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From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <635074543.20161125135600@chriswilson.tv>
In-Reply-To: <635074543.20161125135600@chriswilson.tv>
Date: Sat, 26 Nov 2016 10:35:35 -0000
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Subject: Re: LF: H Bridge power MOSFET based amps question
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Chris
The Decca 5501 amp modules use a H bridge class D configuration followed by 
a series resonant PA circuit and this is  probably one of the most reliable 
amplifiers about. Use suitable rated FET'S to give the desired RF output.
Also the protection cct used avoids disaster in the event of an antenna 
failure short/open cct,
I have built lots of class D 1Kw push pull amps over the years for LF/MF but 
they are vunerable to damage with load variations whereas the Decca bridge 
amps survive. I never had a failure with the Decca, and have been using them 
for many years.
Another approach which is worth a consideration is the class E amp single 
ended, these are very reliable and avoids the crossover switching problems 
with the push pull system. Setting up procedure is critical but there after 
very stable,
Hope these observations help you

de mal/g3kev

-----Original Message----- 
From: Chris Wilson
Sent: Friday, November 25, 2016 1:56 PM
To: rsgb_lf_group@blacksheep.org
Subject: LF: H Bridge power MOSFET based amps question

25 November 2016


I have built a 1kW Class D amp for none linear digital transmissions
around 136kHz.

My Dave Pick designed amp uses four MOSFETS, two in parallel each side
driven by a dedicated driver chip. I see the odd LF amp design using
what's described as a H Bridge MOSFET array. The one I most recall
most, by Andy Talbot used rectified 240 volt single phase mains to run
it. My question is why use an H Bridge rather than paralleling more
MOSFETS in what to me is a more conventional format? Does it overcome
voltage or current limitations? Is it due to adding more capacitance
with paralleled devices? Or does it allow a linear amp to be built?

I realize a meaningful reply might be quite lengthy, so I assure you I
have Googled it, but most stuff related to in the articles is
industrial motor control.

Secondly, in a Class D none linear amp, how is the gate resistor value
chosen, and is there ever a case for not using one at all?

Thirdly, am I alone in thinking the terms Drain and Source seem back
to front and the names would intuitively be reversed so a Drain was
called a Source and vice versa? :)

Thanks. 2E0ILY


-- 


Best regards,
Chris  2E0ILY                           mailto:chris@chriswilson.tv