Accurate Measurement of Dynamic on-State Resistances of GaN Devices under Reverse and Forward Conduction in High Frequency Power Converter

Because of trapped charges in GaN transistor structure, device dynamic ON-state resistance RDSon is increased when it is operated in high frequency switched power converters, in which device is possibly operated by zero voltage switching (ZVS) to reduce its turn-ON switching losses. When GaN transistor finishes ZVS during one switching period, device has been operated under both reverse and forward conduction. Therefore its dynamic RDSon under both conduction modes needs to be carefully measured to understand device power losses. For this reason, a measurement circuit with simple structure and fast dynamic response is proposed to characterise device reverse and forward RDSon. In order to improve measurement sensitivity when device switches at high frequency, a trapezoidal current mode is proposed to measure device RDSon under almost constant current, which resolves measurement sensitivity issues caused by unavoidable measurement circuit parasitic inductance and measurement probes deskew in conventional device characterisation method by triangle current mode. Proposed measurement circuit and measurement method is then validated by first characterising a SiC-MOSFET with constant RDSon. Then, the comparison on GaN-HEMT dynamic RDSon measurement results demonstrates the improved accuracy of proposed trapezoidal current mode over conventional triangle current mode when device switches at 1MHz

Intrinsic magnetic properties in R(Fe1−xCox)11TiZ(R=Yand Ce;Z=H,C,and N)

To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, R(Fe1−xCox)11TiZ with R=Y, Ce and interstitial doping (Z=H,C,N). The magnetization M, Curie temperature TC, and magnetocrystalline anisotropy energy K calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to K reveal that all three Fe sublattices promote uniaxial anisotropy in YFe11Ti, while competing anisotropy contributions exist in YCo11Ti. As observed in experiments on R(Fe1−xCox)11Ti, we find a complex nonmonotonic dependence of K on Co content and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated TC enhancements are in the sequence of N\u3eC\u3eH, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of R(Fe1−xCox)11TiZ generally decreases with C and N; although, for R=Ce, C doping is found to greatly enhance it for a small range of 0.

General Relativistic Instability Supernova of a Supermassive Population III Star

The formation of supermassive Population III stars with masses $\gtrsim$ 10,000 Msun in primeval galaxies in strong UV backgrounds at $z \sim$ 15 may be the most viable pathway to the formation of supermassive black holes by $z \sim$ 7. Most of these stars are expected to live for short times and then directly collapse to black holes, with little or no mass loss over their lives. But we have now discovered that non-rotating primordial stars with masses close to 55,000 Msun can instead die as highly energetic thermonuclear supernovae powered by explosive helium burning, releasing up to 10$^{55}$ erg, or about 10,000 times the energy of a Type Ia supernova. The explosion is triggered by the general relativistic contribution of thermal photons to gravity in the core of the star, which causes the core to contract and explosively burn. The energy release completely unbinds the star, leaving no compact remnant, and about half of the mass of the star is ejected into the early cosmos in the form of heavy elements. The explosion would be visible in the near infrared at $z \lesssim$ 20 to {\it Euclid} and the Wide-Field Infrared Survey Telescope (WFIRST), perhaps signaling the birth of supermassive black hole seeds and the first quasars.Comment: 23 pages, 4 figures (accepted to ApJ

Origin of magnetic anisotropy in doped Ce2Co17 alloys

Magnetocrystalline anisotropy (MCA) in doped Ce2Co17 and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the formation of 1-5 structure that exhibits a large uniaxial anisotropy, such that Zr is the most effective element to enhance MCA in this system

Intrinsic magnetic properties in R(Fe1−xCox)11TiZ(R=Yand Ce;Z=H,C,and N)

To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, R(Fe1−xCox)11TiZ with R=Y, Ce and interstitial doping (Z=H,C,N). The magnetization M, Curie temperature TC, and magnetocrystalline anisotropy energy K calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to K reveal that all three Fe sublattices promote uniaxial anisotropy in YFe11Ti, while competing anisotropy contributions exist in YCo11Ti. As observed in experiments on R(Fe1−xCox)11Ti, we find a complex nonmonotonic dependence of K on Co content and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated TC enhancements are in the sequence of N\u3eC\u3eH, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of R(Fe1−xCox)11TiZ generally decreases with C and N; although, for R=Ce, C doping is found to greatly enhance it for a small range of 0.

SiC/GaN power semiconductor devices theoretical comparison and experimental evaluation

SiC and GaN power transistors conduction loss and switching losses are compared in this paper. In order to compare performance of the same power rating device, a theoretical analysis is given to compare SiC device conduction loss and switching losses change when device maximal blocking voltage reduces by half. Then static and dynamic characteristics of commercial SiC and GaN power transistors are compared and it is shown that GaN-HEMT would still have smaller ON-state resistance and inter-electrode capacitance in comparison with a 600V SiC device. After that, switching losses E8w of a GaN-HEMT is measured and compared with that of a 1200V SiC-JFET and a 600V SiC-MOSFET, in which it is shown that E8w of a GaN-HEMT is smaller than a SiC power transistor with the same power rating

SiC and GaN power transistors switching energy evaluation in hard and soft switching conditions

SiC and GaN power transistors switching energy are compared in this paper. In order to compare switching energy Esw of the same power rating device, a theoretical analysis is given to compare SiC device conduction loss and switching losses change when device maximal blocking voltage reduces by half. After that, Esw of a 650V GaN-HEMT is measured in hard switching condition and is compared with that of a 1200V SiC-MOSFET and a 650V SiC-MOSFET with the same current rating, in which it is shown that Esw of a GaN-HEMT is smaller than a 1200V SiC-MOSFET, which is smaller than 650V SiC-MOSFET. Following by that, in order to reduce device turn-ON switching energy, a zero voltage switching circuit is used to evaluate all the devices. Device output capacitance stored energy Eoss are measured and turn-OFF switching losses are obtained by subtracting Eoss, which shows that GaN-HEMT is sill better than SiC device in terms of switching losses and 1200V SiC-MOSFET has smaller switching losses than 650V SiC-MOSFET