A few years ago, a customer used our TCNT1000 reflective sensor in an application. This reflective sensor combined an emitter and detector in a small, surface mount package. The emitter irradiated a shaft with a black strip embedded in it. As the shaft rotated, the reflection from the plastic strip versus the metal shaft was large enough to determine and control the speed of the shaft. The customer neglected to consider the pulse duration and ambient operating temperature when setting the forward current. Over time, they began to experience some field failures. The emitter junction temperature was too high and, as the emitter degraded, the radiant flux declined. The controller no longer reliably received shaft speed and the application failed.
Infrared Emitter – Pulsed Operating Mode
Vishay’s Infrared Emitters are offered in a wide variety of packages, peak wavelengths, radiant intensities and angle-of-half intensities. These emitters can operate in a steady (continuous) or pulsed mode. Each emitter datasheet includes an absolute maximum ratings table, Table 1. The TSHG6400, an infrared, 850 nm emitting diode in GaAlAs double hetero (DH) technology with high radiant power and high speed, molded in a clear plastic package, datasheet is shown as an example. The maximum allowed forward current, for example, IF = 100 mA boxed in red in Table 1, is defined for a continuously operated emitter. The peak forward current is defined for a pulsed emitter, boxed in green in Table 1. The duty cycle for the peak forward current is included in the table; tp/T = 0.5 with a pulse length of tp = 0.1 ms. In this case the pause or off time T is equal to 0.2 ms.
Because the pulse duration and forward current may differ for each application, each emitter datasheet also includes a Pulse Duration vs. Forward Current graph, Figure 2. This graph provides the maximum forward current for multiple duty cycles: 50%, 20%, 10%, 5%, 2% and 1%. As shown in the graph, for pulse durations of 0.1 ms or shorter, the peak forward current can be as high as 1 A or equal to the maximum surge forward current. As the pulse duration increases beyond 0.1 ms, each curve, regardless of the duty cycle, shows a decreasing forward current. This decrease in forward current reduces the junction temperature of the emitter. Without a decrease in current, the emitter could overheat and result in rapid degradation.
To determine the maximum forward current for a pulse of 2 ms and a duty cycle of 10%.
tp = 2 ms, T = 20 ms
tp / T = 0.1 or 10%
IFmax = 400 mA
From the graph in Figure 3, a maximum current of 400 mA is possible. If the pulse duration were less than 0.1 ms, the maximum current would have been approximately 600 mA. Be smart! Include some safety margin when setting the application duty cycle. A pause of 20 ms will allow the emitter junction temperature to cool, ensuring long life. The average current can be calculated.
Iavg = Imax ÷ (T/tp)
Iavg = 400 mA ÷ (20 ms/2 ms)
Iavg = 400 mA ÷ 10
Iavg = 40 mA
Note that the graphs above are valid for ambient temperatures less than 50°C. What about temperatures greater than 50°C?
High Temperature Operation
Learn higher temperature operation in the next edition of the Opto Squad Adventures.