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

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The increasing sophistication of electronic systems in automobiles presents unique challenges and opportunities for power system designers. Most automotive modules need low voltages like 3.3 V and 5 V. The voltage conversion from battery to such lower voltages using linear regulators means a significant power dissipation. High power dissipation makes the thermal management difficult and expensive. The higher power requirement of faster processors and ASICs have steered the power-conversion method from simple, low-cost, inefficient linear regulators to the more complex but higher-efficiency switching converters.

The size of a switching converter depends on the switching frequency. The passive components like power inductors and capacitors become smaller with higher switching frequency. These high-efficiency converters reduce power dissipation by eliminating bulky and expensive heatsinks. The entire power supply can shrink significantly when using switching converters. These advantages make the switching converter an obvious choice for power management of body electronics, infotainment and engine-control modules.

Selecting the proper switching frequency is critical because a switching converter poses its own set of problems. For example, electromagnetic radiation caused by the switching converter can interfere with other electronics. The AM radio receiver is sensitive to the interference between 530 kHz and 1700 kHz. The switching converter's fundamental switching frequency and third harmonics can contribute significant interference to other electronics around the power supply. The even harmonics cancel each other out, and the odd harmonics higher than fifth order typically have little energy, which is usually easy to filter out. Selecting a switching frequency greater than 1800 kHz eliminates fundamental and other harmonic interference from the AM frequency band.

However, a high switching frequency increases the power loss, partly offsetting the advantage of using a switching converter. The switching losses increase with higher input voltages, as they are proportional to the square of the operating voltage. The automotive environment typically demands high-voltage processes (40 V or higher) for power controller ICs to withstand overvoltage transients such as load dump. High-voltage processes use relatively larger geometries and higher gate thicknesses. The resulting longer channel lengths lead to longer propagation delays. Obviously, high-voltage processes are inherently slow and could be very inefficient as the transition losses increase due to longer rise and fall times of the switch.

Certain fabrication processes at Maxim are suited for extremely high-speed converters at moderate voltage levels. A recent example is the MAX5073, a dual-output 2.2-MHz buck or boost converter that can tolerate up to a 23-V input. An effective switching frequency of 4.4 MHz is achieved by using ripple phase operation. The switching converters are supposed to be immune to the interference present on the power source. As far as automotive applications are concerned, high-voltage controllers are not an absolute necessity when designing these switching converters. This article describes the most common automotive power disturbances and ways to protect the low-voltage electronics from them.

More on Buck Converters

• Buck-Converter Design Demystified
• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

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