2x N-ch MOSFET in series, PWM small motor speed control

Thread Starter

tonigau

Joined May 2, 2011
14
I'm repairing a small rotary hand tool(engraver/grinder), 12V battery operated for a friend who decided to run it from a reverse connected 12V power supply - result dead MOSFETs & snubber diode. These MOSFETS (NCE30H15K) are cheap (x10 for 2.5$) & the MCU on board is still operating (buttons operate speed select LED's) so is worth a go.
After de-soldering the 2x TO-252 MOSFETs, diode & cleaning carbonized gunk can see are series connected.
+12V-->Motor-->D-S-->D-S-->Gnd. Gates are driven individually from gate driver IC & PWM from MCU.

Q1. Is this series connection just for increased voltage withstand or something else. NCE30H15K are 30V 150A 4mohm., motor is 1.8A no load.
Maybe cheaper x2 lower volt devices or optimize RDSon ?

Q2. I can't identify supply for the 5 pin Gate driver yet, markings looks like N53238E ? . . .


Gate driver IC.jpg
 

dl324

Joined Mar 30, 2015
10,044
Q1. Is this series connection just for increased voltage withstand or something else. NCE30H15K are 30V 150A 4mohm., motor is 1.8A no load.
Maybe cheaper x2 lower volt devices or optimize RDSon ?
It makes no sense. You have a 12V supply, so source/drain breakdown is unlikely to be an issue. Putting two devices in series will just increase on resistance which wouldn't make sense.

To be certain of what you're describing, it would be helpful if you traced the circuit and posted a schematic.
 

Wolframore

Joined Jan 21, 2019
1,627
N channel mosfets are placed in series to make sure the body diode wont affect reverse current... usually this is done with drains connected. Not sure if this is your case but I’ve seen this before. This way the only ways to turn the channel on is when the gate voltage is correct.

i don’t think it’s correct in your case, if you search N531 Sot it comes up with a chinese part that look the same and states it’s an Igbt.

https://www.1688.com/huo/detail-607964579355.html?spm=a262i4.9164788.zhaohuo-list-offerlist.10.63825aba7nW0hW
 
Last edited:

Thread Starter

tonigau

Joined May 2, 2011
14
Ok, I missed a track. there is quite a bit of damage... only the top MOSFET is driven from gate driver, other gate drive is from another part of circuit, the bottom MOSFET is not PWM'd , might be a fail safe .
Sorry for the stupid question.


MOSFET ckt block diagram.gif
 
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