JPH04312350A - Input voltage drop detecting circuit - Google Patents

Abstract

PURPOSE:To prolong output voltage ensuring time at input voltage drop under light load and to lower power dissipation in an input voltage drop detecting circuit for a switching power supply. CONSTITUTION:Switching pulses (a) for a performing ON/OFF control of a switching element are integrated through an integrating circuit 6. An integrated output (c) and a reference voltage (b) are then subjected to level comparison through a comparator 7. Input voltage drop is detected based on the comparison result (d). Since the saw-tooth signal (c) obtained through integration of the switching pulse (a) is subjected to level comparison with the reference voltage (b), time for reaching the reference level is prolonged when the load is light even if the input voltage drops and thereby the output voltage ensuring time is prolonged. Both the integrating circuit and the comparing circuit are constituted to dissipate low power.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input voltage drop detection circuit, and more particularly to an input voltage drop detection circuit for detecting a drop in input voltage of a switching power supply.

0002

2. Description of the Related Art A conventional input voltage drop detection circuit for detecting a drop in input voltage of a stabilized DC power supply has a configuration shown in FIG. A stabilized power supply circuit 3 receives AC voltage from input terminals 1 and 2 and outputs a stabilized DC voltage to output terminals 4 and 5, and a rectifier circuit rectifies and smoothes the AC input from AC input terminals 1 and 2. A smoothing circuit 9 compares this smoothed output with a built-in reference voltage and outputs an input voltage drop abnormality signal 8 when the smoothed output decreases and falls below the reference voltage as the input AC voltage decreases. The input power supply drop detection circuit 10 includes an input power supply drop detection circuit 10.

FIG. 4 is an operating waveform diagram of the circuit shown in FIG.
The smoothed output voltage also decreases. When this smoothed output voltage becomes lower than the reference voltage, the level comparison circuit 10 detects this and outputs, for example, a high level input voltage drop abnormality signal 8.

[0004] Normally, in order to secure the output voltage guarantee time in the event of instantaneous input power interruption required for a power supply, a stabilizing power supply circuit 3 is provided.
The minimum input voltage and built-in capacitor capacity of the conventional input power supply drop detection circuit are determined by the stabilized power supply circuit 3.
It operates independently of the voltage of the internal smoothing circuit.

That is, regardless of the magnitude of the load on the stabilized power supply circuit, input voltage abnormality detection is performed only based on the rectification and smoothing level of the AC input voltage, so there is a drawback that accurate input voltage drop detection cannot be performed. Furthermore, since the input voltage of the detection circuit is high, there is a problem that power consumption is large.

[0006]

OBJECTS OF THE INVENTION An object of the present invention is to provide an input voltage drop detection circuit that can accurately detect an input voltage drop while taking into account the magnitude of the load on the circuit and can be configured with very little power.

[0007]

According to the present invention, there is provided an input voltage drop detection circuit for a switching power supply that outputs a DC stabilized voltage by controlling the pulse width of a switching pulse that controls on/off of a switching element, and which integrates the switching pulse. an integrator that generates a tooth-shaped wave signal, and a comparator that compares the levels of the tooth-shaped wave signal and a reference voltage;
An input voltage drop detection circuit is obtained, which is characterized in that a drop in input voltage is detected by this comparison output.

[0008]

Embodiments Examples of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of one embodiment of the present invention. The stabilized power supply circuit 3 inputs alternating current or unstable constant direct current from input terminals 1 and 2 and outputs stabilized direct current to output terminals 4 and 5 by the action of a built-in pulse width control circuit.
An on-pulse a having the same timing as the on-pulse that makes the internal switching element conductive is output.

The integration circuit 6 integrates this on-pulse,
This integrated claw-tooth waveform is input to the non-inverting input terminal of the comparator circuit 7. Further, the reference voltage b is input to the inverting input terminal of the comparator circuit 7, and the comparator circuit 7 sends out an input voltage drop abnormality signal when the voltage value of the output c of the integrating circuit 6 is greater than the reference voltage value.

FIG. 2 is a time chart showing the operation of the circuit shown in FIG. 1. When the input power supply decreases, the width of the on-pulse of the switching element in the stabilized power supply circuit 3 increases, and the minimum input of the stabilized power supply circuit 3 increases. When the voltage drops to the voltage value, the width of the on-pulse a becomes maximum, and an input voltage drop abnormal signal is sent out.

When the load is light, even if the input voltage drops, it takes a long time to reach the minimum input voltage.
The guaranteed time for the output voltage is extended, and accurate voltage drop detection can be performed taking into account the size of the load.

[0013]

As explained above, according to the present invention, when an input voltage drop occurs, the input voltage drop abnormal signal is not sent until the lowest input voltage value of the stabilized power supply circuit. This has the advantage that the output voltage guarantee time can be extended, and detection circuits such as an integrating circuit and a comparator circuit can be configured with very little power.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the embodiment circuit of FIG. 1;

FIG. 3 is a block diagram of a conventional input voltage drop detection circuit.

[Explanation of symbols]

3 Stabilized power supply circuit 6 Integral circuit 7 Comparison circuit

Claims (1)

[Claims] 1. An input voltage drop detection circuit for a switching power supply that outputs a DC stabilized voltage by controlling the pulse width of a switching pulse that controls on/off of a switching element, the circuit integrating the switching pulse and generating a hook-tooth wave signal. An input voltage drop detection device comprising: an integrator that generates a signal, and a comparator that compares the levels of this tooth-tooth wave signal with a reference voltage, and detects a drop in input voltage based on the comparison output. circuit.