"Eeyore" wrote in message
...


Or use schottkies; they don't snap at all.

Yes I found the 11DQ10 and MBR1100 which could do the job. Vrrm needs to
be ~70V.
Would schottkies still benefit from snubbing ?

Graham


Some of my experiences with SB diodes from my monitor repairing days might
be informative.

First you should note the ESD warnings most manufacturers put on SB diodes,
they are very sensitive to excessive reverse voltage. Some monitor
manufacturers used SB diodes to protect the gate in the SMPSU MOSFET as it
doesn't suffer junction storage delay if ringing drives it into forward
conduction and they reliably fail S/C if the PIV is exceeded, on some blown
PSUs in which the MOSFET channel had been destroyed the 1N5817 had
successfully protected the O/P of the UC3842 long enough for the fuse to
break.

My experience with snubbing SB diodes is probably not all that relevant,
some monitors were badly designed with underrun heaters - the cure was to
upgrade the heater rectifier to an SB type, as the SMPSU was invariably a
flyback type the peak reverse voltage was often many times the rectified
voltage, which made it harder to find SB diodes with adequate PIV. The cure
was to snub the flyback peak with a diode, capacitor and resistor, the diode
and capacitor forming an extra O/P with opposite polarity to the heater
supply and the resistor to load it and pull down the flyback peak.

As SB diodes seem somewhat less robust against excessive reverse voltage,
and fail S/C more reliably. It is important to consider carefully before
replacing regular diodes with SBs, what are the chances of spikes exceeding
the PIV? And what other damage will happen if the rectifiers fail S/C?