MOSFET prevents battery damage
A MOSFET protects reverse-connected batteries from excessive current.
Santosh Bhandarkar, Wep Peripherals, Mysore, India; Edited by Martin Rowe and Fran Granville -- EDN, February 5, 2009
Sealed-lead-acid batteries, which find wide use in power-electronics products, such as UPS (uninterruptible-power supplies), inverters, and emergency lamps, supply power to the load whenever utility power is unavailable. When you restore utility power, a charger supplies the power to the load and charges the batteries (Figure 1).
You can add a diode to protect a load from current resulting from a reverse-connected battery. The diode, however, won’t protect a reverse-connected battery from the charger circuit. If the charger is on, a potentially dangerous current can flow into a reverse-connected battery. The battery voltage, which normally opposes the charging voltage, now aids it, which lets a higher current flow into the battery.
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If you add an N-channel MOSFET to the circuit, you can protect the battery from this damaging condition (Figure 2). The MOSFET conducts only when the battery is correctly connected, which lets the battery charge or discharge. In this condition, the transistor gets forward-biased, which switches on the MOSFET. If the battery is reverse-connected, the transistor and MOSFET turn off, thus preventing current flow. This simple circuit provides reverse-battery protection in both charger and battery paths, thereby protecting the battery, the charger, and the load. You can use a microcontroller to measure battery current and make a decision on appropriate action, as well.
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Since one of the objectives is to protect against a reverse connected battery, a signal diode is needed in series with the base bias resistor since the 2N2907 data sheet indicates the following base-emitter junction reverse bias limit:
V(BR)EBO* Emitter-Base Breakdown Voltage
(IC = 0)
IE = -10 uA
Absolute Max: -5 V
The diode will only allow base current when the battery is connected properly.
Art Du Rea - 2010-9-2 19:37:00 PST -
I beleive the circuit has a potential problem: Q1 and Q2 are connected in a positive-feedback loop. If the charger power is on, with the battery disconnected, any leakage in Q2 will provide base current to Q1, which will further turn on Q2, until the circuit latches on. This would depend on the Hfe and Gm of the parts involved, so some units might have the problem and others not. It might only show up after the circuit has been charging and Q2''s temperature has risen, making it intermittent also- a nasty combination. The solution,however,is simple - add a resistor from Q1''s base to the positive rail to give the leakage current an alternate path.
Ronald Keeney - 2009-10-3 13:47:00 PDT
