Transcript Lecture 12

Lecture #12 Circuit models for Diodes,
Power supplies
Reading:
Malvino chapter 3, 4.1-4.4
Next: 4.10, 5.1, 5.8
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Then transistors (chapter 6 and 14)
1
Circuit models
• Now that we have studied the physics
underlying how a diode works, we are
going to hide all of it in a circuit model
Why?
• If we create a circuit model, then we can
draw and analyze electronic circuits
without getting lost in the details.
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IV curve for an ideal diode
• The IV curve for a ideal diode is to have
zero current in the reverse direction, and
no resistance when forward biased
Current 
Voltage →
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Real diode IV curve
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Idealized devices
• We have encountered the idea of ideal
devices before:
• A voltage source is like a battery, but
produces a perfect voltage regardless of
current:
And the ideal current
source, a current
regardless of voltage
~
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The ideal diode
• We now add another ideal device, the ideal diode.
A real diode
drawn as the same symbol
sometimes in a circle to
make it clear that it is not
a ideal diode
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The ideal diode as a switch
• The ideal diode behaves as a switch:
• If current is being pushed through in the
forward direction the switch is closed.
• If a reverse bias voltage is applied, the
circuit is closed.
Reverse Bias:
Forward Bias:
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Ideal diode vs real diode IV curve
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Ideal diode vs real diode IV curve
We could improve our model for
real diode by not closing the switch
until the voltage gets about 0.7
volts into the forward bias.
We can do this in a circuit by
making a circuit model
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The ideal diode
To make a somewhat better model of a real diode:
+
~
0.7 volts
-
We use an ideal diode in series
with an ideal voltage source
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Ideal diode vs real diode IV curve
We could improve our model further
by sloping the IV curve for the
region where forward current is
flowing
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Improved diode model
To make an even better model of a real diode:
R
We use an ideal diode in series
with an ideal voltage source and
a resistor. The resistance needed for
the model is given by the inverse’
of the slope of the IV curve
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+
~
0.7 volts
-
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Key point: the model can change
• Which model you use for a device can change
depending on
– What the mode of operation of the device is
– how accurately you need to model the device
For example: A hand analysis of a power supply
would probably use an ideal diode, and then
break the problem into two time periods
– When the diode is forward biased
– When the diode is reverse biased
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Higher accuracy models
• If a diode was to be used at high frequencies
(hundreds of megahertz or higher) then the
model would have to account for the movement
of charge in and out of the depletion zone, a
capacitive effect.
It is important to use a model which is accurate
enough to account for the necessary effects,
without using so complicated a model that it is
difficult to understand what is going on!
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Applications
•
•
•
•
•
•
•
•
Applications of diodes include
Power supply rectifiers
Demodulators
Clippers
Limiters
Peak detectors
Voltage references
Voltage multipliers
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Half-wave rectifier
• A single diode can be used to take an
alternating current, and allow only the
positive voltage swing to be applied to the
load
~
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An AC input is sinusoidal
1.5
1
0.5
0
0
5
10
15
20
-0.5
-1
-1.5
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The diode blocks the negative
voltages
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
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Full-wave rectifier
• If we add an additional diode, it does not pass current at
the same time as the first diode, but the load is now
disconnected during the negative half cycle.
• What if we could flip the connection and use the negative
half wave?
~
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Full-wave rectifier
• The result is called a full wave rectifier
~
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Full-wave rectified voltage
1.2
1
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0.6
0.4
0.2
0
0
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Transformers
• In order to use a full wave rectifier, the source
and the load must be able to float with respect to
each other
• One way to isolate AC power is to use a
transformer. A transformer is a couple of coils of
wire which transfer power by a changing
magnetic field.
• By having different numbers of windings, or
turns of wire, a transformer can step up or step
down an AC voltage.
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Transformers
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• The voltage across the secondary of the
transformer (the output windings) is:
N2
V2 
V1
N1
• But this only works for changes in the
voltage—and therefore for AC only
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Filtering
• A transformer and a full wave rectifier will
produce a voltage which is always
positive, but varies with time
• In order to power electronic devices, we
need to smooth out the variations with
time.
• Another way to look at this is that we need
to store energy temporarily while the input
voltage changes sign.
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Power supply filter capacitor
• If we add a capacitor in parallel with the load, it will
charge up when power is available from the voltage
source, and then it will slowly discharge through the load
when the diodes are off.
~
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R
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Full wave rectified, with filtering
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0.8
0.6
0.4
0.2
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Ripple
The result is a DC voltage, with some residual variations at twice the
frequency of the AC power. The variation is called ripple.
1.2
1
0.8
0.6
0.4
0.2
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0
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