SLUAAC5 August   2021 UCC28700 , UCC28701 , UCC28702 , UCC28703 , UCC28704 , UCC28710 , UCC28711 , UCC28712 , UCC28713 , UCC28720 , UCC28722 , UCC28730 , UCC28740 , UCC28742 , UCC28910 , UCC28911


  1.   Trademarks
  2. Introduction
  3. Brief Review of DCM FM, AM, FM Flyback Control Law
  4. Input (VIN) and Output (VOUT) Voltage Sensing for UVLO and OVP Fault Protection
  5. Input Under Voltage Lockout (UVLO) Protection
  6. Output Overvoltage (OVP) Protection
  7. Not Recognizing a UVLO or OVP Fault
  8. Separate Bias Supply Startup Issue and Resolution
  9. Not Having a Clean Aux Winding Signal
  10. Removing Aux Winding Ringing to Resolve False Triggering of OVP and UVLO Faults
  11. 10Noise on CS Pin Tripping Over Current Protection (OCP)
  12. 11Summary
  13. 12References


Remember when designing a DCM flyback controller that uses the transformer aux winding for fault sensing to sample the input and output voltage. It is critical that the aux winding waveform is as clean as possible with as little ringing on it as possible. Some designers struggle with this ringing caused by parasitic inductances and capacitances that cause false OVP or UVLO faults. This application note describes how to prevent false OVP and UVLO faults, A RCD clamp and or a snubber across the flyback converter’s output rectifier is recommended to dampen the aux winding ringing.