Re: LF: W1VD amp help - more waveforms

[N1BUG <paul@n1bug.com>]

Hi Rik,

I don't recall that having been mentioned previously in this
discussion. I am sure I have read that somewhere in my studies over
the past year but it obviously had not taken root as it seemed all
new to me when I read your message today. :-) Your explanation of
how the resistor and diode work is helpful.

Today's experiments to update those who are following this story:

I moved the driver board so the gate leads are much shorter. I used
thicker wire for the gate leads. Result: absolutely no change.

At Jay's suggestion I put additional 1000pf caps in a parallel with
the existing 1000pf timing caps on the driver board. Result: no
change that I can be certain of. If anything, the timing could be
just a tiny bit *worse* than before but this might be just my eyes
playing tricks.

Paul



On 04/11/2018 10:42 AM, Rik Strobbe wrote:
> Hello Paul,
> 
> I was on holiday last week and didn't pay much attention to what
> passed in the reflector. So my appoligies if I tell things that
> have been told earlier.
> 
> The "both sides conducting" issue is typical for the power
> MOSFETs as the turn off delay is much larger than the turn on
> delay. Exactly for that reason most class D push-pull PA's have a
> gate resistor to delay the turn on a bit and a "reverse" diode to
> bypass the resistor at turn off (so the turn off is not extra
> delayed). If you see the one side turning on befor the other side
> is turned off it migh be worth to increase the gate resistors
> (from 10 Ohm to 15, 18 or even 22 Ohm). This will increase the
> turn on delay and maybe solve the issue.
> 
> 73, Rik  ON7YD - OR7T
> 
> 
> ________________________________________ Van:
> owner-rsgb_lf_group@blacksheep.org
> <owner-rsgb_lf_group@blacksheep.org> namens N1BUG
> <paul@n1bug.com> Verzonden: woensdag 11 april 2018 16:10 Aan:
> rsgb_lf_group@blacksheep.org Onderwerp: Re: LF: W1VD amp help -
> more waveforms
> 
> Hi Rik,
> 
> Of course that is true and I can see a difference in the gate 
> waveform with Vdd on and off. But it doesn't explain why under
> same conditions (Vdd off) Jay and I see quite different gate
> waveforms.
> 
> I just did another test, gate waveforms with Vdd on.
> 
> http://www.n1bug.com/gates12-wdv13.jpg
> 
> Depending how I interpret this, I can almost believe I see both 
> sides conducting for a very brief instant.
> 
> Looking at this critically, I can see that one starts to turn
> off before the other starts to turn on. So there is a delay
> being generated by the driver circuit. But it's not clear that
> one is fully stopped conducting before the other starts. Seems as
> if the decay time is not fast enough. Maybe it's the length of
> those gate leads messing things up after all...
> 
> Jay's article says the 10 ohm resistors and 1N4148 diodes slow
> gate turn on and speed up gate turn off. Given that both my turn
> on and turn off seem slower than Jay's, can it be my 1N4148's are
> somehow not the same as his?
> 
> Paul