I have an rc aircraft application where I need to run a relatively small brushed motor, on 1s Lipo voltage, for maybe 20-30 seconds max, and then it will be shut down. This cycle will be repeated at the beginning of each flight. Each flight is roughly 6-8 minutes total so the duty cycle of the motor is very low. I know from measurements that during the time the motor is running it is drawing approximately 4.5A.
I have some 2802 (RC Switch with Small Low-Side MOSFET) on hand and they are rated at roughly 3A max. I know that using a single one in the above application didn’t burn it up after one cycle but it is clearly using it outside the specs. Is it feasible to use two of them in parallel for this specific application and have reasonable confidence that they won’t routinely fail? I know there is an RC switch available with a medium low-side MOSFET that will handle up to 15A, but I happen to already have the small ones on hand.
Thanks for any feed back. I can order a medium switch but, if it’s feasible to use two small ones in parallel that is a more immediate solution.
We generally would not recommend using two RC switches in parallel like that since they might not share the load evenly and the timing of the two turning on and off (some of the more stressful moments with motors) will not be exactly the same. Your application has short enough bursts with a long enough rest though, that you might be able to get away with it.
Hey Claire, I’ve seen plenty of designs, especially in audio power amps, where output MOSFETs are wired in parallel, and I’ve read in The Art of Electronics that that arrangement is acceptable because MOSFETs have a negative temperature coefficient, i.e. they pass less current the hotter they get, so if one MOSFET is shouldering more of the load than others, it will soon heat up and shoulder less of it. In other words, parallel MOSFETs balance themselves out automatically if they all dissipate heat the same way. In mejmia’s situation, if he can guarantee that both switches are either full on or full off at any given instant (i.e. the timing issue you identified is minimized), then wouldn’t this principle apply?
Thank you both. I’d seen the various concerns and thoughts about paralleling mosfets but confess to not having enough independent knowledge to make an informed decision so I appreciate the information.
I am “close coupling” the two switches, literally piggybacking them, so they should see as close to the same signals and thermal environment as I can provide If they smoke at some point it won’t be the end of the world, I’ll have learned something, and I’ll be buying the larger switch!
You are right, theoretically MOSFETs should share loads well while paralleled. In practice though, differences in the FETs characteristics or immediate environment, even if they are the same part number, can cause some unevenness. As you mentioned, in a case like this where the FETs are in steady state and not getting too close to their maximum current ratings (and thus maximum heat ratings) it should be fine. If either of you are interested in more about the consideration involved in paralleling MOSFETs, I found this NXP application note which has more details.
mejmea
It sounds like you are talking about directly soldering one RC switch on top of the other so all of their pins are connected. This would directly connect their GOOD and OUT pins and thus directly connect the output pins of two microcontrollers. That is generally bad practice and we do not recommend it, though this is another case where even though the outputs won’t switch at exactly the same time, you can probably get away with it.
I did lay one switch right on top of the the other (with a couple layers Kapton tape in between) and soldered the leads right through the holes that were then lined up. The only pins I connected in parallel are:
RC IN, VRC, and their associated GND
LOAD HIGH, LOAD LOW, and
LOAD HIGH and its associated GND
I additonally jumpered VCC to VRC on both switches to provide logic power supply in that way.
I did not join the GOOD or OUT pins as I didn’t need them (I could check the RC signal with a scope and I can tell if the MOSFETS is switching via the yellow LEDs).
I understand it’s still probably not a recommended method but in the FWIW category it seems to be working so far through half a dozen or so uses. There is a brief current spike at motor start that still exceeds the 6A capacity of the dual switches so I may yet be back to buy the medium version if that leads to these failing.
In the meantime, these two small switches replaced a failed OEM motor ESC that is no longer available (and was literally at least 10 times their size) so they have reduced aircraft weight and congestion inside the electronics/battery compartment quite nicely.
