Power conversion system

Abstract

To improve power conversion efficiency.SOLUTION: An isolation transformer, a first bridge circuit connected to a primary side of the isolation transformer, a second bridge circuit connected to a secondary side of the isolation transformer, and a control device that executes volts alternating current control in a discontinuous current mode for controlling a period during which both phases on the primary side and the secondary side of the isolation transformer are zero voltage, when power is transmitted from one bridge circuit of the first bridge circuit and the second bridge circuit to the other bridge circuit in the discontinuous current mode, the control device turns off the upper and lower arms of any one leg of the other bridge circuit among the first leg and the second leg of the first bridge circuit and the third leg and the fourth leg of the second bridge circuit, and alternately switches the upper and lower arms of each of the remaining legs with a dead time interposed therebetween, the dead time being a time during which the upper and lower arms of each of the remaining legs are turned off.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, there is known a power conversion system including an isolation transformer, a first bridge circuit connected to the primary side of the isolation transformer, a second bridge circuit connected to the secondary side of the isolation transformer, and a control device that performs discontinuous current mode phase control that controls a period during which both AC voltages on the primary side and secondary side of the isolation transformer become zero voltage (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2024-082491 Summary of the Invention [Problem to be solved by the invention]

[0004] The discontinuous current mode includes a period in which both the AC voltages on the primary and secondary sides of the isolation transformer are zero. Therefore, ideally, reverse power, which is transmitted in the opposite direction to the power transmission direction determined by the phase relationship between these AC voltages, does not occur. However, when switching the upper and lower arms in each bridge circuit, a dead time is actually provided in which both the upper and lower arms are turned off to prevent shoot-through current from flowing through the upper and lower arms. This dead time can cause the AC current flowing through the isolation transformer in the opposite direction to the ideal direction, reducing the power conversion efficiency of the power conversion system.

[0005] An object of the present disclosure is to improve power conversion efficiency. [Means for solving the problem]

[0006] The present disclosure provides: An isolation transformer, a first bridge circuit connected to a primary side of the isolation transformer and including a first leg and a second leg connected in parallel, the first leg and second leg including upper and lower arms connected in series; a second bridge circuit connected to the secondary side of the isolation transformer and including a third leg and a fourth leg in parallel, each of which includes an upper arm and an lower arm connected in series; a control device that performs discontinuous current mode phase control to control a period during which both AC voltages on the primary side and the secondary side of the isolation transformer are zero voltage, The control device, when transmitting power from one of the first bridge circuit and the second bridge circuit to the other bridge circuit in the discontinuous current mode, turns off the upper and lower arms of any one of the other bridge circuits among the first leg to the fourth leg, and alternately switches the upper and lower arms of each of the remaining legs with a dead time in between during which the upper and lower arms of each of the remaining legs are turned off. [Effects of the Invention]

[0007] According to the present disclosure, power conversion efficiency can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a circuit diagram showing a configuration example of a power conversion system according to a first embodiment. [Figure 2] FIG. 3 is a waveform diagram showing an example of an operation of phase control in a discontinuous current mode executed in the power conversion system according to the first embodiment. [Figure 3] FIG. 10 is an operational waveform diagram of a basic switching pattern of phase control in discontinuous current mode. [Figure 4] FIG. 1 is a diagram showing a current path for each period in a basic switching pattern of discontinuous current mode phase control. [Figure 5] FIG. 1 is a diagram showing a current path for each period in a basic switching pattern of discontinuous current mode phase control. [Figure 6] FIG. 10 is an operational waveform diagram in a first switching pattern of phase control in discontinuous current mode. [Figure 7] FIG. 10 is a diagram showing a current path in each period in a first switching pattern of phase control in discontinuous current mode. [Figure 8] FIG. 10 is a diagram showing a current path in each period in a first switching pattern of phase control in discontinuous current mode. [Figure 9] FIG. 1 is a diagram showing a current path for each period in a basic switching pattern of discontinuous current mode phase control. [Figure 10] FIG. 10 is an operational waveform diagram in the second switching pattern of phase control in discontinuous current mode. [Figure 11] FIG. 1 is a diagram showing a current path for each period in a basic switching pattern of discontinuous current mode phase control. [Figure 12] FIG. 10 is an operational waveform diagram in the third switching pattern of phase control in discontinuous current mode. [Figure 13] FIG. 1 is a diagram showing a current path for each period in a basic switching pattern of discontinuous current mode phase control. [Figure 14] FIG. 10 is an operational waveform diagram in the fourth switching pattern of phase control in discontinuous current mode. [Figure 15] FIG. 10 is a waveform diagram showing an example of the overall operation of the power conversion system according to the second embodiment. [Figure 16] FIG. 10 is a diagram illustrating an example of power transmission characteristics in the power conversion system according to the second embodiment. [Figure 17] FIG. 10 is a diagram for explaining a method for specifying a leg to be turned off to prevent a reverse current. DETAILED DESCRIPTION OF THE INVENTION

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

[0010] First Embodiment FIG. 1 is a diagram showing a configuration example 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 the first bridge circuit 111 and the 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 has a primary winding 31 and a secondary winding 32, and is a transformer in which the primary winding 31 and the secondary winding 32 are magnetically coupled. The turns ratio of the primary winding 31 and the secondary winding 32 is set as appropriate.

[0013] In this specification, unless otherwise specified, the turns ratio of the primary winding 31 and the secondary winding 32 may be considered to be 1:1. However, when the turns ratio of 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 into 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 into 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 mean the voltage values converted to the primary side. That is, when the number of turns of the primary winding 31 of the isolation transformer 102 is n1 and the number of turns of the secondary winding 32 is n2, the voltage value obtained by multiplying the actual secondary-side DC voltage by the coefficient n1 / n2 (the voltage value converted to the primary side) is the secondary-side DC voltage E2. The same applies to the secondary-side AC voltage v2. Also, in the following description, the low-voltage side and the high-voltage side mean the side where low voltage is generated and the side where high voltage is generated among the primary side and the secondary side of the isolated DC / DC converter 110. For example, if there is a relationship of E1 < E2 between the primary-side DC voltage E1 and the secondary-side DC voltage E2 converted to the primary-side value, the primary side is the low-voltage side and the secondary side is the high-voltage side, and if there is a relationship of E1 > E2, 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 101a and a low-side arm 101b are connected in series, and a leg 12 in which a high-side arm 101c and a low-side arm 101d are connected in series. The arm 101a is an example of a first arm, the arm 101b is an example of a second arm, the arm 101c is an example of a third arm, and the arm 101d 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 101a and 101b and an intermediate connection point b1 between the arms 101c and 101d. 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 103a and arms 101a to 101d.

[0020] The capacitor 103a 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 103a).

[0021] The arms 101a to 101d 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] The leg 11 includes a configuration in which the arms 101a and 101b are connected in series between the pair of DC buses 43p and 43n, and the leg 12 includes a configuration in which the arms 101c and 101d are connected in series between the pair of DC buses 43p and 43n. Each of the arms 101a to 101d 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. The arms 101a to 101d may include a diode connected inversely between the main terminals. If the arms 101a to 101d are MOSFETs, the diode may be a parasitic diode. FIG. 1 illustrates freewheeling diodes D1, D2, D3, and D4.

[0023] When the arms 101a and 101d are turned on and the arms 101b and 101c are turned off, the first bridge circuit 111 electrically connects the intermediate node a1 to the positive bus 43p and electrically connects the intermediate node b1 to the negative bus 43n. As a result, the first bridge circuit 111 sets the voltage v1 between the intermediate node a1 and the intermediate node b1 to a positive voltage "E1." E1 is the voltage value between the pair of DC buses 43p and 43n. When the arms 101a and 101d are turned off and the arms 101b and 101c are turned on, the first bridge circuit 111 electrically connects the intermediate node a1 to the negative bus 43n and electrically connects the intermediate node 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 101a and 101c are turned on and the arms 101b and 101d 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 101a and 101c are turned off and the arms 101b and 101d 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 43n 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 101e and a low-side arm 101f are connected in series, and a leg 14 in which a high-side arm 101g and a low-side arm 101h are connected in series. The arm 101e is an example of a fifth arm, the arm 101f is an example of a sixth arm, the arm 101g is an example of a seventh arm, and the arm 101h 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 101e and 101f and an intermediate connection point b2 between the arms 101g and 101h. The second bridge circuit 112 may include a reactor 104b in the bridge portion 23 that is connected in series to 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 103b and arms 101e to 101h.

[0031] The capacitor 103b 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 103b).

[0032] The arms 101e to 101h are secondary-side switching elements, and specific examples thereof include semiconductor switching elements such as MOSFETs and IGBTs, similar to the arms 101a to 101d.

[0033] The leg 13 includes a configuration in which the arms 101e and 101f are connected in series between the pair of DC buses 44p and 44n, and the leg 14 includes a configuration in which the arms 101g and 101h are connected in series between the pair of DC buses 44p and 44n. Similar to the arms 101a to 101d, the arms 101e to 101h each have a first main terminal, a second main terminal, a control terminal, and a diode. Fig. 1 illustrates freewheeling diodes D5, D6, D7, and D8.

[0034] When the arms 101e and 101h are turned on and the arms 101f and 101g 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 between the intermediate node a2 and the intermediate node b2 to a positive voltage "E2." E2 is the voltage value between the pair of DC buses 44p and 44n. When the arms 101e and 101h are turned off and the arms 101f and 101g 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 101e and 101g are turned on and the arms 101f and 101h are turned off, the second bridge circuit 112 electrically connects both the intermediate node a2 and the intermediate node b2 to the positive bus 44p, thereby making the voltage v2 substantially zero. When the arms 101e and 101g are turned off and the arms 101f and 101h are turned on, the second bridge circuit 112 electrically connects both the intermediate node a2 and the intermediate node b2 to the negative bus 43n, thereby making the voltage v2 substantially zero.

[0036] The control device 106 controls the first bridge circuit 111 and the second bridge circuit 112. The control device 106 generates drive pulses Ga to Gd for driving the arms 101a to 101d of the first bridge circuit 111, respectively, and drive pulses Ge to Gh for driving the arms 101e to 101h of the second bridge circuit 112, respectively. The drive pulses Ga to Gh are drive signals for controlling the on / off of corresponding arms among the arms 101a to 101h.

[0037] 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 that controls the on / off switching of the arms 101a to 101d of the first bridge circuit 111 in accordance with drive pulses Ga to Gd. The drive circuit 105b is a secondary side drive circuit that controls the on / off switching of the arms 101e to 101h of the second bridge circuit 112 in accordance with drive pulses Ge to Gh.

[0038] 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.

[0039] The control device 106 controls the phases of the edges of the drive pulses Ga to Gh for driving the arms 101 a to 101 h, 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 Ga to Gh, thereby controlling the power transmission of the isolated DC / DC converter 110 (power transmission between the first bridge circuit 111 and the second bridge circuit 112).

[0040] The control device 106 controls the phase of the edge of the drive pulse to perform discontinuous current mode phase control, which controls the period during which both the primary-side AC voltage v1 and the secondary-side AC voltage v2 of the isolation transformer 102 are zero voltage. By generating a period during which both the primary-side AC voltage v1 and the secondary-side AC voltage v2 are zero voltage, the control device 106 avoids the generation of power (reverse power) transmitted in the opposite direction to the power transmission direction determined by the phase relationship between the primary-side AC voltage v1 and the secondary-side AC voltage v2.

[0041] FIG. 2 is a waveform diagram showing an example of the operation of the discontinuous current mode phase control executed in the power conversion system according to this embodiment.

[0042] In the power conversion system 100, the first bridge circuit 111 and the second bridge circuit 112 output a primary-side AC voltage v1 and a secondary-side AC voltage v2 having the same cycle (wavelength). In the following description, the terms phase angle and phase difference are used, and the phase angle and phase difference refer to the relative lengths of various periods, where one cycle (one wavelength) of the primary-side AC voltage v1 and the secondary-side AC voltage v2 is expressed as 2π.

[0043] In the operation example of FIG. 2, the low-voltage side primary-side AC voltage v1 changes from zero voltage to a voltage of a first polarity (positive polarity in this example), and then, after a period of phase angle θ-γ, the high-voltage side secondary-side AC voltage v2 changes from zero voltage to a voltage of the first polarity. Then, the primary-side AC voltage v1 and the secondary-side AC voltage v2 maintain the first polarity for a period of phase angle π-θ. Then, the primary-side AC voltage v1 and the secondary-side AC voltage v2 simultaneously change from the first polarity to zero voltage and maintain the zero voltage for a period of phase angle γ. Then, the primary-side AC voltage v1 changes from zero voltage to a voltage of a second polarity (negative polarity in this example), which is opposite to the first polarity, and then, after a period of phase angle φ-γ, the secondary-side AC voltage v2 changes from zero voltage to a voltage of the second polarity. Then, the primary-side AC voltage v1 and the secondary-side AC voltage v2 maintain the second polarity for a period of phase angle π-θ. Then, the primary-side AC voltage v1 and the secondary-side AC voltage v2 simultaneously change from a voltage of the second polarity to zero voltage and maintain zero voltage for a period of phase angle γ. Then, the primary-side AC voltage v1 changes from zero voltage to a voltage of the first polarity.

[0044] 2, the current i flowing through the primary winding 31 of the isolation transformer 102 increases during a period of phase angle φ-γ when the primary AC voltage v1 is a voltage of the first or second polarity and the secondary AC voltage v2 is zero. Thereafter, during a period when both the primary AC voltage v1 and the secondary AC voltage v2 are voltages of the same polarity, the current i flowing through the primary winding 31 of the isolation transformer 102 decreases and reaches zero, and this operation is repeated.

[0045] Then, during the period of phase angle γ after the current i flowing through the isolation transformer 102 decreases and becomes zero, both the primary side AC voltage v1 and the secondary side AC voltage v2 become zero voltage, and the current i flowing through the primary winding 31 of the isolation transformer 102 stops changing. As a result, polarity reversal, in which the direction of flow of the current i is reversed, does not occur, and therefore the generation of power transmitted in the opposite direction to the power transmission direction determined by the phase relationship between the primary side AC voltage v1 and the secondary side AC voltage v2 (reverse power) is avoided.

[0046] In the operation example of FIG. 2, the power P transmitted from the primary side to the secondary side depends on the peak value of the current i flowing through the isolation transformer 102. The peak value of the current i depends on the length of the period of the phase angle φ - γ. The control device 106 controls the period of the phase angle γ during which both the AC voltages on the primary and secondary sides of the isolation transformer 102 become zero voltages, and controls the phase angles φ and γ, thereby controlling the transmission power P between both the primary and secondary sides. This transmission power P is P=(E1E2 / (ωL))(A(1 - A)(π - γ) 2 ) / (2π) ··· Equation 1 represented by. A is A = low - voltage - side DC voltage / high - voltage - side DC voltage < 1 ··· Equation 2 represented by.

[0047] By the way, as described above, since the discontinuous current mode generates a period during which both the primary - side AC voltage v1 and the secondary - side AC voltage v2 become zero voltages, the reverse power transmitted in the direction opposite to the power transmission direction determined by the phase relationship of those AC voltages ideally does not occur. However, when switching the upper and lower arms of each leg, a dead time is provided in which both the upper and lower arms are in the off state for the purpose of preventing the generation of through - currents flowing through the upper and lower arms. Due to this dead time, a pulse dropout of the primary - side AC voltage v1 occurs, and the AC current i may flow in the direction opposite to the ideal direction. This current flowing in the reverse direction (reverse current) is reactive current that increases conduction loss, and thus the power conversion efficiency of the power conversion system 100 may decrease.

[0048] FIG. 3 is an operation waveform diagram in the basic switching pattern of phase control in the discontinuous current mode. FIG. 3 shows the waveforms during the power - transmission operation in which power is transmitted from the first bridge circuit 111 to the second bridge circuit 112 when E1 < E2.

[0049] When the control device 106 is performing phase control in discontinuous current mode, it alternately switches the upper and lower arms of each of the legs 11 to 14, with dead times in between, in which the upper and lower arms are in an off state, in accordance with the driving pulses Ga to Gh having the switching pattern shown in FIG.

[0050] In this example, when the drive pulses Ga to Gh are at a high level, the corresponding arms are in an ON state, and when the drive pulses Ga to Gh are at a low level, the corresponding arms are in an OFF state (the same applies to the other drawings).

[0051] In an ideal case where the upper and lower arms are switched simultaneously without dead time (see the dashed line in Figure 3), there is no pulse dropout in the primary AC voltage v1, so the peak value (absolute value) of the current i flowing through the isolation transformer 102 rises to a first level and then begins to decrease. However, when dead time exists (see the solid line in Figure 3), pulse dropout occurs in the primary AC voltage v1, and the peak value (absolute value) of the AC current i rises to a second level lower than the first level before rising to the first level, and then begins to decrease. If the peak value (absolute value) of the current i does not rise to the first level, as shown in Figure 3, the AC current i may flow in the opposite direction (reverse current) to the ideal direction.

[0052] Fig. 4 is a diagram showing the current path in each period in the basic switching pattern of discontinuous current mode phase control. The arrows in Fig. 4 represent the path of the transmission current flowing between the first bridge circuit 111 and the second bridge circuit 112 in each period T1 to T5 in Fig. 3. In Fig. 4, "ON" added to each arm 101a to 101h means that the arm is in the on state, and "OFF" added to each arm 101a to 101h means that the arm is in the off state (the same applies to other drawings showing current paths).

[0053] During the period T1, the arms 101a and 101d are in the ON state, so the primary AC voltage v1 is a positive voltage "E1," and the arms 101e and 101g are in the ON state, so the secondary AC voltage v2 is zero. As a result, the current flowing in the forward direction (first direction) through the primary winding 31 increases.

[0054] When the period T1 transitions to the period T2, the circuit enters a dead time in which both the arm 101g and the arm 101h are in the OFF state, and the path of the current flowing on the secondary side switches from the arm 101g to the diode in the arm 101h. Therefore, during the period T2, the current flowing on the secondary side passes through the arm 101e via the diode in the arm 101h, and the secondary-side AC voltage v2 becomes a positive voltage "E2." On the other hand, during the period T2, the primary-side AC voltage v1 is maintained at the positive voltage "E1," and the current flowing in the forward direction through the primary winding 31 decreases.

[0055] When the period T2 transitions to the period T3, the arm 101h turns on, and the dead time in which both the arm 101g and the arm 101h are in the off state ends. Since the arm 101h, through whose diode a current flowed during the period T2, turns on, the primary side AC voltage v1 and the secondary side AC voltage v2 change very little during the transition from the period T2 to the period T3.

[0056] The decrease in current i flowing in the positive direction through primary winding 31 continues during period T3. Current i crosses zero at the timing of the transition from period T3 to period T4. At this zero crossing, the direction of current i reverses, and the current flowing through primary winding 31 changes from positive to negative. During period T4, current i flowing in the negative direction through primary winding 31 (reverse current) increases.

[0057] When the period T4 transitions to the period T5, a dead time occurs in which both the arms 101e and 101f are in the OFF state, and the path of the current flowing on the secondary side switches from the arm 101e to the diode in the arm 101f. Therefore, during the period T5, the current flowing on the secondary side passes through the arm 101h and the diode in the arm 101f, and the secondary-side AC voltage v2 becomes zero. On the other hand, when the period T4 transitions to the period T5, a dead time occurs in which both the arms 101c and 101d are in the OFF state, and the path of the current flowing on the primary side switches from the arm 101d, which is in the ON state, to the diode in the arm 101d. During the period T5, the current flowing on the primary side passes through the arm 101a via the diode in the arm 101d, and therefore the primary-side AC voltage v1 is maintained at the positive voltage "E1" during the period T5. Therefore, the current i flowing in the negative direction (reverse current) in the primary winding 31 decreases toward zero.

[0058] 5 is a diagram showing the current path in each period in the basic switching pattern of phase control in discontinuous current mode. The arrows in Fig. 5 represent the path of the transmission current flowing between the first bridge circuit 111 and the second bridge circuit 112 in each period T6 to T10 in Fig. 3.

[0059] During period T6, the arms 101b and 101c are in the ON state, so the primary AC voltage v1 is a negative voltage "-E1," and the arms 101f and 101h are in the ON state, so the secondary AC voltage v2 is zero. As a result, the current flowing in the negative direction (a second direction opposite to the first direction) through the primary winding 31 increases.

[0060] When the period T6 transitions to the period T7, a dead time occurs in which both the arms 101g and 101h are in the off state, and the path of the current flowing on the secondary side switches from the arm 101h to the diode in the arm 101g. Therefore, during the period T7, the current flowing on the secondary side passes through the diode in the arm 101g via the arm 101f, and the secondary-side AC voltage v2 becomes the negative voltage "-E2." On the other hand, during the period T7, the primary-side AC voltage v1 is maintained at the negative voltage "-E1," and the current flowing in the negative direction through the primary winding 31 decreases.

[0061] When the period T7 transitions to the period T8, the arm 101g turns on, and the dead time in which both the arm 101g and the arm 101h are in the off state ends. Since the arm 101g, through whose diode a current had flowed during the period T7, turns on, the primary side AC voltage v1 and the secondary side AC voltage v2 change very little during the transition from the period T7 to the period T8.

[0062] The decrease in current i flowing in the negative direction through primary winding 31 continues during period T8. Current i crosses zero at the timing of the transition from period T8 to period T9. At this zero crossing, the direction of flow of current i reverses, and the current flowing through primary winding 31 changes from negative to positive. During period T9, current i flowing in the positive direction through primary winding 31 (reverse current) increases.

[0063] When the period T9 transitions to the period T10, a dead time is entered in which both the arms 101e and 101f are in the OFF state, and the path of the current flowing on the secondary side switches from the arm 101f to the diode in the arm 101e. Therefore, during the period T10, the current flowing on the secondary side passes through the arm 101g via the diode in the arm 101e, and the secondary-side AC voltage v2 becomes zero. On the other hand, when the period T9 transitions to the period T10, a dead time is entered in which both the arms 101c and 101d are in the OFF state, and the path of the current flowing on the primary side switches from the arm 101c, which is in the ON state, to the diode in the arm 101c. During the period T10, the current flowing on the primary side passes through the arm 101b via the diode in the arm 101c, and therefore the primary-side AC voltage v1 is maintained at the negative voltage "-E1" during the period T10. Therefore, the current i flowing in the forward direction through the primary winding 31 (reverse current) decreases toward zero.

[0064] As described above, in the basic switching patterns of discontinuous current mode phase control (FIGS. 3, 4, and 5), a reverse current occurs due to pulse missing in the primary side AC voltage v1, which may reduce the power conversion efficiency of the power conversion system 100.

[0065] The control device 106 according to the present disclosure executes phase control in discontinuous current mode according to a modified switching pattern different from the basic switching pattern, thereby avoiding the generation of reverse current and improving the power conversion efficiency of the power conversion system 100. For example, consider the case where the control device 106 transmits power from one of the first bridge circuit 111 and the second bridge circuit 112 to the other in discontinuous current mode. In this case, the control device 106 turns off the upper and lower arms of any one leg of the other bridge circuit among legs 11 to 14, and sandwiches a dead time during which the upper and lower arms of the remaining each leg are turned off, and alternately switches the upper and lower arms of the remaining each leg. The control device 106 rectifies the other bridge circuit by maintaining one side leg of the other bridge circuit (the bridge circuit that receives power) in an off state. The control device 106 thus executes phase control in discontinuous current mode according to a modified switching pattern in which one side leg of the other bridge circuit is turned off, thereby avoiding the generation of reverse current and improving the power conversion efficiency of the power conversion system 100.

[0066] FIG. 6 is an operation waveform diagram in the first switching pattern of phase control in discontinuous current mode. FIG. 6 shows waveforms during the power transmission operation of transmitting power from the first bridge circuit 111 to the second bridge circuit 112 when E1 < E2. The first switching pattern is an example of the above-described modified switching pattern.

[0067] When the control device 106 is executing phase control in discontinuous current mode, the control device 106 turns off the upper and lower arms (arm 101e and arm 101f) of leg 13 according to the drive pulses Ge and Gf of the first switching pattern shown in FIG. 6. On the other hand, when the control device 106 is executing phase control in discontinuous current mode, the control device 106 sandwiches a dead time during which the upper and lower arms of the remaining each leg 11, 12, 14 are turned off, and alternately switches the upper and lower arms according to the drive pulses Ga to Gd, Gg, and Gh of the first switching pattern shown in FIG. 6.

[0068] 7 is a diagram showing the current path in each period in the first switching pattern of phase control in discontinuous current mode. The arrows in Fig. 7 represent the path of the transmission current flowing between the first bridge circuit 111 and the second bridge circuit 112 in each period T1 to T4 in Fig. 6.

[0069] During period T1, arms 101a and 101d are in the ON state, so primary AC voltage v1 is a positive voltage "E1." Meanwhile, during period T1, arm 101e is in the OFF state and arm 101g is in the ON state, so the current flowing on the secondary side passes through arm 101g via the diode in arm 101e. Therefore, secondary AC voltage v2 becomes zero voltage. Therefore, the current flowing in the forward direction (first direction) through primary winding 31 increases.

[0070] When the period T1 transitions to the period T2, the circuit enters a dead time in which both the arms 101g and 101h are in the off state, and the path of the current flowing on the secondary side switches from the arm 101g to the diode in the arm 101h. Therefore, during the period T2, the current flowing on the secondary side passes through the diode in the arm 101e via the diode in the arm 101h, and the secondary-side AC voltage v2 becomes the positive voltage "E2" (strictly speaking, it is a voltage that is lower than the positive voltage "E2" by the amount of the forward voltages of the diodes in the arms 101h and 101e). On the other hand, during the period T2, the primary-side AC voltage v1 is maintained at the positive voltage "E1," and the current flowing in the forward direction through the primary winding 31 decreases.

[0071] When the period T2 transitions to the period T3, the arm 101h turns on, and the dead time in which both the arm 101g and the arm 101h are in the off state ends. Since the arm 101h, through whose diode a current flowed during the period T2, turns on, the primary side AC voltage v1 and the secondary side AC voltage v2 change very little during the transition from the period T2 to the period T3.

[0072] The current i flowing in the positive direction through the primary winding 31 decreases until it reaches zero and attempts to cross zero (flow in the negative direction) at the start of period T4. However, during period T4, the arms 101e and 101g are in the off state, so the current does not flow in the negative direction through the arms 101e and 101g, and the current i (reverse current) flowing in the negative direction through the primary winding 31 is prevented. During period T4, the second bridge circuit 112 is open, so the secondary-side AC voltage v2 appears at the same voltage value as the primary-side AC voltage v1.

[0073] Fig. 8 is a diagram showing the current path in each period in the first switching pattern of phase control in discontinuous current mode. The arrows shown in Fig. 5 represent the path of the transmission current flowing between the first bridge circuit 111 and the second bridge circuit 112 in each period T6 to T9 in Fig. 8.

[0074] During period T6, arms 101b and 101c are in the ON state, so the primary AC voltage v1 is a negative voltage "-E1." Meanwhile, during period T6, arm 101f is in the OFF state and arm 101h is in the ON state, so the current flowing to the secondary side passes through arm 101h and the diode in arm 101f. As a result, the secondary AC voltage v2 becomes zero voltage. Therefore, the current flowing in the negative direction (second direction opposite to the first direction) through primary winding 31 increases.

[0075] When the period T6 transitions to the period T7, the circuit enters a dead time in which both the arms 101g and 101h are in the off state, and the path of the current flowing on the secondary side switches from the diode in the arm 101h to the diode in the arm 101g. Therefore, during the period T7, the current flowing on the secondary side passes through the diode in the arm 101f via the diode in the arm 101g, and the secondary-side AC voltage v2 becomes the negative voltage "-E2" (strictly speaking, the voltage is higher than the negative voltage "-E2" by the amount of the forward voltage of the diodes in the arms 101g and 101f). Meanwhile, during the period T7, the primary-side AC voltage v1 is maintained at the negative voltage "-E1," and the current flowing in the negative direction through the primary winding 31 decreases.

[0076] When transitioning from period T7 to period T8, arm 101g turns on, so the dead time during which both arm 101g and arm 101h are off ends. Since arm 101g, through which current was flowing in the diode during period T7, turns on, the primary-side AC voltage v1 and the secondary-side AC voltage v2 hardly change during the transition from period T7 to period T8.

[0077] The current i flowing in the negative direction in the primary winding 31 decreases until it reaches zero and attempts to cross zero at the start of period T9 (attempts to flow in the positive direction in the reverse direction). However, in period T9, since arms 101f and 101h are off, the current does not flow in the reverse direction through arms 101f and 101h, and the current i (reverse current) flowing in the positive direction in the primary winding 31 is blocked. In period T9, since the second bridge circuit 112 is open, the same voltage value as the primary-side AC voltage v1 appears in the secondary-side AC voltage v2.

[0078] Thus, when E1 < E2 and during the power transmission operation of transmitting power from the first bridge circuit 111 to the second bridge circuit 112, by executing phase control in the discontinuous current mode according to the first switching pattern (Figs. 6, 7, and 8), the generation of reverse current is avoided. Therefore, the power conversion efficiency of the power conversion system 100 is improved.

[0079] Fig. 9 is an operation waveform diagram in the basic switching pattern of phase control in the discontinuous current mode. Fig. 9 shows the waveforms during the regenerative operation of transmitting power from the second bridge circuit 112 to the first bridge circuit 111 when E1 < E2. Considering the same as in the case of the basic switching pattern in Fig. 3, as shown in Fig. 9, an alternating current i may flow in a direction reverse to the ideal direction (reverse current).

[0080] Fig. 10 is an operation waveform diagram in the second switching pattern of phase control in the discontinuous current mode. Fig. 10 shows the waveforms during the regenerative operation of transmitting power from the second bridge circuit 112 to the first bridge circuit 111 when E1 < E2. The second switching pattern is an example of the above-described modified switching pattern.

[0081] When the control device 106 is performing phase control in discontinuous current mode, according to the drive pulses Gc and Gd of the second switching pattern shown in FIG. 10, the upper and lower arms (arm 101c and arm 101d) of leg 12 are turned off. On the other hand, when the control device 106 is performing phase control in discontinuous current mode, according to the drive pulses Ga, Gb, Ge to Gh of the second switching pattern shown in FIG. 10, the upper and lower arms of each of the remaining legs 11, 13, and 14 are turned off with a dead time interposed therebetween, and the upper and lower arms are alternately switched.

[0082] Since it is the same as the above case of the first switching pattern, a detailed description is omitted. By maintaining one leg (in this case, leg 12) of the bridge circuit on the power receiving side in an off state, the flow of reverse current is blocked. Thus, when performing the regeneration operation of transmitting power from the second bridge circuit 112 to the first bridge circuit 111 at E1 < E2, by executing phase control in discontinuous current mode with the second switching pattern (FIG. 10), the generation of reverse current is avoided. Therefore, the power conversion efficiency of the power conversion system 100 is improved.

[0083] FIG. 11 is an operation waveform diagram in the basic switching pattern of phase control in discontinuous current mode. FIG. 11 shows the waveform during the power running operation of transmitting power from the first bridge circuit 111 to the second bridge circuit 112 when E1 ≥ E2. Considering in the same way as in the case of the basic switching pattern of FIG. 3, as shown in FIG. 11, an alternating current i may flow a current in the reverse direction (reverse current) with respect to the ideal direction.

[0084] FIG. 12 is an operation waveform diagram in the third switching pattern of phase control in discontinuous current mode. FIG. 12 shows the waveform during the power running operation of transmitting power from the first bridge circuit 111 to the second bridge circuit 112 when E1 ≥ E2. The third switching pattern is an example of the above modified switching pattern.

[0085] When the control device 106 is performing phase control in the discontinuous current mode, it turns off the upper and lower arms (arms 101g and 101h) of leg 14 in accordance with drive pulses Gg and Gh of the third switching pattern shown in Fig. 12. On the other hand, when the control device 106 is performing phase control in the discontinuous current mode, it alternately switches the upper and lower arms of the remaining legs 11 to 13, with dead times in between, during which the upper and lower arms are turned off, in accordance with drive pulses Ga to Gf ​​of the third switching pattern shown in Fig. 12.

[0086] As this is similar to the first switching pattern described above, detailed description will be omitted. However, by maintaining one leg of the bridge circuit receiving power (leg 14 in this case) in the off state, the flow of reverse current is prevented. In this way, when E1≧E2, during power running operation in which power is transmitted from the first bridge circuit 111 to the second bridge circuit 112, the occurrence of reverse current is prevented by executing phase control in discontinuous current mode using the third switching pattern (FIG. 12). This improves the power conversion efficiency of the power conversion system 100.

[0087] Fig. 13 is an operating waveform diagram in the basic switching pattern of phase control in discontinuous current mode. Fig. 13 shows waveforms during regenerative operation in which power is transmitted from the second bridge circuit 112 to the first bridge circuit 111 when E1≧E2. Considering the same as the basic switching pattern in Fig. 3, as shown in Fig. 13, there are cases in which the AC current i flows in the opposite direction (reverse current) to the ideal direction.

[0088] Fig. 14 is an operational waveform diagram of the fourth switching pattern of phase control in discontinuous current mode. Fig. 14 shows waveforms during regenerative operation in which power is transferred from the second bridge circuit 112 to the first bridge circuit 111 when E1 ≥ E2. The fourth switching pattern is an example of the above-mentioned modified switching pattern.

[0089] When performing phase control in discontinuous current mode, the control device 106 turns off the upper and lower arms (arms 101a and 101b) of leg 11 in accordance with drive pulses Ga and Gb of the fourth switching pattern shown in Fig. 14. On the other hand, when performing phase control in discontinuous current mode, the control device 106 alternately switches the upper and lower arms of the remaining legs 12 to 14, with dead times in between, during which the upper and lower arms are turned off, in accordance with drive pulses Gc to Gh of the fourth switching pattern shown in Fig. 14.

[0090] Since this is similar to the first switching pattern described above, detailed description will be omitted. However, by maintaining one leg of the bridge circuit receiving power (leg 11 in this case) in the off state, the flow of reverse current is prevented. In this way, when E1≧E2, during regenerative operation in which power is transferred from the second bridge circuit 112 to the first bridge circuit 111, the occurrence of reverse current is prevented by executing phase control in discontinuous current mode using the fourth switching pattern (FIG. 14). This improves the power conversion efficiency of the power conversion system 100.

[0091] In this way, the control device 106 may change the leg of the bridge circuit that receives the power, the upper and lower arms of which are turned off, depending on which is higher: the primary side DC voltage E1 applied to the first bridge circuit 11 or the secondary side DC voltage E2 applied to the second bridge circuit 112.

[0092] Second Embodiment Next, a second embodiment will be described. In the second embodiment, the description of the same configuration, action, and effect as in the first embodiment will be omitted by citing the above description.

[0093] The control device 106 according to the second embodiment controls the phases of the edges of the drive pulses Ga to Gh to perform continuous current mode phase control, which controls the period (phase angle δ) during which the AC voltages on the primary and secondary sides of the isolation transformer 102 are opposite in polarity. This controls the transmission power P exchanged between the primary and secondary sides.

[0094] The control device 106 may switch between executing discontinuous current mode phase control and continuous current mode phase control depending on the magnitude of the control variable D. The control device 106 executes discontinuous current mode phase control, which controls the period during which both the primary side AC voltage v1 and the secondary side AC voltage v2 are zero voltage, by controlling the phases of the edges of the drive pulses Ga to Gh. Alternatively, the control device 106 executes continuous current mode phase control, which controls the period during which the primary side AC voltage v1 and the secondary side AC voltage v2 are opposite in polarity, by controlling the phases of the edges of the drive pulses Ga to Gh. The control device 106 controls the transmission of transmission power P exchanged between the primary side and secondary side of the isolated DC / DC converter 110 by switching between the discontinuous current mode phase control and the continuous current mode phase control depending on the magnitude of the control variable D.

[0095] The control variable D is a control variable determined according to the target transmission power. The larger the control variable D, the larger the transmission power P between the primary side and the secondary side. In a preferred embodiment, the control variable D is provided to the control device 106 from a host device that controls the power conversion system 100. In another preferred embodiment, the control variable D is determined by the control device 106 based on the operating status and load status of the isolated DC / DC converter 110.

[0096] The control variable D can take a value between 0 and 3π / 2. When the control variable D is greater than or equal to 0 and less than or equal to π, the controller 106 performs phase control in discontinuous current mode. When the control variable D is greater than or equal to π and less than or equal to 3π / 2, the controller 106 performs phase control in continuous current mode.

[0097] Fig. 15 is a waveform diagram showing an example of the overall operation of the power conversion system according to the second embodiment. Fig. 16 is a diagram illustrating the power transmission characteristics in the power conversion system according to the second embodiment. With reference to these figures, the operation of the second embodiment can be summarized as follows.

[0098] In the second embodiment, when the control variable D is within the range of 0≦D≦π, discontinuous current mode phase control is performed. In this discontinuous current mode, there is no period (phase angle δ) during which the primary side AC voltage v1 and the secondary side AC voltage v2 have opposite polarities. In the discontinuous current mode, as the control variable D increases, the period during which the AC voltages on both the low-voltage side and the high-voltage side maintain zero voltage (phase angle γ=π−D) decreases, and the period during which the AC voltage on the high-voltage side maintains zero voltage (phase angle φ=π−AD) decreases.

[0099] In discontinuous current mode, no reverse power occurs, and the transmitted power P exchanged between the primary and secondary sides is P=((E1E2) / (ωL))A(1-A)D 2 / (2π) ...Formula 3 It is expressed by:

[0100] In the discontinuous current mode, no reverse power is generated, so power can be transmitted with little loss when the DC voltage difference between the primary and secondary sides is small or when the load is light.

[0101] In the second embodiment, when the control variable D is within the range of π≦D≦3π / 2, phase control in continuous current mode is performed. In continuous current mode, there is no period (phase angle γ) during which both the primary side AC voltage v1 and the secondary side AC voltage v2 are zero voltage. In continuous current mode, as the control variable D increases, the period (phase angle δ=D-π) during which the primary side AC voltage v1 and the secondary side AC voltage v2 are opposite in polarity increases, and the period (phase angle φ=(1-A)(2π-D) during which the high-voltage side AC voltage maintains zero voltage decreases.

[0102] In the continuous current mode, power is transmitted during the period when the primary AC voltage v1 and the secondary AC voltage v2 are in opposite polarity. The transmitted power P between the primary and secondary sides is expressed as follows:

[0103] P=((E1E2) / (ωL))(a(A)D 2 +b(A)D+c(A)) / (2π) ...Formula 4 where a(A), b(A), and c(A) are as follows:

[0104] a(A)=-(1+A 2 )...Equation 5 b(A)=(4A 2 -A+3)π Equation 6 c(A)=-2(2A 2 -A+1)π 2 ...Formula 7 In this continuous current mode, a certain level of power can be transmitted over a wide voltage range, and power can be transmitted with high efficiency even under heavy loads.

[0105] As shown in Figure 16, when the control variable D is changed from 0 to 3π / 2, the power P transmitted from the primary side to the secondary side changes continuously. Furthermore, when the control variable D is π, the operation mode of the second embodiment is at the mode boundary between the discontinuous current mode and the continuous current mode. At this mode boundary, the phase angles δ, φ, and γ calculated assuming the discontinuous current mode match the phase angles δ, φ, and γ calculated assuming the continuous current mode. Therefore, the current waveform of the isolation transformer changes continuously during the transition between the discontinuous current mode and the continuous current mode.

[0106] As described above, according to the second embodiment, discontinuous current mode phase control or continuous current mode phase control is performed depending on the magnitude of one control variable D, so that power can be transmitted between the primary side and the secondary side with high efficiency over a wide voltage range.

[0107] Furthermore, according to the second embodiment, both the discontinuous current mode phase control and the continuous current mode phase control are executed using one control variable D, which has the effect of simplifying the control and making it stable.

[0108] 17 is a diagram illustrating a method for identifying a leg to be turned off to prevent reverse current. The control device 106 determines whether to perform continuous current mode phase control or discontinuous current mode phase control depending on the magnitude of the absolute value of the control variable D. The control device 106 determines to perform discontinuous current mode phase control when the absolute value of the control variable D is less than 180° (less than π). The control device 106 determines to perform continuous current mode phase control when the absolute value of the control variable D is 180° or more (greater than π).

[0109] The control device 106 determines whether to cause the power conversion system 100 to operate in power running mode or in regenerative mode depending on the polarity of the control variable D. When the control variable D is equal to or greater than zero, the control device 106 determines to cause the power conversion system 100 to operate in power running mode. Power running mode is an operation in which power is transmitted from the first bridge circuit 111 to the second bridge circuit 112. When the control variable D is less than zero, the control device 106 determines to cause the power conversion system 100 to operate in regenerative mode. Regenerative mode is an operation in which power is transmitted from the second bridge circuit 112 to the first bridge circuit 111.

[0110] The control device 106 turns off the upper and lower arms of one leg of the bridge circuit on the power receiving side depending on whether the primary-side DC voltage E1 or the secondary-side DC voltage E2 is higher. More specifically, the control device 106 turns off the upper and lower arms of one leg of the bridge circuit on the power receiving side by combining the three conditions shown in FIG. 17. The control device 106 executes discontinuous current mode phase control according to the second switching pattern (FIG. 10) when |D| < 180°, D < 0, and E1 < E2. The control device 106 executes discontinuous current mode phase control according to the fourth switching pattern (FIG. 14) when |D| < 180°, D < 0, and E1 ≥ E2. The control device 106 executes discontinuous current mode phase control according to the first switching pattern (FIG. 6) when |D| < 180°, D ≥ 0, and E1 < E2. The control device 106 executes discontinuous current mode phase control according to the third switching pattern (FIG. 12) when |D| < 180°, D ≥ 0, and E1 ≥ E2.

[0111] The present invention is not limited by the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, replacements, changes, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

Explanation of Reference Numerals

[0112] 100 Power conversion system 102 Insulation transformer 106 Control device 110 Insulation DC / DC converter 111 First bridge circuit 112 Second bridge circuit

Claims

1. An isolation transformer, a first bridge circuit connected to a primary side of the isolation transformer and including a first leg and a second leg connected in parallel, the first leg and second leg including upper and lower arms connected in series; a second bridge circuit connected to a secondary side of the isolation transformer and including a third leg and a fourth leg connected in parallel, the third leg and fourth leg including upper and lower arms connected in series; a control device that performs discontinuous current mode phase control to control a period during which both AC voltages on the primary side and the secondary side of the isolation transformer are zero voltage, the control device, when transmitting power from one of the first bridge circuit and the second bridge circuit to the other bridge circuit, in the discontinuous current mode, turns off an upper and lower arms of any one of the other bridge circuits among the first leg to the fourth leg, and alternately switches the upper and lower arms of each of the remaining legs with a dead time in between during which the upper and lower arms of each of the remaining legs are turned off.

2. 2. The power conversion system according to claim 1, wherein the control device changes a leg of the other bridge circuit, the upper and lower arms of which are to be turned off, depending on which of a primary side DC voltage applied to the first bridge circuit and a secondary side DC voltage applied to the second bridge circuit is higher.

3. 3. The power conversion system according to claim 1, wherein the control device determines whether to perform the discontinuous current mode phase control or to perform continuous current mode phase control for controlling a period during which the AC voltages on the primary side and the secondary side of the isolation transformer have opposite polarities, and when it determines to perform the discontinuous current mode phase control, turns off an upper and lower arms of one leg of the other bridge circuit.

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