[Engineering Essentials]
Stop The Waste In Your Battery-Charger Conversion
As portable devices add functionality, the ability to recharge their batteries—and do so without wasting additional energy—becomes more important.
The gate-drive current requirement increases with decreasing tsw. If the gate driver can’t deliver the required current to minimize on or off transition time, efficiency will suffer. IC designers have come up with FET packaging and IC designs specifically for SMPS applications. Most of the time these are N-channel FETs, which feature very low capacitance, very fast turn on and off, and low RDS(ON) specs. It pays to look through the selection guides on vendor Web sites to find the switching FETs that match your design requirements.
OTHER EFFICIENCY FACTORS When analyzing the power loss in your battery charger, don’t ignore items that aren’t directly in the power-input to battery conversion path. The power consumption of some of these items can add up quickly.
The power to drive LED indicators is small per device, but can add up to a substantial total in a multibay battery charger. For example, if the charger supports six battery charge bays, and each bay has one status LED and five charge-state LEDs in a bar-graph arrangement, the entire charger powers 36 LEDs. If each of these LEDs is lit with 10 mA of current, drawn from a 12-V dc supply, they consume ~3.5 W when they’re all lit. One can reduce the power to the LED indicators by using highintensity devices and lower current, pulsing the LEDs, and turning off non-essential indicators when charge is complete.
A fan can be used in the charger to move hot air out of the enclosure and keep it away from the battery being charged. Of course, the fan itself and its associated drive and control circuits also consume power. Use thermostatic control to turn the fan off when it’s not needed and/or to modulate the fan RPM relative to the battery or enclosure temperature.
Many chargers use microprocessors for control, and these ICs usually require a 3.3- or 5-V dc power supply. The efficiency of this supply should be considered in the overall charger efficiency calculation.
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