Method and apparatus to digitally control turn-off time of synchronous rectifiers in isolated topologies for switched mode power supplies

Abstract

Presented is a circuit and method capable to digitally control and, in particular, to control the switching of one or two MOSFETs used as rectifiers in switched mode power supply isolated topologies. Basic circuit implementation of the presented technique is also introduced. A controller has a fixed frequency square wave signal main clock input, generically switching from a low to a high value in two different time intervals. The controller has one or two square wave outputs, swinging from low to high in phase or in opposite with respect to the clock signal. The digital control method is able to generate output signals timed to anticipate output transitions from high to low level with respect to the clock signal transitions. In the control scheme, one or two other secondary inputs set the amount of anticipation time of the respective transitions of the outputs. In isolated SMPS topologies with the PWM controller located on the primary side, the clock signal is derived from the secondary output of the isolation transformer, while the outputs are meant to drive one or two MOSFET gates. The method is able to provide the proper gate drive signals, solving all the known problems in controlling synchronous rectification.