JPH04244778A - Overcurrent detecting circuit - Google Patents

Abstract

PURPOSE:To obviate the necessity of auxiliary power supply by utilizing the voltage of a DC output circuit in an overcurrent detecting circuit for the DC output circuit such as a switching power supply. CONSTITUTION:A DC output circuit is connected in parallel with a series circuit of a transmission element (e.g. a photocoupler), a resistor and a shunt regulator and a series circuit of a resistor and a constant voltage diode provided with a parallel circuit of two series resistors. The joint of two resistors constituting the parallel resistors of the constant voltage diode is connected with the reference terminal of the shunt regulator and a current detecting resistor is inserted in series with the DC output circuit between the shunt regulator and the anode terminal of the constant voltage diode.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detection circuit provided in a DC output circuit such as a switching power supply.

0002

[Prior Art] Overcurrent detection in a DC output circuit of an isolated switching power supply is performed by providing an overcurrent detection circuit consisting of a current detection resistor, an error amplifier, a reference voltage source, and a plurality of resistors in the DC output circuit. A transfer element (for example, a photocoupler) was activated to detect overcurrent. FIG. 2 is a block diagram of an overcurrent detection circuit in a DC output circuit based on the prior art. In FIG. 2, the non-inverting input terminal of the error amplifier 14 is connected to the reference voltage source VREF via a resistor 16.
13 is connected, and the power supply side terminal of the current detection resistor 4 connected in series to the DC output circuit is also connected via the resistor 17. Further, the inverting input terminal of the error amplifier 14 is connected to the negative terminal of the reference voltage source VREF 13 and the load side terminal of the current detection resistor 4 via a resistor 15. When the DC output circuit current is I1 and the resistance value of the current detection resistor 4 is R1, the voltage drop across the current detection resistor is I1 R1. Therefore, when the DC output circuit current I1 increases, the voltage drop I1 R1
becomes large and switches the output signal of the error amplifier 14 from "H" to "L", so the photocoupler 3 is turned on and P
The output current is limited by narrowing the output pulse width of the WM control circuit 11.

0003

By the way, as is clear from FIG. 2, the error amplifier 14 not only requires a driving power source, but also a reference voltage source 13. Therefore, an auxiliary power supply must be provided, which complicates the circuit configuration. The present invention aims to provide an overcurrent detection circuit that does not require an auxiliary power source by utilizing a DC output circuit voltage.

0004

[Means for Solving the Problems] In order to achieve the above-mentioned object, an overcurrent detection circuit according to the present invention includes a series circuit consisting of a transfer element (for example, a photocoupler), a resistor, and a shunt regulator in a DC output circuit in a switching power supply circuit. In addition, a series circuit of a constant voltage diode and a resistor each having a parallel circuit consisting of two series resistors is connected in parallel,
Further, a connecting point of two resistors constituting the parallel resistance of the voltage regulator diode is connected to a reference terminal of the shunt regulator, and a current is detected in a DC output circuit between the anode terminals of the shunt regulator and the voltage regulator diode. A resistor is inserted.

[0005]

[Operation] The voltage divided by the parallel resistance provided in the constant voltage diode is supplied to the reference terminal of the shunt regulator. Since the current detection resistor is connected in series with the DC output circuit, when the circuit current changes, the voltage drop across the current detection resistor also changes in proportion to the circuit current. Since the voltage drop caused by this current detection resistor is added to the voltage applied to the reference terminal of the shunt regulator, the combined applied voltage to the reference terminal of the shunt regulator changes depending on the circuit current. When this voltage becomes larger than the reference voltage of the shunt regulator, the shunt regulator is activated, so the photocoupler is also turned on and PWM
The switching element is controlled via the control circuit.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of an overcurrent detection circuit according to the present invention. The main components of the isolated switching power supply, such as the DC output circuit 10, PWM control circuit 11, photocoupler 3, and current detection resistor 4, are the same as those in the conventional overcurrent detection circuit shown in FIG. 2, so their explanation will be omitted. .

[0007] The constant voltage diode 2 includes a parallel resistor consisting of two series resistors 7 and 8, and is connected in parallel to the DC output circuit via a resistor 6. Further, a series circuit consisting of a transfer element 3, a resistor 5, and a shunt regulator 1 is also connected in parallel to the DC output circuit, and a reference terminal of the shunt regulator 1 is connected to a connection point between the resistors 7 and 8. The current detection resistor 4 is inserted between the anode terminals of the shunt regulator 1 and the constant voltage diode 2, and is also connected in series to the DC output circuit. Therefore, the voltage drop I1 R1 in the current detection resistor also changes in proportion to the DC output circuit current I1, but this voltage drop is added to the voltage divided by the parallel resistance of the voltage regulator diode 2, so that the reference terminal of the shunt regulator 1 The voltage applied to will vary depending on the DC output circuit current I1. When the added voltage becomes larger than the reference voltage at the reference terminal of the shunt regulator 1, the shunt regulator 1
operates, and a current I2 flows through the transfer element 3, resistor 5, and shunt regulator 1. As a result, the transmission element 3 is activated and controls the switching element via the PWM control circuit 11. As is clear from FIG. 1, the overcurrent detection circuit according to the present invention is supplied with the necessary power from the DC output circuit, and therefore does not require an auxiliary power supply, and therefore the circuit configuration is simplified. Note that the above-mentioned overcurrent detection circuit is not only applicable to the DC output circuit of a switching power supply, but can also be applied to all DC voltage sources such as input circuits by adding circuit components such as diodes and resistors. This is a current detection circuit.

[0008]

[Effects of the Invention] As described above, the overcurrent detection circuit according to the present invention includes a transfer element connected in parallel to a DC output circuit,
Since it is constructed from a shunt regulator, a constant voltage diode, a current detection resistor connected in series with the DC output circuit, etc., the above-mentioned circuit components do not require an auxiliary power source. Therefore, it is possible not only to avoid complication of the circuit configuration associated with the auxiliary power supply, but also to save power consumption, and to realize a compact overcurrent detection circuit.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an overcurrent detection circuit showing an embodiment according to the present invention.

FIG. 2 is a block diagram of an overcurrent detection circuit according to the prior art.

[Explanation of symbols]

1 Shunt regulator 2 Constant voltage diode 3 Transmission element (photocoupler) 4 Current detection resistor 5, 6, 7, 8 resistance 11 PWM control circuit

Claims (1)

[Claims] 1. In a switching power supply circuit that feeds back a DC output to a PWM control circuit that controls switching elements provided in the power supply circuit, a constant voltage regulator is connected to the DC output circuit through a resistor and divides the applied voltage by parallel resistors. a voltage diode, a shunt regulator connected via a resistor to a transfer element connected to the DC output circuit and inputting a divided voltage of the voltage regulator diode to a reference terminal, and an anode of each of the shunt regulator and the voltage regulator diode; A current detection resistor connected in series with the DC output circuit between terminals and adding the voltage drop due to the DC output circuit current to the reference terminal of the shunt regulator with the divided voltage of the constant voltage diode. Features an overcurrent detection circuit.