UCC28730-Q1: Flyback application circuit design

Gary

UCC28730-Q1 and UCC28740-Q1 can both work in this application.  The biggest difference is how they regulate the output voltage, and what the corresponding voltage regulation accuracy will be.

UCC28730-Q1 uses primary side regulation (PSR), where the output voltage is regulated from the auxiliary winding.  The advantage of this is that the PSR control eliminates the opto/TL431 feedback, which reduces total components and improves surge capability since the only component crossing the isolation barrier is the transformer and maybe a y-capacitor.  Since the output voltage is not directly regulated, achieving good cross-regulation (which is the voltage on one output as the other output power varies) can be difficult.

UCC28740-Q1 uses secondary side regulation (SSR), where a TL431/opto feedback is used to regulate the output.  The advantage of this is that the direct SSR control allows for much better cross regulation, especially if you have feedack from both outputs.  The limitation of this is that the opto/TL431 does add additional components.

Typically TI PSR controllers in the UCC287xx family can achieve 5% regulation on one output.  With multiple outputs it gets worse.  If the regulation falls outside of the requirement, they can look to use DC/DC converter (buck/LDO) to improve or to switch to SSR.

Please note that the transformer design has a large impact on the performance, we recommend you share this training video on how to optimize transformer design for multiple output designs https://training.ti.com/multiple-output-flybacks-how-improve-cross-regulation?context=1134585-1139235-1134623

Best Regards,

Eric