JPH06217533A - Output overcurrent detecting device for switching power unit - Google Patents

Abstract

PURPOSE:To improve the detecting accuracy of an output overcurrent detection circuit against the fluctuation of an input voltage by adding an input voltage correction circuit to the detection circuit. CONSTITUTION:By using an input voltage correction circuit composed of resistors R1, R2, and R3 and a Zener diode ZD1 provided in an output overcurrent detection circuit 2A, an input correcting voltage VR3 which increases as an input voltage Vin increases is generated from the resistor R3 and an error amplifier A1 adds the voltage VR3 to the detecting voltage of an input current Iin generated by a resistor R0. Another error amplifier A2 compares the sum voltage with an output current setting voltage Voc and, when the former is higher than the latter, the oscillation of a PWM control circuit 3 is stopped and a switching element (FET) Q1 is turned off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for detecting an overcurrent at its output in order to protect a switching power supply device. In the drawings below, the same reference numerals indicate the same or corresponding parts.

[0002]

2. Description of the Related Art FIG. 5 shows a structural example of a switching power supply device of this type. In the figure, 1 is a main power supply circuit, Vin is an input DC voltage obtained by rectifying an AC input (not shown), Iin is an input current, C1 is a smoothing capacitor on the input side, T1 is a transformer, and Q1 is a primary winding of a transformer T1. Input voltage Vin to the line
FE as a switching means for repetitively intermittently applying
T and 3 are PWM control circuits for controlling the gate of the FET Q1 by a pulse width modulation method.

D1 is a Schottky barrier diode for rectifying the secondary voltage of the transformer T1, D2 is a Schottky barrier diode as a commutation diode for the secondary current, and L1 is a choke coil for smoothing the secondary rectified voltage. , C2 are smoothing capacitors, Vout is an output DC voltage, and Iout is an output current. Reference numeral 4 is a detection circuit for the output voltage Vout, and this detection value is a photocoupler P.
It is given to the PWM control circuit 3 via C1 and the control circuit 3
Is the O of FET Q1 so that the output voltage Vout is constant.
Control the N / OFF ratio.

2 is an output overcurrent detection circuit provided on the primary side. As an output current detection circuit, it is easy to directly detect the output current Iout with a low resistance, but it is not general as a switching power supply due to the following reasons. Considering to simplify the auxiliary power supply of the control unit, it is better to place the control unit on the primary side (input side). Therefore, if the current detection unit is placed on the secondary side, the current detection value must be transmitted to the control unit with insulation.

When the output capacitance becomes large, the detection by the low resistance becomes impossible due to the loss. Generally, the detection by the current transformer is performed, but there is no suitable detection point on the secondary side (output side). I can not use it. Therefore, in
It cannot support a large capacity power supply. Since the output current is Vin / Vout times as high as the input current, the low resistance detectable range is narrowed.

For this reason, the output overcurrent detection circuit on the primary side as shown in FIG. 5 is used. The output current detection circuit 2 on the primary side utilizes the fact that the input current is proportional to the output current as shown in the following equation (1), and thus the output overcurrent Iout
(Oc) is replaced with the input overcurrent Iin (oc) for detection.

[0007]

## EQU1 ## Iin [oc] = (1 / .eta.) (Vout / Vin) .Iout (oc) ... (1) where .eta .; power supply efficiency Vin; input voltage Vout; output voltage Conventional output overcurrent detection of FIG. The circuit 2 has an input current Iin
And a low resistance R0 for detecting the input current detection voltage, and an error amplifier A2 for comparing the input current detection voltage detected by the circuit with the output overcurrent setting voltage Voc.

The protection operation of the overcurrent detection circuit 2 is to stop the oscillation of the PWM control circuit 3 of the switching power supply by the output of the error amplifier A2 when the input current detection voltage exceeds the output overcurrent set value Voc. Stop the power output.

[0009]

The AC power supply on the input side of the switching power supply is rated input voltage [eg AC100].
V] is allowed to vary [Example: AC85V ~
121V]. In the conventional overcurrent detection circuit, when the AC input is at the rated voltage, the output overcurrent setting voltage Voc = R0 × Iin
If set as [oc], the output overcurrent Ioc will be constant when the AC input fluctuates from the rated input voltage lower limit value (AC85V) to the rated input voltage upper limit value (AC121V) within the allowable input voltage range. The input current detection voltage of
As shown in FIG. 6, R0 (Ioc + ΔIin) to R0 (Ioc
However, there is a problem in that the detection accuracy is poor with respect to the input voltage fluctuation, because the change of −ΔIin) occurs.

Therefore, an object of the present invention is to provide an output overcurrent detection device for a switching power supply device that can solve this problem.

[0011]

In order to solve the above-mentioned problems, an output overcurrent detecting device according to a first aspect of the present invention uses a voltage (Vin) of an input DC power source which fluctuates within a predetermined range (by a PWM control circuit 3 or the like). Controlled) switching means (FET Q1
Is repeatedly applied intermittently to the primary winding of the transformer (T1 or the like), and the voltage generated in the secondary winding of the transformer at this time is rectified (via the diode D1 or the like) (choke coil L1, smoothed). In a switching power supply device that generates an output DC power supply (of a voltage Vout) that is smoothed (via a capacitor C2 or the like) and stabilized (via an output voltage detection circuit 4 or the like), a current (Iin) input from the input DC power supply. Input current detection means (resistor R0, current transformer CT20 for detecting and outputting an input current signal proportional to
Etc.) and the current output from the output DC power supply (Io
ut) corresponding to a predetermined overcurrent value (Iout (oc)), and a value of an input current signal corresponding to the value of the input current corresponding to the voltage of the fluctuating input DC power supply, and a predetermined reference signal value. Correction signal output means for outputting a signal (input correction voltage VR3, etc.) of a difference from (output overcurrent setting voltage Voc, etc.), input current signal output by the input current detection means, and output by the correction signal output means A means (error amplifier A2 etc.) for turning off the switching means when it is determined that the added value (via the error amplifier A1 etc.) with the difference signal exceeds the reference signal value is provided.

Further, in the output overcurrent detection device of claim 2,
2. The output overcurrent detection device according to claim 1, wherein the correction signal output means divides the voltage of the input DC power supply into a predetermined ratio (R1, R2, etc.) and the divided voltage into a predetermined value. A Zener diode (ZD1, etc.) that clips with a Zener voltage is provided, and a signal proportional to the current (IZ, etc.) of this Zener diode (voltage of the resistor R3, etc.)
Is the signal of the difference.

[0013]

By adding an input voltage correction circuit to the conventional output overcurrent detection circuit, the detection accuracy for input voltage fluctuations is improved. Since the starting point and inclination of this correction voltage can be freely set, strict correction can be performed.

[0014]

1 shows a circuit of a switching power supply device as a first embodiment of the present invention. In the figure, the output overcurrent detection circuit is replaced with 2A as compared with FIG. The detection circuit 2A shows an embodiment in which a low resistance R0 is used for input current detection. This output overcurrent detection circuit 2A has the following 4
It consists of a circuit.

An input current detection circuit for setting the input current detection voltage by the resistor R0, and input correction voltages for the resistors R1 to R
3, an input voltage correction circuit set by the Zener diode ZD1, an addition circuit (composed of the resistors R4 to R7 and the error amplifier A1) for adding the input current detection voltage and the input correction voltage, the addition result and the output overcurrent set value Voc
Of the error amplifier A2.

By setting the sum of the desired input current detection voltage and the input correction voltage at the output overcurrent point [Iout [oc]] to the output overcurrent set value Voc by each of these circuits, the output When the overcurrent point is exceeded, the output of the power amplifier is stopped by stopping the oscillation of the PWM control circuit by the output of the error amplifier A2 of the comparison circuit. The detailed setting and operation of each circuit will be described below.

Input current detection circuit: As shown in FIG. 2, the input current detection voltage is set to V1 at the rated input lower limit value.
The resistor R0 is set so that [Voc> V1 = R0 × Iin [oc]]. Iin [oc] at this time is given by the following equation (2).

[0018]

[Formula 2] Iin [oc] = (1 / η) (Vout / (rated input voltage lower limit value)) · Iout (oc) (2) Further, the input current detection voltage V2 that should be at the rated input voltage upper limit value Is calculated by the following equation (3).

[0019]

[Equation 3] V2 = R0 × Iin ′ [oc] …… (3) However, Iin ′ [oc] = (1 / η) (Vout / (rated input voltage upper limit value)) · Iout (oc) …… (4 ) Therefore, the input current detection voltage that should be is Voc>V1> V
There is a relationship of 2, and it is in a proportional relationship with the input voltage as can be seen from the expression (1).

Input voltage correction circuit: The operation of the input voltage correction circuit will be described. The voltage used as the correction voltage is R3
Is a voltage VR3 generated across both ends of the
caused by z. Iz is Vz> R2 / [R1 + R2]
It does not flow under the condition of x Vin. Iz is Vz <R2
Under the condition of / (R1 + R2) × Vin, it is given by the following equation (5).

[0021]

## EQU4 ## Iz = (Vin-Vz) / R1 (5) where Vz; ZD1 Zener voltage Therefore, the correction voltage VR3 is obtained by the following equation (6).

[0022]

[Equation 5] VR3 = R3 × Iz = R3 × [Vin−Vz]
/ R1 (6) Next, a method of setting the correction voltage by the input voltage correction circuit will be described. First, as shown in FIG. 3, a correction voltage (Voc-) required at the rated input voltage lower limit value and the rated input voltage upper limit value is obtained.
Plot V1) and (Voc-V2), connect them, and find the starting point VA of the correction voltage on the extension. Vz =
Input voltage Vin satisfying R2 / (R1 + R2) × Vin
Corresponds to VA, the starting point voltage VA is given by the following equation (7).

[0023]

VA = (R1 + R2) / R2 × Vz (7) Therefore, VA can be determined by setting the resistors R1 and R2 and the Zener diode ZD1 so as to satisfy the equation (7). Next, the slope of the correction voltage is set. The required Iz is calculated at the lower limit of the rated input voltage, and the resistance R3 is set. Using the formula (6), VR3; (Voc-V1),
Vin; where R is the lower limit of the rated input voltage, the resistance R3 is given by the following equation (8).

[0024]

[Formula 7] R3 = (Voc-v1) × R1 / ((rated input voltage lower limit value) -Vz) (8) The resistors R1 to R3 and the Zener diode ZD1 are set by the above equations (7) and (8). By doing so, the necessary input correction voltage can be obtained. Since the addition circuit and the comparison circuit of, are well-known circuits, description thereof will be omitted.

Next, FIG. 4 shows a circuit of a switching power supply device as a second embodiment of the present invention, and 2B shows an output overcurrent detection circuit using a current transformer CT20 for input current detection. When the input current is small, it can be detected by the low resistance R0 described in FIG. 1. However, when the output is medium or large in capacity, the input current Iin becomes large. Therefore, the detection by the current transformer CT20 is necessary as shown in FIG. Become. D20 is a Schottky barrier diode that rectifies the secondary current of the current transformer CT20, C21 is a smoothing capacitor that smoothes this rectified output, and R23 is a load resistor. When the input current is detected by the current transformer, the input current detection voltage (voltage across the resistor R23 in this case) can be directly added to the input correction voltage VR3 as shown in FIG. Yes, the circuit is simple.

[0026]

In the switching power supply device for detecting the output overcurrent on the secondary side by the input current on the primary side, in order to make the detection level of the output overcurrent constant, the input current is changed according to the magnitude of the input voltage. However, in the present invention, a correction voltage that depends on the input voltage is added to the detection level of the input current that changes in this way to obtain a constant reference voltage. The voltage generated by the correction circuit and added with the correction voltage to the input current detection voltage is compared with the reference voltage, and when the added voltage exceeds the reference voltage, the operation of the switching power supply device is stopped. The input voltage correction circuit described above is capable of performing correction quite strictly because the starting point and slope of the correction voltage can be set freely. Therefore, the accuracy of the output overcurrent detection can be improved with respect to the input voltage fluctuation.

[Brief description of drawings]

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

FIG. 2 is a characteristic diagram showing the relationship between the input voltage and the input current detection voltage of FIG.

FIG. 3 is a characteristic diagram showing the relationship between the input voltage and the input correction voltage of FIG.

FIG. 4 is a circuit diagram showing a configuration as a second embodiment of the present invention.

FIG. 5: Conventional circuit diagram

FIG. 6 is a characteristic diagram showing the relationship between the desired input voltage and the input current detection voltage shown in FIG.

[Explanation of symbols]

 1 Main Power Supply Circuit 2A, 2B Output Overcurrent Detection Circuit 3 PWM Control Circuit 4 Output Voltage Detection Circuit Vin Input Voltage Vout Output Voltage Iin Input Current Iout Output Current C1, C2, C21 Smoothing Capacitor T1 Transformer Q1 FET D1, D2, D20 Shot Key barrier diode ZD1 Zener diode L1 Choke coil R0-R7, R23 Resistance A1, A2 Error amplifier CT20 Current transformer

Claims (2)

[Claims] 1. A voltage of an input DC power source which fluctuates within a predetermined range is repeatedly applied intermittently to a primary winding of a transformer through a switching means, and at this time, a voltage generated in a secondary winding of the transformer is rectified. In a switching power supply device that generates an output DC power supply that is smoothed and stabilized, an input current detection unit that detects and outputs an input current signal that is proportional to the current input from the input DC power supply, and is output from the output DC power supply. Outputs a signal corresponding to a predetermined overcurrent value of the current and a difference between a value of the input current signal corresponding to the value of the input current corresponding to the fluctuating input DC power supply voltage and a predetermined reference signal value. Correction signal output means, and the addition value of the input current signal output by the input current detection means and the difference signal output by the correction signal output means is determined to exceed the reference signal value. Output overcurrent detection device of the switching power supply apparatus characterized by comprising a means for turning off said switching means. 2. The output overcurrent detection device according to claim 1, wherein the correction signal output means divides the voltage of the input DC power supply into a predetermined ratio, and the divided voltage into a predetermined Zener. An output overcurrent detection device for a switching power supply device, comprising: a zener diode that clips with a voltage, wherein a signal proportional to the current of the zener diode is used as the difference signal.