12.8 Synchronized floating current mirror for maximum LED utilization in multiple-string linear LED drivers
High-voltage linear drivers for multiple-string LEDs have been widely used in general lighting due to their low cost, simplicity, low electromagnetic interference (EMI), and high reliability. However, in conventional multiple-string LED drivers, the LED strings are turned on and off sequentially depending on the magnitude of input voltage (VIN) in order to achieve high power factor (PF) and low total harmonic distortion (THD) [1-3]. The turn-on duty cycles of each string in a four-string LED driver are roughly 80, 60, 40 and 20% respectively, given that VLEDs are identical. Due to its low duty cycle operation, the total brightness is reduced and the brightness of each LED string becomes uneven. This problem becomes more noticeable in long bar-type or surface-type lamps when a triac dimmer is applied. This low utilization of LEDs is a major drawback of multi-string LED drivers. One approach to increase turn-on duty cycle uses diodes and capacitors in each LED string to sustain voltage when VIN is not enough [4]. The problem is that it requires large capacitors due to long turn-off time. Another method is to change the parallel/serial connection of LED strings to match the total voltage drop of LEDs to VIN [5]. Due to its high voltage operation, it is very challenging to make an appropriate, low-cost, and simple solution for all VIN ranges. In order to overcome these problems, a floating current mirror (FCM) IC fabricated in an 85V 0.18um BCDMOS process is presented. As long as VIN is higher than the minimum required LED voltage, FCM enables to turn on all LED strings for all VIN ranges not by using large capacitors, but by modifying only the functional aspects of LED connections. In addition, this approach also prevents EMI problems by keeping an appropriate di/dt during commutation.
A soft self-commutating method using minimum control circuitry for multiple-string LED drivers
Light-emitting diodes (LEDs) are widely used in general lightings due to their several advantages including high efficiency, high reliability, long life, and environmental friendliness. Recently, various converter-free methods for non-isolated LED drivers with multiple LED strings connected in series have been introduced, enabling both a higher efficiency and power factor (PF) as well as lower total harmonic distortion (THD) [1-3]. In multiple-string LED drivers, the efficiency and PF are enhanced as the number of LED strings increases because of a low overhead voltage. However, as the operational voltage range decreases, it is difficult to find a proper commutation time using input voltage sensing approaches due to input voltage noise and LED voltage variation [4]. Other concerns are EMI and EMC noise caused by high di/dt and dv/dt in hard commutations. When the LED current is high, negative effects of hard commutation become worse and the required di/dt control circuits are more complicated [5]. To meet EMI and EMC regulations for lightings without adding on-board EMI filters, soft commutation is essential. In order to overcome these problems, we propose a soft self-commutating method using a Source-Coupled Pair (SCP) and reference voltages. The conventional control circuits required for an appropriate commutation time and soft commutation are no longer necessary. The fabricated 6-string LED driver IC is capable of achieving high efficiency (92.2%), high PF (0.996) and low THD (8.6%) under the 22W/110V AC condition.
Kidney and urinary tract diseases in the newborn
Kidney and urinary tract diseases in the newborn / , Kidney and urinary tract diseases in the newborn / , کتابخانه دیجیتال جندی شاپور اهواز