DC / DC converter

Abstract

In this DC / DC converter that converts a DC voltage into a predetermined DC voltage, an isolation transformer TR1 has a primary winding L1 connected to an input-side DC power supply 2, and a secondary winding L2 connected to an output smoothing capacitor C1 via a rectifier diode D1. A switching element M1 is connected between the primary winding L1 and a ground potential. A control IC 10 drives the switching element M1. A start / stop switch SW1 is connected between the ground potential, and a drive signal line 32 that connects between a control terminal of the switching element M1 and a drive terminal of the control IC 10.

Description

DC / DC Converter

[0001] The present disclosure relates to an isolated flyback DC / DC converter.

[0002] As a small and relatively inexpensive isolated DC / DC converter, the flyback DC / DC converter is widely popular (see, for example, Patent Document 1). In a general flyback DC / DC converter, a load switch (see SW0 in FIG. 1) is inserted into the high-side DC input line, and the power supply to the control IC is turned on / off by on / off control of the load switch by an external signal to start or stop the control IC. This method is often used.

[0003] When the DC input is a high voltage, it is common to use a Pch-MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) for the load switch. However, a Pch-MOSFET that can handle high breakdown voltage and large current is expensive. When the DC input voltage is 300 V or more, a Pch-MOSFET having a breakdown voltage of 300 V or more is required. Such a Pch-MOSFET is expensive and the commercially available products are limited. Also, when a Pch-MOSFET is inserted into the DC input line, losses due to the on-resistance of the Pch-MOSFET occur, and the efficiency of the converter decreases. Therefore, a Pch-MOSFET with a low on-resistance is required.

[0004] Japanese Patent Application Laid-Open No. 2002-345235

[0005] However, it is difficult to obtain a Pch-MOSFET that can handle high breakdown voltage and large current and has a low on-resistance at a low cost.

[0006] The present disclosure has been made in view of such circumstances, and an object thereof is to provide a technology for realizing start / stop control of a control IC of a DC / DC converter at a low cost.

[0007] To solve the above problems, a DC / DC converter in one aspect of the present disclosure is a DC / DC converter that converts a DC voltage to a predetermined DC voltage, comprising: an isolation transformer having a primary winding connected to an input DC power supply and a secondary winding connected to an output smoothing capacitor via a rectifier diode; a switching element connected between the primary winding and ground potential; a control IC for driving the switching element; a drive signal line connecting the control terminal of the switching element and the drive terminal of the control IC; and a start / stop switch connected between the switch and ground potential.

[0008] According to this disclosure, it is possible to achieve low-cost start-up and stop-down control of a DC / DC converter control IC.

[0009] This figure shows the configuration of a flyback DC / DC converter according to a comparative example. This figure shows the configuration of a flyback DC / DC converter according to an embodiment. Figure 3(a) shows an example of a circuit configuration in which the output port of the drive circuit is of the open-drain type. Figure 3(b) shows an example of a circuit configuration in which the output port of the drive circuit is of the push-pull type.

[0010] Figure 1 shows the configuration of a flyback DC / DC converter 1 according to a comparative example. The flyback DC / DC converter 1 shown in Figure 1 is an isolated DC / DC converter that converts a DC voltage supplied from a DC power supply 2 to a predetermined DC voltage, and is capable of both boosting and stepping up voltage. The DC power supply 2 may be a storage battery or a circuit that rectifies and smooths an AC voltage supplied from the commercial power grid. Hereinafter, in this specification, we assume that the DC power supply 2 is a storage battery that discharges at a voltage of 300V or higher, and we assume an example in which the flyback DC / DC converter 1 steps down the high-voltage DC voltage supplied from the DC power supply 2 to a low-voltage DC voltage such as 12V or 24V.

[0011] The flyback DC / DC converter 1 shown in Figure 1 comprises an isolation transformer TR1, a first switching element M1, a first rectifier diode D1, an output smoothing capacitor C1, a control IC 10, a bias circuit 20, and a load switch SW0. The isolation transformer TR1 has a primary winding L1 and a secondary winding L2. One end of the primary winding L1 is connected to the positive terminal of the DC power supply 2, and the other end of the primary winding L1 is connected to ground potential via the first switching element M1.

[0012] The control IC 10 is an IC for driving the first switching element M1. For example, an N-channel MOSFET is used for the first switching element M1. The source terminal of the first switching element M1 is connected to ground potential, the drain terminal is connected to the primary winding L1, and the gate terminal is connected to the drive terminal DRV of the control IC 10 via a drive signal line 32.

[0013] The secondary winding L2 of the isolation transformer TR1 is connected in parallel with the output smoothing capacitor C1. A first rectifier diode D1 is connected in the path between the secondary winding L2 and the output smoothing capacitor C1. For example, an electrolytic capacitor is used for the output smoothing capacitor C1.

[0014] The isolation transformer TR1 has an auxiliary winding L3 for generating the power supply voltage for the control IC 10. The auxiliary winding L3 is connected in parallel with the bias power supply capacitor C2. A second rectifier diode D2 is connected in the path between the auxiliary winding L3 and the bias power supply capacitor C2. The bias power supply capacitor C2 is a capacitor for holding the voltage generated by the auxiliary winding L3, and for example, an electrolytic capacitor is used. In this specification, the auxiliary winding L3, the bias power supply capacitor C2, and the second rectifier diode D2 are collectively referred to as the bias circuit 20. The voltage line 33 of the bias power supply capacitor C2 is connected to the power supply terminal VDD of the control IC 10.

[0015] The control IC 10 includes a startup circuit 11 and a drive circuit 12. The drive circuit 12 performs the switching operation of the first switching element M1 based on the bias voltage generated by the bias circuit 20 and the PWM signal. When an N-channel MOSFET is used for the first switching element M1, a voltage of about 10V must be applied between the gate and source of the first switching element M1 in order to turn it on. In this case, the bias voltage is set to about 10V (for example, 12V).

[0016] When a high-level signal corresponding to the bias voltage is input to the control terminal of the first switching element M1 from the drive circuit 12, the first switching element M1 turns on, and when a low-level signal corresponding to the ground potential is input, the first switching element M1 turns off.

[0017] During the ON period of the first switching element M1, the voltage induced in the secondary winding L2 of the isolation transformer TR1 causes the first rectifier diode D1 to enter a reverse bias state, so no current flows through the secondary winding L2. The primary winding L1 is excited by the DC input voltage Vin, and excitation energy is stored in the isolation transformer TR1.

[0018] During the off-period of the first switching element M1, the voltage induced in the secondary winding L2 of the isolation transformer TR1 causes the first rectifier diode D1 to enter a forward bias state, and the excitation energy stored in the isolation transformer TR1 is released as current from the secondary winding L2. The voltage rectified by the first rectifier diode D1 is smoothed by the output smoothing capacitor C1 and supplied to the load (not shown) as a DC output voltage Vout.

[0019] In the PWM flyback DC / DC converter 1, a PWM signal is generated within the control IC 10 so that the deviation between the DC output voltage Vout and the target voltage is zero. The PWM flyback DC / DC converter 1 has two operating modes: a current discontinuous mode and a current continuous mode. In the current discontinuous mode, the forward current on the secondary side reaches zero during the off period of the first switching element M1, resulting in a period during which no current flows. In the current continuous mode, the forward current on the secondary side does not reach zero during the off period of the first switching element M1, and current continues to flow on the secondary side.

[0020] This specification focuses on starting and stopping the flyback DC / DC converter 1, and the control method for the flyback DC / DC converter 1 is not particularly limited; any control method may be employed.

[0021] The start circuit 11 of the control IC 10 is connected to the high-side input line 31, which connects the DC power supply 2 and the primary winding L1, via the high-voltage terminal HV of the control IC 10. A load switch SW0 is inserted in the high-side input line 31 before the branching point to the high-voltage terminal HV of the control IC 10.

[0022] In the flyback DC / DC converter 1 of the comparative example, when the load switch SW0 is turned on by an external signal, power is supplied to the startup circuit 11 from the high-side input line 31. The startup circuit 11 has a constant current source internally. Based on the DC input voltage Vin supplied from the high-side input line 31, the startup circuit 11 charges the bias power supply capacitor C2 with a weak current to a voltage that can drive the first switching element M1. When the switching operation of the first switching element M1 by the control IC 10 starts, charging of the bias power supply capacitor C2 from the auxiliary winding L3 begins, and the startup circuit 11 stops outputting the weak current.

[0023] As described above, in the flyback DC / DC converter 1 of the comparative example, when the DC input voltage Vin is high, it is necessary to use a P-channel MOSFET with high voltage resistance, high current capacity, and low on-resistance for the load switch SW0. Such P-channel MOSFETs are difficult to obtain, and even if they can be obtained, they are expensive components, thus increasing costs.

[0024] Figure 2 shows the configuration of the flyback DC / DC converter 1 according to the embodiment. In the flyback DC / DC converter 1 according to the embodiment, the expensive load switch SW0 is omitted. Instead, a start / stop switch SW1 and a high-voltage switch SW2 are added.

[0025] The start / stop switch SW1 is connected between the drive signal line 32 and the ground potential. For example, an N-channel MOSFET is used for the start / stop switch SW1.

[0026] The high-voltage switch SW2 is connected between the high-side input line 31 and the high-voltage terminal HV of the control IC 10. For example, a P-channel MOSFET is used for the high-voltage switch SW2. The high-voltage terminal HV of the control IC 10 is generated with high impedance and designed so that only a small current flows from the high-side input line 31 to the start circuit 11 in the control IC 10.

[0027] Therefore, while the P-channel MOSFET needs to have a high voltage rating, it does not need to be capable of handling high currents. Also, since it is not inserted into the high-side input line 31, a high on-resistance does not affect the efficiency of the flyback DC / DC converter 1. Furthermore, since the current flowing from the high-side input line 31 to the start circuit 11 is small, the heat generated is also small, and there is little need to use a low on-resistance MOSFET. Consequently, the P-channel MOSFET used in the high-voltage switch SW2 can be a lower-spec and cheaper P-channel MOSFET than the one used in the load switch SW0.

[0028] As a general rule, while the control IC 10 is stopped, the start / stop switch SW1 is controlled to the ON state by the first external signal, and the high-voltage switch SW2 is controlled to the OFF state by the second external signal. While the control IC 10 is running, the start / stop switch SW1 is controlled to the OFF state by the first external signal, and the high-voltage switch SW2 is controlled to the ON state by the second external signal. The first and second external signals are generated by a microcontroller (not shown) or other higher-level controller.

[0029] When the control IC is started, the high-voltage switch SW2 is turned on and the start / stop switch SW1 is turned off. Voltage is supplied to the high-voltage terminal HV of the control IC 10 from the high-side input line 31, and charging of the control IC's ground-referenced bias power supply capacitor C2 begins, and the bias voltage input to the power supply terminal VDD of the control IC 10 gradually increases. When the bias voltage reaches the start voltage of the control IC 10, the current flowing from the high-voltage terminal HV is minimized, the high-voltage terminal HV enters standby mode, and the control IC 10 starts up. After the control IC 10 has started up, the high-voltage switch SW2 may remain in the ON state or may be turned off.

[0030] After the control IC 10 is started, the square wave voltage induced in the auxiliary winding L3 of the isolation transformer TR1 is rectified by the second rectifier diode D2 of the bias circuit 20 and smoothed by the bias power supply capacitor C2, so that the bias voltage input to the power supply terminal VDD of the control IC 10 stabilizes at a constant value. When a PWM drive signal is output from the drive terminal DRV of the started control IC 10 to the gate terminal of the first switching element M1, the switching operation of the first switching element M1 begins.

[0031] When the control IC is stopped, the high-voltage switch SW2 is turned off and the start / stop switch SW1 is turned on. When the start / stop switch SW1 is turned on, the PWM drive signal output from the drive terminal DRV of the control IC 10 is fixed to ground potential, so the switching operation of the first switching element M1 stops. When the switching operation of the first switching element M1 stops, voltage is no longer supplied to the primary winding L1 of the isolation transformer TR1, no voltage is induced in the secondary winding L2, and the DC output voltage Vout turns off. Also, when the switching operation of the first switching element M1 stops, the bias circuit 20 also stops, the bias voltage in the control IC 10 decreases, and when the bias voltage falls below the start voltage of the control IC 10, the control IC 10 also stops.

[0032] In this embodiment, the drive terminal DRV, which serves as the output port of the drive circuit 12, must be of the open-drain or open-collector type.

[0033] Figure 3(a) shows an example of a circuit configuration in which the output port of the drive circuit 12 is of the open-drain type. In the open-drain type drive circuit 12 shown in Figure 3(a), a pull-up resistor R1 and a second switching element M2 are connected in series between the power line connected to the power terminal VDD of the control IC 10 and the ground line connected to the ground terminal GND. An N-channel MOSFET is used for the second switching element M2, the source terminal of the second switching element M2 is connected to the ground line, the drain terminal is connected to the pull-up resistor R1, and a PWM signal is input to the gate terminal. A drive signal line 32 is connected to the connection point of the pull-up resistor R1 and the second switching element M2. Note that the pull-up resistor R1 may be installed outside the package of the control IC 10.

[0034] Figure 3(b) shows an example of a circuit configuration where the output port of the drive circuit 12 is a push-pull type. In the push-pull type drive circuit 12 shown in Figure 3(b), a third switching element M3 and a second switching element M2 are connected in series between the power line connected to the power terminal VDD of the control IC 10 and the ground line connected to the ground terminal GND. A P-channel MOSFET is used for the third switching element M3, and the pull-up resistor R1 shown in Figure 3(a) is replaced with a P-channel MOSFET.

[0035] The source terminal of the third switching element M3 is connected to the power line, and its drain terminal is connected to the drain terminal of the second switching element M2. A PWM signal is input to the gate terminals of both the second switching element M2 and the third switching element M3. A drive signal line 32 is connected to the connection point between the third switching element M3 and the second switching element M2.

[0036] In this embodiment, the start / stop switch SW1 is turned on to stop the control IC 10, and the output port of the drive circuit 12 is directly connected to the ground potential. In the push-pull method shown in Figure 3(b), when the third switching element M3 is on, the power line of the control IC 10 and the ground potential are forcibly short-circuited, which can cause a large current to flow and potentially lead to failure of the control IC 10. In contrast, in the open-drain method shown in Figure 3(a), a pull-up resistor R1 is interposed between the power line of the control IC 10 and the ground potential, which prevents the flow of short-circuit current.

[0037] As explained above, according to this embodiment, the load switch SW0 inserted into the high-side input line 31 can be omitted, eliminating the need to use expensive and hard-to-obtain P-channel MOSFETs. An inexpensive N-channel MOSFET can be used for the start / stop switch SW1. The high-voltage switch SW2 does not need to handle large currents and does not need to have low on-resistance, so a P-channel MOSFET with lower specifications and lower cost can be used than the P-channel MOSFET used for the load switch SW0.

[0038] Furthermore, the conversion efficiency of the flyback DC / DC converter 1 is not reduced due to losses caused by the on-resistance of the P-channel MOSFET. Therefore, it is possible to achieve low-cost and low-loss start-up and stop-down control of the control IC of the flyback DC / DC converter 1.

[0039] The present disclosure has been described above based on embodiments. The embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications are also within the scope of the present disclosure.

[0040] In the embodiment described above, an example was explained in which MOSFETs are used for the first switching element M1, the start / stop switch SW1, the high-voltage switch SW2, and the second switching element M2. However, a bipolar transistor may be used for at least one of the first switching element M1, the start / stop switch SW1, the high-voltage switch SW2, and the second switching element M2. In that case, "gate," "source," and "drain" are read as "base," "emitter," and "collector," respectively. Alternatively, an IGBT (Insulated Gate Bipolar Transistor) may be used instead of a MOSFET. In that case, "gate," "source," and "drain" are read as "gate," "emitter," and "collector," respectively.

[0041] The embodiments may be specified by the following items.

[0042] [Item 1] A DC / DC converter (1) that converts a DC voltage to a predetermined DC voltage, comprising: an isolation transformer (TR1) having a primary winding (L1) connected to an input DC power supply (2) and a secondary winding (L2) connected to an output smoothing capacitor (C1) via a rectifier diode (D1); a switching element (M1) connected between the primary winding (L1) and ground potential; a control IC (10) that drives the switching element (M1); and a start / stop switch (SW1) connected between a drive signal line (32) connecting the control terminal of the switching element (M1) and the drive terminal of the control IC (10) and the ground potential. With this, an expensive load switch using a Pch-MOSFET can be omitted. [Item 2] The DC / DC converter (1) described in Item 1, wherein the start / stop switch (SW1) is controlled to the ON state by an external signal while the control IC (10) is stopped, and the start / stop switch (SW1) is controlled to the OFF state by an external signal while the control IC (10) is started. With this, the control IC (10) can be started and stopped using an inexpensive start / stop switch (SW1) such as an Nch-MOSFET. [Item 3] The DC / DC converter (1) according to Item 1, wherein the isolation transformer (TR1) further has an auxiliary winding (L3) for generating the power supply voltage of the control IC (10), the DC / DC converter (1) further comprises a bias power supply capacitor (C2) for holding the voltage generated by the auxiliary winding (L3), the control IC (10) includes a start circuit (11) for charging the bias power supply capacitor (C2) to a voltage capable of driving the switching element (M1) based on the voltage supplied from the DC power supply (2), and the DC / DC converter (1) further comprises a high-side input line (31) connecting the DC power supply (2) and the primary winding (L1), and a high-voltage switch (SW2) connected between the high-voltage terminal of the control IC (10) connected to the start circuit (11).According to this, by turning off the high-voltage switch (SW2) while the control IC (10) is stopped, it is possible to prevent the accumulation of unnecessary charge in the bias power supply capacitor C2 and inside the control IC (10) while the control IC (10) is stopped. [Item 4] The DC / DC converter (1) described in Item 3, wherein while the control IC (10) is stopped, the start / stop switch (SW1) is controlled to the ON state by a first external signal, and the high-voltage switch (SW2) is controlled to the OFF state by a second external signal, while the control IC (10) is started, the start / stop switch (SW1) is controlled to the OFF state by the first external signal, and when the control IC (10) is started, the high-voltage switch (SW2) is turned on by the second external signal. According to this, the control IC (10) can be started and stopped more safely with a start / stop switch (SW1) using an inexpensive Nch-MOSFET and a high-voltage switch (SW2) using a relatively inexpensive Pch-MOSFET.

[0043] This disclosure is applicable to isolated flyback DC / DC converters.

[0044] 1 Flyback DC / DC converter, 2 DC power supply, 10 Control IC, 11 Startup circuit, 12 Drive circuit, 20 Bias circuit, M1 First switching element, TR1 Isolation transformer, L1 Primary winding, L2 Secondary winding, L3 Auxiliary winding, D1 First rectifier diode, D2 Second rectifier diode, C1 Output smoothing capacitor, C2 Bias power supply capacitor, SW0 Load switch, SW1 Start / stop switch, SW2 High voltage switch, M2 Second switching element, M3 Third switching element, R1 Pull-up resistor, 31 High-side input line, 32 Drive signal line.

Claims

1. A DC / DC converter that converts a DC voltage to a predetermined DC voltage, comprising: an isolation transformer having a primary winding connected to an input DC power supply and a secondary winding connected to an output smoothing capacitor via a rectifier diode; a switching element connected between the primary winding and ground potential; a control IC for driving the switching element; and a start / stop switch connected between the control terminal of the switching element, a drive signal line connecting the drive terminal of the control IC, and the ground potential.

2. The DC / DC converter according to claim 1, wherein, while the control IC is stopped, the start / stop switch is controlled to the ON state by an external signal, and while the control IC is running, the start / stop switch is controlled to the OFF state by an external signal.

3. The DC / DC converter according to claim 1, wherein the isolation transformer further comprises an auxiliary winding for generating a power supply voltage for the control IC, the DC / DC converter further comprises a bias power supply capacitor for holding the voltage generated by the auxiliary winding, the control IC includes a startup circuit for charging the bias power supply capacitor to a voltage capable of driving the switching element based on the voltage supplied from the DC power supply, and the DC / DC converter further comprises a high-side input line connecting the DC power supply and the primary winding, and a high-voltage switch connected between the high-voltage terminal of the control IC connected to the startup circuit.

4. The DC / DC converter according to claim 3, wherein, while the control IC is stopped, the start / stop switch is controlled to the ON state by a first external signal, and the high-voltage switch is controlled to the OFF state by a second external signal; while the control IC is started, the start / stop switch is controlled to the OFF state by the first external signal; and when the control IC is started, the high-voltage switch is turned ON by the second external signal.

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