Re: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure?

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

There is Miller feedback present, where the drain change of voltage is fed back to the gate via internal capacitiance.  The effect is to magnify the Cdg value by the voltage gain, so its bound to give something.

Also the Cgs - which fo rlot voltage high current FETs is huge, varies with Vds, so all sorts of things ae happening.  And bearing in mind a FET under switching in in a very non-linear region, its just not going to be worth trying to understand / explain every blip in the waveform

Raab, in his varius discussion od Class-E at HF using low cost mosfets, in fact suggests sinewave drive.  the theory being that the removal of hard edges lowers teh drive requirment anyway, and the actual switching of a class-E transmitter, since it occurs at zero drain volts, doesn't have to be hard switched.

Not sure how well that argument applies to class D push pull though.   I think for any QRO LF transmitter these days I'd always go the Class-E route now. http://www.g4jnt.com/QRO_500kHz_PA_Breadboard.pdf

 They're more difficult to set up in the first place, and narrow band, but they are simpler and more efficient when finished, and have a substatial part of the low pass filtering already present in the tank circuitry

Andy  G4JNT

On 10 August 2015 at 17:35, Michael Probert <mikegw4hxo@solva.co.uk> wrote:

Hello Andy and Chris,

Have been following your discussion with interest. I too experienced the "hand grenade effect" of removing the drive whilst using DC coupling. Dave YXM put me right some years ago.

Thanks Andy for the further explanation.

I have always been perplexed by the distortion of the gate pulse that occurs when the drain volts are applied.
This sometimes appears to show up as a slight instability on tail of the drain pulse. Probably of no importance but never heard anyone mention it.

Would appreciate any comments

Mike -GW4HXO
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Subject: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure?

Hello Andy,

Monday, August 10, 2015


> No, capacitive coupling with diode restorationends up with near-enough
> the same voltage applied to the gate as you had in thr first place.


> A 0/12v output from the driver chips ends up at -0.7 /+ 11.3 V
> (assuming 0.7V diode drop)




> As for the question of the dual drive with resistors.    The resistors
> may as well say in, just to smooth off edges and reduce dissipation in
> the driver.     i doubt you'll notice any difference.


> Andy  G4JNT

OK, thanks and sorry for the naive questions, lots to learn, I was amused by my friend calling power FET's "Fire Emitting Transistors". I see his point now, they do go with a good bang and sizzle! The wife smelt them downstairs and asked wearily if it meant a longer wait before a job she attached great importance to, but I classed as a boring irrelevance, got done...







--
Best regards,
 Chris                            mailto:chris@chriswilson.tv