[Design View / Design Solution]
Develop Affordable Mixed-Signal Battery-Charger Designs
Using these techniques, your battery-charger designs can take advantage of the best of both the analog and digital worlds.
DESIGNING BATTERY CHARGERS By keeping certain system considerations in mind, an appropriate charge-management system can be developed. For example, linear charging solutions are employed when a well-regulated input source is available. In these applications, linear solutions offer ease-of-use, size, and cost advantages.
For a wide input-voltage range, such as the unregulated ac-dc wall cube or the automotive dc input, switching regulators lower the internal battery-charger power dissipation to an acceptable level. Switching-regulator topology defines the organization of the regulator's switches and passive filtering components. This difference in organization distinguishes topologies, offering a tradeoff between complexity, efficiency, noise, and output-voltage range. Many converter topologies exist, while only a few are popular for battery chargers in the 5- to 50-W range.
The buck or "step-down" regulator is one popular topology for battery-charging applications. Like other solutions, the buck regulator has some advantages and disadvantages:
Advantages:
It's a low-complexity, single-inductor topology.
For synchronous applications, conversion efficiency can reach 90%.
Disadvantages:
The buck-regulator, MOSFET-switch integral body diode creates a path to discharge the battery when input voltage isn't present. An additional blocking diode is therefore necessary, adding another component and, hence, voltage drop to the system (Fig. 4a).
Buck-regulator input current is pulsed or "chopped" (Fig. 4b). This topology generates high electromagnetic interference (EMI) at the input of the power supply. Thus, most buck regulators require additional input EMI filtering.
The buck regulator can only regulate output voltages that are lower than the input voltage. Some applications have a wide input-voltage range that spans the necessary output-voltage range. This is more common for multiple-cell Li-ion charger applications.
A single fault mode (buck switch short) creates a short circuit from input to battery. For NiMH applications, which lack internal battery protection, this poses a safety concern.
The buck regulator requires a high-side drive (for n-channel MOSFET switches). This is more complex than low-side topologies.
External switch-current sensing in pulse-width-modulation (PWM) controller applications is complex. Limiting switch current is important for fault modes such as shorted batteries or load. Without a high-speed switch-current limit, the battery charger can be destroyed during a shorted condition.
Single-ended primary inductive converter (SEPIC) regulators also are a popular topology in battery-charging applications. SEPIC regulators hold a number of advantages over buck regulators and other topologies, though there are a few disadvantages.
Advantages:
The blocking diode is built into the battery-system topology so no additional components or losses occur (Fig. 5a).
The input current pulled from the source is continuous (smooth) compared to the "choppy" input currents of buck regulators (Fig. 5b).
Input to output is isolated, protecting the load or battery from a switch short.
The SEPIC regulator topology has step-down or step-up (buck-boost) capabilities.
The SEPIC switch is low-side, simplifying the gate drive and current sensing in the switch.
The secondary-side average inductor current is equal to battery current, enabling the sensing of current not in series with the low side of the battery.
Disadvantages:
The SEPIC topology requires two inductors or a "coupled" inductor.
It also requires a single coupling capacitor, which can be expensive for high-power (greater than 50 W) or high-voltage (VIN greater than 100 V) applications.
i need two chargers one multiple from 1.5volts dc to 110ac
Anonymous -May 30, 2009
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