JP2002084744A - Overcurrent protection circuit of switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide protection against overcurrent on the load side in case of significant change in input voltage with respect to the overcurrent protection circuit of a switching power supply. SOLUTION: The overcurrent protection circuit of the switching power supply controls turn-on and -off of the switching transistor Q1 connected with the primary winding n1 of a transformer T through a control circuit CONT so that output voltage Vout obtained by rectifying the induced voltage in the secondary winding n2 of the transformer T and supplied to a load is equal to a preset value. The overcurrent protection circuit is provided with a current detecting means comprising a resistor R1 and the like that detects any current passed through the primary winding n1 of the transformer T through the switching transistor Q1, a capacitor C3 that adds detection signals from the current detecting means, and a comparator COMP that compares a detection signal passed through the capacitor C3 with a comparison reference voltage Vr, and adds detection signals exceeding the comparison reference voltage Vr as overcurrent detection signals to the control circuit CONT to cause it to exercise drop control of the output voltage Vout.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply for supplying a stabilized DC voltage to a load, and when an overcurrent flowing through the load is detected, the output voltage is dropped to protect the load from the overcurrent. The present invention relates to an overcurrent protection circuit for a switching power supply.

[0002]

2. Description of the Related Art FIG. 4 is an explanatory view of a conventional example, where T is a transformer, n1 is a primary winding, n2 is a secondary winding, Q1 is a switching transistor, R1 and R2 are resistors, COMP is a comparator, and CONT. Is a control circuit, D is a diode, C1, C
2 is a capacitor, RL is a load, Vr is a comparison reference voltage, V
“in” indicates a DC input voltage, and “Vout” indicates a DC output voltage, and shows a case of a flyback converter configuration.

The DC output voltage Vou supplied to the load RL
t is detected by the control circuit CONT, and the switching transistor Q1 is turned on and off so as to have a set value, and a current flowing through the primary winding n1 of the transformer T according to the turning on and off of the switching transistor Q1 is expressed by a resistor R
1 is detected as a voltage across the two terminals and input to the + terminal of the comparator COMP via the resistor R2. And the comparator COMP
Is compared with the comparison reference voltage Vr input to the-terminal. When the switching transistor Q1 is turned off after being turned on, a voltage is induced in the secondary winding n2 of the transformer T by the energy accumulated in the transformer T during the on period, and the voltage is rectified by the diode D and the capacitor is rectified. C2 and charges the terminal voltage of the capacitor C2 to the output voltage Vo.
ut is supplied to the load RL.

The comparison reference voltage Vr input to the negative terminal of the comparator COMP is set so that the voltage across the resistor R1 indicates an overcurrent state, and when a large current flows due to a short-circuit state of the load RL or the like. , The current flowing through the primary winding n1 of the transformer T also increases, resulting in an overcurrent state. As a result, the voltage across the resistor R1 exceeds the comparison reference voltage Vr, and the output signal of the comparator COMP becomes "1". Assuming that the output signal of "1" of the comparator COMP is an overcurrent detection signal, the control circuit CONT latches this overcurrent detection signal, and shortens the ON period of the switching transistor Q1 to zero. Therefore, the induced voltage of the secondary winding n2 of the transformer T decreases or becomes zero, and the output voltage Vou
In this case, t is dropped so as to prevent damage due to overcurrent.

[0005]

When the DC input voltage Vin from a rectified power supply or the like is constant, the current flowing through the primary winding n1 of the transformer T changes with the change in the load RL. Then, when an overcurrent state corresponding to a short circuit of the load RL or the like occurs, as described above, the current flowing through the primary winding n1 of the transformer T is detected by the resistor R1, and the ON period of the switching transistor Q1 is shortened or set to zero. , The output voltage Vout is dropped. When the input voltage Vin changes, for example, when the input voltage Vin increases, the load RL
Is constant, the current flowing through the primary winding n1 of the transformer T decreases. Conversely, when the input voltage Vin decreases, the current flowing through the primary winding n1 of the transformer T increases.

Accordingly, the current flowing through the primary winding n1 of the transformer T when the load RL is in an overcurrent state such as a short circuit also differs according to the input voltage Vin. For example, as shown in FIG. 5, when the load RL is in an overcurrent state such as a short circuit while the output voltage Vout is kept constant with respect to a change in the input voltage Vin, when the input voltage Vin is high, When a current of Iov2 flows through the load RL, an overcurrent detection signal is input to the control circuit CONT, and the output voltage Vou
t, and when the input voltage Vin is low, when a current of Iov1 (<Iov2) flows through the load RL, an overcurrent detection signal is input to the control circuit CONT to drop the output voltage Vout. Will be.

The switching power supply has an input voltage Vin.
Can be selected from a relatively wide range such as 80 V to 250 V, and the output voltage Vout is set to 5 V, for example.
Is required to be a constant voltage. As described above, the detection of the overcurrent state of the load RL when the input voltage Vin is high is different from the detection of the overcurrent state of the load RL when the input voltage Vin is low. The current capacity must be configured to be acceptable for the maximum overcurrent, and there is a problem that the cost increases. Therefore, it is conceivable to detect the input voltage Vin and correct the current detection value by the resistor R1, but there is a problem that the circuit configuration becomes complicated. SUMMARY OF THE INVENTION It is an object of the present invention to enable the same overcurrent detection for a relatively wide range of input voltage Vin with a simple configuration.

[0008]

Referring to FIG. 1, the overcurrent protection circuit of the switching power supply according to the present invention rectifies the induced voltage of the secondary winding n2 of the transformer T and supplies it to the load. An overcurrent protection circuit of a switching power supply device that controls on / off of a switching transistor Q1 connected to a primary winding n1 of a transformer T by a control circuit CONT so that an output voltage Vout becomes a set value, A current detecting means for detecting a current flowing through the primary winding n1 of the transformer T via the switching transistor Q1, a capacitor C3 for applying a detection signal from the current detecting means, and a detection signal via the capacitor C3 for comparing the detection signal via the comparison reference voltage Vr And the comparison reference voltage Vr
The overcurrent detection signal is sent to the control circuit C when the detection signal exceeds
In addition to the ONT, a comparator COMP for performing droop control of the output voltage Vout is provided.

The current detecting means may include a current transformer for detecting a current flowing through the primary winding of the transformer via the switching transistor, and a diode connected to a secondary winding of the current transformer.

[0010]

FIG. 1 is an explanatory view of a first embodiment of the present invention. The same reference numerals as those in FIG. 4 denote the same parts, and FIG.
3 is a capacitor. The illustration of the load RL is omitted. The current flowing in the primary winding n1 of the transformer T during the ON period of the switching transistor Q1 is detected by the resistor R1, and the comparator C is connected via the capacitor C3 and the resistor R2.
The signal is input to the + terminal of the OMP and compared with the comparison reference voltage Vr input to the-terminal.

The current flowing through the primary winding n1 of the transformer T when the input voltage Vin is low has, for example, a waveform shown as I _L in FIG. 2, and the current when the input voltage Vin is high is I
_The waveform shown as _H is obtained. Incidentally, Ton, Toff is on of the switching transistor Q1, a period of off, T is turned on, the control period of the off, V _R is the voltage across the resistor R1,
Vr indicates a comparison reference voltage.

When the load is constant, it is necessary to supply the same energy through the primary winding n1 of the transformer T regardless of the input voltage Vin. Therefore, when the input voltage Vin is low,
The ON period Ton is long, and the OFF period Toff is short.
Conversely, when the input voltage Vin is high, the on-period Ton is short and the off-period Toff is long. And the input voltage V
When in is low, the off period Toff is short,
Before the current supplied from the secondary winding n2 due to the energy stored in the transformer T becomes zero, the switching transistor Q
1 turns on. Thereby, it becomes possible remaining or energy in the transformer T, the current waveform of I _L. That is, it contains a DC component. On the other hand, when the input voltage Vin is high, the ON period Ton of the switching transistor Q1 is short, so that the current supplied from the secondary winding n2 due to the energy stored in the transformer T is the OFF period Tof.
It becomes zero at f. That is, the energy stored in the transformer T is released, resulting in an I _H current waveform.

[0013] The voltage across the resistor R1 is V _R becomes large in the case of current I _L, it becomes smaller in the case of current I _L,
When the DC component is blocked by the capacitor C3, the current I _L
In this case, the AC component _VA is almost the same as the voltage V _R across the resistor R1 for the current I _H. Therefore, by comparing with the comparison reference voltage Vr, even if the input voltage Vin changes relatively significantly, an overcurrent state of the same load is detected.
The overvoltage protection can be performed by drooping the output voltage Vout. That is, the overcurrents Iov1 and Iov in FIG.
2 can be set to the same value to perform overcurrent protection.

FIG. 3 is an explanatory view of a second embodiment of the present invention. The same reference numerals as in FIG. 1 denote the same parts, CT denotes a current transformer, D1 and D2 denote diodes, and R3 and R4 denote resistors. . The current detecting means in the embodiment shown in FIG. 1 is a resistor R1, but the current detecting means in this embodiment is connected to a current transformer CT and a secondary winding of the current transformer CT. It has a configuration including a resistor R3 and a diode D1.

As described with reference to FIG. 2, a current corresponding to the current flowing to the primary winding n1 of the transformer T flows through the secondary winding of the current transformer CT as described with reference to FIG. Since this current flows through the resistor R3, the voltage across the both ends is applied to the circuit of the capacitor C3 and the resistor R4 via the diode D1, and the voltage across the resistor R4 is applied to the + terminal of the comparator COMP via the diode D2. And compares it with the comparison reference voltage Vr input to the-terminal.

In this case, the voltage across the resistor R3 has positive and negative polarities and is similar to the waveforms of I _L and I _{H in} FIG. 2. Therefore, by blocking the DC component via the capacitor C3, Even if the input voltage Vin changes, the overcurrent flowing on the secondary side of the transformer T is detected as the same value,
Overcurrent protection can be provided.

[0017]

As described above, according to the present invention, the output voltage Vout which is obtained by rectifying the induced voltage of the secondary winding n2 of the transformer T by the diode D and supplying it to the load becomes the set value. The transformer T is controlled by the control circuit CONT.
Switching transistor Q connected to the primary winding n1
1 is turned on and off, a current flowing through the primary winding n1 of the transformer T via the switching transistor Q1 is detected by current detecting means such as a resistor R1 and the like. COMP, is compared with the comparison reference voltage Vr, and a detection signal for detecting an overcurrent state is input to the control circuit CONT, and the output voltage V
In this case, the output voltage Vout is maintained at the set voltage even when the selection range of the input voltage Vin is relatively wide, and the output voltage Vout is maintained in the same overcurrent state.
There is an advantage that the overcurrent protection can be performed by dropping t.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a first embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of the embodiment of the present invention.

FIG. 3 is an explanatory diagram of a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a conventional example.

FIG. 5 is an explanatory diagram of overcurrent protection characteristics.

[Description of Signs] T Transformer n1 Primary winding n2 Secondary winding Q1 Switching transistor D Diode C1, C2, C3 Capacitor R1, R2 Resistance CONT Control circuit COMP Comparator Vr Comparison reference voltage Vin Input voltage Vout Output voltage

Claims (2)

[Claims] An output voltage supplied to a load by rectifying an induced voltage of a secondary winding of a transformer is set to a set value.
In an overcurrent protection circuit of a switching power supply for controlling on / off of a switching transistor connected to a primary winding of the transformer by a control circuit, a current flowing through the primary winding of the transformer via the switching transistor is detected. Current detection means, a capacitor for adding a detection signal by the current detection means, and a comparison signal for comparing the detection signal via the capacitor with a comparison reference voltage. When the detection signal exceeds the comparison reference voltage, an overcurrent detection signal is generated. An overcurrent protection circuit for a switching power supply, comprising: a comparator for performing droop control of the output voltage in addition to the control circuit. 2. The method according to claim 1, wherein the current detecting means includes a current transformer for detecting a current flowing through a primary winding of the transformer via the switching transistor, and a diode connected to a secondary winding of the current transformer. 2. The overcurrent protection circuit for a switching power supply according to claim 1, wherein: