Soft Recovery Diodes Lower Transformer Ringing by 10-20X
In linear power supplies with solid state rectifiers, the diode switch-off transient generates a current rate-of-change, dI/dt; excellent diodes generate small dI/dt and poor diodes generate large dI/dt. When dI/dt is large, it produces substantial voltage spikes across the leakage inductance of the transformer secondary (V = L*dI/dt). These voltage spikes stimulate the secondary RLC resonant circuit into oscillatory ringing, wich in turn can have a detrimental effect on the audio quality.
In this article Mark Johnson presents measured data on power transformer secondary ringing, produced by 48 different semiconductor diodes. Diode performance is tabulated to facilitate selection for a particular purpose. A design procedure and equations are given for a secondary snubber that eliminates almost all ringing effects.