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I need to switch off a 20V 60A DC current electronically. I don't want to use a relay, due to the environment (vibrations etc.).

It is not a switching supply, just a switch that turns everything off when given a signal.

I've been trying to design a circuit that basically passes the load current through a MOSFET which can be switched off.

I've been looking at power MOSFETs and IGBTs and am having trouble understanding their specifications. For instance this MOSFET has a Maximum Continuous Current rating of 100A, but when I look at the Safe Operating Area graph, at 20V DC it will only do around 1.5A. Similarly for other FETs and IGBTs.

Is the SOA graph what I need to be designing with, or can I use the Max current specifications?

Are there transistors made specifically for DC applications that can handle a large DC current, or am I using the wrong components/design approach here?

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  • \$\begingroup\$ At those ratings you're going to want one of those discrete modules instead of anything that starts with "SOT" or "TO". \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 5 '15 at 23:41
  • \$\begingroup\$ By that do you mean the tin-can types? \$\endgroup\$ – nik Aug 6 '15 at 0:34
  • \$\begingroup\$ Something like Microsemi's APTM10UM01FAG, although that particular one is slight overkill. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 6 '15 at 0:36
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It's not quite that bad - I read about 2.5A at 20V for DC. But it does seem a bit low compared to the 131W max dissipation.

However if you switch in 10us the graph intersects at 400A.

Once the switch is on it will not have 20V across it. Just RDSon * I = 2.8mOhm * 60 = 168mV. This will result in a dissipation of ~10W. Well within the device capability.

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  • \$\begingroup\$ Ah perfect, That's what I was doing wrong. For some silly reason I was working with the full voltage across it. \$\endgroup\$ – nik Aug 6 '15 at 0:33
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I suggest that you use Mosfets and not the IGBTs because you will waste much less voltage and hence have less heat dissipation problems . When you select your mosfets you are best to go for a reasonably low Drain Source voltage because lower on resistances are possible . IN fact if your DC 20volt is regulated and you don't have an inductive load you could use mosfets with VDS 30Volt . NOW you must check the PEAK current and rate the mosfets accordingly if your load is resistive then 60 amps is all that flows . Because the low volt mosfets are cheap its better to parallel a few TO220 devices and minimise heatsinking needs . The job I did was 24V 60A peak for glowplugs which worked fine for customer . 60V devices were used because the deisel heater went in a mobile home. I used photovoltaic stack for highside drive because at the time and possibly still now P chan wasn't cost effective so you probably need to think about some sort of highside drive . The PVI1500 gave a low parts count which was good for low volume thru hole production. NOW the photovoltaic stack isn't without its limitations which are stated on my www.badbeetles.com website.

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