The Influence of High Temperature Aging of Optocouplers on Signal Transmission

LED light intensity attenuation: Long term high temperature can lead to an increase in defects in semiconductor materials (such as GaAs) inside the LED, a decrease in electron hole recombination efficiency, and a gradual decrease in light output power over time
Degradation of response of photosensitive components: The carrier lifetime of the base region of the photosensitive transistor shortens with increasing temperature, resulting in a slower response speed (rise/fall time) to optical signals and a decrease in conversion efficiency (current transfer ratio, CTR)
Packaging and material aging: High temperature can accelerate the thermal aging of optocoupler packaging materials (such as epoxy resin), leading to packaging cracking, sealing failure, and contamination of internal components (such as dust and moisture), further exacerbating performance degradation
2, The specific impact of high-temperature aging on signal transmission
After high-temperature aging of optocouplers, the accuracy, stability, and real-time performance of signal transmission will be compromised, specifically manifested as:
1. Weakening or disappearance of signal amplitude
The attenuation of LED light intensity can lead to insufficient light signal intensity received by the photosensitive element, resulting in a significant decrease (or even zero) in the output current (Ic) or output voltage (Uo) of the optocoupler. For example, in an analog signal isolation circuit, after the input signal (such as sensor voltage) is transmitted through an optocoupler, the amplitude of the output signal may decrease to a level that cannot be recognized by subsequent circuits.
2. Signal distortion and delay
Nonlinear distortion: The current transfer ratio (CTR) of the photosensitive element decreases with increasing temperature, and the linear region of CTR shrinks, causing the linear relationship between input-output signals to be disrupted (such as a sine wave signal becoming a trapezoidal wave).
Increased transmission delay: The light emission response time (Turn On Time) of LEDs and the light reception response time (Rise Time) of photosensitive elements increase with temperature, resulting in an increase in signal transmission delay (Delay Time). In high-speed digital signal transmission (such as PWM control, data bus), delay may cause signal synchronization failure.
3. Electrical isolation failure
The function of optocouplers is electrical isolation (without direct electrical connection between input and output circuits), but high temperature aging may cause packaging cracking or internal insulation materials (such as SiO?)? )Breakdown causes leakage current between the input and output circuits. At this point, the input signal will directly interfere with the output circuit (such as the feedback loop of the power module), causing distortion of the output signal
4. CTR fluctuations and unstable output
CTR (Ic/IF, where IF is the LED driving current) is a key parameter of optocouplers, and its value decreases nonlinearly with increasing temperature. For example, a certain type of optocoupler (such as EL827) has a CTR of 200% at 25 ℃, and when the temperature rises to 85 ℃, the CTR may decrease to below 100% (the specific value depends on the device model). The fluctuation of CTR can cause instability in the optocoupler output signal (such as the feedback signal of a switching power supply), which in turn can cause drift in the system output (such as the power supply voltage)