Switching power supply device

A switching power supply device, comprising:

• a switching output stage configured to receive electric power from a primary circuit system and supply an output voltage and an output current to a load provided in a secondary circuit system while electrically isolating between the primary and secondary circuit systems;
• an output feedback circuit provided in the secondary circuit system and configured to generate a secondary-side feedback current commensurate with the output voltage and the output current;
• a photocoupler configured to convert the secondary-side feedback current into an optical signal by use of a light-emitting element provided in the secondary circuit system and convert the optical signal into a primary-side feedback current by use of a light-receiving element provided in the primary circuit system;
• and
• a controller IC provided in the primary circuit system and configured to control the switching output stage according to the primary-side feedback current passing through an output feedback terminal of the controller IC,
• wherein
• the output feedback circuit includes:
• a shunt regulator configured to control the secondary-side feedback current according to a control voltage applied to a control terminal of the shut regulator;
• a first resistor connected between an output terminal of the output voltage and the control terminal;
• a second resistor connected between the control terminal and a ground terminal;
• a sense resistor configured to convert the output current into a sense voltage;
• and
• a first diode of which an anode is connected to an application terminal of the sense voltage or of a voltage commensurate therewith and of which a cathode is connected to the control terminal.