JPS59132013A - Constant voltage device - Google Patents

Abstract

PURPOSE:To improve the overcurrent drooping characteristic and to make the device small-sized and low-cost by adding an overcurrent protecting circuit where the second reference voltage set to a value lower than the first reference voltage and an input voltage are compared with each other in the second voltage detecting circuit. CONSTITUTION:An input DC voltage IN passes smoothing circuits 1-3 and is subjected to switching control in transistors TRs 4 and 5 and passes a transformer 6 to output an AC voltage. This AC voltage passes a saturable reactor 9, etc. and passes a rectifying circuit 10 and a smoothing circuits 11-13 and is outputted OUT. The output of the rectifying circuit 10 is compared with the first reference voltage 15 in a comparator 14, and the control winding of the saturable reactor 9 is controlled by this error signal, and the fluctuation of the output OUT is so suppressed that it becomes a constant voltage. In this case, the overcurrent protecting circuit (surrounded with a broken line in the figure) is added where the input DC voltage IN is divided by a resistance 23 and the second reference voltage set to a value lower than the first reference voltage 15 and the output of the rectifying circuit 10 are compared with each other in a comparator 22, thus making the drooping start point of the overcurrent uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a constant voltage device that is used in a DC/DC converter or the like and is provided with an overcurrent protection circuit that improves overcurrent drooping characteristics.

Conventionally, there is a constant voltage circuit for a DC/DC converter that utilizes a saturable reactor, as shown in the circuit diagram of FIG. In this circuit, an input DC power supply (LN) is supplied to switching transistors 4 and 5 through a smoothing circuit consisting of a choke coil 1 and a capacitor 2.3, and is switched by these transistors 4.5 to become an AC power supply. This AC power is outputted via the transformer 6.

The output voltage from this transformer 6 is supplied to a choke coil 7, a resonant capacitor 8, and a saturable reactor 9.
It is connected to a bridge rectifier circuit 10 and rectified. The output of this rectifier circuit 10 is outputted (OUT) via a smoothing circuit (filter) consisting of a choke coil 11 and capacitors 12 and 13.

Here, the linear choke coil 7 determines the maximum current when the load is short-circuited, the resonance capacitor 8 determines the maximum current when the output voltage becomes uncontrollable, and the saturable reactor 9 determines the maximum current when the output voltage becomes uncontrollable. By supplying current to the windings, the DC magnetic field is controlled and the AC impedance is varied. Therefore, by changing the control current of the saturable reactor 9, the impedance ratio with the choke coil 7 can be changed, so even if the output voltage of the transformer 6 fluctuates, control can be performed to suppress this fluctuation.

Next, the output of this rectifier circuit 10 is supplied to a voltage detection circuit 14, the voltage output is compared with the set voltage of a reference voltage source 15, and the error voltage is amplified.
6. This voltage-current conversion circuit 16 operates together with a current detection resistor 17 and a control transistor 18 to convert the output voltage of the voltage detection circuit 14 into a current output.

When this current output is supplied to the saturable reactor 9, it controls its AC impedance as described above. Therefore, even if the output voltage of the transformer 6 changes, this constant voltage circuit changes the impedance ratio between the choke coil 2 and the saturable reactor 9 to keep the voltage across the saturable reactor 9 constant, and the rectifier circuit 10 The DC output voltage of can be kept constant.

On the other hand, as in the case of a load short circuit, the output current is
When the current value exceeds the value determined by the resonant capacitor 8 and the resonant capacitor 8, the control current that applies the DC magnetic field to the saturable reactor 9 hardly flows. The output current characteristics with respect to the output voltage at this time exhibit a drooping characteristic as shown in FIG. This short circuit current Ios is expressed by the following equation, where input voltage Ei, choke coil inductance L, and repetition frequency f are given.

f Therefore, as the input voltage Ei increases, the short circuit current IO3
There was a drawback that there were a lot of In other words, in a constant voltage circuit using a conventional saturable reactor, during overcurrent operation, the overcurrent drooping characteristic has a large difference between the current value at which drooping starts and the short-circuit current value, and the current value at which drooping starts is equal to the input voltage. The disadvantage is that it changes depending on the Therefore, when the load current is large, the short-circuit current is also large, and there is a problem that the loss in the diodes in the bridge rectifier 10 and the output choke coil 11 increases, and the temperature rises.At the same time, it is necessary to provide a large margin for the current capacity. was there.

The purpose of the present invention is to eliminate these drawbacks, control the saturable reactor to reduce the short circuit current when an overcurrent is flowing due to a load short circuit, etc., and improve the overcurrent droop characteristics. The object of the present invention is to provide a constant voltage device.

The configuration of the voltage regulator of the present invention includes a choke coil whose one end is connected to an AC input power supply, and a control current that is connected between the other end of the choke coil and the ground terminal and supplied to the control winding. A saturable reactor whose impedance is variable, a rectifier circuit that connects both ends of this saturable reactor as input and rectifies the AC input, and the output voltage of this rectifier circuit and a first reference voltage are compared and the difference between them is calculated. The first step is to amplify the voltage and control the output voltage to be a constant output voltage.
and a second reference voltage, the absolute value of which is lower than the first reference voltage, and the output voltage of the rectifier circuit is compared with a second reference voltage whose absolute value is lower than the first reference voltage, and the difference voltage is determined to have a polarity opposite to that of the first voltage detector. a second voltage detector that amplifies the error voltage as an error voltage, a gate circuit that outputs an output from one of the first and second voltage detectors, and drives the saturable reactor from the output of the gate circuit. and a drive circuit for forming the control current, and when an overcurrent flows through the load, the second voltage detection circuit is operated to shunt the overcurrent to the saturable reactor to reduce the current to the load. It is characterized by

The present invention will be explained in detail below with reference to the drawings.

FIG. 3 is a circuit diagram of a DC/DC converter including an embodiment of the present invention, and the circuit surrounded by the broken line is an additional overcurrent protection circuit. This overcurrent protection circuit has a second reference voltage that divides the input voltage using a variable resistor 23 and compares the output voltage of the rectifier circuit 10 with a second reference voltage that is set lower than the first reference voltage of the voltage source 15. Any output from the voltage detection circuit 22, this voltage detection circuit 22, and the first voltage detection circuit 14 is output and connected to the diode 20°2.
1 and a gate circuit consisting of an output resistor 24.

In this constant voltage circuit, when the output voltage increases or decreases with respect to the first reference voltage (for example, -, 5, 2V), the error voltage drives the first voltage detection circuit 14 to output the output voltage, as in the conventional case. The Yoshicho saturation reactor 9 is controlled so that the output voltage is constant.

On the other hand, when an overcurrent flows through the load and the output voltage drops (
For example, when it becomes -4■), the second voltage detection circuit 2
However, since this voltage detection circuit 22 works to generate a force of opposite polarity to the first voltage detection circuit 14, it works in the direction of causing current to flow through the saturable reactor 9, causing an output short-circuit current. lower.

The operation during this overcurrent will be explained in more detail.

First, when the load current starts to increase, the first voltage detection circuit 1
The output voltage of 4 decreases, and no current flows through the saturable reactor 9. When the load current increases further and exceeds the rated current, the output of the first voltage detection circuit 14 is grounded.
If it is not nearby, almost no current will flow through the saturable reactor 9. Then, due to the characteristics of the constant voltage circuit using this saturable reactor 9, the output voltage will drop a little 1.
This output voltage begins to drop and reaches the second reference voltage (-4V).
When the voltage is below, the second voltage detection circuit 22 operates and the control current flows through the saturable reactor 9 again.

When a control current flows through this saturable reactor 9, the alternating current impedance of this saturable reactor decreases, and current begins to flow through its winding.

When viewed from the choke coil 7 side, the AC impedance of the saturable reactor 9 and the impedance of the load are connected in parallel, so if the AC impedance of the saturable reactor decreases, the overcurrent that should flow to the load will be transferred to the saturable reactor side. The current flowing to the load can be reduced by that amount. As a result, the output voltage also drops rapidly.

The overcurrent characteristic of this embodiment has a fold-back shape with respect to the output current, as shown in the characteristic diagram of FIG.

Note that the current flowing to the saturable reactor 9 is limited to some extent by the current detection resistor 17, so the current flowing to the saturable reactor 9 does not increase at all, so the load current increases near the output voltage of O■. It becomes its expanding shape.

Further, since the second reference voltage of the second voltage detection circuit 22 is obtained by dividing the input voltage, the higher the input voltage, the earlier the operation start voltage of the second voltage detection circuit 22 starts. As shown in the output current of FIG. 4, the starting point of drooping can be made almost constant regardless of the input voltage.

Although this embodiment has been described using the input voltage as the second reference voltage, even when a constant voltage is used as the second reference voltage, the point at which the output current starts to droop varies somewhat depending on the input voltage. It is clear that this can reduce the overcurrent flowing into the load.

As explained above, according to the present invention, when the output of the power supply is in an overload state, the control current can be supplied to the saturable reactor by the second voltage detection circuit different from the one for constant voltage control. It is possible to improve current droop characteristics and realize a good overcurrent protection function. 1. As a result, the current flowing to the rectifier diode included in the bridge rectifier circuit 10 and the smoothing choke coil 11 can be reduced when the load is short-circuited. Therefore, the temperature rise of these parts can be suppressed and the reliability of the circuit can be improved, and the margin of current capacity given to these parts and the output side wiring can be reduced, making it possible to reduce the size and cost of power supplies. It is effective.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a constant voltage circuit using a conventional saturable reactor, Fig. 2 is a graph showing overcurrent drooping characteristics of the conventional circuit, and Fig. 3 is a DC/DC converter including an embodiment of the present invention. FIG. 4 is a graph showing the overcurrent characteristics of the circuit of FIG. 3. In the figure, 1.7...Choke coil, 2,3...
・Capacitor, 4, 5...Switching transistor, 6...Transformer, 8...Resonance capacitor, 9...Saturable reactor, 10.
...Bridge type rectifier circuit, 12.13...
・Output capacitor, 11... Output choke coil, 14.22... Voltage detection circuit, 15...
... Reference voltage source, 16... Voltage-current conversion circuit, 17... Current detection resistor, 18...
Control transistor, 20.21...Switching diode, 23...Variable resistor, 24...
・Output resistance.

Claims (1)

[Claims] 1) A choke coil whose one end is connected to an AC input power supply, and a control current that is connected between the other end of the choke coil and the ground terminal and is supplied to a control winding to reduce diimpedance. A variable saturable reactor, a rectifier circuit that connects both ends of this saturable reactor as input and rectifies the AC input, and compares the output voltage of this rectifier circuit with a first reference voltage and amplifies the difference voltage between them. a first voltage detector that controls the output voltage to be a constant output voltage; a second voltage detector that compares a second reference voltage lower than a reference voltage of the first voltage detector and amplifies the difference voltage as an error voltage having a polarity opposite to that of the first voltage detector; The gate circuit includes a gate circuit that outputs one of the second voltage detectors, and a drive circuit that forms the control current that drives the saturable reactor from the output of the gate circuit. A constant voltage device, characterized in that when the current flows, the second voltage detection circuit is operated to shunt the overcurrent to the saturable reactor, thereby reducing the current flowing to the load. 2) The constant voltage device according to claim 1, wherein the second reference voltage is the input voltage.