JPH03293964A - Overcurrent protective method for power unit and overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To suppress the increase of output currents by narrowing the on pulse width of a switching element at the time of overcurrent occurrence and also lowering the reference voltage for overcurrents in a DC-DC converter, or the like. CONSTITUTION:The overcurrent detecting circuit 15 of a control circuit 14 is composed of an operating circuit 15B, etc., and compares the output of a current peak detecting circuit 16 with the reference voltage for overcurrents. If the current of a switching element increases by the occurrence of an output short accident, or the like, a control circuit 14 narrows the on pulse width of the switching element 4. The capacitor 15F, which is charged in on period, discharges electricity during the period when there is no on pulse, so the charge voltage of the capacitor 15F falls. Accompanying this, the voltage of the reference input terminal of an operating circuit 15B falls, and the difference between the detection signal and the reference voltage for overcurrents enlarges. This way, the increase of output currents resulting from the delay of the control circuit 14, etc., is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention provides an overcurrent protection method and overcurrent protection method for effectively limiting overcurrent in a power supply circuit such as a DC-DC converter or a power supply circuit. Regarding circuits.

[Conventional technology]

FIG. 3 shows a conventional power supply circuit having an overcurrent protection function, in which 1.2 is a DC input terminal, 3 is a transformer having a primary winding and a secondary winding, and 4 is 1 of the transformer 3. A switching element such as an enhancement field effect transistor connected in series with the next winding.

5 is a current detection resistor for detecting the current flowing through the switching element 4; 6 is a rectifier diode connected in series with the secondary winding of the transformer 53; 7 is a flywheel diode;
8 and 9 are inductors and capacitors that make up the smoothing circuit,
10 and 11 are output terminals, 12 is an output voltage detection circuit, 13
1 is a control circuit for controlling the on-pulse width of the switching element 4, 15 is an overcurrent protection circuit for the filI control circuit 14, 16 is a current This is a peak detection circuit.

The current peak detection circuit 16 is as shown in FIG.

It consists of a backflow prevention diode D, a capacitor C2, and resistors Rh and Rz, and a voltage corresponding to the peak value of the current flowing through the current detection resistor 54 is applied to the backflow prevention diode D,
The capacitor C is charged via the resistor WR, and is input as a detection signal to the detection terminal of the arithmetic circuit 15b. The arithmetic circuit 15^ of the overcurrent protection 3 circuit 15 calculates the detection signal and the reference voltage of the quasi-voltage source 15B, and when the former exceeds the latter, outputs an overcurrent signal, The width of the on-pulse applied to switching element 4 is narrowly limited to suppress the current and prevent the current flowing through switching element 4 from increasing abnormally.

However, if the excitation current flowing through the transformer 53 is ignored, the current flowing through the switching element 4 is as shown in FIG. The top of the current waveform rises almost linearly, and therefore the time j In t Z, L 3. is the middle of each current waveform.
The current value I of L a,... is almost the output current! , so as the width of the current 1 flowing through the switching element 4 becomes narrower as shown in FIG. 6, the current value ■1 and the output current ! .

increases.

Also, during the period until the flywheel diode 7 recovers its reverse characteristics, a surge current flows through the diode 7! , flows through the switching element 4 and becomes fllI
Since there is a risk of the control circuit 14 malfunctioning, 1, in order to prevent this, the current peak detection circuit 1 as described above is installed.
6 is used.

The current peak detection circuit 16 includes a capacitor C and a resistor R+.
This charging time constant is used to prevent the control circuit 14 from malfunctioning.

A surge current l of the current flowing through the current detection resistor S5.

The value is selected so as to absorb the spike voltage V generated in the detection voltage V, resulting in the voltage shown by the chain line.

[Problem that the invention seeks to solve]

Therefore, in the conventional circuit, as the on-pulse width of switching element 4 becomes narrower, capacitor C and resistor 23R
, etc., due to the time delay due to the charging time constant becomes noticeable, and the peak value of the current flowing through the switching element 4 further increases as shown in FIG. Along with this, the output current also increases, and a current two to three times or more than the rated current is supplied to the load, and there is a drawback that an unnecessary and harmful current flows through the switching element 4.

[Means for solving problems]

In the present invention, in order to eliminate such a drawback, when an overcurrent condition occurs and the on-pulse width of the switching element becomes less than the set width, the difference between the overcurrent reference voltage and the detection signal is reduced as the on-pulse width becomes narrower. It is characterized by forcibly increasing the size.

[For production]

The difference between the overcurrent reference voltage and the detection signal increases as an overcurrent condition occurs and the on-pulse width of the switching element becomes narrower, so that the overcurrent detection signal operates with a larger overcurrent detection signal than the actual overcurrent detection signal. Therefore, even if there is a time delay such as in the 't4f211 circuit, the current is more strictly limited, and overcurrent can be limited more effectively than in the past.

〔Example〕

Embodiments of the overcurrent detection method and overcurrent detection circuit for a power supply device according to the present invention will be described below.

First, an embodiment of the present invention will be described with reference to FIG. 1. In the figure, the same symbols as those shown in FIG. 3 indicate the same members.

The overcurrent detection circuit 15 of the control circuit 14 includes an overcurrent reference voltage source 15A, an arithmetic circuit 15B, a resistor D15C connected between the reference terminal of the 1 arithmetic circuit 15B, and one end of the overcurrent reference voltage source 15A. and a diode 150 . connected in series between the control electrode of the switching element 4 and the other end of the overcurrent reference voltage source 15 . Resistor n 15E, capacitor 15F, resistor 1 connected in parallel with the capacitor
It consists of a charging/discharging circuit consisting of 5G and a resistor 15E.
The connection point between the capacitor 15F and the resistor 15G is connected to the connection point between the reference terminal of the arithmetic circuit 15B and the resistor 515G.

Next, the operation of this overcurrent detection circuit will be explained. Under normal conditions, the width of the on-pulse applied to the t and 1 m electrodes of the switching element 4 is large, and the period during which the capacitor 15F is charged via the diode 150 and the resistor W15E. is equal to the width of the on-pulse, so the voltage across capacitor 15F is relatively high. Therefore, in this state, the capacitor 15F
The charging voltage does not lower the overcurrent reference voltage.

Next, when the current flowing through the switching element 4 increases beyond a set value due to an output short-circuit accident or the like, the overcurrent detection circuit 15 operates to narrow the width of the on-pulse of the switching element 4. When the width of the on-pulse becomes smaller, the period during which no on-pulse exists at the control electrode of the switching element 4 becomes longer, and the capacitor 15F of the charging/discharging circuit that was charged during the on-pulse period is discharged via the resistor 515G during this period. The charging voltage of 15F decreases as the width of the on-pulse applied to the switching element 4 becomes narrower.

Based on this, the overcurrent reference voltage applied to the input terminal -r- is lowered by the arithmetic circuit 15B, further increasing the difference between the detection signal and the overcurrent reference voltage. therefore,'
It is possible to effectively suppress an increase in output current caused by the time delay and circuit operation of the tA control circuit 14.

Next, the embodiment shown in FIG. 2 is an example of a circuit configuration in which the detection signal is further increased as the width of the on-pulse becomes narrower.

The base of the P conductivity type transistor i5H is connected to the resistor 15E.
The switching element 40, such as an enhancement type field effect transistor, is connected to the control electrode through the control electrode, and its emitter is connected to the control power source 151, and its collector is connected to the capacitor 15F through the resistor 115E. The connection point between the capacitor 15E and the capacitor 15F is coupled to the detection input terminal of the arithmetic circuit 15B.

The transistor 1511 is off during the period when an on-pulse is applied to the control electrode of the switching element 4, and is on during the period when no on-pulse is present at the control electrode. Therefore, as the width of the on-pulse applied to the switching element 4 becomes narrower due to the overcurrent state, the transistor 1
51] becomes longer, and as a result, capacitor 1
Since the time to charge 5F becomes longer, its charging voltage increases, increasing the voltage at the detection input terminal of the playback circuit 15B.

This makes it possible to effectively suppress an increase in the output current caused by the time delay and circuit operation of the control circuit 14.

In the above embodiments, a DC-DC converter has been described, but the present invention can be applied in the same way to a converter such as an I7 converter, and when an AC power source is used, a rectifier, a circuit, etc. are connected to the input terminal. It is natural that they are connected.

〔effect〕

As described above, according to the present invention, when an overcurrent occurs, the on-pulse width of the switching element is made equal to or less than the set width, and as the on-pulse width becomes narrower, the difference between the overcurrent Takigawa reference voltage and the detection signal is forcibly reduced. Since the current is made large, current can be extremely effectively limited even if there is a time delay in the 74m circuit.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of an overcurrent protection circuit of a power supply device according to the present invention, FIG. 2 is a diagram showing another embodiment of the present invention, and FIG. 3 is a diagram showing an overcurrent protection circuit of a conventional power supply device. Diagram showing the protection circuit,
FIG. 4 is a diagram showing an example of a current peak detection circuit, and FIGS. 5 and 6 are diagrams showing current waveforms and voltage waveforms of each part of the power supply device. 1.2-Input terminal. 4-Switching element. io, ii - output terminal. 14-Control circuit. 16-Current peak detection circuit. 3-Transformer 5-Current detection resistor 12-Output voltage detection circuit 15-Overcurrent detection circuit

Claims (5)

[Claims] (1) When the detection signal detected by one or more switching elements, the current detection means, and the current detection means and the overcurrent reference voltage are calculated, and the detection signal exceeds the overcurrent reference voltage, the switching In a power supply equipped with an overcurrent protection circuit that suppresses overcurrent by limiting the on-pulse width of an element, when the on-pulse width of the switching element becomes equal to or less than a set width, the overcurrent reference is adjusted as the on-pulse width becomes narrower. A method for overcurrent protection for a power supply device, comprising increasing a difference between a voltage and the detection signal. (2) The overcurrent protection method for a power supply device according to claim (1), wherein the overcurrent reference voltage is lowered as the on-pulse width of the switching element becomes narrower. (3) The overcurrent protection method for a power supply device according to claim (1), wherein as the on-pulse width of the switching element becomes narrower, a voltage is applied to the detection signal to make the detection signal larger. (4) One or more switching elements, current detection means, and an arithmetic circuit that calculates a detection signal detected by the current detection means and an overcurrent reference voltage, the detection signal being the overcurrent reference voltage. In a power supply equipped with an overcurrent protection circuit that suppresses overcurrent by limiting the on-pulse width of the switching element when the on-pulse width exceeds An overcurrent protection circuit for a power supply device, characterized in that the overcurrent protection circuit is connected to a control electrode of the switching element via the switching element. (5) One or more switching elements, current detection means, and an arithmetic circuit that calculates a detection signal detected by the current detection means and an overcurrent reference voltage, the detection signal being the overcurrent reference voltage. In a power supply equipped with an overcurrent protection circuit that suppresses overcurrent by limiting the on-pulse width of the switching element when the on-pulse width of the switching element exceeds An overcurrent protection circuit for a power supply device, characterized in that the charge/discharge circuit is connected to a current source via a transistor controlled by a control voltage of a control electrode of the switching element.