# Switch capable of very high current?

I need to design a circuit that can turn a DC motor on and off. The motor is like a winch motor on a truck an can draw high amounts of current, in some cases up to 200Amps. The motor runs at standard alternator voltage ~14VDC. If I want to be able to switch this motor on and off from a digital circuit what devices are out there that I could use for the switching? Are there MOSFET-like devices that can handle 200 amps?

• related: High power three phase ac motor controller switches 200 A 12 VDC on and off many times each second. – davidcary Jul 4 '11 at 2:48
• If you don't find many decent mosfets, what about using a mosfet to drive a big relay? you still get digital control without the hassle of having to deal with how to switch the BIG fet and how to heatsink it etc. – Sam May 5 '16 at 7:42

The solenoid DC switch, used on golf carts and car starters can do the job. Try google for "Solenoid Relay Switch Continuous Duty Golf Cart 300 Amp". There are many parts in range of $20..$30 each.

I'm going to disagree with some things already said. I think thyristors are not a good fit for this application. This is because their forward voltage will be a significant fraction of the only 12V supply. Not only will that require a lot of heat to be dissipated somehow, but will reduce the drive to the motor.

A relay could work. The issue there is you need a very robust relay to not only conduct 200A, but also break the circuit with a inductive load without frying or welding the contacts.

Since the voltage is low, I would look at multiple N channel FETs in parallel as low side switches. That won't be cheap either, but switching 200A with inductive load isn't going to be cheap however it's done. Let's say you can get 20A 20V FETs with 15mOhm Rdson (I didn't look just making up something vaguely plausible). 10 in parallel would give you the 200A rating in theory with 1.5mOhm on resistance. That will still dissipate 60W total, but at least will be spread out over 10 devices. However, the FETs won't share the load exactly equally and you want some margin. In this case I'd use maybe 15 of these FETs in parallel. That cuts down both the total dissipation and the dissipation of each one. Since the drains are connected together, you can bolt them all to the same large chunk of corrugated aluminum.

You also need a place for the inductive kickback current to go. Since your voltage is low, this is best done with a bunch of Schottky diodes in parallel in reverse accross the motor. Schottky diodes don't share current well, but with a separate wire to each and if you're only turning off the motor occasionally (once every few seconds), it should work out. You can put deliberate 50mOhm or so resistors in series with each Schottky diode. They only conduct for a short time when the motor is switched off, so you can get away with mostly using peak current instead of average sustained current numbers. I'd derate by 25% at least anyway though.

• you're absolutely right. For 14V the thyristor is not a good idea. Must have missed this, too much focused on the 200A, I guess. – stevenvh Jun 29 '11 at 16:35
• why do they have to be Schottky's? – stevenvh Jun 29 '11 at 16:46
• @stevenvh: Schottkys have a lower forward drop, which at 200A will make a significant difference in heating. They also turn off much faster, which could be a issue if the FETs are ever turned on while the inductor is still discharging. About the only advantage of full silicon diodes would be lower reverse leakage, but at this low voltage the Schottkys should be plenty good enough in that area. – Olin Lathrop Jun 29 '11 at 18:50
• Not to mention the fact that most thyristors can't be turned off, they come on and stay on until power is removed – Sam May 5 '16 at 7:40
• @OlinLathrop I know this is a bit dated but would a thyratron work for something like this? Assuming you have another circuit that can turn up/down the grid voltage to shut it off. – Mr X Feb 11 at 21:10

If your circuit isn't affected by a large inductive load, you could use a relay with a high switched current rating. Digikey has a bunch of likely candidates like this one -> Digikey Relay

It's rated for 500A with a coil rating of 130mA/12VDC. A little pricey, but might be along the lines of what you're looking for.

• Those relays are often known as contactors. To find more, go to the Relays - Power category, and filter by Automotive and Contactor. – Kevin Vermeer Jun 30 '11 at 15:40

I've never seen MOSFETs which can handle 200A. In this kind of application they more often use thyristors (SCRs), some types can switch current of several kA.

The disc-shaped thyristors cope with the high current by having a large contact area for anode and cathode (top and bottom of the disc). At the same time they drain the produced heat away.

edit
Olin points out that this is a low voltage application, and he's absolutely right. Must have missed this, too much focused on the 200A.
Anyway, since it's so low voltage, the voltage drop over the thyristors will give this solution a low efficiency; you won't get the full voltage for the motor.
I'll leave this part of my answer, however, because it may be interesting for other user looking for a very high current solution.

Sean rightly mentions relays (for this type of relays the name is actually contactors). They have the advantage that they will dissipate less power, but can do nasty things when switching on or off. (Switching 200A is not for the faint of heart.)

• I've seen these used with high power AC switching since you have to reverse bias the gate to turn it off. How would you do that with a DC circuit? – Joel B Jun 29 '11 at 16:44
• @Joel - They're GTO (Gate Turn-Off) devices. – stevenvh Jun 29 '11 at 17:33
• One problem is an SCR drops about 0.7V, and up to 1.5V, in normal operation. That could be as much as 300W. Much more than a power MOSFET. – Thomas O Jun 29 '11 at 18:55
• @Thomas - Yes, I'm aware of that. But the actual problem is that the 0.7V (or more!) are relatively high compared to your 14V. That's also what Olin pointed out, and what I added to my answer. For industrial use (machines working on three-phase line voltage) they're very common, even if the voltage drop for contactors for example is much lower. – stevenvh Jun 29 '11 at 19:01

In an electrical boat application we have similar current switching on/off with Czonkas. I think I've seen them mentioned on another post that I'll add shortly.

UPDATE: link High current mechanical relays

Yes there are. Here are some FETs that will do what you want:

• The IRF1324S-7PPbF will handle 240A of continuous current with 0.8mΩ on resistance.
• The STV200N55F3 from ST can handle 200 A with only a 1.8 mΩ on resistance.

Both available for under $10 from Digikey in quantities that won't sell out anytime soon. • the first one looks like a great find! I just wonder how they drain 40W from an SMD. The second one won't do: at 200A it would dissipate 1280W! You can't even cool this is a closed-circuit water cooling. A fast-flowing river may work :-) – stevenvh Jun 29 '11 at 16:34 • @stevenvh - Yeah I pulled that card too quick. I edited my answer and changed to a different one. – Joel B Jun 29 '11 at 16:49 • The ST is obsolete, but apparently still available. What I'm more concerned about is the $R_{DS(ON)}$ you quote. I've tried to explain already several times why you can't count on that, and that you always should work with maximum values. ("typical" is for sales engineers, "maximum" for design engineers.) If you calculate dissipated power you'll find 72W, I get 100W ($R_{DS(ON)}$=2.5mΩ). If your cooling is dimensioned for 72W and the product fails due to overheating, you can't even complain at ST! Same for the IRF. – stevenvh Jun 29 '11 at 18:03 • It's worth noting that ratings like that are generally meaningless. For example, IIRC, with a TO-220 package, the legs melt at ~80A. The 240A rating is based on a theoretical model of the silicon only at 25°C (the calculation completely ignores the constraints from the packaging). Realistically, to manage 200A continuously, you will need a significant number of them in parallel. – Connor Wolf Jun 30 '11 at 5:23 • @ConnorWolf According to the datasheet, the max current is 429A (silicon limited) and 240A (package limited). So this has already been taken into account. That being said, yes it would probably be wise to have an extra unit in parallel. – nick Dec 28 '15 at 18:27 Just get a vacuum switch from kilovac in Carpinteria Ca. You can get a 12 volt or 24 volt coil. Much simpler and easier to implement. Here's why: Using MOSFETs in motor apps is tricky as the off time is critical due to very high voltages being produced by the inductance of the motor and wires. Schottky diodes work but you might still need an RC network to keep the back emf from blowing the MOSFETs. Also driving MOSFETs is not trivial you need a good gate driver and since you will have many in parallel the input capacitance is high enough to be a problem if the gate driver does not have a low enough output impedance. Also the circuit needs to be well made electrically and mechanically. PCB traces need to be wide enough and short enough to handle the current. Unless you want a project, get a vac relay and be done. • Welcome, try and use correct grammar and punctuation here. – Voltage Spike Dec 28 '16 at 20:06 I very agree with Connor Wolf. Yes, there are plenty MOS devices at very low Rds-on and very high currents. An example could be IRFS7730 with a theoretical 246A and a practical 60A (at 80A leads will melt), but i will recommend instead a really good case, the new models of D2PAK casing with 5 or 6 source pins! these really have at least 150 Amps, real for good. An example is IRFS7534-7 with its five source pins!. But don't abuse of one case only: put several in parallel, to reduce RDS-on and dissipation, otherwise you'll fry them. Calculate power dissipated using I2R and ensure your motor is fed during startup when it will absorb 8-10 time more than its nominal current. And don't forget to absorb the flyback current from motor with plenty of Schottky diodes (for example 16 pcs of 8A/24V in parallel) when the motor should stop. Otherwise the MOS will be exposed to flyback currents from motor and will burn. Try using this MOSFET as a switch. IXTN660N04T4 It is rated at 660A continuous current provided that you can cool it. It has a 0.85 milli-ohm on resistance. So at 200A the drop across it would be 0.17V and 34W of heat would be generated. The device has a large isolated pad on the back side that has a junction to pad thermal resistance rated at 0.144 C/W. The pad has screw holes for mounting. So you could theoretically put that isolated pad right on the frame of the truck to sink out as much heat as you need. The source drain and gate connections on the part are made with screws and ring lugs. It is available for$19.6 on Digikey.

http://www.digikey.com/product-detail/en/ixys/IXTN660N04T4/IXTN660N04T4-ND/6053919

IXYS corporation sells other similar MOSFETs if you need a different package style.

To prevent inductive kickback from destroying the device when you cut power to the motor you should install several automotive grade TVS diodes in parallel with the motor and reverse biased between the output of the MOSFET and ground.