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Two-stage AC-DC converters are considered as a prominent solution for DC-type electric vehicle (EV) chargers. However, this kind of architecture suffers from high switching losses with large heatsink and DC-link capacitor volume. To relieve this issue, this paper presents a new hybrid modulation for DC-type EV chargers, where a two-phase clamped discontinuous pulse-width-modulation (DPWM) in the front-end circuit is cooperated with the variable frequency triangular-current-mode (TCM) zero voltage switching (ZVS) or its simplified implementation, i.e., boundary-ZVS (B-ZVS) strategy, in the back-end circuit. The former can stop the switching actions in the front-end stage during two-thirds of the grid period, while the AC currents are at their highest values, which can yield to the best switching loss reduction and deliver high power factor operation. Besides, TCM-ZVS or B-ZVS modulations can achieve ZVS turn-on action for all semiconductors during all operating range in the DC-DC stage to further reduce the power losses on the semiconductors. With such characteristics, the proposed strategies can reduce the switching losses of the system to the best extent, and thus allow an enhancement of the system power density by improving the power conversion efficiency. The proposed strategy is described, analyzed, validated, and benchmarked in a 5 kW SMD SiC MOSFET-based two-stage AC-DC converter. A 99% power efficiency can be achieved with the solution implementing the TCM-ZVS strategy at an output voltage of 400 V and rated power. [ABSTRACT FROM AUTHOR]

The urban rail transit system has the characteristics of wide voltage fluctuation, intermittent and strong impact load, limited heat dissipation capacity. The high-speed flywheel energy storage system (FESS) is used to provide network voltage stability and regenerative braking energy recovery due to its high power density, unlimited number of charge-discharge cycles, fast response and environmental friendliness. When considering the design of high-speed FESS converter with 3.6 kHz fundamental frequency and 330 kW rated power used in 1500 VDC urban rail transit system, the two-stage topological converter reduces the efficiency and increase the cost and size, meanwhile, the series-connected converters and the multilevel converter increase the difficulties of control and insulation, so the two-level and I-type three-level converters of high-speed FESS are designed and evaluated. Several comparisons are made by detailed experimental data from the perspective of the traction load characteristics of the 1500 VDC urban rail transit system and the technical and economical requirements of the high-speed FESS. It is found that the three-level converter has significant advantages in matching the traction load characteristics for its higher duty cycle capability, as well as improving the technical and economical performances such as efficiency, du/dt value, THD, voltage stability, and reducing the cost by 40% when compared to the two-level converter. While in the future MW class high-speed FESS development, the maximum output power capacity of the three-level FESS is relatively lower than that of the two-level FESS. The theoretical analyses were verified by experiments. [ABSTRACT FROM AUTHOR]

Aiming to shed light on the operation of fully-digital transmitters, we present the first symbol-rate baseband model that makes explicit the inter-symbol interference introduced by the RF pulse-width modulation (RF-PWM). The proposed model expresses the transmitter output as symbols plus interference, paving the way for the derivation of in-band performance metrics of the communication system. Then, we find an approximation for the error vector magnitude that allows us to evaluate the impact of the oversampling ratio, the statistics of the transmitted symbols and the shaping pulse in the transmitter performance. Another advantage of the model is that simulation times are much shorter since it does not require interpolation to appreciate the PWM distortion. To obtain the model, we first assume a generic expression of the continuous-time signal to be transmitted, in terms of the information symbols and the reconstruction filter. This allows us to express the nonlinear differential equation that models the intrinsic distortion also in terms of the sent symbols. Then, we elaborate on the formulation of the continuous-time RF-PWM distortion to procure an expression of the received signal sampled at symbol rate. The article concludes with a numerical validation of the model, and a simulation of the bit error rate of the system. [ABSTRACT FROM AUTHOR]