Re: LF: W1VD amp help - more waveforms

[Andy Talbot <andy.g4jnt@gmail.com>]

I'm a bit late coming to this discussion, but that ringing looks exactly like that you get when there is too much leakage inductance in the transformer.  It is common to see such waveforms in SMPSUs and they have to be snubbed with a, well, a snubber network,  Usually a C / R / Diode network to catch and dump the spikes.   As Jay says, the windings do look a bit 'loose' and they'll certainly be giving some leakage inductance.   

I've never used toroids in high power LF PAs but have always used more traditional shaped cores like ETD types and pot cores - not least because they are cheaper than torroids, but more importantly, they enclose the  winding in ferrite much more , so reducing leakage.

But there again, I wouldn't do an LF transmitter by switching into a transformer in the first place.  Far better to start off with resonant circuitry such as a tank directly onteh active devices, THEN use a transformer when the waveform is sinusoidal, or nearly so.   That way, leakage inductance doesn't give spikes; it just contributes a bit of spurious L that you can absorb into the reactive components and tune out.   Hence, I'd only ever go for the series tank or class-E solution.  

http://www.g4jnt.com/PowerAmps.htm

Andy  G4JNT

On 9 April 2018 at 18:41, <jrusgrove@comcast.net> wrote:

Paul

Thanks for the photos and information. I was more interested in the superimposed gate waveforms than the drains, though. A pix of the LPF as well would be appreciated ... think there was one last week. Just for the record ... my doubler was never mounted inside the amplifier ... so you are covering new ground with that. I would think having it external to the amplifier would be less problematic. For testing, stick to driving it with the HP3325A for now ... which is what you're most likely already doing.

Can't find much fault with the construction ... other than the somewhat loose primary winding on the transformer. In mine, the primary winding was nestled inbetween the turns of the secondary. This is important to minimize leakage reactance ... which also contributes to excessive ringing.

Think I would continue to concentrate on getting the ringing down before going to high power.

One other thing ... have you double checked that you have the correct capacitance in the LPF?

Jay W1VD






----- Original Message ----- From: "N1BUG" <paul@n1bug.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, April 09, 2018 11:34 AM

Subject: Re: LF: W1VD amp help - more waveforms

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