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I am working on a switch for my quadcopter. I designed a basic circuit with MOSFET as the switching device. I am using an LDO to supply the gate voltage to the Mosfet. The circuit diagram is attached below. enter image description here

The switch SW3 is connected to the ground so that gate voltage is zero when SW3 is in OFF position, when SW3 is in ON state the quadcopter turns on. I did multiple flight tests and everything was working fine.

To validate the leakage current, I left the circuit connected with SW3 in OFF position for a few hours and found out that Mosfet has been damaged with drain and source shorted internally. Can someone help me out in finding the reason why this could happen?

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You should have a diode (1N4004-type) from the output of the 2940 to its input to discharge the output when you flip the switch to ground. I'm surprised you haven't blown the 2940 (maybe you have). Connect Input 2940 to diode cathode, anode diode to Output 2940.

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I usually add zener diode between gate and source of MOSFET, to eliminate possibility of higher voltage. Also i recommending to 10k resistor insted of 100k, because MOSFET will be too long in high resistance state. You need to turn it off as soon as possible.

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Can someone help me out in finding the reason why this could happen?

  1. You need input and output capacitors on your LDO regulator - read the data sheet
  2. The load for the MOSFET is highly likely to produce back emfs when you deactivate the MOSFET and these will cause excessive DRAIN voltages that can easily damage the MOSFET - add a reverse protection diode across the load (quad-copter). It needs to be rated at the current of the copter.
  3. The recommended input voltage range for your LDO regulator is 6 to 26 volts and your 25 volt supply is close to the upper recommended limit.

Input voltage greater than typically 30 V will cause the LM2940 output to be disabled. When operating with the input voltage greater than the maximum recommended input voltage of 26 V, the device performance is not ensured. Continuous operation with the input voltage greater than the maximum recommended input voltage is discouraged.

  1. The minimum load for the LDO regulator is 5 mA - you are effectively running into an open circuit.

Output current range 5 mA to 1 A

The last two points can cause the LDO to produce excessive output voltages and may damage the gate insulation in the MOSFET.

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  • \$\begingroup\$ Sorry for being sloppy in the circuit digram. I do have input and output capacitors in the LDO and the manufacturer of the MOSFET in the datasheet says there is a freewheeling diode across drain and the source. I want to point out that the damage happened while the switch was in the off position. I had successfully operated the quad several times before this eventuality. \$\endgroup\$ – neo89 Jun 30 '20 at 15:03
  • \$\begingroup\$ I will look into point 3 and 4 thoroughly. Thanks a lot. \$\endgroup\$ – neo89 Jun 30 '20 at 15:05
  • \$\begingroup\$ That freewheeling diode will not protect against back emfs due to motor load current being suspended quickly. Damage will happen when the switch goes into the OFF position. \$\endgroup\$ – Andy aka Jun 30 '20 at 15:05

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