P-MOSFET unexpected fail - gate protection considerations - Page 1
Forum section: electronics / beginners

author: fsky
author_rank: Contributor
timestamp: September 14, 2015, 03:29:09 pm
content: Hello!I've made a circuit for cutting off power to the BLDC motors. I supply whole thing from about 40V battery, and I do expect about 5 amps of current load, so I have chosen quite large N-mosfet for that application - AUIRF540 in D2PAK. Using this transistor, I'm cutting off ground trace for motors. I'm attaching a schematic underneath.I have used Zener diode, gate resistor, pull down resistor and two ceramic caps for gate protection, however it blew the gate short anyway twice. It was working for few hours of testing, mutiple switching on and off, and suddenly it's constantly shorted. Is my gate protection circuit not enough? Is drain current a factor I didn't take into consideration? Maybe I should worry about some ESD stuff? Will be grateful for suggestions.// asbktempnoticed, I made a wrong desctiption. Sorry! Corrected now.

author: bktemp
author_rank: Super Contributor
timestamp: September 14, 2015, 03:37:07 pm
content: The circuit does not match your description: It shows a N-channel mosfet and uses a positive gate voltage.The AUIRF5410 will get quite warm: It dissipates 4Wat 5A.

author: fsky
author_rank: Contributor
timestamp: September 14, 2015, 04:24:08 pm
content: Thank you for noticing, I wrote the description wrong. I have used an N-type mosfet, as it is in the schematic. That's why I had to cut off the ground, as I didn't want to generate voltage above battery positive rail. As for a transistor power dissipation - it's not a problem (N-type mosfet which I actually used have much lower Rdson resistance).

author: nitro2k01
author_rank: Frequent Contributor
timestamp: September 14, 2015, 04:41:04 pm
content: Have you tried the following on the output side to dampen inductive kickback from the motor? Capacitor. Reverse diode.

author: fsky
author_rank: Contributor
timestamp: September 14, 2015, 06:22:40 pm
content: Quote from: nitro2k01 on September 14, 2015, 04:41:04 pmYou are suggesting that inductive kickback trough the motor could cause transient voltage rise on power line, which could hit the gate of the transistor? Wouldn't my protection circuit be sufficient? Could you elaborate more on your idea?

author: Marco
author_rank: Super Contributor
timestamp: September 14, 2015, 07:04:51 pm
content: I'm not familiar with how BLDC motors work, are there diodes to let the motor free run when you press this button or does all the mechanical energy get dumped into your poor little MOSFET through avalanching? (In the second case you'll need a snubber to keep drain voltage under control, the energy needs to be burned in a resistor instead of your MOSFET.)

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 14, 2015, 07:50:27 pm
content: Change the 1k resistor in series with the gate to 100 ohms; get rid of both capacitors shunting the gate; change the 220k shunt resistor to 10k.Reason why: MOSFETs intended for switching applications (which is most of them) hate spending any time in their linear region of operation.10k for the shunt resistor is a good all-around choice to prevent static charge buildup and ensure the MOSFET is normally off. The zener diode (or TVS diode) protects against overvoltage. The 100 ohm series resistor allows for fast charging of the gate and Miller capacitances while also decoupling the gate from transients on the supply.

author: singapol
author_rank: Frequent Contributor
timestamp: September 14, 2015, 09:29:16 pm
content: You should read up on how to protect mosfets from transients/surge and overvoltage by TVS diodes,MOVs ( metal oxide varistor) and NTC resistor. Not to forget snubbers and their design. Everytime you turn on/off ,there will be voltage and current surges many times the capacity of the motor.