JPS6318767B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a regulated power supply device whose purpose is to protect series controlled transistors by limiting current when the load becomes heavy.

A conventional stabilized power supply circuit is shown in FIG. In the figure, 1 is a DC power supply, 2 is a power switch, 3 is an error detection transistor that detects fluctuations in the output voltage across the load 16, 4 is a drive transistor that controls the series control transistor 5, and 6 is the error detection transistor 3. Reference voltage generating element (Zener diode) connected to the emitter, 7 is a bleeder resistor, 8 is a resistor connected to the collector of the drive transistor 4, 9 and 10 are voltage dividing resistors for detecting the output voltage, 11 and 12 denotes a voltage dividing resistor for making the error detection transistor 3 conductive when the power switch 2 is closed, and 13 and 14 denote diodes. Also, 15 is a circuit load, 16 and 18 are capacitors,
17 indicates resistance.

Its operation is such that when switch 2 is closed, the voltage divided by resistors 11 and 12 passes through diode 13 and is applied to the base of error detection transistor 3, forward biasing between the base and emitter of error detection transistor 3 and outputting a voltage to the collector. Flow an electric current. The path is from the emitter to the base of the series control transistor 5, from the emitter to the base of the drive transistor 4, and from the collector of the error detection transistor 3.
As this current flows, the drive transistor 4 and the series control transistor 5 are also brought into conduction. After conduction of the series control transistor 5, a stabilizing voltage is applied across the load 16. On the other hand, the stabilized output voltage is divided by resistors 9 and 10 and applied to the base of error detection transistor 3. Since this voltage is normally higher than the voltage divided by the resistors 11 and 12, the diode 13 is reverse biased. Diode 14 is also normally reverse biased.

In the circuit shown in Figure 1, when the circuit load of 15 is lowered and the current is increased, the output voltage, that is, the capacitor 16
The voltage across the capacitor 16 decreases, and the base voltage of the error detection transistor 3 decreases by the proportion divided by the resistors 9 and 10. On the other hand, since the emitter of the error detection transistor 3 has a Zener diode 6, the output of the voltage across the capacitor 16 decreases. It will continue to influence you. That is, the forward voltage of the error detection transistor 3 becomes larger than before, the collector current of the error detection transistor 3 increases, and the collector current of the drive transistor 4, that is, the base current of the series control transistor 5 increases. By lowering the voltage between the collector and emitter of the capacitor 16, the output voltage of the capacitor 16 returns to its previous value and becomes constant.

When the load of 15 is further reduced, the current of resistor 8 increases, the collector-emitter voltage of drive transistor 4 becomes saturated near zero, the voltage drop of series control transistor 5 increases, and the output voltage finally decreases. . In this state, there is no function as a stabilized power supply, and the power loss of the series control transistor 5 reaches 4 to 5 times that of normal use, causing the series control transistor 5 to be destroyed by heat generation and surrounding components to be damaged by the heat. There is a danger of In addition, if the capacitor 16 at the output end is short-circuited in Figure 1, the resistor 11
The voltage divided by and 12 is further applied to the diode 13.
The voltage is divided by the diode 14 and applied to the base of the error detection transistor 3, but the forward voltage of the diode 14 is equal to the diode forward voltage between the base and emitter of the error detection transistor 3, and the emitter is connected to the resistor 7 and the Zener diode. 6 are connected in parallel, the error detection transistor 3
The collector current does not flow, and the drive transistor 4 and series control transistor 5 are also cut off, and the power loss of the series control transistor 5 becomes close to zero, so there is no need to consider thermal problems.

However, as mentioned before, when the output end cannot be completely shorted with a low resistance, for example, the capacitor 18 of the decoupling circuit 17 and 18 is shorted, the collector to emitter of the drive transistor 4 is saturated and the series control transistor 5 is As the voltage drop increases, the output voltage decreases and the error detection transistor 3
Although the base potential of the error detection transistor 3 drops, the diode 13 becomes forward biased from a certain voltage and supplies the voltage divided by the resistors 11 and 12 to the base of the error detection transistor 3. In other words, the bias from the diode 13 also biases the base of the drive transistor 4, and the series control transistor 5 does not reach the cutoff state, and the output voltage drops, but the state in which current can flow through the low resistance 17 is maintained and the series control The power dissipation in transistor 5 becomes very large, causing destruction and thermal problems to surrounding components.

The present invention eliminates the above-mentioned conventional drawbacks, and one embodiment thereof is shown in FIG. In the figure, the same parts as in FIG. 1 are given the same numbers. The difference from Figure 1 is that the 150mW class low power transistor 1
The collector of 9 is connected to the midpoint between resistors 11 and 12, the emitter is connected to ground, and the base is connected to the midpoint of resistors 20 and 21 for biasing. A capacitor 22 is connected between the base of the transistor 19 and the ground to provide a time constant until the base potential becomes constant after the power switch 2 is closed.

The operation starts after the power switch 2 is closed, in the same way as shown in Fig. 1 under normal conditions, and after the output voltage becomes constant, the resistor 20 and capacitor 22
determine the constant. Once started, resistors 11 and 12
Since the midpoint is shot between the collector and emitter of the transistor 19, the state is the same as if the diode 13 were not present. Now, if the capacitor 16 at the output end is short-circuited, the potential between the base and emitter of the error detection transistor 3 becomes zero, no collector current flows, and the collector currents of the drive transistor 4 and series control transistor 5 also become small and close to zero. The power loss of the series control transistor 5 is close to zero and poses no problem. Furthermore, if the capacitor 18 on the output side of the low resistance 17 is short-circuited, the collector to emitter of the drive transistor 4 will be saturated, the voltage drop of the series control transistor 5 will increase, the output voltage will drop, and the base voltage of the series control transistor 3 will further decrease. As a result, the collector current of the error detection transistor 3 is reduced, and the emitter current of the series control transistor 5, that is, the base current of the series control transistor 5 is reduced. The decrease in the base current of the series control transistor 5 is a feedback loop that causes the output voltage of the series control transistor 5 to decrease, and the output current is much smaller than that shown in Fig. 1, although the voltage drop of the series control transistor 5 is large. Since the collector current is small, the power loss is small, and the series control transistor 5 will not be thermally destroyed, and the heat will not adversely affect peripheral components. In addition, the area of the heat sink of the series control transistor 5 can be reduced, and the diode 1
Since 4 can also be deleted, the practical effect is extremely large.

As explained above, according to the present invention, even if the output end is short-circuited and a large current flows, the series control transistor can be effectively protected, and other peripheral components will not be adversely affected. Furthermore, the shape of the heat sink for the series control transistors can be made smaller, and its practical value is extremely great.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional power supply device, and FIG. 2 is a circuit diagram of a power supply device according to an embodiment of the present invention. 1...DC power supply, 2...Power switch, 3,
4, 5...Transistor, 6...Reference voltage generating element, 13...Diode, 7, 8, 9, 10, 1
1, 12, 17... Resistor, 16, 18... Capacitor, 15... Circuit load, 19... Transistor, 20, 21... Resistor, 22... Capacitor.

Claims (1)

[Claims] 1 A series control transistor inserted in series between a DC power source and a load, an error detection transistor that detects an error based on fluctuations in the stabilized output voltage, and amplifying this error voltage to determine the conductivity of the series control transistor. A reference voltage generating element is connected to the emitter of the error detection transistor, and the error detection is performed through a first resistor and a forward diode that divides the DC voltage on the output side of the power switch. A starting voltage is supplied to the base of the transistor, and a fourth switching transistor is provided.
Connect the emitter of the switching transistor to ground, the collector to the voltage dividing point of the output voltage on the output side of the power switch, and the base to the point where the output voltage on the output side of the power switch is divided by a second resistor. A power supply device characterized in that a capacitor is connected between the base of the fourth transistor and ground.