10R in gate drive

When they draw connection diagrams for power FETs or FET drivers in datasheets, they usually put 10 Ohm resistor in series with the gate.What is the rationale behind that? How did they come up with almost universal value of 10 Ohm?

I could understand slowing down edges for EMC reason, or limiting gate current, or dumping stray inductance to avoid oscillation on transitions. That depends on particular application. But why always 10 Ohms ?

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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Vladimir Vassilevsky
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I generally use 33, or maybe 100 for small fets or when an opamp drives a fet gate. Or zero if the driver is very fast and I want extreme switching speed.

Here I used 270s to slow down the edges to a trapezoid, to reduce noise into nearby analog stuff.

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John Larkin                  Highland Technology Inc 
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John Larkin

It's a knob you can turn to vary the switching time. It's a knob that can be a lifesaver when you're working in the EMI chamber at 3:00AM.

Why not? I generally use zero-ohm resistors, just as place holders. I want the transitions as fast as possible, unless I can't pass EMI.

Reply to
krw

I use 4.7.

But as you've gathered from the other responses, it's an EMI thing. It's there so you can make a tradeoff between dissipation in the transistor due to longer turn-off time and EMI from turning the thing off sharply. The real issue is that most of the time it's not a big deal, but when it does matter, you want to have the spot on the board to put a resistor of your choosing.

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Tim Wescott

"Just cause" >:-} ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     | 
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Jim Thompson

I acquired certain favorite resistor values based on attractive color codes, and I still use them for surface-mount parts. 4.7 (ohms, K, M) looks nice. I never cared much for 1.2 or 8.2.

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John Larkin         Highland Technology, Inc 

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John Larkin

On big MOSFETs it can help control what nastiness gets back into the gate driver by slowing the tr/tf so that inductances don't have as much effect, as well as limiting the current on negative undershoot.

Reply to
Spehro Pefhany

Gate resistors are for wimps. Dare-devils never use any. Honestly, I've never understood why people spend money on sub-1ohm gate drivers that can blast 5-10A of current into a gate, only to hang such a spoiler up front. That's like driving a Corvette at 30mph.

Of course, as others have said, it's always good to have a "chicken exit" in case you can't make it pass EMC. That chicken exit takes the form of a zero ohms resistor in front of the gate. Usually EMC failures of that kind are due to some other not so optimal aspects of the design but when time is of the essence man's gotta do what man's gotta do (John Wayne).

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Joerg

Lots of mosfets will oscillate on rising/falling edges, especially if the gate drive is *not* super fast. But a series resistor will often stop hundereds-of-MHz RF oscillations with little effect on switching times.

Yup, giving up a couple percent efficiency is worth passing EMI.

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John Larkin         Highland Technology, Inc 

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John Larkin

Then the gate drive is too wimpy or the layout is messed up. In cases where there really is a need to suppress things a bead is often better. It can be selected so it rolls off where needed while still delivering zippy edges below 10MHz or so.

Unless that makes the unit glow :-)

No joke, I have a design right now where I'd be in hot doo-doo if I gave up 2-3%. No gate resistors there. Engineers need white-knucjkle rides once in a while.

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Reply to
Joerg

is

OK, that does it, no free beer for you.

In cases

Beads are typically a few uH gross inductance. At amps per us, the drop can add up. Probably the best gate thing would be a series R+L, critically damped. That might actually improve switching speed.

I don't need risk there; we get plenty of thrills in other places.

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John Larkin         Highland Technology, Inc 

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John Larkin

is

value

Dang ... :-(

A peaker?

I often don't get a choice. The usual, customer says we need xx watts and it has to be 30% smaller than a pack of cigarettes, no fan, no airflow. And we want this to work in the Sahara desert.

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Joerg

It's nice to have some kind of fuse between the driver and a mosfet. One of the failure modes of a mosfet is a short between the drain or source.

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Reply to
Nico Coesel

is

Not really, the tendency of MHz oscillations when driving MOSFETs is mainly caused by the non-linearity of the gate capacitance. You need resistance to damp it. Adding reactance doesn't really cure the problem, although it may mask the symptoms.

Regards, Klaus

Reply to
Klaus Bahner

is

value

He, he, right now I have quite the opposite: a 2mW bucker going from 3V down to 100mV, with a lot of bells and whistles. Aimed overall efficiency greater than 80-85%...

Now that's fun :-)

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Fred.
Reply to
Fred Bartoli

Sounds like large inductances and slooooow clocking. Probably a logic gate could be used where it's guaranteed that cross conduction is miniscule. Maybe CD4000 logic? But that's going to behave like molasses at 3V.

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Reply to
Joerg

Good ferite beads are rather lossy, they don't behave like pure inductors. That's the trick, to have them behave lossy only where you want them to be. But one has to spend a long time selecting the right kind and then bench-testing.

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Reply to
Joerg

A fuse has way too much inductance, at least for a fast switcher. It's often best to make sure that if things go kablouie the damage remains limited to the switcher area. Yeah, a meltdown can take out the gate driver but that's typically not a big deal.

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Regards, Joerg 

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Reply to
Joerg

gate.What is

value

transitions.

OK, who wants a 100 mv power supply?

A cmos gate could be your synchronous switcher, with a lot of inductance downstream.

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John Larkin         Highland Technology, Inc 

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John Larkin

Well, 2mW at 100mV is 20mA and the CD4000/74HC/yada yada/... won't cut the mustard at all. Plus, to add more fun, it is a current source, has to be ultra low cost (qty counted in 10^6/y), fast start/stop able with the startup energy being part of the power budget, etc... I already have the (quite neat) answer. I think I'll call it the one buck bucker :-) Sorry, can't disclose anything (but the name :-)

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Fred.
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Fred Bartoli

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