Jason T. Stauth

Resonant Switched-Capacitor Converters for Sub-module Distributed Photovoltaic Power Management

This paper discusses the theory and implementation of a class of distributed power converters for photovoltaic (PV) energy optimization. Resonant switched-capacitor converters are configured in parallel with strings of PV cells at the sub-module level to improve energy capture in the event of shading or mismatch. The converters operate in a parallel-ladder architecture, enforcing voltage ratios among strings of cells at terminals normally connected to bypass diodes. The balancing function extends from the sub-module level to the entire series string through a dual-core cable and connector. The parallel configuration allows converters to handle only mismatch power and turn off if there is no mismatch in the array. Measurement results demonstrate insertion loss below 0.1% and effective conversion efficiency above 99% for short-circuit current mismatch gradients up to 40%. The circuit implementation eliminates large power magnetic components, achieving a vertical footprint less than 6 mm. The merits of a resonant topology are compared to a switched-capacitor topology.

A Resonant Switched-Capacitor IC and Embedded System for Sub-Module Photovoltaic Power Management

The viability of grid-connected photovoltaic (PV) energy has improved dramatically in recent years: large increases in manufacturing capacity have driven reductions in cost and higher efficiencies, improving lifetime cost of energy (LCOE). Mismatch loss remains an important consideration in PV systems and a range of power electronic solutions have been proposed to recover losses due to shading, dust/debris, factory mismatch and aging. This paper presents a high-voltage CMOS IC and embedded system based on a resonant switched-capacitor converter. The solution is integrated into the junction box to balance power flow in parallel with sub-module strings of PV cells. A custom dual-core cable and connector extend the balancing function to multiple PV modules connected in series, improving energy production of large-scale PV arrays in the case of shading or mismatch. The converter is based on a resonant switched-capacitor (ReSC) topology that achieves effective conversion efficiency over 99% for a wide range of mismatch, insertion loss below 0.1%, a vertical footprint less than 6 mm, and weight less than 1 Oz.

Resonant-Switched Capacitor Converters for Chip-Scale Power Delivery: Design and Implementation

There is an increasing need for power management systems that can be fully integrated in silicon to reduce cost and form factor in mobile applications, and provide point-of-load voltage regulation for high-performance digital systems. Switched-capacitor (SC) converters have shown promise in this regard due to relatively high energy-density of capacitors and favorable device utilization figures of merit. Resonant switched-capacitor (ReSC) converters show similar promise as they benefit from many of the same architectures and scaling trends, but also from ongoing improvements in mm-scale magnetic devices. In this study, we explore the design and optimization of 2:1 step-down topologies, based on representative capacitor technologies, CMOS device parameters, and air-core inductor models. We compare the SC approach to the ReSC approach in terms of efficiency and power density. Finally, a chip-scale ReSC converter is presented that can deliver over 4 W at 0.6 W/mm2 with 85% efficiency. The two-phase, nominally 2:1 converter supports input voltages from 3.6-6.0 V, and is implemented in 180-nm bulk CMOS with die-attached air-core solenoid inductors.

Partial-Shading Assessment of Photovoltaic Installations via Module-Level Monitoring

Distributed maximum power point tracking (DMPPT) is a topic of much interest in improving photovoltaic (PV) system performance. This study uses measured performance data at the module level for 542 PV systems to estimate lost system performance due to partial shade. Because each of the monitored systems is equipped with module-level dc power optimizers, an estimate is made of the overall system shading loss and the performance improvement that the system has received from this use of DMPPT. The estimate of shade extent and performance improvement predicted by this approach is verified experimentally against a system that has site survey images, and measured production with and without module-level electronics. Summary data for this analysis across 542 systems find an average power loss of 8.3% due to partial shading, which would have increased to 13% were the systems not equipped with panel-level optimizers. It is estimated that on average, 36% of the power lost from partial shading has been recovered through use of module-level dc power electronics.

A high-voltage CMOS IC and embedded system for distributed photovoltaic energy optimization with over 99% effective conversion efficiency and insertion loss below 0.1%

Solar photovoltaic (PV) energy has increased in importance in recent years as a viable alternative to carbon-producing sources of energy. In an effort to drive PV energy towards grid parity, there is a need to improve the power electronics and architecture for grid-connected systems. Traditional PV systems use a central inverter to manage multiple strings of series-connected PV modules. With mismatch among the PV cells, the energy production of the array suffers in several ways: 1) in series strings, current is limited to the lowest-performing cell in the string, 2) if current is forced to exceed this level, external bypass diodes need to turn on throwing away power available in the string and incurring conductive losses, 3) with bypass diodes on, total string voltage may deviate from maximum power voltage (Vmpp), reducing energy production of all modules in the string.

20.2 A variable-conversion-ratio 3-phase resonant switched capacitor converter with 85% efficiency at 0.91W/mm 2 using 1.1nH PCB-trace inductors

Switched-capacitor (SC) converters have shown significant promise for monolithic integration in a variety of mobile computing applications due to the relatively high energy-densities of modern capacitor technologies and the emergence of deep-trench technology [1-4]. Compared to more traditional buck and boost topologies, the SC approach provides better utilization of active and passive components, and is especially favorable when using submicron or deep-submicron CMOS technology because low-voltage devices can be configured in cascaded or hierarchical structures to interface across wide conversion ratios [5].

Resonant and multi-mode operation of flying capacitor multi-level DC-DC converters

Multi-level converter architectures have been explored for a variety of applications including high-power DC-AC inverters and DC-DC converters. In this work, we explore flying-capacitor multi-level (FCML) DC-DC topologies as a class of hybrid switched-capacitor/inductive converter. Compared to other candidate architectures in this area (e.g. Series-Parallel, Dickson), FCML converters have notable advantages such as the use of single-rated low-voltage switches, potentially lower switching loss, lower passive component volume, and enable regulation across the full VDD-VOUT range. It is shown that multimode operation, including previously published resonant and dynamic off-time modulation, form a single set of techniques that can be used to extend high efficiency over a wide power density range. Some of the general operating considerations of FCML converters, such as the challenge of maintaining voltage balance on flying capacitors, are shown to be of equal concern in other soft-switched SC converter topologies. Experimental verification from a 24V:12V, 3-level converter is presented to show multimode operation with a nominally 2:1 topology. A second 50V:7V 4-level FCML converter demonstrates operation with variable regulation. A method is presented to balance flying capacitor voltages through low frequency closed-loop feedback.

Optimum Biasing for Parallel Hybrid Switching-Linear Regulators

Hybrid combinations of switching and linear regulators have been proposed for both audio amplifiers and dynamic supply modulators for radio frequency (RF) power amplifiers (PAs). Such topologies may provide benefits in terms of efficiency, dynamic range, and speed of dynamic response compared to pure linear regulators or class-D switching amplifiers. This paper presents a framework for analyzing the bias constraints of switching and linear voltage regulators operated in a parallel-hybrid configuration. Particular emphasis is given to polar and envelope tracking RF power amplifier (RF PA) applications. Ideal expressions are derived for the optimum current contribution of the switching regulator under quasi-static operating conditions. In contrast to previous work, it is shown that the optimum mean current contribution of the switching regulator is not necessarily the dc current to the load. Explicit expressions for theoretical maximum efficiency are derived for envelope waveforms that result from two-tone and sinusoidal amplitude modulation of the RF carrier; IS-95 CDMA and IEEE 802.11a/g wireless LAN envelope waveforms are treated in simulation and experiment. Theoretical predictions are validated with measured results.

Multilevel Power Point Tracking for Partial Power Processing Photovoltaic Converters

This paper presents a multilevel control and maximum power point tracking (MPPT) scheme for variable conversion ratio partial power processing photovoltaic (PV) dc-dc converters. A general system model is derived and linearized in state-space form for a switched-inductor (buck-boost dc-dc) converter with an arbitrary number of series-connected stages. The proposed control law is then used as the basis for a multilevel MPPT scheme that can optimize many series-connected PV cells, substrings, or modules simultaneously. The MPPT algorithm is shown to converge in a timeframe independent of the number of PV units and can work in concert with the central inverter MPPT algorithm without conflict. To validate the control and optimization scheme, we present a hardware prototype designed to fit in the junction box of conventional PV modules, operating at the submodule level. While the control law is developed specifically for the buck-boost topology, the MPPT algorithm is generalized and could be applied to many partial-power processing topologies provided there is capability to regulate the voltage differences or voltage ratios among adjacent PV stages.

A coupled-inductor multi-level ladder converter for sub-module PV power management

The viability of solar photovoltaic energy has increased in recent years due to continuing efficiency improvements and cost reductions. However, there remains a need for improvements in power electronic circuits and architectures, especially to deal with sources of mismatch loss in real-world environments. This work presents a circuit implementation and multi-objective control scheme for a four-level DC-DC converter that provides sub-module energy optimization for photovoltaic systems. The work builds on past approaches using switched-inductor (SL) topologies that manage power flow in parallel with series-connected PV strings. We describe the use of coupled-magnetics to reduce current ripple and improve efficiency compared to past SL approaches. The converter works by enforcing voltage ratios among adjacent PV sub-strings, allowing independent sub-module maximum power point tracking (MPPT). A state-space model of the switched-inductor topology is presented to provide a foundation for a PI control scheme. Circuit simulations are compared to measurement results for a four-stage prototype integrated in the junction-box of a 245 Wp PV module.