Re: LF: W1VD amp help - more waveforms

[N1BUG <paul@n1bug.com>]

Yep, voltage breakdown of the FETs was my worry. However I may be
making a false assumption. I assumed the voltage spikes in the
ringing would vary linearly with applied drain voltage? Perhaps that
isn't necessarily so?

With 13V on the drains the first spike here is 'about' 64V.

Efficiency seems fine. I wouldn't try to put an exact figure to it
what with measurement accuracy in three places (voltage, current, RF
power) but it 'appears' to be in the mid 90s.

Paul


On 04/09/2018 11:02 AM, jrusgrove@comcast.net wrote:
> Alan
> 
> The main concern about the ringing is that the positive extreme stays below 
> the drain source 
> breakdown voltage ... with some 'headroom'. Otherwise ...
> 
> Jay W1VD
> 
> 
> ----- Original Message ----- 
> From: "Alan Melia" <alan.melia@btinternet.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Monday, April 09, 2018 10:21 AM
> Subject: Re: LF: W1VD amp help - more waveforms
> 
> 
>> Paul
>> I think you need to step back from this a little. One point, design/cable 
>> routing at 137kHz is 
>> normally not that critical and it is unlikely it is having much effect. Many 
>> us have ''haywired'' 
>> PAs at these frequencies without major troubles....even using croc-leads!!
>>
>> You are unhappy about the drain waveforms.......why?? the ringing is on the 
>> ''off'' device and so 
>> doesnt really matter in terms of efficiency. Consider what is happening. The 
>> FET drive goes off 
>> and the drain current starts to drop inductance in the drain feed causes the 
>> voltage to rise to 
>> try to keep the current flowing. Then on top of this the other transistor 
>> switches on and the 
>> transformer connection for the off side sees a rising induced voltage out of 
>> the transformer. 
>> There is bound to be overshoot but it does not absorb much power. It may be 
>> the generous ''dead 
>> zone'' (whilst a good idea) that is producing this picture.
>>
>> The critical part is the the flat zero volt line when the FET is on. The 
>> current pulse rings a 
>> bit, but the current drops in the second half of the pulse.....why?? This, I 
>> think, is really the 
>> loss of power. Does Jay's show this? The point I am making is don't get 
>> drawn up blind alleys 
>> because a trace just does not look ''pretty''.
>>
>> I think we have mentioned this before but the transient performance of the 
>> power supply may be a 
>> factor. There should not be great changes in the current draw because you 
>> are switching the PSU 
>> load from one transistor to the other but there can be higher speed 
>> transients that can affect 
>> power supply regulators in unexpected ways.
>>
>> As Andy said efficiency at 12v is not very meaningful because of the high 
>> rds of these FETs. It is 
>> usefull to run up a PA at low volts say for safety when the antenna needs to 
>> be tuned.
>>
>> Finally the scope probes are placing an. albeit small, capacitance across 
>> the drains (I am 
>> assuming they are correctly trimmed) The waveform can often be affected by 
>> where the ground is 
>> picked up from.
>>
>> Stick with it because you are close to being ''there''
>>
>> Alan
>> G3NYK