[Lowfer] LF: Re: FET change, hmmm

[DAVE PICK]

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 <paul at n1bug.com mailto:paul at n1bug.com > 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 G3YXM
> > 
> >             > > > On 30 November 2018 at 12:58 N1BUG <paul at n1bug.com mailto:paul at n1bug.com
> > >             mailto:paul at n1bug.com mailto:paul at n1bug.com > 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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