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 Customized AC Hybrid LED Driver With Flicker Reduction for High Nominal Range Applications
This brief presents a customized ac hybrid LED driver for high nominal range (220–240 V<sub>AC</sub>) applications using average current control. A fixed offset hysteretic window approach is proposed to reduce the switching frequency variation. In order to enable low-side current sensing, a special substrate connection is presented. The designed LED driver is fabricated with a 0.18-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, 700-V ultra-high voltage CMOS process. Measurement results show that under 230-V<sub>AC</sub>, 50 Hz input, the peak efficiency of the customized design is over 94% and the power factor is greater than 0.92 while suppressing the photometric flicker and delivering 17-watt power to the LEDs.
Analysis and design considerations of LLCC resonant Dc-Dc converter with precise current sharing for two-channel LED driver
In this paper, a LLCC resonant converter with automatic precise current-sharing is proposed for two-channel LED driver. With a resonant capacitor connected in series with the transformer secondary winding, the currents of the two channels are precisely shared even for different LED strings with different voltage drops. The circuit operation principle, DC gain deduction and design considerations are discussed. A prototype of 140W two-channel LED driver is built to verify the proposed current sharing circuit.