Relay or MOSFET for high current?

Thread Starter

electromeow

Joined Oct 12, 2021
3
Hi,
I have to switch a high current(DC 12V@~60A approx.) for a battery-supplied circuit.
But its on-status resistance should be as low as possible. Because you know the Ohm's Law, the I is going to increase up to even 100 amps sometimes, so a very small resistance creates a big difference.
I don't need a fast switching. My purpose is to turn the circuit off when my battery indicator(made with an opamp as a comparator, it is going to make the signal low when battery voltage is low enough) makes the signal low, to protect the battery from overdischarge.
I have two options:
1) Using a high power relay
The disadvantages of this option are space(it is going to get underwater in a waterproof enclosure and a high power takes up a huge space in the enclosure) and cost.
2) A power MOSFET
I'm not sure this is a perfect option. I would like to use 3 IRFZ44N MOSFETs in parallel(so the RDS(on) will affect the circuit less). But I'm worried about using MOSFETs in parallel.

Which to use for such a purpose? Which one is going to create the minimum resistance?
 

crutschow

Joined Mar 14, 2008
28,538
The power relay will have very low contact resistance and provides isolation between the control signal and the output (do you need that?), but is less reliable (how often will it need to switch?).

The MOSFETs will have a higher resistance but that can be made very small by using low Rds(on) ones (some, e.g. IRFH6200, are a milliohm or less).
MOSFETs can be usually used in parallel.
Since their on-resistance increases with temperature, they will tend to balance the load among them.
 

ElectricSpidey

Joined Dec 2, 2017
2,024
You are still going to need a large heatsink for the MOSFETs so I would consider that when comparing size.

You also need to consider where that heat is going to go in an enclosure.
 

Thread Starter

electromeow

Joined Oct 12, 2021
3
The power relay will have very low contact resistance and provides isolation between the control signal and the output (do you need that?), but is less reliable (how often will it need to switch?).

The MOSFETs will have a higher resistance but that can be made very small by using low Rds(on) ones (some, e.g. IRFH6200, are a milliohm or less).
MOSFETs can be usually used in parallel.
Since their on-resistance increases with temperature, they will tend to balance the load among them.
I don't need isolation, because I have only one power source, the battery pack. It won't switch often, maybe once 3 hours. Its gate/coil will be connected to my battery indicator(made with an LM324 as comparator and a few voltage dividers), it is going to be on the on-status most of the time. But it will close the rest of the circuit when battery voltage gets low enough. I don't know which MOSFET to use. IRFZ44N looks like fine but I'll take a look at some logic level MOSFET models with low on-resistance and high current capacity. If I would use the MOSFET option, I'm going to connect a few in parallel so the on-resistance is going to make a value of a few miliohms, which is pretty OK for me. Last year we supplied the power through a cable 40 meters long instead of a battery inside, and its resistance(like 0.6 ohms) became a pain.
Then I'll look at some relays if they are available in my country or could order them from overseas.
 

Thread Starter

electromeow

Joined Oct 12, 2021
3
You are still going to need a large heatsink for the MOSFETs so I would consider that when comparing size.

You also need to consider where that heat is going to go in an enclosure.
Heat sinks with MOSFETs take a smaller space.
The heat is going to fly into the air, so the air in the enclosure is going to heat up. But it isn't a problem because its enclosure will be opened after a 3-4 hours(max). The battery isn't endless, we are going to take the vehicle out of water and open the lids of enclosure. So I suppose it won't create a big problem.
 

bassbindevil

Joined Jan 23, 2014
236
I built a ESC (electronic speed control) for a friend with an electric-powered model airplane a while ago, and built it with 4 parallel MOSFETs instead of an H-bridge since he didn't need reverse. With the throttle at 100%, voltage drop across the MOSFETs was less than across the radio-controlled relay he'd previously been using, maybe because I built it with thicker wires. It was almost 30 years ago, but I believe those were IRFZ44P; I can't remember how much current the motor pulled (or if I even tested that).

You may need way more than 3 MOSFETs; I got a PWM speed controller from eBay for another friend's trolling motor which pulled nearly 40 amps when tested in a rain barrel. The controller was rated for 60A, and has 8 paralleled NCE7190 MOSFETs, N-channel, 90A and 6.8 milliohms at 10V gate drive. So the MOSFET capacity of 720 A is over 10 times the nominal rated controller capacity. Maybe PWM switching of a motor requires oversized MOSFETs compared to on/off switching.
 

BobTPH

Joined Jun 5, 2013
4,280
Heat sinks with MOSFETs take a smaller space.
The heat is going to fly into the air, so the air in the enclosure is going to heat up. But it isn't a problem because its enclosure will be opened after a 3-4 hours(max). The battery isn't endless, we are going to take the vehicle out of water and open the lids of enclosure. So I suppose it won't create a big problem.
The heating of the air is not the problem. It is the heating of the MOSFET that you need to worry about. A heatsink moves the heat from the MOSFET to the air. It has to do this fast enough to keep the MOSFET from being destroyed.

Bob
 

crutschow

Joined Mar 14, 2008
28,538
Maybe PWM switching of a motor requires oversized MOSFETs compared to on/off switching.
Yes, there are switching losses as well as on-resistance losses in a MOSFET.
The switching losses are determined by the speed of the switching, which is why the high-capacitance gates of switching MOSFETs are driven by high-current, high-speed drivers.
 
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