How is the diagnosis of the commonly used driver ports of the controller implemented?

For ordinary controllers, the hardware interfaces usually include analog input interface, digital input interface, PWM input interface, and the output interfaces also have high-side drive, low-side drive, H-bridge drive, and communication interfaces include CAN, LIN, Ethernet, sent interface, FlexRay, etc. The power supply interface includes 5V power output, 9V power output, and there are some special interfaces, such as the single-cell voltage sampling interface of BMS, which is a special analog input interface; there is also a high-side combined relay drive interface; there is also a bidirectional interface that integrates the input and output of smart accessories.

For all the above types of interfaces, diagnosis is required in application scenarios where there are wiring harnesses connected from the controller to the outside. Some of the diagnosis is performed directly through the corresponding chip, and some of the diagnosis is performed reasonably based on the application scenario and the information obtained by the software.

Because colleagues often raise questions about diagnostic issues, I simply organize the diagnostic principles of common interfaces. For pins connected by wiring harnesses, the fault types are usually divided into three categories: one is the wiring harness short circuit to ground, the second is the wiring harness short circuit to power, and the third is the wiring harness open circuit.

Here we will share the diagnostic status of the two most commonly used ports, namely high-side driver and low-side driver.

First, let's look at the low-side driver. The low-side driver is divided into On state and Off state. The necessity of diagnosis in the two states is briefly explained. When in the On state, the driver port turns on the MOSFET, so that the other end of the load is connected to the ground, thus forming a loop. At this time, it is not easy to diagnose the short circuit to the ground fault. When in the Off state, the voltage of the driver port is the voltage pulled up by the external load to the power supply. At this time, the driver port short-circuit to the power supply will not enable the load, and it is also not easy to diagnose the short circuit to the power supply fault. Therefore, from the above simple analysis, we do not need to diagnose all faults at any time, but to diagnose useful fault information at critical times.

The above figure is the internal topology of a bottom-side driver chip that we often use. In the off state, that is, when the MOSFET is not turned on, the diagnosis is based on the voltage of the port. Under normal circumstances, when the low-side driver is not enabled, the voltage of the driver port should be the voltage pulled up by the external load, that is, VBAT, which is generally 12V. If this harness is short-circuited to the ground, the voltage of the port must be 0V or a voltage very close to 0V. At this time, the external load will be enabled. Of course, this kind of fault needs to be diagnosed. Similarly, when this harness is open-circuited, what is its port voltage? Without the pull-up of the external load, the voltage should not be 12V, but the MOSFET is not turned on, and the voltage should not be 0V, so the chip is designed with a fast pull-up voltage. When the circuit is open, the voltage will be maintained in the Vopen state. Of course, this voltage has no load capacity.

So based on the above description, in the off state, the diagnosis is based on the port voltage. For example, for the two comparators shown in the figure, when the port voltage is lower than VTGND, the port is considered to be short-circuited to ground. When the port voltage is at Vopen, the port is considered to be in an open circuit state.

Similarly, in the On state, the diagnosis is also based on the port voltage. In the On state, the port voltage will become 12V, which is equivalent to 12V directly loaded to both ends of the MOSFET. The current will become very large, and the chip will overheat and then shut down the output. When the internal comparator detects that the port voltage is greater than a certain threshold, it will consider that the port is short-circuited to the power supply and set the relevant internal register flag.

To summarize, when the low-side drive is turned on, when the internal comparator detects that the port voltage is greater than a certain value, it can diagnose a short circuit to the power supply fault; when the low-side drive is not enabled, when the port voltage is lower than a certain value (1.9V for this chip), it is considered to be a short circuit to ground fault, and when the port voltage is around 2.5V, the port is considered to be an open circuit fault.

For intelligent high-side driver chips, the diagnostic principle is similar, except that in the off state, the circuit is diagnosed to power supply fault and open circuit fault, and in the on state, the short circuit to ground fault is diagnosed. The diagnostic principle is similar to that of low-side driver. The most basic principle is to compare the voltage of the port to determine the fault state.

However, the most commonly used high-side driver chip with the best cost performance is ST's VNDxxxx or VNQxxxx. I have carefully compared this type of chip when selecting it. The most important point is that the diagnostic principles are somewhat different, but the main driving principles are similar. Today I will just pick one, VN5050J-E.

The main function of the STATUS pin is to diagnose the status of the high-side driver and the status of external faults. The chip manual only introduces one situation that it can diagnose, which is open circuit diagnosis. This is the situation in the following picture, which is the On state and the Off state respectively.

In the On state, if the current of the output port is less than K*Iref, the voltage of the STATUS port will be pulled down through the comparator. At this time, the MCU can detect the level of STATUS to know that an open circuit fault has occurred.

In the Off state, a pull-up resistor is required outside the driver port. At this time, since the high-side driver port is in a high-impedance state, the port voltage is basically equal to the pull-up voltage. At this time, the internal comparator detects that the port voltage is greater than Vol, and sets the level of the Status port to low. When the MCU detects that the level of the diagnostic port is low, it knows that an open circuit fault has occurred.

For the open circuit fault diagnosis of the diagnostic port, the basic principle is as mentioned above. Of course, the circuit design of STATUS and the selection of the pull-up resistor of the output port need to be selected according to the actual load type, and it is not necessarily a universal selection.

Of course, the above is just a little bit of the basics of high-side driver diagnosis of open circuit faults. If you need to diagnose the fault of the port short circuit to the power supply, or the fault of the port short circuit to the ground, you need to add an analog detection port to the port to detect the status of the port. The diagnosis mentioned here is the diagnostic status of the digital port, referred to as digital high-side driver. There is also a type of high-side driver port, the diagnostic port is a current signal, according to the pull-down resistor of the diagnostic port, the current model is converted into a voltage signal for diagnosis, which is called analog high-side driver. I won't go into details, the principles are similar, you can refer to ST's chip application note, which has detailed explanations.

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