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EE462L, Spring 2014 Electronic Components

EE462L, Spring 2014 Electronic Components. 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. I Amps. Area A. But first, wires.

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EE462L, Spring 2014 Electronic Components

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  1. EE462L, Spring 2014Electronic 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. I Amps Area A But first, wires I Amps flowing uniformly through cross section A square meters yields current density J Amperes/m2 Rated J about 100-200 Amperes/cm2 • #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.

  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? 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. Answer – by compression fittings. This principle is used on a smaller scale in house wiring with twist nuts.

  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!

  11. ! Distortion in ENS voltage i leads v Distortion in the voltage is exaggerated in the current waveform 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

  12. ! i lags v Distortion in the voltage is attenuated in the current waveform 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

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

  14. i Gate + v – Anode Cathode ! on “Fire the gate” with a current pulse to turn on the thyristor i v off Forward breakdown (avoid!) 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 Controllability? -Controlled turn on, uncontrolled turn off.

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

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

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