Elimination of Filter Inductor in Switching Cell AC–AC Converters Using Magnetic Integration

Khan, Ashraf Ali,Honnyong Cha,Ahmed, Hafiz Furqan,Heung-Geun Kim IEEE 2016 IEEE transactions on power electronics Vol.31 No.9

<P>The switching cell (SC) direct pulse widthmodulation ac-ac converters have several advantages; in particular, they do not experience commutation issues, are not susceptible to reverse-recovery losses of body diode of switching devices, produce output waveforms of good quality, are more immune to EMI noise, and have smaller input/output current ripples. The magnetic components of the SC ac-ac converters consist of coupled inductors and a filter inductor. This paper proposes an integrated inductor which integrates allmagnetic components of the SC ac-ac converters into onemagnetic assembly. The leakage inductance of the proposed integrated inductor is utilized to eliminate the filter inductor in buck, boost, and buck-boost-type SC ac-ac converters. The proposed integrated inductor improves the power density and reduces cost and losses because it requires fewer magnetic cores, coils, and soldering connections. The proposed integrated inductor is especially suitable for high-current applications because it can avoid the magnetic core saturation caused by current, and can eliminate the air gap in the core. The proposed integrated inductor can realize high leakage inductance; therefore, the input and output current ripples can be reduced significantly. To verify the proposed integrated inductor, a 2.3-kW prototype inductor was built and tested.</P>

Dual-Buck-Structured High-Reliability and High-Efficiency Single-Stage Buck–Boost Inverters

Khan, Ashraf Ali,Cha, Honnyong IEEE 2018 IEEE transactions on industrial electronics Vol.65 No.4

<P>In this paper, dual-buck structured single-stage buck–boost inverters that use power MOSFETs to achieve high efficiency are presented. The proposed inverters require fewer switches, and half of the switches have reduced current stresses. They have no shoot-through problem; therefore, high system reliability can be obtained. The dead-time in PWM signals can be minimized or eliminated, which improving the quality of the output ac voltages and increasing the efficiency. In the proposed inverters, MOSFETs can be used without reverse-recovery issues of their body diodes to boost the efficiency and increase the switching frequency. To further improve the efficiency and stability of the proposed single-phase inverter, PWM strategies combining high- and low-frequency modulation are presented. A hardware prototype of the single-phase inverter was built and tested using resistive, inductive, and nonlinear loads. Experimental results for a 300-W prototype inverter show a 97.4% peak efficiency with a 25 kHz switching frequency.</P>

A New Reliable Three-Phase Buck-Boost AC–AC Converter

Khan, Ashraf Ali,Cha, Honnyong,Ahmed, Hafiz Furqan IEEE 2018 IEEE transactions on industrial electronics Vol.65 No.2

<P>This paper proposes a new bidirectional direct pulse width modulation (PWM) three-phase buck-boost ac–ac converter. It requires six switches and three inductors and can provide wide range buck-boost voltage operation. It has high reliability due to no short-circuit and open-circuit risks in the converter. Therefore, it solves the commutation problem without using lossy snubber circuits, soft commutation strategies, and coupled inductors. It can be designed with metal–oxide–semiconductor field-effect transistors (MOSFETs) without their body diodes conducting, which eliminate reverse recovery issues and losses of body diodes. In addition, to reduce the ripples in input and output voltages and currents, the proposed converter can be operated with a PWM strategy-II. The PWM strategy-II is a type of interleaved PWM strategy with dead- and overlap-times operating modes. To validate the effectiveness of the proposed converter extensive analysis followed by detailed experimental results are provided.</P>

Magnetic Integration of Discrete-Coupled Inductors in Single-Phase Direct PWM AC–AC Converters

Khan, Ashraf Ali,Honnyong Cha,Heung-Geun Kim IEEE 2016 IEEE transactions on power electronics Vol.31 No.3

<P>Recently, novel direct pulse width modulated (PWM) ac-ac converters using coupled inductors with switching cell structure have been introduced as an alternative to the conventional acac converter topology. The converters can effectively solve the commutation problem associated with the conventional direct PWM ac-ac converters. However, the two coupled inductors used in the topology make the converters less attractive for use in practical applications. This paper proposes magnetic integration of the discrete-coupled inductors in single-phase ac-ac converters. The dc offset fluxes generated by the common-mode currents are cancelled out perfectly when the windings on the common core are appropriately arranged, resulting in ac flux only. The proposed inductor shows significant improvements over discrete-coupled inductors, in that the proposed inductor does not have an air gap, it exhibits symmetrical winding currents, and it has smaller volume with large current handling capability. The proposed inductor can handle an unlimited amount of current due to the absence of core saturation caused by the current and is well suitable for high power ac-ac conversion applications. The performance evaluation and comparison of the experimental results show that the net volume of the coupled magnetic components can be reduced by more than 50% with the proposed integration scheme.</P>

Three-Phase Three-Limb Coupled Inductor for Three-Phase Direct PWM AC–AC Converters Solving Commutation Problem

Khan, Ashraf Ali,Cha, Honnyong,Kim, Heung-Geun IEEE 2016 IEEE transactions on industrial electronics Vol.63 No.1

<P>In this paper, a family of three-phase direct pulse-width modulated (PWM) ac-ac converters consisting of buck, boost, and buck-boost converters is proposed. The proposed converters have no commutation problem even if all of the switches are turned on or off simultaneously. They do not use lossy snubbers and do not sense the voltage polarity for commutation, and produce high-quality output voltage waveforms. The proposed converters allow the use of power MOSFETs and fast recovery freewheeling diodes independently. The use of power MOSFETs as active switches and freewheeling diodes with extremely fast recovery features lower the switching losses and enable us to reduce the volume of passive components by increasing switching frequency. The input (or output) filter inductor experiences twice of the converter switching frequency and thus can be designed with minimum size. To increase power density of the proposed converters, a three-phase three-limb coupled inductor is proposed. Three-phase coupled inductor integrates three separate coupled inductors of the proposed converters in one three-limb core. In comparison with separate coupled inductors, the three-phase coupled inductor has a smaller size with large current-handling capability. Experimental results obtained for the boost-type converter show the robustness of the proposed three-phase ac-ac converters.</P>

Integrated Magnetic Core Switching Cell AC-AC Converters

Ashraf Ali Khan,Honnyong Cha,Hafiz Furqan Ahmed,Heung-Geun Kim 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

The switching cell (SC) direct PWM ac-ac converters solve the commutation problem of traditional direct PWM ac-ac converters in a very effective way. The magnetic components of the SC ac-ac converters consist of two coupled inductors and an AC inductor. In order to make the SC ac-ac converters more useful and practical, this paper presents the integration of magnetic components in these converters in one magnetic core. The proposed integration method can significantly improve the power density of the SC ac-ac converters. In addition the cost of magnetic components and inductor coils can be reduced. In order to prove the validity of the proposed integration scheme a 600 W prototype inductor is built and tested.

An Improved Single-Phase Direct PWM Inverting Buck–Boost AC–AC Converter

Khan, Ashraf Ali,Cha, Honnyong,Ahmed, Hafiz Furqan Institute of Electrical and Electronics Engineers 2016 IEEE transactions on industrial electronics Vol.63 No.9

<P>A single-phase inverting buck-boost ac-ac converter is proposed in this paper. The proposed converter has no shoot-through and dead-time problems and, like conventional dc-dc converters, it can be operated with simple pulse width modulation (PWM) control. It offers high frequency and high efficiency operation because high speed mosfet can be used as switching device without the reverse recovery issues and losses of its body diode. The proposed converter features quasi-continuous input and output currents, high input power factor, low total harmonic distortion of input and output currents, and high efficiency. A 460 W laboratory prototype was constructed and tested with interleaved and complementary PWM controls using a resistive and inductive load. Detailed experimental results are provided to show the novelty of the proposed converter. Experimental results confirmed that the proposed converter can obtain 93.8% efficiency at 60 kHz switching frequency.</P>

A Highly Reliable and High-Efficiency Quasi Single-Stage Buck–Boost Inverter

Khan, Ashraf Ali,Cha, Honnyong,Ahmed, Hafiz Furqan,Kim, Juyong,Cho, Jintae Institute of Electrical and Electronics Engineers 2017 IEEE transactions on power electronics Vol. No.

<P>To regulate an output ac voltage in inverter systems having wide input dc voltage variation, a buck-boost power conditioning system is preferred. This paper proposes a novel high-efficiency quasi single-stage single-phase buck-boost inverter. The proposed inverter can solve current shoot-through problem and eliminate PWM dead time, which leads to greatly enhanced system reliability. It allows bidirectional power flow and can use MOSFET as switching device without body diode conducting. The reverse recovery issues and related loss of the MOSFET body diode can be eliminated. The use of MOSFET contributes to the reduction of switching and conduction losses. Also, the proposed inverter can be operated with simple pulse width modulation (PWM) control and can be designed at higher switching frequency to reduce the volume of passive components. The detailed experimental results are provided to show the advantages of the proposed inverter. Efficiency measurement shows that using simple PWM control the proposed inverter can obtain peak efficiency of 97.8% for 1.1-kW output power at 30-kHz switching frequency.</P>

Cascaded Dual-Buck AC–AC Converter With Reduced Number of Inductors

Khan, Ashraf Ali,Cha, Honnyong,Baek, Ju-Won,Kim, Juyong,Cho, Jintae Institute of Electrical and Electronics Engineers 2017 IEEE transactions on power electronics Vol. No.

<P>This paper proposes a new type of cascaded ac-ac converter with phase-shift control and reduced number of inductors. It can attain high voltage levels by using standard low-voltage rating semiconductor devices. The proposed converter is resistant to current shoot-through and does not require pulse-width modulation (PWM) dead-time; these lead to greatly enhanced system reliability and effective utilization ofPWMvoltages. Moreover, it does not require current/voltage polarity sensors, lossy snubbers, or dedicated PWM strategies for commutation. These features make it possible to design the converter with reduced control complexity, and obtain output voltage with less distortion. The cascaded units in the proposed converter share the inductors; therefore, number of inductors, inductors footprints, and magnetic volume can be reduced. The phase-shift PWM control is also presented. It increases the effective frequency of the converter by the number of cascaded units, which decreases the size of the passive components and/or the current and voltage ripples. In order to demonstrate the advantages of the proposed converter, detailed comparative simulations and experimental results of the proposed 2-unit, 3-unit, and 4-unit cascaded converters are provided.</P>

Dual-Buck AC–AC Converter With Inverting and Non-Inverting Operations

Khan, Usman Ali,Khan, Ashraf Ali,Cha, Honnyong,Kim, Heung-Geun,Kim, Juyong,Baek, Ju-Won Institute of Electrical and Electronics Engineers 2018 IEEE transactions on power electronics Vol.33 No.11

<P>In this paper, a novel buck–boost ac–ac converter is proposed. The basic switching unit of the proposed converter is a unidirectional buck circuit; therefore, it has no shoot-through worries. It can achieve safe commutation without using <I>RC</I> snubbers or soft commutation strategies. It can be implemented with power MOSFETs without their body diodes conducting and, for current freewheeling external diodes, can be used to minimize the reverse recovery issues and related loss. It has a bipolar voltage gain with both inverting and non-inverting operations. The non-inverting operation can be used to compensate voltage sag, and inverting operation can be used to compensate voltage swell. Therefore, the proposed converter as a dynamic voltage restorer is capable of compensating for both voltage sag and swell in a wide range. The detailed theoretical analysis followed by detailed experimental results of a 300-W prototype converter is provided.</P>