The source, drain and gate are immediately shorted. There is an inductive load at the drain of the MOSFET and the source is connected to ground. The gate is powered by a voltage divider.
However, my MOSFET doesn't blow when I disconnect the gate.
Could anyone tell me why this could happen and how I can protect my MOSFETs in general?
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Harish
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I created a better question as Stack won't surface my older questions anymore – Harish Jan 23 '22 at 00:24
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The other question doesn't really help either. – Harish Jan 23 '22 at 00:34
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Please don't repost the same question, that's considered spam here. You can improve your old question by editing it. – marcelm Jan 23 '22 at 11:55
3 Answers
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The inductive load will try to keep current flowing as the FET switches off, and will generate a huge voltage if necessary to achieve this. It’s normal practice to add a freewheel diode across the load, but it’s also possible to mitigate the inductive spike by switching the FET off relatively slowly so that the voltage spike is reduced, or by adding a capacitor across the load if you prefer to radiate the excess energy as RF emissions.
Frog
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My MOSFET doesn't blow when I disconnect the gate. So I don't think that's it. I tried adding a diode but it still doesn't help. – Harish Jan 23 '22 at 01:16
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When you disconnect the gate from the resistors then the high gate-source capacitance keeps the Mosfet turned on for a very long time then it very slowly turns off. You are feeding the inductor with unlimited DC current so the inductor is simply a very low resistance that passes such a high current that the Mosfet is overloaded. – Audioguru Jan 24 '22 at 15:55
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Why does it fail?
If it did't fail from an abrupt turn off and excess energy dumped into the zener diode, then it must have failed some other way such as thermal.
We don't know if it got hot, but generally you expect temperature rise as follows:
V * I * Tja (thermal resistance) 'C/W = 'C temperature rise of junction
What was the current and DC resistance of the inductor? (DCR) and RdsOn of the FET? I = V/(DCR + RdsOn) and V=I*RdsOn
What is Tja? Lookup datasheet and determine if a heatsink is required. Also, consider if perhaps you have exceeded the current rating of the inductor.
Tony Stewart EE75
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The inductor is a dead short at DC that overloads the Mosfet when it turns on. You are feeding the gate with only 4V so the Mosfet is only partially turned on then it gets too hot.
Audioguru
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Vgs(threshold) is 4V. That doesn't mean it is fully turned on at 4V. The transfer characteristic on the datasheet suggests 5 or 6V should do the job. – Yellow Yeti Jan 23 '22 at 09:27
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The datasheet for the IRFZ44 Mosfet says that the Vgs threshold voltage for some of them is 2V up to 4V for some of them at a drain current of only 0.00025A which is far from being turned on, they are almost turned off. The datasheet does not say anything about a Vgs of 4V, instead it says it is turned on with a Vgs of 10V. You should use an IRLZ44 (see the "L"?) that has a threshold voltage from 1V to 2V and is guaranteed to conduct at least 21A with a Vgs of 4V. – Audioguru Jan 24 '22 at 15:50