Post on 10-Jan-2022
QUESTIONS
β’ What is the Ohm Law formula for calculating voltage across a resistor ?
β’ Given the current Kirchoffs Law:
Οπ ππ = π, while
π = π, π, π, and
ππ = πππ¨, ππ = πππ¨,
what is the value of ππ ? ππ = βππππ¨
β’ What is the equivalent resistance, between
points A and C, for the given circuit? 10π
β’ What is the equivalent capacitance?
π. πππππ
9
ππππ
ππ
π¨
πͺ
π©
π¨ π©πππ πππ
πππ
ππ 6π
ππ
ππ
π = π β πΉ
SEMICONDUCTOR DIODE
β’ Diode is an element utilizing pn junction,
placed in a case with connectors
Diode symbol
11
PERFECT DIODE OPERATION
β’ Perfect diode can be modeled as a directional valve
β’ With forward polarization
(anode potential higher than cathode`s)
the circuit is closed
β’ With reverse polarization
(cathode potential higher than anode`s)
the circuit is open
12
ACTUAL DIODE OPERATION
β’ With forward polarization
β’ Conduction begins when the voltage reaches knee voltage
β’ A voltage drop occurs β depending on the particular diode and the current drawn,
thevoltage is usually ~0.7V, it can be always read from the characteristics
β’ Power dissipated is calculated as P=U*I
β’ With reverse polarization
β’ Reverse current of a few Β΅A flows
β’ Power dissipated is βnegligibleβ
β’ Exceeding the maximum reverse
voltage can break the diode
13
DIODE CAPACITANCE
β’ Consider a diode polarized in reverse direction
β’ An isolator layer occurs between both
p and n parts
β’ A parasitic capacitance is introduced
β’ When the polarization is inverted
(voltage goes to positive) the capacitanse
must be first discharged
14
TEMPERATURE DEPENDENCE
β’ Given a constant current flow, the voltage drop across a conducting diode
drops by 2mV/1degC
β’ The warmer the diode, the lower the voltage drop, and thus the power
dissipated.
β’ With rising temperature, the reverse current rises too.
15
DIODE MARKING
β’ p-semiconductor side is an anode
β’ n-semiconductor side is a cathode
β’ The current flows from anode,
to cathode (A-βΊK)
β’ On an actual device a stripe
always marks the cathode side
16
DIODE PARAMETERS I
β’ Maximum repetitive reverse voltage VRRM [V]
β’ Reverse voltage that can be periodically applied to the diode.
β’ Maximum DC reverse voltage VR or VDC [V]
β’ Reverse voltage that can be constantly applied to the diode.
β’ Maximum forward voltage VF [V]
β’ Forward voltage drop given for the nominal current flowing forward.
β’ Maximum (average) forward current IF(AV ) [A]
β’ Maximum forward current. The limit is a result from thermal power dissipation capabilities.
β’ Maximum peak/surge forward current IFSM [A]
β’ Peak current that can be applied to diode according to a specified pattern (for example a single pulse for a given duration)
17
DIODE PARAMETERS II
β’ Total power dissipation PD [W]
β’ The amount of power that can be dissipated from diode to the outside.
β’ Maximum operating junction temperature TJ [Β°C]
β’ Maximum temperaturΔ, that the junction can operate at, without breaking.
β’ Thermal resistance R(T) [Β°C/W]
β’ Parameter for calculation of the internal temperature at given powerdissipated.
β’ Reverse (leakage) current IR [Β΅A]
β’ Reverse current flowing at given negative voltage.
β’ Typical junction capacitance CJ [pF]
β’ Typical capacitance, usually expressed in pF.
β’ Reverse recovery time trr [Β΅s]
β’ Time for the diode to regain blocking capabilities after voltage polatitychange.
18
SCHOTTKY DIODE
β’ m-s metal semiconductor junction
β’ Low capacitance
β’ Short recovery time
β’ Lower forward voltage drop
β’ Lower breakdown voltage
β’ Used in high frequency applications
Schottky diode
symbol19
DIODE MODELS
Real Ideal Simplified Linearized
According to Shockleyβs
equation
No reverse current, no
forward voltageForward voltage
considered
Linear approximation
of the characteristics
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EXAMPLE
β’ Ideal model
β’ Simplified model
β’ Relative error
mAkk
V
RR
VI
VVVV
VVV
R
SS
SRR
DD
03.38.15.1
10
10
0
21
21
21
=+
=+
=
==+
==
mAk
VV
RR
VVI
VVVV
VVVVV
VVV
SS
SRR
DRDRS
DD
61.23.3
4.1104.1
4.1
7.0
21
21
2211
21
=
β=
+
β=
β=+
+++=
==
%1.16%10061.2
03.361.2% =
β=
Assuming ideal model, may lead to large
errors ! 23
DIODE TESTING
β’ Resistance measurement
β’ Forward polarization should give a resistance of <1kΞ©
β’ Reverse polarization should give
a resistance of >1MΞ©
or out of scale
β’ Diode testing mode
β’ Polarized in forward direction should give ~0,7V
β’ Reverse polarization should
occur as open circuit
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DIODE CHARACTERISCICS EXAMINATION
β’ Diode and resistor connected inseries
β’ Function generator producing triangle wave
β’ Oscilloscope in XY mode
β’ X channel corresponds to diode voltage
β’ Y channel corresponds to resitor voltage, thus the current flowing through the
resistor & diode
25
ZENER DIODE
β’ Utilizes the phenomenon of Zener effect
β’ Usually works in reverse polarity
β’ Breakdown voltage is very little dependednt on the current flowing
β’ Zener voltage ranges between a few Volts up to tens of Volts
Zener Diode symbols
27
ZENER DIODE PARAMETERS
β’ Zener voltage VZ given for a test current of IZT (mostly 20mA)
β’ Maximum power dissipated PD(max),
β’ Maximum current results from max. Power
β’ Dynamic impedance
β’ Diode model must consider the breakdown !
Z
D
ZMV
PI
(max)=
==
= 28
mA2
mV56
Z
ZZ
I
VZ
VZ
28
VOLTAGE STABILIZER
β’ Zener diodes are mostly used as a source of reference voltage
β’ The circuit given stabilises the output voltage, regardless of the input voltage,
provided:
β’ When V>VZ the diode operates in breakdown area:
β’ then V0βVZ
β’ Diode current I =πβππ§
π
β’ Diode power P = πΌππ§
β’ Resistor limits the current and
prevents the diode from overheating!
V= V0 +RI
29
GRAHICAL ANALYSIS
β’ Operating point moves when changing the source voltage
β’ Output voltage depends on the steepness of the characteristics,
thus it depends on dynamic impedance
30
REFERENCE VOLTAGE SOURCE
β’ Simple Zener diode circuit stabilizes output voltage
β’ Input voltage must by higher than the Zener voltage
β’ Any load applied must be considered in current flow calculation
β’ Power is dissipated in the resistor and
in the diode or the load
(dependent on the load)
β’ The circuit has very low efficiency
31
CUTTING SIGNAL I
β’ Output waveform can be affected using diodes
β’ Diode applied in series:
β’ Diode cuts one of the half-waves (depending on the direction)
β’ The other half wave is affected by the diode voltage drop
33
CUTTING SIGNAL II
β’ Diode in paralell
β’ Problem β circuit is shorted for
forward polarity
β’ Resistor added in series
β’ Diode voltage drop must be considered
34
LIMITING SIGNAL I
β’ Limiting voltage to a certain level with a diode and a voltage source
β’ Limiting both half waves (positive and negative)
35
LIMITING SIGNAL II
β’ The same application utilizing Zener diodes
β’ Single Zener diode will limit the voltage to forward voltage drop i one direction
and to zener voltage (VZ) in another direction
β’ Zener diodes can be connected in series, thus giving voltage cutting at the level of
Vz+0,7V
36
VOLTAGE SPIKES PROTECTION
β’ Voltage spikes protection:
β’ Must withstand big energies for short periods of time
β’ Must react instantly
β’ Zener diodes and both Transil diodes (or TVS) may be used
β’ Transil (TVS - transient voltage supressor) is a specialized diode designed for
suppresing high voltage spikes
β’ Example protection circuit:
Transil symbol
38
VARISTORS
β’ Varistor is non-linear resistor
β’ Characteristics given in the graph
β’ Compared to transil, varistor:
β’ Has longer time of reaction
β’ Can withstand bigger energies
Varistor symbol
39
VOLTAGE SPIKES PROTECTION
β’ Example 1 β redirecting the high voltage spike to
power supply (it has a relatively big capacitance,
so can absorb the energy)
(-0,7V Γ· +5,7V) utilizing standard diodes
β’ Example 2 β electro-static discharge
protection
40
AMPLITUDE DETECTION
β’ Demodulation of a signal (AM radio signal)
β’ Proper selection of components RLC1 allows to filter the signal and find its corresponding
βenvelopeβ
42
VOLTAGE DOUBLER I
β’ Single pulse voltage doubler
β’ Negative half-wave β C1 charges to Vs
β’ Positove half-wave β C2 charges to VS + VC1
β’ The resulting voltage is
the input voltage doubled
β’ Voltage drops must be
considered
43
VOLTAGE DOUBLER II
β’ Double pulse voltage doubler
β’ Negative half-wave β C1 charges to VS
β’ Positive half-wave β C1 charges to VS
β’ C3 voltage is sum of VC1 + VC2
β’ The resulting voltage is
the input voltage doubled
β’ Voltage drops must be
considered
44
OPTICAL APPLICATIONS
β’ Optical radiation detection (photodiode)
β’ Optical radiation emission (LED)
β’ Electrical power generation (solar cells)
47
PHOTODIODE
β’ Works in reverse polarity
β’ Current proportional to light flux
Photodiode symbol
48
LED I
β’ LED β light-emitting diode
β’ Color (wavelength) depends on the
semiconductor type used
β’ Radiation intensity is proportional
to the current flowing
49
LED symbol
LED II
β’ pn-junction is enclosed in a package
(shorter lead is cathode)
β’ Dual color LED`s are twosiodes
enclosed in a single package
50
SELECTING LED RESISTOR
β’ Diodes are usually designed for the current of 5-20mA
β’ Resistor selection, given the source voltage, diode voltage drop
and diode nominal current
( )
( )
1.8...2.0
20
8
8 1.8310
20
F
F
out pk
out pk F
S
F
V V
I mA
V V
V V V VR
I mA
=
=
=
β β= = =
51
LED APPLICATIONS
β’ Simple applications:
β’ Diode as an indicator
β’ 7-segment display:
β’ Lighting etc.
52
SUMMARY
Forward direction
Reverse direction
β’ Anode potential higher
than cathode
β’ Voltage drop of ~0.7V
β’ Current limited by power
dissipation
β’ Cathode potential higher
than anode
β’ Reverse current of ~Β΅A
β’ Breakdown voltage
β’ p = anode
β’ n = cathode,
marked by a strip on the
package
53