2025-03-21
The electrical properties of leds are the basis of their work
The electrical properties of leds are the basis of their work
The electrical characteristics of LED are the basis of its work, which directly affects its luminous performance, driving mode and reliability. The following is a detailed analysis of the electrical characteristics of LED:
1. Unidirectional conductivity
How it Works:
- LED is a diode that has unidirectional conductivity, that is, it can only be switched on and emitted under a forward voltage.
- Under reverse voltage, the LED hardly conducts, and too high reverse voltage may cause LED damage.
Forward voltage (Vf) :
- The forward voltage is the minimum voltage required for LED conduction, usually between 2V-4V, depending on the material and wavelength:
Red LED: about 1.8V-2.2V.
Green LED: about 2.2V-3.0V.
Blue LED: about 3.0V-3.6V.
White LED: about 3.0V-3.6V (based on blue chip excitation phosphor).
2. Current-voltage characteristics (I-V characteristics)
I-V curve:
- The current-voltage characteristics of LED are nonlinear, and the current increases rapidly after the forward voltage reaches the threshold.
- Below the threshold voltage, LED almost no light; When the threshold voltage is exceeded, the luminous intensity increases with the increase of current.
Threshold voltage (Vth) :
- The threshold voltage is the minimum forward voltage when the LED begins to glow, usually slightly lower than the rated forward voltage.
3. Current-light intensity characteristics
The relationship between light intensity and current:
- The light intensity of LED is approximately proportional to the forward current, but too much current will lead to reduced light efficiency and shortened life.
- Typically, leds operate at a current of 20mA to 350mA, depending on power and package.
Saturation phenomenon:
- When the current exceeds a certain value, the light intensity increases slowly, and even saturation occurs, because the non-radiation recombination inside the chip increases.
4. Dynamic resistance
Dynamic Resistance (Rd) :
- Dynamic resistance refers to the resistance of the LED in the on-state, which is usually very small (a few ohms to tens of ohms).
- The dynamic resistance decreases with the increase of current.
5. Reverse feature
Reverse leakage current:
- Under the reverse voltage, the LED will have a small leakage current, usually a few microamps to tens of microamps.
- Excessive reverse voltage can cause a sharp increase in leakage current and even breakdown the LED.
Reverse breakdown voltage (Vr) :
- The reverse breakdown voltage is the maximum reverse voltage that the LED can withstand, usually 5V-20V.
- Beyond the reverse breakdown voltage, the LED may be permanently damaged.
6. Effect of temperature on electrical characteristics
Forward voltage changes with temperature:
- When the temperature rises, the forward voltage of the LED will decrease slightly (about 2mV/°C).
- This is because the energy gap of semiconductor materials changes with temperature.
Current varies with temperature:
- The increase in temperature will cause the dynamic resistance of the LED to decrease and the current to increase.
- If there is no constant current drive, it may lead to thermal runaway.
7. Driving mode
Constant current drive:
- Leds require constant current drive to ensure stable light output and long life.
- Constant current drive can avoid the impact of current fluctuations on LED performance.
PWM dimming:
- PWM (Pulse width modulation) is a commonly used LED dimming method, which controls brightness by adjusting the duty cycle.
- PWM dimming does not change the color and spectral characteristics of the LED.
8. Test and evaluation of electrical characteristics
Forward voltage test:
- Use a digital multimeter or dedicated tester to measure the forward voltage of the LED.
Reverse leakage current test:
- Measure the leakage current of the LED at reverse voltage.
I-V characteristic test:
- The current-voltage curve of the LED is measured using a semiconductor characteristic analyzer.
Sum up
The electrical characteristics of LED are the basis of its work, covering many aspects such as one-way conductivity, I-V characteristics, current-light intensity characteristics, and so on.
The electrical characteristics of LED are the basis of its work, covering many aspects such as one-way conductivity, I-V characteristics, current-light intensity characteristics, and so on.
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