Download EE362L, Fall 2006

EE462L, Spring 2014
Electronic 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
I Amps flowing uniformly through
cross section A square meters yields
current density J Amperes/m2
Area A
I Amps
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.
3
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 – 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.
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.
5
Buzzards like transmission lines and cause
many short circuits
6
But crime doesn’t pay!
7
And Nature may take revenge!
8
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….
9
Sometimes Nature may not distinguish
between technologies that are environmentally
friendly and those that are not!
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11
!
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
i leads v
Distortion in
ENS voltage
Voltage
Current
Distortion in the voltage is exaggerated in the current waveform
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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
i lags v
Voltage
Current
Distortion in the voltage is attenuated in the current waveform
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!
Diodes
• Power
i
• Schottky
• Zener
+v–
Anode
Reverse
breakdown
200V
Cathode
i
• Switching
Current rating
10-20A
• Note – the voltage and current
ratings are not simultaneous
v
About
0.8 – 1.0V
• Controllability? - Uncontrolled
turn on, uncontrolled turn off.
Typical power diodes that we use
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!
Thyristors
(a.k.a. silicon controlled rectifiers, SCRs)
Gate
i
+v–
Anode
Cathode
on
i
• 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
“Fire the gate” with a current pulse to turn on
the thyristor
v
off
Controllability? - Controlled
turn on, uncontrolled turn off.
Forward
breakdown
(avoid!)
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Triacs
(for symmetric AC operation)
!
Gate
i
• Two antiparallel triacs in one package
• Positive gate current fires one,
negative gate current fires the other
Application of triac in 120Vac light dimmer circuit
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Power MOSFETs
(a high-speed, voltage-controlled switch)
!
D: Drain
D
If desired, a series
blocking diode can be
inserted here to prevent
reverse current
G
G: Gate
Switch closes
when VGS ≈ 4Vdc
S: Source
S
N channel MOSFET equivalent circuit
Controllability? - Controlled turn on, controlled turn off.
(but there is an internal antiparallel diode)
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