DC / DC converter

Abstract

To provide a highly efficient DC / DC converter reducing switching loss and reactor current.SOLUTION: A DC / DC converter 1000 comprises: a transformer T; a first bridge circuit 100 that converts a first DC voltage into an AC voltage and supplies it to a primary winding L3 side; a second bridge circuit 200 that converts an AC voltage supplied from a secondary winding side into a DC voltage and outputs a second DC voltage; an inductor comprising a first inductor L1 between the first bridge circuit 100 and the transformer T, and a second inductor L2 between the second bridge circuit 200 and the transformer T; and a control unit 10 that controls the first bridge circuit 100 by a first switching control signal and controls the second bridge circuit 200 by a second switching control signal. The second switching control signal has a controllable phase difference relative to the first switching control signal, and when the absolute value of the phase difference is in a range of 90° or more and 180° or less, the control unit 10 increases the switching frequency as the absolute value increases.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a DC / DC converter. [Background technology]

[0002] 2. Description of the Related Art Conventionally, a DAB (Dual Active Bridge) type bidirectional DC / DC converter having full bridge circuits on the primary and secondary sides of an isolation transformer has been known (for example, Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Katsuya Hirachi, "Derivation of important formulas for the ΔAB method", [online], February 14, 2015, Internet<URL:http: / / hirachi.cocolog-nifty.com / kh / files / 20150214-1.pdf> [Non-Patent Document 2] Katsuya Hirachi, "Two disadvantages of DAB converters and their solutions", [online], January 16, 2019, Internet<URL:http: / / hirachi.cocolog-nifty.com / kh / files / 20181112-1A.pdf> Summary of the Invention [Problem to be solved by the invention]

[0004] In a DAB bidirectional DC / DC converter, there is a mode in which a reverse flow of power occurs during the cycle of the operating mode, generating a reactor current, which may result in a decrease in power transmission efficiency. Furthermore, in a DAB bidirectional DC / DC converter, there is a region in which soft switching is not possible under light loads, and depending on the state of the input and output voltages, there is a risk of a shoot-through current occurring due to a reverse inductor current (for example, see Non-Patent Document 1).

[0005] Therefore, an object of the present disclosure is to provide a highly efficient DC / DC converter with reduced switching loss and reactor current. [Means for solving the problem]

[0006] The DC / DC converter according to the present disclosure comprises: a transformer having a primary winding and a secondary winding; a first bridge circuit that converts a first DC voltage into an AC voltage in response to a first switching control signal and supplies the AC voltage to the primary winding; a second bridge circuit that converts an AC voltage supplied from the secondary winding side into a DC voltage in response to a second switching control signal having the same switching frequency as the first switching control signal, and outputs a second DC voltage; an inductor including a first inductor between the first bridge circuit and the transformer and a second inductor between the second bridge circuit and the transformer; a control unit that controls the first bridge circuit by the first switching control signal and the second bridge circuit by the second switching control signal; Equipped with the second switching control signal has a controllable phase difference with respect to the first switching control signal; When the absolute value of the phase difference is increased within a range of 90° to 180°, the control unit increases the switching frequency.

[0007] The DC / DC converter according to the present disclosure comprises: a transformer having a primary winding and a secondary winding; a first bridge circuit that converts a first DC voltage into an AC voltage in response to a first switching control signal and supplies the AC voltage to the primary winding; a second bridge circuit that converts an AC voltage supplied from the secondary winding side into a DC voltage in response to a second switching control signal having the same switching frequency as the first switching control signal, and outputs a second DC voltage; an inductor including a first inductor between the first bridge circuit and the transformer and a second inductor between the second bridge circuit and the transformer; a control unit that controls the first bridge circuit by the first switching control signal and the second bridge circuit by the second switching control signal; Equipped with the second switching control signal has a controllable phase difference with respect to the first switching control signal; The control unit controls the phase difference and the switching frequency so that the output current change rate with respect to the phase difference in the output current of the second DC voltage becomes negative within a range from a minimum angle at which soft switching is possible to 180°. Effect of the Invention

[0008] According to the present disclosure, it is possible to provide a highly efficient DC / DC converter with reduced switching loss and reactor current. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a circuit diagram of a DC / DC converter. [Diagram 2] 4 is a timing chart illustrating an operation of the DC / DC converter. [Diagram 3] 13 is a graph showing an output current I2' and a reactor current effective value ILR versus a phase difference φ of a DC / DC converter according to a reference example. [Figure 4] 11 is a graph showing a soft switching region of a DC / DC converter according to a reference example. [Diagram 5] 1 is a graph showing an output current I2' and a reactor current effective value ILR versus a switching frequency f when a phase difference φ is fixed at 90° in a DC / DC converter. [Figure 6] 4 is a graph showing an output current I2', a reactor current effective value ILR, and a frequency f versus a phase difference φ of the DC / DC converter according to the present embodiment.

Embodiment for Carrying out the Invention

[0010] Hereinafter, this embodiment will be described with reference to the drawings. For members having the same reference numerals as those already described in the description of the embodiment, the description thereof will be omitted for convenience of explanation. <Circuit Configuration of DC / DC Converter>

[0011] FIG. 1 is a circuit diagram of a DC / DC converter. The DC / DC converter 1000 includes a first bridge circuit 100, a second bridge circuit 200, a transformer T, and a switching control unit 10. The DC / DC converter 1000 is configured to perform bidirectional power conversion by controlling the phase difference between the reference phase and the control phase, with the first bridge circuit 100 as the reference phase and the second bridge circuit 200 as the control phase.

[0012] The first bridge circuit 100 is a full-bridge circuit composed of a first leg in which switching elements Q1 and Q2 are connected in series and a second leg in which switching elements Q3 and Q4 are connected in series. An input power supply E1 and an input capacitor C9 are connected to the input terminals IO11 and IO12 of the first bridge circuit 100, and a first DC voltage V1 is input.

[0013] The switching elements Q1 to Q4 are semiconductor switching elements such as IGBTs (Insulated Gate Bipolar Transistors) and FETs (Field Effect Transistors). Freewheeling diodes D1 to D4 and capacitors C1 to C4 are connected in parallel to the switching elements Q1 to Q4, respectively. The midpoint M1 of the first leg and the midpoint M2 of the second leg of the first bridge circuit 100 are connected to the primary winding L3 of the transformer T. Also, a first inductor L1 is connected in series between the midpoint M1 of the first leg and the primary winding L3 of the transformer T. The first inductor L1 may be a coil connected to the primary winding L3 or the leakage inductance of the primary winding L3. The current flowing from the midpoint M1 of the first leg to the first inductor L1 is defined as the reactor current IL.

[0014] The switching control unit 10 transmits switching control signals to the gate terminals of the switching elements Q1 to Q4 respectively, and controls the switching elements Q1 to Q4 at the switching frequency f. Thereby, the first DC voltage V1 input to the first bridge circuit 100 is converted into a first AC voltage.

[0015] The transformer T includes a primary winding L3 and a secondary winding L4. The primary winding L3 and the secondary winding L4 are magnetically coupled, and the first AC voltage applied to the primary winding L3 is voltage-converted into a second AC voltage applied to the secondary winding L4.

[0016] Since the circuit configuration of the second bridge circuit 200 is the same as that of the first bridge circuit 100, the description thereof is omitted. A load Lо and an output capacitor C10 are connected to the output terminals IO21 and IO22 of the second bridge circuit 200, and a second DC voltage V2 is output to the load Lо.

[0017] The switching control unit 10 transmits switching control signals to the gate terminals of the switching elements Q5 to Q8 respectively, and controls the switching elements Q5 to Q8 at the switching frequency f. Thereby, the second AC voltage input to the second bridge circuit 200 is converted into a second DC voltage V2. <Operation of the DC / DC Converter>

[0018] FIG. 2 is a timing chart for explaining the operation of the DC / DC converter, showing the case where the phase of the control phase is delayed by a phase difference φ from the phase of the reference phase. In order from the top, it shows the reactor current IL flowing through the first inductor L1, the switching control signal waveforms of the switching elements Q1 and Q4, the switching control signal waveforms of the switching elements Q2 and Q3, the switching control signal waveforms of the switching elements Q5 and Q8, and the switching control signal waveforms of the switching elements Q6 and Q7. The horizontal axis represents time. For the sake of explanation, the switching period T is divided into four modes of Mode1 to Mode4.

[0019] In Mode1, the switching controller 10 controls the switching elements Q1 and Q4 to be turned on from OFF and the switching elements Q2 and Q3 to be turned off from ON at time t0. At this time, the switching elements Q5 and Q8 are maintained in the OFF state, and the switching elements Q6 and Q7 are maintained in the ON state. As a result, the reactor current IL increases from -Ib to Ia.

[0020] Next, in Mode 2, the switching controller 10 controls the switching elements Q5 and Q8 to be turned on and the switching elements Q6 and Q7 to be turned off at time t1, which is delayed from time t0 by the phase difference φ. At this time, the switching elements Q1 and Q4 are maintained in the on state, and the switching elements Q2 and Q3 are maintained in the off state. As a result, the reactor current IL changes from Ia to Ib.

[0021] Next, in Mode 3, at time t3, which is T / 2 after time t0, the switching control unit 10 controls the switching elements Q1 and Q4 to change from ON to OFF and the switching elements Q2 and Q3 to change from OFF to ON. At this time, the switching elements Q5 and Q8 maintain the ON state, and the switching elements Q6 and Q7 maintain the OFF state. As a result, the reactor current IL decreases from Ib to -Ia. Note that T represents the period of the switching frequency f.

[0022] Next, in Mode 4, the switching control unit 10 controls the switching elements Q5 and Q8 to change from ON to OFF and the switching elements Q6 and Q7 to change from OFF to ON at time t4, which is delayed from time t3 by the phase difference φ. At this time, the switching elements Q1 and Q4 are maintained in the OFF state, and the switching elements Q2 and Q3 are maintained in the ON state. As a result, the reactor current IL changes from -Ia to -Ib.

[0023] By repeating the operations of Modes 1 to 4, the phase of the control phase is delayed from the phase of the reference phase by the phase difference φ, and power conversion from the first bridge circuit 100 to the second bridge circuit 200 is performed. <How to calculate the effective value of reactor current>

[0024] 1, the current input to the first bridge circuit 100 is defined as input current I1, and the current output from the second bridge circuit 200 is defined as output current I2. If the current obtained by converting the output current I2 from the second bridge circuit 200 to the first bridge circuit 100 (hereinafter also referred to as conversion to the primary side) is defined as output current I2', the output current I2' is expressed by the following formula. Note that n1 and n2 in the formula represent the number of turns of the primary winding L3 and the number of turns of the secondary winding L4, respectively. I2'=(n2 / n1)I2 (formula 1)

[0025] If the voltage obtained by converting the second DC voltage V2 to the primary side is defined as a second DC voltage V2', the second DC voltage V2' is expressed by the following formula. V2'=(n1 / n2)V2 (Equation 2)

[0026] If the second inductor L2 is converted into the primary side and its combined value with the first inductor L1 is defined as reactor L, reactor L can be expressed by the following formula. L = L1 + (n2 / n1) 2 L2 (Formula 3)

[0027] The output current I2' is expressed by the following formula using the first DC voltage V1, the reactor L, and the phase difference φ. Note that ω in the formula represents the angular velocity of the switching, and the phase difference φ is expressed in radians.

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[0028] Regarding the reactor current IL shown in FIG. 1, a current Ia which is the terminal value of Mode 1 and the initial value of Mode 2 shown in FIG. 2, and a current Ib which is the terminal value of Mode 2 and the initial value of Mode 3 are respectively expressed by the following equations using a first DC voltage V1 and a second DC voltage V2′.

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[0029] The effective value ILR of the reactor current IL is expressed by the following formula using the currents Ia and Ib.

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[0030] To explain the soft switching region of the DC / DC converter according to this embodiment, first, the soft switching region of a DC / DC converter according to a reference example for comparison will be explained using Figures 3 and 4. In the DC / DC converter according to the reference example, the switching frequency f is a fixed value and the phase difference φ is a variable value.

[0031] Fig. 3 is a graph showing the output current I2' and reactor current effective value ILR versus phase difference φ of the DC / DC converter according to the reference example. The solid line shows the waveform of the output current I2', and the dashed line shows the waveform of the reactor current IL. The vertical axis shows the output current I2' and the reactor current IL, and the horizontal axis shows the phase difference φ between the reference phase and the control phase. Note that the phase difference φ in Fig. 3 is expressed in degrees (°).

[0032] As shown in Figure 3, the output current I2' is 0 when the phase difference φ is -180°, 0°, and 180°, is a minimum value when the phase difference φ is -90°, and is a maximum value when the phase difference φ is 90°. As the absolute value of the phase difference φ increases, the reactor current effective value ILR increases.

[0033] Moreover, the step-up ratio k from the first DC voltage V1 to the second DC voltage V2' is expressed by the following formula. k=V2' / V1 (formula 8)

[0034] The soft switching conditions of the DC / DC converter according to the reference example are expressed by the following two formulas when classified according to the step-up ratio k.

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[0035] Fig. 4 is a graph showing the soft switching region of the DC / DC converter according to the reference example. The vertical axis indicates the phase difference φ, and the horizontal axis indicates the boost ratio k. The soft switching region SA that satisfies the conditions of formula 9 and formula 10 is indicated by diagonal lines. In Fig. 4, the boundary line of the condition of formula 9 is indicated by symbol B1, and the boundary line of the condition of formula 10 is indicated by symbol B2.

[0036] 3 and 4, in the range where the phase difference φ is close to 0°, the through current from the secondary side to the primary side is small and the reactor current effective value ILR is small as shown in Fig. 3, but the soft switching range SA is limited as shown in Fig. 4. Also, in the range where the phase difference φ is greater than 90°, soft switching is possible over the entire range of the step-up ratio k as shown in Fig. 4, but the through current from the secondary side to the primary side is large and the reactor current effective value ILR becomes large as shown in Fig. 3. If the reactor current effective value ILR becomes large, the entire device including the DC / DC converter becomes large. <Soft switching region of the DC / DC converter according to this embodiment>

[0037] Fig. 5 is a graph showing the output current I2' and reactor current effective value ILR versus switching frequency f when the phase difference φ is fixed at 90° in a DC / DC converter. The solid line shows the waveform of the output current I2' expressed by equation 4, and the dashed line shows the waveform of the reactor current effective value ILR expressed by equation 7. The vertical axis shows the output current I2' and reactor effective value ILR, and the horizontal axis shows the switching frequency f. In Fig. 5, when the switching frequency f is increased, the output current I2' and the effective value ILR of the reactor current IL decrease.

[0038] Therefore, the inventors have focused on controlling switching so that the switching frequency f increases when the switching frequency f is set to a variable value and the absolute value of the phase difference φ is increased. The soft switching region of the DC / DC converter according to this embodiment will be described below with reference to FIG.

[0039] First, the switching frequency f, which is a variable value, is defined as a function of the phase difference φ as follows: Here, the function g is a function that increases g(|φ|) as the absolute value |φ| of the phase difference φ increases.

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[0040] As described above, when the absolute value |φ| of the phase difference φ is increased, g(|φ|), that is, the switching frequency f, increases, and the output current I2' decreases according to equation 12. Moreover, according to equations 5 to 7, when the switching frequency f increases, the reactor current effective value ILR also decreases.

[0041] 6 is a graph showing the output current I2', reactor current effective value ILR, and switching frequency f versus phase difference in the DC / DC converter according to this embodiment. The solid line indicates the waveform of the output current I2' expressed by equation 12, the dashed line indicates the waveform of the reactor current effective value ILR expressed by equation 7, and the dashed-dotted line indicates the switching frequency f. The vertical axis indicates the output current I2', reactor current effective value ILR, and switching frequency f, and the horizontal axis indicates the phase difference φ.

[0042] 6, the switching frequency f shown in Equation 11 is a linear function of the phase difference φ as follows: Here, the parameter a is a value greater than 0.

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[0043] Here, in the range where the phase difference φ is positive, let the initial values of the phase difference φ and the switching frequency f be the phase difference φ1 and the switching frequency f1, respectively. Similarly, in the range where the phase difference φ is negative, let the initial values of the phase difference φ and the switching frequency f be the phase difference -φ1 and the switching frequency f1, respectively. Also, let the switching frequency f when the phase difference φ is 180° or -180° be f2. Note that when a > 0 in Formula 13, when |φ1| < |φ2| for the phase differences φ1 and φ2, set the switching frequencies f1 and f2 such that f1 < f2.

[0044] At this time, the parameters a and b in Formula 13 are respectively represented by the following mathematical expressions. [Number] (Formula 14) [Number] (Formula 15)

[0045] Also, the change rate (differential value) of the output current I2' with respect to the phase difference φ is represented by the following mathematical expression. [Number] (Formula 16)

[0046] In this embodiment, the phase difference φ1, the switching frequencies f1 and f2 are set so that the parameter a represented by Formula 14 is positive, and the parameters a and b are determined. Thereby, even in the range where the absolute value of the phase difference φ is 90° or more and 180° or less, soft switching control becomes possible while suppressing the reactor current IL.

[0047] Furthermore, the first phase difference φ1 is set to a phase difference greater than 0° and smaller than 90° that is determined based on the step-up ratio k and is equal to or greater than the minimum angle at which soft switching is possible, and the second phase difference φ2 is set to 180°. The phase differences φ1, φ2, and switching frequencies f1, f2 are set so that the differential value expressed by Equation 16, that is, the rate of change of the output current I2' with respect to the phase difference φ in the output current I2', becomes negative. FIG. 6 shows the output current I2' and reactor current ILR with respect to the phase difference φ when set in this way. These set values ​​are expressed as four coordinate points P1 (φ1, f1), P2 (-φ1, f1), P1' (180°, f2), and P2' (-180°, f2) in FIG. 6. In Fig. 6, there is no maximum value of the output current I2' when the phase difference φ is 90° as shown in Fig. 3, and in the range of the phase difference from φ1 to 180° or less, as the phase difference φ increases, the output current I2' decreases, so control is possible from values ​​where the phase difference φ is smaller than 90°. Also, in the reference example shown in Fig. 3, in the region where the phase difference is smaller than 90°, the ratio of the output current I2' to the reactor current ILR is small, and the reactor current ILR is small. Similarly, in the present embodiment, in the region where the phase difference is smaller than 90°, the ratio of the output current I2' to the reactor current ILR is small, and the reactor current ILR can be reduced.

[0048] As described above, the DC / DC converter 1000 of this embodiment controls the switching frequency f to be increased when the absolute value of the phase difference φ is increased within the range of 90° to 180°. This enables soft switching control while suppressing the reactor current IL even when the absolute value of the phase difference φ is within the range of 90° to 180°. Furthermore, in the range of the minimum angle at which soft switching is possible to be greater than or equal to 180°, the phase difference φ and the switching frequency f are controlled so that the rate of change of the output current I2' with respect to the phase difference φ becomes negative. This reduces the reactor current ILR, enabling the DC / DC converter to be made smaller and more efficient.

[0049] Although the embodiment of the present invention has been described above, it goes without saying that the technical scope of the present invention should not be interpreted as being limited by the description of the embodiment. The embodiment is merely an example, and it is understood by those skilled in the art that various modifications of the embodiment are possible within the scope of the invention described in the claims. The technical scope of the present invention should be determined based on the scope of the invention described in the claims and its equivalents. [Explanation of symbols]

[0050] 10: Switching control section 100: First bridge circuit 200: Second bridge circuit 1000:DC / DC converter T: Trans Q1 to Q8: Switching elements D1 to D8: Freewheeling diodes C1 to C8: Capacitors C9: Input capacitor C10: Output capacitor E1: Input power supply L1: First inductor L2: Second inductor L3: Primary winding L4: Secondary winding Lо:Load V1: First DC voltage V2: Second DC voltage

Claims

1. a transformer having a primary winding and a secondary winding; a first bridge circuit that converts a first DC voltage into an AC voltage in response to a first switching control signal and supplies the AC voltage to the primary winding; a second bridge circuit that converts an AC voltage supplied from the secondary winding side into a DC voltage in response to a second switching control signal having the same switching frequency as the first switching control signal, and outputs a second DC voltage; an inductor including a first inductor between the first bridge circuit and the transformer and a second inductor between the second bridge circuit and the transformer; a control unit that controls the first bridge circuit by the first switching control signal and the second bridge circuit by the second switching control signal; Equipped with the second switching control signal has a controllable phase difference with respect to the first switching control signal; The control unit increases the switching frequency when the absolute value of the phase difference is increased within a range of 90° to 180°.

2. a transformer having a primary winding and a secondary winding; a first bridge circuit that converts a first DC voltage into an AC voltage in response to a first switching control signal and supplies the AC voltage to the primary winding; a second bridge circuit that converts an AC voltage supplied from the secondary winding side into a DC voltage in response to a second switching control signal having the same switching frequency as the first switching control signal, and outputs a second DC voltage; an inductor including a first inductor between the first bridge circuit and the transformer and a second inductor between the second bridge circuit and the transformer; a control unit that controls the first bridge circuit by the first switching control signal and the second bridge circuit by the second switching control signal; Equipped with the second switching control signal has a controllable phase difference with respect to the first switching control signal; The control unit controls the phase difference and the switching frequency so that the output current change rate with respect to the phase difference in the output current of the second DC voltage becomes negative within a range of an angle equal to or larger than the minimum angle at which soft switching is possible and equal to or smaller than 180°.

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