Switching power supply device

A switching power supply device comprising:

• a main circuit for converting an input voltage into an output voltage, including:
• a transformer isolating an input side from an output side;
• a main switching element connected with an input side of said transformer;
• and rectifying switching elements connected with an output side of said transformer;
• a controller for supplying a switching pulse with a duty ratio depending on said output voltage to said main switching element and said rectifying switching elements, so as to synchronize a switching operation of said rectifying switching element with that of said main switching element;
• an output detector for outputting, as a voltage detection signal for controlling an operation of said controller, said voltage detection signal indicating a comparative result between a detected voltage obtained from said output voltage and a reference voltage;
• a drive detector for detecting whether said main switching element starts a switching operation after application of said input voltage;
• and
• a rectifying switching element operation limiter for, in response to said detection signal from said drive detector, disabling a switching operation of said rectifying switching element if said main switching element does not start a switching operation and enabling a switching operation of said rectifying switching element if said main switching element starts a switching operation;
• wherein said output detector comprises:
• a reference voltage generating circuit for, after application of said input voltage, maintaining said reference voltage in a first threshold if said main switching element does not perform a switching operation and maintaining said reference voltage in a second threshold if said main switching element performs a switching operation;
• and
• output detection circuit for, after application of said input voltage, outputting said voltage detection signal so as to prevent said controller from supplying a switching pulse if said detected voltage is higher than said reference voltage.