Use of N-channel power MOSFET on BQ78PL114

Posted on Feb 6, 2014

I have finally finished my first prototype PCB but I am a bit worried about the amount of heat generated when using the BMSat max power in myapplication (ultralight hybrid electric vehicle). At 30A cont/50A burstI have almost 20Wloss. It seems to me that it would be possible to reduce power consumption to less than half of that without increasing

Use of N-channel power MOSFET on BQ78PL114
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costsusing: The use of N-channel MOSFETs enables a muchbetter selection of verylow Rds parts. Itwould obviouslyrequire a different driver design, but as far as I can see, there is nofundamental reasons why this should not work It isused on TI battery AFEs like the bq29330. Using MOSFETs with integrated current sensing would again reduce the number for available devices but reduces the power loss ofthe current current (!) sensor. This would either require low-side switching or current sensing level shifting to be compatible with the BQ78PL114 current sensor inputs. Here is suggested circuit for implementing a charge pump and driver in aSOT23-6 capsule. It requires only a few parts more than the bq78PL116 reference design. The suggestion make use of a new high voltage ideal diode driver from Nat. Semi that handles 75V (100V peak). It works by monitoring voltage on both sides of the FET via the IN and OUT terminal. It will turn on the gate when the IN pin is raised minimum 30 mV higher than OUT pin using a charge pump supplying 30uA gate current. The gate voltage is then raised until the IN and OUT levels are identical or until the gate voltage is clamped by an internal zener diode. If the IN pin voltage becomes 30 mV less than OUT pin the gate charge will be shorted to drain within 50-100ns using a built-in high current FET. This is controlled by the voltage divider R1/R4 (R7/R3) that feed OUT with a voltage around half...

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