Today, I went to a customer's company for a technical exchange, because their business is related to cars. When I talked about the high reliability of our products, I mentioned to them the reliability design experience we had when we developed automotive electronics.
I introduced the concepts of “XXXX SDS”, “water resistance”, “wet current”, and “WCCA” worst case analysis to the customer.
As a car company with a history of more than a hundred years, XXXX has summarized its long-term practical experience and formed a system design specification, which is used as an important basis for reliability design.
In the car when the switch is disconnected, and can not only consider the minimum value of 100kΩ of insulation resistance, especially some parts of the car is easy to contact with water, such as door switches, when the switch becomes wet, the insulation performance of the disconnection decreases, if memory serves me correctly, it should be designed according to the switch detection circuit of about 1kΩ, or there may be a disconnection but was mistakenly detected as an abnormal situation of closure, leading to serious consequences. consequences.
For mechanical contacts of the switch or relay, its contacts in the long-term working process, easy to contaminate with dust and generate insulation layer, if the switch closure, the switch detection circuit provided by the current flowing through the contacts is large enough, or the relay control current is large enough, it can be in the contact closure of a moment to produce a small spark, the contact surface of the dust impurities burned, the use of the contacts in the long term use with After the use of contacts in a long period of time can still keep bright as new, so as to extend the service life of the switch or relay.
This current is called wet current, and according to the Ford SDS requirements, the current flowing through the contacts when the switch detection circuit is closed needs to be more than 10mA for wet current purposes.
Therefore, the resistor for detecting the circuit pull-up to the car battery voltage needs to be selected as a 1.3kΩ/1206 resistor. The maximum car battery voltage is considered to be 16V, the maximum power is considered to be 216mW, and the maximum power that the resistor in the 1206 package can withstand is 250mW.
I shared two cases with them.
The first one is an automobile BCM, the speed sensor sends the speed value to the BCM through pulse signal to realize the function such as 20km/h auto lock.In the input circuit of the speed signal, an RC low-pass filter is used to process the signal, in which a 10uF electrolytic capacitor is used. According to the worst-case scenario analysis, the device needs to consider the initial error, the environmental error and the aging error;
The capacitance value has an initial error of maximum +/-30% and a maximum deviation of +30% in high temperature environments.Its capacitance value changes by at least 60%. Considering that the minimum battery voltage is 9.6V, the maximum frequency that can be detected in the limit case is only about 10Hz, and the 20km/h speed pulse signal will be completely filtered out by the filter circuit, and it cannot be detected by the BCM properly, resulting in abnormal function.
Because the controller was shipped before I was onboarded for the WCC analysis, the yard gave feedback on this issue shortly after the analysis was completed.
The second was a Jiang remote central control mainframe, which outputs a switching signal to control the automatic window closer to realize the function of remote window closing.
The car encountered a problem in the road test, the vehicle bumps open the window automatically rises back.
The car factory suspected that the problem was the central control host and asked us to go there to investigate.
I scrutinized the interface circuit with the window closer and found no possible factors contributing to this phenomenon.
I asked the car factory to come to the automatic window closer schematic diagram, immediately found the problem, I analyzed to the car factory in the circuit of each device parameter value, and with the Ford design plan and its back of the principle of pointing out its unreasonable.
That afternoon, the automatic window closer manufacturer was notified by the car factory to come to deal with.
So, the general public, this simple circuit should be considered which factors, how to choose the device parameters and worst-case analysis?
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