Can mosfet gate capacitance cause inrush current?

Discussion in 'The Projects Forum' started by urb-nurd, Feb 2, 2015.

Jul 9, 2014
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I hooked up an lm2596 DC-DC buck converter to a gate drive optocoupler i have, then hooked the output of this up to the gate of my mosfet.
I switched the gate voltage on and the the mosfet conducted for maybe half a second before shutting off.
I have tested the fet and it is fine, but my LM2596 is no longer working.

I thought maybe the inrush current from the gate capacitance surpassed the 2A rating and caused the failure.
The gate capacitance is 210nF and i did not use any gate resistors.

I have ordered more converters, but until they arrive. I need to ensure i don't let this happen again.
Both my pocked and my watch are getting irritated with my tendency for wastage.

2. #12 Expert

Nov 30, 2010
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Yebbut...the total gate charge is insignificant compared to the filter capacitor on your power supply..isn't it?

Jul 9, 2014
269
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Indeed, but i appear to be missing your point.

4. #12 Expert

Nov 30, 2010
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If the gate capacitance is 210 nf and the power supply capacitor is 10 uf, the gate only needs 21% of what's in your filter capacitor. That isn't enough to slap the power supply output to ground and smoke it.

I think you have a different problem.

Jul 9, 2014
269
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i have a 100nf bypass cap on my opto, and as you said - i have a 220uf cap to supply DC to the lm2596.
Though if i need to pass some of this to my gate, it still has to pass through the converter.

The converter was set to output 10V, with the resistance at the gate below an ohm.
oversimplyfying with ohms law: 10V/ 0.5 ohms = 20A
Am i missing something?
Thanks #12

6. #12 Expert

Nov 30, 2010
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Yes. Put 100 uf on the output of the power supply so the capacitor can provide 98% of the current you need in a millisecond or less. Then put a 10 uf (electrolytic or tantalum) and a 0.1 uf capacitor (ceramic or metal film) right at the supply connections to the gate driver.

Still, the idea that your power supply lasted half a second suggests that something else went wrong. The gate should have been charged in less than a millisecond, not half a second.

Last edited: Feb 2, 2015

Jul 9, 2014
269
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ahhhhhhh you cheeky bugger you!
I am following you now hahaha
Thanks for the solution! didn't even see it coming

8. #12 Expert

Nov 30, 2010
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I'm not trying to be obtuse. We just think differently. That's why having about 20 helpers here is the major strength of this forum. Somebody will eventually "connect" with you.

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Jul 9, 2014
269
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ill need to investigate what else could of caused the issue.
I was happy enough believing that the thermal breakdown lagged the inrush current causing the delay to failure, then you come in with all your suggestions of logic.

Apr 5, 2008
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Hello,

When you post a schematic of the current set-up, others may also have a look and give you some suggestions how to improve the project.

Bertus

Jul 9, 2014
269
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A schematic certainly would help!
I should probably fully prepare my questions before posting.
My bad. Ill get a schematic up when i get back to my house

12. #12 Expert

Nov 30, 2010
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You might try about 100 ohms in series with the gate. That will hold the inrush down to 1/10 of an amp and the gate will have the mosfet turned on in about 21 microseconds.

See why 1/2 second indicates a different problem?

Jul 9, 2014
269
3
I did have my gate properly set up with a series gate resistor and another high value to ground, but i figured i would just quickly check the operation with the converter before bed - bad idea i guess.

I can see why 1/2 seconds indicates a larger problem than the inrush, though to be fair my perception of that time could vary massively given the thermal time constant of the filament coupled with my poor comprehension of small time periods.
There must be some short i am missing, because i cant really see how else this could happen - i tested the set up with PWM while checking the duty cycle on my scope, it was the addition of the gate capacitance and/or the load that cause failure (obviously). So the issue must lay with my configuration.

14. #12 Expert

Nov 30, 2010
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I understand. My day job has sharpened my time judgement skills because I very often listen for clicks, whirrs, and moans that tell me the starting sequence of machinery. A lot of them happen in less than one second, but a detectable amount more than zero seconds. It's not fair that I would expect you to have sharpened the same skills.

15. cmartinez AAC Fanatic!

Jan 17, 2007
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Driving mosfets has been one of life's big mysteries to me... right up there along with bees and birds ...
anyway, just wanted to say that I'm learning a lot from this thread, thank you all

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16. #12 Expert

Nov 30, 2010
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In this case, I used the old rule of thumb for charging a capacitor, "63% in one time constant". 210 nanoCoul x 100 ohms is 21 microseconds, and 6.3 volts is enough to get almost any mosfet into its conduction zone.

17. cmartinez AAC Fanatic!

Jan 17, 2007
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Would that be enough to get it fully conductive and out of it's linear zone too?

18. #12 Expert

Nov 30, 2010
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Pick one and look at the datasheet.

Here's one now! An antique IRF510. With 6.3 volts Vgs it is rated for 6+ amps while the front page only claims 5.6 amps.
I think that compares well with this 2 amp example.
Is it maxed out?
No. It keeps getting better to above 10V Vgs.
Does that matter?
It depends on what you need.
Still, I'm not sure I understood your question.

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19. cmartinez AAC Fanatic!

Jan 17, 2007
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It is my understanding that every mosfet has a so called "linear region" in which it is not entirely conductive due to insufficient charge (or is it voltage?) being delivered to the gate fast enough. If a mosfet is operated this way, especially when doing high-speed switching, it will overheat and potentially fail. That's more or less what my question is about.

20. #12 Expert

Nov 30, 2010
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Current into a capacitor causes voltage to rise. That's what happens to a mosfet gate which is secretly nothing but a capacitor, as far as input current is concerned.
Of course a mosfet can be operated in the linear range or as a hard switch, just as a bipolar transistor can. It's just less convenient to do linear with a mosfet because of the gate voltage cost. Let's look at a very different mosfet.

This one is a, "logic level" type. The front page rating is 4.5 Vgs for, "on" and 10 Vgs for 3.5 milliohms. Then we go straight to the graphs to get a better feel for those numbers. Figure 3: This one is done turning on at 4 volts with a quick pulse rating of 130 amps. That is usually a useless number for us amateurs, so go to Fig. 12 and see Rds dropping like a rock between 3 volts and 4 volts. You're in the 5 to 7 milliohm range at 4 volts. If you go to 6 volts, you can promise 5 milliohms at operating temperatures. Still, the curve keeps curving!

What do you call, "completely on" when the curve never stops curving?
It depends on the amount of current you need.
Let's try 7 milliohms @ 4 Vgs and 125C and (first page) 50 C/Watt temperature gain.
Limit is 150C, room temp is 25C so 125C/50 = 2.5 watts.
P=I^2R
2.5W/.oo7 = 357
square root it for 18.9 amps
and, from E=IR we have .132 volts from drain to source.
Look at Fig. 8
We just about hit the dotted line on the left edge.

Let's go looking for trouble with Fig. 12.
Suppose your gate is rising through 3 volts and you have 20 milliohms.
2.5 watts/.02 ohms is = I^2
Current limit is 11.1 amps and Vds is .22V
Look at Fig. 8 again. We're in the middle of a safe area but we've calculated a limit.
That graph can only be true with a heat sink.
That's where you get into trouble and that's why you want to get out of the switching range as quickly as possible.

Bipolar transistor graphs are considerate enough to tell the SAO at DC.
This mosfet datasheet wants to play, "pulses only".
You can see that you really have to dig to get the information out of this data.
The best I can do is drag you through the process so you see that, "on" is related to your needs for current.

• irf8734N-MOS30V_logiclevel.pdf
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Last edited: Feb 3, 2015
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