Re: LF: H Bridge power MOSFET based amps question

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

Quite simple.    Same number of FETs for given power, you either parallel them in half-bridge,  or use singly in H bridge

But H-bridge give you four times the impedance, and high Z is always good news, easier to work with, even if it does mean a higher value L in the tank.

My 320V (DC) design, of course, had a high load resistance to start with at that voltage, so half-bridge was quite adequate.   Still working fine after nearly 20 years http://www.g4jnt.com/137tx.pdf

Andy  G4JNT

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