Current resonance DC / DC converter

Abstract

To provide a current resonance DC / DC converter which is relatively versatile and capable of performing synchronous rectification control in a simple manner.SOLUTION: A current resonance DC / DC converter 10 includes a primary circuit 1 including switching elements Q1 to Q4, a first resonance circuit 2, an insulating transformer Tr, a secondary circuit 3 including switching elements Q5 to Q8, and a control part 4. When transmitting power in a forward direction from the primary circuit 1 to the secondary circuit 2, the control part 4 detects a change rate of driving current flowing through the first resonance circuit 2 and determines the turn-off timing of switching elements Q5 to Q8 for synchronous rectification in accordance with the change rate.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a current resonance type DC / DC converter.

Background Art

[0002] In an LLC-type current resonance type DC / DC converter, by replacing the rectifying element of the secondary rectifier circuit with a switching element and performing synchronous rectification control on the rectifier circuit, the conduction loss of the rectifying element can be reduced, and the power conversion efficiency can be improved. In particular, in a current resonance type DC / DC converter of LLC type or CLLC type capable of bidirectional operation, a switching circuit is provided on the primary side and the secondary side, and since the switching circuit serves as both a drive circuit and a rectifier circuit, there is no need to replace the rectifying element with a switching element, and the originally provided switching element can be utilized. Therefore, in a current resonance type DC / DC converter, the advantage of performing synchronous rectification control is great.

[0003] For synchronous rectification control in a current resonance type DC / DC converter, a current method of detecting the current flowing through the switching element of the rectifier circuit and performing on / off control, and a voltage method of detecting the voltage between the terminals of the current path of the switching element of the rectifier circuit and performing on / off control are known.

[0004] However, in an LLC / CLLC type current resonance type DC / DC converter, since the driving frequency of the switching element becomes 100 [kHz] or more, in the case of the current method, at least a current detector provided in each leg of the rectifier circuit and a comparator for comparing the detected current at high speed or a digital processor for processing the detected current at high speed are required, resulting in a problem of cost increase. Also, although the synchronous rectification current changes greatly and a wide measurement range is required, in the case of the current method, it is necessary to detect the current near zero (since detecting the current immediately before the current value becomes zero leads to an improvement in efficiency), and there is a problem that it is difficult to detect because the current value is easily affected by noise near zero.

[0005] In the case of the voltage method, there is a problem that the voltage between the terminals of the switching element cannot be accurately detected due to the influence of the parasitic inductance of the switching element. In addition, for example, since the battery voltage for an electric vehicle is as high as 450 [V], there is no detection element with high breakdown voltage that can be used, and in the case of the voltage method, there is a problem that the application is limited.

[0006] Patent Document 1 describes a method of comparing a reference voltage signal such as a CR charge / discharge waveform generated by a reference power supply with a detection signal of a resonant current using only the detection signal of the resonant current detected on the primary side, and performing on / off control of a switching element for synchronous rectification. However, since the actual resonant current varies due to variations in the resonance constants caused by the resonance inductance, resonance capacitor, and exciting inductance, it is difficult to fabricate the reference power supply to exactly match the operation of the actual circuit, and it is not practical to fabricate a reference power supply corresponding to each actual individual circuit. Therefore, the method described in Patent Document 1 has low versatility.

[0007] Patent Document 2 describes a method of turning on a switching element for synchronous rectification on the secondary side in synchronization with a drive signal on the primary side, and calculating and turning off the off-time. Accurate circuit constants are required for the calculation of the off-time, but since the actual circuit constants vary due to variations in the characteristics of the components, it is difficult to specify the accurate circuit constants. Therefore, the method described in Patent Document 2 is not practical and has low versatility.

[0008] Patent Document 3 describes a method of turning on and off a synchronous rectifier element based on a timing signal obtained by generating an integrated signal obtained by integrating the voltage generated at both ends of the primary coil of a transformer and an integrated signal proportional to that signal, and comparing these signals with the primary resonance current signal in a resonant DC / DC converter. However, in the method described in Patent Document 3, the integrating circuit for generating the integrated signal is a fixed constant circuit. Since a resonant DC / DC converter has a large variation in excitation inductance, it is necessary to adjust the integrating circuit for each product. However, it is difficult and unrealistic to adjust the integrating circuit for each product, including the fact that the excitation current changes when the input voltage changes. Also, when the driving frequency is higher or lower than the resonance current, the waveform of the current flowing through the primary side is different, and simply comparing with the signal obtained by integrating the voltage of the primary coil alone cannot turn on and off the synchronous rectifier element at the correct timing. In particular, when performing not only frequency modulation control but also phase shift control, burst control, boost control, etc., the resonance current waveform becomes intermittent, making it even more difficult to determine the timing for turning on and off the synchronous rectifier element.

[0009] Patent Document 4 describes a method of predicting the next zero-crossing point of the actual resonance current based on the measurement time from a predetermined point to the point when the peak of the detected current waveform (resonance current waveform) is detected, and a method of detecting the peak of the detected current waveform based on the differential value of the detected current waveform in a wireless power receiving device. However, these methods are for a magnetic resonance circuit and predict the zero-crossing point on the premise that the received current on the secondary side is a sine wave. On the other hand, in a current resonance type DC / DC converter, even in the case of a current resonance type with strong transformer coupling, the resonance current varies greatly around the resonance frequency depending on the driving frequency and does not become a sine wave.

[0010] When the method described in Patent Document 4 is applied to a current resonance type DC / DC converter, as described above, since the resonance current does not become a sine wave, simply detecting the peak of the resonance current cannot predict the next zero-crossing point of the resonance current. In particular, when performing not only frequency modulation control but also phase shift control, burst control, boost control, etc., the resonance current waveform becomes intermittent, making it difficult to even detect the peak of the resonance current. In addition, when the method described in Patent Document 4 and the method described in Patent Document 3 are combined, even if the peak of the resonance current can be detected, the zero-crossing point of the synchronous rectification current cannot be predicted thereby, and synchronous rectification control cannot be easily executed.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] The present invention has been made in view of the above circumstances, and an object thereof is to provide a current resonance type DC / DC converter that is relatively versatile and can easily execute synchronous rectification control.

Means for Solving the Problems

[0013] In order to solve the above problems, a current resonance type DC / DC converter according to the present invention includes a main circuit section and a control section, wherein the main circuit section an isolation transformer including a primary coil and a secondary coil, A primary-side circuit including a plurality of first switching elements, A first resonance circuit provided between the primary-side circuit and the primary-side coil and including a first resonance inductor and a first resonance capacitor, A secondary-side circuit connected to the secondary-side coil and including a plurality of second switching elements, A current resonance type DC / DC converter comprising: The main circuit section, Under the control of the control section, the primary-side circuit operates as a drive circuit, and the secondary-side circuit operates as a synchronous rectification circuit to perform forward power transmission from the primary-side circuit to the secondary-side circuit, The control section during the forward power transmission, Detects a change rate of a drive current flowing through the first resonance circuit, and determines a turn-off timing of the second switching element for synchronous rectification from the change rate.

[0014] In the current resonance type DC / DC converter, The change rate is a change amount of a differential value of the drive current, The control section during the forward power transmission, If the change amount is equal to or greater than a predetermined first threshold value, the second switching element for synchronous rectification can be configured to be turned off.

[0015] In the current resonance type DC / DC converter, The control section during the forward power transmission, After the polarity of the differential value changes from positive to negative after driving the drive circuit, or after the polarity changes from negative to positive after driving, the change amount can be calculated.

[0016] In the current resonance type DC / DC converter, The change rate is a change rate of the drive current per unit time, The control section during the forward power transmission, If the rate of change per unit time is equal to or greater than a predetermined second threshold value, the second switching element for synchronous rectification can be configured to be turned off.

[0017] In the current resonance type DC / DC converter, for example, The rate of change per unit time is the amount of change in the second derivative value of the drive current.

[0018] In the current resonance type DC / DC converter, During forward power transmission, the control unit can be configured to turn off the second switching element for synchronous rectification at a timing that is at least a predetermined margin time earlier than the timing when the amount of change in the derivative value becomes equal to or greater than the first threshold value.

[0019] In the current resonance type DC / DC converter, During forward power transmission, the control unit can be configured to turn off the second switching element when the derivative value at the start of the flow of the resonance current after the turn-on of the first switching element is less than a predetermined minimum value.

[0020] In the current resonance type DC / DC converter, During forward power transmission, the control unit If the turn-off timing of the first switching element is earlier than the turn-off timing of the second switching element determined from the rate of change, the second switching element for synchronous rectification can be configured to be turned off at the turn-off timing of the first switching element.

[0021] In the current resonance type DC / DC converter, The main circuit section can be configured to perform reverse power transmission from the secondary side circuit to the primary side circuit under the control of the control unit, with the secondary side circuit operating as the drive circuit and the primary side circuit operating as the synchronous rectification circuit.

[0022] In the current resonance type DC / DC converter, The main circuit section is provided between the secondary circuit and the secondary coil, and further includes a second resonance circuit including a second resonance inductor and a second resonance capacitor. The control unit during the reverse power transmission can be configured to detect a change rate of a drive current flowing through the second resonance circuit, and determine a turn-off timing of the first switching element for synchronous rectification from the change rate.

Advantages of the Invention

[0023] According to the present invention, it is possible to provide a current resonance type DC / DC converter that is relatively highly versatile and can easily execute synchronous rectification control.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0025] Hereinafter, embodiments of a current resonance type DC / DC converter according to the present invention will be described with reference to the accompanying drawings.

[0026] [First Embodiment] FIG. 1 shows a current resonance type DC / DC converter 10 according to the first embodiment of the present invention. The current resonance type DC / DC converter 10 is an LLC type DC / DC converter, and includes a main circuit section, a control section 4, and first detection means 5.

[0027] The main circuit section includes terminals T1, T2, a capacitor C11, terminals T3, T4, a capacitor C21, a high-frequency isolation transformer Tr (hereinafter abbreviated as "transformer Tr"), a primary side circuit 1, a first resonance circuit 2, and a secondary side circuit 3. The main circuit section performs forward power transmission from terminals T1, T2 to terminals T3, T4 under the control of the control section 4.

[0028] For example, DC terminals of a bidirectional PFC (power factor correction) circuit are connected to terminals T1, T2, and a DC input voltage V1 is input. A capacitor C11 for reducing input ripple during forward power transmission is connected between terminals T1, T2.

[0029] For example, a load such as an electric vehicle battery is connected to terminals T3, T4. A DC output voltage V2 is output from terminals T3, T4. A capacitor C21 for reducing output ripple during forward power transmission is connected between terminals T3, T4.

[0030] Transformer Tr is composed of one or more high-frequency insulated transformers. The primary coil (primary winding) of transformer Tr is connected to the primary circuit 1 via the first resonance circuit 2. The secondary coil (secondary winding) of transformer Tr is connected to the secondary circuit 3.

[0031] The primary circuit 1 includes a first leg and a second leg connected in parallel, and each leg includes an upper arm and a lower arm connected in series, which is a full-bridge circuit. The upper arm of the first leg includes the switching element Q1, the lower arm of the first leg includes the switching element Q2, the upper arm of the second leg includes the switching element Q3, and the lower arm of the second leg includes the switching element Q4. Diodes D1 to D4 are connected in parallel in the reverse direction in the current paths of the switching elements Q1 to Q4, and capacitors C1 to C4 are connected in parallel. The switching elements Q1 to Q4 correspond to the "first switching elements" of the present invention.

[0032] The collector (or drain) terminals of the switching elements Q1 and Q3 (the high-voltage side connection points of the first leg and the second leg) are connected to the terminal T1, and the emitter (or source) terminals of the switching elements Q2 and Q4 (the low-voltage side connection points of the first leg and the second leg) are connected to the terminal T2. The connection point between the emitter terminal of the switching element Q1 and the collector terminal of the switching element Q2 and the connection point between the emitter terminal of the switching element Q3 and the collector terminal of the switching element Q4 are connected to the first resonance circuit 2.

[0033] As the switching elements Q1 to Q4, for example, power semiconductor switching elements capable of high-frequency switching such as IGBT (Insulated Gate Bipolar Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor) can be used. The same applies to the switching elements Q5 to Q8 described later.

[0034] Diodes D1 to D4 are reflux diodes and may be built-in diodes of switching elements Q1 to Q4, external diodes, or both. The same applies to diodes D5 to D8 described later. Also, capacitors C1 to C4 are partial resonance capacitors and may be parasitic capacitances of switching elements Q1 to Q4, external capacitors, or both. The same applies to capacitors C5 to C8 described later.

[0035] The first resonance circuit 2 includes a first resonance inductor (an induction coil, sometimes simply called a coil) Lr1 and a first resonance capacitor (sometimes called a capacitor) Cr1. The first resonance circuit 2 is provided between the primary side circuit 1 and the primary side coil of the transformer Tr. In the present embodiment, the first resonance inductor Lr1 and the first resonance capacitor Cr1 constitute a series resonance circuit together with the exciting inductance of the transformer Tr during forward power transmission. Although the exciting inductance of the transformer Tr is included in the primary side coil of the transformer Tr and is not shown in the figure, it may be a coil having an individual core.

[0036] As long as the first resonance inductor Lr1 and the first resonance capacitor Cr1 are connected to the primary side coil of the transformer Tr to form a series resonance circuit, their arrangement is arbitrary. For example, the first resonance inductor Lr1 and the first resonance capacitor Cr1 may be arranged separately on the primary side and the secondary side of the transformer Tr. Also, the first resonance inductor Lr1 may be the leakage inductance of the transformer Tr, a coil having an individual core, or both. The first resonance capacitor Cr1 may be composed of individual capacitors, the parasitic capacitance of the switching elements Q1 to Q4, or both. The same applies to the second resonance inductor Lr2 and the second resonance capacitor Cr2 of the second embodiment described later.

[0037] The secondary circuit 3 is a full-bridge circuit including a third leg and a fourth leg connected in parallel, and each leg includes an upper arm and a lower arm connected in series. The upper arm of the third leg includes a switching element Q5, the lower arm of the third leg includes a switching element Q6, the upper arm of the fourth leg includes a switching element Q7, and the lower arm of the fourth leg includes a switching element Q8. Diodes D5 to D8 are connected in parallel in the reverse direction to the current paths of the switching elements Q5 to Q8, and capacitors C5 to C8 are connected in parallel. The switching elements Q5 to Q8 correspond to the "second switching elements" of the present invention.

[0038] The collector (or drain) terminals of the switching elements Q5 and Q7 (the high-voltage side connection points of the third leg and the fourth leg) are connected to the terminal T3, and the emitter (or source) terminals of the switching elements Q6 and Q8 (the low-voltage side connection points of the third leg and the fourth leg) are connected to the terminal T4. The connection point between the emitter terminal of the switching element Q5 and the collector terminal of the switching element Q6 is connected to one end of the secondary coil of the transformer Tr, and the connection point between the emitter terminal of the switching element Q7 and the collector terminal of the switching element Q8 is connected to the other end of the secondary coil of the transformer Tr.

[0039] The control unit 4 includes a processing unit (including a storage unit) that generates a control signal for performing on / off control of the switching elements Q1 to Q8, and a driving unit (not shown) that turns on and off the switching elements Q1 to Q8 based on the control signal. The control unit 4 may be composed of a digital circuit such as a microprocessor or a digital signal processor, or may be composed of an analog circuit, or may be composed of a circuit combining a digital circuit and an analog circuit. The control unit 4 may further include a detection unit (not shown). The detection unit includes, for example, a detection circuit such as a current sensor and / or a voltage sensor that detects a current value and / or a voltage value necessary for the control of the control unit 4, and its peripheral circuits. In this embodiment, the detection unit does not include the first detection means 5.

[0040] During forward power transmission, the control unit 4 operates the primary-side circuit 1 as a drive circuit and the secondary-side circuit 3 as a synchronous rectification circuit. Specifically, during forward power transmission, the control unit 4 performs frequency modulation control to modulate the drive frequency for the switching elements Q1 to Q4 of the primary-side circuit 1 according to the input-output characteristics of the LLC method as the on-off control, and performs synchronous rectification control for the switching elements Q5 to Q8 of the secondary-side circuit 3. Although details will be described later, the synchronous rectification control of this embodiment is different from the conventional synchronous rectification control in the turn-off timing of the switching elements Q5 to Q8 of the synchronous rectification circuit.

[0041] The first detection means 5 is a current detector that detects the current (drive current) flowing through the first resonance circuit 2. As will be described later, the drive current includes a resonance current and an exciting current. The first detection means 5 detects the drive current, generates a detection signal (for example, a voltage signal) according to the current value of the drive current, and outputs it to the control unit 4. The position of the first detection means 5 can be arbitrarily changed as long as it can detect the drive current.

[0042] Fig. 2 shows the control timing of the switching elements Q1 to Q8 during forward power transmission. The control unit 4 during forward power transmission repeatedly performs the control of modes 1 to 4. The primary-side circuit 1 operates as a drive circuit by frequency modulation control, and the secondary-side circuit 3 operates as a synchronous rectification circuit by synchronous rectification control.

[0043] The control unit 4 determines the drive frequencies of the switching elements Q1 to Q4 by frequency modulation control, and turns on and off the switching elements Q1 and Q4 at the same timing, turns on and off the switching elements Q2 and Q3 at the same timing, and turns on and off the switching elements Q1 and Q2 alternately with a 180° phase difference with a predetermined dead time (not shown), and turns on and off the switching elements Q3 and Q4 alternately with a 180° phase difference with a predetermined dead time (not shown). The on-duty of the switching elements Q1 to Q4 is set to, for example, 50%. Strictly speaking, the on-duty of the switching elements Q1 to Q4 decreases or increases by the dead time from 50%.

[0044] The control unit 4 turns on the switching elements Q5 and Q8 in synchronization with the turn-on timing of the switching elements Q1 and Q4 by synchronous rectification control (at times t1 and t5), and turns on the switching elements Q6 and Q7 in synchronization with the turn-on timing of the switching elements Q2 and Q3 (at time t3).

[0045] Also, the control unit 4 turns off the switching elements Q5 and Q8 at a timing earlier than or the same as the turn-off timing of the switching elements Q1 and Q4 (at time t3) (at time t2), and turns off the switching elements Q6 and Q7 at a timing earlier than or the same as the turn-off timing of the switching elements Q2 and Q3 (at time t5) (at time t4). For this reason, in the period of mode 2 from time t2 to t3 and the period of mode 4 from time t4 to t5, the switching elements Q5 to Q8 of the synchronous rectification circuit are off.

[0046] Fig. 3 shows the current path diagrams in each mode of Fig. 2. (A) in Fig. 3 shows the current path diagram in mode 1, (B) shows the current path diagram in mode 2, (C) shows the current path diagram in mode 3, and (D) shows the current path diagram in mode 4, respectively.

[0047] In mode 1 of Fig. 3(A), when the switching elements Q1 and Q4 of the primary side circuit 1 turn on at time t1 (time t5), a resonant current is generated in the first resonant circuit 2. Similarly, when the switching elements Q5 and Q8 of the secondary side circuit 3 turn on at time t1 (time t5), synchronous rectification starts in the secondary side circuit 3, and the load current flows to the terminals T3 and T4.

[0048] In mode 2 of FIG. 3(B), when the switching elements Q5 and Q8 turn off at time t2, synchronous rectification ends in the secondary circuit 3. In the primary circuit 1, the resonant current converges and only the exciting current flows. Therefore, even if the switching elements Q5 and Q8 in the secondary circuit 3 remain on, no load current will flow, and it may seem that there is no effect. However, if the switching elements Q5 and Q8 are kept on during the period of mode 2, a current reversal phenomenon will occur immediately after the synchronous rectification current has finished flowing, resulting in problems such as a decrease in power conversion efficiency. Therefore, the control unit 4 of the present embodiment turns off the switching elements Q5 and Q8 for synchronous rectification at the time (time t2) when the load current due to the synchronous rectification current has finished flowing.

[0049] In mode 3 of FIG. 3(C), when the switching elements Q2 and Q3 of the primary circuit 1 turn on at time t3, a resonant current in the reverse direction is generated in the first resonant circuit 2. Also at time t3, when the switching elements Q6 and Q7 of the secondary circuit 3 turn on, synchronous rectification starts in the secondary circuit 3, and the load current flows through the terminals T3 and T4.

[0050] In mode 4 of FIG. 3(D), when the switching elements Q6 and Q7 turn off at time t4, synchronous rectification ends in the secondary circuit 3. In the primary circuit 1, the reverse resonant current converges and only the reverse exciting current flows. The control unit 4 of the present embodiment turns off the switching elements Q6 and Q7 for synchronous rectification at the time (time t4) when the load current due to the synchronous rectification current has finished flowing, thereby avoiding problems such as a decrease in power conversion efficiency due to the current reversal (backflow) phenomenon.

[0051] Note that both the resonant current and the exciting current flow in the primary circuit 1 and the first resonant circuit 2, but only the load current (synchronous rectification current) corresponding to the resonant current flows in the secondary circuit 3. Therefore, even without detecting the current in each leg of the secondary circuit 3, if the switching elements Q5 to Q8 of the secondary circuit 3 are turned off when the resonant current becomes zero and only the exciting current remains, the switching elements Q5 to Q8 can be turned off when the load current has finished flowing.

[0052] Also, if the turn-off timing of the switching elements Q5 to Q8 in the secondary circuit 3 is too early, the secondary circuit 3 becomes a diode rectifier circuit instead of a synchronous rectifier circuit, and the synchronous rectifier current flows through the diodes D5 to D8. Therefore, the power conversion efficiency decreases due to the conduction losses of the diodes D5 to D8. Therefore, it is desirable that the turn-off timing of the switching elements Q5 to Q8 in the secondary circuit 3 be at the point when the resonance current becomes zero, that is, at the point when the load current has finished flowing, as in the present embodiment.

[0053] Fig. 4 shows waveforms for one cycle of the drive current (resonance current and exciting current) during frequency modulation control. Fig. 4(A) is a waveform diagram when the drive frequency ≤ resonance frequency, and Fig. 4(B) is a waveform diagram when the drive frequency > resonance frequency. In Fig. 4, the vertical axis represents the current value and the horizontal axis represents time. The times t1 to t4 on the horizontal axis correspond to the times t1 to t4 shown in Fig. 2.

[0054] In Fig. 4, the solid line is the waveform of the resonance current and the dashed line is the waveform of the exciting current. As shown in Fig. 4, when detecting the current flowing through the first resonance circuit 2, that is, the drive current, a current waveform obtained by adding the resonance current and the exciting current is detected. The resonance current is a current due to the resonance of the resonance inductance of the first resonance inductor Lr1 and the resonance capacitance of the first resonance capacitor Cr1, and the exciting current is a current flowing through an exciting coil (exciting inductance) (not shown) of the transformer Tr.

[0055] As shown in Fig. 4(A), when the drive frequency ≤ resonance frequency, the resonance current ends earlier than half a cycle. On the other hand, as shown in Fig. 4(B), when the drive frequency > resonance frequency, the resonance current becomes longer than half a cycle.

[0056] Note that when boost control (for example, control to form a short - circuit in the secondary - side circuit 3 to perform a boosting operation) is performed, the resonance current ends earlier than the waveform shown in Fig. 4(A). Also, when phase - shift control (for example, control to shift the on - off timing of the first leg and the on - off timing of the second leg of the primary - side circuit 1 by a predetermined phase - shift amount) or burst (intermittent) control (for example, control to provide a standby period during which the on - off operation of the switching elements Q1 to Q4 is not performed) is performed, the resonance current ends earlier than the waveform shown in Fig. 4(A), and the waveform becomes such that it is interrupted halfway, and after the interruption, only the exciting current flows (illustration is omitted).

[0057] Fig. 5 shows a waveform diagram of the differential value of the drive current (resonance current + exciting current) flowing through the first resonance circuit 2 in the period around half a cycle before and after the resonance current ends. Fig. 5(A) corresponds to Fig. 4(A), and Fig. 5(B) corresponds to Fig. 4(B). In Fig. 5, the horizontal axis is the differential - processing time near the times t2 and t3, and the vertical axis is the value obtained by differentiating the drive current, expressed in degrees [deg]. That is, it is a diagram representing the waveform of the differential value near the time when the resonance current stops flowing in the positive direction and starts flowing in the negative direction.

[0058] In Fig. 5(A), the differential - processing time 3 corresponds to the time t2 in Fig. 4(A), and the differential - processing time 5 corresponds to the time t3 in Fig. 4(A). The differential waveform shown in Fig. 5(A) was about - 55° at the differential - processing time 1, but changed steeply in a very short time from the differential - processing time 2 and became zero at the differential - processing time 3 (time t2). That is, in a short time around the time t2 in Fig. 4(A), the differential waveform changes greatly from a large negative value to zero in the positive direction. The control unit 4 detects this steep change amount (change amount of positive differential value of about + 55°) in this short time, determines that the resonance current has ended and changed to only the exciting current, and turns off the switching elements Q5 to Q8 for synchronous rectification. Although not shown, at the time t4 when the resonance current flows in the reverse direction, in a short time around the time t4, the differential waveform changes greatly from a large positive value to zero in the negative direction.

[0059] When the driving frequency ≤ the resonance frequency, the control unit 4 detects that the differential value of the driving current changes from a very large negative value to a positive value (or zero) or from a very large positive value to a negative value (or zero) in a very short time, recognizes that the resonance current becomes zero, the driving current becomes the same value as the exciting current, and the load current becomes zero, and thereby recognizes that this is the timing to turn off the switching elements Q5 to Q8 for synchronous rectification.

[0060] In FIG. 5(B), the differential processing time 4 corresponds to the times t2 and t3 in FIG. 4(B). The differential waveform shown in FIG. 5(B) was about -36° at the time of differential processing time 1, but changes steeply in a very short time from differential processing time 3 and becomes about -83° at differential processing time 4 (times t2 and t3). That is, in a very short time around the times t2 and t3 in FIG. 4(B), the differential waveform changes greatly in the negative direction from a negative value to a larger negative value. The control unit 4 detects this steep change amount (a change amount of a negative differential value of about -47°) in this very short time, determines that the resonance current has ended and changed only to the exciting current, and turns off the switching elements Q5 to Q8 for synchronous rectification. Although not shown, at the time t4 when the resonance current flows in the reverse direction, in a very short time around the time t4, the differential waveform changes greatly in the positive direction from a positive value to a larger positive value.

[0061] When the driving frequency > the resonance frequency, the control unit 4 detects that the differential value of the driving current changes from a negative value to a larger negative value or from a positive value to a larger positive value in a very short time, recognizes that the resonance current becomes zero, the driving current becomes the same value as the exciting current, and the load current becomes zero, and thereby recognizes that this is the timing to turn off the switching elements Q5 to Q8 for synchronous rectification.

[0062] In addition, when the control unit 4 turns off the switching elements Q1 to Q4 of the driving circuit simultaneously (for example, during the standby period of burst control, etc.), the switching elements Q5 to Q8 for synchronous rectification are also turned off simultaneously.

[0063] In the above description, the control unit 4 detects the change amount of the positive differential value when the driving frequency ≤ the resonance frequency, and detects the change amount of the negative differential value when the driving frequency > the resonance frequency. Instead of this, the absolute value of the change amount of the differential value may be detected. Further, the change amount of the differential value of the driving current is an example of the "change rate of the driving current" of the present invention.

[0064] As the detection process of the change amount of the differential value, the control unit 4 may adopt analog processing by an analog circuit, digital processing by a digital circuit, or a combined process of analog processing and digital processing.

[0065] FIG. 6 shows a differential circuit using an operational amplifier OP as an example of an analog circuit that performs the detection process of the change amount of the differential value. This differential circuit has a differential characteristic determined by the input capacitor Cs and the feedback resistor Rf, but a series resistor Rs is provided to limit the gain (amplification factor) for oscillation prevention. The input voltage Vs input to the input terminal is the output voltage (detection signal) of the first detection means 5. The output voltage Vo becomes the differential value of the driving current. By comparing the waveform (differential waveform) of the output voltage Vo with a comparison reference voltage using a comparator, turn-off signals for the switching elements Q5 to Q8 for synchronous rectification can be generated. Note that the output voltage Vo may be compared with a positive comparison reference voltage and a negative comparison reference voltage, or the output voltage Vo may be compared with the comparison reference voltage after passing through an absolute value circuit.

[0066] Alternatively, the output voltage (detection signal) of the first detection means 5 is taken into the digital circuit (such as a microprocessor or a digital signal processor) of the control unit 4, and the differential algorithm by digital processing is executed to detect the change amount of the differential value. When performing digital processing, when the switching elements Q1 and Q4 are turned on, a positive resonance current flows, so a positive change in the differential value can be detected. When the switching elements Q3 and Q2 are turned on, a negative resonance current flows, so a negative change in the differential value can be detected. Therefore, positive and negative comparisons can be determined in advance.

[0067] (First Control Example) Fig. 7 shows the control flow of the first control example of on-off control when digital processing is adopted.

[0068] The control unit 4 that starts the on-off control process in step S100 reads and obtains the threshold value of the differential value stored in advance in the storage unit in the control unit 4 (S101).

[0069] Next, the control unit 4 reads the input / output information such as the current value and / or voltage value required for control from the detection circuit through the A / D conversion circuit or the like into the processing unit in the control unit 4, determines the driving frequency by frequency modulation control, and drives the primary circuit 1 at a duty ratio of 50% as shown in Fig. 2 (S102).

[0070] During frequency modulation control, the control unit 4 compares the output value (output current value, output voltage value, or output power value) included in the input / output information with the target value (target output current value, target output voltage value, or target output power value), and determines the driving frequencies of the switching elements Q1 to Q4 so that the output value approaches the target value. In frequency modulation control, when the output value is smaller than the target value, the control unit 4 decreases the driving frequency to increase the output. On the other hand, when the output value is larger than the target value, the control unit 4 increases the driving frequency to decrease the output. Note that the target value is either instructed from a separate upper-level device or read from the storage unit as a previously specified value.

[0071] The control unit 4 turns on the synchronous rectification switching elements Q5 to Q8 of the secondary circuit 3 by synchronous rectification control in accordance with the driving of the primary circuit 1 (S103). Specifically, the control unit 4 turns on the switching elements Q5 and Q8 in synchronization with the turn-on timing of the switching elements Q1 and Q4, and turns on the switching elements Q6 and Q7 in synchronization with the turn-on timing of the switching elements Q2 and Q3.

[0072] Next, the control unit 4 acquires, as a voltage signal, the current value of the drive current (resonant current + exciting current) flowing through the first resonant circuit 2 from the first detection means 5, performs differential processing in the processing unit within the control unit 4, acquires the differential waveform of the voltage signal, and acquires the change amount of the differential value from the differential waveform (S104). For example, the control unit 4 stores the differential values acquired by the differential processing in the storage unit for a certain period for each unit time of performing the differential processing, and calculates the change amount of the differential value from the stored differential values.

[0073] When the differential waveform is as shown in Fig. 5(A), that is, when the drive frequency ≤ resonant frequency in frequency modulation control, or when resonance is terminated halfway by boost control, phase shift control, etc., and when the switching elements Q1 and Q4 are on and a positive resonant current flows, the change amount of the differential value is, for example, a positive differential value change amount of about +55° during the period from differential processing times 1 to 3. Note that when the switching elements Q3 and Q2 are on and a negative resonant current flows, the change amount of the differential value becomes a negative differential value change amount. When the differential waveform is as shown in Fig. 5(B), that is, when the drive frequency > resonant frequency in frequency modulation control and when the switching elements Q1 and Q4 are on and a positive resonant current flows, the change amount of the differential value is, for example, a negative differential value change amount of about -47° during the period from differential processing times 1 to 4. Note that when the switching elements Q3 and Q2 are on and a negative resonant current flows, the change amount of the differential value becomes a positive differential value change amount.

[0074] The control unit 4 compares the change amount of the differential value acquired in step S104 with the threshold value acquired in step S101 (S105). When the change amount of the differential value is greater than the threshold value (YES in S105), the control unit turns off the switching elements Q5 to Q8 for synchronous rectification of the secondary side circuit 3 and stops the synchronous rectification control (S106). When the change amount of the differential value is less than or equal to the threshold value (NO in S105), if the switching elements Q1 to Q4 of the primary side circuit 1 are turned off (YES in S107), the control unit turns off the switching elements Q5 to Q8 for synchronous rectification (S106). If the switching elements Q1 to Q4 of the primary side circuit 1 are not turned off (NO in S107), the control unit returns to step S104 and repeats the subsequent processing.

[0075] Next, the control unit 4 determines whether to continue the on-off control (S108). For example, if there is an end command from a higher-level device, it is determined not to continue the control (NO in S108), and the on-off control is terminated (S109). On the other hand, the control unit 4 determines to continue the control if there is no end command from a higher-level device, for example (YES in S108), and repeats the processing after step S102.

[0076] By controlling in this way, differential processing is performed on the signal regarding the current value of the drive current (resonance current + excitation current) flowing through the first resonance circuit 2, and by comparing the change amount of the differential value obtained by the differential processing with a predetermined threshold value, the switching elements Q5 to Q8 for synchronous rectification in the secondary circuit 3 are turned off. For this reason, the detection means necessary for controlling the turn-off of the switching elements Q5 to Q8 for synchronous rectification is only the first detection means 5, and cost reduction can be achieved. In addition, since the control unit 4 handles the differential value of the drive current, the point in time when the resonance current ends can be detected as a large change amount of the differential value, so the turn-off timing of the switching elements Q5 to Q8 for synchronous rectification can be easily determined, and synchronous rectification control can be simply executed.

[0077] Also, when the operating frequency > resonance frequency, the control unit 4 turns off the switching elements Q5 to Q8 for synchronous rectification in accordance with the turn-off timing of the switching elements Q1 to Q4 in the primary circuit 1 in synchronization with the half cycle of the drive frequency (half cycle of the drive current) (YES in step S107), so that the switching elements Q5 to Q8 for synchronous rectification can be surely turned off.

[0078] Note that the threshold value obtained in step S101 may be one value (including positive and negative values as a set) or a plurality of values. In the case of a plurality of values, for example, a threshold value when the drive frequency ≤ the resonance frequency and a threshold value when the drive frequency > the resonance frequency can be provided. Further, the change amount of the differential value may be compared with its absolute value as the threshold value, or in the case where the drive frequency ≤ the resonance frequency and in the case of performing boost control, when a positive resonance current flows, the change amount of the differential value is set as a positive threshold value, and when a negative resonance current flows, it is compared with a negative threshold value. In the case where the drive frequency > the resonance frequency, when a positive resonance current flows, the change amount of the differential value is set as a negative threshold value, and when a negative resonance current flows, it may be compared with a positive threshold value.

[0079] Also, in step S104, the range of the differential waveform (detection range of the change amount) used to calculate the change amount of the differential value can be appropriately changed according to the shape of the differential waveform and the like. For example, when the differential value after driving the drive circuit (after turning on the switching elements Q1 to Q4) is positive, the control unit 4 may detect the change amount from the point when the differential value once becomes negative to the point when it greatly changes to another positive value or negative value. When the differential value after driving the drive circuit (after turning on the switching elements Q1 to Q4) is negative, the control unit 4 may detect the change amount from the point when the differential value once becomes positive to the point when it greatly changes to another positive value or negative value.

[0080] (Second control example) FIG. 8 shows the control flow of the second control example of the on-off control when digital processing is adopted.

[0081] The control flow of the second control example, in addition to the processing of the first control example shown in FIG. 7, advances the turn-off timing of the switching elements Q5 to Q8 for synchronous rectification by a margin time determined in advance based on the change amount of the differential value of the previous time, thereby absorbing the detection delay of the change amount of the differential value and reliably stopping the synchronous rectification control before the synchronous rectification current becomes zero.

[0082] The control unit 4 that started the on / off control process in step S200 reads and acquires a differential value threshold value and a margin time that are stored in advance in the storage unit within the control unit 4 (S201). The margin time is a time corresponding to time delays such as the detection delay of the first detection means 5 and the processing of the control unit 4 (for example, the differential processing in step S204, the comparison processing in step S205, and the stop processing in step S209).

[0083] Since the processes of steps S202 to S205 are the same as the processes of steps S102 to S105 in FIG. 7, the description thereof is omitted.

[0084] In the comparison process of step S205, when the change amount of the differential value is larger than the threshold value (YES in S205), the control unit 4 calculates the timing at which the change amount of the differential value exceeds the threshold value, and stores the timing in the storage unit (S206). The above timing corresponds to the time (hereinafter referred to as the first time) from after the driving of the drive circuit in step S202 (after the turn-on of the switching elements Q1 to Q4) to the current time (the comparison processing time in step S205).

[0085] In the comparison process of step S205, when the change amount of the differential value is less than or equal to the threshold value (NO in S205), the control unit 4 calculates the first time, which is the elapsed time this time, and calculates the time (hereinafter referred to as the second time) obtained by subtracting the margin time acquired in step S201 from the previous timing (the first time when the previous threshold value was exceeded) stored in the storage unit, and compares the first time this time with the second time. That is, the control unit 4 determines whether the current time (the comparison processing time in step S205) has reached the timing of the time (margin time) obtained by subtracting the margin time from the timing corresponding to the previous timing (S207).

[0086] When the current time reaches the timing of the margin time (when the first time ≥ the second time) (YES in S207), the control unit 4 proceeds to the process of step S209. When the current time has not reached the timing of the margin time (when the first time < the second time) (NO in S207), if the switching elements Q1 to Q4 of the primary side circuit 1 are turned off (YES in S208), the control unit 4 proceeds to the process of step S209. If the switching elements Q1 to Q4 of the primary side circuit 1 are not turned off (NO in S208), it returns to step S204 and repeats the subsequent processes.

[0087] Since the processes of steps S209 to S211 are the same as the processes of steps S106 to S109 in FIG. 7, the description is omitted.

[0088] By controlling in this way, it is possible to easily prevent the turn-off timing of the switching elements Q5 to Q8 for synchronous rectification from being delayed due to the detection delay of the first detection means 5 or the delay such as the differential processing of the control unit 4, and it is possible to surely prevent the reverse current (backflow) phenomenon of the current due to the delay of the turn-off timing.

[0089] (Third control example) FIG. 9 shows the control flow of the third control example of the on-off control when digital processing is adopted.

[0090] In the first control example and the second control example, the control unit 4 determines that the resonance current has ended and only the exciting current has changed based on the change amount of the differential value of the drive current (the steep change amount in a short time), and turns off the switching elements Q5 to Q8 for synchronous rectification. However, when the load becomes light and the resonance current becomes small, it is assumed that the difference between the change amount of the slope of the resonance current and the change amount of the slope of the exciting current becomes small, and it becomes difficult to detect the steep change amount in a short time.

[0091] Therefore, in the third control example, when the differential value of the drive current at the start of the resonance current flow after the drive of the drive circuit (after the turn-on of the switching elements Q1 to Q4) is less than a predetermined minimum value, the control unit 4 turns off the switching elements Q5 to Q8 for synchronous rectification and operates the secondary circuit 3 as a diode rectifier circuit.

[0092] If the input voltage is constant, the magnitude of the exciting current is determined by the exciting inductance of the transformer Tr, and since the slope of the exciting current is constant, as long as the resonance current is slightly larger than the exciting current, the resonance current is sufficiently small, and it can be expected that the synchronous rectification current on the secondary side is also sufficiently small. For this reason, even if the secondary circuit 3 is operated as a diode rectifier circuit, the conduction losses of the diodes D5 to D8 are within an acceptable range.

[0093] Specifically, the control unit 4 that started the on-off control process in step S300 reads and acquires a threshold value of the differential value and a minimum value of the differential value stored in advance from the storage unit in the control unit 4 (S301). The minimum value of the differential value is a predetermined value larger than the slope of the current waveform due to only the exciting current. This value corresponds to, for example, the slope of the drive current waveform flowing through the first resonance circuit 2 when the resonance current is minimum.

[0094] Since the processes of steps S302 to S304 are the same as the processes of steps S102 to S104 in FIG. 7, the description thereof is omitted.

[0095] In step S305, the control unit 4 compares the differential value acquired in step S304 with the minimum value acquired in step S301 (S305). If the differential value is smaller than the minimum value (YES in S305), the switching elements Q5 to Q8 for synchronous rectification of the secondary circuit 3 are turned off to stop the synchronous rectification control (S306). The process of step S306 is the same as the process of step S106 in FIG. 7.

[0096] When the differential value is equal to or greater than the minimum value (NO in S305), the control unit 4 proceeds to the process of step S307. Since the processes of steps S307 to S310 are the same as the processes of steps S105, S107 to S109 in FIG. 7, the description thereof is omitted.

[0097] By controlling in this way, it is possible to surely turn off the switching elements Q5 to Q8 for synchronous rectification even during low load. Note that since the exciting current also changes depending on the input voltage V1, when the input voltage V1 changes, the control unit 4 may change the minimum value of the differential value in accordance with the change in the input voltage V1. Further, when the change amount of the differential value is equal to or greater than the minimum value (NO in S305), the process may proceed to the process of turning on the secondary side circuit 3 in synchronization with the primary side circuit 1 (S303).

[0098] As described above, in the current resonance type DC / DC converter 10, instead of detecting the synchronous rectification current flowing through the synchronous rectification circuit, the drive current (resonance current + exciting current) flowing through the first resonance circuit 2 on the primary side is detected, and the time point when the resonance current ends and only the exciting current remains is determined from the change amount of the differential value of the detection signal. For this reason, in the current resonance type DC / DC converter 10, it is not necessary to detect the current value for each leg of the secondary side circuit 3, and the detection means necessary for controlling the turn-off of the switching elements Q5 to Q8 for synchronous rectification is only the first detection means 5, and cost reduction can be achieved. Moreover, since the control unit 4 does not detect zero current that is difficult to detect in a wide current range, but detects the change amount of the differential value of the drive current, the turn-off of the switching elements Q5 to Q8 can be controlled based on a steep and large change amount in a short time. That is, in the current resonance type DC / DC converter 10, synchronous rectification control can be easily executed.

[0099] Compared with the voltage detection method of the synchronous rectification circuit, the current resonance type DC / DC converter 10 does not need to detect the voltage for each leg of the synchronous rectification circuit and does not need to use high withstand voltage components (detection means), so in addition to achieving cost reduction, it can also be applied to power supply circuits for electric vehicles etc. with a wide voltage range. That is, the current resonance type DC / DC converter 10 has relatively high versatility.

[0100] As described above, the current resonance type DC / DC converter 10 is configured to detect the actual drive current (resonance current + excitation current) flowing through the first resonance circuit 2, and determine the point in time when the resonance current ends based on the change amount of the differential value of the detection signal, rather than determining the point in time when the resonance current ends based on the resonance constant of the resonance circuit. Therefore, the current resonance type DC / DC converter 10 does not need to perform special calculations to determine the point in time when the resonance current ends, and can easily execute synchronous rectification control even if there are variations in the resonance constant or the circuit constants are unknown.

[0101] In the configuration of the second control example, by setting a margin time in advance, it is possible to easily prevent the turn-off timing of the switching elements Q5 to Q8 for synchronous rectification from being delayed due to the detection delay of the first detection means 5 or the delay in the differential processing of the control unit 4, and it is possible to surely prevent the reverse current (backflow) phenomenon of the current due to the delay of the turn-off timing.

[0102] In the configuration of the third control example, by setting the minimum value of the differential value of the drive current at the start of the flow of the resonance current, it is possible to surely turn off or keep off the switching elements Q5 to Q8 for synchronous rectification even at low load by simple control (comparison processing between the differential value and the minimum value).

[0103] The configurations of the first control example to the third control example are significantly different from the configuration of detecting the peak value of the resonance current by a differential value or the like to predict the zero-crossing point. Even when the drive frequency deviates from the resonance frequency, or when the resonance current becomes a non-sinusoidal intermittent waveform due to phase shift control, burst control, boost control, etc., it is possible to turn off the switching elements Q5 to Q8 for synchronous rectification at the point in time when the synchronous rectification current becomes zero.

[0104] Note that, in the above description, the current resonance type DC / DC converter 10 that only performs forward power transmission has been described. However, the current resonance type DC / DC converter 10 may perform reverse power transmission from terminals T3 and T4 to terminals T1 and T2. However, in order to obtain the same effect as in the forward power transmission during the reverse power transmission, a detection means (current detector) for detecting the drive current flowing between the secondary circuit 3 and the secondary coil of the transformer Tr during the reverse power transmission is required.

[0105] [Second Embodiment] Fig. 10 shows a current resonance type DC / DC converter 10A according to the second embodiment of the present invention. The current resonance type DC / DC converter 10A is a bidirectional CLLC type DC / DC converter, and includes a main circuit section to which a second resonance circuit 6 is added, a control section 4A, a first detection means 5, and a second detection means 7. The current resonance type DC / DC converter 10A performs forward power transmission from terminals T1 and T2 to terminals T3 and T4 and reverse power transmission from terminals T3 and T4 to terminals T1 and T2.

[0106] The main circuit section of the second embodiment has the same configuration as the main circuit section of the first embodiment, except that a second resonance circuit 6 is provided on the secondary side of the transformer Tr. The second resonance circuit 6 has the same configuration as the first resonance circuit 2, and includes a second resonance inductor Lr2 and a second resonance capacitor Cr2.

[0107] The second detection means 7 is a current detector that detects the current (drive current) flowing through the second resonance circuit 6 during reverse power transmission. The drive current includes a resonance current and an exciting current, as described in the first embodiment. The second resonance circuit 6 detects the drive current during reverse power transmission, generates a detection signal (for example, a voltage signal) corresponding to the current value of the drive current, and outputs it to the control section 4A. The position of the second detection means 7 can be arbitrarily changed as long as it can detect the drive current during reverse power transmission.

[0108] The control unit 4A is configured to perform the same control during reverse power transmission as the control performed by the control unit 4 during forward power transmission in the first embodiment. The control unit 4A during reverse power transmission operates the secondary-side circuit 3 as a drive circuit and the primary-side circuit 1 as a synchronous rectifier circuit. Specifically, the control unit 4A during reverse power transmission performs frequency modulation control to modulate the drive frequency for the switching elements Q5 to Q8 of the secondary-side circuit 3 according to the input / output characteristics of the LLC method, and performs synchronous rectification control for the switching elements Q1 to Q4 of the primary-side circuit 1. The control unit 4A performs differentiation processing on the detection signal of the second detection means 7, and turns off the switching elements Q1 to Q4 for synchronous rectification based on the change amount of the differential value obtained by the differentiation processing.

[0109] Since the equivalent circuit of the current resonance type DC / DC converter 10A is the same as the equivalent circuit of the current resonance type DC / DC converter 10 in the first embodiment, the control in the first embodiment (the control of the first control example to the third control example) can also be applied to the current resonance type DC / DC converter 10A according to this embodiment. Therefore, according to the current resonance type DC / DC converter 10A, the same effects as those in the first embodiment can be obtained both in the case of forward power transmission and in the case of reverse power transmission.

[0110] As described above, the embodiments of the current resonance type DC / DC converter according to the present invention have been described, but the present invention is not limited to the above embodiments.

[0111] The current resonance type DC / DC converter according to the present invention includes a main circuit section and a control section. The main circuit section includes an isolation transformer including a primary coil and a secondary coil, a primary circuit including a plurality of first switching elements, a first resonance circuit provided between the primary circuit and the primary coil and including a first resonance inductor and a first resonance capacitor, and a secondary circuit connected to the secondary coil and including a plurality of second switching elements. In the current resonance type DC / DC converter, under the control of the control section, the primary circuit operates as a drive circuit, the secondary circuit operates as a synchronous rectification circuit, and forward power transmission from the primary circuit to the secondary circuit is performed. During the forward power transmission, the control section measures a drive current flowing through the first resonance circuit, detects a change rate of the drive current, and determines a turn-off timing of the second switching element for synchronous rectification from the change rate. If so, the configuration can be changed as appropriate.

[0112] For example, in the first embodiment and the second embodiment, as the change rate of the drive current, the change amount of the differential value of the drive current was taken as an example for explanation, but it is not limited thereto. For example, as the change rate of the drive current, the change rate per unit time of the drive current may be used. The change rate per unit time is, for example, the change amount of the second-order differential value (the further differential value of the differential value).

[0113] By using the change amount of the second-order differential value, it is possible to easily detect the end timing of the resonance current that changes greatly in a very short time. For example, in FIG. 4(A), the change of the resonance current from positive to negative or from negative to positive is a temporally gentle change, but the change at the time when the resonance current ends and only the exciting current remains changes greatly in a very short time. Therefore, the change amount of the second-order differential value becomes larger than the change amount of the differential value. Thus, the end timing of the resonance current can be easily detected, and the switching elements Q5 to Q8 for synchronous rectification can be surely turned off. The same applies to the case of reverse power transmission.

[0114] In the first to third control examples, when using the rate of change of the drive current per unit time, the control unit 4 detects the rate of change per unit time (for example, the change amount of the second derivative value), detects the timing when the resonance current becomes zero, and detects the timing when the synchronous rectification current becomes zero. Therefore, the change in the current value when the resonance current is flowing and the change in the current value when the resonance current is zero (when only the excitation current is flowing) can be clearly distinguished, and synchronous rectification control can be easily performed.

[0115] Also, in the first and second embodiments, the timing to start the synchronous rectification control can be changed as appropriate. For example, during forward power transmission, after driving the drive circuit (after turning on the switching elements Q1 to Q4), boost control may be performed to turn on any one of the switching elements Q5 to Q8 so as to form a short-circuit circuit in the secondary-side circuit 3, and the synchronous rectification control may be started after the boost control. In this case, the peak of the resonance current becomes larger and the period during which the resonance current flows becomes shorter, but the change in the resonance current becomes larger, so the rate of change of the drive current (the change amount of the derivative value or the change amount of the second derivative value) becomes larger. The same applies to the case of reverse power transmission.

Description of Reference Numerals

[0116] 1 Primary-side circuit 2 First resonance circuit 3 Secondary-side circuit 4, 4A Control unit 5 First detection means 6 Second resonance circuit 7 Second detection means 10, 10A Current resonance type DC / DC converter

Claims

1. A current resonance type DC / DC converter comprising a main circuit section and a control section, wherein the main circuit section includes an isolation transformer including a primary coil and a secondary coil, a primary circuit including a plurality of first switching elements, a first resonance circuit provided between the primary circuit and the primary coil and including a first resonance inductor and a first resonance capacitor, and a secondary circuit connected to the secondary coil and including a plurality of second switching elements, and in the main circuit section, under the control of the control section, the primary circuit operates as a drive circuit and the secondary circuit operates as a synchronous rectification circuit to perform forward power transmission from the primary circuit to the secondary circuit, and during the forward power transmission, the control section detects a change rate of a drive current flowing through the first resonance circuit, and determines a turn-off timing of the second switching element for synchronous rectification from the change rate characterizing the current resonance type DC / DC converter.

2. The change rate is a change amount of a differential value of the drive current, and during the forward power transmission, the control section turns off the second switching element for synchronous rectification if the change amount is equal to or greater than a predetermined first threshold value characterizing the current resonance type DC / DC converter according to Claim 1.

3. During the forward power transmission, the control section calculates the change amount after the polarity of the differential value changes from positive to negative or after the polarity changes from negative to positive after driving of the drive circuit characterizing the current resonance type DC / DC converter according to Claim 2.

4. The change rate is a change rate per unit time of the drive current, and during the forward power transmission, the control section turns off the second switching element for synchronous rectification if the change rate per unit time is equal to or greater than a predetermined second threshold value characterizing the current resonance type DC / DC converter according to Claim 1.

5. The change rate per unit time is a change amount of a second differential value of the drive current characterizing the current resonance type DC / DC converter according to Claim 4.

6. During the forward power transmission, the control section turns off the second switching element for synchronous rectification at a timing that is at least a predetermined margin time earlier than a timing at which the change amount of the differential value becomes equal to or greater than the first threshold value ​ The current resonance type DC / DC converter according to claim 2, characterized in that...

7. During the forward power transmission, the control unit turns off the second switching element when the differential value at the start of the flow of the resonance current after the turn-on of the first switching element is less than a predetermined minimum value. The current resonance type DC / DC converter according to claim 2, characterized in that...

8. During the forward power transmission, the control unit turns off the second switching element for synchronous rectification at the turn-off timing of the first switching element when the turn-off timing of the first switching element is earlier than the turn-off timing of the second switching element determined from the rate of change. The current resonance type DC / DC converter according to claim 1, characterized in that...

9. The main circuit unit under the control of the control unit, the secondary circuit operates as the drive circuit, and the primary circuit operates as the synchronous rectification circuit to perform reverse power transmission from the secondary circuit to the primary circuit. The current resonance type DC / DC converter according to claim 1, characterized in that...

10. The main circuit unit is provided between the secondary circuit and the secondary coil, and further includes a second resonance circuit including a second resonance inductor and a second resonance capacitor. During the reverse power transmission, the control unit detects the rate of change of the drive current flowing through the second resonance circuit and determines the turn-off timing of the first switching element for synchronous rectification from the rate of change. The current resonance type DC / DC converter according to claim 9, characterized in that...

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