SWITCHMODE™ Power Supply Reference Manual

SMPSRMThe Active ClampAn active clamp is a gated MOSFET circuit that allowsthe controller IC to activate a clamp or a snubber circuitat a particular moment in a switching power supply’scycle of operation. An active clamp for a flybackconverter is shown in Figure 30.In Figure 30, the active clamp is reset (or emptied of itsstored energy) just prior to the turn–off transition. It isthen disabled during the negative transition.Obviously, the implementation of an active clamp ismore expensive than other approaches, and is usuallyreserved for very compact power supplies where heat isa critical issue.UnclampedSwitch Voltage(V SW )Clamped SwitchVoltage (V SW )V inSwitchCurrent (I SW )+V DR–I CL+GNDI SWV SW–DriveVoltage (V DR )DischargeChargeClampCurrent (I CL )Figure 30. An Active Clamp Used in a One Transistor Forward or a Flyback Converterhttp://onsemi.com29