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PCB layouts with different polygon shapes and insulation distances were prepared and their surface flashover voltage subjected to different ramping rates were measured at different temperatures and different low gas pressures for emulated high-altitude conditions. The dependence of the surface flashover voltage on effects of gas pressure, surface insulation distance, temperature, and polygon shape was studied. Modulation efforts which include tailoring the local surface conductivity and local topography modification were performed to increase the flashover of PCB, and the related mechanism was studied. The results showed that the voltage ramping rate plays an important role in determining flashover voltage. The flashover voltage is lower when the sample is subjected to a rapid voltage ramping rate than with a slow ramping rate. This phenomenon is more prominent towards ambient pressure. The increase in temperature results in a decrease in flashover voltage at 100 kPa, while at 20 kPa and 10 kPa, the influence of temperature on flashover becomes less significant. Through-holes designed in the PCB have a positive role in increasing flashover voltage at lower pressures. However, at 100 kPa, the holes no longer contribute to any higher flashover voltage. Modification of local surface conductivity has no contribution in increasing the flashover voltage, while a surface coating with a surface conductivity of $10^{-10}$ S dramatically decreases the flashover voltage. The work presented in this paper provides a reference for the design and modification of PCB layouts for use in future aerospace hybrid propulsion systems.

With ever-increasing power-density requirements, technologies such as energy storage systems and electric-vehicles can benefit greatly from interfacing medium-voltage (MV)-AC grid like 13.8kV or 30kV using high-AC/low-DC voltage converter. Using modular high-AC/low-DC voltage converter can help increase power-density and efficiency, while reducing total conversion steps and providing flexibility. Full-bridge modular multilevel converters (FB-MMC) and solid-state transformers are existing solutions for such operations, but suffer from limitations of high semiconductor requirements, large submodule capacitors and/or many high-frequency transformers. Three new hybrid-MMC (HMMC) topologies are proposed in this paper as alternative solutions for such high-AC/low-DC voltage operations. Each of the three developed HMMCs utilizes a unique combination of low-frequency high-voltage switches and fast-switching low-voltage switch based submodules to generate multilevel-AC voltage. HMMCs are compared extensively to state-of-the-art FB-MMC and are shown to have semiconductor savings of over 27%, 38% lower submodule capacitor size, and 53% lower losses for 13.8-kV-AC/6-kV-DC operation. Due to these benefits like higher efficiency, significantly smaller submodule capacitance requirements, and fewer semiconductors, HMMCs can be an excellent option for high-AC/low-DC applications. Practical considerations like snubber and DC split-capacitor requirement are also elaborated for developing and commercializing HMMCs. Comparison results are verified using a 17.5 kW three-phase MV laboratory prototype.

Modeling and simulation of the converter dominated systems have recently emerged as a monumental challenge. These simulations are useful for predicting instability and system's harmonic content, consequently assisting in developing better control and filters. But, as more and more converters are incorporated into the system, time-domain simulation models have a tradeoff of either limited accuracy or fewer simulated converters. On the contrary, frequency-domain modeling methods like linear time-invariant models cannot capture the system's time-varying nature and complex converter interactions. The recently developed harmonic state space (HSS) modeling approach has emerged to provide an alternate avenue to model and simulate these converter-dominated systems. HSS models, while accurate, until recently have not been able to reliably model the non-linear pulse width modulator. This paper solves this challenge and develops a new modulator model for harmonic domain models, including HSS. The proposed large-signal modulator is computationally less demanding and can reliably capture the harmonic spectrum of complex multi-converter systems till converter switching frequencies. Several optimization methods are introduced to utilize the unique multi-harmonic nature of HSS, making simulations faster than conventional time-domain methods. Results are verified with extensive simulations and experiments. The application, challenges, and future scope for this method are also highlighted.

In this study, we aim to provide a deep convolutional network based femoral neck fracture detection system on radiographs for emergency patients. We retrospectively collected 1,491 frontal pelvic radiographs from three institutions and assigned them to the following data sets: primary dataset (710 radiographs, to fine-tune and validate the initial model called the Digital Radiography Fracture Detection System [DR-FDS]), internal test set 1 (189 radiographs) and 2 (235 radiographs), and external test set 1 (189 radiographs) and 2 (168 radiographs). Per-bounding box recall and precision and per-image sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (AUC) were computed. We randomly extracted 300 radiographs from the above test sets and compared their effect on the diagnostic accuracy and efficiency of fine-tuned model-assisted and unassisted clinicians. The fine-tuned DR-FDS showed a better overall performance in detecting femoral neck fractures than did the initial DR-FDS. The fine-tuned DR-FDS achieved AUC values of 0.9526 (95%CI, 0.9048–0.9767) and 0.9633(95%CI, 0.9346-0.9797) in internal test sets 1 and 2. In external test sets 1 and 2, this model also achieved promising results with AUC values of 0.9231 (95%CI, 0.8779–0.9520), and 0.9937 (95%CI 0.9739–0.9985), respectively. The clinicians showed a statistically significant increase in specificity, sensitivity, and accuracy for the identification of minimal/undisplaced fracture and a decrease in the average reading time. The object detection model that is fine-tuned has high sensitivity and specificity and the universal ability to detect and locate femoral neck fractures on pelvic radiographs.

Random numbers play a vital role in communications and cryptography. However, most existing true random number generators have difficulty in satisfying the requirements of high-speed communications due to their complexity and bulkiness, or low speed limitations due to equipment bandwidth. Then, the all-electron true random number generator was presented based on GaAs/Al0.45Ga0.55As superlattices conducted under direct current bias at room temperature, which not only possesses the characteristics of miniaturization but also generates random numbers at rates up to the Gbit/s. However, the bit rate of random number generators based on superlattices is still much slower compared to chaotic laser random number generators. In order to generate higher-rate random numbers, we modified a DC-excited superlattice and then redirected the signal generated by the superlattice back into itself, thus introducing a self-feedback, and achieved a self-feedback superlattice true random number generator. This improvement makes the superlattice have a more detailed signal shape, a more effective signal amplitude, and with lower power consumption. Therefore, these advances made the self-feedback superlattice more suitable for generating random numbers. Moreover, we propose a new post-processing method, called the adjacent bits reversal exclusive-or. This method can reduce the sequence bias and correlation without discarding any random bit. The random number obtained by the self-feedback superlattice at a sampling rate of 10 GS/s passed the triple standard deviation test and the random number standard test (NIST SP 800-22), indicating that it possessed good statistical properties as a miniaturized random number generator.