F. Javier Diaz

Teaching Resonant Converters: Properties and Applications for Variable Loads

This paper summarizes a lesson on resonant converters included in the new course, Advanced Power Conversion Techniques, within the M.Sc. program in electrical engineering at the University of Cantabria, Spain. The course has recently received the National Accreditation for official programs. The M.Sc. program, along with the Ph.D. thesis, forms a Ph.D. program in industrial engineering. The contents motivate the students to find suitable resonant converter applications of industrial interest. A new approach to teaching resonant converters is presented that focused on their properties at certain frequencies, such as for voltage sources, current sources, or sink and power source operation with very variable loads. The analysis of the converters enables the identification of these operating points and, by means of a sensitivity analysis with respect to the components of the resonant tank, explains the robustness of their behavior. Using as a starting point the fulfillment of the desired property, we propose that the students develop open-loop designs, with stable behavior, or closed-loop designs, with very small variation in the operating point. Two practical application examples are shown: electronic ballasts for high-intensity discharge lamps and power supplies for electrical discharge machining.

Digitally Controlled Low Frequency Square Wave Electronic Ballast with Resonant Ignition and Power Loop

This paper proposes a digital controller for a low-frequency square-wave (LFSW) electronic ballast that includes the ignition sequence, a double control loop, and the selection of the positive and negative operation modes. The whole ballast is a two-stage circuit, where the first part is a power factor correction (PFC) stage and the second is a full-bridge (FB) converter used for both ignition and square-wave drive. Ignition is achieved by approaching the resonant frequency of the LC filter when the lamp is in the off state and the FB is working as a resonant inverter. After ignition, the converter operates as an LFSW inverter by controlling the FB to act alternately as a buck converter supplying positive or negative current. While ignition occurs at the LC filter resonance frequency (fo = 20 kHz), the buck converter switching frequency (fsw = 200 kHz) is selected significantly higher than fo to attenuate high-frequency harmonics and avoid exciting acoustic resonance. Lamp stability is achieved by controlling the inductor current of the LC filter, and power mode control is achieved by adjusting the average current and voltage supplied by the PFC stage. The solution is experimentally validated for different types of 150-W high-intensity discharge lamps. A coupled-inductor single-ended primary inductance converter operating in continuous conduction mode is used for the PFC stage.

Haar wavelet based processor scheme for image coding with low circuit complexity

A hardware-oriented image coding processing scheme based on the Haar wavelet transform is presented. The procedure computes a variant of the Haar wavelet transform that uses only addition and subtraction operations, after that, an optimized methodology performs the selection and coding of the coefficients, tailored for it with the main aim of attaining the lowest circuit complexity hardware implementation. A selection strategy, which does not require the previous ordering of coefficients, has been used. A non-conventional coding methodology, which uses an optimized combination of techniques adapted to the various groups of coefficients, has been devised for the coding of the selected coefficients leading to a compressed representation of the image and reducing the coding problems inherent in threshold selection. The compression level reached for images of 512x512 pixels with 256 grey levels is just over 22:1, (0.4bits/pixel) with a normalized mean square error, nrmse, of 2-3%, with subjective qualities which can be classified as good. The whole compression circuitry has been described and simulated at HDL level for up to 4 consecutive images, obtaining consistent results. The complete processor (excluding memory) for images of 256x256 pixels has been implemented using only one general-purpose low-cost FPGA chip, thus proving the design reliability and its relative simplicity.

Converter With Four Quadrant Switches for EDM Applications

This paper presents a high-frequency rectifier stage using four quadrant switches as synchronous rectifiers for an electrical discharge machining (EDM) power supply. The EDM impulse generator is a series-parallel resonant inverter operating in current-mode. The device specifications and drive requirements of the converter are studied with the objective of generating unipolar and bipolar current waveforms suitable for different types of EDM operations.

DC and Pulsed DC TIG Welding with a Scalable Power Supply

A multiple two-phase resonant converter module connected in parallel is the chosen architecture for a TIG welding power supply. Connection to the utility is performed through a front-end power factor correction stage. Fine tuning is performed by overlapping the control signals of the resonant converter phases. Output ripple is reduced by interleaving each module and the arc strike is achieved with no extra circuitry and at low voltage. DC and multiple pulsating operations from 20 Hz to 5 kHz with different pulse widths have been tested. The resulting arc stability, welding parameters and efficiency are analyzed. The experimental results show the welding performance and near unity power factor.

Current Sensorless Power Factor Correction based on Digital Current Rebuilding

A new digital control technique for power factor correction is presented. The main novelty of the method is that there is no current sensor. Instead, the input current is digitally rebuilt, using the estimated input current for the current loop. Apart from that, the ADCs used for the acquisition of the input and output voltages have been designed ad-hoc. Taking advantage of the slow dynamic behavior of these voltages, almost completely digital ADCs have been designed, leaving only a comparator and an RC filter in the analog part. The final objective is obtaining a low cost digital controller which can be easily integrated in an ASIC along with the controller of paralleled and subsequent power sections.

Microcontroller power mode stabilized power factor correction stage for electronic ballast applied to metal halide lamps

This paper presents the design considerations, control strategy and resulting performance of a two-stage ballast system; power factor correction (PFC) and resonant inverter (RI), for metal halide lamps (MHL). Medium and long term stability, with regard to utility line perturbations and lamp aging, is achieved by the PFC stage controlled in power mode, while the short term stability is assured by the high output impedance of the LCC inverter, operating in open loop at constant switching frequency. Power mode control requires over voltage and short circuit protection. The LCC resonant inverter is designed in such a way that the maximum power supplied to the lamp is inherently limited, provided that the input voltage is also limited. Hence, the protection requirements are reduced to limit only the maximum voltage supplied by the PFC stage to the inverter stage. By boosting the PFC output voltage with respect to the nominal value during the protection mode two benefits are achieved: 1) the ignition performance using the resonance of the LCC circuit is improved, since it is not necessary to reach the highest voltage gain frequency which is more sensitive to frequency or component tolerance. Hot re-ignition performance is also improved since the required voltage gain is also less sensitive to component thermal deviation. 2) The warm-up time is clearly reduced in comparison to electromagnetic ballast or other non-controlled ballast since the RI input voltage increases when the lamp power is below the target value to reach the power level imposed by the reference

New specification for the PFC controller in HID lamps electronic ballast

The ballast system and the lamp are subjected to the utility disturbances. In this way, HID lamps are very sensitive to voltage supply fluctuations that produce an effect on the human visual perception by changing the light emission, known as flicker. New design considerations and a control strategy for a two-stage ballast system, power factor correction (PFC) and inverter stage, for high intensity discharge (HID) lamps are presented along with a novel voltage fluctuation detection consistent with the maximum human light flickering perception. A correct actuation over the voltage loop of the PFC stage reduces the lamp sensitivity to flicker. In this case, a novel digital control technique for the PFC stage is used. It requires neither current sensor nor high speed analog-to-digital converter, which are two traditional bottlenecks for the implementation of digital controllers in such application.

PCB of a buck converter for laboratory practical classes in power electronics

This paper presents a laboratory PCB (Printed Circuit Board) used in the practical classes of power electronics in the new Degree in Industrial Electronics and Automation taught at the University of Cantabria, where students are trained in the design and measurement of power converters. This laboratory circuit reduces the time spent assembling the circuit and can focus the study on different aspects of the converters. Students place the buck converter to be designed on the developed PCB. This converter is regulated as a voltage source or current source when it is used to supply a LED matrix. The PCB was developed in order to rationalize the students time in the laboratory to achieve their practical objectives. This laboratory circuit is ready for performing measurements of voltage and current on different devices. Furthermore, it is ready for introducing perturbations to measure transfer functions of the converter, so as to design the appropriate regulator.

Anti-flicker digital PFC controller for HID lamp electronic ballast

HID lamps are sensitive to voltage supply fluctuations producing an effect on human visual perception, known as flicker. The fluctuations are typically caused by the repetitive variation in the power consumed by loads, or by the connection and disconnection of significant loads. New improvements are presented in the power factor correction (PFC) stage for electronic ballasts of high intensity discharge (HID) lamps. A universal low frequency voltage fluctuation detection algorithm is proposed, consistent with the maximum human light flickering perception. A correct actuation on the output voltage loop of a current sensorless digital control for the PFC stage reduces the lamp sensitivity to utility fluctuations. Simulation and experimental results show how lamp flicker is avoided and power factor correction is achieved without measuring the input current.