JPH0261218B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an overcurrent protection circuit for a DC power supply that limits load current during overload in order to prevent damage to the power supply due to abnormal current.

(Prior Art) Conventionally, as this type of overcurrent protection circuit, for example, a constant current protection circuit as shown in FIG. 4 having constant current drooping characteristics as shown in FIG. 5, and a constant current protection circuit as shown in FIG. There is a wrap-around type protection circuit as shown in FIG. 6 which has the wrap-around drooping characteristic as shown in FIG.

As shown in FIG. 4, the constant current type protection circuit 2a of the DC power supply 1 consists of a load power supply I flowing through a load resistor RL connected to power supply output terminals B1 and B2, a reference power supply E, and resistors R1 to R3. The DC power supply 1 is controlled via the automatic control circuit 3 by the output of the error amplifier A that compares the bias current i flowing in the direction of the dashed arrow in the current path, and the error amplifier A
is composed of an inverting amplifier, whose output terminal C
is at H level, the automatic control circuit 3 is configured to cause the DC power supply 1 to perform a current limiting operation.

The voltage e1 generated in the resistor R1 in the direction of the arrow in the figure by the bias current i is e1=i・R1−I・R1=R1(i−I), and the voltage e1 is generated in the resistor R2 in the opposite direction to the arrow in the figure. The voltage e2 is represented by e2=i.R2.

The voltage Vab between points a and b is Vab＝e1＋e2 =R1(i-I)+i・R2 =i(R1+R2)−I・R1……(1) It is indicated by.

As shown in equation (1), the constant current protection circuit 2
When the load current I is small, Vab is at a positive potential, so the output terminal C is at L level,
The DC power supply 1 does not perform source operation, and when the load current I increases and Vab becomes a negative potential, the output terminal C
becomes H level, and the DC power supply 1 is configured to perform current limiting operation.

In the above formula (1), i=E/(R1+R2+R3)...(2) It is indicated by.

Here, if the degree of amplification of the error amplifier A is very high, it can be approximately considered that the current limiting operation starts when the voltage Vab becomes zero, so (1), (2) From the formula, 0=E/R1+R2+R3(R1+R2)-I・R1, and the starting current value Is for current limiting operation can be expressed as Is=E/R1+R2+R3(R1+R2/R1)...(3).

That is, in the constant current type protection circuit 2a, as shown in FIG. Current droop characteristics can be obtained.

In addition, the wrap-around protection circuit 2b shown in FIG.
The constant current type protection circuit 2a uses the output voltage Vout instead of the reference power source E of the constant current type protection circuit 2a, and the other configurations are the same as the constant current type protection circuit 2a.

In this wrap-around type protection circuit 2b, the starting current value Is for current limiting operation is expressed as Is=Vout/R1+R2+R3 (R1+R2/R1)...(4) As shown in Figure 7, the point of current value Is is s
When the current limiting operation is started at , a winding type drooping characteristic is obtained in which the output voltage Vout decreases and the load current I also decreases.

However, the above conventional constant current type protection circuit 2
Although circuit a has the advantage of being suitable for storage battery chargers, pulse loads, power supplies for parallel operation, etc., it has the disadvantage of being stressful, and the conventional wrap-around protection circuit 2b described above Although the charger has the feature that the current I is small and the stress on the DC power supply 1 is small, the charger
It has the disadvantage of not being suitable for pulse loads, etc.
Conventionally, there was a problem in that the two types of protection circuits 2a and 2b described above had to be prepared as protection circuits for the DC power supply, and they had to be used depending on the purpose of use.

(Problems to be Solved by the Invention) The present invention has been made by focusing on such conventional problems, and solves the above problems by providing both constant current drooping characteristics and entrainment drooping characteristics. The purpose is to solve the problem.

(Means for Solving the Problems) The gist of the present invention for achieving the above object is as follows:
In a DC power supply whose load current is controlled by the output of an error amplifier that compares a load current and a bias current, a first bias current based on a reference voltage of a reference power supply is used as a current path through which the bias current flows. a first current path through which the voltage flows, and a second current path through which a second bias current based on the output voltage of the power supply output terminal and the reference voltage flows; A switching element is provided in the second current path, which is non-conductive while the voltage is lower than the output voltage, and conductive when the voltage across the terminal is higher than the output voltage, thereby closing the second current path. The present invention resides in an overcurrent protection circuit for a DC power supply, which is characterized by:

(Function) In the above configuration, while the voltage across the ends is smaller than the output voltage, only the first bias current flows through the first current path, thereby obtaining a constant current drooping characteristic. When the voltage becomes larger than the output voltage, a second bias current also flows through the second current path, thereby providing a winding droop characteristic.

(Example) Hereinafter, each example of the present invention will be described based on the drawings. Note that the same parts as in the conventional example described above are given the same reference numerals and redundant explanations will be omitted.

1 and 2 show a first embodiment of the invention.

As shown in FIG. 1, in the overcurrent protection circuit 2 of the DC power supply 1, a first bias current i1 based on the reference voltage of the reference power supply E flows as a current path through which the bias current of the error amplifier A flows. First current path 4
and the reference voltage and power supply output terminals B1 and B2.
A second bias current i based on the output voltage Vout of
A second current path 5 through which 2 flows is provided.

The first current path 4 is formed by resistors R1 to R4 and a reference power source E.

The second current path 5 includes a reference power source E, resistors R4, R
5. It is formed by a block diode D as a switching element and power output terminals B1 and B2. This block diode D is connected to the reference power supply E
and the voltage across resistor R4 (this is resistor R3, R
It is non-conductive while Vd (hereinafter referred to as the voltage at both ends) is smaller than the output voltage Vout, and becomes conductive when the voltage at both ends Vd becomes larger than the output voltage Vout.

The operation will be explained below.

(a) Only the first bias current i1 flows through the first current path 4 while the voltage Vd at both ends is smaller than the output voltage Vout.

At this time, the voltage Vab between points a and b is expressed as Vab=i1(R1+R2)-I.R1 (5) from the above equation (1). Here, the first bias current i
1 is expressed as i1=E/(R1+R2+R3+R4)...(6).

Also, the voltage at both ends Vd is Vd＝R1＋R2＋R3／R1＋R2＋R3＋R4・E……(7) It is indicated by.

As shown in equation (5) above, overcurrent protection circuit 2
When the load current I is small, Vab is at a positive potential, so the output terminal C of the error amplifier A
is at L level, DC power supply 1 does not perform current limiting operation, and when the load current I increases and Vab becomes a negative potential, the output terminal C becomes H level, and DC power supply 1 performs current limiting operation. It is composed of

The starting current value Is for current-limiting operation is given by equation (3) above. Is=Vd/R1+R2+R3 (R1+R2/R1)...(8) It can be expressed as

That is, in FIG. 2, the point s of the current value Is
The current limiting operation is started at , and if the voltage Vd between both ends is constant, constant current drooping characteristic 6 is obtained under the condition of Vd<Vout.

(b) Since the output voltage Vout is expressed as Vout=I・RL, if the load resistance RL is gradually decreased,
The output voltage Vout also gradually decreases. When the voltage Vd at both ends becomes larger than the output voltage Vout, the block diode D becomes conductive, so the second current path 5 is closed, and the positive terminal of the reference power supply E, the resistors R4 and R5, the block diode D, and the power supply output terminal B1 are connected. , the load resistor RL, the power supply output terminal B2, and the negative terminal of the reference power supply E, the second bias current i2 flows.

At this time, if the forward voltage drop of block diode D is ignored, the voltage across both ends Vd is Vd=Vout −R5 {Vout・(R4+RX)−RX・E/R4・RX+R4・R5+
RX・R5} ... is shown as (9). However, RX=R1+R2+R3.

As shown in equations (7) and (8) above, under the condition of Vd>Vout, the winding operation starts at point t in Figure 2, the load resistance RL decreases, the output voltage Vout decreases, and the load An entrainment droop characteristic 7 in which the current I also decreases is obtained.

Further, by selecting the constants of the resistors R1 to R5, it is possible to arbitrarily set the load current I when the output is short-circuited, as shown by the broken line in FIG.

Next, a second embodiment of the present invention will be described based on FIG.

This second embodiment has a resistor R in addition to the first embodiment.
By setting the constants of the resistors R6 and R7, the starting point t of the winding operation in FIG. 2 can be easily set.

Here, the starting voltage Vt of the curling droop characteristic is
In the region of constant current drooping characteristic 6, the potential at point e in FIG. 3 is a potential point equal to the potential Vd at point d, and the following relationship holds true.

Vd/Vout=R7/R6+R7 (Effects of the Invention) According to the overcurrent protection circuit for a DC power supply according to the present invention, while the voltage Vd at both ends is smaller than the output voltage Vout, only the first bias current i1 A constant current drooping characteristic is obtained by flowing in the current path, and the voltage at both ends is
When Vd becomes larger than the output voltage Vout, the second bias current i2 also flows in the second current path, resulting in a drooping characteristic, so it can be used for storage battery chargers, pulse loads, power supplies for parallel operation, etc. At the same time, the load current I becomes smaller when the load is short-circuited, and the stress on the DC power supply becomes smaller.

[Brief explanation of the drawing]

1 and 2 show a first embodiment of the present invention, FIG. 1 is a circuit diagram, FIG. 2 is a voltage/current characteristic diagram, and FIG. 3 is a second embodiment of the present invention. The circuit diagram, Fig. 4 shows a conventional constant current type protection circuit, Fig. 5 shows its characteristic diagram, Fig. 6 shows a conventional winding type protection circuit, and Fig. 7 shows its characteristic diagram. be. 1...DC power supply, 2...Overcurrent protection circuit, 4...
...First current path, 5...Second current path, A...Error amplifier, B1, B2...Power output terminal, D...Block diode (switching element), R4...
Resistance, i1...first bias current, i2...second bias current, Vd...voltage across both ends, Vout...
Output voltage.

Claims (1)

[Claims] 1. In a DC power supply whose load current is controlled by the output of an error amplifier that compares a load current and a bias current, a first bias based on a reference voltage of a reference power supply is used as a current path through which the bias current flows. A first current path through which a current flows and a second current path through which a second bias current based on the output voltage of the power supply output terminal and the reference voltage flows; The second switching element is non-conductive while the voltage is lower than the output voltage, and conductive when the voltage across the terminal is higher than the output voltage to close the second current path.
An overcurrent protection circuit for a DC power supply characterized by being provided within the current path.