MOSFET failure modes... (overvoltage)

[meteor]

Hi!


I'm not getting anywhere in the Castle Creations group about the effect of overvoltage on a MOSFET, versus over-amperage...

I'm looking to use a Phoenix 35 or 45 on 6S Li-Po (about 21V nominal), at a very small proportion of it's current capacity.

Say, maximum 16A on a Phoenix 45 (35% rated current)


Supposedly, the FETs are rated at 25V, but the no-load voltage of the pack would be 25.2V.

As above, the flying voltage would be substantially lower than that, and the BEC would be disabled. (obviously!)


Do most MOSFETS have any latitude, such as a "nominal" rating versus an "absolute maximum".

Will they simply "pop" with a 0.2V (0.8% !) over-voltage situation?



Thanks.

[Mr.RC-CAM]

What are the FET part numbers?

[pmackenzie]

The problem is the switching spikes.
I had a friend try to go to 14 cells on a Smile 6-12. Lasted three flights and then let go.
Pat Mackenzie

[The Other Dave]

Pat is right.

Depending on the motor the switching spikes can be greater than 2x
the supply voltage.

Most power fets have an intrinsic body zener diode that will clamp
the voltage but you don't really want to rely on them for long
term clamping.

The design might also have a snubber system but don't bet on it.

Dave

[meteor]

Would extra/larger battery-side low-ESR caps help?

What about battery/ESC lead length/gauge? (super-short, 12ga)



Also, does the current play any role? (1/3 rated current?)



Thanks.

[pmackenzie]

What are the FET part numbers?
I looked at one of my CC45s, and the fets are labelled as IR 515H and IR 440H. I can't find them on IR's web site.

Meteor, voltage ratings are generally considered as max, not working. It is up to the designer to build in a safety margin.
Why do you want to run 6S at such a low current?
Why not just buy a controller that is rated for 6S?
Pat MacKenzie

[Mr.RC-CAM]

The IRF7832 is rated to 30V. If your max voltage (including any resulting switching spikes) is 25.2VDC, then you should be OK. However, there may be a bank of P-channel FETS too, so you need to look at their specs.

[JimDrew]

Actually, the parts you show are the same. You are looking at the date codes as being different. Both parts are IRF7832, which is a 20amp N-Channel MOSFET.

[z-matrix]

I looked at one of my CC45s, and the fets are labelled as IR 515H and IR 440H. I can't find them on IR's web site.

Meteor, voltage ratings are generally considered as max, not working. It is up to the designer to build in a safety margin.
Why do you want to run 6S at such a low current?
Why not just buy a controller that is rated for 6S?
Pat MacKenzie

don't bet they will work at the maximum rated voltage, at 100C junction themperature, the break down voltage is significantly lower of every mosfet's,

increasing the themperature, decreasing breakdown voltage, and look out for gate drivers, i saw some esc designs does not capable of operation above the rated voltage, you can damage mosfets by putting gate source voltage above the maximum rated, you simply burn through the gate insulation material.
(example irfz44 at 25V gaate source voltage)

[vintage1]

don't bet they will work at the maximum rated voltage, at 100C junction themperature, the break down voltage is significantly lower of every mosfet's,

increasing the themperature, decreasing breakdown voltage, and look out for gate drivers, i saw some esc designs does not capable of operation above the rated voltage, you can damage mosfets by putting gate source voltage above the maximum rated, you simply burn through the gate insulation material.
(example irfz44 at 25V gaate source voltage)



Thats an interesting failure mode..never thought of gate source breakdown...since the larger voilatge is usually drain/gate...but if the design uses bottsrappping to gain extra gate drive..it could blow the gate/source as well.

Like all semiconductors, they are produced and tested to a minimum spec. SOME may be well above that, some may be bang on.
Yesr ago we usied to buold with BC107's - th manufacturer we used, produced a single chip type that was 'selected' out for some special characteristics, and what was left over was stamped 'BC107' they all did about 60v breakdown, although the spec for a 107 was 45v.

But we were not allowed to use them for 60v operation because 'one day we might change manufacturers'

[pmackenzie]

Actually, the parts you show are the same. You are looking at the date codes as being different. Both parts are IRF7832, which is a 20amp N-Channel MOSFET.
Well now I feel stupid. I am usually pretty good at separating the date code from the part number. :o
What must have tripped me up is that they are arranged in rows by the date code. One row of the 515, one row of the 404 and another of the 515.
If there are Pfets they are on the bottom of the board, inside the PCB "sandwich".
Pat Mackenzie

[AndyOne]

Is that a short circuit I see between gate and source, pins 3 and 4 on the left hand MOSFET?

Andy.

[meteor]

Why do you want to run 6S at such a low current?
Why not just buy a controller that is rated for 6S?
Pat MacKenzie

Economics and application...

1) I have a CC35 and a '45

3) Low(er) Kv Mega 16/25/4 (3S2P Li-Po not practical)

2) I have existing 8C 2100 3S Li-Po packs, and I bought some CommonSense 3S 8C 2000's (16A max..)


Super-simple, super-cheap 6S power system! ;)

[pmackenzie]

Is that a short circuit I see between gate and source, pins 3 and 4 on the left hand MOSFET?

Andy.
No, the esc is fine.
Probably some fluff or a scratch in the heat shrink.
Pat MacKenzie

[pmackenzie]

Economics and application...

1) I have a CC35 and a '45

3) Low(er) Kv Mega 16/25/4 (3S2P Li-Po not practical)

2) I have existing 8C 2100 3S Li-Po packs, and I bought some CommonSense 3S 8C 2000's (16A max..)


Super-simple, super-cheap 6S power system! ;)
Makes sense, but it won't be so cheap if you blow up the controller.
Pat MacKenzie

[meteor]

I got the real answer from Patrick at Castle Creations!

Apparently it's not the FETs at all! :rolleyes:

The actual issue is not with the FETs, but with the back-emf detection circuit. It has a maximum input voltage of approx. 24V, and will not function correctly with a fully charge 6S pack (startup will fail, possibly damaging the ESC.)

The FETs are rated at 30V, and breakdown (at room temp) at about 34V.

Patrick