The steadily increasing power demand in automobiles has led in the recent past to a substantial review of the hitherto adopted philosophy of generation and distribution of electrical energy, resulting in the introduction of a novel concept, the 42V-PowerNet. Within such architecture low-voltage (< 100 V) PowerMOSFET's of new generation are used extensively, carrying out many and very diverse functions. While coping with the cost restrictions of the automotive industry, the transistors must comply with very tough requirements, typical of military and space applications (the relation between the maximum acceptable cost-per-power in the respective market-segments is about 500 to 1). That implies very stressful working conditions while, at the same time, considerable operational lifetimes must be guaranteed. Work dedicated to the improvement of the performance of the switches and to the identification of possible failure mechanisms acquires a fundamental importance for the definition of the new electrical network. The present investigations have been directed at the analysis of the high-frequency switching behaviour of the devices and of their use as linear regulators. As regards the first group of activities, the achievement of significant results was bound to the ability of reducing the study of relatively complex and different power conversion circuits to that of a basic cell, representative of a whole spectrum of critical operational conditions. That led to the consideration of the half-bridge connection of transistors switching an unclamped inductive load. Within such topology currents up to 70 A, over a drain-source voltage of 42 V, must be switched at a frequency of 170 kHz, at ambient temperatures up to 140 deg C, using the body-diode of the devices as a freewheeling element. A thorough characterisation of the electrical evolution of the transistors during the switching transitions was carried out, using the power dissipation as an indicator of performance. The experimental analysis relied on hardware solutions carefully designed to minimise the circuit parasitics and the influence of the observer, according to methods of common adoption in power electronics. Moreover, since it is difficult to test all possible conditions and effects inside the devices are not visible, measurements alone do not provide sufficient insight and extensive use was also made of circuit simulation with an accurate physics-based compact model of the devices. In particular, a pre-existing model had to be modified and extended with the description of the parasitic Bipolar Junction Transistor, including an original modelling of the various components of the gate-source capacitance.
Performance and reliability of PowerMOSFETs in the 42V-PowerNet
2004
133 Seiten, Bilder, Tabellen, 72 Quellen
Theses
English
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