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Addressing low EMI automotive power design issues

Posted: 16 Feb 2015     Print Version  Bookmark and Share

Keywords:EMI  power ICs  electromagnetic interference  LT8640  power conversion 

As automobiles continue to add an ever increasing number of electronic systems to enhance safety, comfort, efficiency and performance while minimising harmful exhaust emissions, it comes as no surprise that they call for physically smaller power solutions with dramatically higher power levels. Additionally, with the proliferation of more EMI-sensitive systems within the vehicle, reducing EMI emissions from switching power supplies is of paramount importance, thereby creating more challenges for switching regulator IC design.

According to Strategy Analytics, "the demand for enabling semiconductor devices is expected to grow at a compound average annual growth rate (CAAGR) of five per cent per year over the next seven years, with the total market worth over ₹2.48 lakh crore ($41 billion) by 2021, compared to ₹ 1.75 lakh crore ($28.9) billion in 2014. The Strategy Analytics analysis also identifies that demand for microcontroller and power semiconductors will drive over 40 per cent of revenues."

Strategy Analytics provides a very quantitative description of forecasting the growth of electronics content in automobiles, but more interesting is the prevalent role that power ICs play in this growth. These new power IC designs must offer the following:

1) Robust performance across a wide range of voltages, including handling of transients in excess of 36V
2) Ultra low electromagnetic interference (EMI) emissions
3) The highest efficiency possible to minimise thermal issues and optimise battery run time.
4) The smallest solution footprints, demanding very high power densities with switching frequencies of 2MHz or greater needed to keep the switching noise out of the AM Radio band while keeping solution footprints very small
5) Ultra low quiescent current (<10µA) to enable always-on systems such as security, environmental control and infotainment systems to stay engaged without draining the vehicle's battery when its engine (alternator) is switched off

The goals for the increased performance levels of power ICs are to enable the design of increasingly complex and numerous electronic systems. Applications fueling this growth are found in every aspect of 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.

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 1. Note that the proper power IC (the LT8640 in this case) accurately regulates the 3.3V output through both of these scenarios.

Figure 1: 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. So minimising EMI is imperative.

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