Current Equalization in Photovoltaic Strings With Module Integrated Ground-Isolated Switched Capacitor DC–DC Converters

Switched Capacitor (SC) DC-DC converters are transformer and inductor less DC-DC converters that are used to step up, step down or invert a supply voltage where the power requirements are usually less than 5W. The outputs of the currently available SC converters are referenced with respect to the input supply ground. Though SC converters are ideally suited for many low power miniaturized industrial applications due to their small size, lack of ground isolation in SC converters prevents their use in low power applications that require ground isolation due to ground noise sensitivity of the load. The work presented in this paper introduces the concept of ground isolation in a buck SC converter without the use of isolation transformers like conventional DC-DC converters. Switched capacitors driven by opto-isolated drives are employed to transfer power from the source to the load. The introduction of ground isolation in SC converters significantly widens its scope of application in industry. A state space averaged (SSA) model of the SC converter is developed to study its dynamic and steady state characteristics. The validity of the SSA model is verified with measurements on the hardware model.

Traditionally, switched-capacitor (SC) converters have suffered from high transient currents, which limit both the efficiency and power density of such converters. Soft-charging operation can be employed to eliminate the current transients and greatly improve the power density of SC converters. In this approach, a second-stage magnetic converter is cascaded with the SC stage to act as a controlled current load. Another approach is to use resonant SC converters with zero-current switching. This paper shows that resonant and soft-charging operations of SC converters are closely related, and a technique will be proposed, which achieves either operation by adding a single inductor to existing SC topologies. In addition, since most preexisting resonant or soft-charging SC converters were devised in an ad-hoc manner, this paper formulates an analytical method that can determine whether an existing conventional SC converter topology is compatible with the proposed approach. A number of common SC topologies are analyzed, including Dickson, series-parallel, ladder, Fibonacci, and doubler configurations. Through comparison to simulated results, as well as experimental work, the proposed method is validated and a family of high-performance SC converters is obtained.

A cooling technique for photovoltaic (PV) string based on jet impingement device is proposed in this study to eliminate non-uniform cooling across the surface of the PV panel. Non- uniformity on the PV panel arises due to the uneven solar flux distribution and temperature variations across the cell. This study identifies water jet impingement cooling as a promising solution to provide uniform cooling of PV panels for the climatic data of Middle Eastern region. The complete cooling model incorporates optical, radiation, thermal, electrical and jet cooling model for the overall performance analysis of the PV system. Jet impingement cooling model for PV string is developed in this paper to carry out the heat transfer analysis for single and multiple jet nozzles to predict the cell temperature, power output and conversion efficiency of PV strings. Detailed thermal model based on energy balance approach is also developed to assess the effect of cell temperature on the PV string using average heat transfer correlations. Finally, a comparative analysis is carried out for rectangular channel heat exchanger and jet cooling of PV strings subjected to the conditions of Dhahran. The results reveal that the jet cooling has higher efficiency, minimum cell temperature and temperature uniformity.

Switched capacitor (SC) dc–dc converters, which have traditionally been used for on-chip power supplies, are now being considered for medium to high-power applications. This paper presents a method for automatically synthesizing SC converters as well as deriving the expressions for their charge-multipliers. Charge multipliers can be used to calculate important design characteristics such as converter output voltage regulation, efficiency, and component current ratings, which can then be used to appraise different topologies. However, whilst a full appraisal of converters also requires component voltage ratings, this work is the first step in developing an automatic software tool that will employ a search-based algorithm to generate optimum SC topologies for a given application. The method is based on the proposition that all SC converters can be synthesized from a so-called “basic cell”. The automatic derivation and solution of the charge transfer equations for a traditional Fibonacci SC converter is presented as an example of the proposed method, which is validated against existing analytic equations as well as a detailed Spice simulation. The automatic calculation of the charge-multipliers for two other well-known SC converters is also demonstrated, including a new, arbitrarily generated circuit, which again is validated against detailed spice simulations.

Hybrid and resonant switched-capacitor (SC) converters have the potential to achieve higher power densities than conventional dc-dc converters. This work presents an interleaved, 20 V to 120 V, resonant SC converter which could be used in applications that benefit from high power density and efficiency. The resonant topology of this SC converter enables a compact and efficient design that minimizes losses. We use a new split-phase control scheme that incorporates precisely timed additional transition states to increase the converter efficiency. A two-phase interleaved design increases the output power and simultaneously decreases the output ripple through harmonic cancellation. In addition to experimental evaluation of a 95.7% efficient, GaN-based prototype of this interleaved converter, an analysis of current sharing between interleaved phases is performed.

Photovoltaic (PV) systems consist of multiple dc-dc converters, including a front-end converter for PV string control, a bidirectional converter for battery regulation, and voltage equalizer to preclude partial-shading issues, and hence they are prone to be complex and costly. To cope with this issue, this paper proposes the PWM- and PFM-controlled switched capacitor converter (SCC)-based multiport converter (MPC) integrating voltage equalizer. The proposed MPC can be derived by integrating a bidirectional PWM converter, series-resonant converter (SRC), and SCC. The PWM converter and SRC regulate battery and load voltages, respectively, while the SCC plays a role of the voltage equalization. Not only is the system as a whole dramatically simplified but also the circuit element count is significantly reduced thanks to the integration. The 150-W prototype for three PV modules was built for verification. The experimental results demonstrated the extractable maximum power from PV modules was significantly improved by the SCC, while the battery and load voltages were individually regulated by the PWM and PFM controls, respectively.

The switched-capacitor (SC) converters are gaining popularity in some practical use. Its resonant counterpart, with a small inductor as current shaper in the architecture, has the advantage of smoother charging current, smaller voltage and current variation ratio and so on. This paper presents a generic and thorough analysis of the efficiency mechanism of the resonant switched-capacitor converter (RSCC). It decomposes any RSCC into three basic sub-circuits of SRLC circuit, CRLC circuit and SRLC² circuit, which are analyzed and compared with corresponding sub-circuits of SCC, respectively. Then the overall efficiency of a RSCC could be derived. Furthermore, a new calculation method of output impedance is developed in another perspective to reveal the high efficiency mechanism of RSCC. On this basis, the high efficiency operation condition for RSCC could be obtained. Finally, a resonant bridge modular switched-capacitor converter (RBMSCC) is proposed and built for example of extended implementation of RSCC to achieve high efficiency operation. Some useful comments for designing resonant switched-capacitor converters are suggested.

This paper investigates the interest focused on employing parallel connected dc-dc converter with high tracking effectiveness under wide variation in environmental conditions (Insolation) and wide load variation. dc-dc converter is an essential part of the stand alone PV system. Paper also presents an approach on how duty cycle for maximum power position (MPP) is adjusted by taking care of varying load conditions and without iterative steps. Synchronized PWM pulses are employed for the converter. High tracking efficiency is achieved with continuous input and inductor current. In this approach, the converter can he utilized in buck as well in boost mode. The PV system simulation was verified and experimental results were in agreement to the presented scheme.

Efficient analysis techniques for complex switched-capacitor (SC) converters are essential design tools for the development of practical SC converters. Techniques that use state-space equations based on conventional circuit analysis methods have proven effective in modeling the practical performance of SC converters. Iterative methods of design based on these analysis techniques require the formulation of many Kirchhoff voltage and current equations, which is time consuming if derived manually. Here, an algorithm is introduced to automate the creating of the matrices required for state-space-based modeling of SC converters. The state equations are generated algorithmically, given a standard node incidence matrix generated from a user-defined netlist. The algorithm enables a designer to quickly iterate SC converter design solutions based on their predicted performance. The resulting models are compared against manually generated models, simulations, and experimental results.

A grid-tied multistring photovoltaic (PV) inverter with a high-frequency ac (HFAC) link, soft-switching operation, and high-frequency (HF) galvanic isolation is introduced. This single-stage topology can handle an arbitrary number of PV strings with different electrical parameters, locations, and orientations. Using a dedicated maximum power point tracker for each PV string, this inverter can harvest the highest possible power from each string independently, even though they might be at dissimilar irradiance levels and operating temperatures. The isolated HFAC link is formed by the magnetizing inductance of a small-sized HF transformer and two small ac capacitors without the use of bulky short-life electrolytic capacitors. The link is responsible for transferring the PV strings' power to the grid along with creating zero-voltage switching for the power devices. Therefore, the converter has the advantages of high power density, high reliability, as well as high efficiency. After describing the control scheme and the operating algorithm of the proposed multistring inverter, a detailed analysis is carried out. The experimental results of the developed 1-kW two-string prototype with different PV strings at various PV conditions are also shown. The results verify the effectiveness of the topology in tracking the maximum power of each string independently without affecting the operating condition and control of the other string.

This paper presents a fully-integrated reconfigurable switched-capacitor (SC) converter that extracts power from a 1. 5-1.SV input voltage and regulates a 0. 3-1.SV output voltage. The converter employs a proposed output-boosting scheme that increases the output voltage of the conventional 3-ratio SC converter power-efficiently. Therefore, the voltage conversion ratios increase from 3 to 14 without the need for additional cascaded SC converter stages. The proposed converter was designed and simulated with a 0. 1S$\mu$m CMOS process and achieved a peak efficiency of 86.7% and a maximum output current of 0.8 mA.

A power management unit (PMU) with battery extender and output load regulation is presented using 0.18$\mu$m CMOS technology. For designing few sub-circuits of the PMU, subthreshold region of MOSFET is utilized, which significantly contributes to achieve low power operation. The proposed PMU consists of a voltage regulator, a voltage monitor, a battery backup module and two DC-DC Switched Capacitor (SC) Converters. The regulator output voltage V OUT is regulated at 0.95 V and 0.968 V at input voltages of 0.98 V and 1.33 V respectively. To ensure continuous operation at the load circuit, a battery extender module is implemented in case of unavailability of EH source. The power consumption of the PMU at input voltages of 0.98 V and 1.33 V is 46nW and 1.567 $\mu$W respectively. Near threshold operation is ensured at the load side by further regulating the voltage at the output to 0.67 V and 0.5 V using 3:2 and 2:1 SC Converters respectively. The current consumption of the 3:2 and 2:1 SC Converters is 748 nA and 751 nA respectively. The proposed PMU can deliver output current up to 1 mA at 1.33 V of input voltage and is suitable for numerous applications where low power operation is essential as it consumes a small amount of power.

The increasing popularity of DVFS (dynamic voltage frequency scaling) schemes for portable low power applications demands highly efficient on-chip DC-DC converters. The primary aim of this work is to enable increased efficiency of on-chip DC-DC conversion for near-threshold operation of multicore chips. The idea is to supply nominal (high) off-chip voltage to the cores which are then “voltage-stacked” to generate the near-threshold (low) voltages based on Kirchhoff’s voltage law through charge recycling. However, the effectiveness of this implicit down-conversion is affected by the current imbalance among the cores. The paper presents a design methodology and optimization strategy for highly efficient charge recycling on-chip regulation using a push-pull switched capacitor (SC) circuit. A dual-boundary hysteretic feedback control circuit has been designed for stacked loads. A stacked-voltage domain with its self-regulation capability combined with a SC converter has shown average efficiency of 78%–93% for 2:1 down-conversion with ILoad (max) of 200 mA and workload imbalance varying from 0–100%.

In this paper, an ultra-high-speed, low-loss gate driver circuit using a CMOS-type class-D oscillator and a Switched Capacitor (SC) converter directly driven by it are proposed and demonstrated. Conventional buffer drivers suffer from large power losses, making it difficult to operate SC converters in the several hundreds of MHz range. The proposed driver circuit achieves low-loss operation in this frequency range by employing an inductor-based constant-current gate drive that self-oscillates utilizing CMOS-type class-D oscillator. This approach enables a significant reduction of the flying capacitor value in the SC converter. The proposed oscillator driver and SC converter are implemented using only 1.0V transistors in a 65nm CMOS process. The oscillator inductors and flying capacitors are all integrated on-chip. The measurement results show that the SC converter can be driven at 480MHz with a driver power loss of 61mW, achieving a maximum efficiency of 60.2% at an output current of 350mA.

The intent of this paper is to analyze the power consumption or heat dissipation of defective bypass diode (BPD) at open circuit (OC) and maximum power point (MPP) load conditions when the BPD turns to have resistive behavior in a photovoltaic (PV) string and array. We also analyze the thermal impact of defective BPD in a PV string under different irradiances and cell temperatures. It was observed that the power consumption of defective BPD largely depends on load condition and the thermal impact is more severe in OC condition. It was also observed that the thermal impact of defective BPD is larger under either high irradiance level for the same cell temperature or lower cell temperature on the same irradiance. This study indicates that it is necessary to monitor the heat dissipation of BPDs and to evaluate the electrical properties of the BPDs to realize the reliable PV systems.