Low-cost, high-precision digital control solution for battery testing equipment

Battery testing equipment is an important part of the post-processing system of lithium-ion battery production lines and is crucial to the quality of lithium-ion batteries. The core function of battery testing equipment is to perform high-precision constant current or constant voltage charging and discharging of lithium-ion batteries. The traditional control method is mainly based on analog control solutions built using discrete devices. Compared with traditional analog control solutions, digital control solutions using TI's C2000™ as the core will become the mainstream development direction of battery testing equipment in the future due to its low cost, high precision, greater flexibility, and better confidentiality. In this article, we will introduce in detail how to effectively reduce system costs and ensure extremely high current and voltage control accuracy through TI's C2000™ digital control solution.

1. Low cost

The typical structure of TI's C2000 digital control solution is shown in Figure 1: the current/voltage amplifier samples the current/voltage of battery charging and discharging, converts the analog signal into a digital signal through the analog-to-digital converter ADC and sends it to C2000™. C2000 performs loop calculation based on the constant current or constant voltage command and the sampled signal, outputs a PWM with a certain duty cycle to adjust the switching of the MOSFET, and finally enables the buck/boost converter to charge and discharge the lithium battery through constant current or constant voltage according to the command.

Figure 1

Compared with analog solutions, since voltage, current commands and loop control are all generated and completed in C2000, high-resolution digital-to-analog converters (DACs) and error amplifiers are eliminated, effectively reducing system costs.

The TMS320F280049 is a C2000™ 32-bit MCU with a 100MHz main frequency and 256KB flash memory. It can control up to 8 independent channels of synchronous buck/boost converters through high-resolution 16-bit PWM. The digital control solution of the TMS320F280049 can save more than 30% of the BOM cost compared to the traditional analog control solution.

In addition, since lithium-ion batteries are widely used in many fields such as 3C products, electric vehicles, and energy storage, the currents of various types of lithium-ion batteries often vary greatly. This results in the battery testing equipment often needing to select different hardware solutions according to the current size if analog control is used, which increases the R&D cycle and equipment costs. If the C2000 digital control solution is used, it can switch freely between low current or high current mode without changing the hardware: at low current, each of the 8 channels can operate independently; at high current, multiple channels are operated in parallel to output a larger current.

Figure 2

As shown in Figure 2, when multiple channels are connected in parallel, each channel will use the same constant voltage loop, and the constant current loops are independent. Simply connecting the outputs in parallel can achieve a larger output current range. Therefore, compared to analog control, the C2000 digital control solution can adapt to a wider range of test scenarios without changing the hardware, greatly reducing equipment costs.

2. High precision

Battery test equipment can often remove most of the initial system errors through calibration. The remaining error sources that are difficult to calibrate mainly include: temperature drift of current sense resistors, offset and gain temperature drift of current and voltage sense amplifiers, offset caused by input common-mode voltage changes, ADC nonlinearity, and temperature drift of reference voltage sources. In this article, the error values ​​are calculated based on a temperature variation range of ±5°C.

Current Sense Resistor:

The temperature drift of the current sense resistor is an important source of total system error. For CC control, a high-precision current sense resistor with a few milliohms and a low temperature coefficient is required. This article uses a high-precision, current-sensing metal strip SMD power resistor. The resistance of the sense resistor is 5mΩ and the temperature drift value is 10 ppm. Then, the error caused by the temperature drift of the current sense resistor is 50ppm.

Current Sense Amplifier:

In order to reduce the temperature rise and power loss caused by large current, the resistance of the current detection resistor is generally small. Therefore, the input differential signal of the current detection amplifier generally does not exceed tens of millivolts, and an instrument amplifier is often selected for signal conditioning. The errors of the instrument amplifier mainly come from the following two aspects: the drift of the offset voltage and gain when the ambient temperature changes; the offset voltage caused by the change of the input common-mode voltage when the battery voltage changes. Therefore, when selecting an instrument amplifier, you should focus on parameters such as offset voltage drift, gain drift, and CMRR. Table 1 shows the key parameters of several instrument amplifiers recommended by TI for battery testing equipment:

Table 1

INA821 is a high-precision, low-drift instrumentation amplifier with a maximum offset voltage drift of 0.4 µV/°C. A ±5°C temperature offset will produce a 2 µV offset voltage, or a 40ppm full-scale error. The gain drift is 5 ppm/°C, so a ±5°C temperature offset will produce a 25ppm error. The common-mode voltage rejection ratio is 140dB, so when the input common-mode voltage range changes from 0 to 5V, a 0.5µV offset voltage will be generated. At a charging current of 10A, the voltage signal of the full-scale sampling resistor is 50mV, which means that the input common-mode voltage change brings a 10ppm full-scale error.

Voltage Sense Amplifier:

The error sources of voltage detection amplifiers also mainly come from the drift of offset voltage and gain, as well as the offset voltage caused by the change of input common-mode voltage. Therefore, when selecting an instrumentation amplifier, you should also focus on parameters such as offset voltage drift, gain drift, and CMRR.

TLV07 is a cost-sensitive, low-noise, rail-to-rail output, precision operational amplifier. The typical value of offset voltage drift is 0.9 µV/°C, so a ±5°C temperature offset will produce a 4.5µV offset voltage, that is, a 1ppm full-scale error; gain drift is mainly affected by the drift error of the input resistor and the feedback resistor, here 5 ppm/°C is taken, so a ±5°C temperature offset will produce a 25ppm error. The minimum common-mode voltage rejection ratio is 104dB, so when the input common-mode voltage range changes from 0 to 5V, a 31.5µV offset voltage will be generated, that is, a 6ppm full-scale error.

Analog-to-digital converter and voltage reference:

The error of analog-to-digital converter ADC is mainly caused by nonlinearity and drift of reference voltage source. ADS131M08 is a 24-bit, 32kSPS, 8-channel synchronous sampling Δ-Σ high-precision ADC. Since ADS131M08 is a differential input, it can effectively reduce the error caused by crosstalk between channels. From the data sheet, it can be found that the nonlinearity INL of ADS131M08 is only 7.5ppm full-scale error. If the internal reference voltage source is used, the maximum temperature drift is 20 ppm/°C, then the ±5°C temperature offset will produce 100ppm error. If the external reference voltage source REF2025 is used, the maximum temperature drift is only 8 ppm/°C, then the ±5°C temperature offset error will be reduced to 40ppm.

Error summary:

According to the above analysis, the error values ​​caused by each error source are summarized, and the total system error of the battery test equipment under constant current and constant voltage control can be calculated as shown in Table 2. It can be seen that with the C2000 digital control solution, the current and voltage error ranges are within 20,000, achieving extremely high control accuracy.

Table 2

In summary , the use of TI's C2000 digital control solution in battery testing equipment can reduce system costs while ensuring extremely high current and voltage control accuracy, making it very suitable for application in various battery testing solutions.