SLUAA66 May   2020 UCC28740

 

  1.   Practical Feedback Loop Design Considerations for Flyback Converter Using UCC28740
    1.     Trademarks
    2. 1 Introduction
    3. 2 Design Considerations for the Stability
    4. 3 Experimental Results
    5. 4 Summary
    6. 5 References

4 Summary

The TL431 and an optocoupler play an important role for the stable feedback and control loop. The open loop gain of the TL431 is significantly changed by the pullup resistor, and the value of this resistor must be chosen to provide sufficient bias current to the cathode of the TL431 under all operating conditions. When the optocoupler is connected to FB pin of the UCC28740, it will introduce a pole (fp2(opto)) due to Copto. fp2(opto) will affect the total system stability, and the contribution of Copto should be considered for the stable operation. The components including Copto, CFB3, RFB3 and RFB4 are important to determine the dominant pole position (fp2) properly, and fp2 is calculated applying these components in this application report. The location of fp2 has been confirmed by measuring the loop gain and the phase margin using the UCC28740 EVM-525.