Phototransistor optocouplers have continued to evolve since they were first introduced 40 years ago, and remain popular even now. The versions most widely available today use an infrared light emitting diode (IR LED) produced in gallium arsenide (GaAs). They deliver good performance in a number of applications, but are not as effective at low currents, and that can pose a problem in systems that use lower currents to increase efficiency.

Fairchild Semiconductor has implemented a different type of IR LED, made of aluminum gallium arsenide (AlGaAs), that helps address this problem. The AlGaAs IR LED delivers higher speed, improved temperature performance and consistency, and is much more efficient at lower currents compared to conventional GaAs versions. The new IR LED format is available in the FODM8801 OptoHiTâ„¢ series, which offers guaranteed specifications over temperature with LED currents as low as 1mA.

Long-term and Short-term Performances in LED Efficiency

Long-term performance can be a concern with any IR LED, whether it’s made in GaA or AlGaAs. Over the life of an application, the LED can begin to degrade, leading to long-term permanent reductions in LED efficiency, as measured by the current transfer ratio (CTR). Fortunately, with an IR LED operating at low currents, the long-term degradation is minimal. It’s not uncommon, for example, for the long-term CTR change at low current (IF < 5mA) to be as low as 0.1 percent per 1,000 hours, which means degradation doesn’t pose much of a concern.

What can be a problem, though, is the effect that the increases in operating temperature can have on short-term LED efficiency. The CTR can drop dramatically when the junction temperature rises and problems can occur quickly. The ambient temperate can increase by as much as 50 °C in as little as five minutes. This effect often goes unnoticed, since the CTR recovers to its original value when the ambient temperature is restored to its original starting temperature. (Most data sheets only specify at ambient temperature operation.)

Fairchild’s new AlGaAs IR LEDs are less susceptible to changes in temperature. Figure 1 compares an AlGaAs 880 nm IR LED to a standard GaAs 940 nm IR LED, and shows how light output changes when the junction temperature increases. (The graph shows typical data and is not guaranteed.)

Figure 1. Light output reduction (%/°C) versus LED current

When operated at 1mA, the GaAs IR LED light output drops by 1%/°C. This means that for a 50 °C increase in junction temperature, the LED’s output will drop by 50 percent. With the AlGaAs IR LED, the light output drops by only 0.225%/°C, so a rise of 50°C in junction temperature will cause the light output to drop only 11 percent.

Phototransistor Speed and LED Photocurrent

Another factor to consider in long-term performance is the phototransistor speed. Phototransistor speed is determined by LED developed photocurrent, transistor current gain (hfe), and load conditions. As the LED flux drops, the timing changes. If a design works well over the temperature range at low current, then the long-term change will be minimal.

In general, AlGaAs LEDs are two to three times more efficient than their GaAs counterparts and are 4.5 times more temperature stable. When produced in the AlGaAs process, the IR LED lasts longer at lower currents and lower temperatures.

Fairchild’s FODM8801 OptoHiT series is packaged in a proprietary OPTOPLANAR® coplanar packaging technology, and this helps improve performance at low currents, too. Figure 2 shows a cross-section of OPTOPLANAR technology.

Figure 2. Fairchild OPTOPLANAR® coplanar packaging technology

The package delivers low input-to-output capacitance (CIO), with ratings roughly 30 percent lower than with face-to-face package construction, even in small packages like the half-pitch mini-flat package (MFP). An internal thickness of 0.4mm through insulation provides a high degree of insulation robustness. As a result, designers do not have to sacrifice isolation/insulation for reduced package size.

Conclusion

Fairchild’s FODM8801 OptoHiT series, a new generation of phototransistor optocouplers equipped with IR LEDs produced in AlGaAs process technology, deliver longer, more reliable operation at lower currents and lower temperatures. They make it easier for designers to create systems that still function under worst-case conditions.