Analysis of OBC condensation problem

This article analyzes and troubleshoots the occasional functional failure problem that occurs when electric vehicles are used in winter and locks down the root cause of the problem.

According to the cause of the problem, the solution method is proposed and the test verification method is optimized.

In winter, there were many occasional charging indicator lights on the vehicle by mistake. After the fault occurred, it recovered quickly and could not be reproduced. After testing the faulty vehicle and the faulty parts many times, it did not reproduce.

In view of the fault problem, the technical solution of gun insertion signal detection is analyzed and checked, and the possible causes of failure are comprehensively checked. The results are shown in Figure 1.

By analyzing the software, the gun insertion signal is only connected through the AD sampling port of the BATMCU-CC-ET and the microcontroller. An abnormality in the software causes an abnormal configuration of the AD sampling impedance value. At the same time, the on-board charger needs to be in a wake-up condition to cause a low CC impedance. The probability of this situation is extremely low.

In terms of manufacturing, SMT processing causes the impedance of capacitor C232 or resistor R332 to become lower. Considering that the failure modules are distributed in different production cycles, the possibility of this situation is very low. Except for the on-board charger to process the gun insertion detection signal, no other parts on the vehicle use this signal to eliminate the interference caused by other controllers

The external signal cable problem, the gun insertion detection signal is connected by a relatively long cable on the whole vehicle, resulting in abnormal impedance. This situation requires further investigation and analysis. The presence of contaminant ion migration in the signal port causes the impedance between the plug-in signal and the adjacent CP signal to become smaller. Since the CP signal is grounded through the 2.7K resistor, the impedance of the plug-in signal to the ground becomes smaller. This situation also needs to be further investigated. Through the above analysis of technical solutions and fault phenomena, the focus of investigation is on external signal cable problems and impedance changes caused by abnormal signal ports.According to the on-site data detected by the faulty vehicle at the first time, the fault still exists when the wiring harness associated with the charger and the slow charging port is cut off, thereby eliminating the influence of the external signal cable. The problem is located on the charger and the connection plug-in, and the internal signal of the charger is further measured and analyzed, and it is confirmed that the impedance between the terminals of the internal signal board of the charger is abnormal. The signal board of the faulty part is sliced and analyzed, and the results show that the PCB has no abnormality such as via holes.

QC/T895-2011 has a temperature requirement of -20℃ for low temperature operation and 85℃ for high temperature operation. In view of the non-recurrence of the problem, a long-term uninterrupted continuous high and low temperature cycle test was carried out on the five faulty parts. At the cycle temperature of -40 ~ 85 ℃, under the condition of full load, monitor the working status of the on-board charger during the whole process. One of them recurred after 18 days of accumulative testing. After dismantling the test piece, it was found that a conductive substance was produced between the signal solder joints of the abnormal piece. Conduct EDS analysis on the conductive material, and its main components are Sn (35.48%) and Cu (23.83%). From the material composition, it is inferred that the conductive material originates from the pad at the abnormal signal.

From the analysis of the above simulation test results, after the temperature difference is formed inside and outside the on-board charger when it is working at low temperature and full load, water droplets are formed on the signal terminals with lower temperature.

Ionization occurs after power-on, resulting in the migration of metal substances, which in turn causes the occasional phenomenon of resistance changes between pins.

Condensation occurs on relatively cool objects when the air is saturated with water vapor. If the absolute humidity of the air remains unchanged, the air temperature decreases, and when it reaches a certain value, the air humidity will reach saturation.

If the temperature continues to drop, the moisture in the air will be precipitated, a phenomenon called "condensation". The temperature at which the relative humidity of the air reaches 100% while keeping the moisture content in the air and the atmospheric pressure unchanged is called the dew point temperature. When the ambient temperature changes rapidly, a temperature difference is formed between the internal space of the product and the external environment due to the retardation of the heat conduction of the casing.

The high-humidity gas will flow through the ventilation valve, sealing port and other structures, which is called "breathing". In the process of reducing the ambient temperature, when the temperature of the inner space of the product is higher than the external ambient temperature, condensation will occur on the inner wall of the product housing. Condensed water will accumulate or adsorb in the sample under the action of gravity, such as openings, seals and other structures.

Test the temperature difference between the inside and outside of the on-board charger through the high and low temperature test. Test conditions: The on-board charger is placed in a temperature box, full-load output, and the temperature of the temperature box is set to 50°C, -7°C, and -35°C.

During the test, the relative humidity of the environment was 45%, and the temperature of each sampling point was recorded, as shown in Table 1.

The comparison of dew point temperature under different relative humidity and ambient temperature is shown in Table 2.

By analyzing the temperature sampling data in Table 1 when the low temperature is -35°C, it can be seen that the ambient temperature around the signal terminal inside the car charger is about 19°C, and the temperature at the pin solder joint of the signal terminal is about 5°C. By looking up Table 2, it can be known that the dew point temperature of the current condition is 7°C, and the condensing condition is satisfied at the solder joints of the signal terminals.

Therefore, condensation will occur at the pin solder joints of the internal signal terminals of the car charger (sampling point 3 in Table 1).

Through the above theoretical analysis and actual test data, it is confirmed that condensation will occur at the pin solder joints of the internal signal terminals of the on-board charger.