JPH01144359A - Switching power circuit - Google Patents

Abstract

PURPOSE:To simplify a circuit constitution by supplying the overcurrent detection output in addition to the output voltage detection output to a variable constant voltage element controlling a photocoupler which is commonly provided. CONSTITUTION:A switching power circuit 10 is equipped with an adder 17, and the detected output V1 of an output voltage detection circuit 15 provided in relation to a secondary coil 3b is supplied to an error amplifier 16 through the adder 17. Overcurrent detection circuits 25B-25D are respectively provided to each of secondary coils 3a-3d, and its detected output V2 is supplied to the adder 17 to add to said detected output V1. The output of the error amplifier 16 controls the photocoupler 22 which functions as a common signal transfer means 22. A controlling circuit 20 of a primary coil 3a side is constituted of an error voltage detection circuit 21 and so on. Said adding output is thereby supplied to the common signal transfer means 22 through the error amplifier 16, and is guided to a light emission section 22b.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a switching power supply circuit, and particularly to a switching power supply circuit having an overcurrent protection function.

[Prior Art] As is well known, a switching power supply circuit uses a switching element to intermittent a DC voltage applied to a primary coil, thereby producing a stabilized predetermined output voltage on the secondary coil. It was designed to obtain

FIG. 3 is a system diagram showing an example of the main parts of this type of switching power supply circuit 10, in which the primary coil 3 of the power transformer 3
A switching element 11 for switching the DC voltage applied to the primary coil 3a is provided on the a side.

A pulse modulated switching signal is applied to the switching element 11 . Therefore, an oscillation 1112 is provided, and its oscillation output is supplied to a pulse width control circuit 13, and the pulse width of the oscillation output is controlled by a control signal described later.

On the secondary side of the power transformer 3, a single or multiple secondary coils 3b, 3c, and 3d are wound, in this example, a plurality of secondary coils 3b, 3c, and 3d, each of which provides a predetermined output voltage for each load 6B to 6D. It is done like this.

One of the secondary coils 3b is provided with an output voltage horizontal output circuit 15 in order to stabilize each output voltage, and the output thereof is passed through an error amplifier 16 to form a photocoupler. The light is guided to the first light emitting section 22b.

A first light receiving section 22a that receives the optical signal from the light emitting section 22b is provided on the primary coil 3a side.
2a and the error voltage detection circuit 21, the optical signal is converted into an electric signal corresponding to its strength, that is, an error voltage.

The error voltage is supplied to a pulse width control signal generation circuit 22 to form a pulse width control signal corresponding to the error voltage,
The above-mentioned pulse width control circuit 13 is now controlled.

The pulse width of the oscillation output is controlled to be inversely proportional to the output voltage, and by controlling the conduction angle of the switching element 11, the output voltage applied to the load 6B is stabilized at a predetermined value.

In the switching power supply circuit 10 configured in this way,
In order to prevent excessive current from flowing into the loads, overcurrent detection circuits 25B to 2 are installed corresponding to the respective loads 6B to 6D.
5D is provided, and the respective overcurrent detection outputs are guided to the second light emitting section 27b.

The optical signal is received by the second light receiving section 27a. Second
The light receiving sections 2 and 7a include a pulse width controlled signal generating circuit 28.
A pulse width control signal corresponding to the optical signal is added to a pulse width control signal for output voltage stabilization in an adder 29, and is supplied to a common pulse width control circuit 13. .

Therefore, when an overcurrent flows through the load, control is performed so that the pulse width of the oscillation output becomes narrow or completely zero. This prevents excessive current from flowing into the load and protects the power supply.

[Problems to be Solved by the Invention] Incidentally, in the switching power supply circuit 10 configured as described above, in order to stabilize the output voltage and protect the power supply device from damage due to overcurrent, as described above, overcurrent Detection circuits 25B to 25D are provided, and each detection output is transmitted to the primary coil 3a side.

In that case, it is necessary to electrically isolate the primary side and the secondary side of the power transformer, so the output voltage detection circuit 15 side and overcurrent detection circuits 25B to 25D each have a signal transmission device such as a photocoupler. You will need the means.

Therefore, two independent systems of signal transmission means are required.

Therefore, there is a drawback that the number of parts of the photocoupler increases.

Therefore, in the present invention, a switching power supply circuit is proposed in which the circuit configuration is simplified by using the - set of photocoupler so that necessary signals can be transmitted to the primary coil 3a side. It is.

[Technical Means for Solving the Problems] In order to solve the above-mentioned problems, in the present invention, a DC voltage is applied to the first coil on the primary side of the power transformer, and the voltage is applied to the first coil. Generally, a light emitting means is provided on the secondary side of the power transformer to control the connected switching elements with switching pulses, and to change the brightness of the light emitting element in accordance with fluctuations in the DC voltage supplied to the load. A switching power supply circuit configured to receive the light of the light with a light receiving element provided on the primary side of a power transformer, and to control the pulse width of the switching pulse based on the output of the light receiving element, the power transformer In addition to detecting overcurrent on the secondary side of
The present invention is characterized in that an overcurrent detection means is provided which controls the brightness of the light emitting element and controls the pulse width of the switching pulse by giving top priority to the overcurrent detection output.

[Function] Detection output of output voltage detection circuit 15 and overcurrent detection circuit 2
The detection outputs of 5B to 25D are added in an adder 17.

This summed output is supplied to the common signal transmission means 22 via the error amplifier 16. This signal transmission means 22 is composed of a light emitting section 22b and a light receiving section 22a, and the addition output is first
The light is guided to the light emitting section 22b.

On the other hand, the received optical signal is converted into an error voltage in the error voltage detection circuit 21, and a pulse width control signal corresponding to this error voltage is formed. The switching element 11 is pulse width modulated by this pulse width control signal.

Therefore, when the output voltage applied to the load 6B changes,
Accordingly, the conduction angle of the switching element 11 is controlled, and the output voltage is stabilized.

Furthermore, when an overcurrent condition is detected, the excitation state of the light emitting section 22b is also controlled. Therefore, when an overcurrent condition is also detected, the switching element 11 is controlled and the loads 25B to 25D are protected. be done.

In this case, only one set of photo couplers is required as the signal transmission means.

[Example] Next, an example of a switching power supply circuit 1o according to the present invention will be described in detail with reference to FIG.

Portions corresponding to those in FIG. 2 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In this invention, an adder 17 is provided, and the detection output V1 of the output voltage detection circuit 15 provided in relation to the secondary coil 3b is passed through the adder 17 to the error amplifier 16.
It is designed to be supplied to the side.

Each of the secondary coils 3a to 3d has an overcurrent detection circuit 25.
B to 25D are provided, and the overcurrent detection output (2) detected from each is supplied to the adder 17 and added to the above-mentioned output voltage detection output (2).

By the output of the error amplifier 16, the common signal transmission means 2
A light emitting unit 2 constituting a photocoupler 22 functioning as a light emitting unit 2
The excited state of 2b is controlled. Therefore, the light emitting section 22b is commonly used for the respective detection outputs Vl and V2.

On the other hand, the control circuit 2o provided on the primary coil 3a side is configured as follows.

The optical signal received by the light receiving section 22a is sent to the error voltage detection circuit 21.
The error voltage is converted into a predetermined error voltage, and this is supplied to the pulse width control signal generation circuit 22 to form a pulse width control signal corresponding to the error voltage.

Since the pulse width control circuit 13 is controlled by the pulse width control signal, the switching signal is pulse width modulated thereby.

Therefore, when the output voltage fluctuates, the switching signal to the switching element 11 is pulse width modulated to offset the fluctuation, thereby stabilizing the output voltage applied to the load 6B.

In this state, when overcurrent is detected, the detection output V2
is the above-mentioned light emitting section 22 along with the output voltage detection output ■1.
b, so the error voltage has a value that also corresponds to overcurrent detection output (2). Here, when overcurrent is detected, V2>Vl.

As a result, the switching element 11 is also pulse width modulated by this overcurrent detection output (2), so when an overcurrent condition occurs, a pulse width control signal is generated that turns off the switching element 11 in this example. Ru. As a result, the output voltage that can be applied to the loads 6B to 6D becomes zero, and the loads 6B to 6D are protected from overcurrent.

Note that as the control circuit 20 described above, a commercially available IC such as M51977P manufactured by Mitsubishi can be used.

A specific example of this switching power supply circuit 10 is shown in FIG. However, for convenience of explanation, only one secondary coil is shown.

After the commercial AC source 1 has been converted into a predetermined DC voltage by the rectifier circuit 2, it is supplied to the primary coil 3a of the power transformer 3.

A rectifier circuit 4 consisting of a diode 4a and a capacitor 4b is connected to the secondary coil 3b side, so that the rectified output voltage is supplied to a terminal 5.6. terminal 5
, 6 are connected to a load 6B.

As the switching element 11 connected to the primary coil 3a side, an MO3 type FET transistor or the like can be used.

An output voltage and overcurrent detection circuit 30 is provided in association with the secondary coil 3b.

Let's explain the output voltage detection circuit 15 first.

In this example, a resistor 31, a photodiode 22b forming a photocoupler 22, and a variable constant voltage element (variable Zener diode in this example) 32 are connected between a predetermined DC power supply terminal B and one output terminal 6. are connected in series.

On the other hand 1. A pair of voltage dividing resistors 33a and 33b are connected between the output terminals 5 and 6, and the divided voltage obtained at the connection midpoint n is supplied to the control terminal of the variable Zener diode 32.

In relation to the output voltage detection circuit 15, an overcurrent detection circuit 2
5B is provided.

That is, between the power supply terminal B and the output terminal 6, there are a plurality of resistors 35a to 35d connected in cascade, four in this example.
are connected, and the connection middle point p and the connection point r, that is, the output terminal 6
There is a constant voltage element (such as a Zener diode) 3 between
6 is connected, the reference voltage Va obtained at the connection midpoint q is made constant. This reference voltage Va is supplied to the negative terminal of the operational amplifier 37.

On the other hand, the voltage vb on the output terminal 6 side away from the connection point r is supplied to the positive terminal of the operational amplifier 37.

In the present invention, the output of the operational amplifier 37 is supplied to the control terminal (connection point n) of the variable Zener diode 32 via the resistor 41B and the reverse current blocking diode 42B. Therefore, in this example, the adder 17 is configured by wired AND.

The other detection circuits 25C and 25D for overcurrent detection also have the same configuration as described above, and the output of the operational amplifier (detection output V2) controls the variable Zener diode 32 via a resistor and a diode for blocking reverse current. The terminal is configured to be supplied to the terminal.

In the figure, only the resistor 41C for the detection circuit 25C is shown.

Note that the above-mentioned predetermined DC voltage 10B can be derived from the intermediate tap from the secondary coil 3b, and the rectified output of the voltage obtained at the intermediate tap can be used.

In the switching power supply circuit 10 configured as described above, detection and control operations of output voltage and overcurrent will be briefly described below.

In a normal state, the Zener voltage of the variable Zener diode 32 is controlled by the divided voltage (1) obtained at the connection midpoint n.

Here, when the output voltage becomes high due to a decrease in the impedance of the load 6B connected to the terminals 5 and 6, this lowers the Zener voltage of the variable Zener diode 32, and accordingly the photodiode 22b The luminance of the light increases, this light is received by the phototransistor 22a, an error voltage corresponding to the output voltage is reproduced, and the pulse width of the output pulse of the switching element 11 is controlled to be narrowed according to this error voltage, and the output The voltage is stabilized.

The overcurrent protection operation is as follows.

That is, during steady operation, the reference voltage ■a is selected to be larger than the voltage vb at the terminal 6, so the output V2 of the operational amplifier 37 at that time has a voltage exceeding the divided voltage ■1 at the n point. is not supplied, and the Zener voltage of the Zener diode 32 is controlled according to fluctuations in the output voltage.

However, when an overcurrent flows to the load side, the voltage drop across the resistor 35d connected in series to the terminal 6 becomes large, so that the terminal voltage vb becomes larger than the reference voltage Va.

As a result, the output (2) of the operational amplifier 37 is inverted to a high level, becomes higher than the divided voltage v1, and is applied to the control terminal of the variable Zener diode 32 via the resistor 41 and diode 42. As a result, the Zener diode 32 has priority over the divided voltage V1 and outputs the output of the operational amplifier 37.
2 controls the Zener voltage.

That is, when an overcurrent is detected, the overcurrent detection output is output with priority over the output voltage detection output, so the pulse of the output pulse of the switching element 11 is lower than that during the stabilization operation where the error voltage can control tIJ. The width becomes gradually narrower, and as a result, the output voltage decreases more gradually than during the stabilization operation.

As a result, loads 6B to 6D are protected from overcurrent.

[Effects of the Invention] As explained above, according to the configuration of the present invention, in addition to the output voltage detection output, the overcurrent detection output is also supplied to the variable constant voltage element that controls the commonly provided photocoupler. With this configuration, the element (signal transmission means) for insulating the secondary side of the power transformer can be used in common.

As a result, one pair of photocoupler can be omitted, so that the circuit configuration can be made simpler than before.

Therefore, the switching power supply circuit according to the present invention is extremely suitable for application to a switching power supply circuit having an overcurrent protection function that requires electrical isolation between the primary side and the secondary side.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram of essential parts showing an example of a switching power supply circuit according to the present invention, and FIG. 2 is a connection diagram showing a specific example thereof.
FIG. 3 is a configuration diagram of main parts of a conventional switching power supply circuit. 3...-a m source transformer 3a wheel...primary coil 3b...secondary coil 10...switching power supply circuit 11...switching element 12...oscillator circuit 15...output voltage detection circuit 22 ...Photo coupler 22a...Phototransistor 22b...Photodiodes 25B to 25D...Overcurrent detection circuit 30...Detection circuit 32...Variable Zener diode 37...Operational amplifier patent applicant Nippon Chemi-Con Co., Ltd.

Claims (1)

[Claims] (1) Apply a DC voltage to the first coil on the primary side of the power transformer, control the switching element connected to the first coil with a switching pulse, and apply the DC voltage to the secondary side of the power transformer to the load. A light-emitting means is provided to change the brightness of the light-emitting element in accordance with fluctuations in the supplied DC voltage, and light from the light-emitting element is received by a light-receiving element provided on the primary side of the power transformer, and the output of the light-receiving element is A switching power supply circuit configured to control the pulse width of the switching pulse based on the above, and detecting an overcurrent on the secondary side of the power transformer,
A switching power supply circuit comprising overcurrent detection means that controls the brightness of the light emitting element and controls the pulse width of the switching pulse by giving top priority to the overcurrent detection output.