“This white paper discusses the use of high-performance LEDs in automotive-grade optocouplers. The use of high-performance LEDs proves that optocouplers can handle automotive Class 1 applications. The article also outlines Avago’s latest portfolio of products that can handle higher temperature and lower price demands.
This white paper discusses the use of high-performance LEDs in automotive-grade optocouplers. The use of high-performance LEDs proves that optocouplers can handle automotive Class 1 applications. The article also outlines Avago’s latest portfolio of products that can handle higher temperature and lower price demands.
One of the key enablers for optocouplers to handle automotive Class 1 applications is the use of ultra-high performance LEDs. LED light output drops by less than 0.2/°C, helping to meet high temperature specifications and improve LED design and manufacturing. Combined with the packaging design, this has shown very reliable performance. According to the mission profiles of some 15-year-old cars, this equates to about 2000 hours under highly accelerated continuous operating life stress conditions. After 5,000 hours of stress, the current transfer ratio drops by less than 10, and system designers can accommodate these changes to cover a car’s life span of more than 35 years.
Optocouplers in Motor Drive Systems
Rapidly rising oil prices have propelled the growth of energy-efficient hybrid electric vehicles (HEVs) to the center of the automotive industry. High-voltage batteries used in hybrid vehicles require insulation and isolation. In current hybrid designs, optocouplers have been chosen for this task. Figure 1 shows an example of optocoupler placement in a HEV’s motor drive system.
Although Avago Technologies’ automotive optocouplers are the latest addition to the isolation product family, Avago optocouplers have been successfully used in hybrid vehicle programs in industrial grade form for nearly a decade. In today’s market, it is no longer sufficient for all emerging automotive isolation applications, especially those requiring long-term reliable operation at high ambient temperatures up to 125°C.
The main components of an optocoupler consist of a photodetector IC and an LED. In practice, photodetectors can sometimes be recertified for high temperature automotive use with little or no design changes. However, LEDs require more careful consideration. Some competitors and even customers may express concerns or prejudice about using LEDs at such high ambient temperatures. Some of this point of view may be due to the potential effect that LEDs may suffer from a significant drop in light output after prolonged use at high ambient temperatures. The key operational word here is: potential. Since the advent of LEDs, the continued rapid evolution of LED designs and processes has led to dramatic differences in the inherent aging performance of LEDs.
As shown in Figure 2, LED reliability has made great strides in various fields. Higher LED internal quantum efficiency, better LED light extraction, and improved optical channel efficiency enable lower input drive current (IF) requirements. By designing low forward voltage (VF) LEDs, Avago can reduce the input power (IF x VF) to reduce the heat generated by the LEDs. A good current distribution LED design will improve the current distribution within the LED, thereby further improving the intrinsic lifetime.
Summary of Improvements in Light Emitting Diodes
Manufacturing defects in LEDs can be eliminated through better equipment and ongoing process optimization. These improvements have been in progress for over 35 years since Avago’s optocouplers were introduced.
Temperature Coefficient of LED Light Output
The brightness of an LED has a negative coefficient with temperature. To be able to handle ambient temperatures in excess of +105°C (Automotive Class 1 and Class 2), low temperature drift has a significant advantage in minimizing parametric temperature drift.LEDs used in Avago automotive-grade optocouplers typically drop at +125°C
Normalized light output of LEDs at high ambient temperature
The negative temperature coefficient of the LEDforward voltage is mitigated somewhat due to the small drift in the light output.For example, in a 3.3 V system, this would result in a 10% compensation effect, resulting in an overall drift