Semiconductores

Module 1
Power Semiconductor Devices
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Lesson 1
Power Electronics
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Introduction
This lesson provides the reader the following: (i) (ii) (iii) (iv) (v) Create an awareness of the general nature of Power electronic equipment; Brief idea about topics of study involved, The key features of the principal Power Electronic Devices; Anidea about which device to choose for a particular application. A few issues like base drive and protection of PE devices and equipment common to most varieties.

Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact, and robust manner for convenient utilisation. A passenger lift in a modern building equipped with aVariable-Voltage-Variable-Speed induction-machine drive offers a comfortable ride and stops exactly at the floor level. Behind the scene it consumes less power with reduced stresses on the motor and corruption of the utility mains.

Fig. 1.1 The block diagram of a typical Power Electronic converter Power Electronics involves the study of • • • • • • • • Power semiconductor devices - their physics,characteristics, drive requirements and their protection for optimum utilisation of their capacities, Power converter topologies involving them, Control strategies of the converters, Digital, analogue and microelectronics involved, Capacitive and magnetic energy storage elements, Rotating and static electrical devices, Quality of waveforms generated, Electro Magnetic and Radio Frequency Interference, Version2 EE IIT, Kharagpur 3

• Thermal Management The typical converter in Fig. 1.1 illustrates the multidisciplinary nature of this subject.

How is Power electronics distinct from linear electronics?
It is not primarily in their power handling capacities. While power management IC's in mobile sets working on Power Electronic principles are meant to handle only a few milliwatts, large linearaudio amplifiers are rated at a few thousand watts. The utilisation of the Bipolar junction transistor, Fig. 1.2 in the two types of amplifiers best symbolises the difference. In Power Electronics all devices are operated in the switching mode either 'FULLY-ON' or 'FULLY-OFF' states. The linear amplifier concentrates on fidelity in signal amplification, requiring transistors to operate strictly in thelinear (active) zone, Fig 1.3. Saturation and cutoff zones in the VCE - IC plane are avoided. In a Power electronic switching amplifier, only those areas in the VCE - IC plane which have been skirted above, are suitable. Onstate dissipation is minimum if the device is in saturation (or quasi-saturation for optimising other losses). In the off-state also, losses are minimum if the BJT is reversebiased. A BJT switch will try to traverse the active zone as fast as possible to minimise switching losses.

Fig. 1.2 Typical Bipolar transistor based (a) linear (common emitter) (voltage) amplifier stage and (b) switching (power) amplifier

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Fig 1.3 Operating zones for operating a Bipolar Junction Transistor as a linear and a switching amplifier Linearoperation Active zone selected: Good linearity between input/output Switching operation Active zone avoided : High losses, encountered only during transients Saturation & cut-off zones avoided: poor Saturation & cut-off (negative bias) zones linearity selected: low losses Transistor biased to operate around No concept of quiescent point quiescent point Common emitter, Common collector, Transistor drivendirectly at base - emitter common base modes and load either on collector or emitter Output transistor barely protected Switching-Aid-Network (SAN) and other protection to main transistor Utilisation of transistor rating of secondary Utilisation of transistor rating optimised importance

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An example illustrating the linear and switching solutions to a power...