Re: LF: Varistor spike management

[Markus Vester <markusvester@aol.com>]

Hi Chris,

the "overshoot" screenshot seems to indicate that ihe peak is not only a short RF spike but caused by a longer positive excursion of the supply voltage. This is probably caused by the suddenly ending current consumption when the carrier goes off. If the supply line contains significant inductance, it will try to sustain the current and thus increase the voltage. In a switchmode supply this could easily happen in the output ripple filter. A larger electrolytic cap (e.g. 100 uF) across the DC line near the PA should fix it.   

Best 73,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: Chris Wilson <dead.fets@gmail.com>
An: DK7FC <rsgb_lf_group@blacksheep.org>
Verschickt: Fr, 28. Apr 2017 13:38
Betreff: Re: LF: Varistor spike management

28th May 2017

Hi Stefan, remarkable work on VLF, congratulations!!

I see these with all FET's but I acquired very cheaply some Wolfspeed
FETs with a lot of overhead. These spikes sometimes kill lesser ones.
If you follow these links you can see the normal gate and drain
waveforms, and the drain waveform without a Varistor as an OPERA TX
period ends, i took the latter from a series of screenshots showing
the highest level the close TX spike reaches

http://www.gatesgarth.com/wolfspeed-scope-capture-50v.jpg

http://www.gatesgarth.com/wolfspeed-overshoot-50v.jpg

http://www.gatesgarth.com/wolfspeed.pdf





Best regards,
Chris 2E0ILY mailto:chris@gatesgarth.com

My part time LF grabber is at http://www.chriswilson.tv/grabber.html


> Hi Chris,

> Which type of FETs are you using?
> The IRFP460A for example has a built in Z-diode which seems to handle
> the spikes. Anyway i put a 150 Ohm / 220 pF (for LF) series RC paralell
> to the Drain-Source...

> 73, Stefan


--
Best regards,
Chris mailto:dead_fets@gmail.com