32V 15A Relays Power Consumption

Thread Starter

CineDroid

Joined Jan 25, 2020
16
Hi all,

I'm designing a power distribution gizmo and I'm looking to turning it off and on automatically depending on the state of another device it's connected to. The switch should be able to handle 15A of max current and the max voltage would be 32V.

I'm trying to keep the enclosure as compact as possible and I've been looking at solid state relays. I'm running simulations and I realize that power MOSFETS seem to dissipate a lot of energy (17W for a current of 8A on a 24V power supply with a β of 80). That level of power is on par with the devices that my project aims at powering (small monitors, etc.) and I wonder if I'm understanding something wrong and I should keep investigating solid state relays or if you guys think I should go for another solution (electromechanical?).

I'm a beginner, and trying to learn as much as I can, so apologies if the question seems trivial.

Thanks for the insight!
 

ronsimpson

Joined Oct 7, 2019
405
β is for bipolar junction transistors not MOSFETs.
17W for a current of 8A
Numbers ???
MOSFETs have numbers. There is a max D-S voltage. In your case larger than 40V. (50v) also Current rating which needs to be more than the stall current of the motor. (not the operating current). For heat you need to know the RDSon. (resistance when on)
I picked a MOSFET at random. FDD86381. 60 volts, 25A, 0.016 ohms. Power loss =I^2 * ohms =3.6 watts. If that is too much, FDMS86350 is 80 volt, 0.0024ohms. That is about 1/2 watt at 15A.
 

ronsimpson

Joined Oct 7, 2019
405
I looked at some solid state relays and 20 amp relays can have on resistance in the 0.1 ohm to 0.003 ohms range. Depends on who made it and price.
 

Thread Starter

CineDroid

Joined Jan 25, 2020
16
Hi Ron, thanks a lot for jumping in!

I plugged in the values of the FDMS86350 in Falstad and It's giving me a 20.8W power @ 8.6A @24V.

2.4mΩ as Rds(on) and 4V as Threshold, then I'm pretty sure I'm putting the wrong Vgs value (24V). That gives me a β of 20.8 and the "reading" mentioned above (20.8W).

I'll keep the formula in mind for sure but I wonder if I'm mishandling the data in the software, would like to learn as I go.
 

Thread Starter

CineDroid

Joined Jan 25, 2020
16
I realize the software might be inaccurately asking for information. When I lay a MOSFET in it asks me for the β value and offers to calculate it based on Rds(on), Vgs and Threshold.
 

Thread Starter

CineDroid

Joined Jan 25, 2020
16
I was looking at the CPC1909 which has a 0.1Ω on-resistance and with your formula it makes then sense that the relay would dissipate in the 20W range at 15A. All makes sense now. Thanks :)
 

ronsimpson

Joined Oct 7, 2019
405
On resistance is 0.1 worse case, 0.05 typical. Lets say worse case 0.1 ohm 15A so there will be 1.5V across the switch. The relay can only do 6A with no heat sink. A really big heatsink will be needed for 15A.

Is your motor current DC or AC?
FDMS86350
If Dc we can just use a MOSFET. If we can drive the gate with 10 volts the part turns on with only 0.0024 ohms. So the voltage drop will be 0.036 volts and the power loss in the MOSFET is 0.54 watts.
 
Top