Miguel Rodriguez

Architectures and Control of Submodule Integrated DC–DC Converters for Photovoltaic Applications

This paper describes photovoltaic (PV) module architectures with parallel-connected submodule-integrated dc-dc converters (subMICs) that improve efficiency of energy capture in the presence of partial shading or other mismatch conditions. The subMICs are bidirectional isolated dc-dc converters capable of injecting or subtracting currents to balance the module substring voltages. When no mismatches are present, the subMICs are simply shut down, resulting in zero insertion losses. It is shown that the objective of minimum subMIC power processing can be solved as a linear programming problem. A simple close-to-optimal distributed control approach is presented that allows autonomous subMIC control without the need for a central controller or any communication among the subMICs. Furthermore, the proposed control approach is well suited for an isolated-port architecture, which yields additional practical advantages including reduced subMIC power and voltage ratings. The architectures and the control approach are validated by simulations and experimental results using three bidirectional flyback subMICs attached to a standard 180-W, 72-cell PV module, yielding greater than 98% module-level power processing efficiency for a mismatch less than 25%.

High-Frequency PWM Buck Converters Using GaN-on-SiC HEMTs

GaN high electron mobility transistors (HEMTs) are well suited for high-frequency operation due to their lower on resistance and device capacitance compared with traditional silicon devices. When grown on silicon carbide, GaN HEMTs can also achieve very high power density due to the enhanced power handling capabilities of the substrate. As a result, GaN-on-SiC HEMTs are increasingly popular in radio-frequency power amplifiers, and applications as switches in high-frequency power electronics are of high interest. This paper explores the use of GaN-on-SiC HEMTs in conventional pulse-width modulated switched-mode power converters targeting switching frequencies in the tens of megahertz range. Device sizing and efficiency limits of this technology are analyzed, and design principles and guidelines are given to exploit the capabilities of the devices. The results are presented for discrete-device and integrated implementations of a synchronous Buck converter, providing more than 10-W output power supplied from up to 40 V with efficiencies greater than 95% when operated at 10 MHz, and greater than 90% at switching frequencies up to 40 MHz. As a practical application of this technology, the converter is used to accurately track a 3-MHz bandwidth communication envelope signal with 92% efficiency.

A Multiple-Input Digitally Controlled Buck Converter for Envelope Tracking Applications in Radiofrequency Power Amplifiers

Wireless communication transmitters have very low efficiencies due to the use of linear radiofrequency power amplifiers. Several techniques have been proposed over the years to improve the efficiency of these systems. One of the most promising is called the envelope tracking technique, which is based on using a fast switching mode power supply to provide a varying voltage to the power amplifier that tracks the envelope of the transmitted signal. The amplifier can, thus, operate continuously near its theoretical maximum efficiency, greatly improving the overall efficiency of the communication system. This paper proposes a multilevel digitally controlled power supply suitable for this application. It is shown to perform very well, achieving very high efficiency, high-output power capability and tracking bandwidths above 50 kHz. This paper also shows that the proposed system is able to produce a 15% overall increase in efficiency in a complete envelope tracking system.

An Insight into the Switching Process of Power MOSFETs: An Improved Analytical Losses Model

The piecewise linear model has traditionally been used to calculate switching losses in switching mode power supplies due to its simplicity and good performance. However, the use of the latest low voltage power MOSFET generations and the continuously increasing range of switching frequencies have made it necessary to review this model to account for the parasitic inductances that it does not include. This paper presents a complete analytical switching loss model for power MOSFETs in low voltage switching converters that includes the most relevant parasitic elements. It clarifies the switching process, providing information about how these parasitics, especially the inductances, determine switching losses and hence the final converter efficiency. The analysis presented in this paper yields two different types of possible switching situations: capacitance-limited switching and inductance-limited switching. This paper shows that, while the piecewise linear model may be applied in the former, the proposed model is more accurate for the latter. Carefully-obtained experimental results, described in detail, support the analytical results presented.

Performance of Power-Limited Differential Power Processing Architectures in Mismatched PV Systems

Differential power processing (DPP) architectures employ distributed, low power processing, submodule-integrated converters to mitigate mismatches in photovoltaic (PV) power systems, while introducing no insertion losses. This paper evaluates the effects of the simple voltage-balancing DPP control approach on the submodule-level maximum power point (MPP) efficiency. It is shown that the submodule MPP efficiency of voltage-balancing DPP converters exceeds 98% in the presence of worst-case MPP voltage variations due to irradiance or temperature mismatches. Furthermore, the effects of reduced converter power rating in the isolated-port DPP architecture are investigated by long-term, high-granularity simulations of five representative PV system scenarios. For partially shaded systems, it is shown that the isolated-port DPP architecture offers about two times larger energy yield improvements compared to full power processing (FPP) module-level converters, and that it outperforms module-level FPP approaches even when the power rating of DPP converters is only 20-30% of the PV system peak power. In the cases of aging-related mismatches, more than 90% of the energy yield improvements are obtained with DPP converters rated at only 10% of the PV peak power.

Average Inductor Current Sensor for Digitally Controlled Switched-Mode Power Supplies

Current-mode control in digitally controlled switched-mode power supplies typically requires analog-to-digital (A/D) conversion of at least two signals, voltage, and current. The complexity of voltage A/D converters can be reduced using window A/D techniques. In conventional current A/D conversion, however, relatively high resolution is required over a wide range of signals, which results in increased complexity, power consumption, and cost of the controller. This paper proposes a very simple feedback sensor capable of high-resolution average inductor current sensing using two analog comparators and an analog low-pass filter. The approach requires very few external components and employs minimal digital hardware resources. A dynamic model and performance of the average inductor current sensor are experimentally verified on a 12-V input, 19-V output, 50-W boost converter prototype. The applicability of the proposed sensor is demonstrated in a digitally controlled 400-W, 400-V output Boost power factor preregulator.

Multilevel converter for Envelope Tracking in RF power amplifiers

Envelope Tracking techniques are used to increase the efficiency of modern radiofrequency transmitters. These techniques are based on varying the supply voltage of the amplifiers according to the envelope of the signal being transmitted, thus maximizing their efficiency. Converters suitable for Envelope Tracking applications must provide high tracking bandwidths, along with low output voltage ripple. This paper presents a power supply for Envelope Tracking applications based on a multilevel topology: it targets relatively low voltage systems (12–28 volt range) and medium to high power applications (≫ 50 W peak power). It uses several input voltages to generate a multilevel square waveform which can be easily filtered; it achieves low output voltage ripple, high tracking bandwidths, high efficiency and high output power capabilities. This paper first analyzes its steady-state behaviour; then, a design procedure is provided, along with guidelines for appropriate implementation of the proposed system. A three-level prototype was built to test the system, and its performance is shown in the experimental results section. Finally, conclusions are stated.

A Linear Assisted DC/DC Converter for Envelope Tracking and Envelope Elimination and Restoration Applications

In recent years, there has been a great deal of focus on the development of fast and efficient dc/dc converters in an effort to follow the envelope of communication signals for increasing the efficiency of RF power amplifiers by using techniques such as envelope tracking and envelope elimination and restoration. However, the bandwidth and slew rate required by modern communication signals are higher than the maximum ones achievable by switching dc/dc converters made for this purpose. The slew rate of these switching dc/dc converters can be improved by combining it with the use of a linear stage, thus establishing a trade-off between slew rate and overall efficiency. This paper presents a simple way of combining the use of both a switching dc/dc converter, the multiple input buck converter, and a linear stage to obtain slew rates above 100 V/μs, with a moderate decrease in the overall efficiency [81% efficiency tracking the envelope of an enhanced data rates for GSM evolution (EDGE) standard signal]. Finally, a prototype of the complete system (including the digital control for both stages) is presented in this paper.

Simplified Voltage-Sag Filler for Line-Interactive Uninterruptible Power Supplies

There are three types of static uninterruptible power supplies (UPSs): passive standby, line interactive, and double conversion. The last one protects the load against all types of line disturbances, but it is the most expensive and the one with the lowest efficiency. On the other hand, passive-standby and line-interactive UPSs have higher efficiency and lower cost, but they show an important drawback: a switching time from normal to stored-energy mode. As a consequence, there is a notch in the UPS output voltage during this switching time. In a previous paper, the authors proposed a method for filling these voltage notches with a sinusoidal waveform generated by a switch-mode converter. In this one, a simplified notch filler is proposed. It consists of two capacitors, one charged with positive voltage and the other with negative voltage. If the fault occurs in the positive period, the positive-charged capacitor is connected to the load. This connection is then modulated in order to obtain a sinusoidal waveform at the load. In the negative period, the other capacitor is used in the same way.

100 MHz, 20 V, 90% efficient synchronous buck converter with integrated gate driver

This paper describes a synchronous buck converter based on a GaN-on-SiC integrated circuit, which includes a halfbridge power stage, as well as a modified active pull-up gate driver stage. The integrated modified active pull-up driver takes advantage of depletion-mode device characteristics to achieve fast switching with low power consumption. Design principles and results are presented for a synchronous buck converter prototype operating at 100 MHz switching frequency, delivering up to 7 W from 20 V input voltage. Measured power-stage efficiency peaks above 91%, and remains above 85% over a wide range of operating conditions. Experimental results show that the converter has the ability to accurately track a 20 MHz bandwidth LTE envelope signal with 83.7% efficiency.