Paul
If you look at the spec sheets most of these FETs have a large input (gate) capacitance. Some are more than 5000pF.
This is taking the clean edges off your drive waveform.
The FQA34N20 was only 2450pF, so a good choice.
The IRFP260 is at 4000pF, mid way. Seems you have to spend serious money to get the low-capacitance FETs!
Dave
On 30 November 2018 at 14:36 N1BUG <[email protected]> wrote:
Thanks Dave!
Your note prompted me to go back to the bench and look at this
again. I think I just might be starting to get a clue. I'm not the
brightest but I really am making an effort to learn.
I made some more tests with the oscilloscope.
Looking at the output of the IR2110 FET driver *without* FETs
connected, the rise and fall slopes are extremely steep with a
clear, flat dead zone between the end of the fall on one side and
start of rise on the other.
With FETs connected but no Vdd, the slopes are far less steep and
the fall just reaches zero as the rise on the other side begins.
(with the original FETs there was still a flat dead zone here)
Applying minimal Vdd (13V), the slopes become less steep again plus
they have large FET turn-on / turn-off "steps" along the rise /
fall slopes. Now the falling side has not reached zero as the rising
side is starting up.
I don't know how much of this is due to the speed of the FETs and
how much may be softening of the rise / fall due to lead inductance
or other phenomena in my gate circuit / driver board as more current
is sourced / sinked. But I do know the slopes with these FETs are
not as steep as with the previous FETs.
It should be quite interesting to install faster FETs and do these
checks again.
73,
Paul
On 11/30/18 8:29 AM, DAVE PICK wrote:Paul
Try the Infineon IRFP260MPBF they are fast and reasonably priced.
I use Ixys IXFH58N20 but they are expensive... they are the best
though! Can't find any FQA34N20 over here from reputable
sources.
Dave G3YXMOn 30 November 2018 at 12:58 N1BUG <[email protected]
mailto:[email protected] > wrote:
Yesterday I replaced FETs in my 2200 meter amplifier. Since
the FQA34N20 is long discontinued I had no choice but to make
a substitute. I chose the FQA32N20C because I know of one
active station using them in the same amplifier with success.
I notice drain ringing is significantly greater with the new
FETs, but maybe acceptable. I have little choice but to run the
amp and see how it goes.
I also notice the gate voltage decay takes about twice as long
as the old FETs. According to the data sheets, turn off fall
time on the 32N20C is almost twice that of the 34N20. I suppose
that is what I am seeing. I *guess* this may be related to
increased ringing, as it appears one side is not fully off as
the other begins to switch on.
I would appreciate any comments or suggestions, including
suggestions for a FET which might leave a longer dead zone at
the phase transition. I'd think slow to turn on, fast to turn
off would be nice...
Alternatively, if anyone has a stash of FQA34N20 purchased from
a well respected vendor (old stock) and would be willing to
sell some, I may be interested. I've already tried some which
originated from a Chinese source and am not interested in going
down that road again. :-)
73, Paul N1BUG ______________________________________________________________
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