Sebastian Strache

A Comprehensive, Quantitative Comparison of Inverter Architectures for Various PV Systems, PV Cells, and Irradiance Profiles

This paper compares the performance of state-of-the-art inverter architectures for photovoltaic (PV) systems, such as string inverters, power optimizers, or micro inverters, to one another for different locations and PV applications. Since inhomogeneous irradiance patterns significantly influence the performance of PV systems, this study explicitly considers partial shading effects and weather conditions. To improve output energy at shaded locations, an inverter topology applying converters on a submodule level is proposed and evaluated. A MATLAB/Simulink model on the cell level is employed to compare the energy output of different scenarios for 30 years of operation. Additionally, cell mismatch and degradation are considered. Finally, the paper discusses the advantages and drawbacks of each inverter topology regarding different PV applications based on the simulation results. It could be shown that PV systems employing a small number of solar cells (SCs) connected in series feature an enhanced energy output, even for locations without any shading.

Maximum power point tracker for small number of solar cells connected in series

Maximum power point tracking (MPPT) techniques are widely applied in photovoltaic (PV) systems to fully utilize the PV array output power. Major challenges for MPPT are rapidly changing irradiance conditions and partial shading of the PV cells. These issues become even more severe for mobile applications, such as solar cells on the roof of electric vehicles. This paper deals with the implementation of a fast, fully integrated MPPT for a small number of solar cells connected in series with respect to mobile applications. For this special requirements, different open and closed loop MPPTs have been compared and evaluated. The optimal MPPT has hence been implemented. Furthermore, a bypass concept based on MOSFETs instead of bypass diodes is applied to utilize the maximum available power of the solar cells. The MPPT has been built in VHDL and realized in a 150nm CMOS technology. Due to its adaptive step size and its high sampling rate, the MPPT reaches the MPP within less than 0.65 ms with consumes less than 6.2 μW.

Get the LED Out: Experimental Validation of a Capacitor-Free Single-Inductor, Multiple-Output LED Driver Topology

This article presents a new approach for driving multicolor red-green-blue-white ( RGBW) high-current light-emitting diodes (LEDs) for general lighting. The physical advantages in LED applications can be improved by elementary changes in the topology. The presented prototype demonstrator has superior advantages in complexity, form factor, and cost compared to state-of-the-art multicolor LED drivers. A single inductor in parallel to the LED strings is used to supply multiple different LED strings without additional output capacitors. For energy efficiency, the well-known continuous and discontinuous conduction modes (CCM and DCM) are implemented. A novel all-digital current control strategy is used so that no voltage measurements have to be performed, even for light load detection or idle time control in DCM. The irrelevance of any voltage control and the negligence of storage capacitors dramatically reduce the system?s form factor and increase the LED driver reliability. Dimming has been applied with hybrid amplitude and pulsewidth modulation (AM-PWM) control. Thus, color mixing is done by a continuous time integral of the discrete LED light spectra. A discrete prototype demonstrator has been implemented, and further constraints to develop a full integrated circuit (IC) are being evaluated. The discrete demonstrator runs at 12-V dc input voltage with a maximum current of 200 mA to run high-current RGBW LED strings. The peak efficiency of 77.5% is limited by the choice of the applied N-type metal-oxide-semiconductor (NMOS) transistors, which are special and unique on the market for this topology. The high switching frequency of above 1 MHz during CCM ensures a flicker-free color resolution of above 10 b.

Photovoltaic output power improvement applying DC-DC converters on submodule level

This paper investigates the output energy of power optimizers for partial shading conditions and introduces a novel concept for boosting output energy in such situations. A MATLAB/Simulink model on cell level is developed for comparing the output energy of the different circuits to the theoretical maximum. It is shown that for power optimizers even small amounts of shading reduce the output energy significantly. Based on these results, the so called submodule concept is introduced, which overcomes these drawbacks. By integrating ASICs into the PV modules, the number of series connected solar cells is decreased, while additional costs are kept low and high reliability is maintained. The submodule concept provides the largest output energy improvement for low amounts of shading. From an economic point of view this is the most important operating area, since the theoretical harvestable energy decreases with increasing shading. Therefore, the submodule concept is a good and economic solution for increasing the overall energy yield of PV installations.

12.4 A 7.5W-output-power 96%-efficiency capacitor-free single-inductor 4-channel all-digital integrated DC-DC LED driver in a 0.18μm technology

Todays general lighting development is driven by improvements in semiconductor-based systems. It is expected that solid-state lighting (SSL) will dominate general lighting in the near future. Two main challenges that must be met in SSL are the reduction of the bill of materials (BOM), and an increase in functionality. In [1], a floating DC-DC buck controller is presented. This controller adds to the BOM, as every device of the power path is discrete and the ASIC can only drive a single LED string. In contrast to that, [2] offers a high-current fully integrated power stage. However, several external passives are introduced and the technology inhibits stacking multiple LEDs for high luminous efficacy. To overcome this, [3] presents an integrated HV power path with only the inductor as an external component. Ina parallel development, [4] reports an LED driver similar to [3], but that uses a discrete Schottky diode for asynchronous rectification. In fact, [1-4] demonstrate single output LED drivers without additional functionality or full color spectrum. To overcome these drawbacks in light spectrum and control, [5] presents a 3-channel LED driver. However, the external passives are numerous, which significantly impairs the overall BOM.

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 all digital speed adaptive maximum power point tracker for automotive photovoltaic applications

This paper proposes an all digital speed and step size adaptive maximum power point tracker (MPPT) for photovoltaic (PV) power generation, supporting the power supply of vehicles. To harvest significant amounts of energy in this application the MPPT has to deal with fast changing irradiance and severe shading. It has to achieve high tracking speed while maintaining tracking errors below 1%. The presented modified perturb and observe (P&O) MPPT algorithm meets these requirements by applying digital filtering to detect the settling of the input voltage automatically. This enables minimal sampling time for each situation and, hence, very fast convergence to the maximum power point (MPP). The developed algorithm has been implemented on a field-programmable gate array (FPGA) and verified with an experimental setup. It achieves about 9 kHz average switching frequency while maintaining a tracking accuracy above 99.8%.

Advanced digital current prediction for current ripple reduction in DC-DC converters for photovoltaic applications

Photovoltaic applications require very small current and voltage ripples at the output of the solar cells to avoid power losses due to deviations from the maximum power point. Digital control offers optimization potentials to reduce this ripple without increasing the requirements on the analog-to-digital converters or the loop delay. This paper investigates the achievable current ripple reduction for two different digital current prediction algorithms applied to a hysteretic controlled photovoltaic submodule converter. A basic method relying on the continuity of the inductor current and an advanced algorithm, which compensates loop delay have been implemented in VHDL. Both methods are compared in structure and simulation results. The basic current prediction reduces the current ripple by 11 %, whereas the advanced current prediction decreases the ripple in total by 26 %. The whole controller including the advanced current prediction requires less than 0.1mm2 in a 150nm CMOS technology.

A submodular boost converter ASIC for output energy improvements in photovoltaic applications

Photovoltaic (PV) energy harvesting suffers from output energy reduction due to shading, dirt accumulation or manufacturing variability. Typical PV modules consist of 60 solar cells connected in series. Hence, shading a single cell can cause a whole substring of 20 cells to be bypassed. To overcome these drawbacks, this paper proposes the application of boost converters on submodular level, cutting the series connection to only a few cells. System level simulations prove that this architecture features increased output energy compared to state-of-the-art approaches like power optimizers. For reducing component costs and increasing system reliability, an ASIC for this application has been developed in a 150nm CMOS technology. This paper describes the design and the implementation of a highly efficient hysteretic controlled boost converter. Its high switching frequency of more than 500 kHz enables operation on a single phase while achieving an RMS input current ripple of less than 1.5 % required in photovoltaic applications. Measurements prove a RMS current ripple of less than 1 % and a maximum switching frequnecy of 650 kHz.

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.