Design of portable equipment combination of lithium battery power management solutions
Portability portable devices is closely linked with the development of the battery , from the initial lead-acid batteries, nickel -cadmium (Ni-Cd) batteries developed to the nickel-metal hydride (Ni-H), lithium ion (Li-ion) batteries have been to the nearest lithium Polymer (Li-polymer) battery energy density gradually increased, more powerful mobile performance , the disadvantage of the battery is constantly being overcome. This article will introduce a management system designed portable lithium polymer battery . Overall System Architecture The design of the application entity is a portable device used on an industrial , NIOS Altera's FPGA and embedded processor on the II and use USB interface with a computer connected to a large amount of data oriented applications. This device requires 30V DC voltage, so plan to use four 1000mAh lithium polymer battery cells in series ; addition, consideration for waterproof and dustproof , external use only one square USB port (USB B Type Socket), this USB port while both data transfer and charging functions. Figure 1 shows the overall structure . Control of the core , including FPGA and the connected interfaces, display circuit , a low voltage of 3.3V , the high efficiency DC / DC buck chip directly derived from 4-cell lithium battery pack. This voltage is very important, so it is necessary to maintain stable and continuous , unless the battery is low or overcurrent protection, otherwise the voltage has been supplied . Actuators require 30V DC voltage, current about 80mA or so, use a boost DC / DC circuit, which operated from the control center , usually do not work , just before the action needed to open . Charging using an external 20V power supply, connect via USB interface. Using this power is considered to be a large current 0.5C 1C or high-speed charging . Because sharing a common port USB, in order to avoid the access into ordinary USB charging process , a voltage judging circuit judges . Due to meet the needs of the chip market is difficult to find solutions , decided to use the remaining FPGA logic resources to achieve the charger control, add a small amount of analog circuitry to assist . This requires the power supply control circuit can not be interrupted , lithium batteries must always be online and connected to the negative battery needs have been GND. Battery circuit A voltage sampling The most important part of the design of the voltage sampling circuit is required to have high precision and little affected by temperature . The difficulty lies in the design in terms of battery voltage to GND is floating . Many programs have adopted differential op amp voltage is then converted into the right to exclusive use of the ADC input AD conversion program . However, due to the introduction of this program the differential amplifier , resulting in many problems. First, the relatively high voltage op amp is difficult to find ; Secondly , the op amp input voltage power supply and use the same power supply , so that would require op amps feature rail-to- rail input needs of ; again , you may also need a negative supply , use DC / DC and the introduction of noise ; addition, the op amp and makes use of precision matched resistors reduce . To try to simplify the circuit , where the structure of the integral -type ADC, will be transformed into FPGA timing precision voltage measurement accuracy. This is a simple RC charging circuit ( see Figure 2 ) . The workflow is : J1 first closed, releasing the charge on C1 ; then J1 open from R1 to charge on C1 ; voltage comparator U1 voltage on C1 is compared with a reference voltage V2, high power output when the voltage exceeds V2 C1 level. Statistical open from J1 to time between U1 output high , they can determine the size of the voltage V1 . Can be visually seen , V1 is higher, the shorter period of time . The actual circuit is shown in Figure 3, note that this figure depicts only the first battery charge measurement circuit . Wherein , R1 and the capacitor C1 and the resistor is used for integration , Q1 is a common P-MOSFET, where J1 is used to achieve the function of discharging the capacitor , U5 while the reference voltage to achieve the dual function of the voltage comparator . X1 is a discharge control from FPGA, X2 is the switch output , destined for FPGA. Voltage comparator chosen is Maxim 's This circuit consumes only static MAX921 's 4μA current and C1, Q1, Q2 leakage current , which can be negligible, very low power . Another feature of this circuit eliminating the need for optocouplers frequently used , but instead of using a capacitor C2 . Static when the voltage across C2 to achieve a balance , do not consume energy , at this time , X2 0.U5 output voltage is high, because the voltage across C2 is not transient , so X2 voltage is raised . Two Schottky diodes D1 and D2 is the role played limiter . Carefully adjust the value of C2 and R4 will be able to successfully transfer switch information .