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Flyback controllers rely on two different modes of operation, fixed-frequency PWM and quasi-resonant. Fixed frequency PWM is the most familiar. In this mode of operation, the flyback switch has a varying duty cycle at a fixed frequency to control the transfer of power from primary to secondary. Often the current through the transformer will be allowed to go to zero for what is called discontinuous mode operation (DCM) as it simplifies the dynamics of the power supply feedback loop. However, it requires that peak switched current be relatively high, resulting in increased power losses over the alternatives. Another characteristic of fixed-frequency operation is the concentration of switching noise in a narrow frequency band. For this reason, most fixed-frequency control ICs will implement a small frequency jitter about the frequency of operation to slightly spread the frequency spectrum of this noise. Quasi-resonant operation is a variable frequency mode of operation. It works at the boundary between DCM and continuous conduction, transitioning the switch near the exact moment when power transfer is complete. Depending on the output power draw on the power supply (load) this will require the switching frequency to vary. The result of quasi-resonant operation is lower switched current peaks, reducing overall power losses and improving efficiency, however the switching noise is not contained in a narrow frequency band and can make filtering of noise difficult.
