Power Electronics

Low-Cost Lead-Acid Charger Operates at Line Frequency

Thermal design and management present major challenges in product development.

The lead-acid battery charger market is established, mature, and cost-competitive. The target charger application is for deep-cycle batteries found in the “bass boat” marketplace, where fisherman want to fully charge their trolling batteries, overnight. In this industry, thermal and mechanical considerations are critical to handle heat extraction and waterproof the enclosure.

Lead-acid batteries are very rugged, yet they have a characteristic charging profile that benefits and increases their longevity [1]. The proper charging of sealed lead-acid batteries is critical. Undercharging reduces capacity whereas overcharging damages the battery, “boiling” the electrolyte and causing outgassing. [1, 4]

One common technique to implement a bass boat battery charger is to use a high-frequency design. The advantages for this approach are the same as for switchmode power supplies — lightweight, smaller size, and improved efficiency. An additional advantage is the control of the charging algorithm — either discretely or with an integrated chip. This results in a desirable charging profile of the bass boat lead-acid batteries, often referred to as a three-state or four-state charging algorithm [2].

Fig. 1, on page 50, illustrates a timed out three-state charger that maintains the current at some predetermined high rate of charge for a specified time and then reduces it. Fig. 2, on page 50, illustrates a normal volt/current vs. time graph in which current decays with time. Some charging algorithms provide a fast charge characteristic and then transfer to a slow rate of charge. Reference 4 describes the battery charging characteristics for slow vs. fast charging for sealed lead acid batteries. The trade-off is in the additional cost, the same scenario as in the linear vs. switching supply. The high-frequency charger generally costs more than other approaches.

Another common technique is to use phase-control technology for the battery charger. This approach has been in existence for quite some time, and offers cost advantages in high-volume production. The parts count is low. The major power processing portions are the line frequency transformer and the SCRs. The trade-off here is a loss of the optimum-charging algorithm and the increased weight.

An older, mature design for battery charging is the ferroresonant type. It utilizes the transformer inductance with a specialized capacitor to obtain resonance, thus it's very sensitive to line frequency variations. For example, a 60 Hz design will not operate at 50 Hz. The control of battery charging is very poor, with the possibility of outgassing and boiling batteries. Fig. 3, on page 50, illustrates the volts/amps vs. time charging profile for a ferroresonant charger.

Line Frequency Design

Some low-cost, line frequency designs use phase-controlled SCRs to control output voltage. Controlling the SCR turn-on in the rectification bridge maintains the output voltage. Logic gates, counters and analog-to-digital converters, microcontrollers, or ICs can control the SCR [3].

Another means of controlling the output voltage is through the use of one switch instead of controlling all four SCR switches. Using a low RDS(on) MOSFET as the main switch can take advantage of its lower costs in high volume production. This allows the use of low-voltage, high-current Schottky rectifiers as the bridge rectifying elements. They have a lower voltage drop and lower conduction losses compared with a silicon rectifier or SCR. The control of one switch compared to four SCR switches is another advantage. The MOSFET has low losses at line frequency, with on-state losses predominating and is easier to turn on and off than an SCR.

You can use a current sensing element for implementing current mode control with voltage mode control. Using voltage mode and current mode control in the high frequency switching designs is effective. Using them in this line frequency application, the time constants are much longer in this realm, but you can still apply the concepts. You can use a resistor or current transformer as the current sensing element and implement the voltage and current control loops discretely if board space isn't at a premium. This allows the use of available commodity items such as op amps and comparators to manipulate small signals associated with the loops. This design can be interesting with ways to implement the control algorithms.

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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