JP2520768B2 - MOS transistor drive circuit - Google Patents

Description

The present invention relates to an economical drive circuit for a MOS transistor.

MOS (Metal-Oxid Semiconductor) transistor (MO
Since S FET) is capable of high-speed operation, it has been adopted as a switching element of a switching regulator, which has been advanced in switching frequency.

The MOS transistor as such a switching element switches the voltage applied to the primary winding of the transformer, and controls the ON period of the MOS transistor by the control circuit so that the output voltage of the secondary winding of the transformer is stabilized. To control. This control circuit is generally a semiconductor integrated circuit (IC), and generally has a configuration in which a relatively high voltage needs to be applied at startup.

Further, in order to reduce the switching loss of the MOS transistor, it is required to make the rise and fall of the gate voltage steep.

[Conventional technology]

In a conventional example applied to a switching element of a MOS transistor switching regulator, for example, as shown in FIG. 3, a DC voltage of a DC power supply 21 is applied to a primary winding of a transformer 23 via a MOS transistor 22. , The voltage induced in the secondary winding is rectified and smoothed by the rectifying and smoothing circuit 27, the rectified and smoothed DC output voltage is applied, and the DC output voltage is applied to the control circuit 24 and compared with the reference voltage, and the error The ON period of the MOS transistor 22 is controlled according to the voltage, and the capacitor 28 and the resistor 29 are for setting the cycle of the sawtooth wave generator (not shown) in the control circuit 24. Is. Further, the operating voltage V _{CC of} each part is applied to the control circuit 24 via the three-terminal regulator 25, and the power supply voltage V _CC of the transistors 30 and 31 is applied via the three-terminal regulator 26.
₁ is applied.

The control circuit 24 has, for example, the configuration shown in FIG. 4, where 30 and 31 are the aforementioned transistors, 32 is a gate circuit, and 3 is a gate circuit.
3 is a comparison circuit, 34 is a sawtooth wave generator, 35 is a comparison circuit, 36
Is a reference power supply, 41 is a power supply terminal for applying a power supply voltage V _CC , 42
Is an input terminal for inputting a rectified output voltage or a voltage obtained by dividing the rectified output voltage, 43 is a power supply terminal for applying a voltage V ₁ , and
44 is an output terminal connected to the gate of the MOS transistor 22 via a resistor, 45 is a terminal connected to the source of the MOS transistor 22, 46 is a ground terminal, 47 is a terminal to which the capacitor 28 is connected, and 48 is a resistor 29. It is a terminal.

The comparison circuit 33 compares the voltage of the reference power supply 36 with the rectified output voltage, adds the error voltage to the comparison circuit 35, compares the error voltage with the sawtooth wave signal from the sawtooth wave generator 34, and outputs the rectified output voltage. When the voltage is higher than the reference voltage, a signal having a pulse width that shortens the ON period of the MOS transistor 22 is applied to the gate circuit 32, and when the rectified output voltage for reflection is lower than the reference voltage, the MOS transistor 22 is turned on. A signal having a pulse width that lengthens the period is applied to the gate circuit 32,
The output signals of the transistors 30 and 31 operate in a complementary manner to apply a gate voltage to the MOS transistor 22.

The control circuit 24 includes an overvoltage protection circuit, an overcurrent protection circuit, etc., which are not shown, and is made into a semiconductor integrated circuit (IC), and a start voltage is 16V and an operation stop voltage is 10V, for example. It has been selected. Therefore, it is necessary to apply a starting voltage of 16 V or higher at the start of the operation.

For example, when the voltage of the DC power supply 21 is 24V, the three-terminal regulators 25 and 26 are omitted, and the voltage of the DC power supply 21 is set to the power supply voltage V _CC of the control circuit 24 and the voltage applied to the terminal 43, or Only the terminal regulator 25 is provided, and the control circuit
When the power supply voltage V _{CC of} 24 and the voltage applied to the terminal 43 are set to 16 V or more, the gate-source voltage of the MOS transistor 22 changes as shown as V _{GS1 in} FIG. That is, the gate-source capacitance increases the fall time of the gate voltage. In that case, the drain-source voltage and the drain current change as indicated by V _DS1 and I _D1 , respectively, and a switching loss occurs due to the shaded overlapping portion.

Therefore, when the voltage applied to the terminal 43 of the control circuit 24 is reduced to, for example, 10 V by the three-terminal regulator 26, the gate voltage has a relatively short fall time as shown by V _GS2 ,
Along with this, the drain-source voltage and the drain current change as indicated by V _DS2 and I _D2 , respectively. That is, the fall time of the gate voltage is shortened by lowering the voltage V ₁ for generating the gate voltage, and the overlapped portion of the drain-source voltage V _DS2 and the drain current _{ID 2} is reduced. . Therefore, switching loss can be reduced.

Thus, in the conventional MOS transistor drive circuit, the voltage V ₁ applied to the transistors 30 and 31 of the control circuit 24 for outputting the gate voltage is set to a value slightly exceeding the threshold value of the MOS transistor. A three-terminal regulator 26 is provided to reduce the number.

[Problems to be Solved by the Invention]

As described above, when the voltage of the DC power supply 21 is 24 V, the starting voltage of the control circuit 24 is, for example, 16 V or more, and the gate voltage of the MOS transistor 22 for reducing the switching loss is, for example, about 10 V. Each voltage V _CC , V ₁
Two three-terminal regulators with different characteristics in order to obtain
5,26 will be provided. Therefore, there is a drawback that the cost is increased.

Further, the three-terminal regulators 25, 26 only control the output voltage to a predetermined value, and even after applying a voltage equal to or higher than the starting voltage to the control circuit 24 via the three-terminal regulator 25, the voltage is continuously applied. Therefore, even after the start-up, a voltage equal to or higher than the operation stop voltage is required, but a voltage equal to or higher than the start-up voltage is continuously applied, resulting in a large power loss.

The present invention aims to reduce loss with a relatively simple configuration.

[Means for solving the problem]

A drive circuit for a MOS transistor of the present invention applies a start-up voltage required at start-up to a control circuit and drives a gate voltage of a MOS transistor as a desired value.
It will be described with reference to the drawings.

In the drive circuit including the control circuit 4 for applying a voltage from the DC power supply 1 to the primary winding of the transformer 3 via the MOS transistor 2 and stabilizing the output voltage of the secondary winding of the transistor 3, A three-terminal regulator 5 for applying a voltage from 1 to the control circuit 4, an impedance element 6 such as a resistor and a Zener diode connected to the ground terminal of the three-terminal regulator 5, and a transistor connected in parallel to the impedance element 6. 7 and power switch 9
And a time constant circuit 8 including a resistor R1 and a capacitor C1 for turning on the transistor 7 when a voltage is applied from the DC power supply 1 and turning on the transistor 7 after a predetermined time has passed.

[Action]

In the time constant circuit 8, when the power switch 9 is turned on, the capacitor C1 is charged through the resistor R1 and its terminal voltage rises according to the time constant of R1 · C1 and becomes a certain value or more. Turns on. Until then, the impedance element 6 is connected to the ground terminal of the three-terminal regulator 5.
Is connected, the bias voltage by the impedance element 6 is applied to the ground terminal, the output voltage of the three-terminal regulator 5 becomes a value slightly lower than the voltage of the DC power supply 1, and the control circuit 4 has a starting voltage higher than the starting voltage. Can be applied.

When the transistor 7 is turned on and the impedance element 6 is short-circuited, the three-terminal regulator 5 enters a normal operating state, and the voltage necessary for operation can be applied to the control circuit 4.

That is, the time constant circuit 8 and one three-terminal regulator 5 can apply a starting voltage to the control circuit 4 and a voltage that can obtain a desired gate voltage during operation.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an essential part of an embodiment of the present invention, in which a DC power supply 1 has a structure for outputting a DC voltage obtained by rectifying and smoothing a battery or a commercial AC voltage, and turning on a power switch 9. Thus, a DC voltage can be applied to the primary winding of the transformer 3 via the MOS transistor 2. A rectifying / smoothing circuit 12 including diodes D3 and D4, a capacitor C4 and an inductance L is connected to the secondary winding of the transformer 3.

The control circuit 4 has, for example, the configuration shown in FIG. 4, and in this embodiment, the transistors 10 and 1 are used.
The voltage applied to 1 and the operating voltage of each part are supplied via the three-terminal regulator 5. Note that a rectifying and smoothing circuit
The circuit configuration of applying the DC output voltage to the control circuit 4 in order to stabilize the DC output voltage of 12 is omitted in the figure for simplification.

Further, an impedance element 6 composed of a Zener diode D1 is connected to the ground terminal of the three-terminal regulator 5, and a transistor 7 is connected in parallel with the impedance element 6.
A time constant circuit 8 including a resistor R1, a capacitor C1 and a zener diode D2 is connected to the base of the transistor 7. Note that C2 and C3 are capacitors, R2 is a resistor, and 9 is a power switch.

When the power switch 9 is turned on, the DC voltage from the DC power supply 1 charges the capacitor C1 via the resistor R1 of the time constant circuit 8 and also applies the voltage to the control circuit 4 via the three-terminal regulator 5. . At that time, since the terminal voltage of the capacitor C1 is equal to or lower than the Zener voltage of the Zener diode D2, the base current of the transistor 7 is not supplied, the transistor 7 is turned off, and the impedance element 6 is connected to the ground terminal of the three-terminal regulator 5. Is connected. When the impedance element 6 is a Zener diode D1, the Zener voltage is applied to the ground terminal of the three-terminal regulator 5 as a bias voltage.

For example, when the DC voltage of the DC power supply 1 is 24V, the starting voltage of the control circuit 4 is 16V, the operation stop voltage is 10V, the bias voltage by the impedance element 6 is 6V, and the ground terminal of the three-terminal regulator 5 is directly grounded. When the output voltage is 12V, the output voltage of the three-terminal regulator 5 becomes 18V when the transistor 7 is in the OFF state, and a voltage higher than the starting voltage is applied to the control circuit 4 and the control circuit 4 is started. . That is, a sawtooth wave generator (not shown) in the control circuit 4 starts its operation, the pulse width of the transistors 10 and 11 is controlled, and a voltage is applied to the gate of the MOS transistor 2 via the resistor R2. The on / off control of the MOS transistor 2 is started.

The terminal voltage of the capacitor C1 of the time constant circuit 8 rises according to the time constant of R1 and C1, and when the terminal voltage becomes equal to or higher than the Zener voltage of the Zener diode D2, the base current of the transistor 7 is supplied and the transistor 7 is turned on. ,
The impedance element 6 will be short-circuited. As a result, the output voltage of the three-terminal regulator 5 becomes 12 V in the normal operating state. Therefore, a voltage lower than the starting voltage and higher than the operation stop voltage is applied to the control circuit 4, and the power loss in the control circuit 4 can be reduced. In addition, the gate voltage of the MOS transistor 2 output by the transistors 10 and 11 can be reduced to a desired value.
As described with reference to the figure, the switching loss of the MOS transistor 2 can be reduced.

FIG. 2 is a diagram for explaining the operation of the embodiment of the present invention.
Indicates the input voltage of the three-terminal regulator 5, and (b) indicates the output voltage of the three-terminal regulator (voltage applied to the control circuit 4). When the power switch 9 is turned on at time t1, the input voltage of the three-terminal regulator 5 becomes Then, the output voltage rises and becomes Va with a slight time delay as shown in FIG.

This voltage Va is selected to be, for example, 16 V or more of the starting voltage of the control circuit 4. Further, the time T is set by the time constant circuit 8, and this time T can be set to, for example, about 10 ms. At time t2 when this time T has elapsed, the transistor 7 is turned on, so that the output voltage of the three-terminal regulator 5 drops to Vb. The output voltage Vb is selected to be higher than the operation stop voltage of the control circuit 4, for example, 10V.

Therefore, the control circuit 4 is applied with a voltage equal to or higher than the starting voltage only to start the operation immediately after the power switch 9 is turned on, and when the operation after the time T set by the time constant circuit 8 is started, Since a voltage equal to or lower than the starting voltage and equal to or higher than the operation stop voltage is applied, the gate voltage of the MOS transistor 2 can be set to a desired value, the rising and falling of the MOS transistor 2 can be made sharp, and the switching loss of the MOS transistor 2 can be reduced. .

The present invention is not limited to the above-described embodiments, and, for example, the impedance element 6 can be a resistor, and the time constant circuit 8 can have another circuit configuration.

〔The invention's effect〕

As described above, the present invention is based on the MOS transistor 2
A voltage is applied from a DC power supply 1 to a control circuit 4 for applying a gate voltage to a control circuit 4 through a three-terminal regulator 5, and an impedance element 6 such as a Zener diode or a resistor is connected to the ground terminal of the three-terminal regulator 5. A transistor 7 is connected in parallel to the impedance element 6, and the transistor 7 is turned off at the time of start-up so that the output voltage of the three-terminal regulator 5 is equal to or higher than the start-up voltage of the control circuit 4, and the transistor 7 is turned on when the set time of the time constant circuit 8 has elapsed. ,
The output voltage of the three-terminal regulator 5 is used as the operating voltage during operation of the control circuit 4, and a voltage equal to or higher than the starting voltage is applied to the control circuit 4 only at the time of startup, and the desired operating voltage can be reduced during operation. While reducing the power loss during
Set the gate voltage of MOS transistor 2 to the desired value
There is an advantage that the switching loss of the transistor 2 can be reduced.

Further, since only one three-terminal regulator 5, impedance element 6, transistor 7 and time constant circuit 8 are provided, a simple structure is sufficient, and there is an advantage that the drive circuit can be made economical. .

[Brief description of drawings]

1 is a block diagram of an essential part of an embodiment of the present invention, FIG. 2 is an operation explanatory diagram of an embodiment of the present invention, FIG. 3 is a block diagram of an essential part of a conventional example, and FIG. 4 is an essential part of a control circuit. A block diagram and FIG. 5 are explanatory diagrams of a switching operation of a conventional example. 1 is a DC power supply, 2 is a MOS transistor, 3 is a transformer,
Reference numeral 4 is a control circuit, 5 is a three-terminal regulator, 6 is an impedance element, 7 is a transistor, 8 is a time constant circuit, 9 is a power switch, 10 and 11 are transistors, and 12 is a rectifying and smoothing circuit.

Claims (1)

(57) [Claims] 1. A voltage is applied from a DC power supply (1) to a primary winding of a transformer (3) via a MOS transistor (2) to stabilize an output voltage of a secondary winding of the transformer (3). In a drive circuit including a control circuit (4), a three-terminal regulator (5) for applying a voltage from the DC power supply (1) to the control circuit (4), and grounding of the three-terminal regulator (5) An impedance element (6) connected to the terminal, a transistor (7) connected in parallel to the impedance element (6), and the transistor (7) is turned off when a voltage from the DC power supply (1) is applied, A drive circuit for a MOS transistor, comprising: a time constant circuit (8) which is turned on after a lapse of a predetermined time.