Series-connected LEDs Research Articles

High Voltage, Low Current High-Power Multichannel LEDs LLC Driver by Stacking Single-Ended Rectifiers with Balancing Capacitors

In this paper, a new LLC converter for series-connected, high-voltage LEDs is proposed. The proposed LLC converter consists of two stacked, single-ended rectifiers and one balancing capacitor, to compensate for the current deviation of two individual LED strings. The proposed LLC LED driver can use a diode with low voltage stress, even if the secondary LED is connected in series to have a high driving voltage. In addition, even if several series-connected LEDs are changed into two-stacked structures, the balancing capacitor can compensate for the current deviation of the two separated LEDs, as well as the difference in leakage inductance of the two stacked single-ended rectifiers. The balancing capacitor can be made equal to the voltage tolerance of the stacked, single-ended rectifier diodes. The proposed circuit can be easily extended to a series channel LED driver circuit, without increasing the voltage stress. To verify the characteristics and operation of the proposed LLC LED driver, a 260W high-power LED driver is implemented.

A 2.8-MHz 96.1%-Peak-Efficiency 1.4-μs-Settling-Time Fully Soft-Switched LED Driver With 0.08–1 Dimming Range

Present day LED drivers demand both high power efficiency and wide dimming range. However, these two parameters are often a tradeoff, where a wide dimming range requires a high switching frequency, hence the ensuing switching losses. In this paper, we present an LED driver that features both high power efficiency and wide dimming range by means of fully soft switching that substantially eliminates the switching losses, i.e., the switching losses are negligible. We achieve the said fully soft switching by our proposed hysteretic soft-switching controller (HSSC) and the proposed circuits—voltage detector, current sensor, and level shifter. The HSSC enables the fully soft switching, including zero-voltage switching and zero-current switching, by means of turning on / off power switches when their voltage/current is zero. The proposed voltage detector and current sensor, featuring low power, monitor the voltage and current of the power switches. The proposed level shifter, featuring ultra-fast speed, serves to quickly transmit the monitored signal from the said voltage detector and current sensor to the HSSC. The prototype LED driver, realized in a 130-nm BCDLite process, features 6–18 V input voltage range, output current of 0.5–1.3 A, drives 1–3 series-connected LEDs, 12-W maximum output power, 2.8-MHz maximum switching frequency, 96.1% peak power efficiency, 1.4-μs settling time, and 0.08–1 dimming range at 20-kHz dimming frequency. When benchmarked against state-of-the-art LED drivers, our design simultaneously features the widest dimming range, shortest settling time, and requires the smallest inductor. The measurements and benchmark depict the decoupling of the said tradeoff.

A Current-Accuracy-Enhanced Wide-Input-Range DC-DC LED Driver With Feedforward Synchronous Current Control

A wide-input-range (5–115 V) current-accuracy-enhanced dc–dc LED driver is presented in this paper. A feedforward synchronous current control (FSCC) scheme is developed to improve the LED current accuracy over wide ranges of input and output voltages. In the FSCC, a reconfigurable valley current sensor is proposed to provide accurate positive and negative valley inductor current information for the controller corresponding to hard- and soft-switching modes, respectively. A dynamic peak reference generator is developed to automatically adjust the peak current reference value dependent on input and output voltages for reducing the deviations of the average LED current under different input voltages and numbers of output LED, thereby improving the current accuracy. Implemented in a 0.5- $\mu \text{m}$ 120-V CMOS process, the experimental results show that the proposed LED driver IC achieves a current accuracy within ±6.0% of the average LED current of 350 mA over an input voltage range from 5 to 115 V and numbers of series-connected LEDs from 1 to 25 by using only a small inductor of 10 $\mu \text{H}$ . Compared with the state-of-the-art synchronous current control scheme, the variations of the LED current can be reduced by 2.3 and 4.2 times in the hard- and soft-switching modes, respectively, with the proposed LED driver using the FSCC.

Study on Passive Self-Equalization of Parallel-Connected LED Strings

Parallel connection of several LED strings is a practical way to reduce the overall lamp voltage when a large number of emitters are needed. However, this connection might lead to high current mismatches between strings due to the dispersion of characteristics that even binned devices feature. Several techniques have been proposed in order to eliminate or, at least, reduce the forward current mismatch between paralleled strings. Provided that a cost-effective design for commercial lighting is pursued, passive self-equalization of LED strings will be studied in this paper, where the dynamic resistance will be considered as the equalizing resistor. Thus, a statistical study will first be developed on four commercial devices in order to determine the forward voltage deviation in both binned and unbinned LEDs. Then, the dynamic resistance will be accurately determined for the operating current in a wide temperature range by the procedure followed in previous works, since this parasitic effect will be employed as the equalizing resistor. The I-V and R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> -T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> curves will thus be experimentally obtained for a large sample. Once the thermal and statistical characterizations are done, the study of the LED linear model will be performed in order to determine the impact on forward current mismatches that forward voltage and dynamic resistance variations imply in both binned and unbinned LEDs. Finally, experimental results will show that binned LEDs are suitable for self-equalization, even for a tight current-mismatch tolerance, particularly for a significant number of series-connected LEDs in each string.

Design of high-performance integrated dimmable LED driver for high-brightness solid-state lighting applications

This paper presents a high-voltage buck-type dimmable LED driver for high-brightness solid-state lighting applications. A synchronous hysteretic current control (SHCC) scheme is developed to regulate the LED current with high accuracy under different input voltages and various numbers of output LEDs. With a low-power high-speed dynamic level shifter, the proposed HV gate driver achieves short propagation delays and enables high-frequency operation of the high-voltage SHCC LED driver with an on-chip high-side power pFET. Both peak and valley current sensors are also proposed for high-speed LED current monitoring. Without any power diode and sensing resistor in the power stage, the power efficiency of the proposed LED driver can be enhanced significantly. Implemented in a 0.35-µm HV-CMOS process, the proposed driver can take a high input voltage up to 50 V and deliver an average LED current of 1 A for 12 series-connected output LEDs with a maximum output power of about 46 W. When the input voltage changes from 15 to 50 V and the number of series-connected LEDs varies from 1 to 12, the proposed driver achieves peak power efficiency of 98.8 % and the average LED current error of <1.2 %. The proposed LED driver supports high operation frequency of up to 2.2 MHz and high PWM dimming frequency of 20 kHz with a wide duty ratio from 10 to 100 %. The proposed SHCC LED driver achieves significant improvements in power efficiency and current accuracy over state-of-the-art counterparts.