Comparison of the EMI Performance of LED PWM Dimming Techniques for LED Video Display Application

Light-emitting diode (LED) video displays enclose thousands of LEDs which create the resulting image. LED intensity is regulated using pulse width modulation (PWM). Though the individual LED current is small, the total current consumed is large. Simultaneous switching of a large number of LEDs dimmed by PWM can cause serious electromagnetic interference (EMI). Three LED PWM dimming techniques have been studied for potential EMI. Techniques represent different current pulse positioning in time: the aim of study was to compare the spectrum of current produced in LED video display power supply circuits when discussed techniques are used for pixel intensity control. It was assumed that the produced EMI is proportional to the power supply current. The power supply filter and transient load decoupling capacitors were not taken into account. The uniform and nonuniform pixels´ intensities distribution within an image were assumed. Spectrum has been studied on a single pixel and the video display tile containing 16 × 32 LEDs. Resulting time diagrams and frequency responses are presented. The results indicate that, despite expected significant advantage of binary PWM methods, in realistic case all methods´ performance is similar.