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I need to design a solid state switch to disconnect a battery (28V) from a load that can go to a maximum of 70 Amps in a 40C environnement.

An SSR like the Crydom D06D100 could handle this using a very large heatsink and I suppose it would be the same for whichever big MOSFET I find.

How do the following products achieve this objective without using such a big form factor?

https://www.modernracing.net/product/mr-solid-state-battery-disconnect/

https://perfectswitch.com/dc-solid-state-relays/uni-directional-dc-relays/

https://verticalpower.com/index.php/products/pps

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  • \$\begingroup\$ My guess is that they don't have any special magic, and the specs are written assuming the switches are mounted to a cold plate at some given temperature, or assuming you can keep the case at a certain temperature. \$\endgroup\$
    – John D
    Commented Jan 6, 2021 at 4:55

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Probably all those devices have to be mounted on a plate or heatsink. The heat generated due conduction loss is proportional to I²R, where R is the on resistance of the MOSFET. For large current it is better to put in parallel multiple MOSFETS that have a good thermal conduction to a PCB with thick copper layers.

A special charge pump is needed for driving N-MOSFETS. An example is an ideal diode controller like LTC4358

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