Voltage sensing of an active clamp switching power converter circuit using auxiliary winding having an opposite polarity as a primary winding

A control circuit for a switching power converter, the control circuit comprising:

• an auxiliary winding around a core of a transformer having a same polarity as a secondary winding around the core of the transformer, wherein a sensing circuit generates a sensed voltage based on an auxiliary winding voltage across the auxiliary winding;
• and
• a controller to control switching of a power switch to regulate an output voltage and to control switching of an active clamp switch to turn on the active clamp switch following the power switch turning off and to turn off the active clamp switch based on detected timing of the sensed voltage during a first switching cycle satisfying a predefined condition that the sensed voltage is substantially zero after the active clamp switch is turned off such that the power switch is turned on while the sensed voltage is still substantially zero during a second switching cycle subsequent to the first switching cycle,
• wherein the power switch is coupled to a primary winding of the transformer and the active clamp switch controls leakage energy through the primary winding when the power switch is turned off.

A method for controlling a switching power converter, the switching power converter including a transformer coupled between an input terminal receiving an input voltage and an output terminal supplying an output voltage, the transformer including a core, a primary winding around the core coupled to the input terminal, and a secondary winding around the core coupled to the output terminal, an auxiliary winding around the core of the transformer having a same polarity as the secondary winding, a power switch coupled to control current through the primary winding, an active clamp switch coupled to return leakage energy to the primary winding during an off-time of the power switch, and a controller, the method comprising:

• detecting a sensed voltage by the controller in a first switching cycle, the sensed voltage based on an auxiliary winding voltage across the auxiliary winding;
• detecting timing, in the first switching cycle, of the sensed voltage satisfying a predefined condition by detecting an amount of time it takes for the sensed voltage to satisfy the predefined condition that the sensed voltage is substantially zero after the active clamp switch is turned off;
• and
• controlling, in a subsequent second switching cycle, a turn-off time of the active clamp switch relative to a turn-on time of the active clamp switch such that the detected timing of the sensed voltage satisfying the predefined condition in the subsequent second switching cycle is closer to a predefined amount of time such that the power switch is turned on while the sensed voltage is substantially zero during the second switching cycle.