Design of a Microprocessor Controlled Intelligent Battery Charger

As lithium-ion battery chemistries become more prevalent in a variety of electronic product designs, innovative solutions for charging these batteries become increasingly necessary. To achieve maximum system flexibility, a microprocessor can be used to control all aspects of battery charging, including unique charging algorithms designed to increase charging rates and battery life . This approach can also allow for the implementation of higher voltage battery packs.

This article describes how to use a microprocessor to control a wide input voltage DC / DC controller ...

For ease of development, we break the battery charger into two separate boards: a microprocessor controller board and a DC/DC-converter power stage board (see Figure 1). Both the positive and negative battery terminals connect to the power stage board, while the System Management Bus (SMBus) communication lines connect to the microprocessor board. The smart battery sends the desired charging voltage and current information to the microprocessor, which then sends two pulse width modulation ( PWM ) signals to the DC/DC-converter power stage board to set the actual output voltage and current.

To enable the use of standard wide input voltage DC/DC converters, the power stage board is designed with a special feedback circuit (see Figure 2) to properly control the battery charging. The charging sequence followed by the microprocessor is to limit the charging current until the battery voltage approaches its specified maximum voltage. When the maximum voltage is reached, the charging voltage is held constant, allowing the charging current to be gradually reduced until the battery is considered fully charged. At this point, the PWM output signal is turned off.

The initial current limit charge rate has two current levels. When the battery is over-discharged, a very slow charge rate will be used until the battery voltage reaches a level safe enough to accept the standard charge rate.

In the feedback circuit shown in Figure 2, U3:B compares the PWM-current reference voltage (I_PWM1) with the measured current (ISNS1) supplied to the battery . If the PWM reference voltage is higher than the measured current, the amplifier output is high. If the reference voltage is lower, the amplifier output is low.

A resistor divider (R30 and R34) is used to measure the output voltage at the VBATT1 input of U3:A. We compare this voltage to the PWM-output reference voltage (V_PWM1). If this reference voltage is higher, the amplifier output is high. If the reference voltage is lower, the amplifier output is low. The maximum output voltage can be expressed by the following equation:

Figure 1 High-level system block diagram of a wide input voltage smart battery charger

Figure 2. Constant current/voltage-feedback circuit for properly charging a battery.

Figure 3 Overvoltage and reverse polarity protection circuit

Figure 4 Software Flowchart Overview

The D1 diode combines the two amplifier outputs with a logical OR. The lowest voltage feeds the inverting amplifier (U3:D), which makes the error signal polarity correct when using a DC/DC controller (here, TI's TPS40170). The basic operating principle is that the controller tries to send a set current and, if the load can accept that current, the controller regulates to that current level. If the load does not accept the full current, the voltage starts to rise and eventually reaches VOUT(max). When this happens, the voltage loop takes over and regulates the output voltage.

To improve the safety of the solution, the power stage board also needs protection circuitry for overvoltage conditions (up to 100V) and reverse voltage connections (where the positive and negative poles are swapped). Figure 3 shows such a circuit.

When the input voltage is reversed, reverse voltage protection is provided by MOSFETs Q7 and Q9 and D2. This does not allow negative voltages to be applied to the system. Input overvoltage protection is provided by MOSFETs Q8 and Q10. Zener diode D4 sets the voltage at which the circuit starts clamping. Once the Zener voltage is exceeded, the gate-source voltage of the FET begins to drop. This allows the FET to operate in the linear region and allows the microprocessor to continue to be powered. At the same time, the DC/DC converter is turned off and signals SD1 and SD2 are pulled to ground.

The software implementation is just as important as the hardware implementation. A brief software flow chart is shown in Figure 4. The microprocessor interrogates the battery over the SMBus, requesting its desired charging voltage and current. After confirming these values, it sets the two PWM outputs to regulate the output voltage and current to the battery. If at any time the battery issues a charge warning, the PWM outputs are turned off. Additionally, once the battery's state of charge reaches 100% or the fully charged position is set, the PWM outputs are turned off.

Safety is the most important concern during battery charging. All solutions should have several layers of protection. The first layer of protection is the smart battery itself with an internal protection MOSFET. During charging, the microprocessor should communicate with the battery regularly (every 2 seconds is good) to monitor all safety flags in the "battery status" register. Some of the flags that require a response include overcharge warning (OCA), termination of charge warning (TCA), over temperature warning (OTA), and fully charged (FC) status. The microprocessor's on-board analog-to-digital converter can be used as a secondary check for overvoltage or overcurrent events.

in conclusion

By combining a microprocessor with a wide input voltage DC/DC controller, a fully programmable, wide input voltage battery charger can be designed. This article presents a solution that uses TI's low-power MSP430F5510 microprocessor in conjunction with the TPS40170 DC/DC controller to create an architecture that supports input voltages up to 55V. The article describes a special feedback network developed by TI application staff to implement proper battery charging . In addition, a novel solution for overvoltage protection and reverse voltage protection is discussed. The software required to communicate with the smart battery via the SMBus communication protocol can be downloaded from the link in "Reference 1", an application report. Details on the SMBus smart battery charger can also be found in "Reference 1".

References

For more information about this article, visit www.ti.com/lit/litnumber and replace “litnumber” with your specific TI Lit. # to download the Acrobat® Reader® file for the materials listed below.

Literature, Title TI Lit. #

1. "Wide Input Voltage Battery Charger Using SMBus Communication Interface Between MSP430™ MCU and bq Fuel Gauge " in Application Report , by Abhishek A. Joshi and Keith J. Keller