SLUSCM5A August 2017 – February 2018 UCC24612
It is required for the user to setup snubber components C3 and R3 to get the best performance when using the UCC24612EVM.
To setup these components will require knowledge of the Flyback transformer secondary leakage inductance (Lslk) and measuring the secondary resonant ring frequency (fr) in circuit. It is recommended that the SR is not driven while doing this to simplify the process. It is also recommended to do this test at partial load to avoid creating too much heat on the SR body diode because the conduction loss is much higher. TP3 should be disconnected from the Flyback converter to ensure FET Q1 is turned off while setting up the snubber.
The secondary winding capacitance (Cs) then needs to be calculated based on the following equation. Please note for a transformer with a secondary winding leakage inductance of 3.8 µH and a ring frequency of 2 MHz, the parasitic capacitance would be 1.7 nF, for example.
Based on the calculated Cs, Lslk and fr, the snubber resistor R3 can be set to critically dampen the ringing on the secondary, which requires setting the Q of the circuit equal to 1.
Capacitor C3 is used to limit the time the snubber resistor is applied to the aux winding during the switching cycle. It is recommended to set the snubber capacitor C3 with the following equation based on the Flyback converters switching frequency (fSW). For a Flyback converter switching at 85 kHz in the example would require a C3 of roughly 497 pF.
Please note that the calculations for R3 and C3 are just starting points and should be adjusted based on individual preference, performance and efficiency requirements. More snubber design information can be found in "Snubber Circuits Theory, Design and Application".