Current transfer ratio is comparable to BJT’s HFE or beta (β). This is the ratio of the optocoupler’s collector current to the forward current. Current transfer ratio is popular by its abbreviation CTR. CTR is simply the measure of the optocoupler’s transfer efficiency. If we recall the basics, optocoupler is a semiconductor device with a light source and a detector. The light source if often times an LED while the detector is an open base transistor or a photo transistor. When the LED is forward biased it will gives off light and bias the photo transistor. Then the current will start to flow in the photo transistor’s collector. In circuit involving optocoupler, in some cases design engineers failed to fully consider the factors affecting current transfer ratio. As a result, design issue sufficed during mass production. Opto coupler current transfer ratio is dependent to the luminous intensity of the light source, operating temperature and continuous operation or aging.
Factors Affecting Current Transfer Ratio
1. Luminous Intensity of the Light Source
This is dependent to the forward current applied to the light source which is commonly a light emitting diode. The higher the forward current, the brighter is the light and the higher the CTR will be. However, operating at higher forward current will degrade the life of the optocoupler faster, so better consider it as well. Figure 1 shows an example graph of forward current versus the current transfer ratio of a particular optocoupler device. As you can see, at 1mA the CTR is around 100% while it increases to more than 200% when the forward current is reaching 10mA for a VCE of 5V. You have to take note also not to operate in much higher current because it is not anymore helpful in your design since the CTR will start to decrease at some point based from the graph.
In my job as a SMPS design engineer I always used an optocoupler in a circuit that connects the primary and the secondary of the SMPS. I used also this device in the feedback loop. For analysis regarding optocoupler working in a feedback loop read this. Based on my experienced, the usual forward current I set is between 1.5- 3mA. Maybe you will ask why this range. I cannot increase it above 4mA because most of the times the light source or the LED is driven by a digital signal controller which can handle only maximum of 4mA source and sink currents.
2. Ambient Temperature Dependency
One of the factors affecting current transfer ratio of optocoupler is the ambient temperature. The luminous efficiency of the LED has a negative temperature coefficient while a positive coefficient for the photo transistor HFE. This will result to a curve shown in Figure 2.
The same with other semiconductor devices, the optocoupler operation is also affected by higher temperature. A CTR of 100% at a nominal ambient temperature of 25’C will no longer the same at 60’C. If your design is exposed in a higher temperature, always consider the temperature and relative CTR graph for proper guidance. For example in below graph, the CTR at 25’C is at 100% but it decreased to 60% at 100’C.
Another contributor of CTR degradation is aging. Continuous operation of the optocoupler will decrease its life expectancy because the luminous efficiency of the light source will degrade over time. Another factor is that the optical coupling efficiency between the LED and the photo-transistor will degrade over time. For the photo transistor, the photo-electrical conversion efficiency and the DC amplification factor will be affected by continuous operation also.
Figure 4 shows a graph between operating time and relative CTR. As you can see, the CTR starts to de-rate after 20,000 hours.
So if you target a 5 year life of your products, you must consider the CTR that will not compromise within 5 year period. Otherwise, your design will fail in the field and most likely there is higher warranty rate.