Patent ID: 6879501
Filing Date: 2005-04-12
Classification: H02M

Abstract:
1. A switching power supply, comprising: a transformer; a switching element that has an input terminal connected to a first primary winding of the transformer and is fed with a first direct-current voltage via the transformer; an output voltage generation circuit that is connected to a secondary winding of the transformer, and rectifies and smoothes a secondary output voltage of the transformer, whereby a second direct-current voltage smaller than an absolute value of the first direct-current voltage is generated from the first direct-current voltage and the second direct-current voltage is outputted; an output voltage control circuit for stabilizing the second direct-current voltage; a control signal transmission circuit for transmitting a signal of the output voltage control circuit to a primary side; a control circuit for controlling an operation of the switching element; and an auxiliary power supply voltage generation circuit that is connected to an auxiliary winding of the transformer, generates a primary output voltage proportionate to the secondary output voltage and rectifies and smoothes the generated primary output voltage, whereby an auxiliary power supply voltage for feeding a power supply voltage to the control circuit is generated and outputted, said control circuit for controlling an operation of the switching element comprising: a regulator for generating and feeding a power supply voltage of the control circuit from the first direct-current voltage and the auxiliary power supply voltage; an oscillation circuit for generating and outputting a switching signal to be applied to the switching element; a current detection circuit for detecting a current flowing through the switching element and outputting the detected current as an element current detection signal; a feedback signal control circuit for outputting a signal from the control signal transmission circuit as a feedback signal; a comparator for comparing the element current detection signal and the feedback signal, and outputting a comparison signal after comparison; a switching signal control circuit for controlling a current amount and an output of the switching signal based on the comparison signal; a clamping circuit for fixing a maximum value of the element current detection signal; and a clamp voltage variable circuit for changing a clamp voltage of the clamp circuit according to a voltage value of the auxiliary power supply voltage, wherein the clamp voltage variable circuit outputs to the oscillation circuit an oscillation frequency reduction signal for reducing an oscillation frequency of the oscillation circuit, when the clamp voltage is lower than a given value.