Modulation scheme analysis for high efficiency three-phase buck rectifier considering different device combinations

For most medium voltage high power applications, Neutral Point Clamped (NPC) three-level inverter is a preferred choice due to its advantages such as low cost, light weight and compact size. Space vector modulation is widely used in real-time digital control for power converters. It is especially suitable for use in NPC multilevel inverters due to its good harmonic profile, flexibility and easy digital implementation. This thesis focuses on the space vector modulation for high power three-level NPC inverter, where the switching frequency of the semiconductor devices is nominally below 1 OOOHz to reduce the power loss of the switching devices. The conventional space vector modulation (SVM) scheme for the three-level NPC inverter produces even order harmonics in the output voltages, which are not desirable for most industrial applications. In this thesis, the mechanism of even order harmonic generation is analysed. A new space vector modulation scheme, which can eliminate all the even order harmonics, is proposed. The performance of the new design is investigated and simulation results are provided for the verification purpose. The harmonic and THD profiles are compared with those of the conventional SVM scheme. The elimination of even order harmonics is achieved at the expense of a slight increase in the switching frequency. The proposed space vector modulation scheme can be applied to other types of converters for the even order harmonic elimination. An algorithm is developed to mitigate the neutral point potential deviation, which is a common problem in the NPC inverters. The simulation results show that this algorithm is suitable for both conventional and the proposed space vector modulation schemes.

THIS dissertation addresses the technical challenges associated with the operation and control of high-power modular multilevel converters. To improve the performance of modular multilevel converter (MMC), a generalized three-phase mathematical model with common-mode voltage (CMV) is proposed. By using the proposed mathematical model, the magnitude of circulating currents, capacitors voltage ripple, and the ripple in DC-link current during balanced and unbalanced operating conditions can be minimized. The modulation scheme and switching frequency are directly affected the output power quality and the performance of the converter and control method. In this dissertation, a novel sampled average and space vector modulation scheme is proposed. These modulation schemes are suitable to control the MMC with any number of submodules (without modifications), operates at low switching frequency, minimizes the ripple in output current and voltage harmonic distortion, and reduces the output filter size. For reliable operation of MMC, the voltage balancing among submodules is mandatory. This dissertation proposes a generalized single-stage balancing approach with reduced current sensors to control the MMC. The proposed balancing approach is suitable to implement with both phase-shifted and level-shifted pulse width modulation schemes. With the proposed approach, it is also possible to control the MMC with half-bridge and three level flying capacitor submodules. Also, an improved balancing approach often referred as the dual-stage balancing approach is proposed to minimize the voltage harmonic distortion and device power losses. This dissertation also proposes a direct model predictive control (D-MPC) approach to minimize the ripple in submodule capacitors voltage. To implement D-MPC approach, a discrete-time model of MMC with CMV is proposed. With the use of proposed model, the D-MPC approach does not require a cost function to minimize the circulating currents. The computational complexity is one of the major issues in the implementation of D-MPC approach for MMC. In this dissertation, a novel reduced computational MPC approaches named as dual-stage D-MPC and indirect model predictive control (I-MPC) approach are proposed. These approaches significantly minimize the computational complexity and, improve the voltage and current waveform quality while operating at the low switching frequency. Finally, the simulation and experimental studies are presented to validate the dynamic and steady-state performance of proposed methodologies. Index Terms • Modular Multilevel Converters. • Capacitors Voltage Balancing. • Pulse Width Modulation Schemes. • Circulating Currents. • Capacitors Voltage Ripple • Direct Model Predictive Control. • Dual-Stage Direct Model Predictive Control. • Indirect Model Predictive Control. • Total Harmonic Distortion.

In this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.

In this paper, an improved Space Vector Modulation (SVM) scheme is developed for matrix converter based rectifier. The conditions of using traditional SVM include small current ripple and continuous current operation mode (CCM). Thus the converter requires a very large inductor on DC side. To further reduce the size of converter, Discontinues current mode (DCM) is adopted for high efficiency compact design. The developed the modulation scheme works well with both CCM and DCM. The simulation results show that the line side current THD is further improved with proposed modulation scheme, especially for DCM operation.

PFC converters are widely used in industrial drive systems. The modulation scheme and the topology of the converter have a considerable impact not only on the switching losses, but also on the volume of the passive components, i.e. the EMI filter (CM and DM) and the dC link capacitors. In order to evaluate this impact, different modulation schemes for 2L and 3L topologies are analysed in this paper based on comprehensive converter optimizations. In these optimizations also a model for the EMI filter is included and the optimal converter volume including the EMI filter is evaluated for the different topologies and modulation schemes. Thus, significant differences in the converter volume can be observed for the different modulation schemes. Also the impact of SiC semiconductors on the system performance is analysed. The 3L-NPB modulation scheme with a NPC topology offers the best compromise of heat sink, EMI filter and DC link capacitor volume.

Hard-switching-induced switching loss can contribute significantly to the power loss of an isolated bidirectional dual-active-bridge (DAB) dc–dc converter operating at high frequency. An $LC$ -type series resonant DAB converter based on a switch-controlled-inductor (SCI) is proposed to mitigate the loss arising from hard switching under wide-range variations in output voltage and current. Zero-voltage switching is achieved at the primary side (high voltage), while at the secondary side (low voltage), zero-current switching is preferred to reduce excessive ringing due to circulating current and switching loss. In order to achieve reduced conduction loss, a nominal operating point is chosen where the root-mean-square resonant tank current is the minimum. To validate the proposed topology and modulation scheme, an $LC$ -type series resonant DAB converter based on SCI operating at 100 kHz is designed to interface a 400-V dc bus to a supercapacitor-based energy storage. Simulation and experimental results validate the effectiveness of the proposed topology for charging/discharging a supercapacitor with an output voltage variation of between 10 and 48 V and a maximum rated power of 480 W. A maximum efficiency of 94.6% is achieved using the proposed topology and modulation scheme.

This paper presents a novel idea to integrate three of the Space Vector Modulation (SVM) schemes, including the alternating zero sequence, the symmetric sequence and the bus clamped, in only a single FPGA chip which provides many advantages. Flexibility, reliability and very compact system are obviously obtained from this design chip. Moreover, faster design and verification time, design change without penalty are also the benefits resulted in FPGA-Based IC design. The optimized SVM schemes frequencies and programmable deadtime that used to adjust output signals are obtained through either 4/spl times/4 keypad or microprocessor port.

This paper presents the characteristics of a 1200 V, 33 A SiC MOSFET and a 1200 V, 60 A SiC schottky barrier diode (SBD). The switching characteristics of the devices are tested by a double pulse test (DPT) based on a current-source structure at voltage levels up to 680 V and current up to 20 A. In addition, based on these devices, a 7.5 kW, three-phase buck rectifier for a 400 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> architecture data center power supply is designed. The total loss of this rectifier is calculated full load. The results show that the SiC based buck rectifier can obtain low power loss and smaller weight and volume than a Si based rectifier.

This paper describes a novel hybrid modulation scheme for the operation of an isolated high-frequency and high-power direct-power-conversion (DPC) inverter. It enables: (a) removal of the DC-link filter evident in conventional inverters where the diode rectifier stage is followed by low-pass filter and a voltage-source inverter (VSI); (b) significant reduction in switching loss of the inverter by modulating the switches of 2 of the 3 legs of the 6-switch AC/AC converter at 120 Hz while modulating the 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> leg at high frequency. Unlike the VSI approach, in the DPC approach hybrid modulation enables the retention of the sine-wave modulated switching information at the output of the diode rectifier without rather than filtering it. Overall, the following is demonstrated: i) modulation scheme and uniqueness, and analytical derivation of the Fourier transformation of the output voltage; ii) derivation of a reachability condition for the DPC topology that outlines its global stability and performance; iii) demonstration of the switching and conduction losses and harmonics; and iv) scaled experimental validation. It is noted that, the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in literature. The term hybrid modulation scheme is simply chosen because at any given time, only one leg of the inverter output stage carries out forced switching while the other two legs are not switching at all. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a DC and not a pulsating modulated DC and at most only one leg stays on or off permanently in a 60deg or 120deg cycle.

There is a continuous demand for improvements in the quality of switching power devices, such as higher switching frequency, higher withstand voltage capability, larger current-handling capability, and lower conduction losses. However, for single-conduction-mechanism devices (SCRs, GTOs, BJTs, FETs), possessing all these features is probably unrealizable for physical reasons. An attractive solution appears to be double-mechanism devices, in which the features of both a minority carrier device (BJT or SCR) and a majority carrier device (MOSFET) are embedded. Both IGBTs (insulated-gate bipolar transistors) and MCTs (MOS-controlled thyristors) belong to this family of double-mechanism devices and promise to have a major impact on converter circuit signs. The authors deal with the major features of these two devices, pointing out those that are most critical to the design of power converter topologies. In particular, the two devices have been tested both in a chopper and in two resonant link converter topologies, and the experimental results are reported. >

With the fast development of information technology (IT) industry, the demand and market volume for off-line power supplies keep increasing, especially those for telecommunication, computer servers and data centers. As the capital expenditure was measured by the square footage occupied rather than power consumption, the development of high power density converter system is of greater interesting. The rising energy prices have resulted in the cost of power and cooling exceeding the purchase cost in less than two years. Therefore, highly efficient power conversion is required for the power converter system. Generally, the power supply unit (PSU) for power distribution system (PDS) in data center and telecom are the standard two-stage approach which normally consists of power factor correction (PFC) circuit and isolated DC-DC converter. The two-stage power conversion has demonstrated excellent performance and high reliability, since the design can be optimized for each stage. However, limitations to prevent the existing two-stage PSU to fulfill future requirements for the PDS in data center and telecom applications are revealed, and a very promising and fundamentally different approach with the single-stage isolated converter is proposed in this dissertation. The development of single-stage converters with the option of placing the energy storage outside of the PSU creates new degrees of freedom regarding e.g. simplified rectifier racks in telecom and data center. This provides tangible benefits in the form of space saving, better airflow for power unit in rectifier racks and improved lifespan. The three-phase isolated buck matrix-type rectifier, capable of achieving high power density and high efficiency, is identified as an excellent candidate for the medium power level (5 kW~10 kW) single-stage power supply design. Nevertheless, the matrix-type rectifiers are known for their relatively complex modulation and commutation techniques, and lack of ride-through capability such as the stringent case of one phase loss operation. This dissertation work provides comprehensive study on the commutation method and modulation scheme design for the isolated buck matrix-type rectifier. It aims to analyze the operation principle of the rectifier and propose viable modulation and commutation schemes for this rectifier under both three-phase and single-phase operation. The method is verified by the hardware experiments of the PSUs with high efficiency (&gt; 98%) and high power density (&gt; 70 W/in3 ) for 54 V and 380 VDC applications. The prototypes demonstrated in the experiments show the effectiveness of the proposed modulation and commutation schemes for industry.

MOSFET transistors are continuously being an essential part of power electronic converters due to their low switching losses at high switching frequencies. However, the traditional power MOSFET suffers from a relatively low breakdown voltage. In the late 1990's, the CoolMOS transistor was launched by Infineon Technologies based on the super junction technology. This device is capable of blocking a considerably higher source to drain voltage and virtually combines the low on-state resistance of the IGBT with the low switching losses of the MOSFET. The main problem associated with this device is that the reverse characteristics of its internal body diode are found to be snappier than that of the conventional MOSFET. This makes it difficult to use this device in hard switched inverters employing Voltage Source Inverter (VSI) topology. In this paper different factors affecting the reverse recovery behaviour of the internal body diode of CoolMOS power switch are studied. Moreover, a technique to eliminate high reverse recovery peak currents in CoolMOS-based inverters is proposed based on Current Source Inverter (CSI) topology accompanied with silicon carbide Schottky diode. Simulation and experimental tests are carried out to compare between the peak current and the switching losses of CoolMOS employed in both VSI and CSI topologies. The experimental results confirm the improvement achieved using CSI. (6 pages)

This paper describes a technique for shaping the input current to a three-phase diode rectifier using a two-switch series-connected dual boost converter and a three-phase bidirectional switch circuit. Circuits are described for generating a single voltage DC output, "single DC-rail", or a dual output DC voltage using center-tapped capacitors, "split DC-rail". Both rectifier types can be operated with the boost inductors located either on the DC or the AC side of the rectifier. The resultant rectifier circuit configurations have an excellent immunity to the "shoot-through" fault condition and use active switching elements with low per-unit current ratings and low switching losses. These features increase the reliability factor and lower the cost penalty associated with unity fundamental power factor three-phase rectifiers. Test results are presented for the rectifiers using simulation and experimental results.

SummaryTurn-off semiconductor devices have vastly promoted modern power electronics during the last decade. Included are bipolar power transistors. Field Effect Transistors (MOSFETs), Gate Tun-off Thyristors (GTOs) and Insulated Gate Bipolar Transistors (IGBTs). Additional elements are Static Induction Transistors and Thyristors (SITs and SIThs) as well as MOS Controlled Thyristors (MCTs) for higher frequency applications.The prensent status of semiconductor power devices is discussed including switching behavior, losses, gate driver, snubber, overcurrent and protection. The standard control is nowadays pulse width modulation, Resonant circuits are growing in importance because they allow for higher frequency, lower storage components and transformers and reduction of switching losses in semiconductor devices, Main field of application are discussed and future trends given.

This paper proposes a novel modulation scheme for a multilevel Dual Active Bridge (DAB) DC–DC converter with direct power transfer capability in PV applications. The proposed modulation scheme has the ability to shape the high-frequency current waveforms, leading to lower peak/RMS values of current over a wide range of PV voltages. In addition, the multi-level structure allows for the use of low-voltage devices with significantly smaller channel resistance. The power circuit topology is based on a current-fed half-bridge (CF-HB) converter, with a coupled inductor, which facilitates the direct power transfer, resulting in significantly lower conduction losses. In summary, the proposed topology tackles both the conduction and switching losses through a multi-faceted approach by using the novel modulation scheme, the multi-level structure, the direct power transfer, and the inherent soft-switching. Detailed mathematical analysis and extensive experimental results demonstrate the superior performance of the proposed converter.