Valley-mode switching schemes for switching power converters

A switching power converter comprising:

• a transformer including a primary winding coupled to an input voltage and a secondary winding coupled to an output of the switching power converter;
• a switch coupled to the primary winding of the transformer, current through the primary winding being generated while the switch is turned on and not being generated while the switch is turned off;
• and
• a controller configured to generate a control signal to turn on or turn off the switch, the switch being turned on responsive to the control signal being in a first state and the switch being turned off responsive to the control signal being in a second state, wherein:
• the controller is further configured to determine a desired turn-on time of the switch in each switching cycle according to a control mode employed by the switching power converter;
• and
• the controller is further configured to generate in each switching cycle the control signal in the first state to turn on the switch at an actual turn-on time corresponding to one of a plurality of local minimums of a voltage across the switch subsequent to the desired turn-on time determined according to the control mode.