Low-Cost Lead-Acid Charger Operates at Line Frequency

Jun 1, 2001 12:00 PM
By Paul Chinski, Charles Industries, Rolling Meadows, Ill.

Thermal Design

With no fans for air cooling, the design of this battery charger requires careful thought into the mechanical design and cooling of the semiconductor devices. With Schottky rectifiers and a MOSFET, you must devise a means of extracting the heat from these components while keeping manufacturing costs in mind. You may use an extruded heat sink or aluminum plate. This allows you to mount the semiconductor devices onto the heat sink.

The first challenge is to properly mount the devices onto the heat sink while ensuring sufficient pressure for low thermal resistance. To screw each device individually involves more manufacturing time, more screws in the heat sink, and more opportunities for water to penetrate through the heat sink. You can use a bracket to hold down the devices, although screws are still required to screw the bracket onto the heat sink. Clips are another option.

Another means is to attach the semiconductor devices onto a separate plate to serve as heat sink, which you would then mount onto the main heat sink. Whatever means you choose, the basic concept is to remove heat from the semiconductor devices without internally heating the enclosure.

Besides screws in the heat sink, the input and output leads must pass through the enclosure/heat sink. Holes in the heat sink allow the dc battery connector leads and the ac input leads to pass through into the enclosure; however, you must fully seal these for the enclosure to be waterproof.

Originally, a positive temperature coefficient (PTC) resistor was inserted into the laminations of the transformer as an indirect measure of thermal rise and current flow. While this part and its assembly are expensive, a current sensing element was used to measure current directly. This part then required heat sinking. In addition, the use of temperature sensitivity of a small signal diode cuts back current.

Due to the high peak currents, some heat generates with this line frequency design. The temperature rise of the heat sink approaches 50°C. This is acceptable, as long as this application is an overnight charge of the battery.

Performance

By using the discrete, small signal control circuitry, this charger outputs higher currents at higher voltages, compared with other chargers, with the exception of some high frequency chargers (see Fig. 4). You can see the V/I curves for another type of charger by looking at Fig. 5.

The typical set-point voltage in this design at no-load is 13.7Vdc. As the load current increases up to 10A, the output voltage increases to 14.2Vdc. Few, if any, chargers increase the output voltage at higher currents. This feature allows optimum charging of the batteries in a shorter period of time. This means that a low state-of-charge battery will draw more and more current as its voltage increases up to 14.2Vdc. At 10A, the current sensing circuitry is activated and the voltage will then decline. In addition, as the internal temperature increases, the current will, over time, cutback to around 8A. You can set this to a specific value with the discrete control circuitry. Thus, this charger outputs a lot of energy into a battery in a very short period of time. The trade-off is the loss of heat and resulting thermal issues and temperature rise.

With this information, it's always possible to enter into a mature, established market with a new product, if you have a competitive advantage in either cost or performance. This design reviews some of the issues involved in improving the performance and maintaining the cost targets. Using new devices and establishing discrete control circuitry may enhance performance. Several trade-offs exist, including the temperature rise of the enclosure. You may alleviate this with some form of cutback in current. Overall, new product designs can be successful in established markets by attention to the application requirements and a complete team approach including not only the electrical and mechanical engineers, but the manufacturing group as well.

References:

1. Buxton, Joe, and Kester, Walter, “Battery Chargers: Sec. 5,” WebEE Primers.

2. O'Connor, John A., “Simple Switchmode Lead-Acid Battery Charger,” Unitrode Application Note U131.

3. Ramaswamy, Venkat, “Interactive Power Electronics Online Text.”

4. US Government Bureau of Reclamation, Power Program, Facilities Instructions, Standards and Techniques (FIST) Manuals, Storage Battery Maintenance and Principles, December 1997, “Facilities Instructions, Standards, & Techniques — Vol. 3-6: Sec. 2 Lead-Acid Battery Principles.”

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