Once the frequency response of the power stage has been calculated, a controller needs to be used to close the loop. The frequency response of the controller needs to be designed to give an overall control loop with the following characteristics:-
- High gain at low frequencies to give excellent DC set point accuracy
- High loop cross-over frequency to give the best possible transient response to rapid changes in load or input voltage
- Loop phase shift significantly less than 180° at the loop cross-over frequency to give a stable and robust system
The overall loop response of the converter is the sum of the responses of the power stage and the controller. This overall loop response can then be used to predict DC gain, transient recovery time and whether the power supply will over/undershoot under transient conditions.
Compensation of a Current-Mode Flyback Converter with Type 2 Control
The type 2 controller used for the flyback converter here gives very high DC gain, only limited in practice by the finite gain-bandwidth product of the op-amp used in the control circuit. The loop cross-over frequency is around 2kHz and the loop phase shift at this point is only 90°. The result is a very stable system which will show no overshoot during transient loads.