Re: Re: H bridge progress

On 12 Jun, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> A resistor from gate to DRAIN will turn it ON! To turn it off, you want
> a resistor from gate to SOURCE! But, the 2A or so drivers from IR do
> a fine job of turning fets on and off, and also handle driving the high-side
> fet, whose source is swinging through the full DC voltage supply.

Sorry, brain fade here .. yes, Jon is correct.

The circuit I was thinking about uses a resistor from gate to drain to
turn it on relatively quickly, and a transistor to turn it off even
quicker.

It's true, the 2104 does ease the ugly task of driving the high side
FETs, but more than one designer has reported problems with the charge
pump aspect of these devices.

Worries about this problem - perhaps unfounded - have caused me to
always use a more primitive drive circuit, tempting though these
devices are.

While not particularly power efficient ( although not bad ), the
slight asymmetry in the turnon-turnoff behaviour is advantageous.

More to the point, I'm always nervous about designs with single sourced
parts - little guys frequently sent way to the back of the line.

I had a product with a Motorola part that was just wonderful. Come
time to produce a second batch, I was informed that there was
no stock and the earliest delivery date was 9 months from now ..
maybe.

Apparently, Ford had bought up the entire production. Somehow, my
request for a hundred of them just didn't match up against Ford's
request for every one of them in the world and every one to come.

> No, that actually allows current to recirculate (through the reverse diode
> on one of the transistors) indefinitely, reduced mostly by the IR drop of
> the coil itself.

Right again .. man I hate Monday mornings.

New Japan Radio has some application notes at

http://www.njr.co.jp/pdf/ee/ee05002.pdf
http://www.njr.co.jp/pdf/ee/ee05004.pdf
http://www.njr.co.jp/pdf/ee/ee05005.pdf

that give a good description of the plusses and minuses of fast and
slow current decay.

Alan


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Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta