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Date: Mon, 9 Apr 2018 11:02:03 -0400
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Subject: Re: LF: W1VD amp help - more waveforms
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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
>
>
> ----- Original Message ----- 
> From: "N1BUG" <paul@n1bug.com>
> To: "Chris Wilson" <dead.fets@gmail.com>; "N1BUG" <rsgb_lf_group@blacksheep.org>
> Sent: Monday, April 09, 2018 2:22 PM
> Subject: Re: LF: W1VD amp help - more waveforms
>
>
>> Hello Chris,
>>
>> There are plenty of possible causes here I think. When it comes
>> right down to it, I'm not at all happy with the end result of my
>> construction on this amp. If it ends up not working, it's because
>> I'm not a good enough builder. Despite building for 35 years
>> including many QRO tube amplifiers, I felt this pushed my skills to
>> the limit... or beyond? It's not the fault of the design of course,
>> it's me. This requires a very different skill set from what I've
>> developed through the years.
>>
>> The driver board you see is my fourth attempt yet I'm still very
>> unhappy with my layout and on-board routing of connections.
>>
>> I probably made a huge error in design and mounting of the driver
>> board. Everyone seems to agree my gate leads are too long. This may
>> take a while. If I'm starting over I'll have a few more goes at
>> trying to come up with a more sensible layout and on-board routing.
>> I'll probably have to design the new board so it can be soldered to
>> the ground plane for mounting. The only other way I can think of
>> would involve complete deconstruction of the amp to drill and tap
>> new holes in the heat sink for relocated board mounting standoffs.
>>
>> I literally spent two weeks and likely 50 attempts at winding the
>> output transformer, still not happy with it. I'm not sure the
>> primary winding is acceptably tight.
>>
>> Today I'll try swapping out the cables connecting amp to LPF and LPF
>> to dummy load. No reason to suspect bad cables, but I've seen
>> strange stuff happen. The cable between amp and LPF is a bit long at
>> 4 feet, maybe it doesn't like that. I'll have a look on the PSU side
>> of that choke with the scope too. Good idea. Could be some RF there
>> causing the PSU to go insane.
>>
>> I haven't added bypass caps on the driver board yet. I'm out of .1
>> uf caps. I have some 1 uf ceramic but that may be a bit much? I'd
>> probably get shorter leads by tacking them on the back side of the
>> board. Bear in mind I'm testing at 13V which gives about 65W output
>> so there's not massive amounts of RF around yet.
>>
>> Still puzzling over why my gate waveforms look OK (I think?) but
>> drains don't. To my usually wrong way of thinking that suggests the
>> ringing problem is caused by some defect in the drain circuit or
>> outside the amp (cables, LPF). I think I'd like to try and eliminate
>> those possibilities before I go back and completely redesign the
>> driver board and mounting to shorten gate leads. No doubt it'll
>> still come to that but I can hope not a bit longer. :)
>>
>> Paul
>>
>>
>> On 04/09/2018 07:49 AM, Chris Wilson wrote:
>>>
>>>
>>> Hello Paul,
>>>
>>> If my similar amp exhibits ringing on the drains (which I monitor in
>>> real time), it means an antenna mismatch or a dicky connection (PL
>>> type plugs were the cause, cheap junk off Ebay, a change to branded N
>>> types fixed that issue). So if the output transformers are the right
>>> material and wound right, the snubber resistors and caps are correct,
>>> and the gate driver wires are really short, and you are looking into a
>>> decent dummy load, well, it's a mystery! You have put decoupling caps
>>> on all the Vcc pins? If you have a nice PCB and don't want to make
>>> new, and the IC's are socketed you could solder the decoupling caps
>>> direct to the IC pins for a test. Or even if they aren't socketed I
>>> guess :) Without them my amp suffered with RF getting back into stuff
>>> and  "funny  things"  happened. You should also have a `scope about on
>>> the PSU side of the choke.
>>>
>>> Monday, April 9, 2018, 12:29:18 PM, you wrote:
>>>
>>>> Thanks Hugh and everyone else who commented and provided tips on my
>>>> earlier post about this amplifier.
>>>
>>>> I will wait for Jay to have a chance to comment but I *think* I'm OK
>>>> on timing of the phase transitions. However, something is causing
>>>> major ringing (is that the correct term?) on the drains. I believe I
>>>> need to find the cause and do something about that but I have no
>>>> idea what to look at.
>>>
>>>> I've been building and even designing QRO amplifiers for 35 years
>>>> but I'm a vacuum tube HF/VHF/UHF guy! This FET class D/E stuff is
>>>> all new to me and I don't learn new tricks as easily as I once did. :)
>>>
>>>> Paul
>>
>
>