NFET instead of a relay for 12Volt high current brushed motors

Thread Starter

dlatch

Joined May 15, 2016
85
I am wiring a commercial 24th scale slot car track and I want to use big chassis mount NFETs to connect each lane instead of the typical relay. It is a high side connection, I will use a separate power supply to get the 10 volts Vgs.

Why? Just because it's better?. No deterioration over time and way less on resistance. And of course way less power to turn on.

Is there any reason not to do this? Attached is the data sheet for one of these fets

https://www.mouser.com/datasheet/2/205/Viewer-765418.pdf
 

dendad

Joined Feb 20, 2016
3,564
A relay has very low on resistance, and if you use a high current socketed automotive one, the are readily available for replacement.
But the FET will work ok too. It is just a bit more complex to drive.
Have you figured out the circuit you are going to use?
 

jpanhalt

Joined Jan 18, 2008
9,699
Why must the NFET (mosfet) be high side? There are high-side drivers that will allow you to eliminate the "separate" power supply for the gates. What current will the mosfet need to switch?
 

BobTPH

Joined Jun 5, 2013
2,427
All you need is a resistive divider (2 resistors) to keep the gate voltage below its maximum rating.
He said high-side. You need a voltage (not necessarily 10V) above the supply voltage it is switching to drive the gate.

Someone mentioned high side drivers. Are there any that do not require PWM to work, i.e. they can keep the gate higher than then supply voltage without switching the MOSFET itself? It would simply need to be a switched capacitor booster that is run by the chip itself, but I have never seen such a driver.

Bob
 

ebp

Joined Feb 8, 2018
2,332
The FET could be used in the low side, provided that nothing else dictates otherwise (something that might create a "sneak path" that isn't obvious).

Charge pump high side drivers are not usable for aps where the ON time is long. You can use a high side driver with a floating power supply. If you chose the driver wisely it will have undervoltage lockout for the gate drive voltage, which prevents the FET from being partially enhanced and dissipating high power should the gate drive supply voltage fall out of spec for any reason.

For only 12 volts there are probably many FETs in TO-247 that have lower ON resistance than the one in question and cost a lot less. With care the leads could be formed to fit a terminal block so all of the connections could be made without soldering. A TO-247 can be mounted with one screw, but typically would require insulation from the heatsink. At 100 amperes, a 1 milliohm FET only dissipates 10 W, so not much of a heatsink is required. 1200 W, 1.6 HP, seems rather generous for a slot car. There are sub-milliohm FETs available. You need a good relay to get contact resistance below a milliohm.
 

crutschow

Joined Mar 14, 2008
25,116
There are high-side N-MOSFET gate drivers that have an internal oscillator to generate a continuous high-side gate signal for controlling a DC voltage, such as this.
 

ebp

Joined Feb 8, 2018
2,332
There are high-side N-MOSFET gate drivers that have an internal oscillator to generate a continuous high-side gate signal for controlling a DC voltage, such as this.
Spiffy device! Rather expensive, as expected, it being from Maxim, but saves a lot of fuss and bother and expense elsewhere if the maximum voltage rating suits (that is one of those parts I would have gotten all excited about, only to be horribly disappointed by the voltage limit).
 

olphart

Joined Sep 22, 2012
78
Ok, silly question -- If this is for a slotcar track, is it for general lane on/off control?
Sure isn't for car speed control (I still have 1/24 cars & an MRC controller).
If on/off, it's near always switched on under no load, and only sometimes off under load.
A relay is just fine there, the automotive 30+A units will live a Long time.
They're cheap, easy to connect and socketed... Or am I missing something?

P.S. where generally is this track? I'd Love to play.. been ~50 yrs.
 

sdowney717

Joined Jul 18, 2012
648
What is the expected lifespan of a power MOSFET versus a mechanical relay?
How about robustness, how much abuse can they stand versus the relay.
Seeing so much burnt out electronics over the years, I recall few failed relays.
 

Thread Starter

dlatch

Joined May 15, 2016
85
Great stuff all. I am stuck with the high side layout. I'm not wiring it from scratch just improving it. It is already, like all such tracks, set up negative common.

It's only 12 volts and some of the motors are crazy hot so it's hard to get the wiring resistance low enough. It seems a good application for a big fet.

Then again there is nothing more robust than a relay. And although a separate supply to throw the fets isn't much of a complication, it is it still a non standard way to do it and would bind me to service it prompty when it's 2hours away. SO that is the best reason not to deviate from relays.

(Staunton Virginia Olphart. BTW there are tracks in most major metropolitan areas search slot car forums there are two with a list of tracks

I will digest these posts further...I am interested in those high side drivers as I have been building controllers using P fet.

thanks again to everyone for taking the time
 

jpanhalt

Joined Jan 18, 2008
9,699
Great stuff all. I am stuck with the high side layout. I'm not wiring it from scratch just improving it. It is already, like all such tracks, set up negative common.
That is common. Please show your circuit so we are not all guessing.

It's only 12 volts and some of the motors are crazy hot so it's hard to get the wiring resistance low enough. It seems a good application for a big fet.
Why would more resistance of the switch make those motors run any hotter? Higher resistance of your switch would allow less current and less heat (DC).

Then again there is nothing more robust than a relay. And although a separate supply to throw the fets isn't much of a complication, it is it still a non standard way to do it and would bind me to service it prompty when it's 2hours away. SO that is the best reason not to deviate from relays.
I am confused by that. The "it" is referring to the fets, and you seem to be making an argument to use relays. Why would you need to service a high-side driver for the fets any more than any other device? Others here have suggested using relays. Use relays and be done with it!
 
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