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Power Electronics –Regulator
Application
Diode and Diode Circuit
• All materials can be classified (electrically) into three categories:» Conductors.
» Insulators.
» Semiconductors
• Conductors easily allow current to pass through them.
• Insulators do not allow current to pass through them. Semiconductors are a group of material that posses the property of neither insulator nor good conductor, but somewhere in between Example of semiconductors are silicon (Si) and germanium (Ge).
• Pure semiconductors are poor conductors, because the low number of free electrons. However, the resistivity can be reduced (so that it conducts more current) by putting in impurities into the pure semiconductors. The process of introducing a small amount of impurities (during manufacturing) into the semiconductors is called doping.
DOPING
•
• The type of material that is added to the pure semiconductor will determine whether it will become n-type or p-type semiconductor.
• N-type semiconductor is produced if the impurity is either phosphorus (P), arsenic (As), or antimony (Sb) - all from group 5 of periodic table. The
introduction of either one of these impurities into a pure semiconductor produces more free electron in the semiconductor.
• P-type semiconductor is produced if the impurity is either aluminium (Al), boron (B), or gallium (Ga) - all from group 3 of periodic table. The introduction of either one of these impurities into a pure semiconductor produces more "hole" in the semiconductor. A hole is a condition where there is absence of one electron, which gives the effect of more positive charge.
PN JUNCTION
• A p-n junction is piece of semiconductor material in which part of the material is p-type and part is n-type. In order to examine the charge situation, assume that separate blocks of p-type and n-type materials are pushed together. Also assume that a hole is a positive charge carrier and that an electron is a negative charge carrier.
• At the junction, the donated electrons in the n-type material, called majority carriers, diffuse into the p-type material and the acceptor holes in the p-type material diffuse into the n-type material as shown by the arrows in Figure 2.2.
• Because the n-type material has lost electrons, it acquires a positive potential with respect to the p-type material and thus tends to prevent further movement of electrons.
• The p-type material has gained electrons and becomes negatively charged with respect to the n-type material and hence tends to retain holes. Thus after a short while, the movement of electrons and holes stops due to the potential difference across the junction, called the contact potential.
• The area in the region of the junction becomes depleted of holes and electrons due to electron-hole recombination's, and is called a depletion layer, as shown in Figure 2.3.
PN JUNCTION
PN JUNCTION
Transistors
• Transistors often involve power transfer and are usually manufactured from silicon (resistor) material.
• The name ‘transistor’ derives from TRANSfer and resISTOR.
• The general form of a transistor is a crystal (usually silicon) in which two pn junctions are formed.
• The junctions can be npn or pnp.
• The basic transistor has three electrode regions within the one crystal structure (compared to two in the pn junction diode).
• These regions in a transistor are termed as base, collector, and emitter and that there will be three connection terminals
• This form of transistor if often termed a junction transistor or bipolar transistor.
Transistor
Transistors
Silicon Controlled Rectifier
• Silicon Controlled Rectifier (SCR).
• Thyristor is used for requiring high speed & high
power switching.
• Handle V & I up to 1 kV & 1000A
• Anode : high +ve voltage with relative to cathode &
gate at small +ve potential w.r.t cathode.
SCR
Transistors
Power electronics
With the application of sufficient reverse voltage, a p-n junction will experience
a rapid avalanche breakdown and conduct current in the reverse direction.
Zener Diode
Zener Regulator
The constant reverse voltage of the zener diode makes it a valuable component
for the regulation of the output voltage. The current through the zener will change
to keep the voltage at within the limits of the threshold current and the
maximum power it can dissipate
Is
Rs
VoVsIs
VoIsRsVs
VzVo
IoIsIz
IoIzIsKCL
R
VoI
L
L
:
(Any components in parallel with Zener, it will follow Vz)
Shunt Regulator – Overflow currents shunt away from Zener diode
Proton Saga 1.3L
Basic Idea:
Vs = VRS + Vo
IcIzIIsIc
IIcIzIs
R
VoI
Rs
VoVsIs
VoIsRsVVzIsRsVs
VVzVVo
L
L
L
L
BE
BECE
tocompare small as ,
Ic
Is
IL
IZ = IBIC = βIB
Proton Saga 1.5LWhen Vo↑, VBE ↑, IB ↑, IC ↑, IL↓, Vo normal
Iswara
VB
LCOSLCECD
L
OL
ZBEZO
OBEZB
IIVVIVIP
R
VI
VVVV
VVVV
in which )(
constant ,Vo ↑, VBE↓, IB↓,
(IC=IL)↓, Vo normal
Vo ↓, VBE ↑, IB↑,
(IC=IL)↑, Vo normal
LINE REGULALTION
LOAD REGULATION
= IE
(IR = IZ + IB)
(OPEN)
(SHORT)
•In normal operation, VB = VZ
•The current flowing through resistor R is:
•For a fixed Vs (and also Vz), IR is a fixed value
•When IB increases, Iz will decrease
•In order to get a good regulation, Iz must be larger than
a minimum value IZK over the rated range of load current
ZBR
ZSRZSRZSR
ZRS
OBEZB
III
R
VVIVVRIVVV
VVV
VVVV
,,
IR
Darlington Transistors
A typical modern device has a current gain of 1000 or more, so that only a tiny
base current is required to make the pair switch on.
Example:
Typical Darlington transistor has current gain of 1000.
If input current is 10mA, means that output current, IC = 10mA x 1000 = 10A
Darlington transistor combines two bipolar transistors in “Darlington pair")
in a single device so that the current amplified by the first is amplified further
by the second transistor.
This gives it high current gain and takes up less space than using two discrete
transistors in the same configuration.
112222
1121
BBC
BBC
IββIβI
IβII
Conclusion:
Darlington transistor required less base current, IB to produced the required
amount of IL. So less Iz drawn away from zener diode and the stability
of the circuit can be maintained.
WIRA
Concept: Depends on the voltage different between V+ and V-
When Vo↑, VR2 ↑, (V+ -V-)↓, IB↓, (IC=IL)↓, Vo normal
Concept: Depends on the VBE2
When Vo↑, VR2 ↑, VBE2↑, IC2↑, IB1↓, IC1↓, Vo normal
VR2 = VBE2 + VZ(constant)IR3 = IC2 + IB1
New equivalent circuit when the output is accidentally shorted
Analysis: Condition under short circuit
Vo = 0V, no feedback voltage, VBE = 0V
Q2 off.
Now:
200Ω
AmAII
So
II
SINCE
mAR
VVI
BC
BR
BESR
65.85.86100
,
,
5.86200
7.018
11
13
3
3
18v
WP
AVP
IVP
IVP
D
D
CSD
CCED
7.155
65.818
)0(
NDISSIPATIO POWER
1
11
P4 with 75Watts Power dissipation
1A
Protection Network
Current through Q1 Current overflow
Through Diodes
Protection network off
Protection network on
VBE < 0.7V
Equivalent Circuit Under Short Circuit Condition
Under Short Circuit Condition:
Most of the output voltage now dropped across RCS. VRB becomes
Large and shunt away most of the current from IR3.
RCS
BA
BBE V
RR
RV )(
Transistor turns ON before output load short circuit:
Concept: More short; more current shunt away
Heat Sink