The effects of SCM and PWM on the chromaticity and the spectrum of PC white LEDs

When phosphor converted white light emitting diodes were operated under stepwise current modulation (SCM) and pulse width modulation (PWM), the change in chromaticity and spectrum were investigated. In the chromaticity diagram, yellow shift was observed with increasing forward current in SCM while blue shift with increasing duty ratio in PWM In SCM, the peak wavelength around blue color band moved to shorter wavelengths and the efficiency of blue-to-yellow conversion also increased as forward current increased. Meanwhile, a peak wavelength in PWM was almost fixed but the light conversion efficiency decreased as duty ratio increased. The average Δuv was 0.004193 in SCM and 0.003210 in PWM.