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# UCC28742: TL431 Feedback circuit true or not: for the Webench power designer : UCC28742DBVR 47V-49V to 1000.00V @ 0.01A

Part Number: UCC28742
Other Parts Discussed in Thread: TL431,

Dear TI Technical Team,

I am writing to seek clarification on two aspects of a design using the TL431 for feedback and the UCC28742 controller.

TL431 Feedback Circuit:

I have simulated the circuit in LTspice and am concerned about the current flowing through the optocoupler diode. My simulation shows a current of approximately 1.3A, which seems excessively high. Could you please help me identify potential issues in my design that might be causing this high current?

UCC28742 Switching Frequency:

The WEBENCH design for this circuit specifies a switching frequency of 100 kHz. However, the UCC28742 datasheet lists a maximum switching frequency limit. Could you clarify if the 100 kHz switching frequency is appropriate for the UCC28742, and if not, what would be a more suitable frequency for my design?

• Hi Vidhya,

The bias resistor of the LT431 (R4 in your simulation) needs to be way higher than 750ohms as it is designed to limit the bias current of TL431 at no load conditions. It should be designed with equation 32 of Datasheet (see below) at no load conditions.

ICENL is the feedback current at no load (~30uA), CTR_NL is the Current Transfer Ratio of your optocoupler when I_FB=~30uA (no load conditions). VOPTNL is the voltage across the diode at no load conditions, check the optocoupler datasheet and Ropt is 681ohms in your case. VOUTNL is the output voltage at no load conditions (VOUT-ripple)=~1000V.  2V comes is the minimum voltage of the Shunt Regulator LT431.

Thank you