UCC28950: About RCD snubber for UCC28950

Hello

There is an extensive discussion about snubber design at http://www.ti.com/ww/en/power-training/login.shtml just enter the keyword snubber

The 'snubber' used in the EVM is more like a clamp rather than a snubber circuit. This is because the RC time constant of 220nF with 500 Ohms is about 110us which is much, much longer than the duration of the spike being snubbered. The circuit operates as follows: diodes D4 or D6 become forward biased when the spike generated by the transformer reaches the voltage on the capacitor C1. The spike is then clamped as it charges the capacitor. The resistors, R8 and R9 prevent the capacitor from being charged without limit. The RC time constant is about 110us, which is much longer than the duration of the spike. Unfortunately I did not design this circuit myself and the original designer has retired so I'm not certain of the function of R6 (100k). The time constant of R6 with C1 is 20ms - It may be present to ensure C1 gets discharged fully when the power stage is turned off - but to be honest I'm not sure.

Finally - most snubber circuits are designed by using published information as a guide and then iterating to a solution that is an acceptable compromise between power loss, peak voltage control and (sometimes) EMI.

Regards

Colin