Solid state relay output connection: Is there a difference between high-side and low-side switch configurations?

Question

I am planning to use this SSR chip.

I decided to use the "Connection C" configuration which is given pictured in the 2nd page of the datasheet.

However, there is something I don't like about this connection, and that is that the load is connected to the high side. If I wanted a low-side switch configuration, I could simply use a MOSFET instead of this SSR chip.

So, I decided to modify the "Connection C" topology simply by swapping the source and the load. Now it looks like this:

My corresponding schematic is like this:

The voltage to be switched is 36 V. The load will be connected in parallel to the Schottky diode D400.

Intentionally there is no isolation between input and output. Maximum load current is 4 A. Switching frequency is very low; around 1 Hz at most. The ambient temperature will be 50°C at most.

Is there something wrong with my circuit? Will it work as intended in this modified version of "Connection C"?

Answer 1

Maximum load current is 4A. Switching frequency is very low; around 1Hz at most. The ambient temperature will be 50^oC at most."

The G3VM-61FR device uses a photovoltaic generator to produce a floating voltage that directly interfaces to two MOSFETs. The two MOSFETs can be wired antiphase (as in connection scheme A or, can be wired for DC connections using 1 or 2 of the MOSFETs (schemes B and C).

The G3VM-61FR is rated at 5 amps (25°C) and you want to use it at an ambient temperature of 50°C. That means you have to derate it by 100 mA per °C above 25 °C and therefore your maximum current is 5 amps minus 2.5 amps.

That's significantly below the 4 amps you specified for the load. That's the main problem you face. Then you have to consider how warm it might get when continuously driving 2.5 amps into a load and figure out how much the local ambient temperature might rise. It looks to me like you won't get much change from 1.5 amps so, my conclusion is that it is unsuitable for your requirements. Internal heat dissipation is around 200 mW with a 4 amp load and this will warm the device over several minutes. OK, if you are operating the device at 1 Hz, the average dissipation is 100 mW.

Will it work as intended in this modified version of "Connection C"?

Apart from it not being able to supply the load current, it will work fine with load and voltage supply being swapped.

Maybe consider something like a separate MOSFET driven by a photovoltaic driver like the VOM1271. Then you pick a MOSFET to interface with it that has beefier performance.

Answer 2

Just wanted to add a note here that according to the datasheet this relay is good for up to 10A when wired in the C configuration - there shouldn't be any issue driving 4A through it at 1Hz when wired to use both MOSFETs.