Mor Mordechai Peretz

Digital Control of Resonant Converters: Resolution Effects on Limit Cycles

The conditions for limit-cycle oscillations in digitally controlled resonant converters are explored theoretically and are tested by simulation and experiment. The analytical analysis reveals that in a manner similar to digital pulsewidth modulation (PWM) control, limit cycles occur in such systems when the LSB of the control changes the output by a value that is larger than the analog-to-digital converter (ADC) resolution. However, in resonant converters, unlike the case of PWM, limit-cycle oscillations depend on the steady-state control input, since both the power stage gain and the resolution of the digitally generated drive frequency are not constant over the operating frequency range. Consequently, at high gains (close to resonance), the required frequency resolution may not be supported by the digital core. A time-domain behavioral simulation model, developed, and experimentally verified, allows the steady-state behavior of digitally controlled resonant converters to be analyzed, including the phenomenon of limit cycles as well as the closed-loop response. A cycle-by-cycle Powersim (PSIM) simulation model of a digitally controlled resonant converter, developed in this study, includes a digital core realization using C code block. This simulation model enables the exploration of the system in fine details. The proposed method of static analysis and dynamic modeling is experimentally verified on a series-resonant parallel-loaded converter operated in closed-current loop. The digital control algorithm was implemented on a TMS320F2808 DSP core. Very good agreement is found between the analytical derivations, simulations, and experimental results.

Time-Domain Design of Digital Compensators for PWM DC-DC Converters

A time-domain design method for the digital controller of pulsewidth modulation dc-dc converters was developed. The proposed approach is based on the fact that the closed-loop response of a digitally controlled system is largely determined by the first few samples of the compensator. This concept is used to fit a digital PID template to the desired response. The proposed controller design method is carried out in the time domain and, thus, bypasses errors related to the transformation from the continuous to discrete domain and to discretization. The method was tested by simulations and experimentally. Digital PID controllers for experimental buck- and boost-type converters were designed according to the proposed method and implemented on a TMS320LF2407 DSP core. The measured closed-loop attributes were found to be in good agreement with the design goals. The study was further expanded to investigate the possible realistic closed-loop performance that can be obtained from a system that is controlled by a PID template controller, as well as the stability boundaries of the proposed time-domain controller design approach. The results of the study delineate a normalized map of deviation from the target closed-loop performance goals possible for PID control of switch-mode converters and the areas in which the use of this control law is feasible.

A High-Efficiency Resonant Switched Capacitor Converter With Continuous Conversion Ratio

A resonant switched capacitor converter with high efficiency over a wide and continuous conversion ratio is introduced. The efficiency of the topology depends primarily on the conduction losses and is decoupled, to a large extent, from the voltage conversion ratio. This is an advantage over classical switched capacitor converters in which the efficiency is strongly related to the conversion ratio. The operation principle applies three zero current switching (ZCS) states to charge, discharge and balance the remaining charge of the flying capacitor. This results in a Gyrator-behaved voltage-dependent current source with a wide voltage conversion ratio (smaller as well as greater than unity) as well as bidirectional power flow capabilities. The analytical expressions for conversion ratio and expected efficiency are provided and validated by simulation and experiments. The experimental verification of the converter demonstrates peak efficiency of 96%, and above 90% efficiency over a wide range of voltage gains and loading conditions. In addition, the system was found to be highly efficient at the extreme cases of both light and heavy loads.

A self-adjusting sinusoidal power source suitable for driving capacitive loads

A new self-adjusting current-fed push-pull parallel inverter (SA-CFPPRI) is presented and tested by simulation and experimentally. The power source includes a soft switching control circuitry and a controllable inductor. The SA-CFPPRI can drive a capacitive load at any frequency within the design range. It will maintain zero voltage switching of the main transistors and follow the input frequency signal even when the resonant elements and/or the load vary. Possible range of applications for the proposed power source is: piezoelectric transformers and motors, AC bus for a distributed power system and other loads that need to be fed by a sinusoidal waveform. The experimental results of the laboratory unit (160 Vrms at 93 kHz and nominal output power of 5 W) verify the analytical analysis and proposed design procedure.

Enhanced Differential Power Processor for PV Systems: Resonant Switched-Capacitor Gyrator Converter With Local MPPT

This paper introduces an enhanced differential power processor topology and principle of operation for photovoltaic (PV) systems that is based on switched-capacitor technology, featuring local maximum power point tracking (MPPT) capability, zero current switching, high efficiency over wide-operation range, and reduced size. The new converter operates as a voltage-dependent current-source and is regulated by dead-time or frequency control. Local MPPT on the individual PV elements is realized and the operation is demonstrated by simulation and experiments. Differential power processing operation is verified using 150-W prototypes, demonstrating ultimate improvement in the power harvesting capability of above 90% and up to 99% out of the available power in the string, under different insolation levels.

Cold Cathode Fluorescent Lamps Driven by Piezoelectric Transformers: Stability Conditions and Thermal Effect

The envelope impedance (EI) concept was used to develop a systematic and simple approach for studying the conditions for stability of piezoelectric transformers (PT) based ballasts for cold cathode fluorescent lamps (CCFL) systems and the implications of the thermal operating conditions on such systems. The envelope analysis of the CCFL, which was verified experimentally, revealed that the magnitude of the negative EI decreases with temperature. This might cause the system to become unstable at low temperatures due to the fact that the loop gain of the PT-CCFL system encircled the (-1, 0) point in the Nyquist plane. In such cases, the PT-CCFL system may enter a quasi-stable oscillatory operation mode in which the current is AM modulated by a low frequency parasitic signal. This phenomenon is analyzed and explained by considering the temperature effect on the EI of the CCFL. The results of this work provide an insight into the stability issue of the PT-CCFL system, and could help in finding remedies to the instability

Multiple Conversion Ratio Resonant Switched-Capacitor Converter With Active Zero Current Detection

This paper introduces an active method for zero current switching (ZCS) for resonant switched capacitor converters (SCC) with wide dynamic range. The method is demonstrated on a Binary SCC that features wide range of conversion ratios. Due to the resultant high efficiency of the converter operating under soft-switching conditions, it is applicable for higher power levels up to the medium power range (100W). The resonant operation is achieved with single air core inductor and precise commutation at zero current. The sensing signals of the resonant currents are obtained from the flying capacitors rather than form the inductive element in which the current includes a dc component. The zero detection method developed is capable of compensating for both the processing delays (from detection to switching action) and for the large variations of the resonant characteristics (due to transition between sub-circuits) and for any other component variations. A prototype with maximum input voltage of 100V and 19 conversion ratios was built and tested experimentally. The current sensing was implemented with a simple, cost-efficient, passive, sensor. For proper construction of the higher power experimental prototype, a simple and efficient, isolated gate driver was also developed.

Enhanced differential power processor for PV systems: Resonant switched-capacitor gyrator converter with local MPPT

This paper introduces an enhanced differential power processor topology and principle of operation for photovoltaic (PV) systems that is based on switched-capacitor technology, featuring local maximum power point tracking (MPPT) capability, zero current switching, high efficiency over wide-operation range, and reduced size. The new converter operates as a voltage-dependent current-source and is regulated by dead-time or frequency control. Local MPPT on the individual PV elements is realized and the operation is demonstrated by simulation and experiments. Differential power processing operation is verified using 150-W prototypes, demonstrating ultimate improvement in the power harvesting capability of above 90% and up to 99% out of the available power in the string, under different insolation levels.

Time Domain Identification of PWM Converters for Digital Controllers Design

A discrete time-domain based system identification method for PWM DC-DC converters is presented. The proposed procedure is capable of successfully reconstructing the systems model from an arbitrary excitation at the command input. In this study, a step perturbation was applied, which is simple to apply and has an intuitive interpretation of the output response. The effects of switching and quantization noise were overcome by choosing the sampling instance to be after the switching oscillations decay significantly and by averaging the responses of synchronously perturbed sequences. The proposed method was evaluated on Buck and Boost converters. The digital data acquisition procedure was implemented on a TMS320F2407 DSP core. Excellent agreement was found between simulations and experimental results.

Analysis of the Current-Fed Push-Pull Parallel Resonant Inverter Implemented with Unidirectional Switches

A modified current-fed push-pull parallel resonant inverter (CFPPRI), that includes diodes in series with the switches, was analyzed and tested by simulation and experimentally. The modified power source is capable of producing a low THD output voltage within a relatively wide frequency range. A comparison between the modified topology and the basic CFPPRI, in terms of output signal quality and efficiency was carried out. It was found that the modified inverter is more efficient and that it produces an output signal that is less distorted as the running frequency deviates from resonance. Operation with a capacitive load in an off-resonance mode was demonstrated by driving a rotary piezoelectric motor by the modified inverter