EIE209 Basic Electronics Diode circuits Prof.. Tse: Diode circuits Contents • Diode models • Typical diode circuits & applications • Load line concepts for nonlinear load analysis... Tse
Trang 1EIE209 Basic Electronics
Diode circuits
Prof C.K Tse: Diode circuits
Contents
• Diode models
• Typical diode circuits & applications
• Load line concepts for nonlinear load analysis
Trang 2Prof C.K Tse: Diode circuits
What is a diode?
Simplest view (no physics):
— a unidirectional device
that allows current to flow in
one direction but not the
other.
Ideally, we regard a diode as
short circuit when voltage
applied to it in the forward
manner is positive.
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Ideal characteristic
Bias conditions
Forward bias : vd > 0 — current can flow and id > 0
Reverse bias : vd < 0 — current cannot flow and id = 0
vd
id
vd
id
0.7V
with finite forward drop (more realistic)
Ideal diode
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Real characteristic
Take a closer look at the characteristic around the turning point
The i-v characteristic is an exponential function
From physics, we have
Also, the diode can only stand the negative voltage up to a certain threshold VBD , beyond which the diode conducts reverse current (breakdown)
vd
id
0.7V
id = Iss( eq vd /k T
- 1)
–VBD
breakdown
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Which model to use?
The choice depends on the external voltage magnitudes.
Ideal model:
id = 100/100 = 1 A
With 0.7V drop:
id = (100–0.7)/100
= 0.997 A
Ideal model:
id = 10/100 = 100 mA With 0.7V drop:
id = (10–0.7)/100 = 93 mA
Ideal model:
id = 2/100 = 20 mA With 0.7V drop:
id = (2–0.7)/100 = 13 mA
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Example: rectifier circuit
The ideal model is valid if the external voltages are well above 0.7V.
What is the magnitude of vr ? Crude — 50 V
Better — 50 – 1.4 = 48.6 V
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Application examples
half-wave doubler full-wave doubler
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A nonlinear circuit problem
Suppose we wish to find vd , given that the external voltage Vs is not large
enough to validate the use of the ideal diode model
??
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Step 1 : locating the operating points
the operating point (vd,
id) is somewhere on the
diode characteristic curve
We know
is somewhere on the resistor characteristic curve
2
Recall: The characteristic curve/line for a device actually defines where the point (v, i) can lie.
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Step 2 : KVL & KCL constraints
from KCL : id = iR AND from KVL : vd + vR = Vs
We also know
=
+ = Vs
O
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Step 3 : enforcing KVL & KCL
Method: flip the resistor curve horizontally; and
push the two curves together horizontally until the y-axes are Vs apart
add to Vs
push push
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Solution : load line
The flipped resistor line is called the LOAD LINE
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General problem
+
_
R
+
vd –
+ vR – iR
id
Vs
nonlinear device
vd
id
How to find vd and id ?
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Basic load line construction
+
_
R
+
vd –
+ vR – iR
id
Vs
nonlinear device
v
i
Vs
slope = –1/R
vd
characteristic
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Alternative construction
+
_
R
+
vd –
+ vR – iR
id
Vs
nonlinear device
v
i
device characteristic
vd when device is open-circuit
id when device is short-circuit
load line
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Tutorial problem
Find the operating point.