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

Predicting processing parameters in high temperature laser sintering (HT-LS) from powder properties

PublishedThis is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.There are two other ORE records for this publication: http://hdl.handle.net/10871/21707 and http://hdl.handle.net/10871/36733New materials for laser sintering (LS) are usually developed using a trial and-error approach that consists of a series of builds within LS systems. This strategy is time consuming, costly and focuses only on the optimisation of the processing parameters, ignoring the powder properties of the materials under examination. Being able to predict processing parameters on the basis of the powder material properties would enable a faster development of new materials and new applications, while acknowledging a more in-depth understanding of the mechanisms involved in LS. This paper provides new results into the prediction of processing conditions from the material properties. It is here shown that high temperature polymers such as poly ether ether ketone (PEEK) and poly aryl ether ketone (PEK) can be successfully used in LS despite the lack of a super-cooling window. The evaluation of the stable sintering region of PEEK 450PF and the application of the energy melt ratio parameter in relation to the mechanical performance of laser sintered PEEK samples are also provided. Lastly, a new method for estimating the powder bed temperature is proposed.The authors wish to acknowledge Victrex [23] and Invibio Biomaterials Solutions [44] for the supply of materials

Dynamic thermo-mechanical and impact properties of helical auxetic yarns

This paper presents an experimental investigation of the dynamic thermo-mechanical and impact properties of helical auxetic yarns (HAYs). A series of thermoplastic polyurethane (TPU) core fibres fabricated using an extrusion process have been wrapped with either ultra-high-molecular-weight polyethylene (UHMWPE) wrap or stainless steel wire wrap to form helical auxetic yarns. Dynamic mechanical analysis (DMA) measurements indicated that the core/wrap diameter ratio and the initial wrap angle influenced significantly the dynamic thermo-mechanical behaviour of HAYs. The impact test results have shown that the fibre property, impact velocity and the initial wrap angle had great effect on the impact response of a HAY. Importantly, in this work it is shown that an optimal wrap angle can be found to give the best combination of stiffness, energy absorption and auxetic performance of HAYs.This work is supported by the UK Engineering and Physical Science Research Council (EPSRC grant No. EP/J004553/1). The authors would like also to acknowledge their colleagues Yat-Tarng Shyng and the late Dave Baker for technical support

Predicting processing parameters in high temperature laser sintering (HT-LS) from powder properties

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.There are two other ORE records for this item: http://hdl.handle.net/10871/21707; http://hdl.handle.net/10871/24577New materials for laser sintering (LS) are usually developed using a trial and-error approach that consists of a series of builds within LS systems. This strategy is time consuming, costly and focuses only on the optimisation of the processing parameters, ignoring the powder properties of the materials under examination. Being able to predict processing parameters on the basis of the powder material properties would enable a faster development of new materials and new applications, while acknowledging a more in-depth understanding of the mechanisms involved in LS. This paper provides new results into the prediction of processing conditions from the material properties. It is here shown that high temperature polymers such as poly ether ether ketone (PEEK) and poly aryl ether ketone (PEK) can be successfully used in LS despite the lack of a super-cooling window. The evaluation of the stable sintering region of PEEK 450PF and the application of the energy melt ratio parameter in relation to the mechanical performance of laser sintered PEEK samples are also provided. Lastly, a new method for estimating the powder bed temperature is proposed

Transforming growth factor-beta enables NFATc1 expression during osteoclastogenesis.

Osteoclastogenesis is dependent on distinct stimuli that prime and activate osteoclast differentiation. One cytokine needed to prime monocytes for osteoclastogenesis is TGF-beta, which enables and augments RANKL and TNF-alpha-induced osteoclast differentiation. However, the precise time-period during which this occurs and the molecular mechanism mediating this action are unknown. We report here TGF-beta prime monocytes for osteoclast formation within 24h by regulating expression of NFATc1, a key osteoclastic transcription factor. TGF-beta directly induces cytoplasmic NFATc1 expression within 24h, but is unable to stimulate NFATc1 nuclear translocation. Furthermore, RANKL-induced NFATc1 expression is dependent on the presence of TGF-beta during the early stages of osteoclastogenesis. Similarly, TNF-alpha activates osteoclastogenesis by stimulating translocation of TGF-beta-induced NFATc1. In light of these findings, it is apparent that osteoclast formation is dependent on coordinated interactions between TGF-beta and RANKL/TNF-alpha that regulate the expression and intracellular distribution of NFATc1 during early stages of osteoclast differentiation

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

GaN-HEMT dynamic ON-state resistance characterisation and modelling

GaN-HEMTs suffer from trapping effects which might increase device ON-state resistance (RDS(on)) values. Thus, dynamic RDS(on) of a commercial GaN-HEMT is characterized at different bias voltages in the paper by a proposed measurement circuit. Based on the measurement results, a behavioural model is proposed to represent device dynamic RDS(on) values, in which trapping and detrapping time constant is represented by a series of RC network. The model is simulated in PSPICE, of which the simulation results of RDS(on) values are compared and validated with the measurement when device switches in a power converter with different duty cycles and switching voltages. The results show that RDS(on) values of this device would increase due to trapping effects

Developing power semiconductor device model for virtual prototyping of power electronics systems

Virtual prototyping (VP) is very important for power electronics systems design. A virtual prototyping design tool based on different modelling technology and model order reduction is proposed in the paper. In order to combine circuit electromagnetic model with power semiconductor device models, a SiC-JFET behavioural model is presented and implemented in the design tool. A half bridge circuit using SiC-JFET devices is thus represented in the VP software. The presented SiC-JFET behavioural model is then validated by comparing with experimental measurements on switching waveforms

GaN-HEMT dynamic ON-state resistance characterisation and modelling

GaN-HEMTs suffer from trapping effects which might increase device ON-state resistance (RDS(on)) values. Thus, dynamic RDS(on) of a commercial GaN-HEMT is characterized at different bias voltages in the paper by a proposed measurement circuit. Based on the measurement results, a behavioural model is proposed to represent device dynamic RDS(on) values, in which trapping and detrapping time constant is represented by a series of RC network. The model is simulated in PSPICE, of which the simulation results of RDS(on) values are compared and validated with the measurement when device switches in a power converter with different duty cycles and switching voltages. The results show that RDS(on) values of this device would increase due to trapping effects