Sizing Mosfet for Driving a 24v Relay

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rocky79

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Hello,

I am trying to monentarly switch a 24v relay from an existing mosfet (part# NTMD4840NR2G) switch on a PCB. The issue is the break down Vds of the mosfet is 30v, the relay is rated at 24v.
Since I can't go bigger on the the MOSFET unless I redesign the board, do you think will it be ok driving the solenoid with this mosfet provided that I use a flyback diode across the relay?
I think that's not enough voltage margin but would like to hear your feedback, thanks in advance.
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I'm sure you can find a MOSFET in the same package with a higher VDS rating.

If, for some reason, you can't replace the MOSFET, then it should be okay, in theory. I would definitely look at the voltages with a scope to make sure there's no spikes caused by layout-related parasitics, etc.

If this is for a product, I'd be pretty nervous about the lack of margin. (And I'd be pretty nervous BUYING that product knowing the designer cut corners.)
 

Thanks for the reply. I will have to look harder for higher voltage mosfet with same pinout. otherwise, I am going to re-redesign.
 

That Mosfet comes in an SO-8 package. There must be literally many dozens of higher voltage drop-in replacements from both Onsemi and a host of other vendors.
 

Once relays are typically inductive components, is expected to generate high level spikes over the commutation device which drive it. Measured value is often of the same magnitude than supplied voltage, therefore producing somewhat as ~48v ( 2 x 24v) on above circuit.

Other important advice, is that MOSFET are not so well suitable to switch inductive devices, due to its smaller breakout voltage if compared to BJT ones. The reason is the fact that silicon construction process on BJT require more doping regions in series, breaking voltage inside segments in not so dangerous proportion.


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