# MOSFET burnt

#### Kayukayu

Joined May 24, 2021
10
I am supposed to discharge this battery in pulses. Initially the gate is at 0 V. When I applied the 5 volts to the gate, my battery is supposed to discharge while the MOSFET is on. But when I do this, my MOSFET got burnt and my battery voltage first reduced and then began increasing. Please help

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#### ElectricSpidey

Joined Dec 2, 2017
2,753
MOSFET might not be turned on enough, try a logic level MOSFET.

#### BobTPH

Joined Jun 5, 2013
8,688
Your MOSFET needs 10V to turn on fully, so it is partially on at 5V and high resistance and dissipating a lot of power, and thus burns up.

You need a logic level MOSFET if you can only supply 5V to the gate. IRLZ44n would work.

Bob

#### neonstrobe

Joined May 15, 2009
190
You need to check the ON resistance of any FET you plan to use. Look at its data sheet.
To dissipate next-to-no power either the V is low or the I is low. If you want high I you have to have low V. So you need a MOSFET which not only turns on fully at 5V but also has an On resistance which is low (preferably much lower) than your load resistor. If your discharge resistor is 0.47 ohms that means your FET should have an on resistance at least below .047 ohms. What power will that cause it to dissipate? Is it still too high for the device?
There are many MOSFETs now which have much lower on resistances but you need to make sure it gets there at 5V.

#### Audioguru again

Joined Oct 21, 2019
6,621
The datasheet for the IRF3205 Mosfet shows that it needs 10V on the gate to fully turn on.
Since you are giving it only 4V or less then it is only half turned on and overheats.

Logic level Mosfets (IRL540 and many more with the F replaced with L) needs a Vgs of only 5V to fully turn on but that is not enough for your low battery voltage.

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#### Kayukayu

Joined May 24, 2021
10
The datasheet for the IRF3205 Mosfet shows that it needs 10V on the gate to fully turn on.
Since you are giving it only 4V or less then it is only half turned on and overheats.

Logic level Mosfets (IRL540 and many more with the F replaced with L) needs a Vgs of only 5V to fully turn on but that is not enough for your low battery voltage.
Thank you very much for a detailed reply. Will this setup work if I increase the gate voltage to 10? Also, any idea why my battery is charging after it falls from the open circuit voltage?

#### Kayukayu

Joined May 24, 2021
10
The datasheet for the IRF3205 Mosfet shows that it needs 10V on the gate to fully turn on.
Since you are giving it only 4V or less then it is only half turned on and overheats.

Logic level Mosfets (IRL540 and many more with the F replaced with L) needs a Vgs of only 5V to fully turn on but that is not enough for your low battery voltage.
Thank you very much for a detailed reply.

#### LowQCab

Joined Nov 6, 2012
3,960
You are trying to discharge your Battery an EXTREMELY high Amperage,
far more than it is designed to deliver.
You may actually cause your Battery to explode or start a fire.

What type of Battery are you working with ?
( I'm guessing a single Li-Po Cell, since you state that the Voltage is ~4-Volts)
Where did you find a Single Li-Po Cell rated for 4-Amp/Hours ?

Where did you get the 4-Amp/Hour Rating for you Battery ?

What is the Maximum-Discharge-Rating of your Battery ?

How did you decide on an ~8.5-Amp Discharge-Rate ?

Is your Resistor rated in excess of the ~35-Watts that it will be expected to dissipate ?

Are you using an adequate Heat-Sink ?
Even if you intend to supply ~10-Volts to the Gate,
any fraction of a Second spent with the Gate-Voltage at less than 10-V
will cause an extremely-fast spike in temperature without a Heat-Sink.

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#### Kayukayu

Joined May 24, 2021
10
You are trying to discharge your Battery an EXTREMELY high Amperage,
far more than it is designed to deliver.
You may actually cause your Battery to explode or start a fire.

What type of Battery are you working with ?
( I'm guessing a single Li-Po Cell, since you state that the Voltage is ~4-Volts)
Where did you find a Single Li-Po Cell rated for 4-Amp/Hours ?

Where did you get the 4-Amp/Hour Rating for you Battery ?

What is the Maximum-Discharge-Rating of your Battery ?

How did you decide on an ~8.5-Amp Discharge-Rate ?

Is your Resistor rated in excess of the ~35-Watts that it will be expected to dissipate ?

Are you using an adequate Heat-Sink ?
Even if you intend to supply ~10-Volts to the Gate,
any fraction of a Second spent with the Gate-Voltage at less than 10-V
will cause an extremely-fast spike in temperature without a Heat-Sink.
.
.
.
It’s lithium ion 21700 battery. It’s maximum discharge current is listed as 35A in the data sheet.

#### LowQCab

Joined Nov 6, 2012
3,960
That's good to know, But ..........

You should not routinely, and continuously, cause the Battery to be Discharged
at its "maximum" Discharge-Rate,
this will shorten the Life expectancy of the Battery.

This indicates that you should probably at least double the Resistance of your Resistor.
This will reduce the discharge Current to roughly half of what it is now,
or around ~17-Amps.
This will also cut the heat dissipated by the Resistor in half, to roughly ~17-Watts.

There is an easier way to accomplish this job ..............

#### Kayukayu

Joined May 24, 2021
10
The datasheet for the IRF3205 Mosfet shows that it needs 10V on the gate to fully turn on.
Since you are giving it only 4V or less then it is only half turned on and overheats.

Logic level Mosfets (IRL540 and many more with the F replaced with L) needs a Vgs of only 5V to fully turn on but that is not enough for your low battery voltage.
I used a heat sink today, it solved the burning up part of this problem. But during the discharge period of the battery, that is the time during the mosfet is switched ON which is 10 seconds, sometimes my battery terminal voltage starts to increase instead of decreasing.

#### LowQCab

Joined Nov 6, 2012
3,960
You are pulling too much Current.

It's not unusual for Li-Po Batteries to have weird Voltage fluctuations during, and after,
Heavy-Loads, or, Heavy-Charging-Currents, are applied to them.
.
.
.

#### Kayukayu

Joined May 24, 2021
10
You are pulling too much Current.

It's not unusual for Li-Po Batteries to have weird Voltage fluctuations during, and after,
Heavy-Loads, or, Heavy-Charging-Currents, are applied to them.
.
.
.
I have since changed the resistance to 0.64 ohms. Since this battery has a maximum current of 35 amps, shouldn’t it be able to handle the 6,7 amps?

#### LowQCab

Joined Nov 6, 2012
3,960
Yes,
4.2-Volts / 0.64-Ohms = 6.56- Amps.

6.56-Amps X 4.2-Volts = 27.6-Watts,
Do you have a ~30-Watt-rated (or higher) Power-Resistor ?
( Wattage rating may depend on the use of an adequate Heat-Sink )

Do you have a Control-Circuit that will turn-off the FET
when the Voltage gets down to the desired level ? (usually ~3.7-Volts ).

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