JPH0537287Y2 - - Google Patents

Description

[Detailed explanation of the idea] (b) Industrial application fields The present invention relates to a constant voltage power supply circuit.

(b) Prior art In constant voltage circuits used in video tape recorders (VTRs), etc., a resistor and a switching circuit are installed before the series transistor to prevent the collector loss of the series transistor from increasing even if the input voltage fluctuates. There is a configuration in which a parallel circuit of transistors is provided (for example, see Utility Model Application No. 55-24645).

In other words, as shown in Fig. 2, a resistor R1 is connected between the collector of the series transistor Q5 of the constant voltage circuit 1 and the input terminal 2 of the non-regulated DC voltage (commercial power supply rectified and smoothed). A parallel circuit of switching transistor Q1 is inserted.

The switching transistor Q1 is turned on and off by the transistor Q2, but when an input voltage is applied to the terminal 2, the switching transistor Q1 is in the on state, and the resistor R1
Both ends are shorted.

The input voltage to terminal 2 is Zener diode D1
Zener voltage, voltage determined by resistors R8 and R9
When the voltage becomes higher than Vt, transistors Q3 and Q4 are turned on, and transistor Q2 and switching transistor Q1 are turned off. Then, the resistor R1 is inserted before the series transistor Q5.

Therefore, a voltage drop occurs due to the resistor R1, and the collector loss in the series transistor Q5 is reduced.

However, in the configuration as shown in FIG. 2, even when the output of the constant voltage circuit 1 is shorted, the short circuit current is limited because of the presence of the resistor R1. In other words, there is a possibility that the protection circuit for detecting overcurrent may not operate properly when the output is shorted. However, if the value of resistor R1 is made small, the function of reducing collector loss will be impaired.

Furthermore, in a case where a plurality of constant voltage circuits are connected to the resistor R1, a predetermined output voltage may not be obtained depending on the situation. In other words, when a constant voltage circuit controlled by a power switch is connected to a constant voltage circuit that is always in operation, if there is a temporary increase in load current due to turning on the power switch, the voltage will drop due to resistor R1. is getting bigger,
There is a risk that the input voltage to the constant voltage circuit that is in the always-operating state may drop and the rated voltage may not be output.

(c) Problems that the invention aims to solve In other words, in conventional constant voltage circuits that take collector loss reduction into consideration, the protection circuit may not operate if the output is short-circuited, or if multiple constant voltage circuits When a constant voltage circuit is provided, there is a drawback that the load current of one constant voltage circuit increases and the rated output cannot be obtained from other constant voltage circuits. In other words, there is a possibility that an adverse effect may occur due to the resistor being provided at the front stage of the series transistor.

(d) Means for solving the problem The present invention includes a switch means that detects when the output voltage of the constant voltage circuit becomes lower than a predetermined value and short-circuits the resistor inserted at the front stage of the constant voltage circuit. It is equipped with a means to turn it on.

(E) Effect Therefore, this resistor was inserted because it detects an increase in load current due to a decrease in the output voltage of the constant voltage circuit and shorts both ends of the resistor to reduce the collector loss of the series transistor. The harmful effects caused by this can be eliminated.

(f) Examples Examples will be described below with reference to the drawings. FIG. 1 is a circuit block diagram of an embodiment.

In the figure, 2 is a terminal to which an unstable DC voltage obtained by rectifying and smoothing an AC commercial power source is applied, 1 is a constant voltage circuit equipped with a series transistor Q5, and 3 is an output voltage terminal. Also, direct transistor Q
6 may be connected to the second constant voltage circuit 4 (output terminal 5).

R1 is a resistor inserted between the input terminal 2 and the constant voltage circuit to reduce collector loss, and the collector and emitter of the switching transistor Q1 are connected to both ends, so that when turned on, the resistor R1 is short-circuited. It's getting old. The switching transistor Q1 is turned on and off by the first transistor Q.
2. That is, when a DC voltage is applied to the input terminal 2, the first transistor Q2 is turned on by the voltage dividing circuit 6 including the resistors R2 and R3, and therefore the switching transistor Q1 is in the on state. Therefore, both ends of the resistor R1 are short-circuited.

The on-off state of the first transistor Q2 is determined by a control circuit including a second transistor Q3, a second voltage dividing circuit 7, a second Zener diode D2, a third transistor Q4, a third voltage dividing circuit 8, and a first Zener diode D1. Consisting of 9. Resistance R10
The resistor R11 of the first Zener diode D1 is a resistor for limiting the current of the second Zener diode D2.

The voltage at the input terminal 2 is the Zener voltage VD1 of the first Zener diode D1, and the third transistor Q.
4. When the threshold value Vt determined by the voltage division ratio of the third voltage dividing circuit 8 is exceeded (=V _BE · R8 + R9 / R8 + VD1, V _BE is the voltage at which the third transistor turns on), the third voltage divider circuit 8
The transistor Q4 is turned on, and current flows through the second Zener diode D2 via the resistor R11. And point a is the second Zener diode D2
The second transistor Q3 is turned on.

Therefore, the first transistor Q2 and the switching transistor Q1 are both turned off, and the resistor R1 is inserted at the front stage of the constant voltage circuit.
At this time, if the load current is I _L , constant voltage circuit 1,
The input voltage V' at 4 is V'=V-R1·I _L (V is the input voltage at terminal 2). In other words, the loss of the series transistors Q5 and Q6 is shared by the resistor R1.

At this time, if Vt is set as shown below, the function of the constant voltage circuit will not be impaired.

Vt≧Vmin+R1・_IL where Vmin is the minimum input voltage of the constant voltage circuit.

A diode D3 is connected between the output terminal 3 of the constant voltage circuit 1 and point a as shown in the figure. Therefore, when the output voltage decreases, diode D3 becomes conductive, the voltage at point a decreases, and the second transistor Q
3 is off. Therefore, switching transistor Q1 is turned on and short-circuits resistor R1.
In other words, when the output voltage of the constant voltage circuit 1 decreases, the resistor R1 is short-circuited even if the input DC voltage is higher than Vt.

When the output of the constant voltage circuit 1 is short-circuited, a very large load current flows and the output voltage decreases. At this time, the diode D3 becomes conductive, so the resistor R1
Since both ends of the terminal 2 are short-circuited and the current flowing into the terminal 2 is not restricted, there is no possibility that a protection circuit such as a fuse provided on the input side will not operate.

Next, consider a case where a plurality of constant voltage circuits 1 and 4 are connected. The constant voltage circuit 4 is not linked to the power switch and always outputs voltage. And this voltage is applied to the timer microcontroller, tuner, VPS
It is assumed that the constant voltage circuit 1 used in the control circuit is linked to a power switch, and that a capacitive load is also connected to its output.

In such a case, when the power switch is turned on,
A large inrush current temporarily flows from the constant voltage circuit 1 to the capacitive load, and when the resistor R1 is inserted, the voltage drop due to this increases, and the voltage at point b becomes the minimum input voltage to the constant voltage circuit 4. There is a risk that it will fall below. At this time, the voltage at the output terminal 5 drops, causing the timer microcomputer and the like to malfunction.

However, in the configuration shown in Figure 1, when the voltage at the output terminal 3 decreases due to an inrush current, the diode D3 becomes conductive and the resistor R1 is no longer inserted.
1 can be prevented, and there is no possibility that the output voltage of the constant voltage circuit 4 will drop.

Zener voltage of second Zener diode D2
The setting of VD2 determines the conduction of diode D3. When checking the output voltages of a plurality of constant voltage circuits, it is necessary to adjust to the lowest output voltage. Also, if it is set too low, the timing at which resistor R1 is reinserted will be earlier.
It is desirable to set it to the same level as the minimum output voltage.

(g) Effects of the invention As described above, the invention is practical because it can eliminate the adverse effects caused by the resistor inserted for collector loss reduction.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment, and FIG. 2 is a circuit diagram of a conventional example. Q1...Switching transistor, R1...
Collector loss reduction resistor, D2... Zener diode, D3... diode.

Claims (1)

[Scope of utility model registration request] In a constant voltage power supply circuit in which a resistor for reducing collector loss is provided between an unregulated voltage input and a series transistor, a switch means for shorting the resistor is turned on when it is detected that the output voltage has fallen below a predetermined value. A constant voltage power supply circuit characterized by comprising means for setting the state.