Subject: Re: [Lowfer] LF: Re: FET change, hmmm
To: rsgb_lf_group@blacksheep.org
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From: N1BUG <paul@n1bug.com>
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Message-ID: <ee91a690-d49d-c0e0-8384-807b67b8bcf2@n1bug.com>
Date: Sun, 2 Dec 2018 14:28:22 -0500
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Reply-To: rsgb_lf_group@blacksheep.org

Hello Chris, LF

> Paul: W1VD makes the comment that the IR2110 is near its limit gate
> capacitance drive wise, driving the parallelled devices, so a change
> to devices with more gate capacitance is going to be pushing things. I
> know these are dear but the gate and drain waveforms I see with them
> and notably better than the older design FET's, although for me they
> have not been an issue. My free supply of the Wolfspeed FET's has
> stopped, otherwise I would still use them. I lost a few whilst finding
> my feet and ironing out mistakes, and poor layout. A gate capacitance
> of 660pF is pretty impressive ;) Can you upload screen shots of the
> gate and drain waveforms you are now seeing for comparison, they are
> kind of hard to envisage in words ;)

I was so frustrated I did not get out the camera to take photos of
the scope traces. But I promise before I remove these FETs I will do
that and post some waveforms with the original FQA34N20 vs. the
FQA32N20C.

I was not monitoring internal waveforms at the time I stopped on air
testing. In my present setup it is difficult to have the scope
connected to a PA when it is running from the big power supply. I
stopped because when I reached 200W output I could see the output
sine wave after the LPF was starting to fluctuate in a certain way.
I have seen it before with more than one PA, and have come to
associate the particular pattern with excessive ringing. Usually it
comes just before the big bang!

I really don't want to re-engineer the whole PA to get it working if
I can avoid it. By now we have all seen my lack of engineering
skills! ;-)

73,
Paul