Envelope-to: dave@picks.force9.co.uk
Delivery-date: Mon, 03 Oct 2005 11:51:29 +0100
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From: "Andy" <actalbot@southsurf.com>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 3 Oct 2005 11:48:10 +0100
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Subject: LF: Re: Advice (again) please - Driving FETs
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If you drive FETs directly from a driver IC, when the FET blows up the gate
shorts to the high voltage, which feeds back and blows all your driver
circuitry too.  I learned that  the hard way when designing a SMPSU some
years ago.  It used a high / low side driver chip (HiP2500 500V Half-bridge
driver)  with a boot strapping arrangement that raised the mean level of
drive waveform for the upper device.

When the arrangement popped (due to overload) two cheap and cheerful surplus
FETs got destroyed, along with the exotic and expensive driver, and the
SMPSU controller chips.  Both of which cost as much as about 10 of the
mosfets.  After this had  happened twice, I changed to a pair of pulse
transformers straight from the SG3525 SMPSU chip which has the same sort of
totem-pole output drive as the TC4425/6/7 family,  and all was fine after
that.

Certainly will never use direct FET drive in experimental high power designs
again.

Andy  G4JNT

-----Original Message-----
From: OSBORN, Chris <Chris.OSBORN@astrium.eads.net>
To: LF Group (E-mail) <rsgb_lf_group@blacksheep.org>
Date: 2005/10/03 11:14
Subject: LF: Advice (again) please


>
>
>
> I'm still beavering away on my new TX and now have stopped blowing
>up the
> PA devices (fingers crossed).
> I've incorporated some of  the suggestions gratefully received:-
> - Higher voltage FETs - IRF630's
> - gate resistors to prevent vhf oscillation
>
> I have also changed from transformer drive to using a 4426 driver
>IC.
>
> I'm generating 100 watts into my 50 ohm dummy load but
> only getting an efficiency of about 50%.
> The output transformer toroidal core is getting quite hot and I
> suspect that it's permeability is too low as the capacitance
>required
> to bring it to resonance is far higher than those of published
>designs
> in the LF Experimenter's handbook, using the same number of turns.
> These capacitors are running very hot too (as mentioned by PA0SE in
>his LFEH article).
>
> Has anyone a spare 3C85 toroid suitable for a PA stage which they
>would sell to me ?
>
> Alternatively I've looked in the RS catalogue but am not sure what
>is a suitable type.
> They quote a parameter   'AL'      ( example    3225 nH)
>
> From my old college days I remember uo and ur but what is 'AL' ?
>
> 73 Chris G3XIZ
>
>
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