Analysis of high voltage sampling scheme using low voltage ground as reference plane on BMS

I suddenly realized that the Olympics is coming in a few days. This year's US men's basketball team has a very strong lineup. I look forward to having time to watch a few games.

This time we will discuss the solution of using low voltage ground as a reference point in high voltage sampling circuits.

There was an article before that introduced the high-voltage sampling circuit of MODEL 3. The specific link is " High-voltage detection and insulation detection circuit on MODEL 3 BMS " . It mentioned that this high-voltage sampling scheme uses the low-voltage ground as the reference plane, and the high-voltage signal is directly connected to the low-voltage circuit network after being divided by a large resistor. At that time, this scheme was briefly mentioned. Recently, I was doing some comparative analysis of competing circuits, so I took another look at it.

Specifically, there are 6 high-voltage sampling points on the MODEL 3 BMS control board, as shown in the figure below, which is a more common relay connection form.

The general high-voltage detection solution is to use the high-voltage ground as the reference plane, connect the remaining 5 sampling points to the high-voltage ground through voltage-dividing resistors, and then connect the signals on the sampling resistors to the BJB chip located on the high-voltage ground, as shown in the figure below.

However, on the MODEL 3 BMS control board, the 6 sampling points of high-voltage sampling are connected to the low-voltage ground through voltage-dividing resistors . The scheme is similar to the following figure: Use the ADC of the low-voltage circuit part to collect the voltage of these 6 points. The scheme is not complicated and the function should be implemented without any problems.

The sampling circuit is roughly as shown in the figure below, which only shows the sampling circuits of two sampling points: the input signal is pulled up to the reference power supply VREF after passing through the voltage divider resistor, and is output to the ADC of the MCU for collection after passing through the op amp follower circuit. There is no switch on the sampling circuit.

advantage:

The most obvious advantage of this solution is its low cost . It saves the isolation power supply and isolation communication circuit between high and low voltage. Of course, whether the switch on the sampling circuit should be retained can be discussed later. This has already saved a lot of costs.

Another advantage is that it avoids the virtual voltage problem , as shown in the following figure: the virtual voltage at the battery high voltage connector port comes from the loop of the battery, insulation resistance and sampling circuit, but in this solution, this loop does not exist, and there is no virtual voltage.

shortcoming:

One disadvantage of this solution is that the voltage-dividing resistor is connected in parallel across the system insulation resistance, which will definitely reduce the insulation performance of the system. Therefore, the voltage-dividing resistor value of this sampling circuit is generally larger. The single-arm value of Tesla is about 10MΩ. In addition, this resistor must also be distinguished when doing insulation testing.

Another issue is the issue of security protection, which is also a gray area for me at the moment.

To elaborate, safety protection is required between high voltage and low voltage. Generally, our protection plan on PCB is to perform electrical isolation and set a certain size of safety spacing. The electrical isolation methods are generally optical isolation, capacitive isolation, and magnetic isolation. Common isolation devices are transformers, optical MOS, and capacitors, as shown in the figure below.

In the scheme of using low voltage ground to collect high voltage, there is only a certain resistance between high and low voltage. Of course, the resistance can be selected to a certain value for current limiting, but does this meet the safety requirements? The standard GB 4943.1-2022 does mention the method of using resistors as safety protection devices, but we need to wait for a detailed interpretation.

In fact, the insulation detection circuit is also connected to the low-voltage ground through a group of resistors. From a certain perspective, we are actually already using this solution. In addition to Tesla, some BMS in China are also implemented using this solution. After we thoroughly study the standards, we will look back at the feasibility of using resistors for safety protection.

Summarize:

Nothing much to say, I’ve been a bit busy lately; all of the above is for reference only.