Inverting vs NonInverting Mosfet Driver

I have some fuzzy recollection of reading to the effect that noninverting mosfet drivers are more prone to instability due to layout effects.

Confirm?

Just doing a quick sim in my head: With poor layout, when a noninverting mosfet driver turns on (sourcing), the signal ground pops up due to the mosfet gate capacitance (iirc called ground bounce?) then the driver sees a valid (actually invalid) '0' threshold to turn the mosfet drive off.. The driver oscillates.

Is it best to use a noninverting mosfet driver in a smps design?

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D from BC
British Columbia
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D from BC
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Not affirmative :-)

I have used both, depending on what was needed or sometimes what Digikey had in stock. Never a problem with either. Of course I never design anything without a full ground plane. Also, most modern FET drivers have input hysteresis so it's kind of tough to get them to oscillate without deliberate and serious feedback. Which I sometimes did, in order to use them as poor man's switch mode controllers and that trick only works with the inverting kind.

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Regards, Joerg

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Joerg

If the driver output feeds back into the input enough to compromise the logic levels, you're in trouble either way.

John

Reply to
John Larkin

You have to test drive any prospective part, by any mfr, in-circuit. Duals and non-inverters are more prone to misbehaviour. Low voltage logic level inputs are a mistake to be avoided, wherever possible, even with a 'ground plane'. Some parts are even sensitive to output disturbances, never mind ground bounce on the input, regardless of sales blurbs or specsmanship.

For non-inversion, bypass Micrel MIC4424 parts, if you want to avoid grey hair. Similar lower-powered Maxim parts MAX4427A or Micrel TC4427 seemed OK, although I recall a lack of internal UVLO, which required vigilance.

RL

Reply to
legg

Ahhhh.. :)

INteresting. Thanks

I'm using a MIC4452 non-inverting.

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D from BC
British Columbia
Reply to
D from BC

Never had a problem with Micrel MOSFET drivers. What caused the gray hair?

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Regards, Joerg

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Joerg

Basically, I had opposing mosfet outputs turning on by themselves on the low side of a full bridge driver. The duration of conduction could be reduced but not eliminated with agressive supply decoupling. The input of the offender scoped slightly negative during the entire drive fault period, following a positive glitche of 100nS duration, possibly generated by it's partner. No other pin-compatible part behaved this way, in the same physical position.

In it's 'representative schematic', the 4424 input is depicted as analog, with some kind of current hysterisis introduced to the signal, at the receiver's output, which is just plain barmy, IMHO.

RL

Reply to
legg

That almost sounds like a damaged chip. I've mainly used the 4421 but AFAIK they are all the same architecture. Ok, they aren't really shoot-through protected but they ran nice and cool at a few hundred kHz. They do need a really stiff supply with two planes and good X7R caps, else all hell can break loose. It also does if you hang too big a gate capacitance onto it, which I guess is why they also make 6A, 9A and 12A devices.

It actually works, a few hundred mV hysteresis. Not barmy :-)

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Regards, Joerg

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Joerg

t

),

?

Micrel is a bit of a floor sweeping company. I'm amazed they got as far as they got in standard products, mostly ripping off Maxim and LTC designs. [In the dark ages, they used to be a foundry.]

I had a device that was killing the internal nicads, ripped it open, and found a Micrel LDO, the kind that sucked current in dropout since it used a bipolar pass device. PFET pass device took care of the problem.

Reply to
miso

This was not a damaged chip - substitutions and iterations in documentatiomn of the fault established this. All outputs were only required to drive the gate limiting resistor, which dominated gate current control in an assisted switching circuit, where the big fet in question had it's active capacitances discharged in advance by an external switch.

I suspect it might have been the reverse transfer current hitting the output, before it was required to be active high, that scrambled adjacent internal logic of it's partner, somehow, but that is just speculation. I wasn't prepared to rip apart the entire topology in order to investigate further, with functional substitutes on-hand.

It obviously doesn't work well enough to be specified in the part's datasheet, as such. The concept should have been buried on that basis alone.

RL

Reply to
legg

Sorry to hear that, it's really strange. I have used Micrel drivers a lot and they always delivered. Typically sans gate resistor because I like to drive FETs with gusto where permitted.

Well, at least they state in in the text:

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Quote "Following the input stage is a buffer stage which provides ~400mV of hysteresis for the input, ..."

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Regards, Joerg

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Joerg

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