1 EE462L, Spring 2014 Electronic Components

2 Our power electronic switches Diodes (a.k.a. rectifiers) Thyristors (a.k.a. silicon controlled rectifiers, SCRs) Triacs (two antiparallel thyristors in one package) Power MOSFETs

3 But first, wires #22 solid for protoboards (1A) #16 stranded “appliance wiring” for circuits (5-10A) #14 stranded “house wiring” for circuits (10A) Short pieces of #14 tinned solid wire are used for MOSFET connections. I Amps flowing uniformly through cross section A square meters yields current density J Amperes/m 2 Area A I Amps Rated J about Amperes/cm 2

4 Question – if aluminum has a higher resistivity than copper, then why do all power lines use aluminum wires instead of copper wires? (note – in power lines, “wires” are called “conductors”) Answer – larger-diameter aluminum wires make up the difference in resistance, but still have less weight per km and lower cost per km Question – why aren’t power line wires insulated? Answer – insulation blocks the transfer of heat to the air, thus lowering the current rating of the wires. Insulation serves no purpose because air is a very good insulator.

5 Question – but solder doesn’t work with aluminum, so how are electrical connections made? Answer – by compression fittings. This principle is used on a smaller scale in house wiring with twist nuts. Question – so if a power line wire isn’t insulated, why can a bird safely sit on the wire? Answer – because the bird is insulated from ground and not near wires of other phases. However, it is possible for large birds with long wing spans to make a phase-to-phase connection.

6 Buzzards like transmission lines and cause many short circuits

7 But crime doesn’t pay!

8 And Nature may take revenge!

9 We can try to be more friendly with Nature by using power electronics systems (if we generate enough power locally we don’t need as many transmission lines as we need now), but….

10 Sometimes Nature may not distinguish between technologies that are environmentally friendly and those that are not!

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12 Capacitors Linear, but frequency dependent Resists sudden voltage changes with i = C dv/dt Impedance decreases with frequency Stored energy is proportional to squared voltage Distortion in the voltage is exaggerated in the current waveform Distortion in ENS voltage i leads v !

13 Inductors Linear, but frequency dependent Resists sudden current changes with v = L di/dt Impedance increases with frequency Stored energy is proportional to squared current Distortion in the voltage is attenuated in the current waveform i lags v !

14 Diodes Power Schottky Zener Switching v i About 0.8 – 1.0V Reverse breakdown 200V Current rating 10-20A Typical power diodes that we use + v – i AnodeCathode Note – the voltage and current ratings are not simultaneous Controllability? - Uncontrolled turn on, uncontrolled turn off. !

15 Thyristors (a.k.a. silicon controlled rectifiers, SCRs) When forward biased, it becomes “a diode” when a pulse of gate current is injected (“firing the gate”) Then, like a diode, it turns off when the current tries to reverse v i Forward breakdown (avoid!) “Fire the gate” with a current pulse to turn on the thyristor off on + v – i Gate AnodeCathode Controllability? - Controlled turn on, uncontrolled turn off. !

16 Triacs (for symmetric AC operation) Two antiparallel triacs in one package Positive gate current fires one, negative gate current fires the other i Gate Application of triac in 120Vac light dimmer circuit !

17 Power MOSFETs (a high-speed, voltage-controlled switch) G: Gate S: Source D: Drain N channel MOSFET equivalent circuit If desired, a series blocking diode can be inserted here to prevent reverse current Switch closes when V GS ≈ 4Vdc G D S Controllability? - Controlled turn on, controlled turn off. (but there is an internal antiparallel diode) !