Battery switch drive circuit

Abstract

A battery switch driving circuit according to an embodiment of the present invention includes a first battery input terminal and a second battery input terminal, a converter located between the first battery input terminal and the second battery input terminal, a first switch located between the second battery input terminal and the converter and cutting off a power supply input to the second battery input terminal when the first switch is off, and a switch driving unit for turning on the first switch, the switch driving unit including a first capacitor that is charged and discharged in response to the on / off of a second switch operated by a PWM signal, and a second capacitor that is charged in response to the on / off of the second switch by a voltage charged in the first capacitor, thereby turning on the first switch.

Description

[Technical Field]

[0001] The present invention relates to a battery switch drive circuit, and more specifically to a battery switch drive circuit that drives a battery switch using a switch, a capacitor, and a diode. [Background technology]

[0002] In recent years, there has been an increase in battery application fields that use multiple batteries instead of a single one, and use a specific battery depending on the situation, such as in automobiles, automation equipment, medical equipment, and robotics.

[0003] Figure 1 shows a system that uses a 48V battery and a 12V battery, and it can charge the 48V battery using the 12V battery via a DC-DC converter, or charge the 12V battery using the 48V battery. When using a 12V battery, a semiconductor switch (FET, IGBT) can be used to disconnect and isolate the 48V battery.

[0004] A high-side switch driver is required to drive the semiconductor switch in order to turn it on and off, as shown in Figure 1. The gate power supply for the semiconductor switch is created using a push-pull method with an isolation transformer, and the switch can be turned on using that power supply.

[0005] Such a circuit for driving a high-side semiconductor switch requires a buffer to amplify the PWM signal and a transformer for isolation, which has the disadvantage of increasing product cost and size. Summary of the Invention [Problem to be solved by the invention]

[0006] The technical problem to be solved by the present invention is to provide a battery switch driving circuit that uses a switch, a capacitor, and a diode to drive a battery switch. [Means for solving the problem]

[0007] In order to solve the above technical problems, a battery switch driving circuit according to one embodiment of the present invention includes a first battery input terminal and a second battery input terminal, a converter located between the first battery input terminal and the second battery input terminal, a first switch located between the second battery input terminal and the converter and cutting off power input to the second battery input terminal when turned off, and a switch driving unit that turns on the first switch, wherein the switch driving unit includes a first capacitor that is charged and discharged in response to on / off of a second switch that operates with a PWM signal, and a second capacitor that is charged in response to on / off of the second switch by a voltage charged in the first capacitor and turns on the first switch.

[0008] Also, one end of the first capacitor may be connected to the first battery input terminal and the other end may be connected to the second switch.

[0009] The first capacitor may also be connected to the first battery input terminal via a first resistor and a first diode.

[0010] Furthermore, one end of the first capacitor may be connected to one end of the second capacitor, and the other end of the first capacitor may be connected to the other end of the second capacitor.

[0011] In addition, the other end of the first capacitor may be connected to the other end of the second capacitor via a second resistor and a second diode, and one end of the first capacitor may be connected to one end of the second capacitor via a third resistor and a third diode.

[0012] The second capacitor has one end connected to the gate of the first switch and the other end connected to the source, and when charged to a voltage equal to or greater than the gate threshold of the first switch, the first switch can be turned on.

[0013] The first capacitor may have one end connected to a system power input terminal or the second battery input terminal, and the other end connected to the second switch.

[0014] The power supply may further include a clamping circuit that clamps a voltage input from the second battery input terminal to the first battery voltage when one terminal of the first capacitor is connected to the second battery input terminal.

[0015] The power supply also includes a third switch connected to the first battery input terminal via an optocoupler and a fourth resistor and turned on and off in response to a first switch-off signal, the optocoupler forming a closed loop with the second capacitor and the third switch operating when turned on to discharge the voltage of the second capacitor.

[0016] In order to solve the technical problems, a battery switch driving circuit according to another embodiment of the present invention includes a first battery input terminal, a second battery input terminal receiving a battery voltage different from that of the first battery input terminal, a converter located between the first battery input terminal and the second battery input terminal, fourth and fifth switches located between the second battery input terminal and the converter and cutting off power input in opposite directions when turned off, and a switch driving unit that turns on the fourth and fifth switches, wherein the switch driving unit includes a first capacitor that is charged and discharged in response to the on / off of a second switch that operates with a PWM signal, and a second capacitor that is charged in response to the on / off of the second switch by the voltage charged in the first capacitor, thereby turning on the fourth or fifth switch. [Effects of the Invention]

[0017] According to an embodiment of the present invention, by using a switch, a capacitor, and a diode to drive a semiconductor switch, it is advantageous in terms of cost and size, and can be configured in a back-to-back configuration, allowing the switch to be turned off quickly when necessary. [Brief explanation of the drawings]

[0018] [Figure 1] 1 illustrates a battery switch driving circuit according to a comparative example of the present invention. [Figure 2] 1 is a block diagram of a battery switch driving circuit according to an embodiment of the present invention; [Figure 3] 1 is a block diagram of a battery switch driving circuit according to an embodiment of the present invention; [Figure 4] 1 is a circuit diagram of a battery switch driving circuit according to an embodiment of the present invention; [Figure 5] 5 is a diagram for explaining the operation of the embodiment of FIG. 4. [Figure 6] 5 is a diagram for explaining the operation of the embodiment of FIG. 4. [Figure 7] 5 is a diagram for explaining the operation of the embodiment of FIG. 4. [Figure 8] 5 is a diagram for explaining the operation of the embodiment of FIG. 4. [Figure 9] FIG. 10 is a circuit diagram of a battery switch driving circuit according to another embodiment of the present invention. [Figure 10] 10 is a diagram for explaining the operation of the embodiment of FIG. 9. [Figure 11] 10 is a diagram for explaining the operation of the embodiment of FIG. 9. [Figure 12] FIG. 10 is a circuit diagram of a battery switch driving circuit according to yet another embodiment of the present invention. [Figure 13] 13 is a diagram for explaining the operation of the embodiment of FIG. 12. [Figure 14] 13 is a diagram for explaining the operation of the embodiment of FIG. 12. [Figure 15] 13 is a diagram for explaining the operation of the embodiment of FIG. 12. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0020] However, the technical concept of the present invention is not limited to the described embodiments, but can be realized in various different forms, and one or more of the components of the embodiments can be selectively combined or substituted and used within the scope of the technical concept of the present invention.

[0021] Furthermore, unless otherwise clearly defined and described, terms (including technical and scientific terms) used in the embodiments of the present invention may be interpreted as meanings that are commonly understood by a person having ordinary knowledge in the technical field to which the present invention belongs, and commonly used terms such as predefined terms may be interpreted in light of the contextual meaning of the relevant art.

[0022] Furthermore, the terms used in the examples of the present invention are intended to explain the examples and are not intended to limit the present invention.

[0023] In this specification, the singular can include the plural unless otherwise specified in the context, and when it is stated as "A and (and) at least one (or more) of B and C," it can include one or more of all possible combinations of A, B, and C.

[0024] Furthermore, when describing components of the embodiments of the present invention, terms such as first, second, A, B, (a), (b), etc. are used only to distinguish the component from other components, and the term does not limit the nature, order, or sequence of the corresponding components.

[0025] It should be noted that when a component is described as being "coupled," "coupled," or "connected" to another component, it includes not only the case where the component is "coupled," "coupled," or "connected" directly to the other component, but also the case where the component is "coupled," "coupled," or "connected" by yet another component between the component and the other component.

[0026] Furthermore, when it is stated that something is formed or disposed "above (above)" or "below (below)" a component, "above (above)" or "below (below)" includes not only the case where two components are in direct contact with each other, but also the case where one or more additional components are formed or disposed between the two components. Furthermore, when it is expressed as "above (above)" or "below (below)," it can include not only the meaning of the upper direction but also the meaning of the lower direction based on one component.

[0027] FIG. 2 is a block diagram of a battery switch driving circuit according to an embodiment of the present invention.

[0028] The battery switch driving circuit 100 according to one embodiment of the present invention comprises a first battery input terminal 110, a second battery input terminal 120, a converter 130, a first switch 140, and a switch driving unit 150, and may include a second switch 151, a first capacitor 152, a second capacitor 153, and resistors and diodes connected between each component.

[0029] The first battery input terminal 110 is connected to a first battery (not shown) having a first battery voltage and can receive the first battery voltage as an input or output a voltage to the first battery to charge the first battery. The second battery input terminal 120 is connected to a second battery (not shown) having a second battery voltage that is different from the battery voltage input to the first battery input terminal 110 and can receive the second battery voltage as an input or output a voltage to the second battery to charge the second battery. Here, the first battery may be a low-voltage battery, for example, a 12V battery, and the first battery may be a high-voltage battery, for example, a 48V battery. Alternatively, the first battery may be a high-voltage battery and the second battery may be a low-voltage battery, or the two batteries may have the same voltage.

[0030] The converter 130 is located between the first battery input terminal 110 and the second battery input terminal 120, and can convert a first battery voltage input to the first battery input terminal 110 into a second battery voltage and output it to the second battery input terminal 120, or can convert a second battery voltage input to the second battery input terminal 120 into a first battery voltage and output it to the first battery input terminal 110. The converter 130 may be a bidirectional converter composed of a plurality of switches and inductors, and can charge different batteries via the converter.

[0031] The first switch 140 is located between the second battery input terminal and the converter and, when turned off, cuts off the power input to the second battery input terminal 120. The second battery voltage connected to the second battery input terminal 120 may be higher than the first battery voltage connected to the first battery input terminal 110. The second battery voltage may be input and converted to the first battery voltage via the converter 130 to charge the first battery connected to the first battery input terminal 110 or to supply power to a device receiving power from the first battery. When the second battery voltage is input and used, the first switch 140 is turned on; otherwise, the first switch 140 is turned off to prevent the second battery voltage from being applied to the first battery. When the first switch 140 is turned off, the power input to the second battery input terminal 120 can be disconnected.

[0032] The switch driver 150 turns on the first switch 140. In order for the first switch 140 to be turned on or off, a driving power source must be supplied to the first switch 140, and the switch driver 150 supplies the driving power source to the first switch 140 to turn it on.

[0033] 3, the switch driver may include a second switch 151, a first capacitor 152, and a second capacitor 153. The switch driver may include a first capacitor 152 that is charged and discharged in response to the on / off of the second switch 151 that is operated by a PWM signal, and a second capacitor 153 that is charged in response to the on / off of the second switch 151 by the voltage charged in the first capacitor 152.

[0034] One end of the first capacitor 152 may be connected to the first battery input terminal 110 and the other end may be connected to the second switch 151. When the second switch 151 is turned on by a PWM signal, a current path is connected between the first battery input terminal 110, the first capacitor 152, and the second switch 151, and the first capacitor 152 may be charged by the current flowing from the first battery input terminal 110. Here, the first capacitor 152 may be connected to the first battery input terminal 110 via a first resistor R1 and a first diode D1. The first resistor R1 allows current to flow, and the first diode D1 prevents current from flowing in the reverse direction when the second switch 151 is turned off.

[0035] One end of first capacitor 152 may be connected to a system power input terminal (not shown) or the second battery input terminal 120, and the other end may be connected to the second switch 151. As shown in FIG. 3, first capacitor 152 may be connected to the system power input terminal or the second battery input terminal 120, rather than to the first battery input terminal 110, and may be charged via the system power input terminal or the second battery voltage. Here, when one end of first capacitor 152 is connected to the second battery input terminal 120, a clamping circuit (not shown) may be included that clamps the voltage input from second battery input terminal 120 to the first battery voltage. As described above, the second battery may be a high-side battery with a high voltage. Since using a high voltage requires a large rated voltage of the device, a clamping circuit may be formed to lower the second battery voltage to the first battery voltage or a preset voltage. Although a high voltage input is received via the clamping circuit, the battery switch driving circuit can be driven at a low voltage, so that the circuit can be realized using low-cost elements.

[0036] One end of the first capacitor 152 may be connected to one end of the second capacitor 153, and the other end may be connected to the other end of the second capacitor 153. When the second switch 151 is turned off by a PWM signal, the path through which current flows through the second switch 151 is cut off, and the first capacitor 152 and the second capacitor 153 form a closed loop. As a result, the voltage charged in the first capacitor 152 charges the second capacitor 153. Here, the other end of the first capacitor 152 may be connected to the other end of the second capacitor 153 via a second resistor (R2) and a second diode (D2), and one end of the first capacitor 152 may be connected to one end of the second capacitor 153 via a third resistor (Rg) and a third diode (D3). A current can be made to flow through the second resistor (R2) and the third resistor (Rg), and a current can be made to flow through the second diode (D2) and the third diode (D3) in a direction in which the first capacitor 152 charges the second capacitor 153.

[0037] The first switch 140 may be a MOSFET, and the second capacitor 153 has one end connected to the gate of the first switch 140 and the other end connected to the source. When the second capacitor 153 is charged to a voltage equal to or greater than the gate threshold of the first switch 140, the first switch 140 can be turned on. Both ends of the second capacitor 153 may be connected to the gate and source terminals of the first switch 140, respectively. That is, the voltage charged in the second capacitor 153 becomes the gate-source voltage of the first switch 140. The first switch 140 is turned on when the gate-source voltage exceeds a threshold, and when the voltage charged in the second capacitor 153 is equal to or greater than the threshold, the first switch 140 is turned on.

[0038] As described above, the battery switch can be driven using only a switch, a capacitor, a diode, and a resistor, and a battery switch driving circuit that is advantageous in terms of cost and size can be realized.

[0039] 4 is a circuit diagram of a battery switch driving circuit according to an embodiment of the present invention. A 12V battery (Battery 12V) may be connected to a first battery input terminal, and a 48V battery (Battery 48V) may be connected to a second battery input terminal. The converter may be a DC-DC converter, which may be a bidirectional converter formed by two switches (Q1, Q2) and an inductor (L). A first switch (SW1) is formed between the 48V battery and the DC-DC converter. 4, the switch driver for driving the first switch (SW1) includes a second switch (SW2) operated by a PWM signal, a first capacitor (C1) connected to a 12V battery via a first resistor (R1) and a first diode (D1), and a second capacitor (Cg) connected at both ends to the first capacitor (C1), and the first capacitor (C1) and the second capacitor (Cg) may be connected via a second resistor (R2) and a second diode (D2), and a third resistor (Rg) and a second diode (D3). Both ends of the second capacitor (Cg) may be connected to the gate and source of the first switch (SW1), and the other end of the second capacitor 153 may be connected to the DC-DC converter.

[0040] The battery switch drive circuit of FIG. 4 can operate as shown in FIGS. 5 and 6 in response to a PWM signal.

[0041] When the first switch (SW1) is turned off, the 12V battery voltage is applied to the source of the first switch (SW1) through Q1 and L of the DC-DC converter, and the source voltage of the first switch (SW1) is 12V-V f,Q1 When a PWM signal is applied to the gate of the second switch (SW2) to turn on the first switch (SW1), the second switch (SW2) repeats on / off.

[0042] When the second switch (SW2) is turned on by the PWM signal, the 12V battery voltage charges the first capacitor (C1) via the first diode (D1), the first resistor (R1) and the path (P1). At this time, the voltage of the first capacitor (C1) rises as follows:

[0043]

number

[0044] In addition, a leakage current flows through the DC-DC converter (L, Q1), second resistor (R2), second diode (D2), second switch (SW2) and path (P2) as shown in the following equation. This current becomes a loss, so the value of the second resistor (R2) must be set to an appropriate large value.

[0045]

number

[0046] Here, the magnitude of the second resistor (R2) can be set by the user according to the allowable leakage current value allowed in the battery device or system, and the magnitude of the second resistor (R2) may be set to be larger than the first resistor (R1) and the third resistor (Rg).

[0047] As shown in FIG. 6, when the second switch (SW2) is turned off by the PWM signal, the voltage of the first capacitor (C1) becomes greater than the 12V battery voltage, and the first diode (D1) is turned off.

[0048]

number

[0049] The voltage charged in the first capacitor C1 charges the second capacitor Cg via the third diode D3, the third resistor Rg, the second capacitor Rg, the second resistor R2, the second diode D2 and the path P3, and the voltage is charged in the second capacitor Cg, turning on the first switch SW1.

[0050] When the second switch (SW2) is repeatedly turned on and off by the PWM signal, the first capacitor (C1) is charged when the second switch (SW2) is on, and the second capacitor (Cg) is charged with the voltage charged in the first capacitor (C1) when the second switch (SW2) is off, so the gate voltage of the first switch (SW1) rises, and ultimately the voltage across the second capacitor (Cg) becomes as follows:

[0051]

number

[0052] When the voltage across the second capacitor (Cg) becomes equal to or greater than the gate threshold of the first switch (SW1), the first switch (SW1) is turned on, and the source voltage of the first switch (SW1) rises from the 12V battery voltage to the 48V battery voltage.

[0053] Figure 7 shows the circuit used to perform the simulation of Figure 4, and Figure 8 shows the simulation results. From the simulation results, it can be seen that when a PWM signal is applied, the gate-source voltage (SW1_Vgs) of the first switch (SW1) begins to charge, and when this voltage exceeds the threshold (Threshold) of the first switch (SW1), the first switch (SW1) turns on, and the source voltage (SW1_source) of the first switch (SW1) rises from the 12V battery voltage to the 48V battery voltage.

[0054] The first switch 140, which is located between the first battery input terminal 110 and the converter 130 and cuts off the power input to the second battery input terminal 120 when turned off, may instead include fourth and fifth switches located between the first battery input terminal 110 and the converter 130 and cutting off the power input in opposite directions when turned off. In this case, the switch driver 150 can turn on the fourth and fifth switches. By using the fourth and fifth switches, which can cut off the power input in both directions, instead of the first switch 140, the switch structure can be realized as a back-to-back structure. Here, the back-to-back structure refers to a structure that cuts off a bidirectional signal input. The second capacitor 153 of the switch driver 150 is connected to the gate and drain of the back-to-back structure switch formed by the fourth and fifth switches, and can turn on the fourth and fifth switches as described above.

[0055] 9 is a circuit diagram of a battery switch driving circuit according to another embodiment of the present invention, in which two switches are connected in opposite directions to realize a battery switch with a back-to-back structure. The battery switch driving circuit of FIG. 9 operates in the same way as the battery switch driving circuit of FIG. 4, in that when a PWM signal is applied to the second switch (SW2), the first capacitor (C1) is charged, the second capacitor (Cg) is charged, and the fourth switch and the fifth switch are turned on.

[0056] Figure 10 shows the circuit used to perform the simulation of Figure 9, and Figure 11 shows the simulation results. The simulation results show that when a PWM signal is applied, the gate-source voltage (SW1_Vgs) of switch SW1 in the back-to-back configuration of the fourth and fifth switches begins to charge, and when this voltage exceeds the threshold of the gate of the first switch (SW1), the first switch (SW1) turns on, and the drain voltage (SW1_drain) of the first switch (SW1) rises from the 12V battery voltage to the 48V battery voltage.

[0057] In order to turn off the first switch (SW1) when the first switch (SW1) is on, the voltage charged in the second capacitor (Cg) must be discharged, and a switch-off circuit may be included to quickly turn off the first switch (SW1) in an emergency.

[0058] To this end, the circuit includes a third switch (SW3) connected to the first battery input terminal via an optocoupler and a fourth resistor (R3) and turned on and off in response to a first switch-off signal. The optocoupler forms a closed loop with the second capacitor 153 and operates when the third switch (SW3) is turned on to discharge the voltage of the second capacitor 153. Here, the optocoupler is a switch operated by light and is composed of a light-emitting diode (LED) that emits light when current flows and a switch that is turned on by the light emitted from the LED. When the first switch-off signal is applied to the gate of the third switch (SW3), the third switch (SW3) is turned on, and the light-emitting diode of the optocoupler emits light, operating the optocoupler and forming a closed loop with the second capacitor 153 to quickly discharge the voltage charged in the second capacitor 153 and quickly turn on the first switch 140.

[0059] Figure 12 is a circuit diagram of a battery switch driver circuit according to yet another embodiment of the present invention, which is capable of quickly turning off the first switch (SW1). The battery switch driver circuit of Figure 12 operates like the battery switch driver circuit of Figure 4 when turning on the first switch (SW1), but when turning off the first switch (SW1), it drives a switch-off circuit as shown in Figure 13. When a first switch-off signal (Switch-off) is applied to the third switch (SW3), the third switch (SW3) is turned on, and the optocoupler is turned on via the 12V battery voltage, the light-emitting diode of the optocoupler, the fourth resistor (R3), and the path (P4). The third resistor (Rg) may include a fifth resistor (Rg1) and a sixth resistor (Rg2), and when the optocoupler is turned on, the second capacitor (Cg) is quickly discharged through the optocoupler, the fifth resistor (Rg1), and the second capacitor (Cg) path (P5), thereby quickly turning off the first switch (SW1). Here, the voltage of the discharged second capacitor (Cg) is as follows:

[0060]

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[0061] Figure 14 shows the circuit used to perform the simulation of Figure 12, and Figure 15 shows the simulation results. The simulation results show that when the first switch-off signal (Switch-off) is applied and the third switch is turned on, the optocoupler is turned on and the gate-source voltage (SW1_Vgs) of the first switch (SW1) begins to discharge, and when this voltage falls below the threshold (Threshold) of the first switch (SW1), the first switch (SW1) is turned off, and the source voltage (SW1_source) of the first switch (SW1) drops from the 48V battery voltage to the 12V battery voltage.

[0062] As described above, by implementing a battery switch driver circuit using a capacitor, a diode, a resistor, and a switch, a high-side switch can be turned on and off. Also, by using other battery switch driver circuits in embodiments of the present invention, back-to-back configured switches can be turned on and off, and various other types of semiconductor switches (FET, IGBT, etc.) can be turned on and off. Furthermore, a battery switch can be quickly turned off using a switch-off circuit.

[0063] Those skilled in the art will understand that the present invention can be realized in various modified forms without departing from the essential characteristics of the above description. Therefore, the disclosed method should be considered from an illustrative rather than a restrictive perspective. The scope of the present invention is defined by the claims, not the foregoing description, and all differences within the scope of the claims should be construed as being within the scope of the present invention.

Claims

1. a first battery input terminal; a second battery input terminal; a converter located between the first battery input terminal and the second battery input terminal; a first switch positioned between the second battery input terminal and the converter, the first switch cutting off power input to the second battery input terminal when turned off; a third switch connected to the first battery input terminal via an optocoupler and a fourth resistor, and turned on and off in response to a first switch-off signal; a switch driver that turns on the first switch, The switch driving unit a first capacitor that is charged and discharged in response to the on / off of a second switch that is operated by a PWM signal; a second capacitor that is charged by a voltage charged in the first capacitor in response to turning on and off of the second switch, thereby turning on the first switch; one end of the first capacitor is connected to one end of the second capacitor via a fifth resistor, and the other end is connected to the other end of the second capacitor; the optocoupler forms a closed loop with the second capacitor and the fifth resistor, and operates when the third switch is turned on to discharge the voltage of the second capacitor.

2. 2. The battery switch driving circuit according to claim 1, wherein one end of the first capacitor is connected to the first battery input terminal and the other end is connected to the second switch.

3. 3. The battery switch driving circuit according to claim 2, wherein the first capacitor is connected to the first battery input terminal via a first resistor and a first diode.

4. 4. The battery switch drive circuit according to claim 1, wherein the other end of the first capacitor is connected to the other end of the second capacitor via a second resistor and a second diode.

5. 5. The battery switch driving circuit according to claim 4, wherein one end of the first capacitor is connected to one end of the second capacitor via a sixth resistor and a third diode.

6. 6. The battery switch driving circuit of claim 1, wherein the second capacitor has one end connected to the gate of the first switch and the other end connected to the source of the first switch, and when the second capacitor is charged to a voltage equal to or greater than a gate threshold of the first switch, the first switch is turned on.

7. 7. The battery switch driving circuit according to claim 1, wherein one end of the first capacitor is connected to a system power input terminal or the second battery input terminal, and the other end is connected to the second switch.

8. 8. The battery switch driving circuit according to claim 7, further comprising a clamping circuit that clamps a voltage input from the second battery input terminal to the first battery voltage when one end of the first capacitor is connected to the second battery input terminal.

9. a first battery input terminal; a second battery input terminal; a converter located between the first battery input terminal and the second battery input terminal; a fourth switch and a fifth switch positioned between the second battery input terminal and the converter, the fourth switch and a fifth switch cutting off power input in opposite directions when turned off; a third switch connected to the first battery input terminal via an optocoupler and a fourth resistor, and turned on and off in response to a third switch-off signal and a fourth switch-off signal; a switch driver that turns on the fourth switch and the fifth switch, The switch driving unit a first capacitor that is charged and discharged in response to the on / off of a second switch that is operated by a PWM signal; a second capacitor that is charged by a voltage charged in the first capacitor in response to turning on or off of the second switch, thereby turning on the third switch or the fourth switch; one end of the first capacitor is connected to one end of the second capacitor via a fifth resistor, and the other end is connected to the other end of the second capacitor; the optocoupler forms a closed loop with the second capacitor and the fifth resistor, and operates when the third switch is turned on to discharge the voltage of the second capacitor.

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