JPH1022803A - Drive circuit and current direction switching circuit for n-channel mosfet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To satisfactorily turn an FET on and off, even with a presence of difference of a ground potential in regard to a drive circuit which turns an n-channel MOSFET on and off, according to the state of an input terminal that is grounded and opened by a switching element of a control circuit. SOLUTION: A drive circuit of an FET (Tn) is composed of the NPN transistors TR T1 and T2 and the resistors R1 to R4 and turns the Tn on and off, according to voltage Vo of an input terminal which is grounded and opened by a switching terminal To of a control circuit. The drive circuit of such a constitution can increase the threshold of voltage Vo to turn on the TR T1, as long as the resistance value of the resistor R3 is increased and therefore can surely turn on the TR T1, even if the ground potential GND1 of the control circuit is higher than the ground potential GND2 of the drive circuit. On the other hand, the resistance value of the register R4 must be reduced if the resistance value of the R3 is increase. However, it is not required to especially reduce the resistance value of the R4, since the R4 needs only a sufficient base current to turn on the T2. Thus, it is possible to reduce the current (ic) that flows to the control circuit, when the element To is turned on by increasing the resistance value of the R4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an n-channel MOS
The present invention relates to a drive circuit for turning an FET on and off in accordance with an external command, and a current direction switching circuit for switching a current direction flowing to an electric load using the drive circuit.

[0002]

2. Description of the Related Art Conventionally, a driving circuit for an n-channel MOSFET uses a PNP transistor T11 and an NPN transistor T12 as shown in FIG. 6A, and a driving circuit as shown in FIG. Two types of drive circuits are known, one using two NPN transistors T21 and T22 (published by Sogo Denshi Publishing Co., Ltd., Hisashi Takahashi, "How to Use Power Devices and Designing Practical Control Circuits", pp. 94-95). Page).

In the driving circuit shown in FIG.
The NP transistor T11 has an emitter connected to a power supply line on the positive side of the DC power supply (hereinafter, referred to as a power supply line), and a collector connected via a resistor R11 to a power supply line on the negative side of the DC power supply (hereinafter, referred to as a power supply line). Grand Line)
And a connection point between the collector and the resistor R11 is connected to an n-channel M having a source connected to the ground line.
OSFET: Connected to the gate of Tn (hereinafter referred to as FET: Tn). A resistor R12 is connected between the base and the emitter of the PNP transistor T11.
The base of the transistor T11 is connected via a resistor R13 to the collector of an NPN transistor T12 whose emitter is connected to the ground line.

Accordingly, in the driving circuit shown in FIG. 6A, if the NPN transistor T12 is off, the PN
The base current of the P transistor T11 does not flow, and the PNP transistor T11 is turned off. In this state, since no current flows through the resistor R11, the voltage between the gate and the source of the FET: Tn is 0 V, and the FET: Tn is also turned off.
Conversely, if the NPN transistor T12 is on, P
A base current flows through the NP transistor T11, turning on the PNP transistor T11. Then, a current flows through the resistor R11, the voltage between the gate and the source of the FET: Tn becomes substantially the power supply voltage VB, and the FET: Tn is turned on. As a result,
According to the drive circuit of FIG. 6A, the NPN transistor T
By turning on and off 12, FET: Tn is turned on and off.
Can be turned off.

In the driving circuit shown in FIG. 6B, an NPN transistor T21 has a collector connected to an FET:
Connected to the gate of Tn, connected to the power supply line via a resistor R21, and the emitter is connected to the ground line together with the source of the FET: Tn. This drive circuit includes three resistors R22, R23, and R24 connected in series between a power supply line and a ground line. A connection point between the resistor R22 and the resistor R23 on the ground line side is:
It is connected to the base of NPN transistor T21. The other NPN transistor T22 has an emitter connected to the ground line and a collector connected to a connection point between the resistor R24 and the resistor R23 on the power supply line side.

Accordingly, in the driving circuit shown in FIG. 6B, when the NPN transistor T22 is turned off, a current is supplied to the base of the NPN transistor T21 through the resistors R24 and R23, so that the NPN transistor T21 is turned on. State. As a result, the collector-emitter saturation voltage (approximately 0.4 V) of the NPN transistor T21 is applied between the gate and source of the FET: Tn, so that the FET: Tn is turned off. Conversely, when the NPN transistor T22 is in the ON state, the base-emitter voltage of the NPN transistor T21 divides the collector-emitter saturation voltage (about 0.4) of the NPN transistor T22 by the resistors R22 and R23. This is lower than the forward voltage (about 0.6 V) of the PN junction required for the NPN transistor T21 to turn on, so the NPN transistor T21 turns off. As a result, the power supply voltage V is applied between the gate and the source of the FET: Tn.
B is applied, and the FET: Tn is turned on. Therefore, FIG.
Also in the drive circuit of FIG.
Is turned on and off, the FET: Tn can be turned on and off.

On the other hand, a MOS type FET has a small on-voltage with respect to a load of about several amperes as compared with a bipolar transistor or the like, and has a simple voltage drive.
It is often used as a switching element for turning on and off an energization path to various electric loads.For example, a high-side switch is provided at both ends of the electric load to switch the direction of current flowing through the electric load such as a DC motor at high speed. In a current direction switching circuit such as an H-bridge circuit having a low-side switch, a MOSFET is generally used as a switching element. And like this, MOSF
When ET is used as a switching element for switching the current direction, the simplest drive circuit configuration is shown in FIG.
The drive circuit shown in (a) or (b) is used.

[0008]

However, in the case where the switching element for switching the current direction is formed of a MOSFET and is driven by using the conventional driving circuit shown in FIG. When the high-side switch is turned on, the low-side switch is turned on and a through current flows, or the switching element is connected to the n-channel M
When configured with OSFET, FET is turned on well.
There is a problem that the off control cannot be performed.

Hereinafter, this problem will be described in detail.
FIG. 7 shows a power supply line (potential: VB
2) and high-side switches TAH and TBH for connecting the two terminals for power supply of the DC motor 2 respectively,
N low-side switches TAL and TBL for connecting an SFET and connecting a ground line (potential: GND2) on the negative side of the DC power supply and two power supply terminals of the DC motor 2 respectively;
High side switch T with MOSFET of channel
AH and the low-side switch TBL, and the high-side switch TBH and the low-side switch TAL are each set as a set, and the FETs of each set are alternately turned on.
Switch the direction of the current flowing through
An H-bridge type current direction switching circuit (H-bridge circuit) 50 for inversion is shown.

In the H-bridge circuit 50, drive circuits 50AH, 5A for the switches TAH, TAL, TBH, TBL are provided.
The 0AL, 50BH, and 50BL are configured by a drive circuit including a PNP transistor T11 and resistors R11 to R13 except for the NPN transistor T12 from the drive circuit illustrated in FIG. NPN transistor T in the circuit
12 is provided on the control circuit 60 side separate from the drive circuit, and on the control circuit 60 side, NPN transistors T12AH, T12AL, T12BH, N12 transistors corresponding to the respective switches TAH, TAL, TBH, TBL.
By turning on / off T12BL, each switch TAH,
TAL, TBH, and TBL are configured to be turned on and off according to the rotation direction of the DC motor 2.

In FIG. 7, the p-channel MOSFETs constituting the high-side switches TAH, TBH have their sources connected to the power supply line side, and have the low-side switches TAH.
The drain is connected to the drain of the n-channel MOSFET constituting the L and TBL (in other words, the terminal of the DC motor 2). Each of these p-channel MOSFETs has an N
The transistor is turned off when the PN transistors T12AH and T12BH are turned on, and is turned on when the NPN transistors T12AH and T12BH are turned off.

However, the H-bridge circuit 50
When the driving circuit shown in FIG. 6A is used as the driving circuit of FIG. 6A, when the high-side switches TAH and TBH are turned on, the low-side switches TAL and TBH are turned on, and the through current flows through the FETs constituting each switch. May flow.

That is, the MOSFET has a parasitic capacitance between the gate and the drain and between the gate and the source due to its structure. For this reason, for example, as shown in FIG. 8, when the high-side switch TAH is turned on from the state where both the high-side switch TAH and the low-side switch TAL are off, the n-channel MOSFET of the low-side switch TAL Although a power supply voltage is applied between the drain and the source, the drive circuit shown in FIG. 6A includes a resistor R11 connecting between the gate and the source of the n-channel MOSFET constituting the low-side switch TAL. Therefore, when the high-side switch TAH is on, the voltage between the gate and the source of the n-channel MOSFET constituting the low-side switch TAL changes the power supply voltage VB, the gate-drain parasitic capacitance Cgd, the gate-source parasitic capacitance Cgs, and Combined impedance of gate-source resistance rgs (resistance value of resistor R11) And, the divided voltage Vg by.

The voltage Vg is equal to the resistance value rg of the resistor R11.
The larger the value of s, the larger the value. However, since the resistor R11 limits the current flowing through the PNP transistor T11 when the PNP transistor T11 is turned on, a resistor having a large resistance value of about 10 kΩ is usually used. As a result, immediately after the high-side switch TAH is turned on, the gate-source voltage of the n-channel MOSFET constituting the low-side switch TAL exceeds the threshold voltage of the FET, and the low-side switch TAL is turned on. Each F constituting low-side switch TAL and high-side switch TAH
Through current flows through the ET and each FET is destroyed.
Such a problem arises.

On the other hand, such a problem can be solved by using a drive circuit including two NPN transistors T21 and T22 shown in FIG. 6B as a drive circuit for the n-channel MOSFETs constituting the low-side switches TAL and TBL. . That is, in the drive circuit shown in FIG.
T: When turning off Tn, the NPN transistor T21 is turned on, and the N-channel current flows between the gate and source of the FET: Tn.
Since the collector-emitter saturation voltage of the PN transistor T21 is maintained at about 0.4, the high-side switches TAH and TBH are turned on, and the low-side switches TAL and TBL are turned on.
Even if a power supply voltage is applied between the drain and the source of the n-channel MOSFET constituting the above, the voltage between the gate and the source does not increase and the low-side switches TAL and TBL do not turn on.

However, like the H-bridge circuit 50 in FIG. 7, the low-side switches TAL and TBL are controlled on the control circuit 60 side.
The drive circuits 50AL and 50BL for the low-side switches TAL and TBL are connected to the circuit shown in FIG.
The NPN transistor T21 and the resistors R21 to R24 are obtained by removing the NPN transistor T22 from the driving circuit shown in FIG.
6 is provided on the control circuit 60 side.
In the case where the NPN transistor T22 in the drive circuit of (b) is provided, the ground potential GND2 on the H-bridge circuit 50 side is used.
If there is a potential difference between the control circuit 60 and the ground potential GND1, the low-side switches TAL and TBL cannot be turned on, or when the low-side switches TAL and TBL are turned on, the low-side switches TAL and TBL are turned on. There is a problem that the current flowing into the control circuit 60 increases.

That is, when the NPN transistor T22 in the drive circuit of FIG. 6B is incorporated in the control circuit side which is formed separately from the drive circuit, the ground potential GND1 on the emitter side of the NPN transistor T22 is driven. It may be higher than the ground potential GND2 on the circuit side. And N
When the PN transistor T22 is in the ON state, the connection point between the resistor R23 and the resistor R24 is grounded to a ground line (potential: GND1), and the base-emitter voltage of the NPN transistor T21 is changed in the order of the PN junction. Since the voltage is lower than the directional voltage to turn off the NPN transistor T21, the ground potential GND1 where the emitter of the NPN transistor T22 is grounded is equal to the source of the FET: Tn (in other words, the emitter of the NPN transistor T21). Is higher than the ground potential GND2, which is grounded, when the NPN transistor T22 is turned on, the voltage Vo (the voltage based on the ground potential GND2) at the connection point between the resistor R23 and the resistor R24, which is the input terminal on the drive circuit side. ) Cannot be sufficiently reduced, and the NPN transistor T21 cannot be turned off. In this case,
FET: Tn is kept off.

Therefore, when the low-side switches TAL and TBL are driven by using the drive circuit of FIG. 6B in the H-bridge circuit 50 of FIG. 7, the ground potential GND1 of the control circuit 60 becomes H level. When the potential becomes higher than the ground potential GND2 of the bridge circuit 50, the low-side switches TAL and TBL cannot be turned on, and the DC motor 2 cannot be energized.

Such a problem is caused, for example, in a system such as an automobile in which various electric loads including an H-bridge circuit and a control circuit are connected to a power supply line connected to a single DC power supply such as a battery. , A big problem. That is, in an automobile, as shown in FIG. 9, power is supplied from a battery to various electric loads via a plurality of power supply paths, but the control circuit and the H-bridge circuit receive power supply through different paths. In such a case, the voltage drop on the power supply path from the battery to the control circuit and the H-bridge circuit depends on the resistance component (r) of the path.
Since V is generated, the potentials of the power supply line and the ground line in each circuit may be significantly different, and the above problem is likely to occur.

However, in FIG. 7, when the ground potential GND1 of the control circuit 60 is different from the ground potential GND2 of the H-bridge circuit 50, the potential VB1 of the power supply line of the control circuit 60 and the potential of the power supply line of the H-bridge circuit 50 Although VB2 is also different, each drive circuit of the H-bridge circuit 50 has its input terminal connected to the NPN transistor of the control circuit 60 side grounded or opened to the ground line of the control circuit 60 side. Since the FET is turned on and off, if the input terminal of the drive circuit is switched between ground and open on the control circuit side as described above, no malfunction occurs due to a difference in power supply voltage.

On the other hand, in the drive circuit shown in FIG. 6B, the ground potential GND1 on the NPN transistor T22 side is used.
Is high, the NPN transistor T22 is turned on and N
In order to turn off the PN transistor T21, the base-emitter voltage of the NPN transistor T21 should be lower than the forward voltage VF of the PN junction. For this purpose, when the NPN transistor T21 is turned off, the connection point voltage V
The resistance ratio of the resistors R23 and R22 that divide o
May be set so as to be sufficiently larger than the resistor R22.

That is, the connection point voltage VoTH (hereinafter referred to as threshold voltage) required to turn on the NPN transistor T21 is represented by a resistor R23 having a resistance of r23, a resistor R22 having a resistance of r22, and a PN junction having a resistance of PN junction. Assuming that the forward voltage is VF, the following equation (1) is obtained. VoTH> {(r23 / r22) +1} .VF (1) As the threshold voltage VoTH is increased, the malfunction of the drive circuit due to the difference voltage between the ground potentials GND1 and GND2 can be prevented. Has a resistor R23
May be increased.

However, as described above, the resistance value r of the resistor R23 is
When 23 is increased and the threshold voltage VoTH is increased,
To enable the NPN transistor T21 to be turned on when the NPN transistor T22 is turned off (when the connection point between the resistor R24 and the resistor R23 is opened), the resistance value r of the resistor R24
24 must be set under the condition of the following equation (2) to reduce the voltage drop across the resistor R24 that occurs when the NPN transistor T22 is turned off.

R24 <[{(VB / VF) -1} .r22-r23] (2) That is, when the NPN transistor T21 is turned on, its base-emitter voltage becomes the forward voltage VF of the PN junction, and the power supply Current ｛= (VB-VF) / () flowing through resistors R24 and R23 to which a voltage (VB-VF) obtained by subtracting this forward voltage VF from voltage VB (= power supply line potential VB2-ground potential GND2) is applied. r23 + r24)｝ must be at least larger than the current (VF / r22) obtained by dividing the voltage VF by the resistance value r22 of the resistor R22. To satisfy this condition, the resistance value r24 of the resistor R24 must be set to the above value. It is necessary to make the resistance value r24 of the resistor R24 sufficiently small by setting according to the equation (2).

Then, as described above, the resistance value r of the resistor R24 is obtained.
If 24 is reduced, the current ic flowing to the NPN transistor T22 side via the resistor R24 when the NPN transistor T22 is turned on increases, and the allowable current that allows a large current to flow through the resistor R24 and the NPN transistor T22 is increased. You have to use a big one, FET: Tn
There is a problem that power consumption for driving is increased.

The present invention has been made in view of such a problem, and the input terminal is grounded / opened via a switching element provided on the external control circuit side. Even if a potential difference occurs between the ground potential on the control circuit side and the ground potential on the drive circuit side in the drive circuit for turning on / off the channel MOSFET, a large current does not flow through the switching element on the control circuit side and the n-channel In a current direction switching circuit using an n-channel MOSFET as a switching element for switching the current direction, both the high-side and low-side switching elements are turned on, and the through current is reduced. It is an object of the present invention to surely prevent the flow of the air.

[0027]

In order to achieve the above object, the invention according to claim 1 has a structure in which a drain is connected to a positive electrode side of a current supply path to an electric load, as shown in FIG. An n-channel MOSFET (Tn) having a source connected to the negative side of the path is turned on / off according to the state of an input terminal which is grounded or opened by a switching element (To) provided in an external control circuit. Channel MOSF
An NPN type first transistor (T1) having a collector connected to the gate of the FET (Tn), and an emitter grounded to the negative electrode side of the DC power supply;
A first resistor (R1) provided between the collector of the transistor and the positive electrode of the DC power supply, a second resistor (R2) provided between the base and the emitter of the first transistor,
Third resistor (R3) connected to the base of the transistor
An NPN-type second transistor having a collector connected to the positive electrode side of the DC power supply, an emitter connected to the base of the first transistor via the third resistor, and a base connected as an input terminal to the switching element of the control circuit. (T2),
A fourth resistor (R4) connected between the base and the collector of the second transistor.

That is, the driving circuit according to the present invention has the configuration shown in FIG.
Is provided with a second transistor of an NPN type at a connection point between the resistor R24 and the resistor R23,
A resistor R is connected between the base and collector of the second transistor.
24, and a resistor R23 is connected to the emitter of the second transistor.
This is achieved by connecting

The drive circuit of the present invention thus configured has the same structure as the drive circuit of FIG. 6B when the switching element (To) on the control circuit side is turned off and the input terminal is opened. , The first transistor (T1) turns on, and the FE
When T (Tn) is turned off and the switching element (To) on the control circuit side is turned on and the input terminal is grounded on the control circuit side, the first transistor (T1) is turned off,
The FET (Tn) is turned on.

In the present invention, the first transistor (T
Since the second transistor (T2) is provided in the path for supplying the base current to (1), the first transistor (T2)
To turn off 1) and turn on FET (Tn),
The second transistor (T2) may be turned off. And
In the case of the drive circuit of the present invention, the threshold voltage VoTH of the input terminal required to turn on the first transistor (T1) is obtained by setting the resistance of the second resistor (R2) to r2 and the resistance of the third resistor (R3). Assuming that the value is r3, the following equation (3) is obtained.

VoTH> {(r3 / r2) +2} .multidot.VF (3) For this reason, as is clear from comparison between the expression (3) and the expression (1), the second resistance (R2) And the third resistor (R3)
If a resistor having the same resistance value as the resistors R22 and R23 of the drive circuit shown in FIG. 6B is used, the threshold voltage VoTH can be increased as compared with the conventional drive circuit, and When the ground potential GND1 on the circuit side becomes higher than the ground potential GND2 on the drive circuit side, a voltage value at which the first transistor cannot be turned off can be increased.
That is, according to the drive circuit of the present invention, a malfunction caused by a difference in ground potential between the control circuit and the drive circuit can be more reliably prevented as compared with the drive circuit shown in FIG.

On the other hand, in the driving circuit shown in FIG.
When the resistance value of the resistor R23 (corresponding to the third resistor (R3) of the present invention) is increased in order to increase the threshold voltage VoTH,
The resistance of the resistor R24 (corresponding to the fourth resistor (R4) of the present invention) must be reduced, and the current flowing from the drive circuit to the control circuit when the switching element on the control circuit is turned on increases. However, in the present invention, the fourth
The resistor (R4) only needs to supply the base current of the second transistor (T2), and the resistance value r4 of the fourth resistor (R4) only needs to satisfy the condition of the following equation (4). 6 (b), the fourth resistor (R4)
Can be increased.

R4 <(1 + hFE). [{(VB / VF) -2} .r2-r3] (4) where hFE: hFE of the second transistor, VB: power supply voltage of the driving circuit. According to this, the current ic flowing from the drive circuit to the control circuit can be sufficiently reduced despite the fact that the threshold voltage VoTH can be set large, and the fourth resistor (R4) and the switching element (To ) Does not need to use a large allowable current.

Next, a current direction switching circuit according to a second aspect of the present invention includes a pair of switching elements composed of MOSFETs connected in series between positive and negative power supply lines of a DC power supply;
A drive circuit that is provided in each of the pair of switching elements, receives power from the power supply line, and conducts / cuts off each switching element; and a drive circuit that is configured separately from the drive circuit, By grounding or opening each end, one of the pair of switching elements is selectively turned on via the drive circuit, and the direction of current flowing to the electric load connected to the connection point of each switching element is changed from the connection point. A control circuit for switching between a first direction toward the electric load and a second direction opposite to the first direction.

A switching element arranged as a low-side switch between a connection point of each switching element and the negative electrode of the power supply line has a drain connected to the connection point and a source connected to the power supply. N-channel MOSFE connected to the negative side of the supply line
T, and the drive circuit of the n-channel MOSFET is further constituted by the drive circuit according to claim 1, comprising first and second transistors of NPN type and first to fourth resistors.

Therefore, according to the present invention, by setting the resistance values of the third resistor and the fourth resistor constituting the drive circuit of the low-side switch to be large, the ground potential of the control circuit is reduced to the ground potential of the drive circuit. Even if it becomes higher, the low-side switch can be reliably turned on and off, and the current flowing from the drive circuit to the control circuit when the low-side switch is on can be reduced.

When the low-side switch is off,
Since the first transistor is turned on and the gate-source of the low-side switch is held at the collector-emitter saturation voltage (about 0.4 V) of the first transistor, immediately after the high-side switch is turned on, the low-side switch is turned on. There is no possibility that the voltage between the source and the gate rises and the low-side switch is turned on, so that a through current can be prevented from flowing through the high-side switch and the low-side switch.

Here, when the driving circuit according to claim 1 is used as a driving circuit for a low-side switch composed of an n-channel MOSFET, like the current direction switching circuit according to claim 2, the H level shown in FIG. Like the bridge circuit, the high-side switch is composed of a p-channel MOSFET, and the conventional drive circuit shown in FIG. 6A can be used as it is as a drive circuit for driving the same.

However, in the drive circuit for the high-side switch shown in FIG. 7, when the switching element on the control circuit side is turned off and the input terminal of the drive circuit is opened, the high-side switch is turned on. Therefore, for example, when only the power supply on the drive circuit side is turned on due to disconnection of the second power supply line for supplying power to the control circuit or the like, the high-side switch is always turned on as an initial state. Would. When the high-side switch is turned on, a positive power supply voltage is applied to the connection point between the high-side switch and the low-side switch to which the electric load is connected. If the insulation between them is insufficient, a leakage may occur and an excessive current may flow to the high-side switch.

Therefore, when the current direction switching circuit according to the second aspect is configured, when the switching element on the control circuit side is in the off state (ie, the input terminal of the drive circuit is in the open state), the high side switch is turned on. It is preferable that both the low-side switch and the low-side switch are turned off. When a p-channel MOSFET is used for the high-side switch, it is preferable that the drive circuit is configured as described in claim 3.

That is, in the current direction switching circuit according to the third aspect, the switching element serving as the high-side switch is configured by a p-channel MOSFET in the current direction switching circuit according to the second aspect. The n-channel MOSFET constituting the switch is always turned off in the initial state where the input terminal of the drive circuit is open.

On the other hand, the driving circuit of the high-side switch
P-channel MOSFET for high-side switch
A PNP type third transistor having a collector connected to the gate of the power supply line and an emitter connected to the positive side of the power supply line, and a third transistor provided between the collector of the third transistor and the negative side of the power supply line; F when off
5th that lowers the gate potential of ET and turns on the FET
A resistor, a sixth resistor provided between the base and the emitter of the third transistor, a seventh resistor connected to the base of the third transistor, and a collector connected to the base of the third transistor via the seventh resistor. An emitter is connected to the negative side of the power supply line, a base is connected as an input terminal to the control circuit, and a fourth transistor of NPN type is connected between the base of the fourth transistor and the positive side of the power supply line. And an eighth resistor.

That is, the driving circuit for the high-side switch is the same as the driving circuit shown in FIG. 6A, except that the eighth circuit is connected between the base of the NPN transistor T12 (corresponding to the fourth transistor) and the positive electrode side of the power supply line. Connect a resistor and N
This is achieved by connecting the base of the PN transistor T12 to the control circuit.

In the drive circuit for the high-side switch thus configured, when the input terminal is grounded on the control circuit side, both the fourth and third transistors are turned off, and the high-side switch is turned off. When the input terminal is open, the fourth transistor and the third transistor are both turned on, and the high-side switch is turned off.

Therefore, according to the current direction switching circuit of the third aspect, when the input terminals of the driving circuits of the high side switch and the low side switch are open, both the high side switch and the low side switch are in the off state. The power supply voltage is applied to the connection point between the high-side switch and the low-side switch when each drive circuit and the control circuit are not connected, or when the control circuit is not operating even if they are connected. If the insulation between the connection point and the ground is insufficient, electric leakage occurs, and it is possible to reliably prevent an excessive current from flowing to the high-side switch, thereby improving safety.

Further, in the current direction switching circuit according to claim 2, the high side switch is an n-channel MOSF.
It can also be composed of ET. When the high-side switch is formed by an n-channel MOSFET, the drive circuit may be configured as described in claim 4.

That is, the high-side switch is connected to the n-channel M
In the case of using an OSFET, the drive circuit can be basically configured in the same manner as the drive circuit according to claim 1 which includes first and second NPN transistors and first to fourth resistors. However, in this case, in order to turn on the n-channel MOSFET constituting the high-side switch, it is necessary to set the gate potential of the n-channel MOSFET even higher than the positive side of the power supply line. Therefore, in the current direction switching circuit according to a fourth aspect, a booster circuit that generates a power supply voltage higher than the DC power supply is provided in the drive circuit according to the first aspect as a drive circuit for the high-side switch. The power supply voltage output line of the circuit and the gate of the high side switch are connected via the first resistor.

Then, like the current direction switching circuit according to the fourth aspect, both the high side switch and the low side switch are formed by n-channel MOSFETs, and the driving circuit is the driving circuit according to the first aspect. The high-side switch and the low-side switch can both be turned off when the input terminal of each drive circuit is in an open state, thereby enhancing safety as in the current direction switching circuit according to claim 3. it can. Also, n-channel MOSFET
In the same size, the on-resistance can be reduced as compared with the p-channel MOSFET, so that compared with the case where the p-channel MOSFET is used for the high side switch,
It is possible to further reduce the power loss that occurs when the electric load is energized.

The current direction switching circuit according to the second to fourth aspects can be used, for example, as a charge / discharge current switching circuit for charging / discharging a charge to a capacitive electric load. As described in claim 5, a pair of switching elements can be applied to an H-bridge circuit provided at both ends of an electric load.

[0050]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is an electric circuit diagram showing a configuration of a current direction switching circuit that switches the direction of current flowing in DC motor 2 in a motor vehicle to rotate DC motor 2 forward / reverse.

As shown in FIG. 2, the current direction switching circuit of this embodiment comprises an H-bridge circuit 10 and a control circuit 20 for the H-bridge circuit 10. The H-bridge circuit 10 shown in FIG. 50, the power supply 2 for the DC motor 2
The high-side switches TAH and TBH connect the terminal and a power line (potential: VB2) drawn from the positive electrode side of a DC power supply (battery) (not shown). The source is connected to the power supply line and the drain is the terminal of the DC motor 2. And a low-side switch TAL, TBL for connecting a ground line (potential: GND2) connected to the negative electrode side of the battery and two power supply terminals of the DC motor 2, respectively. Motor 2
And an n-channel MOSFET whose source is grounded to the ground line. Also, the high-side switches TAH, TBH and the low-side switches TAL, T
The drive circuits 10AH, 10BH, 10AL, 10
BL is provided, and the control circuit 20 grounds or opens the input terminal of each of the drive circuits 10AH to 10BL, thereby connecting the high-side switches TAH and TBH and the low-side switch TAL via the drive circuits 10AH to 10BL. , TBL are turned on and off, respectively.

That is, the control circuit 20 controls each drive circuit 10AH
The collectors correspond to each drive circuit 10AH-1
0BL, the NPN transistor ToA having an emitter grounded to the ground line on the control circuit 20 side.
H, ToAL, ToBH, and ToBL, and each of the driving circuits 10AH to 10AH through these NPN transistors ToAH to ToBL.
By grounding or opening the input terminal of 10BL, a set consisting of a high side switch TAH and a low side switch TBL, a high side switch TBH and a low side switch T
One of the pair consisting of AL is turned on, and a current is supplied to the DC motor 2. By switching the set which is turned on, the direction of the current flowing through the DC motor 2 is switched to rotate the DC motor 2 forward.・ Turn it over.

As shown in FIG. 9, the control circuit 20 has a power supply line (potential:
VB1) and a power supply from a battery via a ground line (potential: GND1). Next, the drive circuits 10AH and 10BH of the high-side switches TAH and TBH are configured in the same manner as the drive circuits 50AH and 50BH shown in FIG. That is, in each of the drive circuits 10AH and 10BH, a collector is connected to the gate of the p-channel MOSFET constituting the high-side switches TAH and TBH, and a PNP transistor T3 whose emitter is connected to the power supply line; a collector of the PNP transistor T3; A resistor R5 connected between the base and the emitter of the PNP transistor T3; and a resistor R7 connected to the base of the PNP transistor T3. The open end of the NPN transistor To in the control circuit 20 is used as an input end of the drive circuits 10AH and 10BH.
Connected to AH and ToBH collectors.

Therefore, in the driving circuits 10AH and 10BH for the high side switch, if the NPN transistors ToAH and ToBH on the control circuit 20 are in the off state, the PNP
The transistor T3 is turned off. As a result, the p-channel MOSFs constituting the high-side switches TAH and TBH
The potential of the gate of ET becomes the ground potential GND1, and the high-side switches TAH and TBH are turned on.
On the other hand, the NPN transistors ToAH, T
If oBH is on, a base current flows through PNP transistor T3, turning on PNP transistor T3. Then, since a current flows through the resistor R5, the p-channel MOS constituting the high-side switches TAH, TBH
The potential of the gate of the FET becomes the same high potential (VB2) as the power supply line, and the high-side switches TAH and TBH are turned off.

That is, in this embodiment, if the control circuit 20 turns off the NPN transistors ToAH and ToBH and opens the input terminals of the drive circuits 10AH and 10BH, the high-side switches TAH and TBH can be turned on. Conversely, if the control circuit 20 turns on the NPN transistors ToAH and ToBH and grounds the input terminals of the drive circuits 10AH and 10BH, the high-side switches TAH and TBH can be turned off.

On the other hand, the drive circuits 10AL and 10BL of the low-side switches TAL and TBL use the drive circuit of the present invention (claim 1) shown in FIG. That is, the driving circuit 10
AL and 10BL are composed of NPN transistors T1 and T2 as first and second transistors and resistors R1 to R4 as first to fourth resistors, similarly to the drive circuit shown in FIG. I have.

Therefore, in the driving circuits 10AL and 10BL for the low-side switch, if the NPN transistors ToAL and ToBL on the control circuit 20 are in the off state, the NPN
When a base current flows through the transistor T2, the NPN transistor T2 is turned on, and when the NPN transistor T2 is turned on, the base current is supplied to the NPN transistor T1 via the NPN transistor T2 and the resistor R3. It turns on, the gate potential of the n-channel MOSFET forming the low-side switches TAL, TBL becomes substantially the ground potential (GND2), and the low-side switches TAL, TBL are turned off. vice versa,
If the NPN transistors ToAL and ToBL on the control circuit 20 side are on, the NPN transistor T2 turns off,
Since the NPN transistor T1 is also turned off, the n-channel MOSF constituting the low side switches TAL and TBL
The gate potential of ET becomes the same potential (VB2) as the power supply line, and the low-side switches TAL and TBL are turned on.

That is, in this embodiment, if the NPN transistors ToAL and ToBL are turned off on the control circuit 20 side and the input terminals of the drive circuits 10AL and 10BL are opened, the low-side switches TAL and TBL can be turned off. Conversely, if the control circuit 20 turns on the NPN transistors ToAL and ToBL and grounds the input terminals of the drive circuits 10AL and 10BL, the high-side switches TAL and TBL can be turned on.

When the low-side switches TAL and TBL are turned off in the H-bridge circuit 10 of the present embodiment thus configured, the NPN transistor T1 in the drive circuits 10AL and 10BL is turned on, and the low-side switches TAL and TBL are turned on. The collector and source of the NPN transistor T1 are connected between the gate and source of the n-channel MOSFET constituting TAL and TBL.
When the low-side switches TAL and TBL are turned off, the high-side switch T
Even if AH and TBH are turned on, the low-side switch T
AL and TBL do not turn on, and it is possible to reliably prevent a through current from flowing when the high-side switches TAH and TBH are turned on.

When the high-side switches TAH and TBH are turned off, the PNP transistor T3 in the drive circuits 10AH and 10BH is turned on, and the gates and p-channel MOSFETs of the high-side switches TAH and TBH are turned on. Since the voltage between the sources is maintained at the collector-emitter saturation voltage of the PNP transistor T3, the low-side switch TAH is turned off when the high-side switches TAH and TBH are turned off.
Even if L and TBL are turned on, the high-side switch TA
H and TBH are not turned on, and a through current can be reliably prevented from flowing when the low-side switches TAL and TBL are turned on.

On the other hand, as in the present embodiment, the drive circuits 10AH to 10BL of the H-bridge circuit 10 and the control circuit 20 are formed separately, and each circuit has a different power supply line (power supply line and ground line). When power is supplied via the power supply, the ground potential GN of the drive circuits 10AH to 10BL is used.
A potential difference may occur between D2 and the ground potential GND1 on the control circuit 20 side. Then, as described above, the driving circuits 10AL, 10BL for the low-side switches TAL, TBL
When the conventional driving circuit shown in FIG. 6B is used, when the ground potential GND1 becomes higher than the ground potential GND2, the low-side switches TAL, TBL
Cannot be turned on, or in order to prevent this, the current flowing from the drive circuit to the control circuit must be increased.

However, the drive circuits 10AL and 10BL for the low-side switch of this embodiment are provided with an NPN transistor T2 as a second transistor in addition to the NPN transistor T1 as a first transistor.
By the N-transistor T2, a current obtained by multiplying the current flowing through the resistor R4 by hFE can be supplied to the resistor R3, so that the resistance values of both the resistor R3 and the resistor R4 can be increased. Drive circuits 10AL, 10BL when the NPN transistors ToAL and ToBL of
Erroneous operation due to the potential difference between the ground potentials GND1 and GND2 can be prevented while suppressing the amount of current flowing into the control circuit 20 from the circuit.

That is, the base voltage of the NPN transistor T2 required to turn on the NPN transistor T1 (that is, the threshold voltage Vo at the input terminals of the drive circuits 10AL and 10BL)
TH) is as described above in (3), where r2 is the resistance value of the resistor R2 and r3 is the resistance value of the resistor R3. Therefore,
As the resistance value r3 of the resistor R3 increases, the threshold voltage VoTH increases, and the ground potentials GND1 and G
Malfunction due to the potential difference of ND2 can be prevented. When the NPN transistors ToAL and ToBL on the control circuit 20 are turned on, the current flowing into the control circuit 20 is limited by the resistor R4. When the NPN transistors ToAL and ToBL are turned off, the resistor R4 is turned off. NPN transistor T
2. It is sufficient if T1 can be turned on, and for this purpose, the resistance value r4 of the resistor R4 may be set so as to satisfy the above-mentioned equation (4).

As a result, the driving circuit 10oAL,
According to 10oBL, the threshold voltage VoTH is determined by the above equation (1), and the resistance r24 of the resistor R24 for limiting the current flowing into the control circuit 20 is limited by the above equation (2). As compared with the driving circuit shown in FIG.
By increasing the resistance value of R4, the input terminal voltage for turning on the low-side switches TAL and TBL can be increased while suppressing the amount of current flowing into the control circuit 20, and a malfunction occurs when the ground potential GND1 increases. Can be satisfactorily prevented.

FIG. 3A shows the driving circuit 10AL of this embodiment in which the resistors R2 and R3 for determining the threshold voltage VoTH are fixed, and a constant voltage generator is connected to the input terminals thereof. In the case where the input terminal voltage is changed from 0 V to the power supply voltage VB2, the direction of the current flowing from the drive circuit 10AL to the constant voltage generation circuit side is set to the positive direction, and the calculation result of the current ic is shown. FIG. 3 (b) is the same as FIG. 6 (b)
In the conventional driving circuit shown in FIG.
Are fixed to the same resistance values as the resistors R2 and R3, and the calculation result of the current ic is shown in the same manner as in FIG. Also, from the calculation results, the resistors R2 and R3 for determining the threshold voltage VoTH in the drive circuit 10AL of the present embodiment and the threshold voltage VoTH in the conventional drive circuit shown in FIG. When the resistors R22 and R23 to be determined are set to the same resistance value, the drive circuit 10AL of the present embodiment can make the threshold voltage VoTH higher, and moreover, the drive circuit 10AL It can be understood that the current ic flowing through the device can be reduced.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.
Various aspects can be taken. For example, in the above embodiment, p-channel MOSFETs are used for the high-side switches TAH and TBH constituting the H-bridge circuit 10, and the drive circuits 10AH and 10BH for turning on and off the switches and the switching elements on the control circuit side are shown in FIG. The drive circuits 10AH and 10AH are configured in substantially the same manner as the drive circuit shown in FIG.
The high-side switches TAH and TBH are turned on when the input terminal of the BH is open. In this case, for example, the input terminals of the drive circuits 10AH and 10BH are connected to the control circuit 20. When the power supply is supplied only to the H-bridge circuit 10 without disconnecting the signal line to be connected or the power supply to the control circuit 20 is not performed, the high-side switches TAH and TBH of the H-bridge circuit 10 are turned on. , And the power supply voltage is constantly applied to both ends of the DC motor 2. In this state, when the harness from the H-bridge circuit 10 to the DC motor 2 comes into contact with the ground line, a large current flows through the high-side switches TAH and TBH, and the high-side switches TAH and TBH are destroyed. Therefore, in order to prevent such a failure from occurring in the H-bridge circuit 10 of the above embodiment, the drive circuits 10AH and 10BH for the high-side switches are used.
It is preferable that the high-side switches TAH and TBH can be turned off when their input terminals are in an open state. To this end, the drive circuits 10AH and 10BH may be configured as shown in FIG. .

That is, as shown in FIG.
Drive circuit 10A for high side switch constituting 0 '
H 'and 10BH' are connected to the gates of the p-channel MOSFETs constituting the high-side switches TAH and TBH, the collectors of which are connected to each other, and the emitters of which are connected to the power supply line, and the collector of the PNP transistor T3 and the ground line. A resistor R5 connected between
A resistor R6 connected between the base and the emitter of the P-transistor T3, a resistor R7 connected to the base of the PNP transistor T3, and a collector connected through the resistor R7 to the resistor R7.
It is connected to the base of the NP transistor T3, and has an emitter connected to a ground line (potential: GND) common to the control circuit 20.
NP connected to NPN transistors ToAH and ToBH on the control circuit 20 side as an input terminal.
It comprises an N-transistor T4 and a resistor R8 connected between the base of the NPN transistor T4 and the power supply line.

Therefore, the driving circuits 10AH 'and 10BH'
, The NPN transistor To on the control circuit 20 side
When AH and ToBH are on, the NPN transistor T4 turns off, the PNP transistor T3 turns off, and the high-side switches TAH and TBH turn on. Conversely, the NPN transistor T on the control circuit 20 side
If oAH and ToBH are off, the NPN transistor T4 turns on, the PNP transistor T3 turns on, and the high-side switches TAH and TBH turn off.

That is, the driving circuits 10AH 'and 10BH' for the high-side switch shown in FIG. 4 are different from the driving circuits 10AH and 10BH shown in FIG.
And a resistor R8 to invert the logic of the operation, so that when the input terminals of the drive circuits 10AH 'and 10BH' are open, the high-side switches TAH and TBH are turned off, and the power supply system is turned off. This prevents the high-side switches TAH and TBH from being easily broken in the event of an abnormality.

In FIG. 4, the configuration other than the drive circuits 10AH 'and 10BH' for the high-side switch is exactly the same as that of FIG. The drive circuits 10AH 'and 10B
H 'corresponds to the driving circuit according to claim 3, wherein the PNP transistor T3 is a third transistor, the NPN transistor T4 is a fourth transistor, the resistor R5 is a fifth resistor, the resistor R6 is a sixth resistor, and a resistor. R7 is the seventh resistor, resistor R
8 corresponds to the eighth resistor, respectively.

Next, in the above embodiment, p-channel MOSFETs are used for the high-side switches TAH and TBH constituting the H-bridge circuit 10. However, the high-side switches TAH and TBH are similar to the low-side switches 10AL and 10BL. , N-channel MOSFETs can be used. In this case, as shown in FIG. 5, the drive circuits 10AH "and 10B for the high side switches TAH 'and TBH'.
H ″ is connected to the drive circuits 10AL and 10B for the low side switch.
What is necessary is just to comprise similarly to L. However, in this case, in order to turn on the high-side switches TAH 'and TBH', the n-channel MO constituting the high-side switches TAH 'and TBH' must be turned on.
Since the gate potential of the SFET needs to be higher than the potential VB2 of the power supply line, a booster circuit 30 for boosting the power supply voltage is provided in the H-bridge circuit 10 ″.
0 to the power supply voltage output line,
It is necessary to connect a resistor R1 that pulls up the gate potential of T to the power supply side.

When the H-bridge circuit 10 "is constructed in this manner, similarly to the H-bridge circuit 10 'shown in FIG. 4, when the input terminal of each drive circuit is open, the high-side switch TAH', TBH 'and low side switch T
AL and TBL can all be turned off. Further, when both the high-side switch and the low-side switch are configured by n-channel MOSFETs as described above, the n-channel MOSFETs have the same size,
Since the on-resistance can be reduced as compared with the SFET, the power loss on the current path of the DC motor can be further reduced.

The driving circuit 1 for the high-side switch
When 0AH "and 10BH" are configured in this manner, the n-channel Ms constituting the high-side switches TAH 'and TBH'
Since a high voltage from the booster circuit 30 is applied to the gate of the OSFET, Zener diodes ZDAH and ZDBH and diodes DAH and ZDAH are connected between the gate and source of the n-channel MOSFET for gate protection as shown in FIG.
It is preferable to provide a protection circuit composed of DBH.

As the protection circuit, a resistor may be used in addition to the Zener diode. The protection circuit using such a Zener diode or a resistor is provided between the gate and the source of the n-channel MOSFET forming the low-side switches TAL and TBL, or the gate and source of the p-channel MOSFET forming the high-side switches TAH and TBH. It may be provided between the sources.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram illustrating an n-channel MOSFET drive circuit according to the present invention.

FIG. 2 is an electric circuit diagram showing a configuration of a current direction switching circuit (H-bridge circuit) according to the embodiment.

FIG. 3 is a graph showing a calculation result obtained by calculating a relationship between an input terminal voltage and an output current between the drive circuit of the embodiment and a conventional drive circuit.

FIG. 4 shows a p-channel MOSFE for a high-side switch.
FIG. 9 is an electric circuit diagram illustrating another configuration example of the H bridge circuit when T is used.

FIG. 5 shows an n-channel MOSFE for a high-side switch.
FIG. 4 is an electric circuit diagram illustrating a configuration example of an H-bridge circuit when T is used.

FIG. 6 is an electric circuit diagram showing a conventional n-channel MOSFET drive circuit.

FIG. 7 is an electric circuit diagram illustrating a configuration example of an H-bridge circuit using a conventional driving circuit.

FIG. 8 is an explanatory diagram for explaining a problem of a through current generated in the H-bridge circuit shown in FIG. 7;

FIG. 9 is an explanatory diagram illustrating an example of a system in which a difference in ground potential occurs between a control circuit and a drive circuit.

[Explanation of symbols]

10 H bridge circuit 10AH, 10BH, 10AL, 1
0BL ... Drive circuit 20 ... Control circuit TAH, TBH ... High side switch TAL, TBL ... Low side switch T1 ... NPN transistor (first transistor) T2 ... NPN transistor (second transistor) T3 ... PNP transistor (third transistor) T4 ... NPN transistor (fourth transistor) R1 ... resistor (first resistor) R2 ... resistor (second resistor) R3 ... resistor (third resistor) R4 ... resistor (fourth resistor) R5 ... resistor (fifth resistor) R6: resistor (sixth resistor) R7: resistor (seventh resistor) R8: resistor (eighth resistor)

Claims (5)

[Claims] An n-type power supply circuit includes a drain connected to a positive electrode side of a current supply path to an electric load, and a source connected to a negative electrode side of the path.
A drive circuit of an n-channel MOSFET for turning on / off a channel MOSFET according to a state of an input terminal grounded or opened by a switching element provided in an external control circuit, wherein a collector is connected to a gate of the FET. An NPN-type first transistor having an emitter grounded to the negative side of the DC power supply; and a collector provided between the collector of the first transistor and the positive side of the DC power supply, wherein the FET is turned off when the first transistor is turned off. A first resistor for increasing the gate potential of the first transistor to turn on the FET, a second resistor provided between the base and the emitter of the first transistor, a third resistor connected to the base of the first transistor, A collector is connected to the positive side of the DC power supply, and an emitter is connected to the base of the first transistor via the third resistor. A second transistor having a base connected to the switching element of the control circuit as the input terminal; and a fourth resistor connected between the base and the collector of the second transistor. Drive circuit for an n-channel MOSFET. 2. A pair of switching elements composed of MOSFETs connected in series between positive and negative power supply lines of a DC power supply, provided on each of the pair of switching elements, and receiving power supply from the power supply line. A drive circuit for conducting and blocking each switching element; and a drive circuit which is formed separately from the drive circuit, and grounds or opens each input terminal of each drive circuit, thereby switching the pair of switches via the drive circuit. One of the elements is selectively turned on, and the direction of the current flowing through the electric load connected to the connection point of each of the switching elements can be any one of a first direction from the connection point to the electric load and a reverse second direction. A current direction switching circuit comprising: a switching circuit between the connection point and the negative electrode side of the power supply line, of the pair of switching elements; A switching element which is arranged as a low-side switch, the drain is connected to the connection point, a source constituted by the connected n-channel MOSFET to the negative of the power supply lines, further,
A drive circuit for the n-channel MOSFET, a first transistor of an NPN type having a collector connected to the gate of the FET and an emitter grounded to the negative side of the power supply line; a collector of the first transistor and the power supply A first resistor that is provided between the positive electrode side of the line and increases the gate potential of the FET when the first transistor is turned off to turn on the FET; and a first resistor that is provided between the base and the emitter of the first transistor. A second resistor; a third resistor connected to the base of the first transistor; a collector connected to the positive electrode side of the power supply line; and an emitter connected to the base of the first transistor via the third resistor. A second transistor of a NPN type having a base connected to the control circuit as the input terminal; n-channel MOSF to fourth resistor connected between the scan collector, to become configured from wherein
ET drive circuit. 3. A switching element, which is arranged as a high-side switch between the connection point and the positive side of the power supply line, of the pair of switching elements, has a source connected to the positive side of the power supply line. The drain is constituted by a p-channel MOSFET connected to the connection point. Further, a drive circuit of the p-channel MOSFET is connected to a collector of the gate of the FET, and an emitter is connected to a positive electrode of the power supply line. Connected PNP type third
A fifth resistor that is provided between the collector of the third transistor and the negative electrode side of the power supply line and that lowers the gate potential of the FET to turn on the FET when the third transistor is off; A sixth resistor provided between the base and the emitter of the third transistor, a seventh resistor connected to the base of the third transistor, and a collector connected to the base of the third transistor via the seventh resistor An NPN-type fourth transistor having an emitter connected to the negative side of the power supply line and a base connected to the control circuit as the input terminal; a base of the fourth transistor and a positive side of the power supply line 3. The current direction switching circuit according to claim 2, wherein the current direction switching circuit comprises an eighth resistor connected between the first and second resistors. 4. A switching element disposed as a high-side switch between the connection point and the positive side of the power supply line among the pair of switching elements, and a drain is connected to the positive side of the power supply line. The source is constituted by an n-channel MOSFET connected to the connection point, and further, a drive circuit of the n-channel MOSFET is connected to a booster circuit that generates a power supply voltage higher than the DC power supply, and a gate of the FET. A first transistor of an NPN type having a collector connected thereto and an emitter grounded to the negative side of the power supply line; and a first transistor provided between a collector of the first transistor and a power supply voltage output line of the booster circuit. When one transistor is turned off, the gate potential of the FET is raised to a potential higher than the positive side of the power supply line, and the FE A first resistor for turning on T; a second resistor provided between the base and the emitter of the first transistor; a third resistor connected to the base of the first transistor; and a collector connected to a positive electrode of the power supply line. A second transistor of an NPN type having an emitter connected to the base of the first transistor via the third resistor, and a base connected to the control circuit as the input terminal; 3. The current direction switching circuit according to claim 2, comprising: a fourth resistor connected between the base and the collector. 5. The current direction switching circuit according to claim 2, wherein the current direction switching circuit is an H-bridge circuit provided with the pair of switching elements at both ends of an electric load. .