Re: LF: H Bridge power MOSFET based amps question

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

Forget my comments about gate resistors, you said for a non linear (switching) design.  I was talking about linear amps

As the last post said, small values to damp oscillations if any are needed at all - assuming a proper square wave drive, they shouldn't be.

'JNT

On 25 November 2016 at 14:16, Andy Talbot <andy.g4jnt@gmail.com> wrote:

continuing...

The gate resistor just has to swamp teh Xc of the gate capacitance in a linear amp, to make it look clean and resistive at the input.  So, for example, assume 2000pF  Cgs at 475kHz.   Xc = 167 ohms.   SO a shunt R of say 30 to 50 ohms will dominate.    

If you were really daring you might think it possible to resonate out the Gate C and then get really high gain.   ( Don't do this with an expensive FET).   But worth trying the experiment to see - power oscillators take on a completely new meaning if you try it.

drain / source...   emitter / collector.  They're just legacy names.  You just know what they are without thinking of the actual meaning.   Although, I suppose, if you think of electron flow...........

'JNT

On 25 November 2016 at 13:56, Chris Wilson <chris@chriswilson.tv> wrote:

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



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