I'm trying to make a simple passive battery balancing circuit using logic level n-mosfet driven by MCU. However, I've got some issue on how to drive the mosfets by the digital output of the mcu, and also having isolation between mcu ground and the source legs of the mosfet. Otherwise, in the case of the following circuit, the 2nd battery would be short circuited.
Looking forward to hear your thoughts or maybe another alternative to drive the mosfet. I was thinking to use optocoupler, but then I need another isolated supply. Thanks
Best Answer
A low-part-count solution would be using an optodriver such as the TLP190B, which allows you to drive a N-FET referenced to a different potential than your local ground without the need for an extra isolated supply. (The IC works by illuminating a small LED that points at a small photodiode, generating the current to enhance an N-FET's gate).
A possibly cheaper (but higher-part-count solution) would use N-FETs to pull down on the gate of P-FETs. Depending on your P-FETs and how many batteries are in your stack, you would need to be clever with the inclusion of zener diodes to stay within the V_gs,max of the part. The scheme I'm suggesting might look something like the schematic on this page.
Finally, there are a whole slew of battery management ICs that would work, depending on the constraints of your application. Searching for a "two cell battery management ic" would put you on the right track.