Driving Automotive Power Supplies to Higher Frequencies

Sep 1, 2006 12:00 PM
By Nitin Kalje, Senior Scientist, Maxim Integrated Products, Sunnyvale, Calif., and Greg Dygert, Str

Power Line Overvoltage Stress Conditions

Overvoltage (OV) protection devices are used to isolate and protect electronic circuitry from excessive voltage conditions that are conducted via electrical connections to the automotive electrical system, in particular those connections to the main voltage supply. The ability to withstand conducted disturbances is generally known as conducted immunity (CI).

Automotive manufacturers and standards organizations specify various test methods to evaluate the CI of electronic components and systems. While each automotive OEM tends to have specific requirements, the ISO7637 standard provides the basis for many of these. The following is not meant to be a comprehensive description of all CI requirements, but rather a brief summary of the typical OV conditions relevant to automotive applications.

Steady-State OV Conditions

Certain OV conditions are of long enough duration to be considered steady-state from an electronic circuit standpoint. For example, any OV condition that persists comparatively longer than the thermal time constant of an electronic device can be considered steady-state. In these situations, the continuous power dissipation and resulting temperature rise are of primary concern. Conditions that can be considered steady-state include a failed alternator regulator, a double-battery jump-start and reverse battery connections. The following is a brief description of these conditions.

The output of the alternator is regulated with respect to speed, load and temperature by regulating the magnitude of the current in the field winding. This function is typically provided by an electronic circuit (that is, voltage regulator) that pulse width modulates the field winding to achieve a constant, regulated output voltage from the alternator. The output set point of the voltage regulator is typically about 13.5 V. It is possible for the alternator voltage regulator to fail in such a way as to provide full field current irrespective of load or output-voltage conditions. When this happens, voltage in excess of the typical 13.5 V may be applied to the entire system, with the actual voltage level being dependent on vehicle speed, loading and other conditions. The typical OEM failed-regulator test requirement is on the order of 18 V for 1 hour. Most systems are required to operate under these conditions, although certain comfort or convenience functions are allowed to deviate.

Another OV condition that is effectively steady-state is the double-battery jump-start. This typically occurs when a tow truck or service station uses a 24-V system to jump-start a disabled vehicle or charge a dead battery. The typical OEM double-battery test requirement is on the order of 24 V for 2 minutes. Certain engine management and safety-related systems are required to operate under these conditions.

It is possible for a steady-state reverse potential to be applied to the vehicle electrical system during manufacturing or service. In general, most systems are required to survive but not operate under this condition. The typical requirement is -14 V for 1 minute. This can be a challenging requirement for systems with high current or low-voltage drop input requirements.

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