JP2518478Y2 - Overcurrent protection circuit for stabilized power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an overcurrent protection circuit for a stabilized power supply, and more particularly to fluctuations in its output voltage.

[Conventional technology]

As an overcurrent protection circuit for a stabilized power supply having a simple structure, there is one shown in FIG. In the figure, T1 is the main transistor of the stabilized power supply, T2 is the protection transistor, and Rb, Rc.
Is a resistor and Dz is a zener diode.

The overcurrent protection circuit of the conventional stabilized power supply is configured as described above, the output voltage is regulated by the Zener diode Dz and the main transistor T1, and the output current is detected by the resistor Rc. When output current reaches a certain level, resistance
The voltage across Rc exceeds the operation start voltage of the protection transistor T2, and the protection transistor T2 starts to conduct. When the protection transistor T2 starts to conduct, the voltage between the emitter and the base of the main transistor T1 is regulated, so that the collector current of the main transistor T1, that is, the output current is suppressed to a predetermined magnitude.

[Problems to be solved by the invention]

In the conventional stabilized power supply as described above, the output voltage fluctuates due to the voltage drop due to the resistor Rc as the current detection circuit. For example, set the operation start voltage of the protection transistor T2 to 0.6
(V), the voltage across the resistor Rc is 0-0.6 (V)
, And this change appears as a change in the output voltage.

On the other hand, in order to reduce such fluctuations in the output voltage, there is also a constant voltage power source that detects the output voltage and performs feedback control as shown in FIG. However, such a constant voltage power supply requires a feedback control circuit composed of the resistors Rd, Re, Rf and the transistor T3, resulting in a complicated structure and a large number of parts.

The present invention has been made to solve such a problem, and an object thereof is to obtain an overcurrent protection circuit for a stabilized power supply with a simple configuration and a reduced fluctuation in output voltage.

[Means for solving problems]

An overcurrent protection circuit for a stabilized power supply according to the present invention connects a collector and a base of a protection transistor that controls the main transistor between an emitter and a base of a main transistor that form the stabilized power supply, and detects a current. A current detection circuit consisting of a series circuit of a resistor for protection and a diode is connected in series to the output side of the main transistor and between the emitter and base of the protection transistor.

[Action]

In the present invention, when the voltage across the current detection circuit reaches the operation start voltage of the protection transistor, it functions to limit the output current of the main transistor. At this time, the current detection circuit consists of a series circuit of a current detection resistor and a diode, and the voltage drop of the diode is almost constant regardless of the output current unless the output current becomes 0.
The voltage across the current detection circuit changes within the range of (diode voltage drop) ← → (protection transistor operation start voltage), and this change appears as the output voltage, but the fluctuation is small. ing.

〔Example〕

FIG. 1 is a circuit diagram of an overcurrent protection circuit for a stabilized power supply according to an embodiment of the present invention, in which the same reference numerals as those in FIG. 2 indicate the same or corresponding parts. In this embodiment, the Schottky diode Dc is connected in series with the resistor Rc in relation to that shown in FIG. This resistance
The series circuit of Rc and diode Dc corresponds to the current detection circuit of the present invention, and is connected in series with the transistor T1 and between the emitter and the base of the protection transistor T2.

In the overcurrent protection circuit of the stabilized power supply configured as described above, the base current flows into the main transistor T1 through the resistor Rb, the main transistor T1 operates, and the predetermined output voltage Vout appears at the output terminal. . That is, the output voltage Vout appearing at the output terminal is Vout = [Zener voltage Vz of Zener diode Dz]-[Base-emitter voltage Vbe of main transistor]-[Voltage drop Vcr due to resistor Rc]-[Voltage of Schottky diode Dc] Descent Vcd] ... (1) Here, the voltage drop Vcr due to the resistor Rc is almost ignored when the output current is small. When the output current increases and the sum of Vcr and Vcd, that is, the voltage drop of the current detection circuit reaches the operation start voltage of the protection transistor T2, the protection transistor T2 starts to conduct. When this protection transistor T2 starts to conduct, the main transistor
The voltage between the emitter and the base of T1 is regulated, so that the collector current of the main transistor T1, that is, the output current is suppressed to a predetermined value.

Here, if the output current of the main transistor T1 is 1 (A)
If you try to hold it down to a certain level, the voltage drop of the current detection circuit when the maximum current is flowing is 0.6 (V) and the voltage drop Vcd of the Schottky diode Dc is about 0.4 (V). The drop Vcr is 0.2 (V). The resistance Rc at this time is 0.2 (Ω). Therefore, the output voltage Vout fluctuation element represented by the equation (1) is only the voltage drop Vcr due to the resistance Rc, and the fluctuation range is almost 0 (V) ← → 0.2.
(V). On the other hand, in the conventional example shown in FIG. 2, since the Schottky diode Dc of this embodiment is not provided, the resistance Rc
Protection transistor T2 depending only on the voltage drop Vcr
The voltage between the base and the emitter is controlled, and therefore the voltage drop Vcr fluctuates within a range of approximately 0 (V) ← → 0.6 (V). Therefore, in this embodiment, compared with the conventional example shown in FIG. 2, the voltage fluctuation amount is reduced to 1/3, and it can be seen that a stable output voltage is obtained.

It should be noted that, although the example using the npn-type transistor has been shown in the above-mentioned embodiment, it goes without saying that the present invention is not limited to this and is similarly applied to a pnp-type transistor.

[Effect of device]

As described above, in the present invention, the current detection circuit is composed of the resistor and the diode, and the voltage fluctuation is limited to the voltage drop due to the resistance. Therefore, the fluctuation component appearing in the output voltage is smaller than the conventional one. The configuration is also extremely simple in that only a diode is inserted in the conventional one.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an overcurrent protection circuit for a stabilized power supply according to an embodiment of the present invention, and FIGS. 2 and 3 are circuit diagrams of conventional overcurrent protection circuits for a stabilized power supply. T1 ...... Main transistor T2 ...... Protection transistor Rc ...... Current detection resistor Dc ...... Schottky diode

Claims (1)

(57) [Scope of utility model registration request] 1. A collector and a base of a protection transistor for controlling the main transistor are respectively connected between an emitter and a base of a main transistor constituting a stabilized power supply, and a current detection resistor and a diode are connected in series. An overcurrent protection circuit for a stabilized power supply, characterized in that a current detection circuit composed of a circuit is connected in series to the output side of the main transistor and between the emitter and base of the protection transistor.