary negative electrode line N2.

[0052] In Figure 2, "Vrp2" represents the positive rated voltage of the applied voltage applied to the secondary bridge circuit 113. In Figure 2, "Vrn2" represents the negative rated voltage of the applied voltage applied to the secondary bridge circuit 113. In Figure 2, "Ipp1" represents the positive rated peak current of the primary current, and "Ipn1" represents the negative rated peak current of the primary current. In Figure 2, "Vr" represents the reference voltage Vr. Figure 2 shows the passage of time from left to right.

[0053] As shown in Figure 2, at time t1, the operation of the power converter 1 begins, and the initial charging period Tec begins. When the operation of the power converter 1 begins, the initial charging signal generation unit 122 (see Figure 1) generates high-level unmonitored circuit control signals Snm11, Snm14 (see Figure 1) and low-level unmonitored circuit control signals Snm12, Snm13 (see Figure 1) and outputs them to the selection unit 124 (see Figure 1). The initial charging signal generation unit 122 also generates low-level monitored circuit control signals Smc21~Smc24 (see Figure 1) and outputs them to the selection unit 125 (see Figure 1). The control signal generation unit 121 (see Figure 1) generates control signals Sc11~Sc14 (see Figure 1) and outputs them to the selection unit 124, and generates control signals Sc21~Sc24 (see Figure 1) and outputs them to the selection unit 125. Thus, the control signal generation unit 121 generates control signals Sc11~Sc14 and Sc21~Sc24 even during the initial charging period Tec.

[0054] At time t1, capacitor C1 (see Figure 1) on the primary bridge circuit 112 is fully charged (established state) by the power supplied from the primary device 2. On the other hand, at time t1, capacitor C2 (see Figure 1) on the secondary bridge circuit 113 is not charged, and its capacitor voltage Vc2 is, for example, 0V. Therefore, at the start of operation of the power converter 1, a potential difference exists between the capacitor voltage Vc1 of capacitor C1 and the capacitor voltage Vc2 of capacitor C2.

[0055] The capacitor voltage Vc2 is lower than the reference voltage Vr, which is set to, for example, 90% of the positive rated voltage Vrp2. Therefore, the DC voltage monitoring unit 123 (see Figure 1) outputs a high-level monitoring signal Sm to the selection units 124 and 125. As a result, the selection unit 124 selects the non-monitoring circuit control signals Snm11 to Snm14 input from the initial charge signal generation unit 122, and outputs the selected non-monitoring circuit control signals Snm11 to Snm14 as selection signals Ss11 to Ss14 (see Figure 1) to the gate drive unit 112a (see Figure 1) provided in the primary bridge circuit 112. The selection unit 125 selects the monitored circuit control signals Smc21 to Smc24 and outputs the selected monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a (see Figure 1) provided in the secondary bridge circuit 113 (see Figure 1).

[0056] As described above, the gate drive unit 112a generates gate signals Sg11~Sg14 that are in phase with the unmonitored circuit control signals Snm11~Snm14, which are input selection signals Ss11~Ss14, and outputs them to transistors Q11~Q14. Similarly, the gate drive unit 113a generates gate signals Sg21~Sg24 that are in phase with the monitored circuit control signals Smc21~Smc24, which are input selection signals Ss21~Ss24, and outputs them to transistors Q21~Q24. As a result, at time t1, the voltage levels of gate signals Sg11 and Sg14 become high, and the voltage levels of gate signals Sg12 and Sg13 remain low. This causes transistors Q11 and Q14 (see Figure 1) in the primary bridge circuit 112 to be in the ON state (conducting state), and transistors Q12 and Q13 (see Figure 1) to remain in the OFF state (non-conducting state).

[0057] As a result, the positive voltage of the primary side device 2 is applied to one terminal of the primary winding of the isolation transformer 111, and the negative voltage of the primary side device 2 is applied to the other terminal of the primary winding of the isolation transformer 111. Therefore, as shown in Figure 2, at time t1, the applied voltage V111a applied to the primary winding of the isolation transformer 111 is a positive voltage.

[0058] Furthermore, at this time, primary current flows through the primary side of the isolated DC / DC converter 11 via the path "primary side device 2 → transistor Q11 → connection part a1 → inductor L1 → primary winding of isolation transformer 111 → connection part b1 → transistor Q14 → primary side device 2". Therefore, as shown in Figure 2, at time t1, the primary side current I111a flowing through the primary winding of the isolation transformer 111 becomes a current that increases in the positive direction.

[0059] When a primary current I111a flows through the primary winding of the isolation transformer 111, an induced electromotive force is generated in the primary winding. Furthermore, due to mutual induction within the isolation transformer 111, a mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111. The isolation transformer 111 is configured such that the applied voltage V111a to the primary winding and the applied voltage V111b to the secondary winding have the same polarity. Therefore, as shown in Figure 2, at time t1, the applied voltage V111b to the secondary winding of the isolation transformer 111 begins to increase in the positive direction.

[0060] Mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111, causing the applied voltage V111b to increase in the positive direction. As a result, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path: "secondary winding of isolation transformer 111 → connection b2 → freewheeling diode D23 → capacitor C2 → freewheeling diode D22 → connection a2 → inductor L2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. Consequently, as shown in Figure 2, the capacitor voltage Vc2 begins to gradually increase from time t1.

[0061] As shown in Figure 2, at time t2, a predetermined time has elapsed from time t1, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 and Snm14, which change the voltage level to a low level, and unmonitored circuit control signals Snm12 and Snm13, which change the voltage level to a high level, to the selection unit 124. The initial charging signal generation unit 122 also outputs monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, to the selection unit 125.

[0062] As shown in Figure 2, at time t2, the capacitor voltage Vc2 of capacitor C2 provided in the secondary bridge circuit 113 is lower than the reference voltage Vr. Therefore, the DC voltage monitoring unit 123 outputs a monitoring signal Sm with a high voltage level to the selection units 124 and 125. As a result, the selection unit 124 selects the unmonitored circuit control signals Snm11 to Snm14 output from the initial charge signal generation unit 122 and outputs the selected unmonitored circuit control signals Snm11 to Snm14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the monitored circuit control signals Smc21 to Smc24 and outputs the selected monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0063] Therefore, at time t2, the voltage levels of gate signals Sg11 and Sg14 become low, and the voltage levels of gate signals Sg12 and Sg13 become high. As a result, transistors Q11 and Q14 in the primary bridge circuit 112 become off (non-conductive), and transistors Q12 and Q13 become on (conductive).

[0064] As a result, the negative voltage of the primary side device 2 is applied to one terminal of the primary winding of the isolation transformer 111, and the positive voltage of the primary side device 2 is applied to the other terminal of the primary winding of the isolation transformer 111. Therefore, as shown in Figure 2, at time t2, the applied voltage V111a applied to the primary winding of the isolation transformer 111 falls to the same potential as the reference potential (for example, 0V).

[0065] Furthermore, at this time, primary current flows through the primary side of the isolated DC / DC converter 11 via the path "primary side device 2 → transistor Q13 → connection b1 → primary winding of isolation transformer 111 → inductor L1 → connection a1 → transistor Q12 → primary side device 2". Therefore, as shown in Figure 2, at time t2, the primary side current I111a flowing through the primary winding of the isolation transformer 111 is a current that decreases from positive to negative polarity.

[0066] Based on the negative primary current I111a flowing through the primary winding of the isolation transformer 111, a mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111 in the opposite direction to that at time t1. Therefore, as shown in Figure 2, at time t2, the applied voltage V111b to the secondary winding of the isolation transformer 111 begins to increase in the negative direction.

[0067] Mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111, causing the applied voltage V111b to increase in the negative direction. As a result, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path: "secondary winding of isolation transformer 111 → inductor L2 → connection a2 → freewheeling diode D21 → capacitor C2 → freewheeling diode D22 → connection b2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. At time t2, the secondary current flows through capacitor C2 in the same direction as at time t1. Consequently, as shown in Figure 2, the capacitor voltage Vc2 continues to increase.

[0068] As shown in Figure 2, at time t3, a predetermined time has elapsed from time t2, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 and Snm14, which change the voltage level to a high level, and unmonitored circuit control signals Snm12 and Snm13, which change the voltage level to a low level, to the selection unit 124. The initial charging signal generation unit 122 also outputs monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, to the selection unit 125.

[0069] The gate signals Sg11~Sg14 at time t3 have the same voltage as the gate signals Sg11~Sg14 at time t1. Therefore, at time t3, the isolated DC / DC converter 11 operates in the same way as at time t1. As a result, the applied voltage V111a applied to the primary winding of the isolation transformer 111 rises and transitions from a negative voltage to a positive voltage. Also, the primary current I111a flowing through the primary winding of the isolation transformer 111 increases from negative to positive with time t3 being its extreme value. Furthermore, a mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111 in the same direction as at time t1. The capacitor voltage Vc2 at time t3 is higher than the capacitor voltage Vc2 at time t1. Therefore, the applied voltage V111b on the secondary winding of the isolation transformer 111 at time t3 is higher than the applied voltage V111b at time t1. In addition, the secondary current at time t3 flows through capacitor C2 in the same direction as at time t2. As a result, the capacitor voltage Vc2 continues to increase.

[0070] As shown in Figure 2, at time t4, a predetermined time has elapsed from time t3, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 and Snm14, which change the voltage level to a low level, and unmonitored circuit control signals Snm12 and Snm13, which maintain the voltage level at a low level, to the selection unit 124. The initial charging signal generation unit 122 also outputs monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, to the selection unit 125. Furthermore, until the time immediately before time t5, a predetermined time has elapsed from time t4, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 to Snm14, which maintain the voltage level at a low level, to the selection unit 124.

[0071] As shown in Figure 2, from time t4 to just before time t5, the capacitor voltage Vc2 of capacitor C2 provided in the secondary bridge circuit 113 is lower than the reference voltage Vr. Therefore, the DC voltage monitoring unit 123 outputs a monitoring signal Sm with a high voltage level to the selection units 124 and 125. As a result, the selection unit 124 selects the unmonitored circuit control signals Snm11 to Snm14 output from the initial charge signal generation unit 122, and outputs the selected unmonitored circuit control signals Snm12 to Snm14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the monitored circuit control signals Smc21 to Smc24, and outputs the selected monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0072] Therefore, at time t4, the voltage levels of gate signals Sg11 and Sg14 become low, and the voltage levels of gate signals Sg12 and Sg13 remain low. As a result, transistors Q11 and Q14 in the primary bridge circuit 112 are turned off (non-conducting), and transistors Q12 and Q13 remain off (non-conducting). Furthermore, from the time immediately after time t4 to the time immediately before time t5, the voltage levels of gate signals Sg11 to Sg14 remain low. Therefore, during the period from time t4 to the time immediately before time t5, transistors Q11 to Q14 in the primary bridge circuit 112 are turned off (non-conducting). Consequently, during the period from time t4 to the time immediately before time t5, an induced electromotive force with a higher voltage value is generated on the connection side a1 than on the isolation transformer 111 side, based on the energy stored in inductor L1. As a result, regenerative current flows through the path "inductor L1 → connection part a1 → freewheeling diode D11 → primary side device 2".

[0073] Furthermore, during the period from time t4 to the time immediately preceding time t5, the applied voltage V111a applied to the primary winding of the isolation transformer 111 is 0V, so no mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111. However, based on the energy stored in inductor L2, an induced electromotive force with a higher voltage value is generated on the connection part a2 side than on the isolation transformer 111 side. As a result, a regenerative current flows through the path "inductor L2 → connection part a2 → freewheeling diode D21 → capacitor C2". This regenerative current flows to capacitor C2 in the same direction as the secondary current at time t3. Consequently, as shown in Figure 2, even if transistors Q11~Q14 in the primary bridge circuit 112 and transistors Q21~Q24 in the secondary bridge circuit 113 are all in the off state, the capacitor voltage Vc2 continues to increase.

[0074] As shown in Figure 2, the applied voltage V111a applied to the primary winding of the isolation transformer 111 is, in other words, the output voltage output from the primary bridge circuit 112. In this embodiment, the absolute value of the positive voltage level of the applied voltage V111a and the absolute value of the negative voltage level of the applied voltage V111a are the same. Also, the combined period from time t1 to time t2 and the period from time t3 to time t4 is the same as the period from time t2 to time t3. Therefore, for the applied voltage V111a, the sum of the voltage-time product of the positive output voltage for the period from time t1 to time t2 and the period from time t3 to time t4 and the voltage-time product of the negative output voltage for the period from time t2 to time t3 is zero. The applied voltage V111a is output from the primary bridge circuit 112 by the control of transistors Q11 to Q14 by gate signals Sg11 to Sg14. The gate signals Sg11 to Sg14 have voltage waveforms that are in phase with the unmonitored circuit control signals Snm11 to Snm14. Therefore, the applied voltage V111a during the period from time t1 to time t4 can be considered to be the voltage output from the primary bridge circuit 112 as a result of transistors Q11 to Q14 being controlled by the unmonitored circuit control signals Snm11 to Snm14. Accordingly, the unmonitored circuit control signals Snm11 to Snm14 are pulse signals whose pulse width is set such that the sum of the voltage-time products of the positive and negative output voltages output by the primary bridge circuit 112 is zero.

[0075] The primary bridge circuit 112 is driven based on unmonitored circuit control signals Snm11~Snm14, whose pulse width is set so that the sum of the voltage-time products of the positive and negative output voltages output by the primary bridge circuit 112 is zero. This suppresses the change in magnetic flux density generated in the primary winding of the isolation transformer 111 due to the voltage change of the applied voltage V111a from time t1 to time t4. As a result, the power converter 1 can prevent the generation of unexpected electromotive force in the primary winding even from time t1 to time t4, when the potential difference between the capacitor voltages Vc1 and Vc2 is largest during the initial charging period Tec, thereby preventing overcurrent from occurring in the primary bridge circuit 112.

[0076] The applied voltage V111a applied to the primary winding of the isolation transformer 111 is a positive polarity output voltage and a negative polarity output voltage, which are configured as a single unit consisting of a combination of a positive voltage pulse signal and a negative voltage pulse signal during the period from time t1 to time t4.

[0077] Furthermore, as shown in Figure 2, during the initial charging period Pec, the primary current I111a flowing through the primary winding of the isolation transformer 111 is kept within the range of the positive rated peak current Ipp1 and the negative rated peak current Ipn1. This prevents overcurrent from flowing through the primary bridge circuit 112 during the initial charging period Tec.

[0078] The applied voltage V111a applied to the primary winding of the isolation transformer 111 and the primary current I111a flowing through the primary winding of the isolation transformer 111 are controlled by the time and timing of maintaining transistors Q11~Q14, provided in the primary bridge circuit 112, in either an on or off state. The on / off control of transistors Q11~Q14 can be controlled by gate signals Sg11~Sg14, i.e., unmonitored circuit control signals Snm11~Snm14. Therefore, the initial charge signal generation unit 122 is configured to generate unmonitored circuit control signals Snm11~Snm14 that have a pulse width that prevents overcurrent from flowing through the primary bridge circuit 112 (an example of an unmonitored bridge circuit) during the initial charge period Tec, and are configured as a unit of a combination of a positive voltage pulse signal and a negative voltage pulse signal to generate positive and negative polarity output voltages. As a result, the power converter 1 can prevent a DC component from being superimposed on the current flowing through the isolation transformer 111.

[0079] As shown in Figure 2, at time t5, the initial charging signal generation unit 122 generates unmonitored circuit control signals Snm11 and Snm14, which maintain the voltage level at a low level, and unmonitored circuit control signals Snm12 and Snm13, which change the voltage level to a high level, and outputs them to the selection unit 124. The initial charging signal generation unit 122 also generates monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, and outputs them to the selection unit 125.

[0080] At time t5, the capacitor voltage Vc2 of capacitor C2 in the secondary bridge circuit 113 is lower than the reference voltage Vr. Therefore, the DC voltage monitoring unit 123 outputs a high-level monitoring signal Sm to the selection units 124 and 125. As a result, the selection unit 124 selects the unmonitored circuit control signals Snm11 to Snm14 output from the initial charge signal generation unit 122 and outputs the selected unmonitored circuit control signals Snm11 to Snm14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the monitored circuit control signals Smc21 to Smc24 and outputs the selected monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0081] Therefore, at time t5, the voltage levels of gate signals Sg11 and Sg14 become low, and the voltage levels of gate signals Sg12 and Sg13 become high. As a result, transistors Q11 and Q14 in the primary bridge circuit 112 become off (non-conductive), and transistors Q12 and Q13 become on (conductive).

[0082] As a result, the negative voltage of the primary side device 2 is applied to one terminal of the primary winding of the isolation transformer 111, and the positive voltage of the primary side device 2 is applied to the other terminal of the primary winding of the isolation transformer 111. Therefore, as shown in Figure 2, at time t5, the applied voltage V111a applied to the primary winding of the isolation transformer 111 falls to a negative voltage from the reference potential (e.g., 0V).

[0083] Furthermore, at this time, the primary current flows through the primary side of the isolated DC / DC converter 11 along the same path as at time t2. Therefore, as shown in Figure 2, at time t5, the primary current I111a flowing through the primary winding of the isolation transformer 111 becomes a current that decreases from the reference potential (e.g., 0V) toward the negative polarity.

[0084] Based on the negative primary current I111a flowing through the primary winding of the isolation transformer 111, a mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111 in the same direction as at time t4. Therefore, as shown in Figure 2, at time t5, the applied voltage V111b to the secondary winding of the isolation transformer 111 begins to increase in the negative direction.

[0085] Mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111, causing the applied voltage V111b to increase in the negative direction. As a result, secondary current flows through the secondary side of the isolated DC / DC converter 11 along the same path as at time t2. Capacitor C2 is charged by this secondary current. At time t5, the secondary current flows through capacitor C2 in the same direction as at time t4. Consequently, as shown in Figure 2, the capacitor voltage Vc2 continues to increase.

[0086] As shown in Figure 2, at time t6, a predetermined time has elapsed from time t5, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 and Snm14, which change the voltage level to a high level, and unmonitored circuit control signals Snm12 and Snm13, which change the voltage level to a low level, to the selection unit 124. The initial charging signal generation unit 122 also outputs monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, to the selection unit 125.

[0087] At time t6, the gate signals Sg11 to Sg14 change voltage in the same way as the gate signals Sg11 to Sg14 at time t3. Therefore, at time t6, the isolated DC / DC converter 11 operates in the same way as at time t3. As a result, the applied voltage V111a applied to the primary winding of the isolation transformer 111 rises and transitions from a negative voltage to a positive voltage. Also, the primary current I111a flowing through the primary winding of the isolation transformer 111 becomes a current that increases from negative to positive with time t6 as its extreme value. Furthermore, a mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111 in the same direction as at time t3. The capacitor voltage Vc2 at time t6 is higher than the capacitor voltage Vc2 at time t3. Therefore, the applied voltage V111b on the secondary winding of the isolation transformer 111 at time t6 is higher than the applied voltage V111b at time t3. In addition, the secondary current at time t6 flows through capacitor C2 in the same direction as at time t5. As a result, the capacitor voltage Vc2 continues to increase.

[0088] As shown in Figure 2, at time t7, a predetermined time has elapsed from time t6, the initial charging signal generation unit 122 outputs unmonitored circuit control signals Snm11 and Snm14, which change the voltage level to a low level, and unmonitored circuit control signals Snm12 and Snm13, which change the voltage level to a high level, to the selection unit 124. The initial charging signal generation unit 122 also outputs monitored circuit control signals Smc21 to Smc24, which maintain the voltage level at a low level, to the selection unit 125.

[0089] At time t7, the gate signals Sg11 to Sg14 change voltage in the same way as the gate signals Sg11 to Sg14 at time t2. Therefore, at time t7, the isolated DC / DC converter 11 operates in the same way as at time t2. As a result, the applied voltage V111a applied to the primary winding of the isolation transformer 111 falls, transitioning from a positive voltage to a negative voltage. Also, the primary current I111a flowing through the primary winding of the isolation transformer 111 becomes a current that decreases from positive to negative, with time t7 being its extreme value. Furthermore, a mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111 in the same direction as at time t2. The capacitor voltage Vc2 at time t7 is higher than the capacitor voltage Vc2 at time t2. Therefore, the applied voltage V111b on the secondary winding of the isolation transformer 111 at time t7 is higher than the applied voltage V111b at time t2. Furthermore, the secondary current at time t7 flows through capacitor C2 in the same direction as at time t6. As a result, the capacitor voltage Vc2 continues to increase.

[0090] As shown in Figure 2, at time t8, a predetermined time has elapsed from time t7, the initial charging signal generation unit 122 outputs to the selection unit 124 unmonitored circuit control signals Snm11, Snm14, which maintain the voltage level at a low level, and unmonitored circuit control signals Snm12, Snm13, which change the voltage level to a low level. The initial charging signal generation unit 122 also outputs to the selection unit 125 monitored circuit control signals Smc21~Smc24, which maintain the voltage level at a low level. Furthermore, until the time immediately before time t9, a predetermined time has elapsed from time t8, the initial charging signal generation unit 122 outputs to the selection unit 124 unmonitored circuit control signals Snm11~Snm14, which maintain the voltage level at a low level.

[0091] As shown in Figure 2, from time t8 to just before time t9, the capacitor voltage Vc2 of capacitor C2 provided in the secondary bridge circuit 113 is lower than the reference voltage Vr. Therefore, the DC voltage monitoring unit 123 outputs a monitoring signal Sm with a high voltage level to the selection units 124 and 125. As a result, the selection unit 124 selects the unmonitored circuit control signals Snm11 to Snm14 output from the initial charge signal generation unit 122, and outputs the selected unmonitored circuit control signals Snm12 to Snm14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the monitored circuit control signals Smc21 to Smc24, and outputs the selected monitored circuit control signals Smc21 to Smc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0092] Therefore, at time t8, the voltage levels of gate signals Sg11 and Sg14 remain low, and the voltage levels of gate signals Sg12 and Sg13 become low. As a result, transistors Q11 and Q14 in the primary bridge circuit 112 remain off (non-conducting), and transistors Q12 and Q13 also remain off (non-conducting). Furthermore, from the time immediately following t8 to the time immediately following t9, the voltage levels of gate signals Sg11 to Sg14 remain low. Therefore, during the period from time t8 to the time immediately following t9, transistors Q11 to Q14 in the primary bridge circuit 112 remain off (non-conducting). Consequently, during the period from time t8 to the time immediately following t9, an induced electromotive force with a higher voltage value is generated on the connection side a1 than on the isolation transformer 111 side, based on the energy stored in inductor L1. As a result, regenerative current flows through the same path as at time t4: "inductor L1 → connection a1 → freewheeling diode D11 → primary side device 2".

[0093] Furthermore, during the period from time t8 to the time immediately preceding time t9, the applied voltage V111a applied to the primary winding of the isolation transformer 111 is 0V, so no mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111. However, based on the energy stored in inductor L2, an induced electromotive force with a higher voltage value is generated on the connection part a2 side than on the isolation transformer 111 side. As a result, at time t8, a regenerative current flows through the path "inductor L2 → connection part a2 → freewheeling diode D21 → capacitor C2," similar to time t4. This regenerative current flows to capacitor C2 in the same direction as the secondary current at time t7. Consequently, as shown in Figure 2, even if transistors Q11~Q14 in the primary bridge circuit 112 and transistors Q21~Q24 in the secondary bridge circuit 113 are all in the off state, the capacitor voltage Vc2 continues to increase.

[0094] Although the voltage polarity is reversed during the period from time t1 to time t4, even during the period from time t5 to time t8, the unmonitored circuit control signals Snm11 to Snm14 are pulse signals whose pulse width is set such that the sum of the voltage-time products of the positive and negative output voltages output by the primary bridge circuit 112 is zero.

[0095] The applied voltage V111a applied to the primary winding of the isolation transformer 111 is a positive polarity output voltage and a negative polarity output voltage, which are configured as a single unit consisting of a combination of a positive voltage pulse signal and a negative voltage pulse signal during the period from time t5 to time t8.

[0096] Furthermore, as shown in Figure 2, during the initial charging period Tec, the primary current I111a flowing through the primary winding of the isolation transformer 111 is kept within the range of the positive rated peak current Ipp1 and the negative rated peak current Ipn1. This prevents overcurrent from flowing through the primary bridge circuit 112 during the initial charging period Tec.

[0097] The applied voltage V111a applied to the primary winding of the isolation transformer 111 and the primary current I111a flowing through the primary winding of the isolation transformer 111 are controlled by the time and timing of maintaining transistors Q11~Q14, provided in the primary bridge circuit 112, in either an on or off state. The on / off control of transistors Q11~Q14 can be controlled by gate signals Sg11~Sg14, i.e., unmonitored circuit control signals Snm11~Snm14. Therefore, even during the period from time t5 to time t8, the initial charge signal generation unit 122 is configured to generate unmonitored circuit control signals Snm11~Snm14 that have a pulse width that prevents overcurrent from flowing through the primary bridge circuit 112 (an example of an unmonitored bridge circuit) during the initial charge period Tec, and are configured as a unit to generate positive and negative polarity output voltages, which are composed of a combination of a positive voltage pulse signal and a negative voltage pulse signal.

[0098] As shown in Figure 2, during the period from time t4 to just before time t5, transistors Q11 to Q14 in the primary bridge circuit 112 are all in the off state. Transistors Q11 to Q14 are controlled to the off state by gate signals Sg11 to Sg14. The non-monitoring circuit control signals Snm11 to Snm14 are signals that are in phase with the gate signals Sg11 to Sg14. Therefore, during the period from time t4 to just before time t5, transistors Q11 to Q14 can be considered to be in the off state controlled by the non-monitoring circuit control signals Snm11 to Snm14.

[0099] Therefore, the initial charge signal generation unit 122 is configured to generate non-monitoring circuit control signals Snm11 to Snm14 that turn off transistors Q11 to Q14 (an example of at least both of the two switching elements) provided in the primary bridge circuit 112 during a portion of the initial charge period Tec (i.e., the period from time t4 to just before time t5).

[0100] The operation of power converter 1 during the period from time t9 to the time immediately preceding time t10 is the same as the operation of power converter 1 from time t1 to the time immediately preceding time t5, so the explanation is omitted. Also, the operation of power converter 1 during the period from time t10 to the time immediately preceding time t11 is the same as the operation of power converter 1 from time t5 to the time immediately preceding time t9, so the explanation is omitted. Furthermore, the operation of power converter 1 during the period from time t11 to time t12 is the same as the operation of power converter 1 during the period from time t1 to time t4, so the explanation is omitted.

[0101] As shown in Figure 2, assume that at time t13, a predetermined time has elapsed from time t12, the capacitor voltage Vc2 of capacitor C2 provided in the secondary bridge circuit 113 has become higher than the reference voltage Vr. As a result, the DC voltage monitoring unit 123 outputs a low-level monitoring signal Sm to the selection units 124 and 125. Consequently, the selection unit 124 selects the control signals Sc11 to Sc14 output from the control signal generation unit 121 (see Figure 1) and outputs the selected control signals Sc11 to Sc14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the control signals Sc21 to Sc24 and outputs the selected control signals Sc21 to Sc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0102] Therefore, at time t13, the on / off state of transistors Q11 to Q14 in the primary bridge circuit 112 is controlled according to the voltage levels of the gate signals Sg11 to Sg14. As a result, the initial charging period Tec ends at time t13, and the power converter 1 starts the desired operation.

[0103] In this embodiment, the initial charging signal generation unit 122 generates initial charging control signals Sec21 to Sec24 that turn off transistors Q21 to Q24 provided in the secondary bridge circuit 113, which is the monitored bridge circuit, during the initial charging period Tec. Therefore, the secondary bridge circuit 113 supplies current to capacitor C2 using freewheeling diodes D21 to D24 during the initial charging period Tec. During the initial charging period Tec, current continues to flow through capacitor C2 provided in the secondary bridge circuit 113 in the same direction (in this embodiment, from the secondary positive electrode line P2 to the secondary negative electrode line N2). As a result, as shown in Figure 2, the capacitor voltage Vc2 of capacitor C2 continues to rise during the initial charging period Tec. Therefore, as the initial charging period Tec progresses, the difference between the capacitor voltage Vc1 and the capacitor voltage Vc2 of capacitor C1 provided in the primary bridge circuit 112 decreases. As a result, the peak value of the primary current I111a flowing through the primary winding of the isolation transformer 111 decreases as the initial charging period Tec progresses. Therefore, the power converter 1 can prevent overcurrent from occurring in the primary bridge circuit 112 during the initial charging period Tec.

[0104] (Effects of power converters) The effects of the power converter according to this embodiment will be explained with reference to Figures 1 and 2, and with reference to Figure 3. Figure 3 is a diagram showing an example of the voltage waveforms of the primary transformer voltage of the primary bridge circuit and the secondary transformer voltage of the secondary bridge circuit, as well as the current waveform of the primary current, of a conventional power converter. Although not shown in the diagram, the conventional power converter has the same configuration as the power converter 1 according to this embodiment, except that it does not have an initial charge signal generation unit 122, a DC voltage monitoring unit 123, and selection units 124, 125. The primary transformer voltage corresponds to the applied voltage that the primary bridge circuit applies to the primary winding of the isolation transformer, and the secondary transformer voltage corresponds to the applied voltage that the secondary bridge circuit applies to the secondary winding of the isolation transformer. Figure 3(a) schematically shows various waveforms when the capacitors provided in the primary bridge circuit and the capacitors provided in the secondary bridge circuit are fully charged, and the primary transformer voltage and the secondary transformer voltage are approximately the same voltage value. Figure 3(b) schematically shows various waveforms when the capacitor in the primary bridge circuit is fully charged, the capacitor in the secondary bridge circuit is not charged, and the primary transformer voltage is greater than the secondary transformer voltage.

[0105] In Figure 3, "Vt1" shows the voltage waveform of the primary transformer voltage in the primary bridge circuit, and "Vt2" shows the voltage waveform of the secondary transformer voltage in the secondary bridge circuit. In Figure 3, "It1" shows the current waveform of the primary current flowing through the primary winding of the isolation transformer connected to the primary bridge circuit. In Figure 3, "Ipp1" shows the rated peak current on the positive side of the primary current, and "Ipn1" shows the rated peak current on the negative side of the primary current.

[0106] In a DAB converter such as the isolated DC / DC converter 11 provided in the power conversion device 1 according to this embodiment, the difference between the primary transformer voltage in the primary bridge circuit (capacitor voltage of the capacitor provided in the primary bridge circuit) and the secondary transformer voltage in the secondary bridge circuit (capacitor voltage of the capacitor provided in the secondary bridge circuit) is applied to the leakage inductance component of the isolated transformer. Assuming that the excitation current is small and therefore negligible, the change in current flowing through the primary winding of the isolated transformer is determined by this leakage inductance component and the voltage applied to the leakage inductance component.

[0107] When the difference between the primary transformer voltage (primary capacitor voltage) and the secondary transformer voltage (secondary capacitor voltage) is small, the voltage applied to the transformer leakage inductance also becomes small. As a result, as shown in Figure 3(a), the current change in the primary current flowing through the primary winding of the isolation transformer becomes small and falls within the range of the rated peak current Ipp1 and rated peak current Ipn1, so no overcurrent occurs in the primary bridge circuit.

[0108] On the other hand, if the difference between the primary transformer voltage (primary capacitor voltage) and the secondary transformer voltage (secondary capacitor voltage) is large, the voltage applied to the leakage inductance also increases. Therefore, depending on the pulse width of the primary transformer voltage, an overcurrent may occur in the primary bridge circuit. Conventional power converters operate during the initial charging period by the gate signal when performing the desired operation. Therefore, in conventional power converters, the pulse width of the primary transformer voltage during the initial charging period is equal to the period during which the four transistors in the primary bridge circuit switch between on and off states when the power converter is performing the desired operation. As a result, as shown in Figure 3(b), the current change in the primary current flowing through the primary winding of the isolation transformer becomes large and exceeds the range of the rated peak current Ipp1 and rated peak current Ipn1, causing an overcurrent in the primary bridge circuit. Conventional power converters may be damaged by this overcurrent.

[0109] The rise in the primary current flowing through the primary winding of an isolation transformer increases not only with the voltage difference between the primary and secondary capacitors, but also with the larger the capacitances of the primary and secondary capacitors, or with the smaller the impedance of the isolation transformer, including its leakage inductance. Depending on the application of the power conversion device, it may not be possible to reduce the capacitances of the primary and secondary capacitors or increase the impedance of the isolation transformer. Therefore, there are limitations to suppressing this rise in primary current by optimizing the structure of an isolated DC / DC converter.

[0110] In contrast, the power converter 1 according to this embodiment includes an initial charge signal generation unit 122 and a DC voltage monitoring unit 123. During the initial charging period, the power converter 1 can drive the isolated DC / DC converter 11 with non-monitoring circuit control signals Snm11 to Snm14, the pulse width of which is set such that the sum of the voltage-time products of the positive and negative output voltages output by the primary bridge circuit 112 is zero. As a result, the power converter 1 can suppress the rise in the primary current flowing through the primary winding of the isolation transformer 111 during the initial charging period, even if there is a voltage difference between the primary capacitor C1 and the secondary capacitor C2 at the start of operation, or if the capacitances of capacitors C1 and C2 are large, or if the impedance of the isolation transformer 111 is small.

[0111] Furthermore, the power converter 1 includes an initial charge signal generation unit 122, a DC voltage monitoring unit 123, and selection units 124 and 125 as functions of the control device 12. The control device is a component that is also provided in conventional power converters in order to generate the gate signal. In the power converter 1, by providing the initial charge signal generation unit 122, the DC voltage monitoring unit 123, and selection units 124 and 125 as functions of the control device 12, it is possible to prevent the occurrence of overcurrent at the start of operation without using a dedicated circuit.

[0112] As described above, the power conversion device 1 according to this embodiment includes an isolation transformer 111, a primary bridge circuit 112 provided on the primary side of the isolation transformer 111 and having transistors Q11 to Q14, and a secondary bridge circuit 113 provided on the secondary side of the isolation transformer 111 and having transistors Q21 to Q24, which comprises an isolated DC / DC converter 11. The power conversion device 1 includes a DC voltage monitoring unit 123 that monitors the DC voltage of at least one of the primary bridge circuit 112 and the secondary bridge circuit 113, and an initial charging signal generation unit 122 that generates initial charging control signals Sec11 to Sec14 and Sec21 to Sec24 for controlling the isolated DC / DC converter 11 during the initial charging period until the capacitor voltage Vc2 monitored by the DC voltage monitoring unit 123 becomes higher than the reference voltage Vr from the voltage at the start of initial charging. The initial charging signal generation unit 122 generates monitored circuit control signals Smc21 to Smc24 for controlling the secondary bridge circuit 113, one of the primary bridge circuits 112 and secondary bridge circuits 113, whose capacitor voltage Vc2 is monitored by the DC voltage monitoring unit 123, during the initial charging period, and unmonitored circuit control signals Snm11 to Snm14 for controlling the primary bridge circuit 112, one of the primary bridge circuits 112 and secondary bridge circuits 113, during the initial charging period, as initial charging control signals Sec11 to Sec14 and initial charging control signals Sec21 to Sec24. The unmonitored circuit control signals Snm11 to Snm14 are pulse signals whose pulse width is set such that the sum of the voltage-time products of the positive and negative output voltages output by the primary bridge circuit 112 is zero.

[0113] A power converter 1 with this configuration can prevent overcurrent at startup without using a dedicated circuit. Furthermore, since the power converter 1 does not require the addition of a dedicated circuit to prevent overcurrent, it can be made smaller and less expensive.

[0114] <Example 1> A power conversion device according to Modification 1 of this embodiment will be described with reference to Figures 1 and 2, and with reference to Figure 4. This modified power conversion device has the same configuration as power conversion device 1 according to this embodiment, except that the unmonitored circuit control signal is different. For this reason, a detailed explanation of the configuration of this modified power conversion device will be omitted, and it will be explained with reference to Figure 1 as necessary.

[0115] (Operation of the power converter) Figure 4 is a timing chart showing an example of the operating waveform of the power converter according to this modified example. "Sg11", "Sg14", "Sg12", "Sg13", "Sg21", "Sg24", "Sg22", "Sg23", "V111a", "V111b", "I111a", "Vc2", "Vrp2", "Vrn2", "Ipp1", "Ipn1", and "Vr" in Figure 4 have the same meaning as those shown in Figure 2, so their explanation is omitted. Figure 4 shows the progression of time from left to right.

[0116] As shown in Figure 4, in this modified power converter, the voltage waveforms of the gate signals Sg11 to Sg14 are different from those of the power converter 1 according to this embodiment. Therefore, in this modified power converter, the voltage waveforms of the unmonitored circuit control signals Snm11 to Snm14 are different from those of the power converter 1 according to this embodiment.

[0117] Specifically, as shown in Figure 2, in the power conversion device 1 according to this embodiment, the polarity of gate signals Sg11, Sg14 and gate signals Sg12, Sg13 is reversed before and after the period in which the applied voltage V111a applied to the primary winding of the isolation transformer 111 is constant at a reference potential (for example, the period from time t4 to time t5). In contrast, as shown in Figure 4, in the power conversion device according to this modified example, the polarity of gate signals Sg11, Sg14 and gate signals Sg12, Sg13 is not reversed before and after the period in which the applied voltage V111a applied to the primary winding of the isolation transformer 111 is constant at a reference potential. That is, the initial charge signal generation unit 122 provided in the power conversion device according to this modified example generates non-monitoring circuit control signals Snm11, Snm14 and non-monitoring circuit control signals Snm12, Snm13 whose polarity is not reversed before and after the period in which the applied voltage V111a applied to the primary winding of the isolation transformer 111 is constant at a reference potential.

[0118] In this modified power conversion device, a current in a constant direction can be supplied to the capacitor C2 provided in the secondary bridge circuit during the initial charging period Tec without reversing the polarity of the gate signals Sg11, Sg14 and Sg12, Sg13 before and after the period when the applied voltage V111a applied to the primary winding of the isolation transformer 111 is constant at the reference potential. Therefore, the power conversion device in this modified version can obtain the same effects as the power conversion device 1 in this embodiment.

[0119] <Modification 2> A power conversion device according to Modification 2 of this embodiment will be described with reference to Figures 1 and 2, and with reference to Figure 5. This modified power conversion device has the same configuration as power conversion device 1 according to this embodiment, except that the control signals of the monitored circuit are different. For this reason, a detailed explanation of the configuration of this modified power conversion device will be omitted, and it will be explained with reference to Figure 1 as necessary.

[0120] (Operation of the power converter) Figure 5 is a timing chart showing an example of the operating waveform of the power converter according to this modified example. "Sg11", "Sg14", "Sg12", "Sg13", "Sg21", "Sg24", "Sg22", "Sg23", "V111a", "V111b", "I111a", "Vc2", "Vrp2", "Vrn2", "Ipp1", "Ipn1", and "Vr" in Figure 5 are the same as those shown in Figure 2, so their explanation is omitted. Figure 5 shows the progression of time from left to right.

[0121] As shown in Figure 5, in this modified power converter, the voltage waveforms of the gate signals Sg21 to Sg24 are different from those of the power converter 1 according to this embodiment. Therefore, in this modified power converter, the voltage waveforms of the monitored circuit control signals Smc21 to Smc24 are different from those of the power converter 1 according to this embodiment. Specifically, the initial charge signal generation unit 122 generates monitored circuit control signals Smc21 to Smc24 having the same voltage waveform as the unmonitored circuit control signals Snm11 to Snm14 during the initial charge period Tec. In this modified example, the initial charge signal generation unit 122 generates monitored circuit control signals Smc22 and Smc23 having the same voltage waveform as the unmonitored circuit control signals Snm11 and Snm14, and generates monitored circuit control signals Smc21 and Smc24 having the same voltage waveform as the unmonitored circuit control signals Snm12 and Snm13.

[0122] Therefore, as shown in Figure 5, the gate signals Sg21 and Sg24 based on the monitored circuit control signals Smc21 and Smc24 in this modified example have a voltage waveform that maintains a low level at time t1, changes to a high level at time t2, changes to a low level at time t3, and maintains a low level from time t4 until just before time t5. In addition, the gate signals Sg23 and Sg23 based on the monitored circuit control signals Smc22 and Smc23 in this modified example have a voltage waveform that changes to a high level at time t1, changes to a low level at time t2, changes to a high level at time t3, changes to a low level at time t4, and maintains a low level from just after time t4 until just before time t5.

[0123] Therefore, at time t1, transistors Q21 and Q24 in the secondary bridge circuit 113 are in the off state (non-conductive state), and transistors Q22 and Q23 in the secondary bridge circuit 113 are in the on state (conductive state).

[0124] Furthermore, as explained using Figure 2, when transistors Q11 to Q14 provided in the primary bridge circuit 112 are driven by high-level gate signals Sg11 and Sg14 and low-level gate signals Sg12 and Sg13, primary current flows through the primary side of the isolated DC / DC converter 11 via the path "primary side device 2 → transistor Q11 → connection part a1 → inductor L1 → primary winding of isolation transformer 111 → connection part b1 → transistor Q14 → primary side device 2". As a result, mutual induced electromotive force is generated in the secondary winding of the isolation transformer 111, and the applied voltage V111b increases in the positive direction.

[0125] Therefore, at time t1, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path "secondary winding of isolation transformer 111 → connection b2 → transistor Q23 → capacitor C2 → transistor Q22 → connection a2 → inductor L2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. As a result, as shown in Figure 5, the capacitor voltage Vc2 begins to gradually increase from time t1.

[0126] At time t2, transistors Q21 and Q24 in the secondary bridge circuit 113 are ON (conducting), and transistors Q22 and Q23 in the secondary bridge circuit 113 are OFF (non-conducting).

[0127] Furthermore, as explained using Figure 2, when transistors Q11 to Q14 provided in the primary bridge circuit 112 are driven by low-level gate signals Sg11 and Sg14 and high-level gate signals Sg12 and Sg13, a primary current flows through the primary side of the isolated DC / DC converter 11 via the path "primary side device 2 → transistor Q13 → connection b1 → primary winding of isolation transformer 111 → inductor L1 → connection a1 → transistor Q12 → primary side device 2". Therefore, as shown in Figure 5, at time t2, the primary current I111a flowing through the primary winding of the isolation transformer 111 is a current that decreases from positive polarity to negative polarity.

[0128] As a result, mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111, causing the applied voltage V111b to increase in the negative direction. Consequently, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path: "secondary winding of isolation transformer 111 → inductor L2 → connection a2 → transistor Q21 → capacitor C2 → transistor Q22 → connection b2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. At time t2, the secondary current flows through capacitor C2 in the same direction as at time t1. As a result, as shown in Figure 5, the capacitor voltage Vc2 continues to increase.

[0129] Although a detailed explanation is omitted, the power converter in this modified configuration operates similarly to time t1 at time t3 and similarly to time t2 at time t4. Therefore, as shown in Figure 5, the capacitor voltage Vc2 continues to increase during the period from time t3 to time t4.

[0130] The operation of the power converter according to this modified example during the period from time t4 to the time immediately preceding time t5 is the same as the operation of the power converter 1 according to this embodiment during the period from time t4 to the time immediately preceding time t5. Therefore, as shown in Figure 5, the capacitor voltage Vc2 continues to increase during the period from time t4 to the time immediately preceding time t5.

[0131] As shown in Figure 5, in this modified example, the gate signals Sg21 and Sg24 based on the monitored circuit control signals Smc21 and Smc24 have a voltage waveform that changes to a high level at time t5, to a low level at time t6, to a high level at time t7, to a low level at time t8, and maintains a low level from immediately after time t8 until immediately before time t9. In addition, the gate signals Sg23 and Sg23 based on the monitored circuit control signals Smc22 and Smc23 in this modified example have a voltage waveform that maintains a low level at time t5, changes to a high level at time t6, changes to a low level at time t7, and maintains a low level from time t8 until immediately before time t9.

[0132] Therefore, at time t5, transistors Q21 and Q24 in the secondary bridge circuit 113 are in the ON state (conducting state), while transistors Q22 and Q23 in the secondary bridge circuit 113 remain in the OFF state (non-conducting state).

[0133] Furthermore, at time t5, transistors Q11 to Q14 in the primary bridge circuit 112 are driven by low-level gate signals Sg11 and Sg14 and high-level gate signals Sg12 and Sg13. Therefore, at time t5, the primary current flows through the same path as at time t2. As a result, as shown in Figure 5, at time t5, the primary current I111a flowing through the primary winding of the isolation transformer 111 decreases from positive to negative polarity.

[0134] As a result, mutually induced electromotive force is generated in the secondary winding of the isolation transformer 111, causing the applied voltage V111b to increase in the negative direction. Consequently, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path: "secondary winding of isolation transformer 111 → inductor L2 → connection a2 → transistor Q21 → capacitor C2 → transistor Q22 → connection b2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. At time t5, the secondary current flows through capacitor C2 in the same direction as at time t4. As a result, as shown in Figure 5, the capacitor voltage Vc2 continues to increase.

[0135] At time t6, transistors Q21 and Q24 in the secondary bridge circuit 113 are in the off state (non-conductive state), and transistors Q22 and Q23 in the secondary bridge circuit 113 are in the on state (conductive state).

[0136] Furthermore, at time t6, transistors Q11 to Q14 in the primary bridge circuit 112 are driven by high-voltage gate signals Sg11 and Sg14 and low-voltage gate signals Sg12 and Sg13. Therefore, at time t6, the primary current flows through the same path as at time t1. As a result, as shown in Figure 5, at time t6, the primary current I111a flowing through the primary winding of the isolation transformer 111 increases from negative to positive polarity.

[0137] Therefore, at time t6, secondary current flows through the secondary side of the isolated DC / DC converter 11 via the path "secondary winding of isolation transformer 111 → connection b2 → transistor Q23 → capacitor C2 → transistor Q22 → connection a2 → inductor L2 → secondary winding of isolation transformer 111". Capacitor C2 is charged by this secondary current. As a result, as shown in Figure 5, the capacitor voltage Vc2 begins to increase.

[0138] Although a detailed explanation is omitted, the power converter in this modified configuration operates similarly at time t2 at time t7 and similarly at time t3 at time t8. Therefore, as shown in Figure 5, the capacitor voltage Vc2 continues to increase during the period from time t7 to time t8.

[0139] In this modified power conversion device, the period from time t8 to just before time t9 operates in the same way as the period from time t4 to just before time t5. Therefore, as shown in Figure 5, the capacitor voltage Vc2 continues to increase during the period from time t8 to just before time t9.

[0140] Although a detailed explanation is omitted, the power converter according to this modification operates in the same way from time t1 to time t9 during the period from time t9 to the time immediately preceding time t11. Furthermore, the power converter according to this modification operates in the same way from time t1 to time t4 during the period from time t11 to time t12. Therefore, as shown in Figure 5, the capacitor voltage Vc2 continues to increase during the period from time t9 to time t12.

[0141] As shown in Figure 2, assume that at time t13, a predetermined time has elapsed from time t12, the capacitor voltage Vc2 of capacitor C2 provided in the secondary bridge circuit 113 has become higher than the reference voltage Vr. As a result, the DC voltage monitoring unit 123 outputs a monitoring signal Sm with a high voltage level to the selection units 124 and 125. Consequently, the selection unit 124 selects the control signals Sc11 to Sc14 output from the control signal generation unit 121 and outputs the selected control signals Sc11 to Sc14 as selection signals Ss11 to Ss14 to the gate drive unit 112a. The selection unit 125 selects the control signals Sc21 to Sc24 and outputs the selected control signals Sc21 to Sc24 as selection signals Ss21 to Ss24 to the gate drive unit 113a.

[0142] Therefore, at time t13, the on / off state of transistors Q11 to Q14 in the primary bridge circuit 112 is controlled according to the voltage levels of the gate signals Sg11 to Sg14. As a result, the initial charging period Tec ends at time t13, and the power converter 1 starts the desired operation.

[0143] As described above, the power conversion device according to this modified version, like the power conversion device 1 according to this embodiment, can charge capacitor C2 during the initial charging period Tec, bringing the voltage difference between capacitors C1 and C2 to approximately zero, and then transition to the desired operation. As a result, the power conversion device according to this modified version can obtain the same effects as the power conversion device 1 according to this embodiment.

[0144] A power conversion device according to a second embodiment of the present invention will be described with reference to Figures 6 to 8. First, the schematic configuration of the power conversion device according to this embodiment will be described with reference to Figure 6. Figure 6 is a block diagram showing an example of the schematic configuration of the power conversion device 1 according to this embodiment.

[0145] (Outline configuration of a power converter) As shown in Figure 6, the power converter 1 has an additional feature compared to the configuration in Figure 1: the DC voltage monitoring unit 123 inputs information about the capacitor voltage Vc2 it monitors to the initial charge signal generation unit 122.

[0146] (Operation of the power converter) The operation of the power converter 1 according to this embodiment will be explained with reference to Figure 7. In Figure 7, "Δt1", "Δt2", "Δt3", and "Δt4" represent the total pulse width of the combination of positive and negative voltage pulse signals set so that the sum of the voltage-time products of the output voltage V111a is zero.

[0147] In Figure 7, "Vrp1" indicates the positive rated voltage of the applied voltage applied to the primary bridge circuit 112. In Figure 7, "Vrn1" indicates the negative rated voltage of the applied voltage applied to the primary bridge circuit 112.

[0148] In Figure 7, "Vc2(t20)", "Vc2(t22)", "Vc2(t24)", "Vc2(t26)", and "Vc2(t28)" represent the capacitor voltage Vc2 at the times indicated in parentheses.

[0149] As shown in Figure 7, at time t20, before the operation of the power converter 1 begins, the initial charge signal generation unit 122 acquires information on the capacitor voltage Vc2(t20) monitored by the DC voltage monitoring unit 123. At time t20, the capacitor C2 is not charged, and the capacitor voltage Vc2(t20) is, for example, 0V.

[0150] Subsequently, at time t21, the power converter 1 starts operating, and the initial charging period Tec2 begins. The initial charging signal generation unit 122 determines the total pulse width Δt1 of the first combination of positive and negative voltages, which starts at time t21, based on the capacitor voltage Vc2(t20) acquired in advance at time t20. Based on the determined total pulse width t1, the initial charging signal generation unit 122 generates non-monitoring circuit control signals Snm11 to Snm14 for generating gate signals Sg11 to Sg14.

[0151] One example of how the initial charging signal generation unit 122 determines the total pulse width Δt1 of the positive and negative voltage combinations from the capacitor voltage Vc2(t20) acquired in advance is to make the current peak of the primary side current I111a flowing through the primary winding of the isolation transformer 111 during the initial charging period Tec2 equivalent to the positive rated peak current Ipp1 and the negative rated peak current Ipn1. An example of this method is described below.

[0152] Each of the total pulse widths Δt1 to Δt4 of the combination of positive and negative voltages is divided into multiple equal parts. In this example, it is divided into four equal parts. The initial charge signal generation unit 122 generates non-monitoring circuit control signals Snm11 to Snm14 such that a positive or negative voltage pulse signal is output as the applied voltage V111a for the first 25% of the period, a pulse signal with the opposite polarity to that of the first 25% of the period is output for the next 50% of the period, and a pulse signal with the same polarity as that of the first 25% of the period is output for the remaining 25% of the period.

[0153] During the application of a positive voltage pulse signal for the first 25% of the total pulse width Δt1 period (Δt1 / 4), starting at time t21, the change in the primary current I111a flowing on the primary side of the isolation transformer 111 can be calculated using the following formula, based on the leakage inductance component of the isolation transformer 111, the inductance L which is the sum of inductors Ll and L2, the primary side rated voltage Vrp1, and the previously acquired capacitor voltage Vc2(t20) (=0V).

[0154] [Formula 1] Δt1 Current change during the first 25% period = (Vrp1 - Vc2(t20)) / L =Vrp1 / L By utilizing the current change represented by Equation 1, the total pulse width Δt1 of the combination of positive and negative voltages where the current peak of the primary side current I111a coincides with the positive rated peak current Ipp1 can be calculated using the following formula.

[0155] [Formula 2] Vrp1 / L × Δt1 / 4 = Ipp1 → Δt1 = 4 × Ipp1 × L / Vrp1 The calculations in Equations 1 and 2 are performed by the initial charge signal generation unit 122 between the time t20, when it acquires the capacitor voltage Vc2(t20), and the time t21, when it outputs the first combination of positive and negative voltages.

[0156] By outputting the first combination of positive and negative voltages with a total pulse width Δt1 calculated by Equation 2, the capacitor voltage rises to Vc2(t22).

[0157] The initial charging signal generation unit 122 acquires the capacitor voltage Vc2(t22) at time t22, for example, at the same time as the end of the first combination output of positive and negative voltages, and calculates the total pulse width Δt2 of the second combination of positive and negative voltages. Note that the timing for acquiring the capacitor voltage Vc2 can be anytime before the start of the second combination output of positive and negative voltages at time t23.

[0158] During the application of a negative voltage pulse signal for the first 25% period (Δt2 / 4) of the total pulse width Δt2, starting at time t23, the current change of the primary current I111a flowing on the primary side of the isolation transformer 111 can be calculated using the following formula.

[0159] [Formula 3] Δt2 Current change during the first 25% period = -(Vrp1 - Vc2(t22) / L) By utilizing the current change represented by Equation 3, the total pulse width Δt2 of the combination of positive and negative voltages where the current peak of the primary side current I111a coincides with the negative rated peak current Ipn1 can be calculated using the following formula.

[0160] [Formula 4] -(Vrp1-Vc2(t22) / L×Δt2 / 4=Ipn1 → Δt2=4×Ipp1×L / (Vrp1-Vc2(t22)) =Vrp1 / (Vrp1-Vc2(t22))×Δt1 The calculations in Equations 3 and 4 are performed by the initial charge signal generation unit 122 between the time t22 when it acquires the capacitor voltage Vc2(t22) and the time t23 when it outputs the second combination of positive and negative voltages.

[0161] By outputting the second combination of positive and negative voltages with a total pulse width Δt2 calculated by Equation 4, the capacitor voltage rises to Vc2(t24).

[0162] In this embodiment, the initial charging signal generation unit 122, each time it outputs a combination of positive and negative voltages during the initial charging period Tec2, acquires the capacitor voltage Vc2 in advance as described above, and uses that capacitor voltage Vc2 to calculate and determine the total pulse width of the combination of positive and negative voltages so that the current peak of the primary side current I111a is equivalent to the rated peak current Ipp1 or Ipn1. As a result, as shown in Figure 7, even if the capacitor voltage Vc2 rises as the initial charging period Tec2 progresses, the peak value of the primary current I111a flowing through the primary winding of the isolation transformer 111 will be equivalent to the rated peak current Ipp1 or Ipn1. Consequently, the initial charging period Tec2 can be shortened, and the time until the power converter 1 starts the desired operation can be reduced. Therefore, the power converter 1 can achieve high-speed initial charging while preventing overcurrent from occurring in the primary side bridge circuit 112.

[0163] On the other hand, in this embodiment, the initial charging signal generation unit 122 uses arithmetic operations during the initial charging period Tec2, which may increase the computational load on the control device 12. A modified version of this embodiment that can avoid this increase in the computational load on the control device 12 is described below.

[0164] <Variation> A modified power conversion device according to this embodiment will be described with reference to Figure 6 and Figure 8.

[0165] As shown in Figure 8, in the power conversion device 1 according to this modification, in addition to the reference voltage Vr of the capacitor voltage Vc2, reference voltages Vr × 3 / 3 and reference voltages Vr × 1 / 3 are added. Here, new reference voltages are added by dividing Vr into three equal parts, but the additional reference voltages may be set to any value and any number.

[0166] In the modified power conversion device 1, the initial charge signal generation unit 122 generates three initial charge modes corresponding to the magnitude of the capacitor voltage Vc2, and combinations of positive and negative voltages having a total pulse width Δt=ta, tb, tc corresponding to each initial charge mode.

[0167] Specifically, the initial charging mode is defined as follows: when the capacitor voltage Vc2 is 0 or greater and less than Vr × 1 / 3, it is defined as initial charging mode 2 when it is 1 / 3 or greater and less than Vr × 2 / 3, and it is 2 / 3 or greater and less than Vr.

[0168] The calculation of the pulse width Δt=ta corresponding to initial charging mode 1 adopts the condition Vc2=0V (minimum capacitor voltage condition in mode 1), which is the condition in which the peak value of the primary current I111a is largest under the capacitor voltage conditions in mode 1. The pulse width Δt=ta corresponding to initial charging mode 1 is calculated according to the calculation process from equations 1 to 4. [Formula 5] ta = 4 × Ipp 1 × L / (Vrp 1 - 0) = 4 × lppl × L / Vrp1 It can be calculated using this method.

[0169] Similarly, the calculation of the pulse widths Δt=tb,tc corresponding to initial charging modes 2 and 3 assumes the maximum case of the peak value of the primary current I111a by using the minimum capacitor voltage condition for each mode. The pulse widths Δt=tb,tc corresponding to initial charging modes 2 and 3 are calculated according to the calculation process from Equations 1 to 4. [Formula 6] tb = 4 × Ipp 1 × L / (Vrp 1 - Vr × 1 / 3) tc = 4 × Ipp 1 × L / (Vrp 1 - Vr × 2 / 3) It can be calculated using this method.

[0170] The following describes an initial charging signal generation method using the initial charging modes 1 to 3 and the total pulse widths ta to tc corresponding to each initial charging mode in the power conversion device 1 according to this modified example.

[0171] The initial charge signal generation unit 122 acquires information about the capacitor voltage Vc2(t20) at time t20, before the start of the initial charge period Tec2. At time t20, the capacitor C2 is not charged, and the capacitor voltage Vc2(t20) is, for example, 0V.

[0172] The initial charge signal generation unit 122 determines that the initial charge mode is 1 based on the previously acquired capacitor voltage Vc2(t20) (=0V), and outputs a combination of positive and negative voltages with a corresponding total pulse width ta from time t21 when the initial charge period Tec2 begins.

[0173] The initial charging signal generation unit 122 acquires the capacitor voltage Vc2(t22) at time t22, for example, at the same time as the end of the first positive and negative voltage combination output, and determines the initial charging mode for determining the second positive and negative voltage combination. In the case of Figure 8, since Vc2(t22) is still less than Vr × 1 / 3, it is determined to be initial charging mode 1, just as at the time of the first positive and negative voltage combination output. The initial charging signal generation unit 122 outputs a positive and negative voltage combination with a total pulse width ta corresponding to initial charging mode 1 from time t23.

[0174] In this modified power converter, the initial charge signal generation unit 122, each time it outputs a combination of positive and negative voltages during the initial charge period Tec2, acquires capacitor voltage information in advance as described above, uses that capacitor voltage information to determine the initial charge mode, and selects a combination of positive and negative voltages with a total pulse width corresponding to the determined initial charge mode. As a result, the power converter 1 can prevent overcurrent and perform high-speed initial charging of the primary bridge circuit 112 without increasing the computational load of the control device 12.

[0175] The present invention is not limited to the above embodiment and can be modified in various ways. The power converter 1 according to the above embodiment is configured to monitor the voltage of capacitor C2 provided in the secondary bridge circuit 113, but the present invention is not limited to this. For example, the power converter may include a DC voltage monitoring unit that monitors the voltage of capacitor provided in the primary bridge circuit. In this case, the initial charge signal generation unit provided in the power converter outputs an unmonitored circuit control signal to the secondary bridge circuit and a monitored circuit control signal to the primary bridge circuit. As a result, the power converter 1 can obtain the same effects as the power converter 1 according to the above embodiment.

[0176] Furthermore, the power converter may include a DC voltage monitoring unit that monitors the voltage of a capacitor provided in the primary bridge circuit and a DC voltage monitoring unit that monitors the voltage of a capacitor provided in the secondary bridge circuit. In this case, the control device is configured to receive a direction command value that indicates the direction in which power is transferred.

[0177] If the direction command value contains information indicating that power is supplied from the primary bridge circuit to the secondary bridge circuit, the initial charging signal generation unit outputs an unmonitored circuit control signal to the primary bridge circuit and a monitored circuit control signal to the secondary bridge circuit. Furthermore, one of the two selection units selects and outputs either the unmonitored circuit control signal input from the initial charging signal generation unit or the control signal input from the control signal generation unit, based on the monitoring signal output from the DC voltage monitoring unit which monitors the voltage of the capacitor provided in the secondary bridge circuit. The other of the two selection units selects and outputs either the monitored circuit control signal input from the initial charging signal generation unit or the control signal input from the control signal generation unit, based on the monitoring signal output from the DC voltage monitoring unit which monitors the voltage of the capacitor provided in the secondary bridge circuit.

[0178] On the other hand, if the direction command value contains information indicating that power is supplied from the secondary bridge circuit to the primary bridge circuit, the initial charging signal generation unit outputs a monitored circuit control signal to the primary bridge circuit and an unmonitored circuit control signal to the secondary bridge circuit. Furthermore, one of the two selection units selects and outputs either the monitored circuit control signal input from the initial charging signal generation unit or the control signal input from the control signal generation unit, based on the monitoring signal output from the DC voltage monitoring unit which monitors the voltage of the capacitor provided in the primary bridge circuit. The other of the two selection units selects and outputs either the unmonitored circuit control signal input from the initial charging signal generation unit or the control signal input from the control signal generation unit, based on the monitoring signal output from the DC voltage monitoring unit which monitors the voltage of the capacitor provided in the secondary bridge circuit.

[0179] As a result, the power conversion device can obtain the same effects as the power conversion device 1 according to the above embodiment.

[0180] The technical scope of the present invention is not limited to the illustrative and described embodiments, but also includes all embodiments that produce effects equivalent to those aimed at by the present invention. Furthermore, the technical scope of the present invention is not limited to the combination of features of the invention defined by the claims, but can be defined by any desired combination of specific features from all disclosed features. [Explanation of Symbols]

[0181] 1. Power converter 2 Primary side device 3 Secondary side device 11. Isolated DC / DC Converter 12 Control device 111 Isolation transformer 112 Primary side bridge circuit 112a, 113a Gate drive unit 113 Secondary bridge circuit 121 Control signal generation unit 122 Initial charge signal generation section 123 DC Voltage Monitoring Unit 123a Comparator 123b Reference voltage generation unit 124,125 Selection section a1, a2, b1, b2 connection part C1, C2 Capacitors D11, D12, D13, D14, D21, D22, D23, D24 Freewheeling diodes I111a Primary current Ipn1, Ipp1 Rated Peak Current L1, L2 Inductors N1 Primary side negative electrode line N2 secondary negative electrode line P1 Primary side positive electrode line P2 Secondary side positive electrode line Tec initial charging period Q11, Q12, Q13, Q14, Q21, Q22, Q23, Q24 Transistors Sc11, Sc12, Sc13, Sc14, Sc21, Sc22, Sc23, Sc24 Control signals Sec11, Sec12, Sec13, Sec14, Sec21, Sec22, Sec23, Sec24 Sg11, Sg12, Sg13, Sg14, Sg21, Sg22, Sg23, Sg24 Gate signals SM monitoring signal Smc21,Smc22,Smc23,Smc24 Monitored circuit control signal Snm11, Snm12, Snm13, Snm14 Unmonitored Circuit Control Signals Ss11, Ss12, Ss13, Ss14, Ss21, Ss22, Ss23, Ss24 selection signals V111a, V111b Applied Voltage Vc1, Vc2 Capacitor voltage Vrn1, Vrp1, Vrn2, Vrp2 Rated Voltage Vr Reference Voltage

Claims

1. An isolated DC / DC converter having an isolation transformer, a primary bridge circuit provided on the primary side of the isolation transformer and having at least two switching elements, and a secondary bridge circuit provided on the secondary side of the isolation transformer and having at least two switching elements, A DC voltage monitoring unit that monitors the DC voltage of at least one of the primary bridge circuit and the secondary bridge circuit, An initial charging signal generation unit generates an initial charging control signal for controlling the isolated DC / DC converter during the initial charging period from the voltage at the start of operation until the DC voltage monitored by the DC voltage monitoring unit becomes higher than the reference voltage. Equipped with, The initial charging signal generation unit generates, as the initial charging control signals, a monitored circuit control signal for controlling the monitored bridge circuit whose DC voltage is monitored by the DC voltage monitoring unit among the primary and secondary bridge circuits during the initial charging period, and an unmonitored circuit control signal for controlling the unmonitored bridge circuit among the primary and secondary bridge circuits that is not the monitored bridge circuit during the initial charging period. The unmonitored circuit control signal is a pulse signal whose pulse width is set such that, during the initial charging period, the sum of the voltage-time product, which is the product of the time the unmonitored bridge circuit outputs a positive polarity output voltage and the positive polarity output voltage, and the voltage-time product, which is the product of the time the unmonitored bridge circuit outputs a negative polarity output voltage and the negative polarity output voltage, is zero. The initial charging signal generation unit generates a control signal for the monitored circuit that causes the at least two switch elements provided in the monitored bridge circuit to be turned off during the initial charging period. Power converter.

2. The initial charging signal generation unit generates the unmonitoring circuit control signal having the pulse width such that no overcurrent flows through the unmonitoring bridge circuit during the initial charging period. The power conversion device according to claim 1.

3. The initial charging signal generation unit generates the non-monitoring circuit control signal so as to change the total pulse width, which is the sum of the pulse width of the positive polarity output voltage and the pulse width of the negative polarity output voltage, in accordance with the DC voltage monitored by the DC voltage monitoring unit during the initial charging period. The power conversion device according to claim 2.

4. The initial charging signal generation unit calculates the total pulse width using the DC voltage information monitored by the DC voltage monitoring unit during the initial charging period. The power conversion device according to claim 3.

5. The initial charging signal generation unit is: From among a plurality of initial charging modes corresponding to the magnitude of the DC voltage, one initial charging mode is determined according to the magnitude of the DC voltage monitored by the DC voltage monitoring unit during the initial charging period, and the non-monitoring circuit control signal is generated so that the positive polarity output voltage and the negative polarity output voltage are output with the total pulse width corresponding to the determined one initial charging mode. The power conversion device according to claim 3.

6. The initial charging signal generation unit generates the unmonitoring circuit control signal that turns off both of the two switch elements provided in the unmonitoring bridge circuit during a portion of the initial charging period. A power conversion device according to any one of claims 1 to 5.

7. The initial charging signal generation unit generates the monitored circuit control signal during the initial charging period, which has the same voltage waveform as the unmonitored circuit control signal and for the same duration as the unmonitored circuit control signal. A power conversion device according to any one of claims 1 to 6.

8. The DC voltage monitoring unit, The initial charging signal generation unit, A control signal generation unit that generates a control signal for controlling the isolated DC / DC converter after the initial charging period has elapsed, A selection unit that, based on the voltage level of the output signal output from the DC voltage monitoring unit, selects one of the unmonitored circuit control signal and the monitored circuit control signal input from the initial charging signal generation unit and the control signal input from the control signal generation unit and outputs it to the isolated DC / DC converter, Equipped with a control device having A power conversion device according to any one of claims 1 to 7.

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