Sizing Mosfet Gate Resistors

rep

LOL. I cant read....

correct, and the analysis then gets slightly harder.

HTH

next interesting question: what about using a bipolar drive (+/-15V) and resistor? I'll leave the analysis as an exercise for the interested reader

Cheers Terry

the rep

now.

V.

Instinctively I'd treat it as equivalent to 30V unipolar.

Graham

In message , dated Wed,

Surface-mounted devices. With BIG bolts. (;-)

They're both incompetent because the MOSFET Cin is nonlinear, time varying, and a complicated function of gate driver performance. It is not necessary to compute the gate resistor power dissipation to any great accuracy, it is only necessary to bound it. The gate resistor power handling will usually be derated by a factor of two minimum. A more usable estimate simply works on worst-case rise and fall times of Vgs, derived from manufacturer total gate charge requirements and driver characteristics, like so: View in a fixed-width font such as Courier.

. . . 2 . (Vin-Vgs) Vin . P (t)= --------- & Vgs= --- x t . R R Tr . . 2 Tr . V / 2 . in | t 2 Vin Tr . -> E = --- * | (1 - --) dt = --- * -- . R R | Tr R 3 . / . 0 . 2 . Vin Tf . Similarly on fall E = --- * -- . R R 3 . . . . 2 . Vin . P = --- x F x T , T = Tr + Tf , F=frequency . R,AVG 3R T T . . .

That so-called test is not representative of any actual circuit stress on the gate resistors. A simple calculation, as in my first post, shows that the average power dissipation in the gate resistor is inversely proportional to Rg and directly proportional to the transition times. If the transition times are proportional to RgxCin, as one might expect, then the average power dissipated in Rg is independent of Rg and in direct proportion to a F*Cpd*Vg^2 product, where Cpd is a parameter similar to that specified for the CMOS logic families as a means to estimate power dissipation with frequency. The reason why a Cpd cannot be universally applied to power MOSFETs is that Cpd will be load dependent. As others have stated, it may not be power so much as peak current handling capability of the Rg that is in question.

It also assumes that the drive is much faster than the tau of the resistor*gate capacitance.

John

You and me both.

Ed V.

Since an effective transition time is an RMS of the two, the driver need only transition ~10x faster to make its contribution negligible. This is a reasonable constraint for specialized driver ICs and most worthwhile improvised substitutions.

I don't know that I'd describe Cin as "non-linear", but you have a point. There is a region where the gate "eats" charge, but doesn't change voltage. I suspect you can analyze the power dissipated in a far simpler fashion than you suggest.

...Jim Thompson

In message , dated Thu, 31 Aug 2006, Jim Thompson writes

If the device model is good, LT Spice will tell you what the dissipation in the resistor is.

Yep, ANY SPice will calculate it correctly IF the model is good... PSpice Level=7, or HSpice Level=28 or Level=49

...Jim Thompson

The test is to make sure the fets are soldered to the PWB. The problem is the 10 ohm gate resistors for 10 fets are daisy chained to the same signal source.

The other problem is of course I ask more than one question at a time.

Thanks to all. I get quite an education posting here. I will remember to spell check next time.

Ed V.

I'm not so sanguine. Most of the energy is dissipated in very small volumes, before spreading out to the rest of the package. It's easy to blow them out without significant package heating. I agree with Joerg, be careful. I've had to replace more than a few surface-mount resistors in multiple types and designs of commercial products, where I think the failures were due to the poor peak-power capability of the resistor. We're talking from a reference filter in a commercial power supply, to an offline regulator in my tech's Kenmore washing machine's DSP VFD motor controller (after working properly for five years = past the warranty).

A typical SMD resistor seems much more fragile than the older already-fragile film resistors.

It looks like some engineers aren't taking the conservative peak-power specs typically found in a data sheet seriously, perhaps because they got used to better in the old days.

Those "professional thin film MELF resistors" look impressive.

Do you have any specific data on peak-power vs time profiles in cases where you got into trouble with ordinary SMD parts?

An open gate resistor is not neccessarily the source of upset - it is however quite commonly the end result.

Peak power can be a factor in device reliability;

In fixed frequency operation power loss is determined by C * V^2 * f , as the power dissipated in the resistor occurs twice in each cycle. Normal energy loss in a single unidirectional pulse is ( C * V^2 ) / 2 , in joules.

Note that this is independant of R, in so much that the R of the series limiter in question still dominates. If there are other series lossy elements, the total power loss is shared proportional to R.

At 100 KHz, an 0805 part is safe driving a 10V signal into a 10nF gate, ignoring the effect of Cgd, at an 80% power stress rating, provided it has sufficient copper connected to dissipate .125W.

RL

Using total gate charge (including reverse transfer) is as much as it's probably worth, even though this is a nominal value and has to be factored for drain voltage.

If thee are other sources of drain voltage movement, they add further charges to the burden.

RL

The total gate charge spec only assumes 0V origins, but despite other words to the contrary, once the drain has stopped changing voltage, the gate capacitance is quite linear and energy state changes from zero can be simply added.

RL

Depends on whether the driver is resistive or current delivering. The first assumes it's share of the total calculated losses, the second has losses that are more related to average current.

RL

Hello Win,

The least fragile were grampa's old carbon resistors. They could take a punch.

I have come across resistors where there was no peak power spec :-(

I have always liked MELFs. Wonderful parts but often no fun for the assembly plant.

Unfortunately not anymore. I got that data from Beyschlag (now Vishay) and similar data from AVX for ceramic caps because we were stressing some of those as well. Both companies were excellent in furnishing data beyond what's in the spec sheets.

The problem has subsided a bit because we don't have to "beat FETs over the head" anymore to get performance. But it'll come back with the migration to 0402 and smaller because everything has to be the size of an iPod now.

Hello Jim,

Can Spice also show you the location and size of the crater that develops when it "needs to vent"? Happened recently. A volcano appeared on the SOT23 package, a wee puff and then a stench wafted through the room. Miraculously the device was still working to some extent.