Switching power supply with multiple output voltages and an intermittent oscillation pulse in standby

A switching power supply device comprising:

• an oscillation circuit that generates an oscillation signal to switch a current flowing through a primary coil of a transformer;
• an oscillation control circuit that controls the oscillation signal of the oscillation circuit on a basis of a voltage induced to a secondary coil of the transformer such that a DC voltage becomes a predetermined value;
• a plurality of power supply voltage generating circuits each of which generates a different DC voltage on a basis of an output voltage of the secondary coil of the transformer;
• and
• a power supply feedback circuit that detects a variation of the main power supply voltage as an error signal by comparing a main power supply voltage output from a main power supply voltage generating circuit of the plurality of power supply voltage generating circuits with a reference voltage, and supplies the error signal to the oscillation control circuit as a feedback signal,
• wherein an intermittent oscillation pulse output from a microcomputer is supplied to a connection point between a reference voltage line in the power supply feedback circuit and a control line of an error detecting element through a DC cutoff circuit,
• a duty ratio and a frequency of the intermittent oscillation pulse are varied to determine the duty ratio and the frequency of the intermittent oscillation pulse that provides a lowest power consumption in a standby state and a microcomputer is configured to output the intermittent oscillation pulse with the determined duty ratio and frequency, and
• the intermittent oscillation pulse with the determined duty ratio and frequency output from the microcomputer is supplied to the connection point through the DC cutoff circuit in the standby state.

A switching power supply device comprising:

• an oscillation circuit that generates an oscillation signal to switch a current flowing through a primary coil of a transformer;
• an oscillation control circuit that controls the oscillation signal of the oscillation circuit on a basis of a voltage induced to a secondary coil of the transformer such that a DC voltage becomes a predetermined value;
• a first power supply voltage generating circuit that generates a first DC power supply voltage by an output voltage of the secondary coil of the transformer;
• a second power supply voltage generating circuit that generates a second DC power supply voltage lower than the first DC power supply voltage by the output voltage of the secondary coil of the transformer;
• and
• a power supply feedback circuit that detects a variation of the second power supply voltage as an error signal by comparing a reference voltage generated on a basis of the first DC power supply voltage with the second DC power supply voltage, and supplies the error signal to the oscillation control circuit as a feedback signal,
• wherein an intermittent oscillation pulse output from a microcomputer is supplied to a connection point between a reference voltage line in the power supply feedback circuit and a control line of an error detecting element through a DC cutoff circuit,
• a duty ratio and a frequency of the intermittent oscillation pulse are varied to determine the duty ratio and the frequency of the intermittent oscillation pulse that provides a lowest power consumption in a standby state and the microcomputer is configured to output the intermittent oscillation pulse with the determined duty ratio and frequency, and
• the intermittent oscillation pulse with the determined duty ratio and frequency output from the microcomputer is supplied to the connection point through the DC cutoff circuit in the standby state.