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EE462L, Spring 2014Electronic Components

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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

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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/m2

Area A I AmpsRated J about 100-200 Amperes/cm2

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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.

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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.

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Buzzards like transmission lines and cause many short circuits

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But crime doesn’t pay!

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And Nature may take revenge!

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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….

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Sometimes Nature may not distinguish between technologies that are environmentally

friendly and those that are not!

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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

Voltage

Current

Distortion in the voltage is exaggerated in the current waveform

Distortion in ENS voltage

i leads v

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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

Voltage

Current

Distortion in the voltage is attenuated in the current waveform

i lags v

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Diodes• Power

• Schottky

• Zener

• Switching

v

i

About0.8 – 1.0V

Reversebreakdown

200V

Current rating10-20A

Typical power diodes that we use

+ v –

i

Anode Cathode

• Note – the voltage and current ratings are not simultaneous

• Controllability? - Uncontrolled turn on, uncontrolled turn off.

!

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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

Forwardbreakdown

(avoid!)

“Fire the gate” with a current pulse to turn on the thyristor

off

on

+ v –

i Gate

Anode Cathode

Controllability? - Controlled turn on, uncontrolled turn off.

!

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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

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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 VGS ≈ 4Vdc

G

D

S

Controllability? - Controlled turn on, controlled turn off.

(but there is an internal antiparallel diode)

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