Switching mode power supply

A switching power supply having a normal operation mode and a standby operation mode, the switching power supply comprising:

• a power supply section having a main switch coupled to a primary winding of a transformer,
• wherein in normal operation mode the main switch is operable to perform a switching operation at a fixed frequency,
• wherein in standby operation mode the main switch is operable to inhibit the switching operation during a first interval and to perform the switching operation during a second interval, and
• wherein in standby operation mode the main switch is further operable to repeat the first and second intervals periodically at a fixed duty cycle, and
• wherein the switching operation energizes a secondary winding of the transformer;
• a mode discriminator operable to generate a mode signal in response to an external sync signal presented at the primary winding of the transformer and further operable to enter an operation mode in response to the mode signal, the operation mode being selected from a list consisting of the normal operation mode and the standby operation;
• a control voltage generator having
• a first transistor operable to conduct a current having a magnitude responsive to the mode signal, and
• and a first capacitor connected in parallel with a pair of current terminals of the first transistor and
• a switching controller operable to control the main switch to perform the switching operation at the fixed duty cycle responsive to a control voltage on the first capacitor in normal operation mode and further operable to perform the repeating.