Electronic Design

Add Switch-Mode To Battery Charger

Because battery chargers control power, they resemble regulators in some respects. Linear chargers, for example, are less efficient than switching types when operating at high current and there's a large Vin - Vout difference. When powered by a 15-V car battery and delivering 2 A while charging an 8-V, 6-cell NiCd stack, a linear charger must dissipate 14 W. Efficient chargers therefore employ switch-mode dc-dc conversion to derive the battery-charge voltage from Vin.

An efficient fast-charge circuit features a controller chip that typically implements linear regulation with an external pnp transistor (see the figure). However, the circuit substitutes a p-channel MOSFET for the pnp and it implements switch-mode PWM regulation with the help of an inductor, two Schottky diodes, and a dual timer (IC1). The timer's "B" side is configured as a one-shot that receives nominal 70-kHz triggers from the device's free-running "A" side.

During a charge, IC2 monitors the slope of battery voltage versus time. The MAX712 (for nickel-metal-hydride batteries) terminates charging when the slope reaches zero; the MAX713 (for NiCd or NiMH batteries) terminates the charging operation when the slope goes negative. As a backup, an internal timer can be programmed to terminate the charge after 1/4, 1/2, 1, or 2 hours (times a multiple of 1.5X or 2X).

As a further backup, comparator circuits in IC2 monitor the battery temperature. This prevents charging if the battery is too cold and terminating the charge if the battery becomes too hot. In every case, the IC applies a trickle charge of C/16 (125 mA in the figure) after termination of the fast charge.

IC2 can be programmed to charge 1 to 16 cells connected in series. The device simultaneously monitors and regulates battery voltage and charge current (via current-sense resistor R4), and issues signals at the open-drain output DRV. These signals implement pulse-width modulation by altering the duty cycle of Q3's gate drive.

Built with narrow-SO surface-mounted components, the circuits fit easily into notebook computers and other portable equipment. When delivering 2 A, it can charge a stack of six sub-C cells in less than one hour. The efficiency is 89% for Vin = 12 V and Vout = 9 V. The power dissipation is virtually constant at 2.3 W for Vin = 11 to 16 V, and Vout = 3 to 9 V (Vin must exceed the maximum battery voltage by at least 1 V). Heat sinks are unnecessary for the devices because no component dissipates more than 0.5 W.

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