Feedback loop compensation of a current-mode Flyback converter with optocoupler
- RAJ SHARMA
- Apr 2
- 1 min read
The flyback converter is an isolated switching power supply topology widely used for output power levels below 150 W (Figure 1). In addition to providing galvanic isolation between input and output, it generates an output voltage which can be higher or lower than the input voltage. Its range of applications is vast, covering not only offline AC-DC conversion and power factor correction stages, but also high and low-voltage DC-DC converters alike in virtually all industry areas. A few example applications are main and auxiliary supplies for home-appliances, battery chargers for smartphones/tablets, LED lighting, computer/laptop power supplies, power-over-ethernet (PoE), industrial power, auxiliary supplies in motor drives, etc. To keep the output voltage regulated at a target level despite variations in the input voltage or output current, the output voltage needs to be monitored and a related signal fed back to the controller, which can then adjust the duty-cycle of the control transistor regulating with it the flow of energy to the output. This is a closed-loop negative feedback system, with a plant block (formed by the duty-cycle generator and power stage), and with a compensator block, necessary to stabilize and shape the dynamic response of the converter (see Figure 2). Note that the compensator is designed to ensure not only that the feedback loop is stable, meaning that perturbation signals will not generate a sustained oscillation in the system, but also that after a transient event the output voltage will meet the target requirements regarding over- and undershoot, ringing and settling-time imposed by the supplied load.
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