The flyback converter is an isolated power supply topology widely used for output power levels below 150 W. 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.
In order to keep the output voltage regulated despite variations in the input voltage or output current, the output voltage is monitored and a related signal fed back to the controller, which will then adjust the duty-cycle as required to regulate the amount of energy transferred to the load. An optocoupler is typically used to transfer the feedback signal across the isolation barrier, becoming with it part of the feedback loop, and more specifically of the compensator circuit. Therefore, the compensator needs to be designed considering optocoupler parameters like its current-transfer-ratio (CTR) (with its corresponding variations) as well as its collector-emitter parasitic capacitance. If this is not observed, the stability of the power supply may be compromised and the target transient response specification may not be met.
This application note provides an example procedure for designing the feedback loop compensation of a flyback converter with current-mode control and optocoupler-based feedback. 

Main topics:

• Control-to-output transfer function of a current-mode flyback converter
• Analysis and design of a type-2 compensator circuit with optocoupler
• Impact of optocoupler CTR and parasitic capacitance in the system
• How to achieve a fast transient response despite the optocoupler frequency limit
• Analytical, simulation and experimental results of a prototype

Download: ANP113