In this article we will introduce to you an effective method to control the brightness and consequently the power consumption and costs of multi-LED applications. Providing each LED a PWM controller and converting input voltage to LED current via this controller is a traditional brightness control method for multi-LED applications. This application applied voltage-controlled PWM controllers. Usually, led pn junction will be cascade with a current-sensing resistor, which will send feedback voltage signals to controllers. Basically, this type of controllers operated in the voltage mode and maintained the feedback voltage signal at a fixed value, stabilizing the internal current.
Using multiple RGB LEDs to generate white rays is a common application. In addition to red rays, green rays and blue rays, there will be 1-2 more LED chips that used to swift LED white rays into a certain tone. In order to simulate the proportion of red rays, green rays and blue rays in the white ray spectrum, working current for these three different types of LED chips will be fixed at a certain ratio by three controllers. Normally, the output green rays occupy 50-64% of total, 25-40% for red rays, and blue rays only account for 5-15%. In this way, at least three PWM controllers are needed, which increases the production costs virtually. Each controller provides power for each colored LED. In addition, the control system also needs to change the brightness of LED lamps by adjusting the load cycle of PWM controllers.
Another approach is applying single-current driver onto theses cascaded LED chips. The brightness of each LED chip is controlled by FET switches, which is connected with these chips in parallel. When FET is turned on, current will pass through it, stopping the parallel connected LED chip. On the contrary, switching off FETs will drive the current to pass through LED chips. Another system is responsible for the control of on-time ratio of LEDs in each clock cycle, including the three cascaded single-current controlled LEDs. The brightness of each LED will be adjusted by each FET. Compared with the three individual PWM controllers mentioned above, multi-LEDs with one PWM controller but multiple FETs will costs much less and save more power consumption. Removing each PWM controller will reduce the load dramatically and reduce the power consuming accordingly.