[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 resulting current pulses on the secondary winding of the transformer are rectified and filtered. They provide the dc output. The output voltage is regulated by varying the duty cycle or frequency of the pulses on the transformer primary. Control feedback from the secondary to the primary is completed via optoisolators to preserve galvanic isolation.
To satisfy CEC efficiency requirements (and the proposed DOE regulations), acdc supplies with load capacity greater than 45 W must exhibit better than 85% efficiency and consume less than 0.5 W with no load on their output. The spec includes an equation for supplies with higher capacity. In addition, ac-dc supplies with over 75-W capacity require power factor correction (PFC). PFC circuits can reduce conversion efficiency.
If an offline switching topology is used in a single-bay battery-charger design, the main challenge is tight voltage control. Near the end of the charge cycle, a lithiumion (Li-ion) battery charger must maintain a constant voltage across the battery with ~1% tolerance.
For example, a typical single-cell charger maintains 4.2 V ±0.05 V across the battery until the current decreases to a small value to finish the charge cycle. It’s more difficult to achieve this tight voltage control in an offline switching supply than with, for example, a dc-dc buck-converter topology.
One must accomplish both current and voltage control in a Li-ion battery charger during portions of the charge cycle. This is easier to design in a dc-dc converter. On the other hand, a single-stage offline switching charger may have a higher efficiency than an ac-dc supply followed by a buck-converter charger.
Linear regulation is the cheapest and least complex of the charger circuit topologies in general use (Fig. 2). However, because it generally has the lowest conversion efficiency of the charger topologies, it’s typically used only for low-power chargers. The waste heat produced by this circuit is calculated by:
Dissipation = VQ1 × IBAT + RSNS × IBAT2
Take the case of a 2S (two cells in series) Li-ion battery with a nominal 3.8 V across each cell. When charging at 0.8 A, with a 12-V dc supply, its dissipation is:
The conversion efficiency of this charger in active mode is a modest 62.5%, or 6 W into the battery divided by (6 W + 3.6 W) at the input. The voltage drop across the pass transistor, multiplied by the charge current, is the primary loss factor. That’s why linear regulated chargers, though simple, are only useful for low charge currents or when the input dc and battery voltages are similar. Also, the battery voltage must always be lower than the input voltage.
Most circuit designers have used linear- mode converters for constant-voltage power supplies. The only real difference between a constant-voltage output linear power supply and a charge controller—a resistive shunt is added to regulate battery current, and more algorithms are implemented in the controller to control battery voltage and current profiles during charge. Also note that this controller, and most charge controllers, sense battery temperature and include trip points in the control algorithm to shut down or limit charge current at high and low temperature points.
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