diodes - pusan national universitybml.pusan.ac.kr/.../undergraduates/electronics/6_diodes.pdf ·...
TRANSCRIPT
DIODES
Ho Kyung Kim, Ph.D.
School of Mechanical Engineering
Pusan National University
Basic Experiment and Design of Electronics
Outline
• Diode I-V characteristics
• Diode approximations
• Rectifier circuits
• Voltage regulators
• Clamp and limiting circuits
2
Breakdown voltage ~ 20 V but Zener breakdown voltage ~ 5 V
Zener diode
• Zener diodes
– designed for operation in the reverse breakdown region
– heavily doped regions in the ohmic contact to reduce the avalanche breakdown voltage, causing very narrow depletion region an intense electric field
– providing an output reference voltage that is stable despite changes input voltage
– maintaining an essentially constant voltage across its terminals over a specified range of reverse current values
① for vD V conventional forward-biased diode
② for VZ < vD < V reverse-biased diode (open circuit)
③ for vD VZ constant voltage (-VZ)source
VZ
rZrD
V
• Large-signal diode models
– on/off diode model
Diode approximations
Example: half-wave rectifier
– offset diode model
• vD 0.6 V; diode 0.6-V battery
• vD < 0.6 V; diode open circuit
iD
vD0.6 Von off
offset diode model
v
t
0.6 V
0.6 V
Example: half-wave rectifier
• Small-signal diode models
– modeling the diode as a resistor (instead of a short circuit) once it is conducting to account for the slope of I-V curve
– quiescent (operating) point or Q point
vD
RT
VT iD
vD
iD
IQ
VQ
VD / RT
VT
Q point (IQ, VQ)
DTDT vRiV
)1(/
0 kTqv
DDeIi
T
T
D
T
D VR
vR
i11
load-line equation
– piecewise linear diode model
• treating the diode as an open circuit in the off state and as a linear resistor in series with V
in the on state
iD
vD
IQ
VD / RT
VT
Q point
VTV
1 / rD
D
D
D v
i
r
1
kTqV
VID
D
D
Q
ekT
qI
v
i
r
/0
),(
1
onoff
V
rD
diode incremental resistance
v
t
0.6 V
Example: half-wave rectifier
Rectifier circuits
RL
– Bridge rectifier
RL
D1D2
D3 D4
2V
• Capacitor filter
RL
rippleripple
%100(%) Ripple
DC
pp
V
V
VDC
VP-P
12
• DC power supply
– desirable as steady and ripple-free as possible
– transformer + rectifier + filter + regulator
• Voltage regulator
– holding a DC load voltage relatively constant in spite of possible fluctuations (independent of load and supply variations)
• Zener diode as a voltage regulator
– regulating the unregulated DC source, vS, to the value of the Zener voltage, VZ
–
– acting as a sink to the undesired source current;
• to keep the load at VZ, dumping excess currents to ground through the Zener diode
–
RL
RS
iSiZ iL
vS VZ
+
-
L
ZL
R
Vi
ZSL iii
S
ZSS
R
Vvi
• To protect loads against excessive voltages
• To keep the load voltage within a range, - Vmax vL(t) Vmax
Diode limiter (or clipper)
vS RL vL
D1 D2
Vmax Vmax
rS
max)( VtvRr
RS
LS
L
)()( tvRr
Rtv S
LS
LL
max)( Vtv
Rr
RS
LS
L
max)( VtvL
vS RL vL
D1
Vmax
rS
vL
D1
Vmax
rS||RL
)(tvRr
RS
LS
L
D1 is off D1 is on
Diode clamp
CR
VDC
vout