Power converter and method for controlling same

Abstract

The present invention improves the accuracy of detection of a diode mode of a power converter. A power converter (1) comprises a primary-side circuit (10), a secondary-side circuit (20) having a plurality of synchronous rectification switches (21)-(24), and a controller (40). The secondary-side circuit (20) has: a sub-circuit (100) connected in series with one main transistor (24) among the plurality of synchronous rectification switches (21)-(24); and a detection circuit (200) having a diode function unit. The detection circuit (200) detects the voltage across the main transistor (24) and the sub-circuit (100) by using the diode function of the diode function unit, and detects whether the main transistor (24) is in diode mode or not on the basis of a change in the voltage when the main transistor (24) is turned off. The controller (40) increases the synchronous rectification period of each of the plurality of synchronous rectification switches (21)-(24) until the diode mode is not detected.

Description

Power converter and control method thereof

[0001] The present disclosure relates to a power converter and a control method thereof.

[0002] To improve the power conversion efficiency of a power converter, there is a synchronous rectification technology that replaces the rectifying section (diode) with a switching element (synchronous rectification switch). Synchronous rectification switches have lower conduction loss than diodes, improving efficiency. However, if the switching timing is not set appropriately, switching loss increases, resulting in a deterioration in power conversion efficiency. For example, Patent Document 1 discloses a technology that lengthens the on-period (called the synchronous rectification period) of a synchronous rectification switch when it is detected that the synchronous rectification switch is in a mode (called diode mode) in which current flows through the body diode of the synchronous rectification switch when the synchronous rectification switch is turned off. This reduces switching loss and improves power conversion efficiency.

[0003] U.S. Patent No. 7,495,934

[0004] However, in the technology disclosed in Patent Document 1, the detection accuracy of the diode mode depends on the performance of the synchronous rectifier switch, and therefore, if a synchronous rectifier switch that has low detection accuracy of the diode mode is used, there is a problem that synchronous rectification cannot be performed normally.

[0005] Therefore, the present disclosure provides a power converter and the like that can improve the accuracy of detecting the diode mode.

[0006] A power converter according to the present disclosure includes a primary-side circuit having a plurality of switches, a secondary-side circuit having a plurality of synchronous rectifier switches, and a controller that controls the primary-side circuit and the secondary-side circuit. The secondary-side circuit further includes a sub-circuit having a sub-diode or sub-transistor connected in series with a main transistor that is one of the plurality of synchronous rectifier switches, and a detection circuit having a diode function unit that is a diode or transistor. The detection circuit detects the voltage across the main transistor and sub-circuit connected in series using the diode function of the diode function unit, and detects whether the main transistor is in diode mode, in which current flows through its body diode, when turned off, based on a change in the voltage across both ends when the main transistor is turned off. The controller extends the synchronous rectification period, which is the on-period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

[0007] A control method according to the present disclosure is a control method for a power converter, the power converter including a primary-side circuit having a plurality of switches and a secondary-side circuit having a plurality of synchronous rectifier switches. The secondary-side circuit further includes a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor that is one of the plurality of synchronous rectifier switches. The control method detects the voltages across the series-connected main transistor and sub-circuit using the diode function of the diode or transistor diode function unit, detects whether the main transistor is in diode mode, in which current flows through its body diode, when turned off, based on a change in the voltage across both ends when the main transistor is turned off, and extends the synchronous rectification period, which is the on-period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

[0008] These comprehensive or specific aspects may be realized as a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or may be realized as any combination of a system, a method, an integrated circuit, a computer program, and a recording medium.

[0009] According to the power converter and the control method thereof of the present disclosure, the accuracy of detecting the diode mode can be improved.

[0010] FIG. 1 is a circuit configuration diagram showing an example of a power converter according to an embodiment; FIG. 2 is a circuit configuration diagram showing a first example of a synchronous rectifier circuit according to an embodiment; FIG. 3 is a diagram for explaining an example of operation of the power converter in the first example; FIG. 4 is a diagram for explaining another example of operation of the power converter in the first example; FIG. 5 is a circuit configuration diagram showing a second example of a synchronous rectifier circuit according to an embodiment; FIG. 6 is a diagram for explaining an example of operation of the power converter in the second example; and FIG. 7 is a flowchart showing an example of a control method according to another embodiment.

[0011] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0012] The embodiments described below are all comprehensive or specific examples, and the numerical values, shapes, materials, components, arrangement and connection of the components, steps, and order of steps shown in the following embodiments are merely examples and are not intended to limit the present disclosure.

[0013] (Embodiment) Hereinafter, a power converter according to an embodiment will be described.

[0014] FIG. 1 is a circuit configuration diagram showing an example of a power converter 1 according to an embodiment.

[0015] The power converter 1 is an isolated DC-DC converter that boosts or bucks an input voltage to a predetermined output voltage. For example, the power converter 1 is an LLC converter that utilizes LLC resonance due to the leakage inductance, excitation inductance, and resonant capacitor of a transformer. In an LLC converter, frequency control is performed to change the switching frequency of each switch on the primary side, and phase shift control is performed to change the phase difference between the switching of each switch on the primary side, thereby changing the input / output voltage ratio (Gain), thereby enabling the desired power output. Note that the power converter 1 may be a resonant converter other than an LLC converter, or may be another type of buck, boost, or buck-boost converter.

[0016] The power converter 1 has terminals t1, t2, t3, and t4. The terminal t1 is an input terminal, and the terminal t2 is a ground terminal. The terminal t3 is an output terminal, and the terminal t4 is a ground terminal. Note that, since the power converter 1 is an isolated DC-DC converter, the terminals t2 and t4 are electrically isolated.

[0017] For example, the power converter 1 is mounted on a vehicle and applied to an electric vehicle system that drives auxiliary equipment. For example, a high-voltage lithium-ion battery or the like is connected to terminals t1 and t2, and a low-voltage lead-acid battery and auxiliary equipment are connected to terminals t3 and t4. For example, the voltage of a lithium-ion battery is 250 V to 450 V, and the voltage of a lead-acid battery is 10 V to 16 V. To convert a high voltage of 250 V to 450 V to a low voltage of 10 V to 16 V, for example, a power converter 1 such as an LLC converter that supports a wide input / output voltage ratio is used.

[0018] The power converter 1 includes a primary side circuit 10, a secondary side circuit 20, a transformer T, an inductor Lr, capacitors Cr, Cin and Cout, and a controller 40. The capacitors Cin and Cout do not necessarily have to be components of the power converter 1.

[0019] The capacitor Cin is an input capacitor connected between the terminal t1 and the terminal t2, and the capacitor Cout is an output capacitor (smoothing capacitor) connected between the terminal t3 and the terminal t4.

[0020] The primary circuit 10 is connected to the primary side of the transformer T. The primary circuit 10 has a plurality of switches. For example, the primary circuit 10 has switches 11, 12, 13, and 14 as the plurality of switches.

[0021] The switch 11 is a switch provided on a first path connecting the terminal t1 and the terminal t2. The switch 11 is, for example, an N-channel metal oxide semiconductor field effect transistor (MOSFET). The drain of the switch 11 is connected to the terminal t1, and the source of the switch 11 is connected to the drain of the switch 12.

[0022] The switch 12 is provided on the first path and is connected in series with the switch 11. The switch 12 is, for example, an N-channel MOSFET. The drain of the switch 12 is connected to the source of the switch 11, and the source of the switch 12 is connected to the terminal t2.

[0023] The switch 13 is provided on a second path that connects the terminal t1 and the terminal t2 and is different from the first path. The switch 13 is, for example, an N-channel MOSFET. The drain of the switch 13 is connected to the terminal t1, and the source of the switch 13 is connected to the drain of the switch 14.

[0024] The switch 14 is provided on the second path and connected in series with the switch 13. The switch 14 is, for example, an N-channel MOSFET. The drain of the switch 14 is connected to the source of the switch 13, and the source of the switch 14 is connected to the terminal t2.

[0025] The transformer T is an isolation transformer having a primary winding and a secondary winding that are insulated from each other. The primary winding of the transformer T is connected between a node N1 on the first path between the switches 11 and 12 and a node N2 on the second path between the switches 13 and 14.

[0026] The capacitor Cr and the inductor Lr are connected between the node N1 and the primary winding of the transformer T or between the node N2 and the primary winding of the transformer T. The capacitor Cr is a resonant capacitor, and the inductor Lr is a resonant inductor. For example, the capacitor Cr is connected to the node N2, the inductor Lr is connected to the node N1, and the capacitor Cr, the inductor Lr, and the primary winding of the transformer T are connected in series. The inductor Lr may be provided as a separate inductor, or may be provided by utilizing the leakage inductance of the transformer T.

[0027] The capacitor Cr and the inductor Lr may be connected in series at one end (e.g., node N1 side) of the primary winding of the transformer T, or may be connected in series at the other end (e.g., node N2 side) of the primary winding of the transformer T. In this way, the order in which the capacitor Cr, the inductor Lr, and the primary winding of the transformer T are connected between the node N1 and the node N2 is not particularly limited.

[0028] In FIG. 1, the excitation inductance of the transformer T is indicated by an inductor Lm.

[0029] The secondary-side circuit 20 is connected to the secondary side of the transformer T. The secondary-side circuit 20 includes a plurality of synchronous rectification switches. For example, the secondary-side circuit 20 includes switches 21, 22, 23, and 24 as the plurality of synchronous rectification switches. The switches 21, 22, 23, and 24 are each a switch for performing synchronous rectification. To perform synchronous rectification, it is necessary to detect the current flowing through the switch. For example, the current can be directly detected using a Hall sensor or a shunt resistor. However, the power converter 1 performs synchronous rectification by indirectly detecting the current without using a Hall sensor or a shunt resistor. The secondary-side circuit 20 includes, as a component for indirectly detecting the current, a synchronous rectification circuit 30 provided corresponding to the switch 24, for example. Note that the synchronous rectification circuit 30 may be provided corresponding to the switch 21, 22, or 23. Alternatively, the synchronous rectification circuit 30 may be provided for each of two or more of the switches 21, 22, 23, and 24. In other words, a plurality of synchronous rectification circuits 30 may be provided.

[0030] The switch 21 is a switch provided on the third path connecting the terminal t3 and the terminal t4. The switch 21 is, for example, an N-channel MOSFET. The drain of the switch 21 is connected to the terminal t3, and the source of the switch 21 is connected to the drain of the switch 22.

[0031] The switch 22 is provided on the third path and connected in series with the switch 21. The switch 22 is, for example, an N-channel MOSFET. The drain of the switch 22 is connected to the source of the switch 21, and the source of the switch 22 is connected to the terminal t4.

[0032] The switch 23 is a switch provided on a fourth path that connects the terminal t3 and the terminal t4 and is different from the third path. The switch 23 is, for example, an N-channel MOSFET. The drain of the switch 23 is connected to the terminal t3, and the source of the switch 23 is connected to the drain of the switch 24.

[0033] The switch 24 is provided on the fourth path and connected in series with the switch 23. The switch 24 is, for example, an N-channel MOSFET. The drain of the switch 24 is connected to the source of the switch 23, and the source of the switch 24 is connected to the terminal t4.

[0034] The secondary winding of the transformer T is connected between a node N3 on the third path between the switches 21 and 22 and a node N4 on the fourth path between the switches 23 and 24 .

[0035] 1 shows the body diode of each switch, and each body diode is connected in parallel to the corresponding switch in the equivalent circuit. Specifically, the anode of each body diode is connected to the source of the corresponding switch in the equivalent circuit, and the cathode is connected to the drain of the corresponding switch.

[0036] The secondary-side circuit 20 further includes a sub-circuit 100 connected in series with the switch 24, which are included in the synchronous rectification circuit 30, and a detection circuit 200. The switch 24 is an example of a main transistor, and the main transistor is one of the multiple synchronous rectification switches (switches 21, 22, 23, and 24).

[0037] The sub-circuit 100 includes a sub-diode or a sub-transistor. In the example shown in FIG. 1, the sub-circuit 100 includes a sub-diode, diode 103. The anode of the diode 103 is connected to terminal t4, and the cathode is connected to the source of the switch 24.

[0038] The detection circuit 200 has a diode function unit that is a diode or a transistor. The diode function of the diode function unit may be realized by a diode or a body diode of a transistor. The detection circuit 200 detects the voltage across the switch 24 and the sub-circuit 100, which are connected in series, using the diode function of the diode function unit. The detection circuit 200 then detects whether the switch 24 is in diode mode, in which current flows through the body diode, when the switch 24 is turned off, based on a change in the voltage across both ends when the switch 24 is turned off. The operation of the detection circuit 200 will be described in detail below.

[0039] The controller 40 controls the primary side circuit 10 and the secondary side circuit 20. Specifically, the controller 40 controls the switching of the switches 11, 12, 13, and 14 and the switches 21, 22, 23, and 24. For example, the controller 40 controls the switching of the switches 11, 12, 13, 14, 21, 22, 23, and 24 by controlling gate drive circuits (not shown) connected to the gates of the switches 11, 12, 13, 14, 21, 22, 23, and 24 via a PWM generator (not shown) or the like.

[0040] The controller 40 is realized by, for example, a computer including a processor (microprocessor) and a memory. The memory may be a read-only memory (ROM) or a random access memory (RAM), and can store programs to be executed by the processor. For example, the controller 40 may be realized by a microcontroller.

[0041] Using the detection result of detection circuit 200, controller 40 lengthens the synchronous rectification period, which is the on period of each of switches 21, 22, 23, and 24, until the diode mode is no longer detected when switch 24 is turned off. In other words, if the diode mode of switch 24 is detected when switch 24 is turned off, the synchronous rectification period is short, and current still flows from the source to the drain of switch 24 when switch 24 is turned off (in other words, ZCS (zero current switching) has not been achieved). Therefore, the synchronous rectification period is extended until the current no longer flows. Note that, because current also flows through switches 21, 22, and 23 in the same way, the synchronous rectification periods of all switches 21, 22, 23, and 24 can be controlled simply by detecting the diode mode of one synchronous rectification switch (switch 24 in this case).

[0042] Here, the operation of the detection circuit 200 will be described in detail.

[0043] When the switch 24 is turned off and a current flows through the body diode of the switch 24, a voltage corresponding to the forward voltage of the body diode of the switch 24 is generated in the switch 24. Even if the sub-circuit 100 is not provided, the detection circuit 200 can detect whether or not a voltage corresponding to the forward voltage of the body diode of the switch 24 is generated in the switch 24, i.e., whether or not the switch 24 is in the diode mode, from a change in the difference between the voltage generated in the diode function section (forward voltage of the diode function section) when the switch 24 is turned off and the voltage generated in the switch 24.

[0044] For example, the cathode of the diode function unit (diode or body diode) is connected to the cathode of the switch 24. Also, for example, assume that the forward voltage of the body diode of the switch 24 is 0.8 V and the forward voltage of the diode function unit is 0.4 V. When the switch 24 is in the on state, the difference between the voltage (0.4 V) generated in the diode function unit and the voltage (0 V) generated in the switch 24 (i.e., the voltage at the anode of the diode function unit) is 0.4 V. On the other hand, when the switch 24 is turned off, the difference between the voltage (0.4 V) generated in the diode function unit and the voltage (0.8 V) generated in the switch 24 (i.e., the voltage at the anode of the diode function unit) is −0.4 V. By detecting this change in voltage from +0.4 V to −0.4 V when the switch 24 is turned off, it is also possible to detect whether the switch 24 is in diode mode.

[0045] However, the forward voltage of the body diode of switch 24 is small, for example, 0.8 V, and even if a voltage corresponding to the forward voltage of the body diode of switch 24 is generated in switch 24, detection circuit 200 may not be able to detect that switch 24 is in diode mode. In other words, it may not be able to detect small voltage changes such as from +0.4 V to −0.4 V near 0 V.

[0046] Therefore, a sub-circuit 100 having a sub-diode or sub-transistor (diode 103 in FIG. 1 ) is connected in series to the switch 24. Furthermore, the detection circuit 200 detects the voltage across the series-connected switch 24 and sub-circuit 100, and detects whether the switch 24 is in diode mode from a change in the difference between the voltage across the diode function part and the voltage across the switch 24 when the switch 24 is turned off. As a result, the voltage across the switch 24 becomes larger than the forward voltage of the body diode of the switch 24, and it is possible to increase the change in the difference between the voltage across the diode function part and the voltage across the switch 24 when the switch 24 is turned off.

[0047] For example, assume that the forward voltage of diode 103 is 0.8 V. When switch 24 is in the on state, the difference between the voltage (0.4 V) generated in the diode function unit and the voltage across both ends (0.8 V) (i.e., the voltage at the anode of the diode function unit) is −0.4 V. On the other hand, when switch 24 is turned off, the difference between the voltage (0.4 V) generated in the diode function unit and the voltage (1.6 V) generated across switch 24 (i.e., the voltage at the anode of the diode function unit) is −1.2 V. Whether switch 24 is in diode mode can be detected from this relatively large voltage change from −0.4 V to −1.2 V.

[0048] As a result, the accuracy of detecting the diode mode can be improved. By improving the accuracy of detecting the diode mode, synchronous rectification can be performed normally, and power conversion efficiency can be improved.

[0049] It is also possible to detect the current flowing through the body diode of the main transistor when the switch 24 is turned off (i.e., the diode mode of the switch 24) by using a Hall sensor or a shunt resistor. However, using a Hall sensor is large, which increases the size of the power converter. Furthermore, using a shunt resistor results in loss due to the insertion of the resistor. In the present disclosure, the diode mode can be detected without using a Hall sensor or a shunt resistor, thereby improving the accuracy of detecting the diode mode while achieving a smaller size and lower loss of the power converter 1.

[0050] Next, the case where the sub-circuit 100 has a sub-transistor will be described with reference to FIGS.

[0051] FIG. 2 is a circuit configuration diagram showing a first example of a synchronous rectifier circuit 30 according to an embodiment.

[0052] In the first example, the sub-circuit 100 has a sub-transistor, a switch 101. The switch 101 is, for example, an N-channel MOSFET. The drain of the switch 101 is connected to the source of the switch 24, and the source of the switch 101 is connected to the terminal t4.

[0053] 2, the voltage across the switch 24 and the sub-circuit 100 connected in series is V Ｕｎｉｔ , the gate-source voltage of the switch 24 is V ｇｓ , the forward voltage of the body diode of the switch 24 is V ｓｄ , the gate-source voltage of the switch 101 is V ｇｓ＿Ｓｕｂ１ , the forward voltage of the body diode of the switch 101 is V ｓｄ＿Ｓｕｂ１ , the current flowing through the switch 24 and the sub-circuit 100 connected in series is I ｓｅｃ , the diode mode detection signal (signal that becomes High level when the diode mode is detected) by the detection circuit 200 is V Ｄｅｔ It is shown as follows.

[0054] Next, the operation of the power converter 1 in the first example will be described with reference to FIGS.

[0055] 3 is a diagram for explaining an example of the operation of the power converter 1 in the first example. From the top, FIG. 3 shows a detection signal (V Ｄｅｔ ), the voltage across the switch 24 and the sub-circuit 100 connected in series (V Ｕｎｉｔ ), the current (I ｓｅｃ ), the gate-source voltage of the switch 24 (V ｇｓ ), the gate-source voltage of the switch 101 (V ｇｓ＿Ｓｕｂ１ ) is shown in the graph. The same applies to FIG. 4, which will be described later.

[0056] V in Figure 3 ｇｓ＿Ｓｕｂ１ As shown in V in FIG. 3, the switch 101 is temporarily turned off for detecting the diode mode, but is basically in the on state. ｇｓ As shown in Figure 3, before the switch 24 is turned on, the switch 23 is in the on state, and the voltage across the switch 24 and the sub-circuit 100 connected in series is the voltage at the terminal t3 (output voltage). When the switch 23 is turned off and the switch 24 is turned on, both ends of the switch 24 and the sub-circuit 100 connected in series are shorted, and the voltage across both ends becomes 0V. Then, as shown in Figure 3, ｓｅｃAs shown in (a) of FIG. 3, when the switch 24 is turned off while a current is flowing through the switch 24 and the sub-circuit 100 connected in series, the current continues to flow through the body diode of the switch 24. Therefore, as shown in (b) of FIG. 3, the voltage across both ends becomes the forward voltage (-V ｓｄ The detection circuit 200 can detect that the switch 24 is in the diode mode from the change in the voltage across the switch 24 when the switch 24 is turned off.

[0057] However, as described above, the forward voltage of the body diode of switch 24 is small, and as shown in FIG. 3C , even if switch 24 is in diode mode, the detection accuracy of detection circuit 200 may be poor and it may not be possible to detect that switch 24 is in diode mode. Controller 40 lengthens the synchronous rectification period, which is the on-period of switch 24, until diode mode is no longer detected. However, if the detection accuracy of detection circuit 200 is poor and it is not possible to detect that switch 24 is in diode mode, the synchronous rectification period is not extended. The synchronous rectification period is gradually lengthened from a minimum value determined by the switching frequency and phase of switches 11, 12, 13, and 14 in primary-side circuit 10. However, if the detection accuracy of detection circuit 200 is poor and it is not possible to detect that switch 24 is in diode mode, the synchronous rectification period remains at its minimum value without being extended.

[0058] Therefore, when the controller 40 detects that the synchronous rectification period has reached the minimum value while the switch 101 is in the on state, the controller 40 turns off the switch 101 in synchronization with the turn-off of the switch 24. For example, the controller 40 may turn off the switch 101 in synchronization with the turn-off of the switch 24 when the controller 40 detects that the synchronous rectification period has reached the minimum value multiple times in succession. As shown in (d) of FIG. 3, it can be seen that the switch 101 is turned off in synchronization with the turn-off of the switch 24 when the controller 40 detects that the synchronous rectification period has reached the minimum value two or more times in succession. When the switch 24 is turned off while a current is flowing through the switch 24 and the sub-circuit 100 connected in series, the current continues to flow through the body diode of the switch 24 and the body diode of the switch 101. For this reason, as shown in (e) of FIG. 3, the voltage across both ends is the sum (-V ｓｄ -V ｓｄ＿Ｓｕｂ１ 3(f), the detection circuit 200 can detect that the switch 24 is in the diode mode from the large change in the voltage across the switch 24 when the switch 24 is turned off.

[0059] 3(g), since the diode mode has been detected, the controller 40 lengthens the synchronous rectification period, thereby shortening the period during which the switch 24 is in the diode mode and reducing the conduction loss caused by current flowing through the body diode.

[0060] Thus, if it is not detected that switch 24 is in diode mode at all, controller 40 does not lengthen the synchronous rectification period, and the synchronous rectification period is minimized. In this case, the detection accuracy of detection circuit 200 may be poor, causing detection circuit 200 to fail to properly detect diode mode. Therefore, in such a case, the detection accuracy of diode mode can be improved by turning off switch 101 in synchronization with the turning off of switch 24. Note that when a secondary diode is connected to switch 24 as sub-circuit 100, conduction loss occurs due to the secondary diode. However, when switch 101 is connected to switch 24 as sub-circuit 100, the conduction loss generated in sub-circuit 100 can be reduced.

[0061] FIG. 4 is a diagram for explaining another example of the operation of the power converter 1 in the first example.

[0062] As shown in FIG. 4 , the controller 40 may turn off the switch 101 in synchronization with the turning off of the switch 24, and then turn on the switch 101 during the dead time period between the switching of the switches 11, 12, 13, and 14 in the primary side circuit 10.

[0063] When switch 101 is turned off in synchronization with the turn-off of switch 24, if switch 24 is in diode mode, current also flows through the body diode of switch 101, causing conduction loss in the body diode of switch 101. Therefore, by turning switch 101 on during the dead time after turning off switch 101, the conduction loss can be reduced. Note that since the diode mode can be detected as long as the falling edge of the voltage across both ends is detected, the detection of the diode mode is not affected even if switch 101 is turned on immediately after the falling edge of the voltage across both ends is detected.

[0064] Next, a case where the sub-circuit 100 has two or more sub-transistors will be described with reference to Figures 5 and 6. The two or more sub-transistors are connected in series with each other.

[0065] FIG. 5 is a circuit diagram showing a second example of the synchronous rectifier circuit 30 according to the embodiment.

[0066] In the second example, the sub-circuit 100 has switches 101 and 102. Switch 101 is an example of a first sub-transistor included in two or more sub-transistors. Switch 101 is, for example, an N-channel MOSFET. The drain of switch 101 is connected to the source of switch 24, and the source of switch 101 is connected to the drain of switch 102. Switch 102 is an example of a second sub-transistor included in two or more sub-transistors. Switch 102 is, for example, an N-channel MOSFET. The drain of switch 102 is connected to the source of switch 101, and the source of switch 102 is connected to terminal t4. Note that the sub-circuit 100 may have three or more sub-transistors.

[0067] 5, the voltage across the switch 24 and the sub-circuit 100 connected in series is V Ｕｎｉｔ , the gate-source voltage of the switch 24 is V ｇｓ , the forward voltage of the body diode of the switch 24 is V ｓｄ , the gate-source voltage of the switch 101 is V ｇｓ＿Ｓｕｂ１ , the forward voltage of the body diode of the switch 101 is V ｓｄ＿Ｓｕｂ１ , the gate-source voltage of the switch 102 is V ｇｓ＿Ｓｕｂ２ , the forward voltage of the body diode of the switch 101 is V ｓｄ＿Ｓｕｂ２ , the current flowing through the switch 24 and the sub-circuit 100 connected in series is I ｓｅｃ , the detection signal of the diode mode by the detection circuit 200 is V Ｄｅｔ It is shown as follows.

[0068] Next, the operation of power converter 1 in the second example will be described with reference to Fig. 6. Note that Fig. 6 illustrates an example in which controller 40 turns off switch 101 or 102 in synchronization with the turning off of switch 24, and then turns on switch 101 or 102 during the dead time period of the switching of switches 11, 12, 13, and 14 in primary-side circuit 10, as in the description of Fig. 4.

[0069] 6 is a diagram for explaining an example of the operation of the power converter 1 in the second example. From the top, FIG. 6 shows a detection signal (V Ｄｅｔ ), the voltage across the switch 24 and the sub-circuit 100 connected in series (V Ｕｎｉｔ ), the current (I ｓｅｃ ), the gate-source voltage of the switch 24 (V ｇｓ ), the gate-source voltage of the switch 101 (V ｇｓ＿Ｓｕｂ１ ), the gate-source voltage of the switch 102 (V ｇｓ＿Ｓｕｂ２ ) is shown. Note that the forward voltage of the body diode of switch 101 is lower than the forward voltage of the body diode of switch 102 (V ｓｄ＿Ｓｕｂ１ <V ｓｄ＿Ｓｕｂ２ )

[0070] V in Figure 6 ｇｓ＿Ｓｕｂ１ and V ｇｓ＿Ｓｕｂ２ As shown in FIG. 6, the switches 101 and 102 are temporarily turned off for diode mode detection, but are basically in the on state. ｇｓ 6, before the switch 24 is turned on, the switch 23 is in the on state, and the voltage across the switch 24 and the sub-circuit 100 connected in series is the voltage at the terminal t3 (output voltage). When the switch 23 is turned off and the switch 24 is turned on, the voltage across the switch 24 and the sub-circuit 100 connected in series is shorted, and the voltage across the terminals becomes 0V. Then, as shown in I of FIG. ｓｅｃ As shown in (a) of FIG. 6, when the switch 24 is turned off while a current is flowing through the switch 24 and the sub-circuit 100 connected in series, the current continues to flow through the body diode of the switch 24. Therefore, as shown in (b) of FIG. 6, the voltage across both ends becomes equal to the forward voltage (-V ｓｄ The detection circuit 200 can detect that the switch 24 is in the diode mode from the change in the voltage across the switch 24 when the switch 24 is turned off.

[0071] However, as described above, the forward voltage of the body diode of switch 24 is small, and as shown in FIG. 6C , even if switch 24 is in diode mode, the detection accuracy of detection circuit 200 may be poor and it may not be possible to detect that switch 24 is in diode mode. Controller 40 lengthens the synchronous rectification period, which is the on-period of switch 24, until diode mode is no longer detected. However, if the detection accuracy of detection circuit 200 is poor and it is not possible to detect that switch 24 is in diode mode, the synchronous rectification period is not extended. Since the synchronous rectification period is gradually lengthened from a minimum value determined by the switching frequency and phase of switches 11, 12, 13, and 14 in primary-side circuit 10, if the detection accuracy of detection circuit 200 is poor and it is not possible to detect that switch 24 is in diode mode, the synchronous rectification period will not be extended and will remain at its minimum value.

[0072] Therefore, when the controller 40 detects that the synchronous rectification period has reached the minimum value while the switches 101 and 102 are in the on state, it turns off the switch 101 in synchronization with the turn-off of the switch 24. For example, when the controller 40 detects that the synchronous rectification period has reached the minimum value multiple times in succession, it turns off the switch 101 in synchronization with the turn-off of the switch 24. As shown in (d) of FIG. 6, it can be seen that when the minimum value is detected two or more times in succession, the switch 101 is turned off in synchronization with the turn-off of the switch 24. As a result, when the switch 24 is turned off while a current is flowing through the switch 24 and the sub-circuit 100 connected in series, the current continues to flow through the body diode of the switch 24 and the body diode of the switch 101. For this reason, as shown in (e) of FIG. 6, the voltage across both ends is the sum (-V ｓｄ -V ｓｄ＿Ｓｕｂ１ )

[0073] 6(f), the detection circuit 200 may not be able to detect that the switch 24 is in the diode mode even if the change in the voltage across the switch 24 is increased by the forward voltage of the body diode of the switch 101. In this case, the synchronous rectification period is not extended and therefore remains at the minimum value.

[0074] Therefore, as shown in (g) of FIG. 6, if the controller 40 detects that the synchronous rectification period has again reached the minimum value while the switches 101 and 102 are in the on state, it turns off the switch 102 in synchronization with the turn-off of the switch 24. As a result, when the switch 24 is turned off while a current is flowing through the series-connected switches 24 and the sub-circuit 100, the current continues to flow through the body diodes of the switches 24 and 102. For this reason, as shown in (h) of FIG. 6, the voltage across both ends is the sum of the forward voltage of the body diode of the switch 24 and the forward voltage of the body diode of the switch 102 (-V ｓｄ -V ｓｄ＿Ｓｕｂ２ )

[0075] 6(i), the detection circuit 200 may not be able to detect that the switch 24 is in the diode mode even if the change in the voltage across the switch 24 is increased by the forward voltage of the body diode of the switch 102, which is greater than the forward voltage of the body diode of the switch 101. In this case, the synchronous rectification period is not extended and therefore remains at the minimum value.

[0076] Therefore, as shown in (j) of FIG. 6, if the controller 40 detects that the synchronous rectification period has again reached the minimum value while the switches 101 and 102 are in the on state, it turns off the switches 101 and 102 in synchronization with the turn-off of the switch 24. When the switch 24 is turned off while a current is flowing through the series-connected switch 24 and the sub-circuit 100, the current continues to flow through the body diode of the switch 24 and the body diodes of the switches 101 and 102. For this reason, as shown in (k) of FIG. 6, the voltage across both ends is the sum (-V ｓｄ -V ｓｄ＿Ｓｕｂ１ -V ｓｄ＿Ｓｕｂ２ 6(l), the detection circuit 200 can detect that the switch 24 is in the diode mode from the large change in the voltage across the switch 24 when the switch 24 is turned off.

[0077] Then, although not shown, the controller 40 lengthens the synchronous rectification period because the diode mode has been detected, thereby shortening the period during which the switch 24 is in the diode mode and reducing the conduction loss caused by current flowing through the body diode.

[0078] In this way, the voltage across the series-connected switch 24 and sub-circuit 100 can be further increased by using two or more sub-transistors (e.g., switches 101 and 102), and the change in the difference between the voltage across the diode function portion and the voltage across the switch 24 when it is turned off can be further increased, thereby further improving the accuracy of diode mode detection.

[0079] Furthermore, if two or more sub-transistors (for example, switches 101 and 102) are all turned off in synchronization with the turn-off of switch 24, current also flows through the body diodes of the two or more sub-transistors when switch 24 is in diode mode, causing conduction loss in each of the body diodes of the two or more sub-transistors. Therefore, the conduction loss can be reduced by turning off the sub-transistors one by one in synchronization with the turn-off of switch 24 until diode mode is detected.

[0080] The forward voltage of the body diode of switch 101 may be the same as the forward voltage of the body diode of switch 102. In this case, if the diode mode is not detected even when one of switches 101 and 102 is turned off, then both switches 101 and 102 may be turned off.

[0081] (Other Embodiments) As described above, the embodiments have been described as examples of the technology according to the present disclosure. However, the technology according to the present disclosure is not limited to these, and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate. For example, the following modifications are also included in one embodiment of the present disclosure.

[0082] For example, a diode or a transistor may be connected in parallel to the synchronous rectifier switch (e.g., switch 24), and for example, a diode or a transistor may be connected in parallel to the sub-transistor or sub-diode of the sub-circuit 100.

[0083] For example, the present disclosure can be realized not only as a power converter, but also as a control method including steps (processing) performed by components that make up the power converter (e.g., detection circuit 200 and controller 40).

[0084] FIG. 7 is a flowchart showing an example of a control method according to another embodiment.

[0085] The control method is a control method for a power converter, the power converter comprising a primary-side circuit having a plurality of switches and a secondary-side circuit having a plurality of synchronous rectifier switches, the secondary-side circuit further comprising a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor which is one of the plurality of synchronous rectifier switches, and the control method, as shown in FIG. 7 , detects the voltage across the main transistor and sub-circuit connected in series using the diode function of the diode function unit which is the diode or transistor (step S11), detects whether the main transistor is in diode mode, in which current flows through its body diode, when turned off, based on a change in the voltage across both ends when the main transistor is turned off (step S12), and extends the synchronous rectification period, which is the on-period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off (step S13).

[0086] For example, the present disclosure can be realized as a program for causing a computer (processor) to execute steps included in the control method. Furthermore, the present disclosure can be realized as a non-transitory computer-readable recording medium, such as a CD-ROM, on which the program is recorded.

[0087] For example, when the present disclosure is realized as a program (software), each step is performed by running the program using hardware resources such as a computer's CPU, memory, input / output circuits, etc. In other words, each step is performed by the CPU acquiring data from memory or input / output circuits, etc., performing calculations, and outputting the calculation results to memory or input / output circuits, etc.

[0088] In the above-described embodiments, each component included in the power converter may be configured with dedicated hardware, or may be realized by executing a software program suitable for each component. Each component may be realized by a program execution unit such as a CPU or processor reading and executing a software program recorded on a recording medium such as a hard disk or semiconductor memory.

[0089] Some or all of the functions of the power converter according to the above-described embodiments are typically realized as an LSI, which is an integrated circuit. These may be individually integrated into single chips, or some or all of them may be integrated into a single chip. Furthermore, the integrated circuit is not limited to an LSI, and may be realized using a dedicated circuit or a general-purpose processor. It is also possible to use an FPGA (Field Programmable Gate Array) that can be programmed after LSI manufacturing, or a reconfigurable processor that can reconfigure the connections and settings of circuit cells within the LSI.

[0090] Furthermore, if an integrated circuit technology that can replace LSI emerges due to advances in semiconductor technology or other derivative technologies, that technology may naturally be used to integrate the components included in the power converter.

[0091] In addition, this disclosure also includes forms obtained by making various modifications to the embodiments that a person skilled in the art would think of, and forms realized by arbitrarily combining the components and functions in each embodiment within the scope of the present disclosure.

[0092] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0093] (Technology 1) A power converter comprising: a primary-side circuit having a plurality of switches; a secondary-side circuit having a plurality of synchronous rectifier switches; and a controller that controls the primary-side circuit and the secondary-side circuit, wherein the secondary-side circuit further comprises: a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor that is one of the plurality of synchronous rectifier switches; and a detection circuit having a diode function unit that is a diode or a transistor, wherein the detection circuit detects voltages across the main transistor and the sub-circuit connected in series using the diode function of the diode function unit, and detects whether the main transistor is in a diode mode in which a current flows through a body diode when the main transistor is turned off, based on a change in the voltage across both ends when the main transistor is turned off, and the controller extends a synchronous rectification period, which is an on-period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

[0094] When the main transistor is turned off and a current flows through its body diode, a voltage corresponding to the forward voltage of the body diode of the main transistor is generated across the main transistor. Even if a sub-circuit is not provided, the detection circuit can detect whether a voltage corresponding to the forward voltage of the body diode of the main transistor is generated across the main transistor, i.e., whether the main transistor is in diode mode, from a change in the difference between the voltage generated across the diode function section (forward voltage of the diode function section) and the voltage generated across the main transistor when the main transistor is turned off. However, since the forward voltage of the body diode of the main transistor is small, the detection circuit may not be able to detect that the main transistor is in diode mode even if a voltage corresponding to the forward voltage of the body diode of the main transistor is generated across the main transistor. Therefore, in this disclosure, a sub-circuit having a sub-diode or a sub-transistor is connected in series with the main transistor. The detection circuit also detects the voltage across the main transistor and sub-circuit connected in series and detects whether the main transistor is in diode mode from a change in the difference between the voltage across the diode function section and the voltage across the main transistor. This makes the voltage across both ends larger than the forward voltage of the body diode of the main transistor, increasing the change in the difference between the voltage across both ends and the voltage generated in the diode function section when the main transistor is turned off. This improves the accuracy of diode mode detection. Improved diode mode detection accuracy allows synchronous rectification to be performed normally, improving power conversion efficiency.

[0095] It is also possible to detect the current flowing through the body diode of the main transistor when the main transistor is turned off (i.e., diode mode) by using a Hall sensor or shunt resistor. However, using a Hall sensor is large, which increases the size of the power converter. Furthermore, using a shunt resistor results in loss due to the insertion of the resistor. In the present disclosure, the diode mode can be detected without using a Hall sensor or shunt resistor, thereby improving the accuracy of diode mode detection while achieving a smaller power converter and lower loss.

[0096] (Technology 2) The power converter according to Technology 1, wherein the sub-circuit has the sub-transistor, and the controller turns off the sub-transistor in synchronization with the turn-off of the main transistor when the controller detects that the synchronous rectification period has reached a minimum value determined from the switching frequency and phase of the multiple switches in the primary side circuit while the sub-transistor is in the on state.

[0097] If the controller does not detect that the main transistor is in diode mode at all, the controller does not lengthen the synchronous rectification period, and the synchronous rectification period is minimized. In this case, the detection accuracy of the detection circuit may be poor, causing the detection circuit to fail to properly detect diode mode. Therefore, in such cases, the accuracy of diode mode detection can be improved by turning off the secondary transistor in synchronization with the turn-off of the main transistor. Note that when a secondary diode is connected to the main transistor as a secondary circuit, conduction loss occurs due to the secondary diode, but when a secondary transistor is connected to the main transistor as a secondary circuit, the conduction loss generated in the secondary circuit can be reduced.

[0098] (Technology 3) The power converter according to Technology 1, wherein the sub-circuit has two or more of the sub-transistors.

[0099] This allows the voltage across the main transistor and sub-circuit connected in series to be further increased by using two or more sub-transistors, and further increases the change in the difference between the voltage across the diode function section and the voltage across the main transistor when it is turned off, thereby further improving the accuracy of diode mode detection.

[0100] (Technology 4) A power converter according to Technology 3, wherein the forward voltage of the body diode of a first sub-transistor included in the two or more sub-transistors is lower than the forward voltage of the body diode of a second sub-transistor included in the two or more sub-transistors, and when the controller detects that the synchronous rectification period has reached a minimum value determined from the switching frequency and phase of the multiple switches in the primary-side circuit while the two or more sub-transistors are on, the controller turns off the first sub-transistor in synchronization with the turn-off of the main transistor, and when the controller detects that the synchronous rectification period has reached its minimum value again while the two or more sub-transistors are on, the controller turns off the second sub-transistor in synchronization with the turn-off of the main transistor, and when the controller detects that the synchronous rectification period has reached its minimum value again while the two or more sub-transistors are on, the controller turns off the first sub-transistor and the second sub-transistor in synchronization with the turn-off of the main transistor.

[0101] If two or more sub-transistors are all turned off in synchronization with the turn-off of the main transistor, and the main transistor is in diode mode, current also flows through the body diodes of the two or more sub-transistors, causing conduction loss in each of the body diodes of the two or more sub-transistors. Therefore, the conduction loss can be reduced by turning off the sub-transistors one by one in synchronization with the turn-off of the main transistor until diode mode is detected.

[0102] (Technology 5) A power converter according to any one of Technologies 2 to 4, wherein the controller turns off the sub-transistor in synchronization with the turn-off of the main transistor, and then turns on the sub-transistor during a dead time period of switching of the plurality of switches included in the primary side circuit.

[0103] When the secondary transistor is turned off in synchronization with the turn-off of the main transistor, if the main transistor is in diode mode, current also flows through the body diode of the secondary transistor, causing conduction loss in the body diode of the secondary transistor. Therefore, by turning the secondary transistor on during the dead time after turning it off, the conduction loss can be reduced.

[0104] (Technology 6) A control method for a power converter, the power converter comprising: a primary-side circuit having a plurality of switches; and a secondary-side circuit having a plurality of synchronous rectifier switches; the secondary-side circuit further comprising a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor which is one of the plurality of synchronous rectifier switches; the control method detects voltages across the main transistor and the sub-circuit connected in series using the diode function of a diode function unit which is a diode or a transistor; detects whether the main transistor is in a diode mode in which a current flows through a body diode when the main transistor is turned off based on a change in the voltage across both ends when the main transistor is turned off; and extends a synchronous rectification period, which is an on-period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

[0105] This makes it possible to provide a control method that can improve the detection accuracy of the diode mode.

[0106] The present disclosure is applicable to power converters such as LLC converters.

[0107] REFERENCE SIGNS LIST 1 Power converter 10 Primary side circuit 11, 12, 13, 14, 21, 22, 23, 24, 101, 102 Switch 20 Secondary side circuit 30 Synchronous rectification circuit 40 Controller 100 Sub-circuit 103 Diode 200 Detection circuit Cin, Cout, Cr Capacitor Lr, Lm Inductor N1, N2, N3, N4 Node T Transformer t1, t2, t3, t4 Terminal

Claims

1. A power converter comprising: a primary side circuit having a plurality of switches; a secondary side circuit having a plurality of synchronous rectifier switches; and a controller that controls the primary side circuit and the secondary side circuit, wherein the secondary side circuit further comprises a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor that is one of the plurality of synchronous rectifier switches; and a detection circuit having a diode function unit that is a diode or a transistor, wherein the detection circuit detects the voltage across the main transistor and the sub-circuit connected in series using the diode function of the diode function unit, and detects whether the main transistor is in diode mode in which current flows through its body diode when the main transistor is turned off, based on a change in the voltage across both ends when the main transistor is turned off, and the controller extends the synchronous rectification period, which is the on period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

2. The power converter according to claim 1, wherein the sub-circuit has the sub-transistor, and when the controller detects that the synchronous rectification period has a minimum value determined from the switching frequency and phase of the multiple switches in the primary side circuit while the sub-transistor is in an on state, the controller turns off the sub-transistor in synchronization with the turn-off of the main transistor.

3. The power converter according to claim 1, wherein the sub-circuit has two or more of the sub-transistors.

4. The power converter according to claim 3, wherein the forward voltage of the body diode of a first sub-transistor included in the two or more sub-transistors is lower than the forward voltage of the body diode of a second sub-transistor included in the two or more sub-transistors, and wherein the controller, when detecting that the synchronous rectification period has reached a minimum value determined from the switching frequency and phase of the multiple switches in the primary-side circuit while the two or more sub-transistors are on, turns off the first sub-transistor in synchronization with the turn-off of the main transistor, and when detecting that the synchronous rectification period has again reached its minimum value while the two or more sub-transistors are on, turns off the second sub-transistor in synchronization with the turn-off of the main transistor, and when detecting that the synchronous rectification period has again reached its minimum value while the two or more sub-transistors are on, turns off the first sub-transistor and the second sub-transistor in synchronization with the turn-off of the main transistor.

5. The power converter according to any one of claims 2 to 4, wherein the controller turns off the secondary transistor in synchronization with the turning off of the main transistor, and then turns on the secondary transistor during a dead time period of switching of the plurality of switches in the primary side circuit.

6. A control method for a power converter, the power converter comprising: a primary side circuit having a plurality of switches; and a secondary side circuit having a plurality of synchronous rectifier switches; the secondary side circuit further having a sub-circuit having a sub-diode or a sub-transistor connected in series with a main transistor which is one of the plurality of synchronous rectifier switches; the control method comprising: detecting the voltage across the main transistor and the sub-circuit connected in series using the diode function of a diode function unit which is a diode or a transistor; detecting whether the main transistor is in diode mode in which current flows through its body diode when the main transistor is turned off based on a change in the voltage across both ends when the main transistor is turned off; and lengthening the synchronous rectification period, which is the on period of each of the plurality of synchronous rectifier switches, until the diode mode is no longer detected when the main transistor is turned off.

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