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AFE (analog front end), which is an analog front end in Chinese, refers specifically to the battery sampling chip in BMS, which is used to collect cell voltage and temperature.

Before writing this, I thought about it for a long time. There are too many places to write about AFE. A small place can be pulled up for a while; Finally, I decided to start with a question and introduce some AFE-related things in the middle, so that the content will not appear too rigid and dogmatic.

The question is: How to choose a suitable AFE?

The position of AFE in BMS (the picture is from TI's official website)

According to the normal thinking logic, of course, we should analyze our input needs and then make a choice; As a matter of fact, only half of the above sentence is right, because there are not many AFEs available for us to choose from, and all needs need to compromise with reality; Just like a chef, he can make all kinds of flavors to meet customers' needs, but he only has tofu in his hand, which is a kind of food material, so he can't afford it.

The above figure is the internal function block diagram of ADI's LTC6813. At present, the internal structure of AFE that can be accessed on the market is similar, and the differences are more concentrated on hardware resources; The other is the architecture designed for functional safety requirements. In brief, the main differences are the number of sampling channels, the number, type and architecture of internal ADC (ADC can be discussed separately later).

The external input needs we obtain mainly come from two parts: domestic and foreign standards and customer needs. There must be a cross between the two parts, which requires us to distinguish ourselves. The good news is that the requirements of general customers will be stricter than the standards.

The main standard that BMS can refer to is QC/T 897-2011. Since the updated version has not yet come out, let's take this as an example; The most important requirement for AFE is acquisition accuracy (as shown in the figure below), which is our bottom line; The index requirements in the standard are not very strict, and the test conditions are vague. The latest discussion draft is much better. The measurement and verification of voltage acquisition accuracy of AFE seems simple, but how to achieve an accurate voltage reference source, especially in EMC testing, is a problem worth discussing.

There are many demands from customers, and the main factor affecting AFE selection is the configuration of battery modules. For example, how many strings and parallels is the smallest module? How many modules are there altogether? Then there are some detailed requirements, such as sampling accuracy, number of temperature points, functional safety requirements, etc.

In practical applications, we sample the cells connected in parallel as the same cell; Further, the cores are basically parallel before serial to reduce the need for sampling channels; However, our headache is that the number of cells in series in a Module is not fixed, and the total number of cells in series is not uniform. In this case, we need to match the number of cells of each module. Unfortunately, the maximum number of voltage channels of AFE is discontinuous (mainly divided into three grades: about 6s, about 12s and about 18s). Therefore, we need to carefully select the appropriate number of channels to match, so as not to waste or force. Therefore, especially for the third-party independent BMS manufacturer, it is difficult to make a platform version of the product because of its limited voice, sandwiched between the main engine factory and the core factory.

Image source: https://book.liionbms.com

In addition, a special scenario is also involved, that is, AFE sampling across modules (as shown in the figure below). In short, it is to collect the pressure drop on the copper bar together; We may use an independent channel to collect, or put it together with the cell to collect. Either way will involve a negative pressure sampling problem, which requires that the AFE sampling channel can withstand negative pressure. At present, there are few manufacturers that can do this, and many manufacturers' products are evolving in this direction.

In addition to the sampling accuracy, it is easy to ignore the number of temperature sampling channels of AFE. The recommended ratio of the number of temperature channels to the number of voltage channels is 1:2, which should not be smaller, because the number of temperature channels may be insufficient. In addition to collecting external NTC, there may also be some analog signals in the board, so the temperature sampling channel is a relatively tight resource.

At present, there are some AFE resources available for us to choose from (see the table below). Some information may be inaccurate, and some are still in the sample stage, for reference only. It classifies American and non-American devices. In fact, NXP and Resa have a close relationship with the United States, some of which are unclear. The domestic AFE started a little late, and we can see that there are almost no actual drivers.

summary

The main basis for selecting AFE was briefly introduced in the previous section. In fact, if the company's platform has been established, it is difficult to select a new AFE, which only evolves repeatedly on the old platform. It is good and bad for engineers to use mature circuits directly. In the long run, it is better to go through the process from 0 to 1.

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