Lukas Lohaus

A Dimmable LED Driver With Resistive DAC Feedback Control for Adaptive Voltage Regulation

This paper proposes a dimmable energy-efficient light-emitting diode (LED) driver for applications in interior lighting. High efficiency is achieved by an adaptive voltage regulation, which minimizes power losses in the linear current regulator. A digital control mechanism employing a resistive digital-to-analog converter for feeding the analog feedback input of a dc-dc converter is introduced. It is shown that the digital control methodology gives maximum design flexibility and enhances control over regulation speed and stability. In an experimental setup, the proposed concept is verified and evaluated. Operating at an input voltage of 24 V, the LED driver provides a relatively wide output voltage range of 3.5-38 V. Output current is regulated to 700 mA with a steady-state precision of more than 98.8%, whereas pulse width modulation dimming with a frequency of 1 kHz and shortest on-time of 4 μs is employed. A peak efficiency of the complete system of 93.9% is achieved.

A capacitor-free single-inductor multiple-output LED driver

This work presents a promising approach for driving multi-color RGBW high current LEDs for general lighting. Based on a DC input voltage, the applied topology correlates on a DC-DC flyback converter. Physical advantages in LED applications compared to resistive electrical load open room for elementary changes in the converter topology, which makes the presented work superior to state-of-the-art LED drivers in energy-efficiency, complexity, form-factor and cost. The presented capacitor-free single-inductor multiple-output LED driver offers multiple output voltages in order to gain high efficiency. Besides that, a single inductor is used to supply multiple outputs, without the necessity of capacitors. The presented work has been developed in a 0.18 μm technology. With a total output power of 6.5 W, the proposed LED driver light output is up to 600 lm, which is comparable to a standard 60 W light-bulb. The light output is related to a modern Retrofit LED lamp, but also has the enhancement of creating dimmable and tunable white colors.

An efficient dual-stage power supply topology for series connected control devices in intelligent lighting systems

A typical problem in modern intelligent lighting systems is the power supply of smart control devices replacing conventional, series connected light switches or dimmers. This paper proposes a two-stage power supply concept that overcomes the involved issues and compares it to a recently published work [1]. Both concepts can operate without a neutral wire connection at the light switch lead out. The power supply shown in this work consists of a flyback converter as the input stage and a buck converter as the output stage, whereas the concept presented in [1] works with a boost converter as the first stage and a flyback converter as the second stage. The comparison of these two power supply topologies relies on measurement data and evaluates both concepts regarding efficiency, maximum output power, complexity and robustness. Both power supplies are directly attached to the utility grid and hence cope with a high voltage ratio from 230 VRMS alternating current (AC) mains to a constant direct current (DC) output voltage of 3.3 V. For the comparison the power supplies are exemplarily examined in compliance with the Digital Load-Side Transmission (DLT) standard [2]. It is shown that both concepts can handle these highly discontinuous input voltages while providing a maximum output power high enough to operate even power hungry devices, such as liquid crystal display (LCD) backlights or a WiFi module.

A comparison of receiver topologies for Digital Load-Side Transmission in general LED lighting

Digital Load-Side Transmission (DLT) is a two-wire bound power line communication methodology that has been specifically developed for general lighting applications and transmits data over mains. This paper proposes three DLT receiver concepts and compares them to one other regarding robustness, complexity and power consumption. All of these detectors have been realized and tested in hardware. To evaluate robustness in terms of utility grid variations, 24-hour long term measurements have been taken for each concept, when connected to 50 Hz mains. A telegram error rate (TER) has been measured for all setups, while the power consumption of the implemented receivers has been determined by Simulation Program with Integrated Circuit Emphasis (SPICE) simulations. A fully digital approach and an active analog bandpass based structure seem to be the most promising solutions as they feature comparable TERs of 44 and 80 parts per million (ppm), respectively. All presented topologies can be monolithically integrated on a chip to facilitate development of highly integrated, intelligent LED drivers.

Advanced color control for multicolor LED illumination systems with parametric optimization

LED based luminaires are capable of providing innovative functionalities, such as color control and color temperature adaption. In fact, precise control of these two parameters has turned out to be very complex and challenging. This paper presents an advanced control technique for a very accurate and robust adjustment of both color and color temperature. Additionally, the proposed methodology enables system optimization of a LED based luminaire regarding various parameters, e.g. maximum color rendering index (CRI). The control signals for any PWM-switched LED driver can be tuned accordingly by the system controller. The algorithm is tested in simulations for three different luminaire realizations, namely RGB-W, RGB-A and RGB-CAW. The CIEDE2000 color difference formula is applied for computing the perceivable color difference ΔE by the human eye. The developed color control algorithm achieves a superior minimization of the absolute color distance to ΔE ≤ 0.21, which is not even perceivable by the human eye. This performance can be realized over a wide color temperature range from 3200K to 7500 K while luminous output is kept constant. Due to the applied optimization, a CRI of ≥ 90 can be maintained over the same color temperature range for all three scenarios. Simultaneously, the deviation of correlated color temperature ΔCCT is minimized to less than 50 K, which is a precision of more than 98 %. The novel algorithm considers strict hardware limitations, such as finite switching speed or limited computation resolution, to account for implementations on an ASIC or a μC.

A power supply topology operating at highly discontinuous input voltages for two-wire connected control devices in Digital Load-Side Transmission (DLT) systems for intelligent lighting

This paper presents a power supply concept for control devices in Digital Load-Side Transmission (DLT) systems delivering a high energy output. The proposed topology consists of two stages, a boost converter and a flyback converter. As a direct replacement for conventional light switches or dimmers, this power supply works without a neutral wire connection and is supplied with energy compliant to the DLT specification [1] through a switched bypass structure in the series connected DLT luminaire. This method enables the application of DLT systems in existing installations, which typically lack a neutral wire at the light switch lead-out. Although operating at a highly discontinuous and varying input voltage, the proposed two-stage topology provides a continuous high output power of maximum Pout,max = 475 mW. Input voltages of up to 100 V can be handled. The output voltage is regulated to 3.3 V, a typical value to supply microcontrollers with attached periphery, e.g. common WiFi modules [2], [3]. It is shown that the measured output power is high enough to operate a WiFi frontend. Completely new functionality, such as integrating a DLT control device into a smart home infrastructure, is hence realized.

Multiple input, single output, single inductor DC-DC converter architecture providing charge reuse by an efficient high voltage current sink

Integrated gate-drivers for power MOSFETs require multiple supply voltages biasing internal circuitry or providing a high voltage ground to the high-side driver. This paper proposes a novel DC-DC converter architecture fulfilling these demands of modern gate-driver ASICs while increasing system efficiency. In the proposed concept a conventional single inductor DC-DC converter generating a low voltage supply to auxiliary circuits from a high voltage input is extended by using the high-voltage ground of the gatedriver as an additional input to the converter. This input voltage is controlled to remain nearly constant by adapting its load current with an attached current sink. The charge drained from the high-voltage ground by this current sink is reused to boost system efficiency of the converter. The developed concept is also applicable to converters operating in higher power regimes.

A low power high-side current sense SAR ADC for automotive applications

Direct current measurement is one of the most important control characteristics for DC-DC converters. This sensitive method is used for current control of converters, which is much faster and more accurate than voltage control. Unfortunately, the sensitivity of the current measurement is also its major drawback. Especially in automotive applications, electromagnetic interference of the increasing number of electrical appliances severely degrade the performance of analog signal processing. Having proven to be more robust and error-prone, digital signal processing has to be preferred. This work presents a universal high-side current measurement at automotive battery voltage levels up to 24 V with direct analog-to-digital (A/D) conversion in a 180 nm technology. The A/D converter has been realized as 9 bit successive-approximation (SAR) ADC, with a sampling rate of 10 MS/s to detect DC-DC converter switching frequencies above 1 MHz. The combined circuit area of 0.2 mm2 dissipated less than 2 mA at 1.8 V supply voltage. Simulation results of the extracted layout reflect the transient waveforms of a DC-DC LED driver during continuous and discontinuous conduction mode.