Power conversion system

Abstract

This power conversion system is provided with a control device for generating a first drive pulse to supply to a first bridge circuit and a second drive pulse to supply to a second bridge circuit. The control device: causes only one bridge circuit, of the first bridge circuit and the second bridge circuit, to operate in a state in which power transmission is not in operation between the first bridge circuit and the second bridge circuit, thereby detecting variation in delay time of a timing at which the state of respective switching elements of the one bridge circuit changes; and in accordance with the variation, corrects the timing at which to change the drive pulse which is supplied to the one bridge circuit and which is from between the first drive pulse and the second drive pulse.

Description

Power Conversion Systems

[0001] The present disclosure relates to power conversion systems.

[0002] A bidirectional isolated DC / DC converter (DAB converter) known as a DAB (Dual Active Bridge) is a type of DC / DC converter known in the art. This DAB converter has a configuration in which two single-phase bridge circuits are connected via a high-frequency isolation transformer (see, for example, Patent Document 1).

[0003] Patent No. 7060179

[0004] However, in conventional DAB converters, when a voltage containing a DC component is applied to a magnetic component such as an isolation transformer connected between bridge circuits, the magnetic core of the magnetic component connected between the bridge circuits may become biased, making it impossible to transmit power.

[0005] An object of the present disclosure is to suppress biased magnetism of a magnetic core in a magnetic component such as an isolation transformer connected across a bridge circuit.

[0006] The present disclosure provides a power conversion system comprising: an isolation transformer having a magnetic core; a first bridge circuit connected to the primary side of the isolation transformer and having a plurality of switching elements; a second bridge circuit connected to the secondary side of the isolation transformer and having a plurality of switching elements; a control device that generates a first drive pulse to be supplied to the first bridge circuit and a second drive pulse to be supplied to the second bridge circuit; a first drive circuit that drives the first bridge circuit based on the first drive pulse; and a second drive circuit that drives the second bridge circuit based on the second drive pulse, wherein the control device operates only one of the first bridge circuit and the second bridge circuit when power is not being transmitted between the first bridge circuit and the second bridge circuit, thereby detecting a variation in delay time in the timing at which the state of each of the switching elements in the one bridge circuit changes, and correcting the timing at which the drive pulse to be supplied to the one bridge circuit, out of the first drive pulse and the second drive pulse, is changed in accordance with the variation.

[0007] According to the present disclosure, it is possible to suppress biased magnetism of a magnetic core in a magnetic component such as an isolation transformer connected across a bridge circuit.

[0008] FIG. 1 is a circuit diagram showing an example of the configuration of a power conversion system according to a first embodiment. FIG. 2 is a timing chart illustrating an example of a waveform of a first drive pulse provided by a control device to a primary side drive circuit and a switching waveform of a primary side switching element. FIG. 3 is a timing chart illustrating an example of a waveform of a second drive pulse provided by a control device to a secondary side drive circuit and a switching waveform of a secondary side switching element. FIG. 4 is a diagram showing an example of a control sequence of the power conversion system according to the first embodiment. FIG. 5 is a diagram showing a specific example of a control sequence of the power conversion system according to the first embodiment. FIG. 6 is a timing chart showing an example of a circuit operation for detecting variations in switching delay time of a first bridge circuit. FIG. 7 is a diagram showing an example of a generation circuit for generating drive pulses. FIG. 8 is a timing chart showing an example of an operation waveform of a drive pulse. FIG. 9 is a timing chart showing a modified example of a circuit operation for detecting variations in switching delay time of a first bridge circuit.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] <First embodiment> Fig. 1 is a diagram showing an example of the configuration of a power conversion system according to the first embodiment. The power conversion system 100 shown in Fig. 1 includes a bidirectional isolated DC / DC converter (isolated DC / DC converter 110) in which bridge circuits are provided on both sides of an isolation transformer 102. The power conversion system 100 supplies power bidirectionally between a first bridge circuit 111 and a second bridge circuit 112.

[0011] The power conversion system 100 includes an isolation transformer 102 , a first bridge circuit 111 , a second bridge circuit 112 , and a control device 106 .

[0012] The isolation transformer 102 is a transformer having a primary winding 31 and a secondary winding 32 that are magnetically coupled to each other. The turns ratio of the primary winding 31 and the secondary winding 32 is set appropriately. The isolation transformer 102 has a magnetic core 33 around which the primary winding 31 and the secondary winding 32 are wound.

[0013] In this specification, unless otherwise specified, the turns ratio between the primary winding 31 and the secondary winding 32 may be considered to be 1:1. However, when the turns ratio between the primary winding 31 and the secondary winding 32 is other than 1:1, the voltage value on the secondary side or the primary side may be converted to the voltage value on the primary side or the secondary side, and the current value on the secondary side or the primary side may be converted to the current value on the primary side or the secondary side. For example, in the following description, the secondary side DC voltage E2 and the secondary side AC voltage V2 refer to voltage values ​​converted to the primary side. In other words, when the number of turns of the primary winding 31 of the isolation transformer 102 is n, １ , the number of turns of the secondary winding 32 is n ２ In this case, the coefficient n １ / n ２ The voltage value (voltage value converted to the primary side) obtained by multiplying by E1 and V2 is the secondary-side DC voltage E2. The same applies to the secondary-side AC voltage V2. In the following description, the low-voltage side and the high-voltage side refer to the primary side and secondary side of the isolated DC / DC converter 110, respectively, where a low voltage is generated. For example, if there is a relationship E1<E2 between the primary-side DC voltage E1 and the secondary-side DC voltage E2 converted to a primary-side value, then the primary side is the low-voltage side and the secondary side is the high-voltage side; and if there is a relationship E1>E2, then the primary side is the high-voltage side and the secondary side is the low-voltage side.

[0014] The first bridge circuit 111 is a primary-side bridge circuit connected to the primary side of the isolation transformer 102, and exchanges power with the primary winding 31 of the isolation transformer 102. The first bridge circuit 111 has a positive terminal 41p and a negative terminal 41n as primary-side DC terminals electrically connected to an external device (not shown). The first bridge circuit 111 exchanges power with the external device connected to the primary-side DC terminals.

[0015] The first bridge circuit 111 has a positive bus 43p and a negative bus 43n as a primary-side DC bus pair. The positive bus 43p is connected to the positive terminal 41p. The negative bus 43n is connected to the negative terminal 41n. The first bridge circuit 111 switches the polarity of the voltage V1 applied to the primary winding 31 of the isolation transformer 102 by the primary-side DC bus pair 43p, 43n.

[0016] The first bridge circuit 111 is a full bridge circuit having a plurality of legs 11 and 12 connected in parallel.

[0017] The first bridge circuit 111 has, for example, a leg 11 in which a high-side arm Q1 and a low-side arm Q2 are connected in series, and a leg 12 in which a high-side arm Q3 and a low-side arm Q4 are connected in series. The arm Q1 is an example of a first arm, the arm Q2 is an example of a second arm, the arm Q3 is an example of a third arm, and the arm Q4 is an example of a fourth arm. The leg 11 is an example of a first leg, and the leg 12 is an example of a second leg. The high-side arm and the low-side arm may be collectively referred to as upper and lower arms.

[0018] The first bridge circuit 111 is a full-bridge circuit in which a primary winding 31 of an isolation transformer 102 is provided in a bridge portion 21 that connects an intermediate connection point a1 between the arms Q1 and Q2 and an intermediate connection point b1 between the arms Q3 and Q4. The first bridge circuit 111 may include a reactor 104a in the bridge portion 21 that is connected in series to the primary winding 31 of the isolation transformer 102. The intermediate connection point a1 is an example of a first connection point. The intermediate connection point b1 is an example of a second connection point. The bridge portion 21 is an example of a first bridge portion.

[0019] The first bridge circuit 111 includes a capacitor C1 and arms Q1 to Q4.

[0020] The capacitor C1 is connected between the pair of DC buses 43p and 43n on the primary side, and smoothes the voltage between the pair of DC buses 43p and 43n (the voltage of the capacitor C1).

[0021] The arms Q1 to Q4 are primary-side switching elements, and specific examples thereof include semiconductor switching elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors).

[0022] Leg 11 includes a configuration in which arms Q1 and Q2 are connected in series between the pair of DC buses 43p and 43n, and leg 12 includes a configuration in which arms Q3 and Q4 are connected in series between the pair of DC buses 43p and 43n. Each of arms Q1 to Q4 has a first main terminal, a second main terminal, and a control terminal. For example, the first main terminal corresponds to the drain or collector, the second main terminal corresponds to the source or emitter, and the control terminal corresponds to the gate. Arms Q1 to Q4 may include a diode connected in reverse between the main terminals. If arms Q1 to Q4 are MOSFETs, this diode may be a parasitic diode. FIG. 1 illustrates freewheeling diodes D1, D2, D3, and D4.

[0023] When the arms Q1 and Q4 are turned on and the arms Q2 and Q3 are turned off, the first bridge circuit 111 electrically connects the intermediate connection point a1 to the positive bus 43p and electrically connects the intermediate connection point b1 to the negative bus 43n. As a result, the first bridge circuit 111 sets the voltage V1 to a positive voltage "E1". The voltage V1 is the primary side AC voltage between the intermediate connection point a1 and the intermediate connection point b1. E1 is the voltage value (primary side DC voltage) between the pair of DC buses 43p, 43n. When the arms Q1 and Q4 are turned off and the arms Q2 and Q3 are turned on, the first bridge circuit 111 electrically connects the intermediate connection point a1 to the negative bus 43n and electrically connects the intermediate connection point b1 to the positive bus 43p. As a result, the first bridge circuit 111 sets the voltage V1 to a negative voltage "-E1". By operating in this manner, the first bridge circuit 111 switches the polarity of the voltage V1 applied to the primary winding 31 of the isolation transformer 102 by the primary-side DC bus pair 43p, 43n.

[0024] When the arms Q1 and Q3 are turned on and the arms Q2 and Q4 are turned off, the first bridge circuit 111 electrically connects both the intermediate connection point a1 and the intermediate connection point b1 to the positive bus 43p, thereby making the voltage V1 substantially zero. When the arms Q1 and Q3 are turned off and the arms Q2 and Q4 are turned on, the first bridge circuit 111 electrically connects both the intermediate connection point a1 and the intermediate connection point b1 to the negative bus 43n, thereby making the voltage V1 substantially zero.

[0025] The second bridge circuit 112 is a secondary-side bridge circuit connected to the secondary side of the isolation transformer 102, and exchanges power with the secondary winding 32 of the isolation transformer 102. The second bridge circuit 112 has a positive terminal 42p and a negative terminal 42n as secondary-side DC terminals electrically connected to an external device (not shown). The second bridge circuit 112 exchanges power with the external device connected to the secondary-side DC terminals.

[0026] The second bridge circuit 112 has a positive bus 44p and a negative bus 44n as a secondary-side DC bus pair. The positive bus 44p is connected to the positive terminal 42p. The negative bus 44n is connected to the negative terminal 42n. The second bridge circuit 112 switches the polarity of the voltage V2 applied to the secondary winding 32 of the isolation transformer 102 by the secondary-side DC bus pair 44p, 44n.

[0027] The second bridge circuit 112 is a full bridge circuit having a plurality of legs 13 and 14 connected in parallel.

[0028] The second bridge circuit 112 has, for example, a leg 13 in which a high-side arm Q5 and a low-side arm Q6 are connected in series, and a leg 14 in which a high-side arm Q7 and a low-side arm Q8 are connected in series. The arm Q5 is an example of a fifth arm, the arm Q6 is an example of a sixth arm, the arm Q7 is an example of a seventh arm, and the arm Q8 is an example of an eighth arm. The leg 13 is an example of a third leg, and the leg 14 is an example of a fourth leg. The high-side arm and the low-side arm may be collectively referred to as upper and lower arms.

[0029] The second bridge circuit 112 is a full-bridge circuit in which the secondary winding 32 of the isolation transformer 102 is provided in a bridge portion 23 that connects an intermediate connection point a2 between the arms Q5 and Q6 and an intermediate connection point b2 between the arms Q7 and Q8. The second bridge circuit 112 may include a reactor 104b in the bridge portion 23 that is connected in series with the secondary winding 32 of the isolation transformer 102. The intermediate connection point a2 is an example of a third connection point. The intermediate connection point b2 is an example of a fourth connection point. The bridge portion 23 is an example of a second bridge portion.

[0030] The second bridge circuit 112 includes a capacitor C2 and arms Q5 to Q8.

[0031] The capacitor C2 is connected between the pair of DC buses 44p and 44n on the secondary side, and smoothes the voltage between the pair of DC buses 44p and 44n (the voltage of the capacitor C2).

[0032] The arms Q5 to Q8 are secondary-side switching elements, and specific examples thereof include semiconductor switching elements such as MOSFETs and IGBTs, similar to the arms Q1 to Q4.

[0033] Leg 13 includes a configuration in which arms Q5 and Q6 are connected in series between the pair of DC buses 44p and 44n, and leg 14 includes a configuration in which arms Q7 and Q8 are connected in series between the pair of DC buses 44p and 44n. Similar to arms Q1 to Q4, arms Q5 to Q8 each have a first main terminal, a second main terminal, a control terminal, and a diode. Figure 1 illustrates freewheeling diodes D5, D6, D7, and D8.

[0034] When the arms Q5 and Q8 are turned on and the arms Q6 and Q7 are turned off, the second bridge circuit 112 electrically connects the intermediate node a2 to the positive bus 44p and electrically connects the intermediate node b2 to the negative bus 44n. As a result, the second bridge circuit 112 sets the voltage V2 to a positive voltage "E2". The voltage V2 is a secondary AC voltage between the intermediate node a2 and the intermediate node b2. E2 is a voltage value (secondary DC voltage) between the pair of DC buses 44p, 44n. When the arms Q5 and Q8 are turned off and the arms Q6 and Q7 are turned on, the second bridge circuit 112 electrically connects the intermediate node a2 to the negative bus 44n and electrically connects the intermediate node b2 to the positive bus 44p. As a result, the second bridge circuit 112 sets the voltage V2 to a negative voltage "-E2". By operating in this manner, the second bridge circuit 112 switches the polarity of the voltage V2 applied to the secondary winding 32 of the isolation transformer 102 by the secondary-side DC bus pair 44p, 44n.

[0035] When the arms Q5 and Q7 are turned on and the arms Q6 and Q8 are turned off, the second bridge circuit 112 electrically connects both the intermediate connection point a2 and the intermediate connection point b2 to the positive bus 44p, thereby making the voltage V2 substantially zero. When the arms Q5 and Q7 are turned off and the arms Q6 and Q8 are turned on, the second bridge circuit 112 electrically connects both the intermediate connection point a2 and the intermediate connection point b2 to the negative bus 44n, thereby making the voltage V2 substantially zero.

[0036] The power conversion system 100 includes a contactor S1 inserted in series with a positive bus 43p and a contactor S2 inserted in series with a positive bus 44p. When the contactor S1 is turned on, a charging circuit including a resistor Rc and a switch Sc may be provided in parallel with the contactor S1 to equalize the voltage of the capacitor C1 and the voltage of an external device (not shown) connected to the positive terminal 41p and the negative terminal 41n. When the contactor S2 is turned on, a charging circuit including a resistor and a switch may be provided in parallel with the contactor S2 to equalize the voltage of the capacitor C2 and the voltage of an external device (not shown) connected to the positive terminal 42p and the negative terminal 42n.

[0037] An external device (not shown) connected to the positive terminal 41 p and the negative terminal 41 n is, for example, a charging / discharging device capable of charging / discharging a battery, etc. An external device (not shown) connected to the positive terminal 42 p and the negative terminal 42 n is, for example, a DC system capable of charging / discharging energy of a charging / discharging device connected to the primary side.

[0038] The control device 106 controls the first bridge circuit 111 and the second bridge circuit 112. The control device 106 generates drive pulses g1 to g4 for driving the arms Q1 to Q4 of the first bridge circuit 111, respectively, and drive pulses g5 to g8 for driving the arms Q5 to Q8 of the second bridge circuit 112, respectively. The drive pulses g1 to g8 are drive signals for controlling the on / off of corresponding arms among the arms Q1 to Q8. The drive pulses g1 to g4 are commands output from the control device 106 as first drive pulses that the control device 106 supplies to the first bridge circuit 111. The drive pulses g5 to g8 are commands output from the control device 106 as second drive pulses that the control device 106 supplies to the second bridge circuit 112.

[0039] The power conversion system 100 includes a drive circuit 105a and a drive circuit 105b. The drive circuit 105a is a primary-side drive circuit (first drive circuit) that drives a first bridge circuit 111 based on drive pulses g1 to g4 provided by a control device 106. The drive circuit 105a controls the on / off switching of arms Q1 to Q4 of the first bridge circuit 111 in accordance with the drive pulses g1 to g4. The drive circuit 105b is a secondary-side drive circuit (second drive circuit) that drives a second bridge circuit 112 based on drive pulses g5 to g8 provided by the control device 106. The drive circuit 105b controls the on / off switching of arms Q5 to Q8 of the second bridge circuit 112 in accordance with the drive pulses g5 to g8.

[0040] The power conversion system 100 includes a DC voltage detection unit 107a and a DC voltage detection unit 107b. The DC voltage detection unit 107a is a circuit that detects a primary side DC voltage E1 applied to the DC bus pair 43p, 43n of the first bridge circuit 111. The DC voltage detection unit 107b is a circuit that detects a secondary side DC voltage E2 applied to the DC bus pair 44p, 44n of the second bridge circuit 112.

[0041] The power conversion system 100 includes a DC current detection unit 108a and a DC current detection unit 108b. The DC current detection unit 108a is a circuit that detects a primary side DC current I1 flowing through the positive electrode bus 43p of the first bridge circuit 111. The DC current detection unit 108b is a circuit that detects a secondary side DC current I2 flowing through the positive electrode bus 44p of the second bridge circuit 112.

[0042] The control device 106 controls the phases of the edges of drive pulses g1 to g8 for driving the arms Q1 to Q8 based on the primary side DC voltage E1 detected by the DC voltage detection unit 107 a and the secondary side DC voltage E2 detected by the DC voltage detection unit 107 b. The control device 106 controls the phases of the edges of the drive pulses g1 to g8 to control the power transmission of the isolated DC / DC converter 110 (power transmission between the first bridge circuit 111 and the second bridge circuit 112).

[0043] FIG. 2 is a timing chart illustrating the waveform of a first drive pulse provided by a control device to a primary-side drive circuit and the switching waveform of a primary-side switching element. FIG. 3 is a timing chart illustrating the waveform of a second drive pulse provided by a control device to a secondary-side drive circuit and the switching waveform of a secondary-side switching element. As shown in FIGS. 2 and 3, the switch timing at which arms Q1 to Q8 change state is delayed relative to the timing at which the edges of drive pulses g1 to g8 change. The delay times ΔQ1 to ΔQ8 vary among arms Q1 to Q8. This variation depends on individual differences in the characteristics of electronic components used in arms Q1 to Q8 and drive circuits 105a and 105b.

[0044] Because the delay time varies among the arms, a voltage containing a DC component (DC(V1), DC(V2)) is applied to magnetic components such as the isolation transformer 102. T represents one switching cycle of the arms. When a voltage containing a DC component is applied to a magnetic component such as the isolation transformer 102, the magnetic core of the magnetic component may become asymmetrically magnetized, which may result in an inability to transmit power or an overcurrent.

[0045] 4 is a diagram illustrating an example of a control sequence of the power conversion system according to the first embodiment. The control device 106 detects and corrects variations in the switching delay times ΔQ1 to ΔQ8 when no power is being transmitted between the first bridge circuit 111 and the second bridge circuit 112.

[0046] In step S41, in an initial state in which no power transmission operation is performed between the first bridge circuit 111 and the second bridge circuit 112, the control device 106 determines which bridge circuit, the first bridge circuit 111 or the second bridge circuit 112, is to be operated (initial setting). In step S42, the control device 106 determines whether or not there is variation in the delay time between the timing at which the states of the arms of one of the bridge circuits change when only one of the bridge circuits, the first bridge circuit 111 or the second bridge circuit 112, is operated.

[0047] If the control device 106 determines that there is variation, it corrects the timing at which to change the drive pulses g1 to g8 supplied to one of the bridge circuits in accordance with the variation (step S43). The control device 106 repeats the correction in step S43 until it determines that there is no variation. Eliminating the variation prevents a voltage containing a DC component from being applied to magnetic components such as the isolation transformer 102. This makes it possible to suppress biased magnetism in the magnetic cores of the magnetic components.

[0048] If the control device 106 determines that there is no variation, it executes an operation to transmit power between the first bridge circuit 111 and the second bridge circuit 112 (step S44). The control device 106 repeats the power transmission operation until it receives a command to stop operation (step S45).

[0049] If the above-described correction to reduce delay time variations is performed while power is being transmitted between the first bridge circuit 111 and the second bridge circuit 112, and the correction is not completed in time, an overcurrent may flow, damaging the device or causing an overcurrent trip and resulting in a shutdown. Limiting the amount of power transmission to prevent such problems would impair the performance of power transmission, which is the primary role of a DC / DC converter. Furthermore, configuring a high-speed, high-precision current detection system and control system to prevent such problems would result in excessively high costs.

[0050] However, the control device 106 according to the present disclosure detects and corrects variations in switching delay time when power transmission is not in progress, thereby enabling power to be transmitted without overcurrent and preventing device failure or operational shutdown due to overcurrent tripping.

[0051] Fig. 5 is a diagram showing a specific example of a control sequence of the power conversion system according to the first embodiment. The control sequence in Fig. 5 is an example in which a power supply is connected to the input side of the first bridge circuit 111, and the operation of correcting the variation in the switching timing of each bridge circuit is performed using the power charged in the capacitor C1 or the capacitor C2.

[0052] In step S11, in the initial state where no power transmission operation is performed between the first bridge circuit 111 and the second bridge circuit 112, the control device 106 determines which bridge circuit, the first bridge circuit 111 or the second bridge circuit 112, is to operate (initial setting).

[0053] In step S13, the control device 106 charges the capacitor C1 to a given voltage from an external device connected to the primary side via a charging circuit including the switch Sc and the resistor Rc. In step S15, in the initial state where no power is being transferred between the first bridge circuit 111 and the second bridge circuit 112, the control device 106 operates only the first bridge circuit 111 with the contactor S2 cut off.

[0054] In steps S17 and S19, the control device 106 operates only the first bridge circuit 111 to correct variations in the switching timing of the first bridge circuit 111. At this time, the control device 106 either disconnects at least one of the contactor S1 and the contactor S2, or shorts the secondary winding 32 of the isolation transformer 102 by the second bridge circuit 112. As a result, power is not transmitted between the primary-side DC terminal and the secondary-side DC terminal of the isolated DC / DC converter 110.

[0055] After completing the detection and correction of the variation in the switching timing of the first bridge circuit 111, the control device 106 executes the process of step S21. The control device 106 charges the capacitor C1 to the voltage of the primary-side external device (a voltage higher than the arbitrary voltage in step S13) via a charging circuit including the switch Sc and the resistor Rc. After charging the capacitor C1 to the voltage of the primary-side external device, the control device 106 closes (turns on) the contactor S1.

[0056] In step S23, the control device 106 charges the capacitor C2 to a given voltage using the voltage charged in the capacitor C1 via the first bridge circuit 111 and the second bridge circuit 112. In step S25, the control device 106 operates only the second bridge circuit 112 while keeping the contactor S2 open.

[0057] In steps S27 and S29, the control device 106 operates only the second bridge circuit 112 to correct variations in the switching timing of the second bridge circuit 112. At this time, the control device 106 either disconnects at least one of the contactors S1 and S2, or shorts the primary winding 31 of the isolation transformer 102 with the first bridge circuit 111. As a result, power is not transmitted between the primary-side DC terminal and the secondary-side DC terminal of the isolated DC / DC converter 110.

[0058] After completing the detection and correction of the variation in the switching timing of the second bridge circuit 112, the control device 106 executes the process of step S31. The control device 106 charges the capacitor C2 to the voltage of the secondary-side external device (a voltage higher than the arbitrary voltage in step S23) using the voltage charged in the capacitor C1 via the first bridge circuit 111 and the second bridge circuit 112. After charging the capacitor C2 to the voltage of the secondary-side external device, the control device 106 closes (turns on) the contactors S1 and S2.

[0059] The control device 106 starts the power transmission operation, which is the original role of the isolated DC / DC converter 110 (step S33), and repeats the power transmission operation until a command to stop operation is received (step S35).

[0060] The order of correcting the first bridge circuit 111 and the second bridge circuit 112 may be reversed, and the control device 106 may correct the first bridge circuit 111 after completing the correction of the second bridge circuit 112.

[0061] The control device 106 may correct the variation in the switching timing of each bridge circuit by using power from an external power supply connected to the input side of the bridge circuit. For example, the control device 106 charges the secondary-side capacitor C2 using a charging circuit provided on the secondary side to correct the second bridge circuit 112. Thereafter, the control device 106 may charge the capacitor C1 using the charge stored in the capacitor C2 to correct the first bridge circuit 111.

[0062] 6 is a timing chart showing an example of a circuit operation for detecting variations in the switching delay time of the first bridge circuit 111. During this operation, the control device 106 may short-circuit the secondary winding 32 of the isolation transformer 102 by the second bridge circuit 112.

[0063] The delay times from when the drive pulses g1 to g4 change from on to off until when the state of the switching elements Q1 to Q4 changes from on to off are denoted by ΔQ1 to ΔQ4. At this time, the direct current component DC(V1) included in the output voltage V1 of the first bridge circuit 111 is expressed by Equation 1.

[0064] DC(V1)=E1 / T((ΔQ1-ΔQ2)-(ΔQ3-ΔQ4)) Equation 1 The amount of change ΔDC(Iac1) in one switching cycle of the direct current component DC(Iac1) of the alternating current Iac1 flowing through the primary winding 31 of the isolation transformer 102 is given by Equation 2.

[0065] ΔDC(Iac1)=E1 / L((ΔQ1-ΔQ2)-(ΔQ3-ΔQ4)) Equation 2 Here, L represents the inductance of the current path from intermediate node a1 to intermediate node b1 of the first bridge circuit 111, including the isolation transformer 102. More specifically, when intermediate node a2 and intermediate node b2 of the second bridge circuit 112 are open-circuited, L represents the sum of the excitation inductance of the isolation transformer 102 and the inductance of the reactor 104a. When intermediate node a2 and intermediate node b2 of the second bridge circuit 112 are short-circuited, L represents the leakage inductance of the isolation transformer 102 and the sum of the inductances of the reactors 104a and 104b.

[0066] Therefore, by detecting the amount of change ΔDC(Iac1), the control device 106 can detect the variation Tx1 in the delay time of the timing at which the switch state on the primary side switches from on to off. Tx1 is the relative difference in the variations of the arms of leg 11 and leg 12, as shown in Equation 3.

[0067] Tx1=(ΔQ1-ΔQ2)-(ΔQ3-ΔQ4) Equation 3 The control device 106 performs a similar operation for the second bridge circuit 112. The control device 106 detects a change amount ΔDC (Iac2) in one switching cycle of the direct current component DC (Iac2) of the alternating current Iac2 in the secondary winding 32 of the isolation transformer 102. By detecting the change amount ΔDC (Iac2), the control device 106 detects a variation Tx2 in the delay time at which the switch state of each arm of the second bridge circuit 112 switches from on to off.

[0068] FIG. 7 is a diagram showing an example of a generation circuit that generates drive pulses. As a representative example, FIG. 7 illustrates a generation circuit that generates drive pulses g1 and g2 for leg 11. The generation circuits for drive pulses for legs 12, 13, and 14 are similar. The generation circuit generates a carrier wave CarA by adding a reference carrier wave to a phase command value. The generation circuit generates drive pulses g1 and g2 by comparing a command value for the switching duty ratio DutyA of arms Q1 and Q2 with carrier wave CarA. The generation circuit generates drive pulses g1 and g2 that switch with a dead time in which both arms Q1 and Q2 are in an off state.

[0069] 8 shows example operating waveforms of drive pulses g1 to g4 for legs 11 and 12. Similar examples are shown for legs 13 and 14. The generating circuit compares the carrier wave with the comparison signal (duty ratio) for each leg, and determines whether to turn on or off according to the comparison results as follows:

[0070] g1 = on, g2 off (CarA ≦ DutyA) g1 = off, g2 on (CarA > DutyA) g3 = on, g4 off (CarB ≦ DutyB) g3 = off, g4 on (CarB > DutyB) g5 = on, g6 off (CarC ≦ DutyC) g5 = off, g6 on (CarC > DutyC) g7 = on, g8 off (CarD ≦ DutyD) g7 = off, g8 on (CarD > DutyD)

[0071] Normally, the duty ratio is set to 50% to prevent DC components from being superimposed on the inverter output voltage. The control device 106 corrects (adjusts) this duty ratio based on Tx1 and Tx2 as follows, thereby making it possible to make the DC components contained in the output voltages V1 and V2 zero.

[0072] DutyA=50%-(Tx1 / 2T)×100% DutyB=50%+(Tx1 / 2T)×100% DutyC=50%-(Tx2 / 2T)×100% DutyD=50%+(Tx2 / 2T)×100%

[0073] 9 is a timing chart showing a modified example of the circuit operation for detecting the variation in the switching delay time of the first bridge circuit, which differs from the operation example shown in FIG. 6 in that the bridge circuit is operated intermittently.

[0074] The greater the variation in switching delay, the greater the time increase rate of the DC current component of the current flowing through the isolation transformer. Therefore, in the continuous operation shown in FIG. 6 , an overcurrent may occur during the detection operation of the switching timing delay variations Tx1 and Tx2. Furthermore, if the operation time is shortened to prevent an overcurrent, the DC component contained in the AC current cannot be detected in time, and Tx1 and Tx2 cannot be detected correctly. To prevent an overcurrent while correctly detecting Tx1 and Tx2, it is necessary to shorten the detection time of the DC component contained in the AC current. To shorten the detection time of the DC component contained in the AC current, it is necessary to use a current detector with good responsiveness or to set the sampling speed of the control device sufficiently higher than the switching frequency, which results in higher costs.

[0075] The control device 106 detects variations in delay time while intermittently operating the first bridge circuit 111 or the second bridge circuit 112. As shown in FIG. 9 , the control device 106 intermittently operates the first bridge circuit 111 so that periods in which the output voltage V1 is zero voltage occur intermittently, thereby reducing the time increase rate of the DC component of the AC current Iac1. This allows the DC component of the AC current Iac1 to increase more slowly, preventing overcurrent even if the operation time is extended to accurately detect Tx1 and Tx2. This eliminates the constraint on the detection time of the DC component in the AC current detection unit that detects the AC current Iac1, thereby preventing excessive cost increases in the AC current detection unit that detects the AC current Iac1. Similarly, the control device 106 intermittently operates the second bridge circuit 112 so that periods in which the output voltage V2 is zero voltage occur intermittently, thereby reducing the time increase rate of the DC component of the AC current Iac2. Therefore, it is possible to prevent the AC current detection unit that detects the AC current Iac2 from becoming excessively expensive.

[0076] The present invention is not limited to the above-described embodiments. The above-described embodiments can be embodied in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are also included in the inventions described in the claims and their equivalents.

[0077] This international application claims priority based on Japanese Patent Application No. 2024-131194, filed on August 7, 2024, the entire contents of which are incorporated herein by reference.

[0078] 33 Magnetic core 100 Power conversion system 102 Isolation transformer 106 Control device 110 Isolation DC / DC converter 111 First bridge circuit 112 Second bridge circuit

Claims

1. A power conversion system comprising: an isolation transformer having a magnetic core; a first bridge circuit connected to the primary side of the isolation transformer and having a plurality of switching elements; a second bridge circuit connected to the secondary side of the isolation transformer and having a plurality of switching elements; a control device that generates a first drive pulse to be supplied to the first bridge circuit and a second drive pulse to be supplied to the second bridge circuit; a first drive circuit that drives the first bridge circuit based on the first drive pulse; and a second drive circuit that drives the second bridge circuit based on the second drive pulse, wherein the control device operates only one of the first bridge circuit and the second bridge circuit when power is not being transmitted between the first bridge circuit and the second bridge circuit, thereby detecting variations in delay time in the timing at which the state of each of the switching elements in the one bridge circuit changes, and correcting the timing at which the drive pulse to be supplied to the one bridge circuit, out of the first drive pulse and the second drive pulse, is changed in accordance with the variations.

2. The power conversion system according to claim 1, wherein the control device detects the variation by detecting the DC component of the current flowing through the isolation transformer when a voltage is applied to the isolation transformer from one of the bridge circuits.

3. The power conversion system according to claim 1 or 2, wherein the control device detects the variation while intermittently operating the first bridge circuit or the second bridge circuit.

4. The power conversion system according to claim 1 or 2, wherein the control device adjusts the duty ratio of the drive pulse in accordance with the variation.

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