Originally Posted by Joe Perez
I just pulled a spec sheet at random, the Crydom D06D60, which is rated 60 A DC. It claims a max PWM frequency of 1 Khz, with notes that this applies only to resistive loads and requires a de-rating to 75% of rated current at the maximum PWM frequency. (No chart of de-rating vs. frequency is given, nor do they discuss inductive loads.)
Whoops, being a bit late to this thread. The derating is most likely because of the slow switching of the SSR. Those things are most likely galvanically isolated through something like an optocoupler and these things are slow; they will limit the speed at which the MOSFET can turn on and off.
Because the MOSFET turns on and off slowly it will dissipate energy while it is switching, this is why they derate the thing when switching at 1kHz.
Normally I would never consider SSR as they are 'extremely slow' in my book, but when your PWM frequency is only 100Hz they can be used. You just cannot build DC/DC converters with them as those easily need >100kHz switching speed
As for the fuel pump there is no real need for galvanic isolation I think? So instead of using a SSR you could also pick a beefy protected MOSFET, that would have much less switching losses compared to a SSR at >500Hz switching frequency.
I am not sure about how much power that diode has to absorb, but I guess that diode looks suitable. I think the diode only sees the peak winding current for a short period of time.
That TO-247 package has a thermal resistance (junction-air) of 30*C/W, so even if the diode has to dissipate 3-4 watts you are still fine without a heatsink.
Just slap it on a piece of aluminium and check temperature when you are running a 90%-ish duty cycle: if it does not instantly burn your finger it will be fine.