Insulated dc/dc converter, and power adaptor and electronic device using the same

An insulated DC/DC converter, comprising:

• a transformer having a primary winding, a secondary winding, and an auxiliary winding;
• a switching transistor installed between the primary winding of the transformer and a ground line;
• a sense resistor installed in series with the switching transistor;
• a rectifier circuit connected to the secondary winding of the transformer;
• a photocoupler including a light emitting element and a light receiving element;
• a feedback circuit configured to drive the light emitting element of the photocoupler such that an output voltage of the DC/DC converter approaches a target voltage;
• a primary side controller having a feedback terminal which is connected to the light receiving element of the photocoupler and receives a feedback signal from the photocoupler, a current detection terminal which receives a current detection signal corresponding to a voltage drop of the sense resistor, a zero current detection terminal which receives a zero current detection signal corresponding to a voltage generated at one end of the auxiliary winding, and a peak current mode pulse modulator of a quasi-resonant mode configured to generate a pulse signal depending on the feedback signal, the current detection signal, and the zero current detection signal;
• a first voltage dividing resistor installed between the one end of the auxiliary winding and the current detection terminal;
• a second voltage dividing resistor installed between the current detection terminal and the ground line;
• and
• a first resistor and a first capacitor installed in series and on a path parallel with the first voltage dividing resistor, between the one end of the auxiliary winding and the current detection terminal.