Gate drive in datasheet

Whoa, never seen that before.

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Switching times measured at "RG = 0".

I'd really like to have a copy of their drive circuit. I could solve a lot of problems with unlimited current drive and zero inductance, even if it's only 10V.

Tim

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Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
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Tim Williams
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That's because you are not familiar with the struggle to minimize ringing i n circuits containing these devices. The RG refers to the value of damping bootstrap resistance in the gate drive circuit. Since it's application spe cific they revised the original datasheet from RG 2R to RG 0R. The user wil l have to make his own determination of best RG.

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bloggs.fredbloggs.fred

I could have cut them some slack if the idea was they used a low impedance driver rather than a function generator with 50 ohms impedance. That is, maybe they meant Rgen is low, as in Rgen!-=50. . However RG in the datasheet is the intrinsic gate resistance, so it can never be zero.

Reply to
miso

Maybe the 0-ohm drive is offered as a limiting case. That part sure does have a lot of gate capacitance, so the driver is going to work hard.

I always assume that a mosfet is, inherently, infinitely fast silicon with some parasitic Ls here and there. Drive it hard enough, and it will switch fast.

This is generated by a couple of cheap paralleled sot-23 mosfets driving a transmission-line transformer.

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2N7002s are about the same, but only good to 60 volts.

All you need is a ripping gate driver... about 12 cents worth. Under a dollar's worth of silicon in total.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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John Larkin

No, the driver source impedance. Internal RG is specified (0.5 to 1.5 ohms or whatever). Presumably that's what they mean, but that still requires zero external impedance.

How would you create a true zero ohm gate driver circuit?

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

So if it's about ringing, why are they testing it at a clearly impractical condition?

The appnote below only says: "durr, sometimes you need to slow it down with RG, but then it runs hotter". None of their waveforms even solve the problem set out in the title -- they all ring! I'm kind of embarassed by that whole appnote, frankly -- a component manufacturer's engineers should know the secret to quiet, efficient inverters by now. And it ain't "minimize inductance".

At any rate, this doesn't answer my question: how do you implement a true zero ohm gate driver? Are they just out and out lying? I wouldn't put that past a manufacturer like International Rectifier, but TI?

For one example, I've never seen an IXYS datasheet showing RG < 0.2 ohms or so, which happens to be around the output resistance of their driver chips. Makes perfect sense, and it's practical.

Tim

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Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com 
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Reply to
Tim Williams

I know that they mean driver impedance, but they should call it something else besides Rg, which is the intrinsic gate resistance. This is just a bad practice. It looks like apps and design were not coordinated.

You can't make a true zero ohm driver because it is an analog world! ;-) Even your power supply has a finite impedance.

Reply to
miso

I'd have to see the layout to be sure, but it appears they depend on the silicide to keep the gate resistance low. It won't be zero, but this looks like a good scheme.

Reply to
miso

In case anyone was wondering, contacting TI was not illuminating. They said it simply means no external resistor was used. They didn't happen to mention what the driver impedance was. I take that to mean, if you see this figure in a datasheet, ignore it completely.

Tim

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Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com 
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Tim Williams

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