Flyback converter

The first technique, involving an optocoupler, has been used to obtain tight voltage and current regulation, whereas the second approach has been developed for cost-sensitive applications where the output does not need to be as tightly controlled, but many components including the optocoupler can be eliminated from the overall design.

Also, in applications where reliability is critical, optocouplers can be detrimental to the system's MTBF (Mean Time Between Failure).

The third technique, primary-side sensing, can be as accurate as the first and more economical than the second, yet requires a minimum load so that the discharge-event keeps occurring, providing the opportunities to sample the 1:N secondary voltage at the primary winding (during Tdischarge, as per Fig 3).

A variation in primary-side sensing technology is where the output voltage and current are regulated by monitoring the waveforms in the auxiliary winding used to power the control IC itself, which have improved the accuracy of both voltage and current regulation.

This topology is now replacing ringing choke converters (RCCs) in applications such as mobile phone chargers.

Fig. 1: Schematic of a flyback converter
Fig. 2: The two configurations of a flyback converter in operation: In the on-state, the energy is transferred from the input voltage source to the transformer (the output capacitor supplies energy to the output load). In the off-state, the energy is transferred from the transformer to the output load (and the output capacitor).
Fig. 3: Waveform - using primary side sensing techniques - showing the 'knee point'.