JP3032935B2 - Drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit having an improved on / off control characteristic of a switching element.
A switching element in a switching regulator or the like needs to control switching on and off at a high speed to reduce switching loss, and the characteristics of the switching element are being improved.

[0002]

2. Description of the Related Art FIG. 5 is an explanatory view of a conventional example.
A switching element such as an OSFET (field effect transistor); 52, a drive circuit; 53, a power supply voltage terminal;
Are ground terminals, 55 and 56 are complementary transistors, 5
7, 59 are resistors, 58 is an input transistor, 60 is a pulse width control unit, 61 is a transformer, 62 and 63 are diodes, 64 is a choke coil, and 65 is a smoothing capacitor. G indicates a gate, S indicates a source, and D indicates a drain.

The pulse width control unit 60 includes a diode 62,
The switching element 5 is controlled so that the rectified output voltage of the switching element 63, the choke coil 64, and the capacitor 65 becomes a set value.
Control pulse for controlling the ON period of
Add to The pnp-type and npn-type complementary transistors 55 and 56 of the drive circuit 52 are connected between the power supply voltage terminal 53 and the ground terminal 54, and the commonly connected emitter is connected to the gate G of the switching element 51 via the resistor 57. It is connected to the. Therefore, a pulse voltage having an amplitude between the power supply voltage and the ground potential is applied to the gate G.

The switching element 51 is connected in series with a primary winding of a transformer 61 between a power supply voltage terminal 53 and an earth terminal 54. When the switching element 51 is turned on and off, the switching element 51 is connected to the primary winding of the transformer 61. The flowing current is interrupted, and the voltage induced in the secondary winding of the transformer 61 is rectified by diodes 62 and 63, smoothed by a choke coil 64 and a capacitor 65, and supplied to a load (not shown). The ON period of the switching element 51 is controlled by the pulse width control unit 60 so that the rectified output voltage becomes a predetermined value.

FIG. 6 is a diagram for explaining the operation of the conventional example.
Indicates a gate voltage V _GS applied between the gate and the source, (b) indicates a voltage V _DS between the drain and the source, and a drain current _ID , Ton indicates an ON period of the switching element, and Toff indicates an OFF state of the switching element. Indicates the period.

Normally, a control pulse of a nearly ideal rectangular wave is applied to the drive circuit 52 from the pulse width control unit 60. However, the complementary transistors 55 and 56 do not perform an ideal switching operation. Therefore, as shown in FIG. _6A , the rise and fall of the gate voltage VGS change gradually.

If the threshold value of the switching element 51 is V _th , the switching element 51 is turned on when the gate voltage V _GS exceeds the threshold value V _th , and conversely, the gate voltage V _GS
Becomes lower than the threshold value _Vth , the switching element 51 is turned off. Accordingly, a drain current I _D and the drain-source voltage V _DS varies as shown in (b) of FIG. 6, a portion shaded is switching loss.

[0008]

As described above, since the rise and fall of the gate voltage V _GS of the switching element 51 are not ideal, there is a problem that a switching loss occurs, and the rise of the gate voltage V _GS occurs. In order to make the falling characteristics sharp, the complementary transistors 55, 56 and the like of the drive circuit 52 need to have high-speed operation characteristics. An object of the present invention is to improve the switching characteristics of a switching element with a simple configuration.

[0009]

A drive circuit according to the present invention will be described with reference to FIG. 1. In a drive circuit 2 for controlling ON / OFF of a switching element 1, a power supply voltage terminal 3 and a ground terminal 4 are provided. Are connected between the output terminals of the complementary transistors 5 and 6 and the gate of the switching element 1, a capacitor 7 having a larger capacitance than the input capacitance of the switching element 1. Connect.

Furthermore, at between the power supply voltage terminal 3 and the ground terminal 4, the power supply voltage is connected to the power supply voltage determining unit 8 determines whether that falls below a predetermined value, the supply voltage determining unit 8
There when the drive voltage of the switching element 1 degree to the power supply voltage drops below a predetermined value is determined to have decreased, the switch circuit 9 for short-circuiting or directly via the resistor capacitor 7 provided by the control of the power supply voltage determining unit 8 You.

[0011]

A pulse voltage having an amplitude between the power supply voltage and the ground potential is output from the output terminals of the complementary transistors 5 and 6, and the rise from the ground potential and the fall to the ground potential have gentle waveforms. When this output voltage is applied to the gate of the switching element 1 via the capacitor 7, the voltage divided by the input capacitance of the switching element 1 and the capacitance of the capacitor 7 becomes the gate voltage of the switching element 1. That is, the output voltages of the complementary transistors 5 and 6 are level-shifted to be positive and negative gate voltages, and the threshold value of the switching element 1 can be set at the steep rising and falling portions of the gate voltage. Therefore, the switching element 1 can be turned on and off at a high speed.

When the power supply voltage decreases, the voltage applied to the gate of the switching element 1 also decreases, and the switching element 1 cannot be driven sufficiently. In such a state, the switching element 1 cannot be completely turned on, the switching loss increases, and the switching element 1 may be thermally destroyed. However, when the power supply voltage determination unit 8 detects such a decrease in the power supply voltage and short-circuits or directly short-circuits the capacitor 7 via a resistor, the voltage division ratio of the capacitor 7 decreases, and the gate voltage increases. Switching element 1
Can be driven.

[0013]

FIG. 2 is an explanatory view of an embodiment of the present invention.
Is a switching element such as a MOSFET, 12 is a drive unit, 13 is a power supply voltage terminal, 14 is a ground terminal, and 15 and 1
6 is a pnp type and npn type complementary transistor;
Is a capacitor, 18 is an input transistor, 20 is a pulse width control unit, 21 is a transformer, 22, 23 are diodes,
24 is a choke coil, 25 is a capacitor, 26-30
Is a resistor, 31 is a Zener diode, 32 is a comparator, 3
3 is a resistor, and 34 is a photocoupler.

In the case of the MOSFET switching element 11, the input capacitance C _iss corresponds to the sum of the capacitance C _GS between the gate G and the source S and the capacitance C _GD between the gate G and the drain D. The input capacitance C _iss of a general MOSFET is about several thousand pF. A capacitor 17 having a capacitance several times or several tens times or more as large as the input capacitance C _iss is connected between the gate G of the switching element 11 and the output terminals of the complementary transistors 15 and 16 of the drive unit 12.

The comparator 32, the Zener diode 31, and the resistors 27, 28 and 30 constitute the power supply voltage judging section 8 in FIG. 1, and the photocoupler 34 and the resistor 33 make up the switch circuit in FIG. 9. The photocoupler 34 includes a light emitting diode and a photothyristor, and the resistor 29 is a current limiting resistor for the light emitting diode.

A predetermined reference voltage is set by the resistor 27 and the Zener diode 31, and this reference voltage is
The comparator 32 compares the power supply voltage divided by 8 and 30 with the power supply voltage. When the power supply voltage is higher than the predetermined value and the normal state, the output terminal of the comparator 32 becomes high level,
Since the light emitting diode of No. 4 does not emit light, the drive unit 12
Only the capacitor 17 is connected between the switching element 11 and the gate G of the switching element 11.

The drain D of the switching element 11 is connected to the power supply voltage terminal 13 via the primary winding of the transformer 21, the source S is connected to the ground terminal 14, and the drive unit 1
The switching element 11 is turned on from the output terminal of the switching element 2 to the gate G of the switching element 11 via the capacitor 17.
A pulse voltage to be turned off is applied. By the on / off control of the switching element 11, the current flowing through the primary winding of the transformer 21 is intermittent, and a voltage is induced on the secondary winding of the transformer 21. This induced voltage is applied to diodes 22 and 23
, And smoothed by the choke coil 24 and the capacitor 25, and a DC voltage is applied to a load (not shown).

The pulse width control unit 20 compares the DC voltage applied to the load with the set voltage, and applies a control signal having a pulse width corresponding to the error voltage to the drive unit 12. When the input transistor 18 of the drive unit 12 is turned on by the control signal, the transistor 16 of the complementary transistors 15 and 16 is turned on, and when the input transistor 18 is turned off, the transistor 15 is turned on. Therefore,
When the power voltage is V _P, ignoring the collector-emitter voltage of the complementary transistors 15 and 16, the drive unit 1
The output signal of No. 2 is a pulse signal having an amplitude between _VP and 0.

When the output signal of the drive section 12 is applied to the gate G of the switching element 11 via the capacitor 17 when the power supply voltage V _P is normal, the capacitance C of the capacitor 17 and the input capacitance C _{iss of the} switching element 11 are obtained. And the voltage divided by the above is the voltage V actually applied to the gate G.
_It becomes _GS and becomes a voltage of positive and negative amplitude by charging and discharging of the capacitor.

FIG. 3 is a diagram for explaining the operation of the embodiment of the present invention. FIG. 3A shows the gate voltage V _{GS of} the switching element 11,
(B) shows a drain-source voltage V _DS and a drain current _{ID :} , and (c) shows an equalizing circuit. The capacitance C of the capacitor 17 and the input capacitance C _iss of the switching element 11 are connected in series to the output terminal of the drive unit 12 as shown in FIG.

Therefore, the output signal V of the drive unit 12
_DP is _divided by the capacitors C and C _iss and the gate voltage V _GS
Becomes In this case, the relationship of C> C _iss is selected. In particular, by setting C≫C _iss , the amplitude of the gate voltage V _GS becomes a value almost close to the power supply voltage V _P. The gate voltage V _GS is formed by charging and discharging the capacitor, and becomes a positive / negative pulse voltage centered at 0V. For example, assuming that C _iss = 1000 pF and C = 0.1 μF, the relation of C≫C _iss is established, and the gate voltage V _GS has substantially the same amplitude around 0 V as the amplitude of the output signal V _DP of the drive unit 12.

That is, as shown in FIG. 3A, a positive / negative pulse voltage centered at 0 V is obtained, and relatively gentle rising and falling portions shift to a negative polarity region.
In the vicinity of the threshold value _Vth of the switching element 11, steep rising and falling characteristics are obtained. Therefore, the drain-source voltage V _DS and the drain current I _D change as shown in FIG. 3B, the area indicated by the diagonal lines also becomes smaller, and the switching loss can be significantly reduced.

FIG. 4 is a diagram for describing the operation of the power supply voltage at a decline in the embodiment of the present invention, when the power supply voltage V _P is lowered, for example, as shown in (a) of FIG. 4, the power supply until the time t ₁ voltage V _P is V _1, then sequentially decreased, reduced to V ₂ at the time t _2, the input voltage of the operational limits of the drive circuit is assumed to be V _2, the gate voltage V _GS is, (b ). That is, at time t ₂ , the drive unit 1
The output signal _VDP from 2 becomes 0.

The gate voltage V _GS is, for example, at time t ₁₂
When drops below the threshold value V _th at the switching element 11 can not complete transition to the ON state, the drain-source voltage V _DS at the on period because not to 0V, and the switching loss is increased. If such a state continues, the switching element 11 will be thermally destroyed.

[0025] Therefore, in the present invention, monitors the power supply voltage V _P at the power supply voltage determining unit, before the gate voltage V _GS falls below the threshold V _th, controls the switch circuit, a capacitor 17 Is short-circuited directly or via the resistor 33. That is, the comparator 32 compares the reference voltage with the power supply voltage divided by the resistors 29 and 30. When the power supply voltage falls below the reference voltage, the output terminal is set to low level, and the photocoupler 3 is turned on.
A current is passed through the light emitting diode No. 4 via the resistor 29 to turn on the photothyristor and connect the resistor 33 in parallel with the capacitor 17.

The photocoupler 34 at time t ₁₁ In (c) of FIG. 4 are turned on, the shift by a state of connecting a resistor 33 in parallel with the capacitor 17, [Delta] V by a gate voltage V _GS to the positive electrode side Will do. This makes it possible to apply the gate voltage V _GS exceeding the threshold value V _th to the gate G of the switching element 11 up to the input voltage V ₂ at the operation limit of the drive circuit, thereby preventing the switching element 11 from being thermally destroyed. Can be.

The above-mentioned ΔV increases as the value of the resistor 33 increases, and when the value of the resistor 33 becomes zero, the capacitor 17 is completely short-circuited, and ΔV = 0.
That is, the output signal of the drive unit 12 is directly applied to the gate G. Therefore, it is sufficient to set the value of the resistor 33 in consideration of the input voltage at the operation limit of the drive circuit and the input voltage in the steady state.

When switching to a battery power supply due to a power failure or the like and operating in a low power consumption mode, the power supply voltage may be lower than the normal state. The determination unit determines a decrease in the power supply voltage, and short-circuits the capacitor 17 directly or via the resistor 33 by the switch circuit including the photocoupler 34, so that the gate voltage V _GS that can control the switching element 11 to the ON state is applied to the gate G. Can be applied.

Further, the present invention can be applied to a case where the power supply voltage is gradually increased from a low voltage to a normal voltage after the power is turned on, such as a soft start. That is, when the power supply is turned on, when the power supply voltage is lower than the normal voltage but higher than the operation limit voltage of the drive circuit, the switch circuit operates under the control of the power supply voltage determination unit to short-circuit the capacitor 17 directly or via the resistor 33. The output signal of the drive unit 12 is applied to the gate G of the switching element 11 directly or through a parallel circuit of the resistor 33 and the capacitor 17.

Therefore, the center of the amplitude of the gate voltage V _GS is
Since it is shifted to the positive side as in the time t ₁₁ after the (c) in FIG. 4, so that the switching element 11 can be sufficiently drive. When the power supply voltage approaches the normal value, the switch circuit is turned off under the control of the power supply voltage determination unit, and the output signal of the drive unit 12 is output to the capacitor 17.
Is applied to the gate G of the switching element 11 via the gate voltage V _GS . As described above, the switching element 11 can be controlled by the gate voltage V _GS having steep rising and falling characteristics with respect to the threshold value V _th . it can.

[0031]

As described above, according to the present invention, the complementary transistors 5, 6 are connected between the power supply voltage terminal 3 and the ground terminal 4, and the output terminals of the complementary transistors 5, 6 are connected to the switching element. Capacitor 7 between gate 1
The voltage applied to the gate of the switching element 1 is _{divided by} the input capacitance C _iss of the switching element 1 and the capacitance C of the capacitor 7, and is level-shifted to positive and negative pulses. Since the switching element 1 can be controlled using the characteristic portion having a sharp fall, there is an advantage that switching loss can be reduced.

When the power supply voltage drops, the switching element 1
If the gate voltage V _GS becomes such that the gate voltage V _GS cannot be completely turned on, it is determined by the power supply voltage determination unit 8 and the switch circuit 9 is controlled to short-circuit the capacitor 7 directly or via a resistor. As a result, the gate voltage is level-shifted to the positive electrode side, and the switching element 1 is completely turned on, so that an increase in switching loss due to a decrease in gate voltage can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of an embodiment of the present invention.

FIG. 3 is an operation explanatory diagram of the embodiment of the present invention.

FIG. 4 is an explanatory diagram of an operation when the power supply voltage drops according to the embodiment of the present invention.

FIG. 5 is an explanatory diagram of a conventional example.

FIG. 6 is an operation explanatory diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Switching element 2 Drive circuit 3 Power supply voltage terminal 4 Earth terminal 5, 6 Complementary transistor 7 Capacitor 8 Power supply voltage judgment part 9 Switch circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-7-46836 (JP, A) JP-A-2-165722 (JP, A) JP-A-2-284516 (JP, A) JP-A-4- 170108 (JP, A) JP-A-3-18118 (JP, A) JP-A-61-131615 (JP, A) JP-A-61-26328 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03K 17/00-17/693

Claims (1)

(57) [Claims] In a drive circuit (2) for controlling on / off of a switching element (1) , a complementary transistor (5) is connected between a power supply voltage terminal (3) and a ground terminal (4). (6) When the power supply voltage is lower than a predetermined value
A power supply voltage judging unit (8) for judging whether the voltage has dropped below
Connect the complementary transistor (5), between the gate of the output terminal (6) a switching element (1), the switching element (1) large capacitance of the capacitor compared to the input capacitance of ( 7) is connected, and the power supply voltage is determined by the power supply voltage determination unit (8).
When it is determined that the value has fallen below the value, the capacitor
A switch circuit that short-circuits (7) via a resistor or directly
A drive circuit comprising (9) .