Strange MOSFET driver behaviour - Microchip TC4426

IIRC most variants of the IRF540 are around 1700pF on the gate and the TC4426 is really more a 1nF driver. Still, it should have done it in under 100nsec. Sure that you didn't get bad chips or counterfeit ones?

Well, can't be sure of course. Got just 25 from Digikey for the prototype.

Dimiter

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Maybe they saw the word "Transgalactic" in the order and thought, oh, better not send the real stuff :-)

But Digikey is usually a reliable source. Yet they could end up with a bad lot from the mfg. I'd test these things for Rdson, something ain't right there. No chance that some fat inductive spike or ESD gets in?

And ask Jerry the cat whether he has a good alibi for the times the circuit was left unattended ...

On a sunny day (Mon, 10 May 2010 09:53:57 -0700 (PDT)) it happened Didi wrote in :

Yep, stay clear of Microfip.

Hmm, indeed they may have thought they can't be bitten from the other end of the galaxy... :-)

Oh no, nothing. There is a 1uF 1206 decoupling cap at the bottom side of the SO-8, via-d straight into the power/GND pins. And the driver drives just the gate capacitance, drain disconnected.

The same drivers work within < 100nS driving gates of FDS6985 which have something like 500pF gate capacitance, so it is not just R/C. As if they have some active current limiting inside. I won't spend time investigating, after all the 33151 work and have been first team regulars for me for decades. I posted this hoping that someone had the answer readily available :-).

Poor Jerry died aged 14 a few years back, he was the smartest cat I ever knew. He tried to fix a door we had blocked with a rod against his slamming (got around it, located the problem and tried to remove it, can you believe this) when he was < 1 year old... so he might have not been innocent on that either, I would not underestimate him :-). I wish he were still around.... still miss him.

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Original message:

Well, their I2C EEPROMs work OK so I thought I'd give them a chance. But there you go, I should have known better :-). Have yet to identify the mystery of their MCUs popularity, don't these have to work, ROFL.

Dimiter

We've used MAX4420s to drive NTP15N40s through gate drive transformers and they were so fast we had to add gate resistors to slow things down. We switched 400 volts in something like 10 ns, as I recall.

John

Have never tried them, just had a look - look really fast, being single driver per SO-8 is apparently for a reason :-).

But my losses are mostly in the inductors - which I will have to address if I am to offer this for battery operation. One flyback transformer is on an iron powder core and is absolutely marginal at max power (where it won't get, OK), efficiency is 60 to 70% at most. I know you have been through that, is there a lot to gain by using kool mu or MPP? Something like 10W max., I am doing it at about 500 kHz.

Dimiter

Our Rottweiler is 13 and you can see the he's failing. I dread that day but we have to be there for them until the end. Contrary to scientific beliefs animals can do rational thinking and planning. Our Sheperd has done stuff when she was younger, for example stealing a freshly baked roll from underneath an aluminum foil wraper, _then_ putting the wrapper back on. The crumbs in her basket evidenced the mischief though, she didn't have enough time to clean that up. You could literally see that "Darn!" expression on her face.

Powered iron is horrible for switchers; AC losses get huge as flux density increases. MPP is great (more energy storage per volume, much less loss) but expensive. KoolMu is almost as good as MPP and a lot cheaper. Somebody else, maybe Micrometals, has an equivalent to KoolMu.

I've burned the paint off powdered iron cores in buck regulators.

John

^^^^^^^^^^^^

Some new type of weapon? :-)

I am pretty happy with ferrite.

I've exploded an Amidon T200-2. *KABOOM* ... followed by about 120ft worth of wire antenna smacking onto the ground.

Avoid yellow/white mix 26 if possible. It's cheap, so you can give up flux density for dollars, if space and weight allows. I see green/blue mix 52's on motherboards a lot, which apparently has about half the losses of 26, for slightly more price. I haven't seen any other grades used in consumer equipment, just those and ferrite.

I think Micrometals' products are all numbered. I'd guess 52 is the equivalent you speak of, but I don't have a reference for that.

At high frequencies, gapped ferrite looks best. Only 0.4T Bmax, but you can use the whole damn B-H curve, not some teensy fraction of it. At about

Tim

Here are our two Bostons.

He's pretty smart.

Tom

Amazing. If he'd be a human he'd probably be doing hard time already :-))

As was related in an earlier thread, the input hysterisis of the 4426 and similar parts is not enforced on the input signal pin, but after a linear input stage comprised of an nmos fet. This means that input hysterisis cannot be specified for these parts as a percentage of the input signal logic level - so input noise and ground bounce on the devices' supply pins may be an issue.

In a dual driver situation, you might even find that one driver is upsetting the other, so check the the input and output state of the static side when the funny behavior is occuring on the other. There may be symptoms there, that you wouldn't normally look for, or expect.

RL

Well the input is 5V CMOS, well below 10 nS edge. If that is not enough for them I can't imagine what is. Ground bounce if any will be solely inside the chip, like I explained in an earlier post a 1206 1uF (ceramic) is pinned to the power/GND pins, at the bottom side of the board. The connecting vias also connect the thing to the power/GND planes respectively. So it is neither of these, I would not have posted this if things were that easy :-).

The other driver is driven and unused. Does not change state at all, the input is either 0 or 5V (don't remember which it was). Nothing worth noting at it.

Dimiter

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flux

for

I use their 52 material. The people at Micrometals were as kind as they could get, sent me some samples etc. I am not sure I can do that on a ferrite core, at 500 kHz I manage somewhat above 10 (I think I tested at 12) W through a 10mm outer diameter, 3.8mm high toroid. It does get hot at this power though. Can't measure it, too tiny for the IR thermometer, but it is above

Can you make a guess how much there is to gain by using MPP (presumably the lowest possible loss)? (right now my efficiency is around 60% to

I have been using ferrite cores for that sort of thing last 20 years, but at 100 kHz. I could not locate anything which could do the job at 500 like that 52 material toroid does. I am not sure why I did not find anything and opted for the powdered metal, was some years back. Then I may simply have made a mistake.

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As long as you did look...I've only seen trouble on an MIC4424. I realize that a range of these drivers has the same nominal inputconfiguration, but this similarity may encourage a lack of dilligence in characterization.

My reaction, like yours, was simply to change the chip, after noting the pin waveforms accompanying the behavior..

RL

It would be interesting to see how the 18FxxKxx would do in that circuit. I'm guessing >150MHz typically.