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I'm designing a simple cell balancing for charging LiPoFe4 cells in series. In order to prevent overcharging, when on of the cells reach the maximum charing voltage, the microcontroller will enable the mosfet, to bypass the current to the next cell.

Consider the following circuit, where the 3.2V sources represent two cells, only with the top cell presenting the balancing circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

For the mosfet I intend to use, the max threshold gate voltage is 2.5V. Assuming the worst case scenario, the botom cell being fully charger, I'll have a Vs of 3.2V. With the bypass enabled, I'll have Vg of 5V from the microcontroller. Thus, I won't achieve the Vgs = 2.5V, and the mosfet won't transit onto its 'on' state.

The question, what would be the best way of workaroud here, not changing the mosfet? I thought, maybe, add a greater voltage reference and control this one with the microcontroller,but I still think there's something simple I'm missing.

Thanks in advance!

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  • \$\begingroup\$ 1 kOhm isn't much of a current shunt resitor unless you're only charging at 3 mA or so. \$\endgroup\$ – Olin Lathrop Oct 16 '14 at 12:27
  • \$\begingroup\$ @OlinLathrop, sorry, I should have mention not to take on account the actual values of the resistors. This is just a draft, the focus is on the Vgs issue. \$\endgroup\$ – Sergio Oct 16 '14 at 13:09
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You could pull the gate up to the upper cell + terminal with the high value resistor and then pull it down to turn the MOSFET off, for example with a BJT or another MOSFET (open collector/open drain).

However, the "threshold" voltage is not what you want to look at-- you want a gate voltage that will guarantee a certain Rds(on) so as not to cook your transistor. The only iron-clad guarantee on that datasheet is for 4.5V Vgs, so I would suggest using a MOSFET with guaranteed performance (Rds(on)) with 3V or lower Vgs. Also you should take care that the voltage under fault conditions (for example with the battery removed) cannot come close to exceeding the Vgs maximum rating (usually 10-20V, but sometimes lower). The MOSFET gate oxide can be protected with back-to-back zeners (between gate and source), if necessary.

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  • \$\begingroup\$ Thanks for the reply. Just to clarify your answer, the maximum Vgs I would get from pulling the gate to the top terminal would be 3.2V.. So this suggestion would only apply if I changed the mosfet as you suggested after, for one like this, for instance. Did I get it right? What if I added a voltage reference zener and pulled the gate up to the reference? What do you think? \$\endgroup\$ – Sergio Oct 17 '14 at 8:49
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    \$\begingroup\$ Yes, that one would be great (watch the +/-8V abs max gate voltage though). The voltage reference zener sounds good (maybe with the original MOSFET).. just make sure that under every condition (normal, fully charged, either or both battery open, either or both battery shorted) the gate voltage can't exceed the maximum and the MOSFET is either hard on or completely off and you'll be fine. \$\endgroup\$ – Spehro Pefhany Oct 17 '14 at 10:24

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