JPH05300641A - Switching power supply - Google Patents

Abstract

PURPOSE:To transmit an output voltage drop caused by power supply cutoff to a microcomputer, etc., on the main body side of equipment by detecting the interruption of a commercial alternating current by means of a protective detection circuit provided on the secondary side of an output transformer. CONSTITUTION:A direct current generated by rectifying an alternating current through a diode bridge DB is supplied to the primary side of an output transformer T. An output voltage detection circuit VD detects the secondary-side output voltage of the transformer T and a control circuit CC turns on/off a switching transistor Q1 so as to generate a fixed main voltage on the secondary side. A chopper circuit CH converts the main voltage to a sub-output voltage lower than the main voltage. When the main output voltage drops due to a power failure, etc., and the voltage of a protective detection circuit 3 drops to a prescribed value, for example, 18V, a transistor Q3 is turned on and lowers the sub-output voltage following the main voltage. Therefore, the cost of this switching power supply can be reduced, because the cutoff of power supply can be detected with a simple circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-output switching device which is used to construct a power supply unit of various electronic equipment mainly including a microcomputer such as a laser beam printer, a facsimile machine, a copying machine or a plain paper facsimile machine. It relates to a power supply device.

[0002]

2. Description of the Related Art A conventional multi-output switching power supply device for electronic equipment equipped with such a microcomputer is shown in FIG.
The configuration is as shown in. That is, after the commercial alternating current of the commercial alternating current power source (AC) is full-wave rectified by a full-wave rectifier (DB) such as a diode bridge and smoothed by a smoothing device (not shown), it is applied to the primary winding of the output transformer (T). Supplied. Then, the control circuit (CC) outputs a pulse voltage having a pulse width corresponding to the detection voltage input from the output voltage detection circuit (VD) that detects the main output voltage on the secondary side of the output transformer (T), Switching control of the switching transistor (Q1) is performed by this pulse voltage, and when the transistor (Q1) is turned on, a DC current obtained by rectifying and smoothing the commercial AC flows intermittently in the primary winding of the output transformer (T), and A current flowing through the secondary side of the transformer (T) outputs a DC voltage of, for example, 24 V as a main output voltage through a rectifying / smoothing unit (not shown). The main output voltage supplied to this load is maintained constant at 24V by variably controlling the pulse width of the pulse voltage supplied from the control circuit (CC) to the base of the switching transistor (Q1). Further, the main output voltage is converted into 5V by a chopper circuit (CH) and output as a sub output voltage for driving a microcomputer.

By the way, since the microcomputer is driven by the sub output voltage DC 5V as the main power source,
If the sub output voltage drops due to the interruption of commercial AC, etc., it malfunctions. Therefore, when the commercial AC power supply (AC) is cut off for some reason such as a power failure, the input / output port, the register, the memory, etc. are reset before the sub output voltage drops, or the data is saved to retain it. Need to do. However, commercial AC power supply (AC)
As shown in FIG. 4, which shows the output voltage characteristics of FIG.
The main output voltage of 24V gradually decreases as the energy on the primary side of the output transformer (T) attenuates.
The sub-output voltage of 5V sharply drops when the main output voltage drops to a predetermined value by the chopper circuit (CH) in the presence of a load. There is no room to take the access time of about 10 msec required to perform the reset and save processing.

Therefore, as shown in FIG. 3, a protective detection circuit (1) for detecting the connection / disconnection of the commercial AC power source (AC) and transmitting it to the microcomputer is provided. The protection detection circuit (1) includes two voltage dividing resistors (R1) and (R).
2) and the voltage detection regulator IC (2) set a detection voltage of, for example, 70V, and when the commercial AC voltage is reduced to 70V due to the occurrence of a power failure or the like, the voltage detection regulator IC (2) does not operate. As a result of conduction, no current flows in the light emitting diode (FD) of the photocoupler (FC), which causes the phototransistor (FT) and the detection transistor (Q2) of the photocoupler (FC).
Are both turned off, and a DC voltage of 5V of the sub output voltage is output from the signal output terminal to the microcomputer as a commercial AC interruption detection signal.

[0005]

However, in the above-mentioned protection detection circuit (1), the microcomputer of the equipment body starts necessary processing such as reset and save as soon as possible before the sub output voltage drops. As a result, an expensive voltage detection regulator I for accurately detecting a voltage drop due to interruption of commercial AC is provided on the primary side of the output transformer (T).
Requires C (2). Further, in such a switching power supply device, the primary side and the secondary side of the output transformer (T) have to be insulated from each other due to the safety standard, so that the voltage drop is detected by the voltage detecting regulator IC (2) described above. An expensive photocoupler (FC) is required to transmit to the secondary side of the output transformer (T), and further as an electronic component provided on the primary side having a higher voltage than the secondary side of the output transformer (T). It is necessary to use one with a large rated value,
Moreover, the circuit configuration becomes relatively complicated because a signal system for transmitting the interruption detection signal to the microcomputer is required.
Therefore, there is a drawback that the cost is considerably high.

Therefore, the present invention provides a switch power supply device having an inexpensive circuit configuration and capable of transmitting a decrease in the output voltage due to power interruption or the like to a microcomputer or the like on the apparatus body side without requiring a special transmission signal system. Is a technical issue.

[0007]

According to the present invention, as a technical means for achieving the above-mentioned object, a switching power supply device is constructed as follows. That is, DC power is intermittently supplied to the primary winding of the output transformer by switching control of the switching element on the primary side corresponding to the main output voltage of the secondary side of the output transformer, and the main output voltage is kept constant. In a switching power supply device that controls the value to a value, converts the main output voltage to a sub output voltage lower than the main output voltage by a chopper circuit, and outputs the sub output voltage.
The sub output voltage falls when the main output voltage falls by passing a current from the output line of the sub output voltage to the output voltage setting line of the chopper circuit through a switch element that turns on when the main output voltage drops to a predetermined value. It is characterized in that a protection detection circuit for reducing the voltage is provided.

[0008]

Operation: For example, when the power supply is cut off, the main output voltage is
It gradually decreases as the energy on the primary side of the output transformer decays, and when this main output voltage drops to a predetermined value, the switch element of the protection detection circuit begins to conduct and the sub output line passes through this switch element. A current flows through the output voltage setting line of the chopper circuit. Therefore, the sub output voltage of the sub output line is forcibly and gradually begins to drop with the fall of the main output voltage. In this way, the sub output voltage gradually decreases following the falling characteristics of the main output voltage.Therefore, when the sub output voltage, which is the main power supply, drops to a predetermined value in the microcomputer on the equipment main unit side. If the sub-output voltage is reduced to a voltage value at which the microcomputer malfunctions, it is possible to start the processing that requires resetting or saving by automatically detecting that the commercial AC power is cut off. The process of can be ended. Therefore, the interruption of the commercial AC can be detected on the secondary side of the output transformer, and the signal system for transmitting the interruption of the commercial AC unlike the conventional device is not required.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which the same or equivalent parts as in FIG. 3 are designated by the same reference numerals. The only difference is that the protection detection circuit (3) is provided on the secondary side of the output transformer (T) and the output terminal of the commercial AC interruption detection signal is excluded. The protection detection circuit (3) includes a Zener diode (ZD), a backflow prevention diode (D) and 2 between the 24V main output line and the negative side line.
The voltage dividing resistors (R3) and (R4) are connected in series, and when the voltage of the main output line drops to 18V which is the Zener voltage of the Zener diode (ZD), both voltage dividing resistors (R3) and (R3), The voltage at the common connection point (A) of R4), that is, the base voltage of the PNP transistor (Q3) is set to 5 V, and the emitter and collector of the transistor (Q3) are connected to the sub output line of 5 V and 2 connected in series between the sub output line and the negative side line
The output voltage setting line of the chopper circuit (CH) for setting the voltage of the sub output line to 5V by the resistors (R4) and (R5) for voltage division and the variable resistor (VR) for setting the sub output voltage. It is connected.

Next, the operation of the above embodiment will be described. The operation of generating the main output voltage of 24V and the sub output voltage of 5V is the same as in the case of FIG.
Only the operation of the protection detection circuit (3) will be described. When the commercial AC is cut off, the main output voltage gradually decreases as the energy on the primary side of the output transformer (T) attenuates, as shown in FIG. The protection detection circuit (3) in the present invention controls the fall of the sub output voltage by utilizing the fall characteristic of the main output voltage described above. That is, when the main output voltage starts to drop from 24V and becomes 18V or less, the transistor (Q3) starts to conduct because the base voltage of the transistor (Q3) becomes 5V or less, and starts to conduct from the sub output line through the transistor (Q3) to the chopper circuit (CH3). Current flows through the output voltage setting line in). Therefore, the sub output voltage of the sub output line is
As shown in, the output voltage forcibly begins to drop gradually as the main output voltage drops. Incidentally, in the device of FIG. 3, when the main output voltage drops to about 8V, the chopper circuit (C
H) causes the sub output voltage to drop instantaneously. In this way, the sub output voltage gradually decreases following the falling characteristic of the main output voltage, and according to actual measurement, as shown in FIG. 2, the sub output voltage changes from 4.3V to 3.5V. The time (t) required for the decrease is 10 msec.
This time (t) corresponds to the access time required for the microcomputer to perform reset and save processing.
Therefore, in the microcomputer on the device side,
If the sub-output voltage, which is the main power supply, drops from 5V to 4.3V, it automatically detects that the commercial AC is shut off and starts the necessary processing for resetting and saving. The secondary side can detect the interruption of the commercial alternating current, and the signal system for transmitting the interruption of the commercial alternating current as in the conventional device is not required.

[0011]

As described above, according to the switching power supply device of the present invention, since the protection detection circuit provided on the secondary side of the output transformer detects the interruption of the commercial AC, the protection detection circuit is expensive. The circuit configuration is simplified without using electronic parts, and the power supply cutoff can be transmitted to the main body side due to the gradual decrease of the sub output voltage, so no special signal system for transmission to the main body side is required. Therefore, a significant cost reduction can be achieved.

[Brief description of drawings]

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

FIG. 2 is an output characteristic diagram when the power supply is cut off.

FIG. 3 is an electric circuit diagram of a conventional device.

FIG. 4 is an output characteristic diagram when the power supply is cut off.

[Explanation of symbols]

 Q1 switching transistor (switching element) T output transformer CH chopper circuit Q3 PNP transistor (switch element) 3 detection circuit for protection

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical display location H02M 3/28 V 8726-5H H04N 1/00 C 7046-5C

Claims (1)

[Claims] 1. The main output of the output transformer is intermittently supplied to the primary winding of the output transformer by switching control of a switching element on the primary side corresponding to a main output voltage of the secondary side of the output transformer. A switching power supply device that controls a voltage to a constant value, converts the main output voltage into a sub output voltage lower than the main output voltage by a chopper circuit, and outputs the sub output voltage. A switch that is turned on when the main output voltage drops to a predetermined value. A protection detection circuit is provided that lowers the sub output voltage in response to the fall of the main output voltage by causing a current to flow from the output line of the sub output voltage to the output voltage setting line of the chopper circuit via an element. A switching power supply device characterized by the above.