As battery-powered electronic devices become more widely used and more powerful, the need for battery charger designs with good applicability has also emerged. If only standard components are used, battery charger design can become more flexible and cost-effective. However, the use of mixed-signal design will make it easier to add new functions to the system, and it also makes it possible to add differentiated features to the system.
A wide variety of battery chemistries are used in 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 batteries. NiMH batteries are also popular because of their good safety and environmental performance. It is possible to design a mixed-signal universal battery charger that can charge both chemistries.
The charging or discharging rate is related to the battery capacity. The charging rate (i.e., C rate) is the charging or discharging current, which is defined as I=M ×CN. Among them, I is the charging or discharging current (amperes), M is an integer or fractional multiple of C, C is the value of the rated capacity (ampere-hours), and N is the time (hours) for defining C.
A battery with a discharge rate of 1C will discharge its nominal rated capacity in 1 hour. For example, if the rated capacity is 1000 mAh and the discharge rate is 1C, the corresponding discharge current is 1000mA. Similarly, if the discharge rate is 0.1C, the corresponding discharge current is 100mA.
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| Figure 1: Li-ion batteries require a constant or controlled charge current algorithm and a constant voltage charge algorithm. |
Preferred Charging Technology
Li-ion battery chemistry uses a constant or controlled current and constant voltage charging algorithm that is divided into four stages: trickle charge, constant current charge, constant voltage charge, and charge termination (Figure 1). The preferred charging algorithm for NiMH batteries consists of trickle charge, constant current charge, top-off charge, and charge termination (Figure 2).
Stage 1: Trickle Charge. Trickle charge restores charge to a deeply depleted battery. For Li-Ion batteries, when the cell voltage is below approximately 3V, the battery is charged at a maximum constant current of 0.1C. For NiMH batteries, trickle charge begins when the voltage of each cell is below 0.9V.
Stage 2: Constant current charging. For lithium-ion batteries and nickel-metal hydride batteries, when the battery voltage exceeds the trickle charge threshold, the charging current increases and enters the constant current charging state. The constant current charging current should be in the range of 0.1C to 0.2C.
Stage 3: Constant voltage charging. This process is only applicable to lithium-ion batteries. When the battery voltage rises to 4.2V, the constant current charging ends and enters the constant voltage charging state. For best performance, the voltage regulation tolerance should be less than 1%.
Stage 4: Charge termination. It is not recommended to continue trickle charging for lithium-ion batteries, and it is better to choose charge termination. For NiMH batteries, a certain period of trickle charging can ensure 100% battery capacity utilization. When a certain period of top-off charging process is completed, the charging needs to be terminated.
For lithium-ion batteries, the minimum charge current, timer, or a combination of the two are typical methods for terminating the charging process. The minimum charge current method monitors the charge current during the constant voltage charging phase and terminates the charging process when the charge current decreases to the range of 0.02C to 0.07C. The timer method counts at the beginning of the constant voltage charge and terminates the charge after two hours of continuous charging. Charging in this way can fully charge a deeply depleted battery in about 2.5 hours to 3 hours.
Advanced chargers also have some additional safety features. For example, many advanced chargers will stop charging if the battery temperature drops below 0°C or rises above 45°C.
NiMH batteries terminate the charge process based on the -dV/dt reading of the battery pack, the +dT/dt (rate of change of temperature over time) reading, or a combination of both. In this case, temperature sensing is both a viable safety precaution and a method to terminate the charge process.
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| Figure 2: The preferred algorithm for NiMH batteries consists of four stages: trickle charge, constant current charge, top-off charge, and charge termination. |
System Considerations
Input power: Many applications use a very cheap wall outlet as input power. Their output voltage depends greatly on the wide range of AC input voltage and the load current drawn from the wall power supply. Many applications that charge from a car power adapter may also encounter the same problem. The output voltage of a car power adapter generally ranges from 9V to 18V.
Output voltage regulation accuracy: For lithium-ion batteries, output voltage regulation accuracy is critical to maximize battery capacity utilization. If the output voltage regulation accuracy is slightly reduced, the battery capacity will drop significantly (Figure 3). However, for safety and reliability reasons, an arbitrarily high output voltage cannot be set.
Charge termination method: Overcharging is the fatal weakness of lithium-ion batteries and nickel-metal hydride batteries. Accurate charge termination method is essential for a safe and reliable charging system.
Battery Temperature Monitoring: The typical temperature range for rechargeable batteries is 0°C to 45°C. Charging outside this temperature range may cause the battery to overheat. During the charging cycle, the pressure inside the battery increases, causing the battery to swell. Since temperature and pressure are directly related, the combination of high pressure and high temperature inside the battery can cause the battery to mechanically rupture or leak. Charging outside the temperature range of 0°C to 45°C may also damage the performance of the battery or shorten the battery life.
“Battery discharge current” or “reverse leakage current”: In many applications, the charging system remains connected to the battery even when there is no input power. When there is no input power, the charging system minimizes the current draw from the battery. The maximum current draw should be less than a few microamps, and ideally less than 1μA.
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| Figure 3: Capacity loss vs. charge voltage drop shows that even a small drop in output voltage accuracy can result in a significant drop in battery capacity. |
Battery Charger Design
By understanding these system design considerations, we can develop an appropriate charge management system. For example, if a precisely regulated input power source is available, a linear charging scheme can be used. In these applications, linear charging schemes have advantages in terms of ease of use, size, and cost.
For wide voltage range input sources, such as unregulated AC/DC wall power or automotive DC input power, using a switching regulator can reduce the internal battery charger power consumption to an acceptable level. The switching regulator topology defines the composition of the regulator switches and passive filtering components. The difference in composition can distinguish different topologies, thereby making trade-offs between complexity, efficiency, noise and output voltage range. There are many converter topologies, but there are not many topologies suitable for battery chargers in the 5W to 50W range.
The buck regulator is a popular topology for battery charging applications. The input current of the buck regulator is a pulsed current or "discontinuous current" (see Figure 4b). This topology generates high electromagnetic interference (EMI) at the input of the power supply, so most buck regulators require additional input EMI filtering.
A buck regulator can only regulate an output voltage that is lower than the input voltage. However, some applications have a wide input voltage range that can exceed the output voltage range, which is more common in multi-cell Li-ion battery charger applications.
The single fault mode (buck switch short circuit) will cause a short circuit between the input and the battery. This will cause safety issues for NiMH batteries that lack internal battery protection. The buck regulator requires a high-side driver circuit to drive the N-channel MOSFET switch, which is more complicated than the low-side topology.
In pulse width modulation (PWM) controller applications, the sensing of external switch current is very complex. Limiting the switch current is important for fault modes such as battery short circuit or load short circuit. If the high-speed switch current is limited, the battery charger will burn out in the event of a short circuit.
The single-ended primary inductor converter (SEPIC) is also a popular topology for battery charging devices. SEPIC regulators have many advantages over buck regulators and other regulators, although they also have some disadvantages.
To illustrate these basic concepts, let's analyze a specific battery charger design. By breaking this design into two parts, an affordable, intelligent power system can be developed. The battery charger itself is a mixed-signal system. For example, the power system (in this case, the SEPIC regulator) is analog. Turning the power switch on and off at a very high frequency requires some type of analog drive circuit; on the other hand, the charge termination timer, fault management, and switch control are generally digital functions with timers and programmability.
In this example, the input voltage is 6V ~20V; the output voltage is 0V~4.2V for one battery and 0V~8.4V for two batteries; the pre-adjustment current is 200mA; the pre-adjustment threshold is 3V; the constant current charging current is 2A; and the charge termination threshold is 100mA (the current when charging is completed).
The system has the following features: 1) Overvoltage protection (battery removal); 2) Overcurrent protection (battery or load short circuit); 3) Battery temperature sensing to reflect the charging quality; 4) A two-part approach to mixed-signal design. First, a microcontroller that can read the battery pack status (voltage and temperature, as well as program the SEPIC regulator output current) is selected, in this case a PIC12F683 8-pin flash microcontroller; then a high-speed analog PWM controller with built-in MOSFET drivers such as the MCP1630 is added to develop an "analog" programmable current source.
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| Figure 4: When a buck regulator topology (a) is used in a battery charging design, the pulsed current input (b) generates a lot of electromagnetic interference (EMI). |
Design of SEPIC Programmable Current Source
As with all switching regulator designs, the output is controlled by varying the duty cycle, or percentage of time the switch is on (Q1 in Figure 6). To regulate the current into the battery, the charge current must be sensed. As shown in Figure 6, there is no sensing element in series with the battery in this circuit.
The SEPIC regulator has an average output current on the secondary winding (LS) and an average input current on the primary winding (LP). The secondary resistor RSENSE senses the battery charge current, while the high-speed analog PWM reference input programs the desired battery charge current.
As can be seen in Figure 6, the MCP1630 analog PWM controller and driver form a "programmable" SEPIC current source. The PWM and driver provide power for analog current regulation, MOSFET gate drive, and high-speed overcurrent protection. The microcontroller sets the switching frequency (500kHz) of the SEPIC power-train 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 frequency of the hardware PWM is equal to the switching frequency of the SEPIC power-train. The hardware PWM duty cycle sets the maximum duty cycle of the SEPIC power-train.
The 25% duty cycle, 500kHz pulses from the microcontroller hardware PWM set the SEPIC switching frequency to 500kHz and the maximum duty cycle to 75%. Using a simple RC filter circuit, a standard microcontroller I/O pin can generate a software programmable reference voltage that can accurately program the charging current of the constant current SEPIC converter.
At the non-inverting input (VREF), a programmable reference voltage sets the amount of current that the battery is charged. The MCP1630 PWM output duty cycle (VEXT) is adjusted until the VREF input voltage is equal to the voltage at the error amplifier FB input. By adjusting the VREF input voltage, the battery charge current is adjusted accordingly.
The PWM and driver can drive the MOSFET at a frequency of more than 500kHz while monitoring the SEPIC switch current using an internal high-speed (typically 12ns) comparator. If the switch current is too high, the PWM duty cycle will be stopped to limit the battery current. Ultimately, the charge current is regulated based on information such as battery voltage and temperature received from the analog-to-digital converter (ADC).
To develop the constant voltage charging phase, the microcontroller ADC needs to read the battery voltage and update the programmable current source (SEPIC) to maintain the battery voltage at 4.2 V. This process changes faster than the battery voltage during constant current charging.
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| Figure 5: The SEPIC regulator topology (a) draws continuous input current from the source (b), generating very little EMI compared to the buck regulator approach. |
For Li-Ion applications, the charge cycle is terminated when the current required to maintain the battery voltage at a fixed 4.2V drops to a percentage of the battery's C-rate (100mA). This is set in firmware and can be easily changed to match the recommendations of different battery manufacturers. In typical analog chargers, this termination charge current is a percentage of the charge cycle current and is therefore not easily changed.
For NiMH battery applications, the fast charge cycle is terminated when the battery voltage remains constant or decreases over time, or when the battery pack temperature rises above a predetermined value. When the fast charge is terminated, a slow, timed top-off charge process begins. The ADC input and the battery pack thermistor are used together to sense the battery temperature. By reading the voltage at the "TEMP_SENSE" input, the battery temperature can be determined.
When the battery voltage is too high, the interrupt PIC12F683 code will start the overvoltage protection. The SEPIC converter will shut down in less than 1μs, minimizing the overvoltage generated on the battery terminals. The SEPIC converter diode can block any path that allows the battery discharge charge to return to the system charger, and generally only draws quiescent current from the battery voltage sensing path.
This charger design also has some optional features. For example, using a single microprocessor, multiple high-speed analog PWM modules, and out-of-phase switching techniques and input power budget features, a charger bay can be added for multi-bay applications. Because these firmware can calibrate the Li-Ion battery charge termination voltage and charge current, the accuracy of the system can be increased.
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| Figure 6: SEPIC mixed-signal battery charger based on the PIC2F683 microcontroller and MCP1630 analog PWM controller. |
By adopting a mixed-signal approach to developing a battery charger, the battery charger design can take advantage of the best of both analog and digital technologies. The mixed-signal approach can achieve high-frequency operation (500kHz) and high-speed protection (12ns output current sensing speed) and minimize the size of the filtering components. In addition, the system's digital programmable function also allows the selection of the appropriate charging stage and setting of the charging current.
Since these firmware make it easier to program settings and currents, they also enhance the advantages of new battery charging methods. This method also differentiates mixed-signal designs from other designs. This type of design is not limited to Li-Ion and NiMH applications, it opens the door to introduce future rechargeable technologies into the system.
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