Hard-Switching and Soft-Switching Two-Switch Flyback PWM DC-DC Converters and Winding Loss due to Harmonics in High-Frequency Transformers

The flyback pulse-width modulated (PWM) DC-DC power converter is a very important circuit in switching mode power supply (SMPS) converters for low power applications. The main drawback of the conventional single-switch flyback converter is the high turn-off voltage stress suffered by the switch. The high voltage transients are caused by the resonant behavior of the transformer leakage inductance and the transistor output capacitance, resulting in ringing superimposed on the steady-state switch voltage level. This requires a transistor with higher voltage rating. However, a transistor with higher voltage rating has higher on-resistance causing higher conduction loss. The high voltage ringing also increases the switching loss. In addition, the switch voltage stress is not easily predictable because it is difficult to determine the magnitude of ringing during the design stage. The two-switch flyback DC-DC converter is an extended version of the single-switch flyback converter. The circuit arrangement with an addition of a power transistor and two clamping diodes to the conventional single-switch flyback converter leads to the two-switch flyback PWM DC-DC converter, which effectively reduces the switch overvoltage and eliminates the uncertainty of its value. The clamping diodes in the two-switch flyback converter clamps the voltage across each switch to the DC input voltage and also provide a path to return most of the energy stored in the transformer leakage inductance to the DC input source. In the first part of this research, detailed steady-state analyses of the two-switch flyback PWM DC-DC converter for continuous conduction mode (CCM) and discontinuous conduction mode (DCM) are performed. The transistor output capacitance and the transformer leakage inductance are included in the analyses. Design equations for both CCM and DCM operation modes are derived. Furthermore, by incorporating an active clamp circuit into the hard-switching two-switch flyback converter, a new soft-switching two-switch flyback converter, namely, zero-current transition (ZCT) two-switch flyback converter is proposed. The principle of circuit operation, steady-state analysis, equivalent circuits, converter steady-state waveforms, and design procedure of the proposed ZCT two-switch flyback converter is presented. The key features of the proposed soft-switching converter are 1) the voltage stresses of the main switches are reduced to DC input voltage VI, and 2) all the semiconductor devices are turned off under zero-current (ZC) switching condition. Clamping of the switch overvoltages and reduction in switching loss are achieved in the proposed ZCT two-switch flyback converter. Saber Sketch simulation and experimental results of the hard-switching and the proposed ZCT soft-switching two-switch flyback converters are presented to validate the theoretical analyses. High frequency (HF) transformers used in PWM converters, such as flyback transformers conduct periodic nonsinusoidal currents, which give rise to additional winding losses due to harmonics. In the second part of this research, a theory is developed to find the harmonic winding loss in an HF transformer conducting periodic nonsinusoidal current. Dowell\u27s equation is used to determine the winding resistances due to eddy currents as a function of frequency. Both skin and proximity effects are taken into account. Fourier series of the primary and secondary current waveforms in a two-winding flyback transformer and the primary and secondary winding resistances are used to determine the primary and secondary winding power losses at various harmonics for both CCM and DCM cases, respectively. The harmonic winding loss factors FRph and FRsh are introduced. The theory is illustrated by the case study of flyback converter for both CCM and DCM operations. Using the equations developed to find the winding losses due to harmonic..

Renal allograft pathology with C4d immunostaining in patients with graft dysfunction

Renal allograft biopsy is the gold standard for diagnosis of rejection. Incorporation of C4d as a marker for humoral rejection is a major addition for Banff Schema, 2005. We evaluated the pattern of C4d staining in indicated renal allograft biopsies from January 2005 to December 2009. Of the 67 biopsies analyzed, 21 were C4d-positive. They were 11 cases of acute rejection, seven chronic rejection and one biopsy each of acute tubular necrosis, BK virus nephropathy and normal biopsy. Morphologic features like peritubular capillary dilatation, tubulitis and interstitial inflammation were seen more frequently in C4d-positive biopsies and this was statistically significant. C4d positivity was noted in 50% of the chronic rejection cases indicating a humoral component in the pathogenesis of chronic rejection. There was no significant difference in the serum creatinine levels of C4d-positive and -negative patients, either at the time of biopsy or during the follow-up. This study supports the role of C4d immunostaining in confirming histologically diagnosed acute and chronic humoral rejections and in detecting histologically unsuspected cases

Integrated Assessments of the Impact of Climate Change on Agriculture: An Overview of AgMIP Regional Research in South Asia

South Asia encompasses a wide and highly varied geographic region, and includes climate zones ranging from the mountainous Himalayan territory to the tropical lowland and coastal zones along alluvial floodplains. The region's climate is dominated by a monsoonal circulation that heralds the arrival of seasonal rainfall, upon which much of the regional agriculture relies. The spatial and temporal distribution of this rainfall is, however, not uniform over the region. Northern South Asia, central India, and the west coast receive much of their rainfall during the southwest monsoon season, between June and September. These rains partly result from the moisture transport accompanying the monsoonal winds, which move in the southwesterly direction from the equatorial Indian Ocean. Regions further south, such as south/southeast India and Sri Lanka, may receive rains from both the southwest monsoon, and also during the northeast monsoon season between October and December (with northeasterly monsoon wind flow and moisture flux), which results in a bi- or multi-modal rainfall distribution. In addition, rainfall across South Asia displays a large amount of intraseasonal and interannual variability. Interannual variability is influenced by many drivers, both natural (e.g., El Ni-Southern Oscillation; ENSO) and man-made (e.g., rising temperatures due to increasing greenhouse gas concentrations), and it is challenging to obtaining accurate time-series of annual rainfall, even amongst various observed data products, which display inconsistencies amongst themselves. These climatic and rainfall variations can further complicate South Asia's agricultural and water management. Agriculture employs at least 65 of the workforce in most South Asian countries, and nearly 80 of South Asia's poor inhabit rural areas. Understanding the response of current agricultural production to climate variability and future climate change is of utmost importance in securing food and livelihoods for South Asia's growing population. In order to assess the future of food and livelihood security across South Asia, the Agricultural Model Intercomparison and Improvement Project (AgMIP) has undertaken integrated climate-crop-economic assessments of the impact of climate change on food security and poverty in South Asia, encompassing Bangladesh, India, Nepal, Pakistan, and Sri Lanka. AgMIP has funded, on a competitive basis, four South Asian regional research teams (RRTs) and one South Asian coordination team (CT) to undertake climate-crop-economic integrated assessments of food security for many districts in each of these countries, with the goal of characterizing the state of food security and poverty across the region, and projecting how these are subject to change under future climate change conditions