JP7527738B2 - Bidirectional DC/DC Converter - Google Patents

Description

The present invention relates to a bidirectional DC/DC converter equipped with an ARCP (Auxiliary Resonant Commutated Pole) circuit for use in energy storage systems, etc.

A storage battery system stores cheap electricity at night and operates equipment by discharging the electricity from the battery during the day. In addition to the battery, the system is equipped with a bidirectional inverter and a bidirectional DC/DC converter that are connected to the power grid.

Energy storage systems are still expensive, and efforts are being made to lower prices by reducing the number of storage batteries connected in series. Reducing the number of storage batteries connected in series reduces the battery voltage and increases the step-up/step-down ratio of the bidirectional DC/DC converter (step-down ratio increases when charging, step-up ratio increases when discharging), resulting in a problem of deterioration in conversion efficiency. The ARCP circuit has been proposed as a technology to address the deterioration in efficiency.

However, even if an ARCP circuit is used, the step-up/step-down ratio is about five times higher, and further improvements in conversion efficiency are required, considering the trend toward lowering the cost of storage batteries. Therefore, the applicant has proposed bidirectional DC/DC converters equipped with improved ARCP circuits as shown in Figures 23, 24, 25, and 15. The basic operation of each of these circuits is almost the same.

The circuit configuration of FIG. 15 will be described below as an example. In FIG. 15 , _Tp1 is a high-side input/output terminal on the bidirectional inverter side ("one end" of the present invention), _Tn1 is a low-side input/output terminal on the bidirectional inverter side, _Tp2 is a high-side input/output terminal on the storage battery side ("the other end" of the present invention), BT is a storage battery, _Tn2 is a low-side input/output terminal on the storage battery side, L1 is a high-side line, L2 is a low-side line, C1 is a first smoothing capacitor, _C2 is a second smoothing capacitor, _Qm1 is a high-side main switching element, Qm2 is a low-side main switching element, _Dm1 and _Dm2 are anti-parallel connected diodes (parasitic diodes), _Cr1 and _Cr2 are resonant capacitors (parasitic capacitors), Lout is an output (when viewed in the charging direction) DC reactor, 10 is an ARCP circuit, 11 is a commutation control circuit, 12 is a rectifier circuit, Lr is a resonant reactor, _Qt1 is a high-side commutation switching element, Q _t2 is a low-side commutation switching element, _Dt1 and _Dt2 are inverse-parallel connected diodes (parasitic diodes), T1 is a resonant transformer for both charging and discharging, _Lt1 is a primary winding of the resonant transformer T1, and this primary winding _Lt1 is configured as a single-winding winding without a center tap, _Lt2 is a center-tapped secondary winding of the resonant transformer T1, _Lt21 is a first secondary winding in the secondary winding _Lt2 , _Lt22 is a second secondary winding in the secondary winding _Lt2 , D1 is a first rectifier diode, and D2 is a second rectifier diode.

In the case of Fig. 23, it is necessary to insert reverse current prevention diodes _DG1 , _{DG2 into a series circuit in which commutation switching elements Qt1, Qt2 are connected} to primary windings _NH1 , _NL1 of resonance transformer T0 of commutation control circuit 11, which is a component of ARCP circuit 10. In the case of Fig. 25, it is necessary to insert reverse current prevention diodes D3, D4. However, there is a problem in that reverse current prevention diodes consume large power.

In the case of the circuit configuration of FIG. 15, the primary winding L _T1 of the resonant transformer T1 is connected in series with the resonant reactor Lr, so that the diode for preventing reverse current can be omitted.

Next, the operation of the bidirectional DC/DC converter shown in Figure 15 in the discharge mode from the storage battery will be explained using the timing charts (waveform diagrams) of Figures 15 to 20 and Figure 21.

[A1] Steady state before commutation As shown in Fig. 15, when the high-side and low-side main switching elements _Qm1 , _Qm2 and the high-side and low-side commutation switching elements _Qt1 , _Qt2 are all in the off state (timing _t1 in Fig. 21), when the DC reactor Lout releases energy, the output current from the storage battery BT connected in parallel to the second smoothing capacitor C2 flows through the following path: [positive terminal of storage battery BT → DC reactor Lout → parasitic diode _Dm1 connected in anti-parallel to the high-side main switching element _Qm1 → (high-side line L1) → first smoothing capacitor C1 (bidirectional inverter) → (low-side line L2) → negative terminal of storage battery BT]. Here, the first smoothing capacitor C1 is charged by discharging the storage battery BT. The voltage applied between both terminals of the first smoothing capacitor C1, i.e., the voltage between the high-side input/output terminal _Tp1 and the low-side input/output terminal _Tn1 , is output to a bidirectional inverter (not shown) connected between the terminals, and a current flows from the terminal _Tp1 to the terminal _Tn1 via the bidirectional inverter. This operation mode is a steady state before the commutation operation. Before the commutation, no current flows in the commutation control circuit 11, and no current flows in the rectifier circuit 12.

16, at timing _t2 , when the low-side commutation switching element _Qt2 in the commutation control circuit 11 is turned on, the current that flowed through the anti-parallel connected parasitic diode _Dm1 in the above [A1] gradually commutates to the low-side commutation switching element _Qt2 . The commutation current flows through the path [storage battery BT → DC reactor Lout → resonant reactor Lr → primary winding Lt1 of resonant transformer _T1 → low-side commutation switching element _Qt2 → storage battery BT].

In addition, due to power induced in the secondary winding L _T2 from the primary winding L _T1 of the resonant transformer T1, in the rectifier circuit 12, a current flows through a path of [first secondary winding L _T21 → first rectifier diode D1 → first smoothing capacitor C1 (bidirectional inverter) → center tap of the secondary winding L _T2 ] and a path of [second secondary winding L _T22 → second rectifier diode D2 → first smoothing capacitor C1 (bidirectional inverter) → center tap of the secondary winding L _T2 ].

That is, when the low-side commutation switching element _Qt2 is turned on, the DC reactor Lout that had been storing energy starts to release energy. This marks the start of the generation of a resonant current in the resonant circuit system. Here, the charging of the first smoothing capacitor C1 continues.

[A3] Resonance begins As shown in FIG. 17, at timing _t3 , the low-side commutation switching element _Qt2 remains on, and current continues to flow through the path that it did in the state of [A2] above (storage battery BT → DC reactor Lout → resonant reactor Lr → primary winding L _T1 of resonant transformer T1 → low-side commutation switching element Q _t2 → storage battery BT). However, when the current flowing through the resonant reactor Lr and the primary winding L _T1 of the resonant transformer T1 exceeds the current flowing through the DC reactor Lout, the high-side and low-side resonant capacitors C _r1 , C _r2 begin to charge and discharge. That is, current flows through a path of [low-side resonant capacitor C _r2 → resonant reactor Lr → primary winding L _T1 of resonant transformer T1 → low-side commutation switching element Q _t2 → low-side resonant capacitor C _r2 ] and a path of [first smoothing capacitor C1 → high-side resonant capacitor C _r1 → resonant reactor Lr → primary winding L _T1 of resonant transformer T1 → low-side commutation switching element Q _t2 → first smoothing capacitor C1 ]. As a result, resonance begins between the high-side and low-side resonant capacitors C _r1 and _Cr2 and the leakage inductance of the resonant transformer T1.

The high-side resonant capacitor C _r1 is charged, and the low-side resonant capacitor C _r2 is discharged. The waveform of the resonant current i _r flowing through the resonant circuit system consisting of the low-side resonant capacitor C _r2 , the resonant reactor Lr, the primary winding L _T1 of the resonant transformer T1, and the low-side commutation switching element Q _t2 rises sinusoidally from the 0 level, reaches a peak, and then falls back toward the 0 level.

Furthermore, from timing _t3 to timing _t4 , the applied voltage (drain-source voltage) of the low-side main switching element _Qm2 decreases and approaches zero volts at timing _t4 . The current flow in the rectifier circuit 12 is the same as that in [A2] above.

18, at timing _t4 when the voltage across the low-side resonant capacitor C _r2 , i.e., the drain-source voltage applied to the low-side main switching element Q _m2 , becomes zero level, the main switching element Q _m2 is turned on. This turning on of the low-side main switching element Q _m2 utilizes soft switching (zero voltage switching (ZVS)) due to the resonance phenomenon, thereby reducing switching loss.

At this timing _t4 , charging of the high-side resonant capacitor C _r1 and discharging of the low-side resonant capacitor C _r2 end, and the voltage of the first connection node N1 becomes zero V. However, current continues to flow through the path [storage battery BT → DC reactor Lout → resonant reactor Lr → primary winding L _T1 of resonant transformer T1 → low-side commutation switching element Q _t2 → storage battery BT] that flowed in the above states [A2] and [A3].

When the low-side main switching element Q _m2 is turned on, a current is shunted to the main switching element Q _m2 . That is, the current (resonant current) flowing through the resonant reactor Lr gradually decreases, and the current that had previously flowed through the path [storage battery BT → DC reactor Lout → resonant reactor Lr → primary winding L _T1 of resonant transformer T1 → low-side commutation switching element Q _t2 → storage battery BT] transitions to the path [storage battery BT → DC reactor Lout → low-side main switching element Q _m2 → storage battery BT]. The former resonant current gradually decreases, and the latter drain current flowing through the main switching element Q _m2 gradually increases (timing t ₄ to t ₅ ). The waveform of the resonant current i _r flowing through the resonant circuit system falls toward the 0 level after passing the peak.

The current flow in the rectifier circuit 12 is the same as that described above in [A2] and [A3].

19, at timing _t5 , the resonant current passing through the resonant reactor Lr and the primary winding L _T1 of the resonant transformer T1 disappears, and immediately thereafter at timing _t6 , the low-side commutation switching element Q _t2 is turned off. This turning off of the commutation switching element Q _t2 utilizes soft switching (zero current switching (ZCS)), thereby reducing switching loss.

The current from the storage battery BT flows only through the path [storage battery BT → DC reactor Lout → low-side main switching element _Qm2 → storage battery BT]. The resonant current through the resonant reactor Lr and the primary winding L _T1 of the resonant transformer T1 in the commutation control circuit 11 stops flowing, and the current also stops flowing in the rectifier circuit 12, resulting in a transition to a steady state. As a result, energy is stored in the DC reactor Lout.

20, when the low-side main switching element _Qm2 is turned off at timing _t7 , the state returns to the pre-commutation steady state of [A1]. This turning off of the main switching element _Qm2 utilizes soft switching (zero voltage switching (ZVS)), and switching loss is reduced.

“High-efficiency bidirectional insulated DCDC converter”, [online], 2014, Pony Electric Co., Ltd., [Retrieved February 17, 2021], Internet <URL: http://pony-e.jp/High_efficiency_bi-directional_insulated_DCDCconverter.pdf>

In the above description of the conventional example, a timing chart (waveform diagram) is shown in Fig. 21 when the load condition is a standard load. Here, the relationship between the waveform of the current i _m2 flowing through the low-side main switching element Q _m2 and the waveform of the resonant current i _r flowing through the resonant reactor Lr will be considered.

In the period from about timing _t4 to timing _t6 , when the waveform of the resonant current i _r passes its peak and converges to the 0 level, the waveform of the current i _m2 flowing through the main switching element Q _m2 rises, and in conjunction with this, the waveform of the resonant current i _r falls. In other words, the timing of the increase in one and the timing of the decrease in the other overlap each other, achieving an overall balance, and supporting the formation of a sine wave-like waveform of the resonant current i _r .

However, this proper operating mode is only possible under the constraint that the load conditions are standard loads. If the load conditions are light, the operating mode becomes inappropriate. This problem is explained below.

At the time of light load, as shown in Fig. 22, the ON period of the low-side main switching element Q _m2 becomes short, and accordingly, the pulse width of the current i _m2 flowing through the low-side main switching element Q _m2 also becomes short. Nevertheless, since the waveform of the resonant current i _r remains the same as that at the time of the standard load (Fig. 21) with a relatively large time width, the waveform of the resonant current i _r also rises in the same period as the rising period of the waveform of the current i _m2 flowing through the main switching element Q _m2 , and the timing of the increase of one and the timing of the increase of the other overlap with each other, thus destroying the condition at the time of the standard load described above that the timing of the increase of one and the timing of the decrease of the other overlap with each other to maintain balance. In addition, the latter half of the sine half wave from the time when the resonant current i _r passes the peak to the time when it converges to the 0 level protrudes from the "H" level part of the waveform of the current i _m2 flowing through the main switching element Q _m2 .

Such a discrepancy in the phase relationship of the waveform of the resonant current i _r means that the allowable resonance time of the ARCP circuit cannot be secured under light load conditions, and as a result, the operation of the ARCP circuit must be temporarily stopped under light load conditions, resulting in the inconvenience of increased switching loss.

The above-mentioned problems occur under light load in the discharge mode (step-up mode) accompanied by a transition from turning on of the low-side commutation switching element _Qt2 to turning on of the low-side main switching element _Qm2 , and then turning off of the low-side commutation switching element _Qt2 to turning off of the low-side main switching element _Qm2 , as described in FIGS. 15 to 20. However, a similar problem also occurs under light load in the charge mode (step-down mode) accompanied by a transition from turning on of the high-side commutation switching element _Qt1 to turning on of the high-side main switching element _Qm1 , and then turning off of the high-side commutation switching element _Qt1 to turning off of the high-side main switching element _Qm1 .

The present invention was created in consideration of these circumstances, and aims to achieve the effect of reducing switching loss by allowing the ARCP circuit to continue operating without stopping, even when the load conditions become light.

The present invention solves the above problems by taking the following measures:

The bidirectional DC/DC converter according to the present invention comprises:
a first smoothing capacitor connected between the high-side input/output terminal and the low-side input/output terminal on one end;
a second smoothing capacitor connected between the high-side input/output terminal and the low-side input/output terminal on the other end;
a series circuit of a high-side main switching element and an output DC reactor, the series circuit being inserted in a high-side line connecting the high-side input/output terminal on the one end side and the high-side input/output terminal on the other end side;
a low-side main switching element inserted between a low-side line connecting the low-side input/output terminal on the one end side and the low-side input/output terminal on the other end side and a first connection node connecting the high-side main switching element and the DC reactor;
an ARCP circuit including a commutation control circuit and a rectifier circuit, the ARCP circuit being connected between the high side line and the low side line;
The commutation control circuit includes:
a commutation switch series circuit including a high-side commutation switching element and a low-side commutation switching element connected in series between the high-side line and the low-side line;
a resonant reactor having one end connected to the first connection node;
a resonant transformer having a primary winding connected between the other end of the resonant reactor and the commutation switch series circuit,
the rectifier circuit is connected between the high-side line and the low-side line in a state capable of receiving power induced in a secondary winding of the resonant transformer, and is configured to supply a current to the first smoothing capacitor only in one direction from the high-side line side to the low-side line side;
The power supply circuit further comprises a resonant inductance reducing means for reducing the inductance of the resonant reactor under a light load.

The above-mentioned configuration of the present invention provides the following effects:

When the load condition is not a light load but a standard load, the resonant inductance reduction means is inactive, the resonant reactor exerts its inherent effectiveness in the commutation control circuit in the ARCP circuit, and the total inductance value of the resonant circuit system becomes relatively large. Therefore, when the on-duty of the commutation switching element and the main switching element on one side of the high side or low side is relatively large and in a standard load state, the half-wave portion, which has the time width from the 0 level of the resonant current to the peak value and back to the 0 level, falls within the time region from the rising timing of the commutation switching element to the falling timing of the main switching element.

When the load condition changes from a standard load to a light load, it is necessary to shorten the on-time of the high-side or low-side main switching element and lengthen the off-time. As a result, the on-duty of the commutation switching element and the main switching element becomes short, but in the case of conventional technology, the half-wave portion of the resonant current cannot be contained within the time domain from the rising timing of the commutation switching element to the falling timing of the main switching element. In other words, when the load becomes light, it is no longer possible to ensure the resonant allowable time of the ARCP circuit, and it is necessary to deal with this by stopping the operation of the ARCP circuit (increased switching loss).

In contrast, in the present invention, when the load condition changes from a standard load to a light load, the resonant inductance reducing means operates to reduce the inductance of the resonant reactor. When the total inductance value of the resonant circuit system is reduced, the frequency of the resonant current is switched to a higher one, and the time width of the half-wave portion of the resonant current from the 0 level to the peak value and back to the 0 level is shortened (the waveform of the resonant current becomes steeper).

In other words, when the load is light, the on-duty of the commutation switching element and the main switching element is shortened, and the time width of the half-wave portion of the resonant current is also shortened, so that the half-wave portion of the resonant current falls within the time region from the rising edge of the commutation switching element to the falling edge of the main switching element, ensuring the allowable resonance time of the ARCP circuit.

From the above viewpoint, it is preferable that the resonant inductance reducing means is operated at a timing when the time from the rising edge of the commutation switching element to the falling edge of the main switching element exceeds the time width of the half-wave portion of the resonant current.

Here, as a resonant inductance reducing means, it is preferable that a bypass circuit is connected between both terminals of the resonant reactor to bypass and short-circuit the resonant reactor when the load is light. With this configuration, it is possible to short-circuit both terminals of the resonant reactor and to separate the resonant reactor from the resonant circuit system. In other words, the inductance value of the resonant reactor can be nullified in the resonant circuit system.

According to the present invention, a resonant inductance reduction means is provided for reducing the inductance of the resonant reactor under light load conditions, and therefore the time width of the half-wave portion of the resonant current is shortened in response to the shortening of the on-duty of the commutation switching element and the main switching element under light load conditions. As a result, the half-wave portion of the resonant current falls within the time domain from the rising timing of the commutation switching element to the falling timing of the main switching element, and the resonant allowable time of the ARCP circuit can be secured. Even when the load condition becomes light, the operation of the ARCP circuit is continued without being stopped, and the effect of reducing switching loss can be achieved even under light load conditions.

FIG. 1 is a circuit diagram showing a configuration of a bidirectional DC/DC converter according to an embodiment of the present invention; FIG. 1 is a diagram for explaining the discharge operation of the bidirectional DC/DC converter of the embodiment under light load. FIG. 2 is an explanatory diagram of the discharge operation of the bidirectional DC/DC converter of the embodiment under light load (part 2) FIG. 3 is an explanatory diagram of the discharge operation of the bidirectional DC/DC converter of the embodiment under light load (part 3) FIG. 4 is an explanatory diagram of the discharge operation of the bidirectional DC/DC converter of the embodiment under light load. FIG. 5 is an explanatory diagram of the discharge operation of the bidirectional DC/DC converter of the embodiment under light load. FIG. 6 is an explanatory diagram of the discharge operation of the bidirectional DC/DC converter of the embodiment under light load. FIG. 2 is a time chart (waveform diagram) for explaining the operation of the bidirectional DC/DC converter according to the embodiment; FIG. 1 is a diagram illustrating the charging operation of the bidirectional DC/DC converter of the embodiment under light load (part 1) FIG. 2 is an explanatory diagram of the charging operation of the bidirectional DC/DC converter of the embodiment under light load (part 2) FIG. 3 is a diagram illustrating the charging operation of the bidirectional DC/DC converter of the embodiment under light load. FIG. 4 is an explanatory diagram of the charging operation of the bidirectional DC/DC converter of the embodiment under light load. Charging operation of the bidirectional DC/DC converter of the embodiment under light load (part 5) Charging operation of the bidirectional DC/DC converter of the embodiment under light load (part 6) Operational diagram of a conventional bidirectional DC/DC converter (part 1) Operational diagram of a conventional bidirectional DC/DC converter (part 2) Operational diagram of a conventional bidirectional DC/DC converter (part 3) Operational diagram of a conventional bidirectional DC/DC converter (part 4) Operational diagram of a conventional bidirectional DC/DC converter (part 5) Operational diagram of a conventional bidirectional DC/DC converter (part 6) FIG. 1 is a time chart (waveform diagram) for explaining the operation of a conventional bidirectional DC/DC converter; Timing chart (waveform diagram) pointing out the problems of the conventional example Other conventional circuit diagrams (part 1) Other conventional circuit diagrams (part 2) Other conventional circuit diagrams (part 3)

The following describes in detail the bidirectional DC/DC converter of the present invention with the above configuration, using specific examples.

FIG. 1 is a circuit diagram showing the configuration of a bidirectional DC/DC converter according to an embodiment of the present invention. In FIG. 1 , T _p1 is a high-side input/output terminal on the bidirectional inverter side ("one end" of the invention), T _n1 is a low-side input/output terminal on the bidirectional inverter side, T _p2 is a high-side input/output terminal on the storage battery side ("the other end" of the invention), T _n2 is a low-side input/output terminal on the storage battery side, L1 is a high-side line connecting the high-side input/output terminal T _p1 and the high-side input/output terminal T _p2 , L2 is a low-side line connecting the low-side input/output terminal T _n1 and the low-side input _/ output terminal T _n2 , C1 is a first smoothing capacitor connected between the high-side input/output terminal T _p1 and the low-side input/output terminal T n1 on the inverter side, C2 is a second smoothing capacitor connected between the high-side input/output terminal T _p2 and the low-side input/output terminal T _n2 on the storage battery side, Q _m1 is a high-side main switching element (NMOS transistor), Q _m2 is a low-side main switching element (NMOS transistor), D _m1 is a diode (parasitic diode) connected in reverse parallel between both terminals (drain-source) of the high-side main switching element Q _m1 , C _r1 is a resonant capacitor between the same two terminals, _Dm2 is a diode (parasitic diode) connected in anti-parallel between the both terminals (drain-source) of the low-side main switching element _Qm2 , _Cr2 is a resonant capacitor between the same two terminals, and Lout is a DC reactor for output.

In addition, the device connected between the high-side and low-side input/output terminals T _p1 -T _n1 is not limited to a bidirectional inverter, and the device connected between the high-side and low-side input/output terminals T _p2 -T _n2 is not limited to a storage battery BT, but may be any device capable of inputting and outputting DC power.

A series circuit of a high-side main switching element _Qm1 and a DC reactor Lout is inserted in a high-side line L1. The drain of the high-side main switching element _Qm1 is connected to a high-side input/output terminal _Tp1 on the inverter side, the source of the high-side main switching element Qm1 is connected to one end of the DC reactor Lout, and the other end of the DC reactor Lout is connected to a high-side input/output terminal _Tp2 on the storage battery side. The drain of the low-side main switching element _Qm2 is connected to a first connection node N1 which is a connection point between the high-side main switching element _Qm1 and the DC reactor Lout, and the source of the low-side main switching element Qm2 is connected to a low-side line L2.

10 is an ARCP (auxiliary resonant commutation pole) circuit connected between the high side line L1 and the low side line L2, and includes a commutation control circuit 11 and a rectifier circuit 12.

The components of the commutation control circuit 11 are a commutation switch series circuit 11a, a resonant reactor Lr, a resonant transformer T1 (ARCP resonant transformer), a high-side commutation switching element (NMOS transistor), and _a low-side commutation switching element (NMOS transistor). L _T1 is a primary winding of the resonant transformer T1, and the primary winding L _T1 is configured as a single-winding winding without a center tap. L _{T2 is} a center-tapped secondary winding of the resonant transformer T1, and L T21 is a first secondary winding in the secondary winding L T2. One end of the resonant reactor Lr is connected to the first connection node N1, and the other end of the resonant reactor Lr is connected to one end of the primary winding L _T1 in the resonant transformer T1.

The high-side terminal (drain) of the high-side commutation switching element _Qt1 , which is a component of the commutation control circuit 11, is connected to the high-side line L1, the low-side terminal (source) of the low-side commutation switching element _Qt2 is connected to the low-side line L2, and the low-side terminal of the high-side commutation switching element _Qt1 and the high-side terminal of the low-side commutation switching element _Qt2 are commonly connected. The connection point of this common connection is the second connection node N2. Furthermore, the other end of the single-winding type primary winding _Lt1 (the connection point on the opposite side to the connection point with the resonant reactor Lr) is connected to the second connection node N2.

In other words, the high-side commutation switching element _Qt1 and the low-side commutation switching element _Qt2 are connected in series (common connection point N2), and the single-winding primary winding _Lt1 of the resonant transformer T1 has one end connected to the resonant reactor Lr and the other end connected to a second connection node N2, which is the common connection point of the high-side and low-side commutation switching elements _Qt1 , _Qt2 .

In this embodiment, a commutation switch series circuit 11a according to the configuration of the present invention is configured to include a high-side commutation switching element _Qt1 and a low-side commutation switching element _Qt2 connected in series between a high-side line L1 and a low-side line L2.

In the resonant transformer T1, the secondary winding L _T2 magnetically coupled to the single-winding primary winding L _T1 is configured as a center-tapped winding, that is, the center-tapped secondary winding L _T2 is configured to include a first secondary winding L _T21 and a second secondary winding L _T22 , and these windings L ₂₁ and L ₂₂ are connected at a center tap.

The rectifier circuit 12 has a first rectifier diode D1 and a second rectifier diode D2 connected to a center-tapped secondary winding L _T2 in the resonant transformer T1. A center tap connecting the first secondary winding L _T21 and the second secondary winding L _T22 in the secondary winding L _T2 is directly connected to the low-side line L2, one end of the first secondary winding L _T21 opposite to the center tap is connected to the high-side line L1 via the first rectifier diode D1, and similarly, one end of the second secondary winding L _T22 opposite to the center tap is connected to the high-side line L1 via the second rectifier diode D2.

This configuration simplifies the circuit configuration of the rectifier circuit 12 for charging the first smoothing capacitor C1, making it possible to reduce losses in the rectifier diode.

A series circuit of the resonant reactor Lr and the single-winding primary winding L _T1 of the resonant transformer T1 is connected between a first connection node N1 that commonly connects the high-side main switching element Q _m1 , the low-side main switching element Q _m2 , and the DC reactor Lout, and a second connection node N2 that connects the high-side commutation switching element Q _t1 and the low-side commutation switching element Q _t2 .

The rectifier circuit 12 is connected between the high-side line L1 and the low-side line _L2 in a state in which it can receive power induced in the center-tapped secondary winding _L2 from the single-winding primary winding L1 in the resonant transformer T1. The first rectifier diode D1 and the second rectifier diode D2 in the rectifier circuit 12 each supply a current to the first smoothing capacitor C1 only in one direction, from the high-side line L1 side to the low-side line L2 side.

Reference numeral 13 denotes a bypass circuit as a component of the commutation control circuit 11, and has the function of bypassing and shorting the resonant reactor Lr in the commutation control circuit 11 during light load. The bypass circuit 13 is configured as a bidirectional switch in which a first bypass switching element _Qb1 and a second bypass switching element _Qb2 are connected in series with opposite polarity. The sources of the first bypass switching element _Qb1 and the second bypass switching element _Qb2 are commonly connected, and the drains of the first bypass switching element _Qb1 and the second bypass switching element _Qb2 are connected to one end and the other end of the resonant reactor Lr, respectively. A diode (parasitic diode) _Db1 connected in anti-parallel between both terminals of the first bypass switching element _Qb1 forms a bypass path in cooperation with the second bypass switching element _Qb2 , and a diode (parasitic diode) _Db2 connected in anti-parallel between both terminals of the second bypass switching element _Qb2 forms a bypass path in cooperation with the first bypass switching element _Qb1 .

Next, we will explain the operation of the bidirectional DC/DC converter of the embodiment configured as described above.

When the load condition is a standard load, the bypass circuit 13 does not operate and the resonant reactor Lr always functions effectively, so that the bidirectional DC/DC converter operates in the same manner as in the conventional example (FIG. 15). When the load condition is a light load, gate signals are applied to the first and second bypass switching elements _Qb1 , _Qb2 , and both the first and second bypass switching elements _Qb1 , _Qb2 are turned on to short-circuit the resonant reactor Lr, disabling its function (bringing it into an inoperative state). As a result, the total inductance value of the resonant circuit system is reduced.

Below, the operating mode in which the storage battery BT is discharged under a light load condition is explained using Figures 2 to 8.

(A) Operation mode of discharging from the storage battery BT in a light load state (A1) Steady state before commutation As shown in Fig. 2, when the high-side and low-side main switching elements _Qm1 , _Qm2 , the high-side and low-side commutation switching elements _Qt1 , _Qt2 , and the first and second bypass switching elements _Qb1 , _Qb2 in the bypass circuit 13 added for light load are all in the off state (timing _t11 in Fig. 8), the DC reactor Lout discharges energy. As a result, the output current from the storage battery BT connected in parallel to the second smoothing capacitor C2 flows through the path [positive terminal of the storage battery BT → DC reactor Lout → parasitic diode _Dm1 connected in anti-parallel to the high-side main switching element _Qm1 → first smoothing capacitor C1 (bidirectional inverter) → negative terminal of the storage battery BT]. In the steady state before this commutation operation, no current flows through the commutation control circuit 11, and no current flows through the rectifier circuit 12. The other operation modes are the same as those in [A1] above.

(A2) Commutation Start Because the load condition is a light load, the low-side commutation switching element _Qt2 and the first and second bypass switching elements _Qb1 , _Qb2 in the commutation control circuit 11 are turned on at timing _t12 , as shown in Fig. 3. When the low-side commutation switching element _Qt2 is turned on, the resonant circuit system is connected to the storage battery BT, and at the same time, when the first and second bypass switching elements _Qb1 , _Qb2 are turned on, the resonant reactor Lr is bypassed (short-circuited), and the inductance value of the resonant circuit system decreases. In this state, the resonant reactor Lr is disconnected from the resonant circuit system.

Then, the current that flowed through the parasitic diode _Dm1 connected in anti-parallel to the high-side main switching element _Qm1 in the above (A1) gradually commutates to the resonant circuit system on the side of the low-side commutation switching element _Qt2 . As a result, the output current from the storage battery BT flows along the path of [storage battery BT → DC reactor Lout → anti-parallel connected parasitic diode _Dm1 → first smoothing capacitor C1 (bidirectional inverter) → storage battery BT], and also flows along the path of [storage battery BT → DC reactor Lout → first and second bypass switching elements _Qb1 , _Qb2 → primary winding Lt1 of resonant transformer _T1 → low-side commutation switching element _Qt2 → storage battery BT].

The other operation modes are the same as those of (A2) above. That is, due to the power induced from the primary winding L _T1 to the secondary winding L _T2 of the resonance transformer T1, a current flows through the path of [secondary winding L _T2 →first rectifier diode D1, second rectifier diode D2 →first smoothing capacitor C1 →center tap of the secondary winding L _T2] , and charging of the first smoothing capacitor C1 begins.

4, when the current flowing through the first and second bypass switching elements _Qb1 , _Qb2 and the primary winding L _T1 of the resonant transformer T1 exceeds the current flowing through the DC reactor Lout, the resonant capacitors C _r1 , _Cr2 start charging and discharging at timing _t13 . That is, a current flows through a path of [resonant capacitor C _r2 → bypass switching elements _Qb1 , _Qb2 → primary winding L _{T1 of the resonant transformer T1} → low-side commutation switching element Q _t2 → resonant capacitor C _r2 ] and a path of [first smoothing capacitor C1 (bidirectional inverter) → high-side resonant capacitor C _r1 → bypass switching elements _Qb1 , _Qb2 → primary winding L _T1 of the resonant transformer T1 → low-side commutation switching element Q _t2 → first smoothing capacitor C1 (bidirectional inverter)]. As a result, a resonance operation is performed by the resonance capacitors C _r1 and C _r2 and the leakage inductance of the resonance transformer T1. The high-side resonance capacitor C _r1 is charged, and the low-side resonance capacitor C _r2 is discharged.

In this operation mode, as in the case of (A2), the resonant reactor Lr is bypassed by the function of the bypass switching elements Q _b1 and Q _b2 , so that the inductance value of the resonant circuit system is reduced.

5, at timing _t14 when the voltage across the low-side resonant capacitor C _r2 , i.e., the voltage applied to the low-side main switching element Q _m2 , becomes zero level, the main switching element Q _m2 is turned on. This turning on of the low-side main switching element Q _m2 utilizes soft switching (zero voltage switching (ZVS)) due to the resonance phenomenon, and switching loss is reduced.

When the low-side main switching element _Qm2 is turned on and current is shunted to this main switching element _Qm2 , the current (resonant current) flowing through the bypass switching elements _Qb1 , _Qb2 gradually decreases, and the current that had been flowing along the path [storage battery BT → DC reactor Lout → bypass switching elements _Qb1 , _Qb2 → primary winding Lt1 of resonant transformer _T1 → low-side commutation switching element _Qt2 → storage battery BT] transitions to a flow along the path [storage battery BT → DC reactor Lout → low-side main switching element _Qm2 → storage battery BT].

In this mode, the drain current flowing through the low-side main switching element _Qm2 gradually increases, and the resonant current gradually decreases.

During the period from (A2) through (A3) to (A4) above, the resonant reactor Lr is bypassed by the function of the bypass switching elements Q _b1 and Q _b2 , so the inductance value of the resonant circuit system is reduced. As a result, the waveform of the resonant current becomes sharp and the resonance period decreases (the resonant frequency increases). The resonant current increases rapidly, reaches a peak, and then starts to decrease. Towards timing _t14 , the applied voltage (drain-source voltage) of the low-side main switching element Q _m2 decreases, and becomes zero volts at timing _t14 .

In this way, even if a sufficient resonant allowable time for the ARCP circuit cannot be secured due to a light load, the period of the resonant current flowing through the resonant circuit system can be shortened to generate the resonant current appropriately as desired within the shortened resonant allowable time, so there is no need to stop the operation of the ARCP circuit, and therefore the problem of increased switching loss during light loads can be resolved.

(A5) Steady state At timing _t15 immediately after the current in the resonant reactor Lr and the primary winding L _T1 of the resonant transformer T1 disappears, the low-side commutation switching element Q _t2 is turned off as shown in Fig. 6. This turning off of the commutation switching element Q _t2 utilizes soft switching (zero current switching (ZCS)), thereby reducing switching loss.

In this steady state, the current flows only through the path of [storage battery BT → DC reactor Lout → low-side main switching element Q _m2 → storage battery BT]. Therefore, energy is stored in the DC reactor Lout.

7, when the low-side main switching element _Qm2 is turned off at timing _t17 , the state returns to the pre-commutation steady state of (A1). This turning off of the main switching element _Qm2 utilizes soft switching (zero voltage switching (ZVS)), which reduces switching loss.

(B) Operation mode of charging the storage battery BT in a light load state (B1) Steady state before commutation As shown in Fig. 9, when the high-side and low-side main switching elements _Qm1 , _Qm2 , the high-side and low-side commutation switching elements _Qt1 , _Qt2 , and the first and second bypass switching elements _Qb1 , _Qb2 in the bypass circuit 13 added to handle light loads are all in the off state, the DC reactor Lout discharges energy. As a result, the current discharged from the DC reactor Lout flows through the route [DC reactor Lout → storage battery BT → parasitic diode _Dm2 connected in reverse parallel to the low-side main switching element _Qm2 → DC reactor Lout], and the storage battery BT is charged. In this steady state before the commutation operation, no current flows in the commutation control circuit 11, and no current flows in the rectifier circuit 12.

(B2) Commutation Start As shown in Fig. 10, the high-side commutation switching element _Qt1 in the commutation control circuit 11 is turned on, and since the load condition is a light load, the first and second bypass switching elements _Qb1 , _Qb2 in the bypass circuit 13 are turned on. By turning on the first and second bypass switching elements _Qb1 , _Qb2 , the resonant reactor Lr is bypassed (short-circuited), and the inductance value of the resonant circuit system decreases. In this state, the resonant reactor Lr is disconnected from the resonant circuit system.

Then, the current flowing in from the high-side input/output terminal _Tp1 flows through the path [high-side commutation switching element _Qt1 → primary winding LT1 of resonant transformer _T1 → first bypass switching element _Qb1 → inverse-parallel connected parasitic diode _Db2 → DC reactor Lout → storage battery BT → low-side input/output terminal _Tn1 ].

Since the resonant current flows through the primary winding L of the resonant transformer _T1 , the power induced in the secondary winding L causes a current to flow through the path [secondary winding _L → first rectifier diode _D1 , second rectifier diode D2 → first smoothing capacitor C1 → center tap of the secondary winding L], and charging of the first smoothing capacitor _C1 begins. The current from the first smoothing capacitor C1 compensates for the charging of the storage battery BT.

11, when the current flowing through the primary winding L _T1 of the resonant transformer T1 and the first and second bypass switching elements Q _b1 and Q _b2 exceeds the current flowing through the DC reactor Lout, the resonant capacitors C _r1 and _Cr2 start charging and discharging. That is, a current flows through a path of [high-side resonant capacitor C _r1 → high-side commutation switching element Q _t1 → primary winding L T1 of the resonant transformer _T1 → bypass switching elements Q _b1 and Q _b2 → resonant capacitor C _r1 ] and a path of [low-side resonant capacitor C _r2 → first smoothing capacitor C1 (bidirectional inverter) → high-side commutation switching element Q _t1 → primary winding L _T1 of the resonant transformer T1 → bypass switching elements Q _b1 and Q _b2 → resonant capacitor C _r2 ]. As a result, a resonance operation is performed by the resonance capacitors C _r1 and C _r2 and the leakage inductance of the resonance transformer T1. The high-side resonance capacitor C _r1 is discharged, and the low-side resonance capacitor C _r2 is charged.

In this operation mode, as in the case of (B2), the resonant reactor Lr is bypassed by the function of the bypass switching elements Q _b1 and Q _b2 , so that the inductance value of the resonant circuit system is reduced.

12, at the timing when the voltage across the high-side resonant capacitor C _r1 , i.e., the voltage applied to the high-side main switching element Q _m1 , becomes zero level, the main switching element Q _m1 turns on. This turning on of the high-side main switching element Q _m1 utilizes soft switching (zero voltage switching (ZVS)) due to the resonance phenomenon, and switching loss is reduced.

When the high-side main switching element _Qm1 is turned on and current is shunted to this main switching element _Qm1 , the current (resonant current) flowing through the bypass switching elements _Qb1 , _Qb2 gradually decreases, and the current that had been flowing along the path [storage battery BT → first smoothing capacitor C1 → high-side commutation switching element _Qt1 → primary winding Lt1 of resonant transformer _T1 → bypass switching elements _Qb1 , _Qb2 → DC reactor Lout → storage battery BT] transitions to flow along the path [storage battery BT → first smoothing capacitor C1 → high-side main switching element _Qm1 → series reactor Lout → storage battery BT].

In this mode, the drain current flowing through the high-side main switching element _Qm1 gradually increases, and the resonant current gradually decreases.

During the period from (B2) through (B3) to (B4) above, the resonant reactor Lr is bypassed by the function of the bypass switching elements Q _b1 and Q _b2 , so the inductance value of the resonant circuit system is reduced. As a result, the waveform of the resonant current becomes sharp and the resonance period decreases (the resonant frequency increases). The resonant current increases rapidly, reaches a peak, and then starts to decrease. Over time, the applied voltage (drain-source voltage) of the high-side main switching element Q _m1 decreases and then becomes zero volts.

In this way, even if a sufficient resonant allowable time for the ARCP circuit cannot be secured due to a light load, the period of the resonant current flowing through the resonant circuit system can be shortened to generate the resonant current appropriately as desired within the shortened resonant allowable time, so there is no need to stop the operation of the ARCP circuit, and therefore the problem of increased switching loss during light loads can be resolved.

(B5) Steady state Immediately after the current in the primary winding L _T1 of the resonant transformer T1 and the resonant reactor Lr disappears, the high-side commutation switching element Q _t1 turns off as shown in Fig. 13. This turning off of the commutation switching element Q _t1 utilizes soft switching (zero current switching (ZCS)), thereby reducing switching loss.

In this steady state, the current flows only through the path of [first smoothing capacitor C1 → high-side main switching element Q _m1 → DC reactor Lout → storage battery BT → first smoothing capacitor C1]. Therefore, energy is stored in the DC reactor Lout.

14, when the high-side main switching element _Qm1 is turned off, the state returns to the pre-commutation steady state of (A1). This turning off of the main switching element _Qm1 utilizes soft switching (zero voltage switching (ZVS)), which reduces switching loss.

As described above, when the load is light, the resonant reactor Lr is short-circuited to reduce the total inductance value of the resonant circuit system, so that the allowable resonance time of the ARCP circuit can be secured, and even if the load condition becomes light, the operation of the ARCP circuit can be continued without being stopped, and switching loss can be appropriately reduced.

When the allowable resonance time is reduced under heavy load conditions, the peak value of the resonant current increases, resulting in increased conduction loss. However, under light load conditions, the peak value of the resonant current is small, so even if the allowable resonance time is reduced, conduction loss does not become a problem.

In addition, if the light load continues to progress and it becomes difficult to ensure the resonant allowable time in terms of control, in that case, in addition to reducing the inductance value by operating the bypass circuit, the oscillation frequency can be reduced to increase the absolute length of one cycle, thereby relatively shortening the effective period of ARCP operation within that one cycle, thereby ensuring the resonant period (there is no problem with reducing the oscillation frequency, since the resonant allowable time is determined by the resonant inductance value). Furthermore, the oscillation frequency can be reduced first, and then both ends of the resonant reactor Lr can be short-circuited.

When there is a light load close to no load, it is difficult to ensure the allowable resonance time even if the oscillation frequency is lowered to 20 kHz, so the ARCP circuit is stopped and the system switches to simple PWM control.

Because reducing the oscillation frequency to the audible frequency range can cause problems with abnormal noise, it is preferable to keep the minimum oscillation frequency at around 20 kHz. At light loads close to no load, the switching current is small, and if the oscillation frequency is reduced to 20 kHz, the switching loss is not large.

The application of the present invention is not limited to the circuit configuration of FIG. 1, but can be applied to various circuit configurations such as the configurations of FIG. 23, FIG. 24, and FIG. 25.

In addition, the bypass circuit 13 that shorts both ends of the resonant reactor Lr is not limited to the circuit described in the above embodiment, and may be replaced with another circuit as long as the object of the present invention is achieved. Also, instead of being limited to the bypass circuit 13, a resonant inductance reduction means for reducing the inductance of the resonant reactor Lr may be provided. By reducing the inductance of the resonant reactor Lr and shortening the period of the resonant current flowing through the resonant circuit system, the resonant current can be generated appropriately as desired within the shortened resonant allowable time.

The present invention is useful as a technology that allows the ARCP circuit to continue operating without stopping even under light load conditions, thereby achieving the effect of reducing switching loss.

10 ARCP circuit 11 Commutation control circuit 11a Commutation switch series circuit 12 Rectification circuit 13 Bypass circuit (resonant inductance reducing means)
C1 First smoothing capacitor C2 Second smoothing capacitor L1 High side line L2 Low side line Lout DC reactor Lr Resonant reactor L _T1 Primary winding L _T2 Secondary winding N1 First connection node Q _m1 High side main switching element Qm2 Low side main switching element Q _t1 High side commutation switching element Q _t2 Low side commutation switching element T1 Resonant transformer T _p1 High side input/output terminal on one end T p2 High side input _{/output terminal on the other end T n1 Low} side input/output terminal on one end T _n2 Low side input/output terminal on the other end

Claims (3)

a first smoothing capacitor connected between the high-side input/output terminal and the low-side input/output terminal on one end;
a second smoothing capacitor connected between the high-side input/output terminal and the low-side input/output terminal on the other end;
a series circuit of a high-side main switching element and an output DC reactor, the series circuit being inserted in a high-side line connecting the high-side input/output terminal on the one end side and the high-side input/output terminal on the other end side;
a low-side main switching element inserted between a low-side line connecting the low-side input/output terminal on the one end side and the low-side input/output terminal on the other end side and a first connection node connecting the high-side main switching element and the DC reactor;
an ARCP circuit including a commutation control circuit and a rectifier circuit, the ARCP circuit being connected between the high side line and the low side line;
The commutation control circuit includes:
a commutation switch series circuit including a high-side commutation switching element and a low-side commutation switching element connected in series between the high-side line and the low-side line;
a resonant reactor having one end connected to the first connection node;
a resonant transformer having a primary winding connected between the other end of the resonant reactor and the commutation switch series circuit,
the rectifier circuit is connected between the high-side line and the low-side line in a state capable of receiving power induced in a secondary winding of the resonant transformer, and is configured to supply a current to the first smoothing capacitor only in one direction from the high-side line side to the low-side line side;
Further, a resonant inductance reducing means is provided for reducing the inductance of the resonant reactor under a light load,
the resonant inductance reducing means reduces the inductance of the resonant reactor under light load conditions, thereby shortening a period of a resonant current of a resonant circuit system including the resonant reactor and the resonant transformer, and causing the resonant current to occur within a resonant allowable time of the ARCP circuit.
A bidirectional DC/DC converter comprising: The bidirectional DC/DC converter according to claim 1, characterized in that, as the resonant inductance reducing means, a bypass circuit is connected between both terminals of the resonant reactor to bypass and short-circuit the resonant reactor when the load is light. The bidirectional DC/DC converter according to claim 1 or 2, characterized in that the primary winding of the resonant transformer is connected between the high-side commutation switching element and the other end of the resonant reactor, and between the low-side commutation switching element and the other end of the resonant reactor.

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