Feedback Stability Investigation of a LED Driver Circuit with PWM Dimming
Date Issued
2014
Date
2014
Author(s)
Yeh, Chen-Wei
Abstract
In recent years, light-emitting diodes (LEDs) have increasingly been used for back-light illumination for high-performance digital television applications. This is usually a wide-screen application in which serially-connected LED strings and boost converter configuration are normally the choices. For such an application, the lighting evenness and local area dimming are two major requirements. Therefore, a constant-frequency current-mode (CM) driver scheme is usually used for this application. Current-mode control scheme allows better performances in light- evenness and local dimming capability for a large-screen television application.
To achieve dimming, the industry has in general adopted pulsating scheme, or sometimes called burst-mode control in which the LEDs are powered by a specified current level for certain time and completely shut off for the rest of the time in a low-frequency on-off cycle. Compared to the scheme to reduce LED current level but with continuous operation , this allows better conversion efficiency and better for both the driver circuit and the LEDs. In doing so, however, there are two issues that need to be addressed. Since the LED current, is a stepped waveform under the burst-mode dimming mode, the voltage across the LED strings can have overshoot oscillation which must be restricted to certain level to avoid the shortening of LED life time. The other issue is about the driver circuit feedback stability. The focus of the thesis is to design a proper feedback compensation to meet the goal.
In the thesis, a control model is first established by using small-signal averaging method. Then feedback stability issue and the transient voltage overshoot issue were addressed using the current-mode control block diagram and the output impedance characteristics. The analytical results seem complicated , but it was found that the complexity of the analytical results can be simplified into a second-order equation for normal practical situations. This makes the design of the feedback compensation much easier to deal with using the proposed model. The model was verified by simulations and experimental results. The results from the efforts leading to this thesis should be very helpful in designing the feedback circuit for a current-mode boost LED driver circuit.
Subjects
LED驅動電路
昇壓型
脈衝式調光
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-103-R01921024-1.pdf
Size
23.32 KB
Format
Adobe PDF
Checksum
(MD5):1823df8f518d4ab843fc3e23ec6e021b