Control of the output voltage is provided by a voltage regulator circuit located on the secondary side of the isolation barrier. The demand signal from the secondary regulator circuit is transferred across the isolation barrier using an optocoupler and is fed into the FB pin on UCC25640x. This section discusses the whole feedback chain. Figure 7-3 shows the block diagram of the FB chain, and Figure 7-4 shows the typical timing diagram with a normal soft start followed by a ZCS event, and load step into burst mode, and then exiting burst mode.
The feedback chain has the following functions: