Develop Affordable Mixed-Signal Battery-Charger Designs

As battery-powered electronic devices gain in ubiquity and power, the need for easily adaptable battery-charger designs arises. With just standard components, battery-charger designs can become more flexible and more cost-effective. A mixed-signal design facilitates the addition of new, unique features to the system. It also makes it possible to add differentiating features.

Many disparate battery chemistries are used for rechargeable portable applications, including lithium-ion (Li-ion), nickel metal hydride (NiMH), nickel cadmium (NiCd), and lead acid batteries. Li-ion batteries have the highest energy density of all battery types, making them the most portable of all rechargeable technologies. NiMH batteries are popular because they're safe and environmentally friendly. It's possible to design a mixed-signal, universal battery charger that can charge both of these battery chemistries.

The rate of charge or discharge is expressed in relation to battery capacity. Known as the C-rate, this rate of charge equals a charge or discharge current. It's defined as:

I = M ×C_N

where:

I = charge or discharge current in amps
M = a multiple or fraction of C
C = numerical value of rated capacity in amp-hours
N = time in hours at which C is declared

A battery discharging at a C-rate of 1 delivers its nominal rated capacity in one hour. For example, if the rated capacity is 1000 mAh, a discharge rate of 1 C corresponds to a discharge current of 1000 mA. Similarly, a rate of C/10 corresponds to a discharge current of 100 mA.

PREFERRED CHARGE PROFILE (LI-ION AND NIMH) Li-ion battery chemistries use a constant, or controlled, current and a constant-voltage algorithm that can be broken up into four stages: trickle charge, constant-current charge, constant-voltage charge, and charge termination (Fig. 1). The preferred algorithm for NiMH consists of trickle charge, constant current, top-off charge, and charge termination (Fig. 2).

Stage 1, Trickle Charge: Trickle charge restores charge to deeply depleted cells. For Li-ion batteries, when the cell voltage is below approximately 3 V, the cell charges with a constant current of 0.1 C maximum. For NiMH batteries, trickle charge conditions weak batteries when the cell voltage is less than 0.9 V per cell.
Stage 2, Constant-Current Charge: For Li-ion and NiMH batteries, after the cell voltage rises above the trickle-charge threshold, the charge current increases to perform constant-current charging. The constant-current charge should range from 0.2 to 1.0 C.
Stage 3, Constant Voltage: For Li-ion batteries only, constant-current charge ends and the constant-voltage stage begins when the cell voltage reaches 4.2 V. To maximize performance, the voltage-regulation tolerance should be better than 1%.
Stage 4, Charge Termination: The continuation of trickle charging isn't recommended for Li-ion batteries. Instead, charge termination is a good option. For NiMH batteries, a timed trickle charge ensures 100% of battery capacity use. When the timed top-off charge is complete, charge termination is then necessary.

For Li-ion batteries, one of three methods—minimum charge current, a timer, or a combination of the two—typically terminates charging. The minimum charge-current approach monitors the charge current during the constant-voltage stage and terminates the charge when the charge current diminishes in the range of 0.02 to 0.07 C. The timer method determines when the constant-voltage stage begins. Charging then continues for two hours, and the charge terminates. Charging in this manner replenishes a deeply depleted battery in roughly two-and-a-half to three hours.

Advanced chargers employ additional-safety features. For example, with many advanced chargers, the charge stops if battery temperature is less than 0°C or greater than 45°C.

For NiMH batteries, charge termination is based on a –dV/dt reading of the battery pack, a +dT/dt (delta temperature versus time), or a combination of both. In this case, temperature sensing is a possible safety precaution, as well as a termination method.

SYSTEM CONSIDERATIONS To recharge any battery quickly and reliably, a high-performance charging system is required. Key system parameters ensure a reliable, cost-effective solution.

Input Source: Many applications use very inexpensive wall cubes for the input supply. Output voltage depends heavily on the wide-ranging ac input voltage, as well as on the load current drawn from the wall cube. Applications that charge from a car adapter can experience a similar problem. The output voltage of a car adapter typically will range from 9 to 18 V.
Output Voltage-Regulation Accuracy: For Li-ion batteries, output voltage-regulation accuracy is critical to maximizing battery-capacity usage. A small decrease in output-voltage accuracy results in a large decrease in capacity (Fig. 3). However, the output voltage can't be set arbitrarily high because of safety and reliability concerns.
Charge Termination Method: Overcharging is the Achilles' heel of Li-ion and NiMH cells. Accurate charge termination methods are essential for a safe and reliable charging system.
Cell Temperature Monitoring: The temperature range over which a rechargeable battery should be charged is typically-0°C to 45°C. Charging the battery-at temperatures outside of this range may cause the battery to overheat. During a charge cycle, pressure inside the battery increases, causing it to swell. Because temperature and pressure are directly related, the combination of high temperature and high pressure inside the battery can lead to mechanical breakdown or venting inside the battery. Charging the battery outside of the 0°C to 45°C range also may harm battery performance or reduce its life expectancy.
"Battery Discharge Current" or "Reverse Leakage Current": In many applications, the charging system remains connected to the battery, even without input power. The charging system minimizes current drain from the battery when input power isn't present. Maximum current drain should be less than a few microamperes and, ideally, below 1 µA.

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.

SWITCHING BATTERY-CHARGER DESIGN To illustrate these basic concepts, let's take a look at a specific battery-charger design. By partitioning the design into two parts, it's possible to develop affordable, "intelligent" power systems. Battery chargers are, by nature, mixed-signal systems. For example, the power train (in this case, the SEPIC regulator) is analog. Turning the power switch on and off at high frequency requires some type of analog driver circuit. On the other hand, charge termination timers, fault management, and on/off control typically are digital functions that use timers and programmable capability.

This example includes the following specifications:

Input voltage: 6 to 20 V
Output voltage: 0 to 4.2 V for one cell, 0 to 8.4 V for two cells
Preconditioning current: 200 mA
Preconditioning threshold: 3 V
Constant-current charge: 2 A
Charge termination threshold: 100 mA (current at which charge cycle is completed)

Features:

Overvoltage protection (battery removal)
Overcurrent protection (battery or load shorted)
Sense battery temperature for charge qualification

Using a two-part approach to the mixed-signal design, first select a microcontroller that can read the state of the battery pack (voltage and temperature, as well as programming the SEPIC regulator output current). This example uses the PIC12F683 eight-pin flash microcontroller. Next, add a high-speed, analog PWM controller with a built-in MOSFET driver, such as the MCP1630, to develop the "analog" programmable current source.

DESIGNING A SEPIC-PROGRAMMABLE CURRENT SOURCE As with all switching-regulator designs, the output is controlled by varying either the duty cycle or the percentage of switch on-time (Q1 in Figure 6). To regulate current going into the battery, charge current must be sensed. As the circuit diagram shows, there's no sense element in series with the battery.

The SEPIC regulator secondary winding (L_S) carries the average output current. The primary winding (L_P) carries the average input current. Secondary resistor RSENSE senses battery-charge current, while the high-speed, analog PWM reference input programs the desired battery-charge current.

Referring to Figure 6 again, the MCP1630 analog PWM controller and driver creates a "programmable" SEPIC current source. The PWM and driver supply the analog current regulation, MOSFET gate drive, and high-speed overcurrent protection. The microcontroller sets the SEPIC power-train switching frequency (500 kHz) and programs the SEPIC constant current.

The PWM and driver use the microcontroller hardware PWM to set the SEPIC switching frequency and maximum duty cycle. The hardware PWM frequency equals the SEPIC power-train switching frequency, while the hardware PWM duty cycle sets the maximum SEPIC power-train duty cycle.

A 500-kHz pulse with a 25% duty cycle out of the microcontroller hardware PWM sets the SEPIC switching frequency to 500 kHz, with a maximum duty cycle of 75%. A standard microcontroller I/O pin generates a software-programmable reference voltage using a simple RC filter. This programmable reference programs the constant-current SEPIC converter to a precise charge current.

At the non-inverting input (V_REF), the programmable reference voltage sets the amount of battery-charge current. The MCP1630 PWM output duty cycle (V_EXT) adjusts until the voltage at the V_REF input equals the voltage at the FB input of the error amplifier. By adjusting the voltage at the V_REF input, the battery current adjusts accordingly.

The PWM and driver can drive the MOSFET at frequencies greater than 500 kHz while monitoring the SEPIC switch current using an internal high-speed (12-ns typical) comparator. If the switch current is too high, the PWM duty cycle will terminate, limiting the battery current. Finally, the charge current is adjusted based on information such as battery voltage and temperature, received from an analog-to-digital converter (ADC).

To develop a constant-voltage charge phase, the microcontroller ADC reads the battery voltage and updates the programmable current source (SEPIC) to maintain the battery voltage at 4.2 V. This occurs much more quickly than the rate at which the battery voltage changes when it's subject to a constant current.

For Li-ion applications, the charge cycle terminates when the current necessary to maintain the battery voltage at a fixed 4.2 V reduces to some percentage of the battery C-rate (100 mA). This is set using firmware and is easily changed for different battery manufacturers' recommendations. In a typical analog charger, this termination charge current is a percentage of the charge cycle current, so it can't be changed easily.

For NiMH applications, the fast-charge cycle terminates when one or both of two conditions occur—either the battery voltage remains constant or drops with time, or the battery-pack temperature rise is higher than a predetermined value. When fast-charge terminates, a slow, timed top-off charge can begin. An ADC input and battery-pack thermistor together sense battery temperature. By reading the voltage at the "TEMP_SENSE" input, battery temperature can be determined.

Interrupting the PIC12F683 code when the sensed battery voltage is too high achieves overvoltage protection. The SEPIC converter shuts down in less than 1 µs, with minimal voltage overshoot occurring at the battery terminals.

The SEPIC converter diode prevents any path from sending battery discharge back to the system charger. The only quiescent current draw on the battery is from a battery voltage-sensing path, typically less than 5 A.

OPTIONAL FEATURES Using a single microcontroller and multiple high-speed analog PWM modules makes it possible to add charger bays for multi-bay applications, as well as out-of-phase switching techniques and input power-budgeting features. Such firmware increases system precision because they enable the calibration of the Li-ion termination voltages and charge currents.

By using a mixed-signal approach to developing battery chargers, battery-charger designs can exploit the best of both the analog and digital worlds. A mixed-signal approach enables high-frequency operation (500 kHz) and high-speed protection (12-ns current sense to output), and it minimizes the size of filtering components. In addition, the system's programmable digital features allow appropriate determination of stage-of-charge and set charge current.

Because it facilitates the programming of settings and currents, firmware enhances new battery-charging methods. This approach differentiates one mixed-signal design from another. This type of design isn't limited to Li-ion and NiMH batteries, and it leaves the door open for the programming of future rechargeable technologies into the system.