10R in gate drive

Think of it as a place holder. Apps puts the resistor there to put the idea in your head that a resistor may be required. Design would say it is not a part requirement. Apps would say they don't want the damm phone calls due to ground bound, EMI, etc. Marketing will bitch about the extra component making the design look more expensive than it is.

I assure you hours were wasted on deciding to show the resistor or not. I can say this not even knowing what part you are talking about since this kind of crap is par for the course at a semi. Design generally gives in because the argument is mind numbing, and design needs their brain for other stuff.

At Cambridge Instruments, it went in because lots of MOSFETs needed a bit of resistance in the gate lead to prevent high frequency oscillations, and we'd had cases when the prototype had been well- behaved but most of the production parts had oscillated, though I suspect that some of them reflected situations where the prototype had been checked out with a cheap, slow oscilloscope and the oscillations hadn't been noticed.

10R was typically the lowest resistance value of the relevant resistor range - the manufacturer wouldn't offer anything with a lower resistance in that package. As zero-ohm link might have made more sense, but not every resistor range included one.

--
Bill Sloman, Sydney

I pick a value equal to, or somewhat lower than, the (output delta V) / (rated peak I) of the gate driver. So a "3A" driver at 12V would need 4 ohms, so it gets 3.3 or 4.7 ohms. Doesn't impact performance much (it still delivers ~3A peak), and helps ensure good behavior.

Funny thing is, most FETs have about as much internally (if you're lucky enough to find a datasheet specifying internal equivalent Rg; approximated in the SPICE model, if you have one, with a comparable value), so it should actually be halving the performance (crudely speaking). As time goes on I've been ratcheting down the resistors in my designs and haven't had a problem yet.

Well, there was that one breadboard, where I made a 12V 10ns gate driver (not quite as fast as those flat body, wide lead, RF driver chips IXYS-DEI makes, but I have a different design, better than that), running at 3MHz or so. Since it was breadboarded (i.e., dead bugged), I ran nice long, wide striplines out to the power section. Which made a nice tone burst generator when the transition hit the transistor. A teeny ferrite bead cured that with almost no impact on switching speed. Poor little ferrite bead does get awfully hot at 3MHz though.

Tim

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

"Vladimir Vassilevsky"  wrote in message  
news:9fadne4oN-F9-lrNnZ2dnUVZ5j2dnZ2d@giganews.com... 
> 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 
> www.abvolt.com 
> 
>

Damn. I think that's why I favor 4.7, too.

I hadn't thought about that. Pretty is as pretty does?

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

What is

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e

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e

I'm guessing Nico meant the resistor would get the task of acting as a fuse ;)

-Lasse

Good point, but how far do you put your drivers from the MOSFETs? Mine are pretty much right on top of them.

They're all pretty ugly now.

A trick I learned from our TI FAE: When you change resistors, place them upside down so they're easier to see and harder to forget. Doesn't work for caps. :-(

Of course you also need a FET there. But something has to drive it.

One buck is a lot of money, it buys all the ingedients to bake a delicious 1.5 lbs bread (my wife just did that). I bet then they'll want you to make it a 50 centimes buck converter :-)

Just came back from a friend's house and looked into the eyes of a real buck with big antlers and all. One has to be careful, sometimes they go straight into attack mode for no apparent reason.

--
Regards, Joerg 

http://www.analogconsultants.com/

is

Had one of your famous "kabloueis" last weekend. ...on a customer site. ;-( Took out a 50A fuse with much fire and brimstone. Either the controller locked up, the gate drivers locked on or the low-side FETs shorted on a 400W boost supply. Don't think it's possible to tell which.

[snip]

Did you smell like you were challenging him for that doe in the woods behind him ?>:-} ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

gate.What is

value

transitions.

Yes, but the heat flow is going to be measly.

Gaaah! That's horrid. And only a matter of time until all this pops off and falls into the abyss ... bzzzzzt ... *PHOOF*

Seriously, I've seen it on a CRJ-200 jet. The tie-wraps holding the fluorescent wiring along the sides in place had all popped. At least all the ones I could see, hair-cracks. Around half of them had fallen off.

I think some inhouse seminars are in order here. Hopefully they are in a nice area, or at least have good pubs in the vicinity for the evening.

--
Regards, Joerg 

http://www.analogconsultants.com/

is

value

That would be a smelly fuse :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

Ordinary surface mount resistors are substantially faster if you mount them upside down.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

gate.What is

value

transitions.

Figure downtime at roughly a million dollars per hour.

Wonderful in the summer. Outdoor dining, beautiful grounds, frogs croaking.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

I make a point to get the ones with codes printed on them (I'm still stuck in 0603 land unless someone insists -- and doesn't mind me farming out all the board assembly).

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

I'm not going to get manufacturing to design new pick-n-place machines. That's just not going to happen.

The only 0603s we use are in places where I know I'm going to change them (resistor gains, test shunts, etc.). Everything else is 0402. The assembly is farmed out (each board is ~1500 components) but working on them isn't a big deal. Well, at least if no one else has horned in on my Mantis. ;-)

My boss got hit by a buck driving home a couple of months back. He got the antler through the driver's window. It scratched across his throat and bloodied up his ear pretty bad; several stitches worth.

We have four does (two older ones and two yearlings) around our house when I get home in the evening. I'm quite sure there's a buck around somewhere. ;-) Coyotes, too, but they stay in the woods.

The 10R value is more commonly found in higher voltage and off-line circuits, where traditional 20V gate ratings and higher gate thresholds dominate, and where EMC is of regular concern.

Below about 22R, standard film resistors change from carbon to nickel film, in the same product series, and display fusing characteristics that are ~predictable. This can increase the repairability of circuitry under single fault conditions and reduces the extent of secondary effects in single-fault abnormals, where track fusing isn't normally acceptible.

Schematic values in most publications are just suggestions to begin with.

RL