Stefan Waffler

SiC versus Si—Evaluation of Potentials for Performance Improvement of Inverter and DC–DC Converter Systems by SiC Power Semiconductors

Switching devices based on wide bandgap materials such as silicon carbide (SiC) offer a significant performance improvement on the switch level (specific on resistance, etc.) compared with Si devices. Well-known examples are SiC diodes employed, for example, in inverter drives with high switching frequencies. In this paper, the impact on the system-level performance, i.e., efficiency, power density, etc., of industrial inverter drives and of dc-dc converter resulting from the new SiC devices is evaluated based on analytical optimization procedures and prototype systems. There, normally on JFETs by SiCED and normally off JFETs by SemiSouth are considered.

A Novel Low-Loss Modulation Strategy for High-Power Bidirectional Buck

A novel, low-loss, constant-frequency, zero-voltage-switching (ZVS) modulation strategy for bidirectional, cascaded, buck-boost DC-DC converters, used in hybrid electrical vehicles or fuel cell vehicles (FCVs), is presented and its benefits over state-of-the-art converters and soft-switching solutions are discussed in a comparative evaluation. To obtain ZVS with the proposed modulation strategy, the buck+boost inductance is selected and the switches are gated in a way that the inductor current has a negative offset current at the beginning and the end of each pulse period. This allows the MOSFET switches to turn on when the antiparallel body diode is conducting. As the novel modulation strategy is a software-only solution, there are no additional expenses for the active or passive components compared to conventional modulation implementations. Furthermore, an analytical and simulation investigation predicts an excellent efficiency over the complete operating range and a higher power density for a nonisolated multiphase converter equipped with the low-loss modulation. Experimental measurements performed with 12 kW, 17.4 kW/L prototypes in stand-alone and multiphase configuration verify the low-loss operation over a wide output power range and a maximum efficiency of 98.3% is achieved.

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A novel low-loss, constant-frequency, zero-voltage-switching (ZVS) modulation strategy for bi-directional, cascaded, buck-boost DC/DC converters, used in a hybrid electrical vehicle (HEV), is presented and its benefits over state-of-the-art converters and soft-switching solutions are discussed in a comparative evaluation. To obtain ZVS with the purposed modulation strategy, the buck+boost inductance is selected and the switches are gated in a way that the inductor current has a negative offset current at the beginning and the end of each pulse period. This allows the MOSFET switches to turn on when the anti-parallel body diode is conducting. As the novel modulation strategy is a software-only solution, there are no additional expenses for active or passive components compared to conventional modulation implementations. Furthermore, an analytical and simulation investigation predicts an excellent efficiency over the complete operating range and a higher power density for a multi-phase converter equipped with the low-loss modulation. Experimental measurements performed with a converter prototype verify the mode of operation and the ZVS principle.

Boost Converters

The application of soft-switching concepts or Silicon Carbide (SiC) devices are two enabling technologies to further push the efficiency, power density or specific weight of power electronics converters. For an automotive application, such as a dc-dc converter that interconnects the high voltage battery or ultra-capacitor in a hybrid electrical vehicle (HEV) or a fuel cell vehicle (FCV) to the dc-link, costs and failure rate are likewise of importance. Due to increasing requirements on multiple of these converter characteristics the comprehensive multi-objective optimization of the entire converter system gains importance. Thereto, this paper proposes an optimization method to explore the limits of power density as a function of the switching frequency and the number of phases of non-isolated bi-directional multi-phase dc-dc converters operated with soft-switching and Silicon devices and hard-switched converters that take advantage of SiC devices. In addition, the optimization includes an algorithm to determine the chip size required for the semiconductor devices under consideration of the thermal characteristics and efficiency requirements. Based on detailed analytical volume, loss and cost models of the converter components as well as on measurements demonstrative results on the optimum converter designs are provided and evaluated comparatively for the different converter concepts.

A novel low-loss modulation strategy for high-power bi-directional buck+boost converters

Volume and weight limitations for components in hybrid electrical vehicle (HEV) propulsion systems demand highly-compact and highly-efficient power electronics. The application of silicon carbide (SiC) semiconductor technology in conjunction with high temperature (HT) operation allows the power density of the DC-DC converters and inverters to be increased. Elevated ambient temperatures of above 200degC also affects the gate drives attached to the power semiconductors. This paper focuses on the selection of HT components and discusses different gate drive topologies for SiC JFETs with respect to HT operation capability, limitations, dynamic performance and circuit complexity. An experimental performance comparison of edge-triggered and phase-difference HT drivers with a conventional room temperature JFET gate driver is given. The proposed edge-triggered gate driver offers high switching speeds and a cost effective implementation. Switching tests at 200degC approve an excellent performance at high temperature and a low temperature drift of the driver output voltage.

Multi-objective optimization and comparative evaluation of Si soft-switched and SiC hard-switched automotive DC-DC converters

The paper proposes methods to improve the efficiency of a bi-directional, multi-phase buck+boost DC-DC converter for application in Hybrid Electrical Vehicles (HEV) or Fuel Cell Vehicles (FCV). Thereto, the modulation strategy for a highly-compact, 30kW/Liter, constant-frequency soft-switching converter is optimized based on a converter loss model that includes the losses in the power semiconductors and the buck+boost inductor. An algorithm for numerical calculation of the optimum switching times is given, whereas the values for the loss-optimized operation of the converter are stored in a lookup-table that is accessed by the digital controller. In addition, a novel method and control concept to ensure a Zero Voltage Switching (ZVS) of all semiconductor switches by determination of a zero voltage across the MOSFET switches with analog comparators is proposed that results in the lowest inductor RMS currents for ZVS operation at the same time. Furthermore, at low output power an absolute efficiency gain of over 2.8% is achieved by partial operation of the six interleaved converter phases. A detailed description on the control concept that determines the optimum number of activated phases for the current operating point of the converter is given and verified by experimental results. The measurements prove the capability to instantaneously switch the number of active phases during operation without a overshoot or drop in the converter output voltage.

High temperature (≫200°C) isolated gate drive topologies for Silicon Carbide (SiC) JFET

Bi-directional dc-dc converters for automotive applications typically are limited to generate only a small voltage ripple, especially when a ultra-capacitor with a limited ripple current capability is interfaced by the converter. Thus, to minimize the ripple quantities and the converter volume at the same time, interleaved multi-phase dc-dc converters are utilized. However, tolerances of the buck+boost inductors of the interleaved phases generate sub-harmonics in the ripple spectrum that are counterproductive to the advantage of ripple reduction. A method to cancel the fundamental frequency of the voltage ripple is proposed that is applicable to a converter consisting of three or more phases. It is shown that even for a low phase count the overall ripple amplitude is greatly reduced, only by adjustment of the phase shift angles. Experimental results carried out with a three-phase interleaved bi-directional dc-dc converter proof the concept functionality.

Efficiency optimization of an automotive multi-phase bi-directional DC-DC converter

Switching devices based on wide band gap materials as SiC oer a signicant perfor- mance improvement on the switch level compared to Si devices. A well known example are SiC diodes employed e.g. in PFC converters. In this paper, the impact on the system level perfor- mance, i.e. eciency/power density, of a PFC and of a DC-DC converter resulting with the new SiC devices is evaluated based on analytical optimisation procedures and prototype systems. There, normally-on JFETs by SiCED and normally-off JFETs by SemiSouth are considered.

Output ripple reduction of an automotive multi-phase bi-directional dc-dc converter

Soft-switching techniques are an enabling technology to further reduce the losses and the volume of automotive dc-dc converters, utilized to interconnect the high voltage battery or ultra-capacitor to the dc-link of a Hybrid Electrical Vehicle (HEV) or a Fuel Cell Vehicle (FCV). However, as the performance indices of a power electronics converter, such as efficiency and power density, are competing and moreover dependent on the underlying specifications and technology node, a comparison of different converter topologies naturally demands detailed analytical models. Therefore, to investigate the performance of the ARCP, CF-ZVS-M, SAZZ and ZCT-QZVT soft-switching converters, the paper discusses in detail the advantages and drawbacks of each concept, and the impact of the utilized semiconductor technology and silicon area on the converter efficiency. The proposed analytical models that correlate semiconductor, capacitor and inductor losses with the component volume furthermore allow for a comparison of power density and to find the η-ρ-Pareto-Front of the CF-ZVS-M converter.

SiC vs. Si - Evaluation of Potentials for Performance Improvement of Power Electronics Converter Systems by SiC Power Semiconductors

Pulse loads, like solid-state pulse modulators, generate short pulses with a high peak power that exceeds the average power by 100-1000 times depending on the pulse repetition rate. There, the peak power usually is drawn from an energy buffer such as a capacitor bank. The pulse discharges the energy buffer, and it is fully recharged in the time between the pulses by a power supply, which is usually connected to the mains. Due to the worldwide variation in mains voltages and the desired ability to adapt to the capacitor voltage of the modulator, the power supply has to support a wide input and output voltage range. Additionally, the supply should draw a sinusoidal current from the mains while providing energy to the pulse modulator due to electromagnetic interference regulations. Therefore, a general control concept for pulse load applications, which guarantees continuous power consumption from the mains and power factor correction, is described in this paper. Furthermore, measurements of the control principle, which is independent from the converter topology, are presented for a three-phase buck-boost rectifier.