Attached a circuit of part of the push-pull PA.
The center tap of the primary is at AC ground with the
DC supply coming in via a choke.
I've been getting a lot of ringing on the drains, as is
fairly normal I guess. Even with very low leakage inductance
there are large spikes appearing on the drain when the device
is off.
I took a close look at these spikes and realised they were
transients running down the primary and reflecting off the
AC grounded mid-point. For example there's a spike 40nS wide
occurring 350nS after the FET switches off, then another after
a further 350nS, and then a third, each with diminishing
amplitude. This is clearly a transient propagating back and
forth along the primary. My attempts to suppress this spike
with the usual snubber led to some hot resistors but not much
attenuation.
So over a cup of tea I wondered what would happen if I let
the transient go the full distance of the double primary.
I removed C1 to let the center float at AC.
Now the round trip time of the transient has increased a lot
and it's just arriving back at the drain when the FET switches
back on again. Result: no transient. An unexpected side
effect is that the transformer windings are now cool.
I'm not quite sure how the circuit is working without an AC
ground through C1 but the efficiency has improved.
Has anyone else had trouble with such primary transients?
On the scope they look quite different to the decaying
sinusoid of leakage inductance ringing with drain capacitance,
they appear as a succession of spikes with long intervals
between.
--
Paul Nicholson
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