Comprehensive power supply system designs for harsh automotive environments : Page 7 of 7

October 07, 2019 //By Bin Wu and Zhongming Ye, Analog Devices
Comprehensive power supply system designs for harsh automotive environments
Advances in automotive technology have significantly increased the electronic content of modern automobiles to enhance safety, improve the driving experience, enrich entertainment functions, and diversify the power and energy sources. We continue to commit engineering resources to improving power management solutions for the automotive market. Many of the technologies from that effort have resulted in significant advances in power supply efficiency, compactness, robustness, and EMI performance.

Parasitic capacitance and inductance from both the package and PCB layout play important roles in distributing noise, so if the noise is present, it can be difficult to eliminate. EMI prevention is complicated by the fact that switching noise covers the domain from tens of MHz to beyond GHz. Sensors and other instruments subjected to such noise could malfunction, resulting in audible noise or serious system failure. Therefore, stringent standards have been set up to regulate EMI. The most commonly adopted one is the CISPR 25 Class 5, which details acceptable limits at frequencies from 150 kHz to 1 GHz.

Passing automotive EMI regulation at high current usually means a complicated design and test procedure, including numerous trade-offs in solution footprint, total efficiency, reliability, and complexity. Traditional approaches to controlling EMI by slowing down switching edges or lowering switching frequency come with trade-offs such as reduced efficiency, increased minimum on-and off-times, and larger solution size. Alternative mitigation, including a complicated bulky EMI filter, snubber, or metal shielding, adds significant costs in board space, components, and assembly, while complicating thermal management and testing.

Our Silent Switcher technology addresses the EMI issue in an innovative way, enabling impressive EMI performance in high frequency, high power supplies. Second generation, Silent Switcher 2 devices simplify board design and manufacture by incorporating the hot loop capacitors into the packaging. For a buck regulator such as the 42 V/4 A LT8650S, the hot loop consists of an input capacitor and the top and bottom switches. Other noisy loops include the gate drive circuit and boost capacitor charge circuit. In Silent Switcher 2 devices, the hot loop and warm loop capacitors are integrated into the packaging and laid out to minimize EMI. This reduces the effect of final board layout on the EMI equation, simplifying design and manufacturing. Further peak EMI reduction can be achieved by using the optional spread spectrum frequency modulation feature incorporated into these parts, making it even easier to pass stringent EMI standards.


Figure 13. LT8672 and LT8650S configuration for high output current.

Figure 13 exhibits a low IQ, low noise solution for a high current application for automotive I/Os and peripherals. The LT8672 at the front end protects the circuit from reverse battery faults and high frequency ac ripple with only tens of mV of forward voltage drop. The LT8650S switches at 400 kHz with input ranging from 3 V to 40 V, and an output capability of 8 A by operating two channels in parallel. Two decoupling capacitors are placed close to the input pins of the LT8650S. With Silent Switcher 2 technology, the high frequency EMI performance is excellent even without an EMI filter installed. The system passes the CISPR 25 Class 5 peak and average limit with significant margins. Figure 14 shows the radiated EMI average test results over the range of 30 MHz to 1 GHz, with vertical polarization. A complete solution features a simple schematic, minimal overall component count, compact footprint, and EMI performance that is immune to changes in board layout (Figure 15).

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Figure 14. LT8672 and LT8650S EMI performance: 30 MHz to 1 GHz

 


Figure 15. A complete power supply solution for 3.3 V and 5 V outputs
from an automotive battery.

Figure 15. A complete power supply solution for 3.3 V and 5 V outputs from an automotive battery.

Conclusion

Automotive applications call for low cost, high performance, reliable power solutions. The cruel under-the-hood environment challenges power supply designers to produce robust solutions, taking into account a wide variety of potentially destructive electrical and thermal events. Electronic boards connected to the 12 V battery must be carefully designed for high reliability, compact solution size, and high performance. The Power by Linear device catalog includes innovative solutions specifically addressing automotive requirements: ultralow quiescent current, ultralow noise, low EMI, high efficiency, wide operating ranges in compact dimensions, and wide temperature range. By eliminating complexity while improving performance, Power by Linear solutions reduce power supply design time, lower solution costs, and improve time to market.

About the Authors

Bin Wu received his Ph.D. degree in electrical engineering from University of California, Irvine, California in April 2016. From April 2016 to July 2017, he was a post-doctoral research associate in University of Maryland, College Park. After that, he worked at Maxim Integrated, Inc. Since November 2017, he has been an applications engineer with Analog Devices, San Jose. His interests include electrical vehicle power architecture, high power density step-up/step-down dc-to-dc converters, switched capacitor converters, modeling, and renewable energy integration systems. He can be reached at bin.wu@analog.com.

Zhongming Ye is a senior applications engineer for power products at Analog Devices in Milpitas, California. He has been working at Linear Technology (now part of Analog Devices) since 2009 to provide application support to various products including buck, boost, flyback, and forward converters. His interests in power management include high performance power converters and regulators of high efficiency, high power density, and low EMI for automotive, medical, and industrial applications. Prior to joining Linear Technology, he worked at Intersil for three years on PWM controllers for isolated power products. He obtained a Ph.D. degree in electrical engineering from Queen’s University, Kingston, Canada. Zhongming was a senior member of IEEE Power Electronics Society. He can be reached at zhongming.ye@analog.com.

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