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Why mosfet failed in this circuit?

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Newbie level 4
Feb 5, 2013
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I've made a device that has 20 circuits. (one of them is in attach)
The circuit in fig1 is used to switch on-off 10A DC, but when the circuit worked for 4-5 hours, some of the mosfets (2sk3569) burn.(1-2 percent of mosfets)
I use a heat sink for each 2 mosfets same as fig2.
Now I don't know what should I do?:bang:
I thank you in advance for your help.
Best regards:-D:)
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I'm confused!

The circuit shows 20 MOSFETS but only 4 of them are passing any current. Is it a schematic error or are they the ones burning out?

What does '240' signify and is the resistor really rated at 3KW?


A better question will show your schematic and layout and what you computed for losses and measurements.

Watch out for shootthru failures in high loads ( or saturated cores) in complementary MOSFET drivers.

RdsOn I^2Rt losses are within accepted limits ?
Rca thermal values are as expected?
Was it during any special event?

2sk3569 has 0.54 ohm (Rds(on)) at 25 deg, and each mosfet through 1A.
However temperature of ambient is 35 deg.

- - - Updated - - -

The 3KW Resistor is a heater and 240 volt is Battery voltage.

It could be that when the switch is thrown, the gate line goes open circuit for a milli second or so. During this time the conduction of any FET is dependent on the stored voltage on the Cg-s of each FET, so a device with the biggest C g-s stays on the longest, so for a short time may take all the current.
Make the 10 Ks smaller, or remove the diodes and put a 100 ohms across the switch.
It mean you think that when the mosfets turned off, the mosfets were not in safe area operation?

I think I mean that the slowest turning off FET (one still conducting) is dissipating 120V X 5 A = 600W, so the time its dissipating this power must be as short as possible. Could I am wrong, so the circuit worked for 4-5 hours, was the switch operated ?, often?, not at all?
If your switch does really switch between +12v and ground, then get rid of all the diodes!

When the switch is thrown to ground, you only have the resistor to discharge Cgs + Cmiller
10k ohms is way too large to quickly turn off a single MOSFET, and you are turning off a pair.

A relay would be more reliable than this design.

Mosfets have PTC so RdsOn increases 3x when hot.

These devices are obsolete, you may consider some that are 10 mOhm like SiC types or GaN in one device.

You dont show any debounce circuit or wire inductance and resonance is likely during switch off causing SOA issues with high gate R.
- make Rg ~ 100x RdsOn of diode is used for turn off fast .
-there is no Zener OVP transient protection builtin this device.

Slow Switch energy limit should be <<300mJ or 0.3VA-second . Or 363mJ abs max...

Power sharing 3000W in 20 FETs only works all device thresholds are perfectly matched so during switching off they share lagged inductive current.

normally it is switched off fast with a diode.. Not turned on fast as your design attempts and then turns off slow with 1500pF Ciss per device.

Repetitive Switch Energy limit is only 4.5mJ and without Zener or MOV and if switch bounce, You can expect failures.

3000W if switched off in 1us is 3mJ ,
Consider if all FETs on one heatsink and all pins ganged 20x1500pF=30nF would might need Rg of 30 Ohms to switch <1us.

Since these devices are avalanche mode, it is impossible to guarantee thy all switch simultaneously with variances in Rg Ciss product. But perhaps you were trying to slow down the avalanche time to current share during off with a slower response time. Not easy with wide separation of wired devices.

Consider a Relay or a better 3kW switch from Cree, Diodes inc in the 1 ~10 milliohm range in SiC type.
Do they tend to blow during Turn off or Turn On? You have impedances reversed from the norm to gates with low impedance to +12 and high impedance turn off.
How much load inductance?

Do they tend to blow during Turn off or Turn On? You have impedances reversed from the norm to gates with low impedance to +12 and high impedance turn off.
How much load inductance?

Thanks for your help.
I think they do not tend to blow during Turn off or Turn On.
what do you think about amount of the resistor, I think that resistor for between gate and +12V is proper but the resistor that between gate and GND must be about 2K. Is it true?

I must to say that I used a relay instead of S1 and relay actuated by microcontroller (Atmega128) and switch of relay is different (the time that mosfets are on is also different from 10 minutes to 5 hours).
Now in your opinion what I do to switch DC current with mosfet.(current : 20A, voltage : 440V)
I have 2sk3569 for it, but if you have any recommendation please let me know.
My load isn't inductance and it's ohmic.
I think may the heatsink isn't proper for it and it cannot exchange the heat of mosfet because there are 2 mosfets on each heatsink.

Best regards,

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