Frank Schafmeister

Novel Three-Phase AC–AC Sparse Matrix Converters

A novel three-phase ac-ac sparse matrix converter having no energy storage elements and employing only 15 IGBTs, as opposed to 18 IGBTs of a functionally equivalent conventional ac-ac matrix converter, is proposed. It is shown that the realization effort could be further reduced to only nine IGBTs in an ultra sparse matrix converter (USMC) in the case where only unidirectional power flow is required and the fundamental phase displacement at the input and at the output is limited to plusmnpi/6. The dependency of the voltage and current transfer ratios of the sparse matrix converters on the operating parameters is analyzed and a space vector modulation scheme is described in combination with a zero current commutation method. Finally, the sparse matrix concept is verified by simulation and experimentally using a 6.8-kW/400-V very sparse matrix converter, which is implemented with 12 IGBT switches, and USMC prototypes.

Novel three-phase AC-DC-AC sparse matrix converter

A novel three-phase AC-DC-AC sparse matrix converter (SMC) having no energy storage elements in the DC link and employing only 15 IGBTs as opposed to 18 IGBTs of a functionally equivalent conventional AC-AC matrix converter (CMC) is proposed. It is shown that the realization effort could be further reduced to only 9 IGBTs (ultra sparse matrix converter, USMC) in case the phase displacement of the fundamentals of voltage and current at the input and at the output is limited to /spl plusmn//spl pi//6. The dependency of the voltage and current transfer ratios of the systems on the operating parameters is analyzed and a space vector modulation scheme is described in combination with a zero current commutation procedure. Furthermore, a safe multi-step current commutation concept is treated briefly. Conduction and switching losses of the SMC and USMC are calculated in analytically closed form. Finally, the theoretical results are verified in Part II of the paper by digital simulations and results of a first experimental investigation of a 10 kW/400 V SMC prototype are given.

Analytical calculation of the conduction and switching losses of the conventional matrix converter and the (very) sparse matrix converter

To dimension the power semiconductors of a conventional matrix converter (CMC) or a two-stage sparse matrix converter (SMC and/or very SMC) extensive simulations usually have to be performed as the losses of each device are dependent on several operating parameters, including the different ratios of input and output frequency. In this paper analytical expressions with high accuracy are derived for the switching and conduction losses of the CMC, SMC and VSMCs power semiconductors. These expressions directly show the parameter dependencies of the power semiconductor switching and conduction losses and therefore can be used to determine the maximal local or average thermal stress and for the thermal design of the power components. Furthermore, the total converter losses and conversion efficiency can be determined with minimal calculation effort

Novel modulation schemes minimizing the switching losses of sparse matrix converters

The switching losses of a three-phase sparse matrix converter (SMC) operating in the lower modulation range are minimized by employing the lowest and the second largest input line-to-line voltage for the formation of the converter DC link voltage. The resulting current stresses on the power semiconductors and the switching frequency ripple RMS values of the filter capacitor voltages and output currents are calculated by digital simulation and compared to conventional modulation. Finally, a modulation scheme is introduced which allows the generation of reactive input power also for missing active power transfer via the DC link and/or purely reactive load. This is a basic requirement for operating the SMC in boost mode where the output filter capacitor voltages have to be controlled sinusoidally also for no-load operation.

Novel Hybrid Modulation Schemes Significantly Extending the Reactive Power Control Range of All Matrix Converter Topologies With Low Computational Effort

A novel approach based on indirect modulation, which significantly extends the reactive power control range for three-phase ac-ac matrix converters (MCs; applicable to all matrix topologies) and which is implementable with lowest computational effort, is proposed. This new method denoted as hybrid modulation facilitates the formation of reactive input current also for purely reactive load. The derivation of the modulation schemes, which rely on a decoupling of the output voltage and the reactive input current formation, is described in detail. Furthermore, the operating limits, i.e., the maximum reactive input current that could be formed for the given output voltage amplitude and load current amplitude, are determined. Finally, all theoretical considerations are verified by measurements taken on a 6.5-kW Very Sparse MC.

Adaptive digital slope compensation for peak current mode control

Peak current mode control as well as digital control offers a number of benefits. Therefore it is an interesting approach to combine these two techniques in one control structure. Based on microcontrollers with on-chip comparators, this combination is realizable with very low effort. In order to eliminate the drawbacks of peak current mode control, a slope compensation has to be added. This paper presents such a slope compensation implemented on a microcontroller not using an analog ramp signal but instead pre-calculating the desired comparator switch-off threshold. In contrast to conventional analog control, adaptive algorithms can be used to maintain optimal slope compensation over a wide operating range. Problems that occur in practice due to reverse recovery current spike and computing time can be handled with simple measures. The effectiveness of the proposed digital slope compensation is verified by experimental results.

Novel modulation schemes for conventional and sparse matrix converters facilitating reactive power transfer independent of active power flow

Two novel modulation schemes are proposed which facilitate the transfer of reactive power from the load side to the mains side of a three-phase AC-AC sparse matrix converter (SMC) or a conventional matrix converter. The derivation of the modulation schemes, which rely on a decoupling of the output voltage and the input current formation, is described in detail. Furthermore, the operating limits concerning modulation index and reactive current magnitude are determined. Finally, digital simulations and measurements on a 7.5 kW prototype of a very sparse matrix converter verify all theoretical considerations.

Enhanced control scheme for three-phase three-level rectifiers at partial load

A prominent boost-type three-level topology (VIENNA Rectifier I), which proved to represent a cost-effective and highly efficient solution for switched-mode rectifiers is inspected toward its operation at discontinuous conduction mode (DCM). This mode of operation occurs not only at high input voltage in conjunction with low load currents but even at medium loading in the vicinity of mains voltage zero crossings. When this circuit is operated in DCM, additional measures are required for improved behavior to avoid conflicts with requirements on total harmonic distortion and regulations as well as safe operation in terms of voltage balancing and overvoltage protection. A detailed analysis of DCM and associated states is performed enabling determination and location of error voltages. Basic rules for the location of error voltages can be found. This leads to a novel optimized modulation and control scheme, facilitating designs without additional inductance. Selected simulation and measurement results prove the enhanced modulation scheme.

Semi-Digital Interleaved PFC Control with Optimized Light Load Efficiency

A control structure for an interleaved power factor correction (PFC) rectifier with smart combination of analog and digital control parts is presented in this paper. Analog technique is employed to accomplish high control bandwidth while digital control is used for parts of lower dynamic demands. This results in low microcontroller costs though the system is kept flexible. Particularly, the control strategy can be adapted depending on the operation point. Since the light load performance is in recent focus of interest, appropriate algorithms to improve light load efficiency were implemented on a prototype system and are briefly described in this paper. The effectiveness of the proposed semi-digital control approach is verified by experimental results.

Digital control strategy for multi-phase interleaved boundary mode and DCM boost PFC converters

A digital control strategy which provides optimal interleaving of multi-phase boost power factor correction (PFC) rectifiers operating at DCM/CCM boundary (BCM) or in DCM is presented in this paper. In this method two feedforward algorithms are used. The first provides the turn-on time to attain the desired input current and the second computes the switching period time to achieve BCM operation. An extension to operate in DCM is proposed to avoid CCM under all conditions. Regulated DCM is used to realize a new continuous phase shedding method, which is used to limit the switching frequency. The proposed control structure was implemented on a microcontroller. Experimental results show the effectiveness of the control strategy on a boost PFC prototype with 3 parallel rails.