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Lithium battery cells with voltages of 48V and below have a high application rate in micro-hybrid vehicles and industrial energy storage. The system consists of 13 to 15 lithium battery cells. Due to the inherent characteristics of lithium batteries, it is necessary to accurately monitor the number of battery cells to ensure system safety and improve battery efficiency and life.
This reference design realizes the voltage and temperature monitoring of 15 lithium battery cells. While ensuring the accuracy of signal monitoring, it provides the architecture of the main monitoring circuit and the secondary monitoring circuit to achieve a higher level of system protection.
At the same time, this reference design provides a modular and expandable board-level architecture. In addition to the main monitoring circuit module, the secondary monitoring circuit module, and the data interface module, other modules such as active equalization circuit can be expanded to facilitate system prototype development.
The main chips used in this reference design are:
System design considerations efficiency:
The capacity of lithium-ion batteries in notebook computers, mobile phones and similar portable devices is usually very small, with a typical value of several ampere hours. However, the capacity of lithium-ion batteries used for vehicles or energy storage is much higher, usually around tens or even hundreds of ampere hours. Linear test equipment used for small-capacity batteries, if it is also used for high-capacity battery testing, will consume a lot of power during the charging phase, resulting in low efficiency, and will cause serious thermal problems to the device hardware design. The ADI AD8450/1 and ADP1972 solutions are based on the PWM architecture and help solve this problem.
The ADI PWM architecture can also help send more battery energy back to the grid or other test channels for charging. Compared with a linear architecture that discharges battery energy to a resistive load, this is an environmentally friendly and efficient solution.
In order to obtain accurate lithium-ion battery capacity, it is necessary to accurately measure the current and voltage in both charging and discharging modes. Combined with the precision ADC, DAC and other devices in the system, ADI's solution based on AD8450/1 and ADP1972 can achieve high-precision measurement and setting.
Low system cost:
Higher switching frequency supports the use of smaller, lower-priced power components, such as inductors and capacitors
Energy recycling helps reduce operating costs
AD8450/1 has higher accuracy, which can reduce thermal management costs and simplify control loop design
AD8450/1 uses a unique instrumentation amplifier design, the calibration time in the manufacturing process can be reduced by half, and the performance guarantee time can be longer
Integrated solutions make the system size smaller, equipment and maintenance costs lower
System Block Diagram:
The following is the system block diagram from the DC bus to the battery, including microcontrollers, analog front-ends and controllers, PWM controllers, high-voltage MOSFET drivers, power stages (MOSFETs, inductors, capacitors, shunt resistors), voltage/current reading (ADC ) And voltage/current settings (DAC).
Line-to-battery channel plate design. The technical requirements of the module can vary, but the products listed in the table below represent ADI solutions that meet some of the requirements.
|1. Analog front end and controller 2. Buck and boost PWM controller
|4. Analog-to-digital converter
|6. Reference Voltage Source
|7. MOSFET Driver
|8. Power Management
The working principle of the system:
The above figure mainly contains two functions: one is to charge the battery, and the other is to discharge the battery, which is determined by the mode signal of AD8450/1 and ADP1972. Each function has two modes: constant current (CC) mode and constant voltage (CV) mode. Two DAC channels control CC and CV set points. The CC set point determines how much current is in the loop in the CC mode of the two functions of charging and discharging. The CV set point determines the battery potential when the loop enters CV from CC, and is also applicable to both charging and discharging functions.
The precision analog front end and controller AD8450/1 uses the internal difference amplifier PGDA to measure the battery voltage, and uses the internal instrumentation amplifier PGIA and external shunt resistor (RS) to measure the current on the battery. Then, it compares the current and voltage with the DAC set point through an internal error amplifier and an external compensation network (used to determine whether the loop function is CC or CV). After this module, the output of the error amplifier enters the PWM controller ADP1972 to determine the duty cycle of the MOSFET power stage. Finally, the inductance and capacitance that make up the complete loop. The instructions in this section are for the two functions of charging and discharging, because ADP1972 is a step-down and step-up PWM controller.
In this scenario, the ADC obtains the readings of the loop voltage and current, but it is not part of the control loop. The scan rate has nothing to do with the performance of the control loop, so one ADC can measure the current and voltage of a large number of channels in a multi-channel system. The same is true for DACs, so low-cost DACs can be used to set multiple channels. In addition, a single processor only needs to control CV and CC set points, operating modes and management functions, so it can interface with many channels.
ADI has produced the ADP1972 and AD8450 demo boards as shown below, which can be used to verify their efficiency and accuracy. For this asynchronous step-down and step-up power supply system, the DC bus input is 12 V, and the maximum charge/discharge current is 20 A.
Under the maximum rating, 20 A CC mode (both charging and discharging functions), and 3.3 V load conditions, the efficiency of the demo board is approximately 90%. To achieve this value, external diodes, shunt resistors, inductors, and MOSFETs are all optimized.
After calibrating the initial accuracy, the current accuracy includes temperature drift, linearity in the full current range (0 A to 20 A), short-term stability (noise), and CMRR in the full voltage range (0 V to 3.6 V). The result verified on the demo board is that the typical current accuracy of this ADI solution is less than 0.01% (25°C ± 10°C). A similar analysis can be performed on the voltage accuracy. After verification by this demo board, it is also below 0.01%.
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