I'm looking to create a main power switch for my electric bike, which has a motor controller capable of handling 70A continuously, though it usually operates around 30-40A. My initial idea was to connect about 4 to 8 MOSFETs in parallel and control their gates using a low-power switch. However, I've noticed that the MOSFETs I find often have Rds(on) values of 0.2, 0.1, or even 0.07 ohms in their datasheets. With that, I calculated that at 40A using 6 MOSFETs, I would end up with around 27 watts of waste, which seems quite high. Is my understanding correct, or am I missing something?
2 Answers
You can actually find MOSFETs that have a much lower Rds(on) than what you're seeing. Look for something around 50mΩ or lower, as that can significantly reduce heat losses. It's key to check local suppliers or online to find the best options.
You might want to consider using a relay instead. There are some pretty good deals online for 100A 12V relays that only use about 2 watts for the coil. You'd just have to include a buck converter to drive it, which could help save power when it's off. Alternatively, a battery disconnect switch could be a simple and effective option too!
I see, what Rds(on) should I generally aim for then? I've come across 50mΩ but would love a more concrete target.