Operation
The flyback converter is an isolated power converter, therefore the isolation of the control circuit is also needed. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation). Both require a signal related to the output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design.
The first technique involving an optocoupler has been used to obtain tight voltage and current regulation, whereas the alternative approach was developed for cost-sensitive applications where the output did not need to be as tightly controlled but up to 11 components including the optocoupler could be eliminated from the overall design. Also, in applications where reliability is critical, optocouplers can be detrimental to the MTBF (Mean Time Between Failure) calculations.
Recent developments in primary-side sensing technology, where the output voltage and current are regulated by monitoring the waveforms in the auxiliary winding used to power the control IC itself, have improved the accuracy of both voltage and current regulation.
Previously, a measurement was taken across the whole of the flyback waveform which led to error, but it was realized that measurements at the so-called knee point (when the secondary current is zero, see Fig. 3) allow for a much more accurate measurement of what is happening on the secondary side. This topology is now replacing ringing choke converter (RCC) in applications such as mobile phone chargers.
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