Low EMI automotive power designs: Page 2 of 7

February 05, 2015 //By Jeff Gruetter
Low EMI automotive power designs
Jeff Gruetter, Product Marketing Engineer, Linear Technology Corporation considers low EMI automotive power design issues.
the vehicle. For example, new safety systems, including lane monitoring, adaptive safety control, automatic turning and dimming headlights. Infotainment systems (telematics), which continue to evolve and pack more functionality into an already tight space, must support an ever growing number of cloud applications.

Advanced engine management systems with the implementation of stop/start systems and electronics laden transmissions and engine control, plus drive train and chassis management aimed at simultaneously improving performance, safety and comfort. A decade ago, these systems were only found in high-end luxury cars but are now commonplace in automobiles from every manufacturer; further accelerating automotive power IC growth. Figure 1 below shows the multitude of electronic systems that are typically found in today’s cars.


Figure 1. Electronic Systems Proliferation in Automobiles

Transients in Automotive Systems

Although the battery bus voltage in cars is nominally 12V (it varies from 9V to 16V depending on when the alternator is charging). Furthermore, the lead-acid battery voltage is subjected to wide variations during temporary conditions. Cold-crank and stop-start scenarios can pull the battery voltage down to 3.5V, whereas load dump can subject the battery bus to voltages as high as 36V. Therefore, power ICs must be able to accurately regulate an output through wide variations of input voltages. The wide temporary voltage swing during cold-crank/stop-start and load dump for single-cell lead-acid batteries is illustrated in Figure 2. Note that the proper power IC (the LT8640 in this case) accurately regulates the 3.3V output through both of these scenarios.

Figure 2. LT8640 with 36V Load Dump Transient & 4V Cold Crank Scenario
Low EMI Operation

Because the automotive electrical environment is inherently noisy, with many applications being electromagnetic interference (EMI) sensitive, it is imperative that switching regulators don’t exacerbate EMI concerns.  Because a switching regulator is typically the first active component on the input power bus line, and regardless of downstream converters, it significantly impacts overall converter EMI performance.

Design category: 

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