Overcurrent protection circuit
First Claim
1. An overcurrent protection circuit for a switching power supply including a switch device;
- a PWM control circuit for generating a driving pulse for pulse-width-modulation control of the switch device, the PWM control circuit provided with a setting terminal at which a duty cycle of the driving pulse is set to a small value by changing an applied voltage; and
an output rectifying diode for rectifying an AC voltage generated by ON-OFF control of the switch device, the overcurrent protection circuit comprising;
current detecting means for detecting a circuit current flowing through at least one of the switch device and the output rectifying diode and for producing a detected voltage;
voltage superposition means coupled to a first voltage source at one end, for superposing a constant voltage onto the detected voltage detected by the current detecting means; and
means for reducing the width of the driving pulse for the switch device, the reducing means including a transistor which is ON-controlled by a voltage obtained by superposing the constant voltage onto the detected voltage detected by the current detecting means when an overcurrent flows, and a resistor connected between an output of the transistor and a second voltage source, wherein a voltage applied to the setting terminal of the PWM control circuit is reduced due to the voltage drop at the resistor when the transistor is turned on.
1 Assignment
0 Petitions
Accused Products
Abstract
An overcurrent protection circuit provided for a switching power supply has a current detector, voltage superposition device, and circuit for reducing a driving pulse. The current detector detects a circuit current in the switching power supply and converts it to a voltage. The voltage superposition device generates a constant voltage and superposes the voltage onto the detected voltage converted by the current detector. The detected voltage onto which the constant voltage has been superposed is applied between the base and the emitter of a transistor which functions as the circuit for reducing the driving pulse. If an overcurrent flows through the switching power supply, the transistor is turned on and a current flows into the transistor. As a result, a voltage applied to a maximum-duty-cycle setting terminal of a PWM IC provided for the switching power supply and used for pulse-width-modulation control of a switch device is reduced, and the overcurrent is suppressed.
-
Citations
17 Claims
-
1. An overcurrent protection circuit for a switching power supply including a switch device;
- a PWM control circuit for generating a driving pulse for pulse-width-modulation control of the switch device, the PWM control circuit provided with a setting terminal at which a duty cycle of the driving pulse is set to a small value by changing an applied voltage; and
an output rectifying diode for rectifying an AC voltage generated by ON-OFF control of the switch device, the overcurrent protection circuit comprising;current detecting means for detecting a circuit current flowing through at least one of the switch device and the output rectifying diode and for producing a detected voltage; voltage superposition means coupled to a first voltage source at one end, for superposing a constant voltage onto the detected voltage detected by the current detecting means; and means for reducing the width of the driving pulse for the switch device, the reducing means including a transistor which is ON-controlled by a voltage obtained by superposing the constant voltage onto the detected voltage detected by the current detecting means when an overcurrent flows, and a resistor connected between an output of the transistor and a second voltage source, wherein a voltage applied to the setting terminal of the PWM control circuit is reduced due to the voltage drop at the resistor when the transistor is turned on. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
- a PWM control circuit for generating a driving pulse for pulse-width-modulation control of the switch device, the PWM control circuit provided with a setting terminal at which a duty cycle of the driving pulse is set to a small value by changing an applied voltage; and
-
9. An overcurrent protection circuit for a switching power supply including a switch device;
- a PWM control circuit for generating a driving pulse for pulse-width-modulation control of the switch device, the PWM control circuit provided with a setting terminal at which a duty cycle of the driving pulse is changed by changing an applied voltage; and
an output rectifying diode for rectifying an AC voltage generated by ON-OFF control of the switch device, the overcurrent protection circuit comprising;a current detector for detecting a circuit current flowing through at least one of the switch device and the output rectifying diode and for producing a detected voltage; a voltage superposition circuit coupled to a first voltage source at one end, for superposing a constant voltage onto the detected voltage detected by the current detector; and a circuit for changing the width of the driving pulse for the switch device, the width changing circuit including a controlled switch which is ON-controlled by a voltage obtained by superposing the constant voltage onto the detected voltage detected by the current detector when an overcurrent flows, and a resistor connected between an output of the controlled switch and a second voltage source, wherein a voltage applied to the setting terminal of the PWM control circuit is changed due to the voltage drop at the resistor when the controlled switch is turned on. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16, 17)
- a PWM control circuit for generating a driving pulse for pulse-width-modulation control of the switch device, the PWM control circuit provided with a setting terminal at which a duty cycle of the driving pulse is changed by changing an applied voltage; and
Specification