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Posted on 01 October 2019

Highly Reliable Optocouplers

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High Temperature Automotive Applications

The needs for new isolation devices for automotive applications are rapidly expanding, most notable with the adoption of hybrid vehicle technologies.

By Patrick Sullivan, Leong Yik Loong, Andy Poh from Avago Technologies

 

Although Avago´s automotive optocouplers are quite a new market addition, in reality Avago´s optocouplers have been successfully used in hybrid automotive projects for more than ten years, albeit in the form of industrial grade products.

However in today’s market, it is no longer sufficient to address all emerging automotive isolation applications with industrial grade products, in particular applications requiring reliable long term operation at high ambient temperatures up to 125°C (Figure 1).

HEV Electric Motor Drive System

Of course there are already well proven hermetic optocoupler products for use in aeronautical and space readily able to meet these high temperature requirements, unfortunately these are not often able to meet automotive price expectations.

Optocoupler Technology

The primary piece parts of an optocoupler consist of a photodetector IC and an LED (Figure 2).

Optocoupler Construction

In practice the photodetector IC can sometimes be re-qualified for high temperature automotive use with little or no design changes. The LED however requires more careful consideration. Some technological competitors or even customers might express concern or prejudice against the use of optocoupler LEDs at such high ambient temperatures.

To be fair some of this opinion has at least some grounding, in that LEDs have the potential to suffer from significant light output degradation and aging when operated at high ambient temperatures. But the key operative word here is: potential.

Since the advent of the LED, continuous and rapid development in LED design and processing (see Figure 3) has resulted in a massive divergence in the intrinsic aging and temperature drift performance of LEDs used in optocouplers.

LED Design and Process Improvements

In terms of delivering a high quality optocoupler LED, there are two important criteria, the LED design and the manufacturing process. In this regard, in-house III/V R&D and manufacturing capabilities can provide a significant technical advantages when designing and manufacturing LEDs for innovative next generation optocoupler products.

Avago´s latest generation of automotive grade optocoupler LEDs benefit from a number of product enhancements in the areas of:

1. Higher internal quantum efficiency
2. Lower forward voltage
3. Enhanced current spreading design

These product enhancements not only have a direct influence on the intrinsic properties of the LED, they also facilitate secondary benefits e.g reducing the input If current requirement, in turn reducing the internal power dissipation and subsequent junction temperature.

LED Operating Lifetime

The combination of the product improvement factors results in the reliability performance shown in Figure 4.

High temperature Operating Life test

There are no fixed rules for translating this data into an automotive mission lifetime expectation.

However if we take 2K hours as a bench mark, it is interesting to note that the overall current transfer efficiency of the automotive optocoupler remains remarkably unchanged.

The next point of note, is that this data was obtained from LEDs manufactured from a range of wafer lots, so this extremely tight distribution is indicative of a very well controlled manufacturing process.

Parametric Temperature Drift

It is not just LED aging which can be considered potentially problematic for optocouplers operated at high ambient temperatures. Light output drift over an extended temperature range can also indirectly influence important parametric values such as propagation delay.

It is not unusual to find optocouplers with current transfer drifts as high as 60% over a 100°C range.

This is another area in which the intrinsic LED design reaps benefits, Avago’s automotive LEDs achieve better than a 20% light output drift over the same 100°C temperature range. In fact, this is not the end of the story. Since the forward voltage of the LED has a negative temperature coefficient, driving the LED from a fixed voltage source (normal method) results in an additional 10% compensation factor, giving an overall current transfer drift of less than 10% over a 100°C range.

High Voltage Safety

Many emerging automotive isolation applications require safe high voltage insulation as well as simple voltage level shifting capabilities. It is generally considered necessary to provide safety-related insulation for any voltages above 50Vac or 70Vdc. When user safety comes into the equation, the design margins adjust upwards accordingly.

When talking about safe insulation, the commonly used mantra is reinforced insulation. Reinforced insulation is widely synopsized with the use of double insulation. In fact many well established IEC safety standards do not accept single layer thin sheet insulation for reinforced insulation between high voltage circuits and safe low voltage circuits. If multilayer thin sheet material is used, the initial test voltage should be 150% (3 or more layers) or 200% (2 layers) of the test voltage.

Avago Technologies meets the requirement for double insulation in its automotive range of optocouplers using a double insulation composite construction of polyimide tape and silicone.

Additional two basic production safety tests are applied to Avago automotive optocouplers. First test is the UL1577 dielectric test, which involves applying up to 5000Vrms across the device and sensing the leakage current as a pass or fail criteria. The second test is the IEC62019-5-5 optocoupler partial discharge test, which involves applying 1.875 times the rated working voltage and detecting partial discharges as a pass or fail criteria.

Strictly speaking these production tests only give a reliable indication of the capabilities of the isolator when it is new.

It is of course desirable to have an isolator which can provide definitive safe performance over the life time of the end product, this is even more of a concern with the presence of significant aging factors such as high temperature.

Good high voltage lifetime is not achieved by default. On the contrary, it is possible to build an isolator device capable of passing both UL1577 and IEC62019-5-5 testing, but still end up with an isolator with poor immunity to high voltage degradation. Although it should be noted this is more applicable to non optocoupler technology isolators employing very thin insulation materials such as micro magnetic coil based isolators.

Insulation Degradation

A widely acknowledged cause of insulation degradation is the combination of injected space charge acting on micro-voids. By reducing injected space charge or micro-voids, or even better both, you can very effectively reduce this cause of high voltage aging.

"Micro voids" are unavoidable in any insulation material, but the number and size of the "micro voids" can be significantly reduced by careful choice of insulation material. Other than the material content, the process in which the insulation material is fabricated is also very important. For instance, spin on polyimide coating processes generates more micro voids than a preformed homogenous sheet polyimide; snow versus sheet ice. There are test methods for detectable voids in insulation materials, e.g. partial discharge testing. But unfortunately the sensitivity of these test methods is only able to detect larger voids. "Micro voids" involved in the high voltage degradation process are undetectable using conventional measurement equipment.

The second aggravation element in the high voltage aging process is space charge injection. Space charge is injected into the insulation material when under high voltage duress. In terms of the quantity of space charge injected, the principle determining factor is the thickness of the material and the applied electric field, resulting in a KV/mm stress factor. Other significant factors include operating temperature and the type and frequency of the applied high voltage stress.

To ensure continued safe insulation at high operating temperatures, all Avago automotive optocouplers use thick homogenous polyimide insulation materials to simultaneously minimize both "micro voids" and space charge injection.

Conclusion

To conclude plastic-based optocouplers can meet the requirements of high temperature automotive applications.

 

 

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