Mostafa Khazraei

Active Capacitor Voltage Balancing in Single-Phase Flying-Capacitor Multilevel Power Converters

Two active capacitor voltage balancing schemes are proposed for single-phase (H-bridge) flying-capacitor multilevel converters. They are based on the circuit equations of flying-capacitor converters. Consequently, they can be implemented using straightforward control rules. In particular, the first technique is based on an algorithm which follows the standard multilevel modulation. Then, it utilizes a redundant state selection table for capacitor voltage balancing. In the second method, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. The most important advantage of these two proposed methods is that they can be utilized to converters with any desired number of levels in their output voltage. Moreover, the analysis and implementation of both methods are straightforward. Through simulation and experimental implementation, these methods are shown to be effective on capacitor voltage regulation in flying-capacitor multilevel converters.

A generalized capacitor voltage balancing scheme for flying capacitor multilevel converters

Multilevel power electronic converters are the converter of choice in medium-voltage applications due to their reduced switch voltage stress, better harmonic performance, and lower switching losses. Although it has received little attention, the flying-capacitor multilevel converter has a distinct advantage in terms of its ease of capacitor voltage balancing. A number of techniques have been presented in the literature for capacitor voltage balancing, some relying on “self-balancing” properties. However, self balancing cannot guarantee balancing of capacitor voltages in practical applications. Other researchers present closed-loop control schemes which force voltage balancing of capacitors. In this paper, a new closed loop control scheme is proposed which regulates the capacitor voltages for a multilevel flying capacitor converter. The proposed scheme is based on the converter equations and involves implementing simple rules. In particular, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. Through simulation, the method is shown to work on four, eight and nine-level flying capacitor inverters.

Hysteresis-Based Control of a Single-Phase Multilevel Flying Capacitor Active Rectifier

The deployment of a single-phase, multilevel flying capacitor converter as a 1.5-kW active rectifier is discussed in this paper. The challenges associated with this approach include providing unity power factor, regulating the dc output voltage, and maintaining balanced voltages across the flying capacitors. In such applications, the input current is usually controlled via a reference-frame-based control approach. Here, a different approach, which is based on the hysteresis current-mode control scheme, is applied. The simulation and experimental results for both reference-frame-based and hysteresis-based controllers are presented and compared. The results show that the hysteresis-based method exhibits a tighter regulation of the input current and offers easier hardware implementation.

Start-up Procedure and Switching Loss Reduction for a Single-Phase Flying Capacitor Active Rectifier

This paper describes a research conducted on a single-phase flying capacitor active rectifier. An online process that reduces the total number of switching events, which leads to a reduction of switching losses in the converter, is introduced. This method is based on redundant state selection, which is used to regulate the voltage of flying capacitors. The proposed approach is general and can be applied to flying capacitor converters with any number of levels. Furthermore, in order to control the inrush current at the start of operation and avoid extra voltage and current stress for active switches and capacitors, a start-up procedure that precharges the flying capacitors is proposed. With this precharge procedure, no additional hardware is needed. Simulation and laboratory results demonstrate these new concepts.

A hybrid multilevel inverter with both staircase and PWM switching schemes

In this paper, a new method to control multilevel converters is proposed. The considered multilevel converter consists of a three-phase three-level diode-clamped converter (main converter). Two H-bridge cells are then connected in series with each output phase of the main converter. The operation of the main converter and one of the cascading Hbridge cells is based on the staircase switching method. The firing angles of these converters are selected in a way that the dc voltage required for the last H-bridge cell is minimized. The switching pattern of the second H-bridge cell is based on the pulse-width modulation method. This last cell generates the remaining parts of the desired sinusoidal output voltage. The combination of these converters and their switching methods result in an output waveform with low harmonics. Here, a fifteen-level converter is designed based on this approach. Simulation results and laboratory measurements verify the effectiveness of the proposed topology and modulation method.

Modeling and Analysis of Projected Cross Point Control—A New Current-Mode-Control Approach

This paper features the projected cross point control (PCPC) approach which is a recent current-mode control (CMC) technique. PCPC benefits from several advantages including fixed switching frequency, wide stability range, high current loop gain, improved audio susceptibility, and simpler design procedure. An introduction of its principles of operation is followed by detailed discussions about its stability and dynamic response. This paper then describes the development of the small-signal model of PCPC and derives transfer functions, such as current loop gain, audio susceptibility, and output impedance. Finally, simulations and experimental results are presented. Peak, average, and hysteresis CMC schemes are used for comparison.

Power factor correction using projected cross point control (PCPC)

Projected cross point control (PCPC) is a new current-mode control approach which has been recently introduced. The PCPC method benefits from the advantages of traditional fixed-frequency and variable-frequency current-mode control methods. For instance, while PCPC is a fixed-frequency approach, it is stable for the entire range of the duty cycle similar to the variable-frequency hysteresis current-mode control (HCMC) scheme. In this paper, PCPC is used as the control strategy in a power factor correction (PFC) application. The results are compared with PFC methods based on the peak current-mode control (PCMC) and average current-mode control (ACMC) techniques.

Modified projected cross point control — a large signal analysis

In this paper, the small-signal analysis of the modified projected cross point control (PCPC) method is investigated. Modified PCPC is a new current-mode control approach which benefits from several advantages over common current-mode control methods such as peak current-mode control (PCMC) and average current-mode control (ACMC). Followed by the small-signal analysis, different small-signal transfer functions for this method are obtained. To prove the significant superiority of the modified PCPC controller, the small-signal properties of this novel method are compared with those of the PCMC and ACMC approaches.Modified projected cross point control (PCPC) is introduced and discussed in this paper. Compared to the existing PCPC method, the hardware implementation of the proposed modified method is simpler. This modified PCPC method benefits from the advantages of traditional fixed- and variable-frequency current mode control approaches. For instance, as argued in this paper, while modified PCPC is a fixed-frequency approach, it is stable for the entire range of the duty cycle similar to the variable-frequency hysteresis current mode control scheme. Other advantages and large-signal characteristics of modified PCPC are discussed by comparing it with the well-known peak and average current mode control methods.

A new control strategy for a class of multiple-input DC-DC converters

This paper proposes a new control scheme for a class of multi-input dc-dc power converters. The new control method is based on coupling the signals from independent control loops of conventional controller for a multi-port converter. In this paper, the non-restricted double input buck converter is introduced. The properties of this converter and its steady-state equations along with its small-signal model are reviewed. Next, the conventional control method is reviewed and the new control method is introduced based on coupling the voltage and current loops from the conventional controller. Then, the small-signal model of the converter with the new control method is analyzed and the converter transfer functions are derived. Finally, the converter transfer functions are used to find the output impedance of the converter in order to compare the performance of the new controller with the conventional controllers. Simulation results are provided to verify the results predicted by the small signal analysis.

Capacitor voltage regulation and pre-charge routine for a flying capacitor active rectifier

In this paper, a research is carried out on a single-phase flying capacitor active rectifier. In order to regulate the voltage of the flying capacitors, a method based on redundant state selection is developed that generates an offline switching table. In addition, an online process is introduced that, by using the offline table, reduces the total number of switching events in the converter which leads to a reduction of switching losses. This method can be used for a flying capacitor converter with any number of levels. Furthermore, in order to control the inrush current during the start-up procedure and avoid extra voltage and current stresses for active switches and capacitors, a pre-charge routine for the flying capacitors is proposed. Experimental results demonstrate the benefits of the new concepts.