JPH0223062A - Switching power supply controlling circuit - Google Patents

Abstract

PURPOSE:To reduce electromagnetic noise by providing an impedance controlling circuit between the output terminal of a switching controlling circuit and the gate electrode of a power MOSFET. CONSTITUTION:A switching power supply controlling circuit has a voltage variation detection circuit 1 which outputs a pulse width control signal Vpc of a level responsive to the variation in the output power voltage Vcc of a rectifying and smoothing circuit 7, a pulse width modulator 2, a switching control circuit 3 which outputs a switching control signal Vsc, and switching circuit 6. In this case, an impedance control circuit 5 having a junction type FETQJ in which its ON resistance is controlled in response to the level of an overcurrent detection control signal VDC is provided between the output terminal of the control circuit 3 and the gate electrode of the power MOSFETQP of the circuit 6, and an overcurrent detection controlling circuit 4 is provided. Thus, if an overcurrent flows to the FETQP, the circuit 4 immediately increases the ON resistance of the FETQJ.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a switching power supply control circuit, and particularly to a switching power supply control circuit that controls on/off the primary side of a switching power supply transformer using a power MOSFET to obtain a predetermined power supply voltage. Related to power supply control circuits.

[Conventional technology]

Conventionally, this type of switching power supply control circuit, as shown in FIG.
A voltage fluctuation detection circuit 1 outputs a pulse width control signal VPc at a level corresponding to fluctuations in the output power supply voltage VCC of the rectifying and smoothing circuit 7, and a comparator A2 inputting the triangular wave and the pulse width control signal VPC. Pulse train signal V with a pulse width corresponding to the level of this pulse width control signal VPC
A pulse width modulation circuit 2 that outputs p7, and two transistors Q! connected in series between the reference potential terminal (ground terminal) of the primary power supply and the power supply voltage VcI terminal. , Q2, and an output terminal, a reference potential terminal, and a power supply voltage (■) according to the pulse train signal Vp7. , a switching control circuit 3 which outputs a switching control signal Vsc by alternately turning on and off between the terminals, and a power MOSFET Q p which inputs a switching control signal SC to a gate electrode and turns it on and off. A switching power supply transformer Tl with a rectifying and smoothing circuit 7 connected to the secondary side is connected to the reference potential terminal of the primary power supply and the power supply voltage V.
The configuration includes a switching circuit 6 connected in series between the CI terminals.

[Problems to be Solved by the Invention] The conventional switching power supply control circuit described above detects fluctuations in the output power supply voltage ■cc of the rectifying and smoothing circuit 7 through the voltage fluctuation detection circuit 1, the pulse width modulation circuit 2, and the switching control circuit 3. Since the composition is such that the transmission is controlled via these multiple circuits, there is capacitance between the output power supply voltage VCC terminal and the ground potential terminal, and there is also capacitance between the output power supply voltage VCC terminal and the ground potential terminal, and As a result, the control response speed decreases,
Power MOSFET Q for instantaneous heavy loads
, - has the disadvantage of being destroyed.

The purpose of the present invention is to speed up the control response speed and improve power MOSFETs.
An object of the present invention is to provide a switching power supply control circuit that can prevent destruction of an I''ET.

[Means to solve the problem]

A switching power supply control circuit of the present invention includes a voltage fluctuation detection circuit that outputs a pulse width control voltage at a level corresponding to fluctuations in the output power supply voltage of a rectifying and smoothing circuit, and a pulse train having a pulse width corresponding to the level of the pulse width control voltage. a pulse width modulation circuit that outputs a signal; and two transistors connected in series between the first and second power terminals of a primary power source; a switching control circuit that outputs a switching control signal that alternately turns on and off; and a power M that inputs the switching control signal to a gate electrode and turns it on and off.
A switching power supply control circuit comprising an OS FET and a switching circuit in which the rectifying and smoothing circuit is connected to the secondary side and a switching power supply transformer is connected in series between the first and second power terminals of the primary power supply, The output terminal of the switching control circuit and +iir power MOSF
An impedance control circuit including a transistor whose on-resistance is controlled according to the level of the overcurrent detection control signal is provided between the gate electrode of the ET and the switching circuit and the first power supply terminal of the primary power supply. an overcurrent detection control circuit that includes an overcurrent detection resistor connected in series with the switching circuit between the switching circuit and the overcurrent detection control circuit that outputs the overcurrent detection control signal at a predetermined level according to the voltage between the terminals of the resistor; configured.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

This embodiment differs from the conventional switching power supply control circuit shown in FIG. 3 in that the switching control circuit 1i'!
The output terminal of 3 and the power MOS of the switching circuit 6
An overcurrent detection control signal V is provided between the goo 1 and the electrodes of F E'l' Q p. An impedance control circuit 5 including a junction FET Q whose on-resistance is controlled according to the level of C is provided, and this switching It includes an overcurrent detection resistor R7 connected in series with the circuit 6, a constant voltage diode D2, and an operational amplifier Δ1, and outputs an overcurrent detection control signal VOc at a predetermined level according to the voltage between the terminals of this resistor R7. The point is that an overcurrent detection control circuit 4 for outputting is provided.

The overcurrent detection control signal VDC detects the junction type F of the impedance control circuit 5 when an overcurrent flows through the switching circuit 6.
By increasing the on-resistance of E T Q ,
The level changes to increase the impedance between the gate and the gate. That is, when the voltage across the terminals of resistor R7 exceeds a predetermined level determined by voltage regulator diode D2, overcurrent detection control signal VDC is output from the output terminal of operational amplifier A3.
level decreases, and the on-resistance of the junction type FETQJ increases.

By the way, the input capacitance of the power MOSFET Qp is generally about several thousand pF, so the power MOSFET
In order to perform high-speed switching of ETQP, it is important to lower the impedance between the gate electrode and the reference potential terminal and power supply voltage V C1 terminal.

However, if the impedance is lowered excessively, the power M
When the OS FET Qp is turned off, overpower damage may occur. This is because when it is turned off rapidly, the voltage between the train and the source rises rapidly, but since the switching power transformer Tl is an inductive load, the current does not start to fall suddenly, and this power MOS F E
This is because excessive power is applied to T Q p in a short period of time, and the design of the control circuit for the above-mentioned overpower breakdown protection is an important element in switching power supplies.

From the above facts, the power M OS F E T Q p
An effective control circuit for can be considered as follows.

That is, at turn-off and turn-on, the impedance of the gate drive circuit including the impedance control circuit 5 is lowered as much as possible, but at turn-off, the power MOSF
Ideally, E T Q p should be set to a level that does not lead to turn-off destruction, and the efficiency and destruction resistance should both be operated at optimum values.

Incidentally, in the present invention, the impedance of the gate drive circuit is controlled by using the impedance characteristics (on resistance) in the forward and reverse directions of the junction type FETQ, and the impedance of the junction type FETQ, , + Control of power MO
This is performed by an overcurrent detection control circuit 4 that operates based on the source current of S F E T Q p.

Therefore, when an overcurrent flows through the power MOSFET Qp, the overcurrent detection control circuit 4 immediately increases the on-resistance of the junction type FETQ, +, thereby shortening the control path and shortening the protection response time. can do.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

It is desirable that the impedance be low during turn-on and turn-off, and generally it is often lower during turn-off.

In the second embodiment, the impedance control circuit SA has a configuration in which a parallel circuit of a diode D and a resistor R8 is connected in series with the junction type FETQJ, and at turn-off, the junction type FETQ, . The impedance of the gate drive circuit is controlled by the on-resistance, which is the sum of the on-resistance of the junction type FETQJ and the resistor R8 during turn-on.

〔Effect of the invention〕

As explained above, the present invention provides an impedance control circuit equipped with a transistor whose on-resistance is controlled between the output terminal of the switching control circuit and the G1-'+S pole of the power MOSFET, and By providing a resistor for detecting current and controlling the on-resistance of the transistor by the voltage between the terminals of this resistor, the impedance control path of the power MOSFET gate drive circuit is shortened, thereby shortening the protection response time. This has the effect of preventing damage to the power MOS I''ET.

Furthermore, during large current operation, rapid changes in drain current, voltage waveforms, etc. are relaxed and blunted, so that electromagnetic noise can be reduced.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing first and second embodiments of the present invention, respectively, and FIG. 3 is a circuit diagram showing an example of a conventional switching power supply control circuit. DESCRIPTION OF SYMBOLS 1... Voltage fluctuation detection circuit, 2... Pulse width modulation circuit, 3... Switching control circuit, 4... Overcurrent detection control circuit, 5, 5A... Impedance control circuit, 6
...Switching circuit, 7...Rectifier smoothing circuit, A1
...Operation amplifier, A2...Comparator, A3...
・Operation amplifier, DI, D2... Constant voltage diode, D
3...Diode, Ql, Q2...Transistor,
Q,... Junction FET, Qp... Power MOSFE
T, R1 to R8...Resistor, T1...Switching power supply transformer

Claims (1)

[Claims] a voltage fluctuation detection circuit that outputs a pulse width control voltage with a level corresponding to fluctuations in the output power supply voltage of the rectifying and smoothing circuit; and a pulse width modulation circuit that outputs a pulse train signal with a pulse width corresponding to the level of the pulse width control voltage. , comprising two transistors connected in series between the first and second power supply terminals of the primary power supply, and alternately turns on and off between the output terminal and the first and second power supply terminals according to the pulse train signal. A switching control circuit that outputs a switching control signal, a power MOSFET that inputs the switching control signal to a gate electrode to turn it on and off, and a switching power transformer that connects the rectifying and smoothing circuit to the secondary side of the primary power source. In a switching power supply control circuit having a switching circuit connected in series between first and second power supply terminals, an output terminal of the switching control circuit and the power MOSFET
An impedance control circuit including a transistor whose on-resistance is controlled according to the level of an overcurrent detection control signal is provided between the gate electrode of the ET, and a connection between the switching circuit and the first power supply terminal of the primary power supply is provided. An overcurrent detection control circuit is provided, which includes an overcurrent detection resistor connected in series with the switching circuit in between, and outputs the overcurrent detection control signal at a predetermined level according to the voltage between the terminals of the resistor. A switching power supply control circuit featuring: