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.
Optimal Configuration of High-Efficiency Segmented Linear LED Driver With Genetic Algorithm
This paper presents a genetic algorithm (GA)-based optimal configuration for multisegmented linear LED driver to achieve a maximized system efficiency. Based on the theoretical analysis of the turn-ON states, the system efficiency is derived and found to be highly dependent on the segment voltage and reference current parameters. GA with a fitness function of the system efficiency is introduced in detail for the case of a four-segmented LED voltage configuration. With an input voltage of 220 V (rms) and input current of <inline-formula> <tex-math notation="LaTeX">$8\times $ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$10\times $ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$11\times $ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$12\times $ </tex-math></inline-formula> reference for the four turn-ON states, the four optimal segment voltages are calculated to be 160, 80, 39, and 22 V. Saber simulation results show that the GA-based configuration can give a higher efficiency (96.5%) than other typical configurations (<88.6%). Experiments are also implemented, and the results are consistent with the simulations, showing that a high system efficiency of 95.1% with a power factor of 0.98 can be achieved with the GA-based configuration.
A high-efficiency diode-clamped linear amplifier
This paper proposes a new power converter without any switching operation and which works as a linear amplifier. The main circuit of the proposed converter consists of series-connected MOSFETs, series-multi DC power supplies, and clamping diodes. The circuit configuration is similar to a diode-clamped multi-level inverter, except for using complementary power devices, which are n- and p-channel MOSFETs. One of the series-connected MOSFETs operates in an active state just like a linear amplifier, while the other MOSFETs operate in on or off states like an inverter circuit. As a result, the proposed converter achieves an acceptable efficiency as high as 90% without any ripples or harmonics caused by switching operation. Experimental results demonstrate that the proposed converter has the capability to drive a 2.2-kW three-phase induction motor. © 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(6): 47–56, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10120