JPH0779563A - Detector for output voltage of switching power supply - Google Patents

Abstract

PURPOSE:To eliminate errors caused by the fluctuation of the current transmitting ratio of a photocoupler by finding the output voltage of a switching power supply by performing potential insulation with a series circuit of a variable resistor, a photocoupler, and a shunt regulator and adjusting the variable resistor at the time of adjusting the output voltage by comparing the output voltage with a set voltage. CONSTITUTION:The detecting section 2B of the output voltage VO of a switching power supply is constituted by serially connecting a variable resistor R4, the light-emitting diode PC1 of a photocoupler, and a shunt regulator IC1. A reference voltage VO1 for detecting output voltage is generated by connecting resistors R5 and R6 in parallel with the diode PC1 and regulator IC1. The voltage VO1 is divided by the resistors R5 and R6, a reference voltage Vr is inputted to the terminal 5 of the regulator IC1. By adjusting the variable resistor R4, the current IK of the diode PC1 is adjusted to a prescribed value. Therefore, the occurrence of errors caused by the fluctuation of the current transmitting ratio of the photocoupler can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for controlling the output voltage of a switching power supply device at a constant voltage, and a circuit for detecting the output voltage by performing potential insulation with a photocoupler and providing it to an output voltage adjusting section, particularly the output voltage and the detection circuit. The linearity between the voltage and the voltage is improved, and the gain of the output voltage detection circuit is adjusted so that the current transfer ratio (CTR) of the photocoupler varies (individual difference).
The present invention relates to an output voltage detection circuit of a switching power supply device that can be kept constant even if there is.

In the following drawings, the same reference numerals indicate the same or corresponding parts.

[0003]

2. Description of the Related Art FIG. 2 shows a prior application (Japanese Patent Application No. 4-22185) of the present applicant in which an insulation type output voltage detection circuit detects an error between an output voltage detection value and a set value and performs feedback control.
2 shows the circuit configuration of a switching power supply device (also referred to as a switching power supply). The figure includes a power supply unit 1, an output voltage detection unit 2A, and an output voltage adjustment unit 3.
The power supply unit 1 includes a switching transistor Tr1, a choke coil L1, and a smoothing capacitor C1. Here, the switching transistor Tr1 repeatedly turns on and off the input voltage Vin to obtain a constant output voltage Vo, and the choke coil L1 and the smoothing capacitor C1 smooth the intermittent voltage of the switching transistor Tr1.

The output voltage detector 2A has a resistor R4.
~ R6, Photocoupler PC1, Shunt regulator I
It consists of C1. Here, the shunt regulator IC1 is a semiconductor device that varies its inflow current IK so that the input voltage (referred to as a reference voltage, which is 2.5V in this example) Vr to the reference terminal 5 is kept equal to the internal reference voltage Vs. , If the reference voltage Vr tries to increase or decrease with respect to the reference voltage Vs, the inflow current IK
Respectively increase and decrease sharply.

The resistor R4 limits the current of the photodiode of the photocoupler PC1, and the resistors R5 and R6 divide the voltage between the lower end of the resistor R4 and the ground (referred to as output voltage detection reference voltage) Vo1 to obtain the reference voltage Vr. make. In this circuit, the values of the voltage dividing resistors R5 and R6 are sufficiently larger than the internal impedances of the photocoupler PC1 and the shunt regulator IC1, and the current IK1 flowing through the current limiting resistor R4 is considered to be equal to the photodiode current IK of the photocoupler PC1. You may

As described above, the shunt regulator IC1 has a voltage (reference voltage) at the reference terminal 5 thereof.
Since the current IK is varied so as to keep Vr equal to the built-in reference voltage Vs, the output voltage detection reference voltage Vo1 as the voltage across the voltage dividing resistors R5 and R6 is kept constant. On the other hand, this current IK is given by the following equation.

[0007]

## EQU1 ## IK = (Vo-Vo1) / R4 As a result, the photodiode current IK proportional to the change in the output voltage Vo is obtained. The photocoupler PC1 outputs the output voltage detection value V1 in response to the photodiode current IK while insulating the output voltage detection unit 2A and the output voltage adjustment unit 3.

Next, the output voltage adjusting section 3 includes resistors R1 to R3.
It comprises an error amplifier A1 and a PWM control circuit 4. Here, the resistors R1 and R2 give a set voltage Vs2 obtained by dividing the reference voltage Vs1 to the positive input terminal of the error amplifier A1,
3 is provided in series with the phototransistor of the photocoupler PC1 between the reference voltages Vs1 and supplies the negative input terminal of the error amplifier A1 with the output voltage detection value V1 as the voltage across the phototransistor.

The error amplifier A1 compares the set voltage Vs2 with the output voltage detection value V1 and outputs the error voltage of both. Further, the PWM control circuit 4 sets the error output of the amplifier A1 to P
WM control is performed to determine the on-duty Don of the switching transistor Tr1 so that the error voltage becomes zero. In this circuit, the output voltage Vo is Vo = Don
It is controlled by × Vin.

FIG. 3 is a characteristic diagram showing the relationship between the output voltage Vo and the detected voltage V1 when the resistance R4 in FIG. 2 is used as a parameter. In the circuit of FIG. 2, as shown in FIG. 3, the output voltage detection value V1 can be made proportional to the change ΔVo of the output voltage Vo, and the gain (ΔV1 / ΔVo) is adjusted by the resistor R4. It becomes possible.

[0011]

In the constant voltage control circuit of the switching power supply device described above, maintaining the gain of the control loop constant is advantageous in improving the stability of the output voltage against oscillation and the accuracy of the output voltage. is there. The gain of the output voltage detection circuit is represented by the equation (1) where the output voltage fluctuation is ΔVo and the output voltage detection value is ΔV1.

[0012]

(2) (ΔV1 / ΔVo) = − (R3 × CTR) / R4 (1) where CTR is the current transfer ratio of the photocoupler PC1 and is the collector of the phototransistor of the photocoupler PC1. Ratio of current IC and photodiode current IK (IC / IK)
This ratio can vary between the elements of the photocoupler, for example in the range of 50-600%.

According to the circuit configuration of FIG. 2, the output voltage error due to the variation of the CTR of the photocoupler is adjusted by changing the resistor R6 to change the reference voltage Vo1 for detecting the output voltage and adjusting IK1. It was Therefore, since only the CTR changes in the equation (1), the detection gain changes depending on the CTR of the photocoupler, and this gain cannot be kept constant.

Therefore, an object of the present invention is to provide an output voltage detection circuit of a switching power supply device which can solve this problem.

[0015]

In order to solve the above-mentioned problems, an output voltage detection circuit of the present invention is provided with an input DC power supply (V
In) is a switching power supply device that repeatedly outputs and disconnects (in, etc.) through a switching means (transistor Tr1, etc.) to obtain an output DC voltage (Vo, etc.) of a predetermined level, and a current (IK, etc.) that depends on the output DC voltage Photo-coupler (PC1 etc.) that flows to the photodiode of
The output voltage of the phototransistor of this photocoupler (V
1) and a set voltage (Vs2, etc.), and means (variable amplifier A1, PWM control circuit 4) for variably controlling the interrupting ratio of the switching means so as to match both voltages.
And the like), the output voltage detecting means for proportionalizing the current flowing through the photodiode to the difference voltage between the output DC voltage and a predetermined reference voltage (Vo1 or the like), and the output voltage detecting means. Connects the first resistor (R4 or the like), the photodiode, and the shunt regulator (IC1 or the like) in series in a sequentially operable polarity, and applies the output DC voltage across the series circuit, A second and a third resistor (R
5, R6, etc.), and the connection point between the second resistor and the third resistor is connected to the reference terminal of the shunt regulator, and the both ends of the second and third resistors are connected to each other. The first resistor is configured to generate a reference voltage, and the first resistor is configured to generate a current transfer ratio (CT) of the photocoupler.
It is assumed to be variable according to R).

[0016]

The error adjustment of the output voltage Vo due to the variation of the CTR of the photocoupler PC1 is performed by changing the resistor R4 and changing IK1. Therefore, CT
If R doubles, double R4 and increase IK1 to 1.
/ Adjust to 2 times. At this time, from the equation (1), the gain ΔV1 / ΔVo of the output voltage detection circuit is determined by the CTR of the photocoupler.
Can be made constant, and as a result, the stability of the output voltage against oscillation and the output voltage accuracy can be improved.

[0017]

FIG. 1 shows a circuit configuration of a switching power supply device as an embodiment of the present invention. In FIG. 2, R4 is replaced with a variable resistor and R6 is replaced with a fixed resistor as compared with FIG. 2, and accordingly, the output voltage detecting unit 2 is replaced.
A is replaced by 2B.

The output voltage detector 2B is composed of the following three circuits. A reference voltage circuit configured by a shunt regulator IC1 and resistors R5 and R6 to set a reference voltage Vo1 for detecting an output voltage. An error detection circuit that detects an error between the output voltages Vo and Vo1 and converts the error into a current IK1 by a resistor R4. Note that R4 is a variable resistance in order to absorb the variation in the CTR of the photocoupler PC1.

An output voltage transmission circuit that insulates IK1 from the output voltage detection unit 2B by a photocoupler PC1 and transmits it to the output voltage adjustment unit 3 to give it as an output voltage detection value V1 to the error amplifier A1. This circuit is composed of a photocoupler PC1 and a resistor R3. As a setting method of the circuit of FIG. 1, as shown in FIG. 3, when the output voltage is <1>, the output voltage detection value V1 and the set voltage Vs2 are made equal. <2> Each circuit constant is set so that the output voltage detection value V1 changes proportionally from 0 to Vs1 with respect to the output voltage fluctuation range Vo-ΔVo to Vo + ΔVo.

By the way, as a method of adjusting the variation of the current transfer ratio CTR of the photocoupler PC1, in this circuit, the reference voltage Vr, that is, the output voltage detecting reference voltage Vo1 by the function of the shunt regulator IC1 is used.
Is kept constant, the value of R4 × IK1 must be kept constant in order to keep the output voltage Vo (center value) constant. On the other hand, this value has the relationship of the following expression (2).

[0021]

## EQU3 ## R4 × IK1≈R4 × IK = R4 × IC / CTR (2) In this equation (2), the collector current of the phototransistor of the photocoupler PC1 The IC (center value) must be constant in order to keep the output voltage detection value V1 (center value) constant, so that (R4 / CTR) should be constant, that is, the CTR and R4 vary. It is necessary to adjust the variable resistor R4 so that This means that the gain ΔV1 / ΔVo of the output voltage detection circuit is kept constant, as is clear from the equation (1).

[0022]

According to the present invention, there is provided a switching power supply device which repeatedly outputs and disconnects the input DC power supply Vin through the transistor Tr1 as a switching means to obtain the output DC voltage Vo of a predetermined level. The output voltage V1 of the phototransistor PC1 which makes the dependent current IK flow in its own photodiode and the output voltage V1 of the phototransistor of this photocoupler PC1 are compared with the set voltage Vs2, and the interrupting ratio of the switching means is set so that these two voltages match. Error amplifiers A1 and PW as means for variably controlling
In a switching power supply device including an M control circuit 4 and the like, a resistor R4 as an output voltage detecting unit that makes a current IK flowing through the photodiode proportional to a difference voltage between the output DC voltage Vo and a predetermined reference voltage Vo1. R6 is provided with a shunt regulator IC1 and the like, and the error of the output DC voltage Vo due to the variation of the current transfer ratio CTR of the photocoupler PC1 is adjusted by varying the resistor R4 which produces a difference voltage by the current IK. In addition, it is possible to efficiently design and adjust a stable and stable constant voltage control circuit of the switching power supply device while keeping the gain of the output voltage detection circuit constant, thereby improving the reliability and economical efficiency of the switching power supply.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration as an embodiment of the present invention.

FIG. 2 is a conventional circuit diagram corresponding to FIG.

FIG. 3 is a characteristic diagram showing the relationship between the output voltage and the output voltage detection voltage of the circuits of FIGS. 1 and 2.

[Explanation of symbols]

 1 Power Supply Section 2B Output Voltage Detection Section 3 Output Voltage Adjustment Section 4 PWM Control Section 5 Reference Terminal 11 Output Voltage Forced Bias Circuit Tr1 Switching Transistor L1 Choke Coil C1 Smoothing Capacitor A1 Error Amplifier PC1 Photocoupler IC1 Shunt Regulator R1 Resistor R2 Resistor R3 Resistor R4 Variable resistance R5 resistance R6 resistance Vin Input voltage Vo Output voltage Vo1 Output voltage detection reference voltage Vr Reference voltage V1 Output voltage detection value Vs1 Reference voltage Vs2 Setting voltage IK Photo coupler light emitting diode current IC Photo coupler phototransistor current

Claims (1)

[Claims] 1. A switching power supply device for obtaining an output DC voltage of a predetermined level by repeatedly connecting and disconnecting an input DC power supply through a switching means, and a photocoupler for supplying a current dependent on the output DC voltage to its own photodiode. In a switching power supply device comprising means for comparing the output voltage of the phototransistor of the photocoupler with a set voltage and variably controlling the interruption ratio of the switching means so as to match the two voltages, a current is supplied to the photodiode. An output voltage detection means for making a current proportional to a difference voltage between the output DC voltage and a predetermined reference voltage is provided, and the output voltage detection means has a polarity capable of sequentially operating the first resistor, the photodiode, and the shunt regulator. After connecting them in series, apply the output DC voltage across the series circuit, A series circuit of a second resistor and a third resistor is connected in parallel to a series circuit of a diode and a shunt regulator, and a connection point between the second resistor and a third resistor is connected to a reference terminal of the shunt regulator, This second
And a third resistor configured to generate the reference voltage across the resistor, and the first resistor configured to be variable according to a current transfer ratio of the photocoupler. Device output voltage detection circuit.