Disassembling and studying the BMS high-voltage sampling board of BYD Han EV

Latest update time：2024-10-10

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Today I will learn about the design of the BMS high-voltage acquisition board of BYD Han EV. I had a brief look at it when analyzing Han's BDU a long time ago. The installation position of this high-voltage board in the BDU is shown in the figure below. It is a separate product, independent of the control board.

Remove the shunt mounting screws and the two upper and lower fixing screws of the lower housing to remove it from the BDU.

This high-voltage board is shown below. The product size is approximately 140mm*90mm*20mm. This is actually the B side of the PCBA. You will find that the SHUNT is placed on the T side. In this way, it only needs to be passed through the furnace for soldering once, reducing the risk of tin accumulation on the body. It is worth learning from.

Turn the product over and you can see its shell, which is made of black plastic. The specific composition is PA66+15GF. There is a product label on the shell, which is named High Voltage Monitoring Module.

The shell was removed and its PCBA was analyzed. Its T-side is shown in the figure below. The size of the PCBA is approximately 110mm*85mm*16mm, the PCB thickness is 2mm, it is a 4-layer board, green oil ink, the surface treatment is gold plating, and the three-proof paint is basically fully coated; the chip surface on the T-side has been treated with glue. At present, BYD's BMS board has basically done glue protection in places where the pins are dense, which increases the protection against moisture or pollution; there are three connectors on the single board, all of which are straight-plug type.

The B side is shown in the figure below. They are all passive components, and the three-proof paint is basically fully covered. This three-proof paint is a bit hard after curing. The hardness indicates that its molecular structure is relatively dense, which can delay the entry of water vapor. I am also summarizing the issues about the three-proof paint, and there are indeed some new discoveries.

The functional modules of the entire board are roughly divided as shown in the figure below: In fact, the high-voltage boards are relatively clear-cut, and it is easy to find isolation belts. The low-voltage part is mainly isolated power supply and isolated daisy chain communication. The high-voltage part generally has a BJB chip as the core of control and acquisition. In addition, there are some high-voltage switches, voltage-dividing resistors, etc.

The main circuit architecture of this board is as follows: the BJB chip is LTC2949, with an external crystal and an external EEPROM to store calibration parameters; the power chip uses SN6501, and because its output is not stable, LDO is used for voltage stabilization before and after.

Let's look at some design details. First, let's look at the SHUNT acquisition. The SHUNT size is 8420, which may be 50uΩ, and the copper BAR surface is tinned. The B-side acquisition circuit of the PCBA is as follows. An NTC is arranged at the bottom of the SHUNT to detect its temperature. The filter beads arranged on the current sampling line are also located relatively close to the board to avoid damage caused by mechanical stress in the middle. Some window treatments are done on the B side of the SHUNT welding area to increase thermal conductivity. The vias should not be filled with green oil because tin leaks through. The advantage is that the bubble rate of the T-side pad can be reduced, but attention should be paid to the amount of tin.

The high-voltage sampling circuit is controlled by an optical MOS. This optical MOS has been discussed before and is the largest size currently available. A TVS tube is connected in parallel to the output end of the optical MOS for protection against overvoltage conditions.

The high-voltage sampling board of Han EV is very similar to the high-voltage sampling board of Song DM-i (as shown below), except that there are some changes in the connector, and the interior should be made into a platform-wide component. In addition, in the BMS of Haibao, the high-voltage sampling board also has a similar circuit, but it is made into an integrated form with the control board.