Power electronics application

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