Switched mode power supply including a flyback converter with primary side control

摘要

A method and apparatus for controlling a flyback converter are presented. The flyback converter includes a transformer, a semiconductor switch coupled to a primary winding of the transformer, a current measurement circuit coupled to the semiconductor switch, a diode coupled in series to a secondary winding of the transformer, and a controller. The controller is configured to receive a feedback voltage, a reference signal, and the measured primary current and generate a control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current. The semiconductor switch switches on and off cyclically in CCM operation.

主权项

1. A method for controlling a flyback converter that comprises
a transformer having a primary winding, a secondary winding and an auxiliary winding, the primary winding operably carrying a primary current, the secondary winding operably carrying a secondary current, and the auxiliary winding operably providing a feedback voltage; a semiconductor switch coupled in series to the primary winding and configured to switch the primary current on and off in accordance with a control signal, a current measurement circuit coupled to the semiconductor switch or the transformer and configured to measure the primary current, a diode coupled in series with the secondary winding and configured to rectify the secondary current; and a controller coupled to receive the feedback voltage, a reference signal and the measured primary current and configured to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current, wherein the semiconductor switch switches on and off cyclically in CCM operation, wherein the method comprises: regularly interrupting the switching operation such that the secondary current drops to zero while the semiconductor switch is off; sampling the feedback voltage at the time instant the secondary current reaches zero, thereby obtaining a first sampled value; resuming the switching operation; sampling the feedback voltage at the time instant the control signal indicates a switching operation to switch the semiconductor switch on, thereby obtaining a second sampled value; sampling the measured primary current at the time instant the semiconductor switch has switched on, thereby obtaining a third sampled value; and adjusting the reference signal dependent on the first the second and the third sampled values.