Why are DC SSR's (solid state relays) so expensive?

[MikahB]

I understand the small ones are affordable (<=3A load) but for larger ones in the 30A-40A range, they are quite expensive, even in volume.

For a hypothetical 20A DC load (automotive application), using a PWM signal at about 25kHz, are there more cost-effective options than a $70 SSR? This is something that will be produced in volume, not a one-off.

What kind of trouble would a bank of parallel MOSFET's with a good heatsink get in to with this kind of load?

 

[crutschow]

SSRs tend be be expensive because they have an isolated input from the output. If you don't need isolation, then you can just use a MOSFET (you can find one MOSFET rated at 40-50A to handle the 20A load) for you application. You will, of course, need an adequate driver to rapidly charge and discharge the large gate capacitance of such a large MOSFET at 25kHz to avoid overheating the MOSFET.

You would need a different solution anyway, since most SSRs are fairly slow and cannot handle a 25kHz switching frequency.