The tale of a leaky zener

-February 07, 2013

Tales from the CubeI work as design engineer for an automobile manufacturer in India. One of our premium vehicles includes a body-control module (BCM) we designed in-house. As one of its many features, the BCM allows the driver to dim the cockpit back illumination.

When the vehicle was launched in South Africa, the principal dealer went through stringent performance checks of all the features and discovered that the cockpit illumination sometimes displayed erratic flickering; the flickering was especially noticeable during night-time driving and could add to driver discomfort. Although we received no other such complaints from customers in other countries, we were able to repeat the problem at our end, and our design team set out to investigate its cause.

Because the automotive power bus is supplied through an alternator-battery combination, it is prone to a few hundred millivolts of ripples as well as to load-dependent fluctuations when functions such as air-conditioning and engine cooling are switched on and off. The illumination-level-setting input is derived from a potentiometer knob operated by the driver and is fed to one of the analog inputs of the BCM. Another analog input senses the voltage of the power bus. The microcontroller unit (MCU) logic normalizes the potentiometer voltage based on the current status of the power bus.

Read more Tales from the CubeOur test revealed that when the power bus fluctuated during vehicle operation, the cockpit illumination would undergo a faint flickering, noticeable in dark ambience, even though the driver did not touch the illumination knob. Further post-mortem results revealed the culprit to be a 5.1V zener diode that was used to protect analog pins of the CPU from overvoltage. This zener would draw a leakage current above 3V of illumination input. The leakage current would vary along with the fluctuations on the power bus, thus corrupting the illumination command to the MCU and leading to erratic flickering.

For non-precision analog applications in automotive designs (generally encountered in body control applications), zener leakage is not an issue, and a standard zener protection is used. In our case, nobody suspected that simple illumination control would be adversely affected by the leakage, and we had used a standard analog interface circuit for this low-precision application.

We decided there were two possible solutions: Remove the zeners and either connect the external Schottky diodes as reverse pull-ups or, if the currents involved were low, depend on substrate diodes internal to the MCU. Because the currents involved were indeed low, we chose the second option, and the problem was resolved.

Mayank Deo is a design engineer at a leading automobile manufacturer in India.

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