JPH04295222A - Stabilized power supply circuit - Google Patents

Abstract

PURPOSE:To make possible to freely vary the level of current for detecting a stabilized power supply circuit having an overcurrent detecting resistor and to suppress production of heat. CONSTITUTION:The stabilized power supply circuit comprises a differential amplifier 10 for taking out voltage drop across an overcurrent detecting resistor 9, a constant current circuit 12 and an external resistor 14 connected in series between an input terminal 1 and GND 3. Furthermore, a comparator 11 compares the potential at a joint between the constant current circuit 12 and the external resistor 14 with the output potential of the differential amplifier 10 and turns a first output control transistor 13 ON/OFF.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stabilized power supply circuit, and more particularly to a stabilized power supply circuit equipped with an overcurrent protection circuit.

0002

[Prior Art] Conventionally, this type of stabilized power supply circuit is shown in FIG.
As shown, an overcurrent detection resistor 9 and an output control transistor 15 are connected between the emitter of the output stage transistor 5 and the output terminal 2. Here, when an overcurrent flows through the output stage transistor 5, the voltage drop across the overcurrent detection resistor 9 increases, and the output control transistor 15 turns on, reducing the base current of the output stage transistor 5.
It had a function of reducing the current flowing through the output stage transistor 5.

0003

[Problem to be solved by the invention] In this conventional stabilized power supply circuit, the current value at which the overcurrent protection circuit operates is determined by the value of the overcurrent detection resistor, and it cannot be set arbitrarily. . The operating current value of the overcurrent protection circuit of a conventional stabilized power supply circuit with an output current of 1A is approximately 2A. When an overcurrent flows due to an abnormality in the load applied to the output terminal, the overcurrent protection circuit does not operate up to 2A, and a large current flows through the stabilized power supply circuit, causing a problem in that a large amount of heat is generated.

0004

[Means for Solving the Problems] A stabilized power supply circuit according to the present invention includes an output transistor and a first resistor having a collector-emitter path connected in series between an input terminal and an output terminal, and a reference a connection point from which a voltage corresponding to the voltage between the voltage terminal and the output terminal is obtained; an amplifier whose input terminal is connected to a reference voltage source and whose output terminal is connected to the base of the output transistor; a voltage drop detection means for extracting the amount of voltage drop; a comparison voltage generation means connected between the input terminal and the reference voltage terminal;
switch means connected between the base of the output transistor and the reference voltage terminal; and comparison means for receiving the output of the voltage drop detection means and the output of the comparison voltage generation means and determining the conduction state of the switch means. It is equipped with

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be explained with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention. Figure 2 shows
FIG. 3 is a waveform diagram of voltage and current at various parts in an embodiment of the present invention. A resistor 14 is externally connected to the reference voltage setting terminal 4. External resistor 1
Assuming that the resistance value of the reference voltage setting terminal 4 is Rb, and the current value of the constant current circuit 12 is Ib, a constant voltage of Rb×Ib is generated at the reference voltage setting terminal 4. When the output current Ia flows through the output stage transistor 5, if the resistance value of the overcurrent detection resistor 9 is Ra, the voltage drop across the overcurrent detection resistor 9 is Ra×Ia, and this voltage is amplified by the differential amplifier 10. . If the gain of the differential amplifier 10 is 1, the output voltage of the differential amplifier 10 is Ra×I
It becomes a. Constant voltage Rb generated at reference voltage setting terminal 4
Ib and the output voltage Ra×Ia of the differential amplifier 10 are compared by a comparator 11, and the first output control transistor 13 is controlled by the output signal of the comparator 11.

As shown in FIG. 2 during operation of the overcurrent protection circuit, when an overcurrent Ia flows (see diagram (a)), the output voltage Ra×Ia of the differential amplifier 10 is generated at the reference voltage setting terminal 4. It becomes larger than the constant voltage Rb × Ib (Figure (b))
Then, as shown in the diagram (c), the output voltage of the comparator 11 becomes "H", the first output control transistor 13 turns on, and the output stage transistor 5 turns on as shown in the diagram (d). The base of is lowered to zero level, the output stage transistor 5 is turned off, and the output current is cut. In other words, the current flowing through the output stage transistor 5 is prevented from increasing beyond Ia.

In the present invention, assuming that the overcurrent detection resistor 9 is Ra, the current of the constant current circuit 12 is Ib, and the external resistor 14 is Rb, the current Ia for operating the overcurrent protection circuit is Ib/R.
It is expressed as a×Rb. Here, Ib and Ra are constants, and Ia can be arbitrarily set by the value Rb of the external resistor 13. During normal operation, the error amplifier 6 compares the reference voltage 7 and the voltage obtained by dividing the output voltage by the dividing resistor 8, and uses the error voltage to control the output stage transistor 5 to stabilize the output voltage. As shown during normal operation, the voltage Rb×Ib generated at the reference voltage terminal 4 is
0, the output signal of the comparator 11 is "L", and the first output control transistor 1
3 is in the off state.

The operating current Ia of the overcurrent protection circuit of a conventional stabilized power supply circuit with an output current of 1 A class is about 2 A, and this current value cannot be changed. for example,
In the present invention, it is assumed that the value of the external resistor 13 is adjusted and the current Ia at which the overcurrent protection circuit operates is set to 1.5A. The loss of the stabilized power supply circuit is determined by the product of the voltage difference between the input terminal 1 and the output terminal 2 and the output current. Therefore, if the voltage difference between the input terminal 1 and the output terminal 2 is 1V, the loss during overcurrent is reduced by 0.5W compared to the conventional stabilized power supply circuit, and heat generation is reduced.

[0009]

[Effects of the Invention] As explained above, the present invention has a function in which the current value at which the overcurrent protection circuit operates can be arbitrarily set using a single external resistor. In this case, the overcurrent value can be suppressed to a small value, and the heat generation of the stabilized power supply circuit can be suppressed to a small level.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a stabilized power supply circuit showing one embodiment of the present invention.

[Figure 2] A waveform diagram of voltage and current at each part in Figure 1.

[Figure 3
] FIG. 1 is a circuit diagram of a conventional stabilized power supply circuit.

[Explanation of symbols]

1 Input terminal 2 Output terminal 3 GND 4 Reference voltage setting terminal 5 Output terminal transistor 6 Error amplifier 7 Reference voltage source 8 Dividing resistor 9 Overcurrent detection resistor 10 Differential amplifier 11 Comparator 12 Constant current circuit 13 For first output control transistor 14
external resistor

Claims (1)

[Claims] 1. In a stabilized power supply circuit, a current sensing resistor and a differential amplifier that detects a voltage drop due to this resistor;
The device is characterized by having a comparator that receives the output voltage of the differential amplifier and a reference voltage set externally as input signals, and an output control transistor that operates on and off in accordance with the output of the comparator. Stabilized power supply circuit.