JPS5926185B2 - Voltage droop control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage droop control circuit that protects the power supply by lowering the output voltage when the load of a switching controlled constant voltage power supply is short-circuited or in a state close to short-circuited.

A switching control constant voltage power supply has a switching element such as a transistor mounted on it so that the output voltage is constant.
When the load is short-circuited or close to short-circuited, the output voltage drops to protect the switching elements and the like.

In that case, in a configuration in which the input voltage on the primary side of the transformer is turned on and off by a switching element, and the load is connected to the secondary side of the transformer via a rectifier circuit, the load current on the secondary side of the transformer is All that is required is to detect the voltage and control the output voltage to drop if the voltage exceeds the set value. However, it is necessary to connect a photocoupler or a transformer between the switching element control circuit and the load current detection section on the secondary side of the transformer to protect the switching control circuit with a low voltage circuit configuration. The disadvantage is that the equipment is expensive. Therefore, as shown in FIG. 1, it has been proposed to detect the current on the primary side of the transformer and control the output voltage drop.

In the figure, DC is a direct current power supply, Q is a transistor as a switching element, CONT is a control circuit, and C1 to C
3 is a capacitor, R1 to R7 are resistors, OPAI, 0PA
2 is an operational amplifier, T is a transformer, D1 is a rectifier diode, VDET is a voltage detector, RL is a load, Vrl, V
r2 is a reference voltage. Transistor Q is control circuit CO
The on period is controlled by the NT, and the current flowing to the primary side of the transformer T is switched, rectified by the diode on the secondary side of the transformer T, smoothed by the capacitor C2, and DC is supplied to the load RL.

The voltage applied to this load RL is detected by the voltage detector VDET, compared with the reference voltage Vr2 by the operational amplifier 0PA2, and the comparison output is applied to the control circuit CONT, which controls the transistor Q so that the output voltage is constant. Control takes place. Also, the current flowing to the primary side of the transformer Ti is detected by the resistor R1 connected to the emitter of the transistor Q, and is averaged by the resistors R2, R3 and the capacitor Cs, and the reference voltage Vrl is determined by the operational amplifier OPAI.
When it is detected that a current exceeding the set value is flowing, the output of the operational amplifier 0PA1 causes the control circuit CO
Transistor Q is controlled so that the constant voltage control function of NT is stopped and the output voltage drops.

The current from the DC power supply DC, that is, the input voltage 11n, and the resistance R
3, the relationship with the current detection voltage VD input to the operational amplifier 0PA1 becomes a linear relationship as shown in FIG. 2, and the output voltage V applied to the load RL.

As shown in FIG. 3, the relationship between the output voltage VO and the input voltage 11n is as shown in FIG. Each of the above-mentioned relationships does not change even if the voltage of the DC power supply DC fluctuates. Therefore, for example, if the voltage of the DC power supply DC rises above the specified value when the load RL becomes short-circuited, , input current 1 due to output voltage droop
Since 1n is constant regardless of the input voltage, the output power increases, which has the disadvantage of increasing the burden on the transistor Q, transformer T, etc. The present invention improves the conventional drawbacks as described above, and aims to keep the power constant when the voltage droops even if the DC input voltage fluctuates. Examples will be described in detail below. Fourth
The figure is a circuit diagram of an embodiment of the present invention, where the same reference numerals as in FIG. 1 indicate the same parts, and R8 is a resistor.

The voltage of the DC power supply DC is applied to the current detection circuit via this resistor R8. That is, a resistor R8 is connected between the positive electrode side of the DC power supply DC and the connection point between the resistors R2, R3 and the capacitor C3. As a result, the voltage component of the DC power supply DC is superimposed on the current detection voltage VD, so even if the current detection voltage VD indicates less than the set input current, if the voltage of the DC power supply DC rises, the operational amplifier 0
When the input voltage of PA1 rises and becomes equal to or higher than the reference voltage Vrl, the control circuit C is activated by the output of the operational amplifier 0PA1.
ONT is a transistor Q so as to drop the output voltage VO.
will be controlled. For example, as shown in Fig. 5, when the voltage of the DC power supply DC is the specified value V1, and the output voltage is dropped by the input current 11, the voltage increases and V2 (V2
>V1), the output voltage will drop due to the input current 1202×11), and conversely, the voltage will drop below the specified value and V3(
When V3<1), the output voltage drops with the input current 13 (13>11), and the power when the voltage drops becomes constant, making it possible to prevent the transistor Q and the like from being overloaded. Although the above-mentioned embodiment relates to a one-transistor energy storage type power supply circuit, it can also be applied to a constant voltage power supply circuit using switching control such as a two-transistor push-pull type or a step-down type using a transformer. be.

Alternatively, the input current can be detected on the collector side of the transistor Q. Further, although the case where a transistor is used as the switching element has been shown, a field effect transistor or the like may also be used. As explained above, the present invention detects the input current supplied to the primary side of the transformer with a current detection circuit consisting of resistors R1 to R3, capacitor C3, etc., and lowers the output voltage when the current exceeds a set value. When the load RL is short-circuited or in a state close to it, the DC power supply DC, switching element, transformer T, diode D, etc. are protected, and a voltage proportional to the voltage of the DC power supply DC is sent to the current detection circuit via the resistor R8 etc. It is possible to prevent overload of switching elements, etc. by applying the power in a superimposed manner to the output voltage, thereby keeping the power constant even when the output voltage droops due to voltage fluctuations of the DC power supply DC. .

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional switching control constant voltage power supply.
Figure 2 is a curve diagram for explaining the relationship between input current and current detection voltage, Figure 3 is a curve diagram for explaining the relationship between input current and output voltage,
FIG. 4 is a circuit diagram of an embodiment of the present invention, and FIG. 5 is a curve diagram for explaining the relationship between input current and output voltage according to the embodiment of the present invention. DC is a direct current power supply, RL is a load, CONT is a control circuit, Q
is a transistor, T is a transformer, D1 is a diode, R
1 to R8 are resistors, C1 to C3 are capacitors, VDET is a voltage detector, and 0PA1 and 0PA2 are operational amplifiers.

Claims (1)

[Claims] 1 The input current supplied from the DC power supply to the primary side of the transformer is switched by a switching element, the rectified output of the secondary side of the transformer is supplied to the load, and the output voltage applied to the load is kept constant. In a constant voltage power supply configured to control a switching element and drop the output voltage when the input current is equal to or higher than a set value, a voltage proportional to the voltage of the DC power supply is applied to a current detection circuit that detects the input current. A voltage droop control circuit characterized by having a configuration in which the voltage is applied in a superimposed manner.