Hi Chris,
Which type of FETs are you using?
The IRFP460A for example has a built in Z-diode which seems to handle
the spikes. Anyway i put a 150 Ohm / 220 pF (for LF) series RC paralell
to the Drain-Source...
73, Stefan
Am 27.04.2017 16:03, schrieb Chris Wilson:
Hello LF'ers,
In the hope it may help or be of interest my experiments with
containing the level of spikes seen at the end of a TX on the drains
of push pull FET's has shown that the conventional, wire lead
varistor of sufficient capacity to quell these spikes adds
significant capacitance and ringing on the drains is greatly
increased. I thought I'd get some surface mount ones and the
difference is quite remarkable. They add NO additional ringing as
they are physically a LOT smaller for a similar power handling
ability, and thus have much less intrinsic capacitance themselves. I
used these and spikes on the end of a TX where the back emf was
showing up to 400V + are kept at or below 200V, within the
capability of most FETs likely to be used in a medium or large power
amp. They also have that wonderful quality that we all love of being
very cheap!
http://uk.farnell.com/panasonic-electronic-components/erzvf2m121/varistor-smd-120v/dp/1845508
I am not sure if many have looked at their drain wave form when a TX
ceases, I have to assume everyone has some spiking, just what
decides how great it is I am not sure.
|
