Power supply controller and insulated switching power supply

A power supply controller comprising:

• a monitor voltage generation part configured to generate a monitor voltage according to a primary voltage of a transformer that forms an insulated switching power supply;
• a sample/hold part configured to sample/hold the monitor voltage and output a feedback voltage;
• and
• a controller configured to turn on/off a primary current of the transformer by a fixed on-time method according to the feedback voltage,
• wherein the sample/hold part samples/holds the primary voltage at a plurality of different timings and outputs one of a plurality of hold values as the feedback voltage,
• wherein the sample/hold part includes:
• a plurality of sets of analog switches and capacitors configured to output the plurality of hold values by sampling/holding the monitor voltage at a plurality of different timings;
• at least one comparator configured to compare the plurality of hold values to generate a selection signal;
• and
• a feedback voltage output part configured to output the highest one of the plurality of hold values as the feedback voltage, according to the selection signal, and
• wherein when on-duty of the primary current is higher than a predetermined threshold value, the feedback voltage output part outputs a hold value sampled/held at the latest timing as the feedback voltage, without depending on the selection signal.