[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.
Excess energy waste during battery charge is, of course, a bad idea for the environment and poor design practice. In fact, the problem has deepened to the point where it’s now an international regulatory agency issue. Battery-charger efficiency is more challenging to specify and measure than ac-dc power-supply conversion efficiency. It’s usually understood as the amount of energy stored in the battery relative to the energy consumed by the charger during the charge cycle.
But that isn’t a good definition, because battery chargers are often left powered when batteries aren’t actively charging. Power consumed during idle or battery-charge maintenance must also be considered.
When power supplies only need a measurement of the ac power-in and the dc power-out at maximum and zero rated load and at zero load conditions, battery-charger efficiency can’t be measured that easily. Charger efficiency standards are based on summing energy consumption during a specific period of time that includes active charge, maintenance of charge, and standby mode with no battery in the charger.
For example, the Energy Star measurement cycle starts after the battery has been on-charge for 24 hours. At that point, it’s assumed that it has reached full charge. Then, the energy consumed to maintain full charge on the battery is measured for 36 hours with the battery in the charger and an additional 12 hours with the battery removed.
An energy ratio (ER) is calculated by dividing the energy measured in the 48-hour non-active period by the energy that can be extracted by fully discharging the battery. The Energy Star standard contains a table of the acceptable values of ER relative to the battery voltage, with a maximum ER of 20 for 1.2-V batteries to an ER of 3 for batteries of 24 V or higher.
Obviously, the Energy Star standard doesn’t include the conversion efficiency of the charger during active charge, only during charge maintenance and standby modes. The rationale behind this is the observation that most consumer-market battery chargers sit powered but empty or with fully charged batteries inserted for much of their life.
Looking ahead, the California Energy Commission (CEC) and the U.S. Department of Energy (DOE) are both developing standards that include active charge mode efficiency. The DOE mandatory standard is scheduled for publication in 2008 and will be in force by 2011. The CEC spec will either be published sooner or will just tie into the DOE spec.
Also, most chargers have an ac-dc power supply, either embedded inside the charger or as an external desktop unit. These supplies are already subject to the CEC and Energy Star power-supply efficiency standards and will be regulated by the future DOE standards.
CHARGER TOPOLOGIES, CONVERSION EFFICIENCY Most ac-powered battery chargers are designed as offline or two-stage switched-mode power supplies (SMPS) with special controls. Some dc-powered chargers use linear regulation, but they’re generally limited to low-power products (Fig. 1).
Of course, the linear and SMPS charger topologies also accept dc power directly from a generator or backup battery system. These types of chargers must have input protection and switching topologies to accommodate the voltage transients and wide voltage range found in those environments. The offline charger topology is limited to a single-battery charger, since each battery-charger output must be voltage- and current- controlled per the charge state of the associated battery.
THE AC-DC SUPPLY AND OFFLINE CHARGERS There are two major subcategories of ac-dc power supplies: open-frame or brick supplies intended for building into other systems, and packaged desktop or wall-mount supplies. Battery chargers use both types.
Most ac-dc supplies employ a flyback topology, but there are many variations. In a flyback design, the ac line input is rectified to a high-voltage dc, which is then switched as current pulses onto the primary winding of a transformer by one or more MOSFET transistors.
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