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LED links tolerate lower currents

LED links tolerate lower currents

Despite the emergence of alternative isolation approaches, high speed optocouplers remain a popular technology. According to IMS Research, sales of high speed optocouplers (including gate driver optocouplers) are expected to grow at a rate of 14.5% per annum between 2009 and 2016 and sales revenues are already more than those of phototransistors.

However, customers continue to push for better efficiency, higher operating temperatures and minimal failures. They also want increased design margins, which means better specifications. For example, customers want a specified minimum and maximum current transfer ratio (CTR) with less variance. These CTR ranges are ensured through bin testing, where the CTRce is measured under specific test conditions. Customers also want an optocoupler that can operate at lower current levels, so suppliers need to specify how the optocoupler operates at high temperatures.

For example, the optical source is less efficient at low currents. Let's assume the customer is using a classic phototransistor optocoupler like the FOD817 as part of the feedback loop in their isolated dc/dc converter. Over the years, design demands have reduced the current flowing through the phototransistor optocoupler. Although the device may still work, the FOD817 and its equivalents are not specified at these lower currents. This problem has been solved with the introduction by Fairchild of the FOD8801 OptoHiT series, which offers guaranteed specifications over temperature, with led currents as low as 1mA.

Responding to customer demands, Fairchild has also implemented an infrared led based on aluminium gallium arsenide (AlGaAs). LEDs made from AlGaAs support higher speed, improved temperature performance and consistency and are more efficient at lower currents than the GaAs ir leds commonly found in most phototransistor optocouplers. In addition, Fairchild uses its Optoplanar coplanar packaging technology (see fig 1), which delivers an input to output capacitance of approximately 30% less than that of the face to face construction, even in the half pitch mini flat package. The 0.4mm internal thickness through insulation provides a high degree of robustness, which means designers don't need to sacrifice isolation/insulation for reduced package size.

Meeting technology demands
Fairchild's OptoHiT series meets customer demands by providing good CTR linearity at high temperature and at input currents as low as 1mA. It achieves this through the use of AlGaAs ir leds. The FODM8801 consists of an AlGaAs ir led optically coupled to a silicon phototransistor. The device provides a guaranteed – or 'binned' – CTR for saturated and non saturated modes and switching specifications over the extended operating temperature range (-40 to 125°C) for greater design flexibility. Because the FODM8801 is more stable over an extended temperature range, CTR drift and degradation become less of a concern for engineers when they are doing a worst case design.

The FODM8801 also uses Fairchild's Optoplanar packaging technology. Optoplanar's lower package capacitance supports high noise immunity and reliable isolation at high operating temperatures.

The FODM8801 is supplied in a half pitch, mini flat four pin package, saving board space, providing more design flexibility and allowing for overall reductions in system cost. Additionally, it offers an isolation voltage of 3750V for increased reliability.

Devices in the OptoHiT series are suited for industrial applications, including power supplies, smart meters and motor controls, and consumer applications, which include chargers and adapters.

The challenges
Developers are facing challenges with optocoupler devices, including high temperature, reliability and cost reduction.

Many engineers ask about led degradation over the life of an application. LED degradation, based on time, temperature and the magnitude of the led's forward current, reduces the long term CTR. When the led is operated at low currents, long term degradation is minimal. At low led operating currents, an increase in operating temperature has the greatest effect on short term led efficiency. Junction temperature is the dominant factor affecting the LED efficiency.

Instead of being concerned about the long term CTR change, where degradation at low current (less than 5mA) is measured in 0.1% per 1000hrs, consider that a large change can happen in a few minutes due to a temperature rise. For example, the FOD817's CTR can be reduced by as much as 50% when the ambient temperature is elevated by 50°C – an increase in temperature which can occur in as little as five minutes. This effect often goes unnoticed as the CTR recovers to its original value when the ambient temperature returns to its original value. Most data sheets only specify CTR for ambient temperature operation.

Phototransistor speed is determined by the photocurrent developed by the led, transistor current gain and load conditions. As 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.

The good news is that Fairchild's newer phototransistor optocouplers – like the FODM8801, FODM453 and HCPL4503 – use AlGaAs ir leds, which bring better performance (see fig 2).

The output from the AlGaAs ir led drops by only 0.225%/°C when operated at an led current of 1mA. So, a 50°C change in junction temperature means the led's output will drop by only 11%. By comparison, a GaAs led operated with the same current will typically lose 50% of its light output if the junction temperature is elevated by 50°C. Not only are AlGaAs leds more than twice as efficient, they offer better temperature stability than GaAs leds, which have a typical process variation of 4:1. The bottom line is that you can extend the life time of the led by running it at lower current and lower temperature.

Fairchild is also working to improve the datasheet specifications for its optocouplers. For example, three different CTR ranges are specified for the FODM8801. They have the same minimum to maximum ratios, but overlap. The same leds are used, but transistor gain is targeted to offer different CTR bins. This gives customers the option of using the lower gain parts, which are faster, or higher gain parts when gain is more critical.

If more precision/tolerance is needed, then designers may want to consider products with a guaranteed CTR. Here, the factory preselects leds and builds an amplifier with feedback, so there is only the variation of the led and coupling. For example, in the high speed FOD8012, the variance is eliminated by replacing the phototransistor with a cmos ic.

New markets
In addition to the changes in standard optocouplers, new markets are emerging for high speed and gate driver optocouplers where customers require more than the standard 600V of insulation protection. Fairchild's gate driver optocouplers can provide a working voltage (UIORM) of 1414V and an isolation voltage (VISO) of 5000V.

Phototransistor optocouplers have been around for more than 40 years and remain popular. By improving CTR, linearity, extended temperature range and low input drive current, it becomes easier for engineers to do a worst case design.

Author profiles
Joyce Patrick is regional marketing manager, EMEA, optocoupler products. Robert Krause is an optocoupler products applications engineer. Both are with Fairchild Semiconductor.

Joyce Patrick and Robert Krause

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