Fotec DC SSR-100DD issue

[tonigau]

Looking for a clarification of the output switching device in these Fotek DC SSR's - Should be BJT?

A colleague got a bunch of these to control water heaters from a 48V Battery but none? are working (still waiting to clarify any failure mode(s)-smoke, excess heat etc...). Each heater is 30A so well within the current spec. They are 60V rated so not enough safe operating margin for a full charging LA battery & cable inductance (long wires out to tanks). They are mounted on heatsinks.

Ignoring for now the volt margin, he did an autopsy & the switching transistor is a P140NF75, 120A 75V N-ch MOSFET.
I have used FOTEC SSR-40DD devices before & they were BJT switching device.

Unfortunately the device is mostly potted so can't confirm ATM if there is some charge transfer circuit over the isolation barrier for the Gate.
I am suspecting factory loaded MOSFET's by mistake ? more investigation is ongoing...

Here is some photo sent to me...

 

[gophert]

Even Fortec doesn't list this model number on their web site. And, what is it really? A Fortec SSR-100DD as on the card, a or a Turtle SSD-10DD as in the circuitboard?

This SSD is all over ebay and Amazon and Ali-Express. Note the only available technical info as an incomplete sentance in the highlighted text from an Amazon listing.

 

[tonigau]

Maybe I should have said "Fotek" as it is most unlikely to be made by Fotek. It is from an aliex supplier.
> And, what is it really?
It would likely be a knockoff , the PCB marking "SSD-10DD" would be the base board to fit different rated switching devices.

Going into town tomorrow & I will bring it home for some detailed dissection.

 

[tonigau]

Ok, got the DCSSR to look at today, it has a wee transformer powered from the control side, draws about 10mA @ 6V & gives 11.8V for the Gate. The on resistance for the MOSFET is measured 0.077 ohms which is about 10 times the spec going by the device printed part number (STP140NF75) so its obviously fake or reject.
Will put in a better logic level device which should reduce the dissipation.