Automotive transients introduction

A 12-V automotive battery rail is subject to many transients. The purpose of the front-end power conversion stage shown here is to insulate the electronic load from the wide disturbances on the car battery rail. Some of the extreme transients of short duration are taken care of by the transient protection circuit. A subset of these are often tackled in the power stage design. The power stage design typically consists of the reverse battery protection, EMI filter, and dc-dc conversion stage.

The E.M.I. filter limits the emissions from the power stage to the supply line. The EMI as a topic is dealt with in a separate training. This presentation deals with the conducted immunity portion of the front end design.

The main electrical stresses that are handled by the power stage are listed, along with their origins. The tests we are going to discuss are  reverse voltage, jump start, load dump, superimposed alternating voltage, starting profile. There are many test standards that describe the tests to simulate these disturbances on the battery supply line. The most commonly known are ISO16750-2 (and ISO7637-2) from International Standards Organization. LV124 is very common in Europe. In addition each OEM has their own version and variation of these tests. Some of these documents and relevant sections are listed on the right hand side column on this slide. 

The last slide on this video maps the automotive battery rail voltage stresses to voltage levels that the front end power conversion stage can see during its lifetime. The normal operating range is typically 8 or 9V to about 16. Overvoltage during jump start can present up to 24V on the battery rail whereas load dump can produce voltages up to 42V. The power stage connected to the battery rail must be able to function for normal battery range, and depending on the type of load it is supplying may have to work with limitations or in the very least withstand these higher voltages. The negative voltage during reverse polarity connection is handled by the reverse polarity protection circuit.