JPH04372570A - Step-down chopper circuit - Google Patents

Abstract

PURPOSE:To prevent application of an input voltage to a load as it is when a control signal at the time of applying an input power or switching it in a step-down chopper circuit by using an FET as a switching element. CONSTITUTION:An FET 2 is turned ON, OFF in response to a control signal to be supplied from an IC 14, and its output is supplied to a load 11 through a smoothing capacitor 10. A voltage is supplied from a DC power source 1 to the FET 2, the FET 2 is self-biased by a resistor 4 in a state that a control signal is not applied to a gate of the FET 2, and the FET 2 is not turned ON. The switching speed of the FET 2 is not lowered due to the presence of a constant-voltage diode 3.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down chopper circuit that generates a DC voltage of a predetermined value from a DC power supply.

0002

2. Description of the Related Art FIG. 2 is a circuit diagram showing a step-down chopper circuit using a conventional FET. In the figure, 1 is a DC power supply that is an input power source, 2 is a P-channel FET that is a switching element, 3 is a constant voltage diode for generating voltage between the source and gate of FET 2, 24 is a resistor, and 5 is a DC power supply. A cutting capacitor 6 is a resistor for setting the gate current.

Further, 7 is a freewheeling diode for circulating load current, 8 is a resistor for overcurrent detection, 9 is a choke coil for current smoothing, 10 is a smoothing capacitor for voltage smoothing, 1
1 is a load, 12 and 13 are resistors for output voltage detection, 14 is an IC with built-in error amplifier, oscillator, overvoltage protector, overcurrent limiting circuit, etc., 15 is a resistor for oscillation, 1
6 is a capacitor for oscillation, and 17 is a power source for driving the IC 14.

Next, the operation will be explained. When a voltage is applied from the DC power supply 1 to the FET 2 side, a current flows through the constant voltage diode 3. Then, a Zener voltage is applied between the source and gate of FET2, and FET2 is turned on. Next, when power is supplied to the IC 14 from the power supply 17,
IC 14 oscillates at a frequency determined by resistor 15 and capacitor 16.

The signal oscillated by the IC 14 is applied to the gate of the FET 2 via a resistor 6 and a DC cut capacitor 5. FET2 repeats on and off according to the signal. The output of the FET 2, which is a rectangular wave according to the on/off state, is smoothed by a smoothing capacitor 10 and then supplied to a load 11. Furthermore, when the FET 2 is off, the energy stored in the choke coil 9 is supplied to the load side through the freewheeling diode 7.

[0006] IC 14 detects the voltage supplied to load 11 via resistors 12 and 13. The difference between the desired voltage and the actual voltage is detected by an error amplifier within IC14 and
C14 adjusts the oscillation frequency according to the difference.

FIG. 3 shows the configuration around the FET 2 when a resistor 25 is used as a source-gate voltage generating means for the FET 2. The operation when generating a predetermined DC voltage is the same as that shown in FIG.

[0008]

[Problems to be Solved by the Invention] Since the conventional step-down chopper circuit is configured as described above, in the one shown in FIG.
2 is turned on, a voltage equal to the input DC voltage is applied to the load 11, and an overvoltage is applied to the load at the time of startup. Furthermore, when the IC 14 detects overvoltage or overcurrent and stops oscillation during chopper operation, the FET 2 remains on, resulting in another problem in that overvoltage is applied to the load. In addition, in the case shown in FIG. 3, since the source-gate voltage generating means is the resistor 4, the FET 2
There was a problem that the switching speed of FET 2 became slow and the loss in FET 2 became large.

The present invention has been made to solve the above-mentioned problems, and provides a step-down chopper circuit that does not apply overvoltage to the load when input power is turned on, and does not reduce the switching speed of the FET. The purpose is to

0010

A step-down chopper circuit according to the present invention has a constant voltage diode and a resistor connected in parallel between the source and gate of an FET, which is a switching element.

[0011]

The parallel circuit of the resistor and constant voltage diode according to the present invention turns the FET off by biasing the resistor while no control signal is applied from the oscillation circuit, and maintains the switching speed of the FET at a high speed.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 4 is a bias resistor connected in parallel with the constant voltage diode 3 between the source and gate of FET 2, and the other components are the same as those shown in FIG. 2 with the same reference numerals. be.

Next, the operation will be explained. As in the conventional case, FET 2 is turned on and off according to a control signal given from IC 14, and the output of FET 2 is smoothed by smoothing capacitor 10 and then supplied to load 11.

In this case, even if DC voltage starts to be supplied from the DC power supply 1, the FET 2 will not turn on due to the presence of the resistor 4. The IC 14 then begins supplying control signals. When IC14 soft-starts, the output voltage of FET2 also rises softly. Also, even if IC14 detects overvoltage or overcurrent and stops outputting the control signal, FET2
is self-biased by resistor 4, so FET
2 does not turn on. That is, the DC voltage of the DC power supply 1 is not directly applied to the load side.

Furthermore, since the constant voltage diode 3 is connected between the source and gate of the FET 2, the switching speed of the FET 2 is prevented from decreasing.

[0016]

[Effects of the Invention] As described above, according to the present invention, the step-down chopper circuit has a configuration in which a resistor and a constant voltage diode are connected in parallel between the source and gate of the FET, which is a switching element, so that when the input power is turned on, This has the effect of preventing overvoltage from being applied to the load when the control signal is no longer applied due to some abnormality.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a step-down chopper circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional step-down chopper circuit.

FIG. 3 is a circuit diagram showing a switching section of another conventional step-down chopper circuit.

[Explanation of symbols]

1 DC power supply 2 FET 3. Constant voltage diode 4 Resistance 10 Smoothing capacitor 11 Load 14 IC (oscillation circuit)

Claims (1)

[Claims] 1. A step-down chopper circuit comprising a P-channel FET whose source is connected to an input power source, and an oscillation circuit that applies a control signal to the gate of the FET, wherein a step-down chopper circuit is provided that is connected between the source and gate of the FET. A step-down chopper circuit comprising a constant voltage diode and a resistor connected in parallel to the constant voltage diode.