Stable current drives the LED to maintain a fixed brightness

LED driving still faces many challenges. To maintain a constant brightness, it is necessary to drive the LED with a stable current without being affected by the input voltage. Compared with the incandescent light bulb, it is more challenging to simply connect the battery as a power source.

The LED has a forward V-I characteristic, similar to the case of a diode. The turn-on threshold of a white LED is about 3.5 volts. Below this threshold, the amount of current passing through the LED is very small. After this threshold is exceeded, the current will increase exponentially, causing the forward voltage to increase, and the LED becomes a voltage source model with series resistance. However, it should be noted that this model is only valid under the condition of a single DC current operation. If the DC current in the LED changes, the resistance in the model should also be changed to show the new operating current. In the case of a large amount of forward current, the power consumed in the LED will increase the temperature of the device, change the forward voltage drop and dynamic impedance, and when determining the LED impedance, be sure to consider the heat of the environment.

If the LED is driven by a buck regulator, in addition to the DC current, the LED often conducts the AC chain wave current of the inductor, depending on the selected output filter arrangement. This will increase the RMS intensity of the current in the LED, increase its power consumption, and increase the junction temperature, which has a significant impact on the life of the LED.

If a 70% limit is established on the light output as the lifetime of the LED, the life of the LED can be increased from 15,000 hours at 74°C to 40,000 hours at 63°C. The method of judging the power loss in the LED is to multiply the LED resistance by the square of the RMS current, plus the average current multiplied by the forward voltage drop. Since the junction temperature is determined by the average power, even if there is a large amount of chain-wave current, the impact on power consumption is very small. For example, in a buck regulator, the peak-to-peak chain wave current (Ipk-pk=Iout) equal to the DC output current will increase the total power loss by less than 10%.

If it is quite a lot more than this, the supplied AC chain wave current must be reduced to maintain the junction temperature and operating life. There is a practical basic principle here, that is, if the junction temperature is lowered by 10°C, the life of the semiconductor will be doubled. In fact, most designs tend to use much lower chain-wave currents due to the limitation of inductance. In addition, the peak current in the LED should not exceed the maximum safe operating rating specified by the manufacturer.