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Internal Assessment Test 1 – Sept. 2017Sub: Power Electronics Sub Code: 15EE53 Branch: EEE

Date: 21/09/17 Duration: 90 min’s Max Marks: 50 Sem/Sec: 5A & 5B OBEAnswer any FIVE FULL Questions MARKS CO RBT

1 (a) What is power Electronics? Specify various applications of power electronics. [05] CO6 L1

(b) List out the different types of power electronic Converters. Consider that DCsupply is given to a circuit which supplies an AC fed linear load. What is thecircuit that can be used for this operation? Draw the circuit and explain the inputand output waveforms.

[05] CO6 L1

2 (a) Write short notes on the following :1) Ideal Characteristics of a switch.2) Practical Characteristics of a switch.3) Switch Specifications.

[10] CO6 L1

3 (a) The BJT is specified to have β in the range of 8 to 40. The load resistance Rc = 11Ω.The DC supply voltage is VCC = 200V and the input voltage to the base circuit isVB = 10V. If VCE(sat)= 1.0V and VBE(sat) = 1.5V. Calculate

a. The value of RB that results in saturation with a overdrive factor of 5.

b. The forced βf.

c. The power loss PT in the transistor.

[06] CO1 L3

(b) Discuss the Output and transfer characteristics of a power BJT. [04] CO1 L2

4 (a) With a neat diagram explain the switching characteristics of a MOSFET. [07] CO1 L4

(b) Compare power BJT with power MOSFET. [03] CO1 L5

5 (a) What is the need for isolation of drive circuits from power circuits? Explain the gateand base drive isolation circuits with a neat diagram.

[10] CO1 L4

6 (a) Derive the expression for reverse recovery time (trr) of a diode by making use ofthe reverse recovery characteristics.

[07] CO1 L3

6 (b) Write a short note on silicon carbide Schottky diodes. [03] CO1 L1

7 (a) Explain any two base drives that can be used for BJT. [10] CO1 L4

8 (a) Derive the average and RMS output voltage of a single phase full wave diodebridge rectifier (R- Load). Also draw the related output voltage and currentwaveforms.

[10] CO2 L3

APPLICATIONS OF POWER ELECTRONICS

The demand for control of electric power for electric motor drive systems and indus-trial controls existed for many years, and this led to early development of theWard-Leonard system to obtain a variable de voltage for the control of de motor drives.~l"~l~ctI.oniC£.h.aY~evolutionized.the concept Lpo er control for Q.qwer conver-sion .a1l.dl0 control Q ectrical motor drives. _.

- ~ower electronics combin~ power, el~r~nics, and controL~ontroLdeals with thesteady-state and dynamiC characteristics of closed~loop systems. power deals with thestatlc and rotatjng pOwer equipment for the g'eneration, tranSmissioIl:, and djstributionof ~l ctric__eneJ:gy.ElectrOnics deal with the s lid-state devices and circuits for signalprocessing to meet the desjred control objectives. 'Olyer__!lectronics maybe defined asth~ruilic~tions of solid-state electronics for the control and conversion of electricp-ower1The interrelationship of power electronics with power, electronics, and controlis shown in Figure 1.1. .

( Power electronics are based--l2rimarily on the switching gf the power semjcgn-.ductor devjces.)Vith the devel~pment of power semiconductor technology, the power-handlin caQabiHties and the switching speed of the power devices have improvedtremendously. The development of microprocessors and microcomputer t~chnology

. . .. \

has a great impact on the control and synthesizing the control strategy for the powersemiconductor devices. Modern power electronics equipment uses 1)_ power semi-conductors that can be regarded as the muscle, and (2) mjcraeJfrctrOJ~jcs Utat have thegower and intelligence of a brain. '

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~ TYPES OF ,(OWER ELECTRONIC CIRCUITS .

For the cont~l of electric power. or power conditionin~, t~e conversion. o~~lectricpower from one form to another IS necessary and' the switching characteristics of thepower devices permit these conversions, The .static power converters. perform thesefunctions of power conversions.A co~verter may' be consIdered as aswitching matrix.The power electronics circuits can be classified into sixtypes:

~ ':---- -------_ ....--) ;

/ .. Diode rectifiers .. ~~ Ac-dc converters (controlled rectifiers)k\Ac-ac converters (ac voltage controllers)~ Dc-dc converters (de choppers)bpc-ac converters (inverters)

~/ Static switches ..

desupply Vs

FIGURE 1.15~'1;¥

. ; ,

+

(a) Circuit diagram~.~., .: I .. ~~'~~t ~~..

Single-phase dc-ac conve.rie:r.~,f' ,. ,_',

",', I 'Vg'l"Vi2 .' : .'. d ."I, -, , .

O~~~~~--~~~~--·t; i.,';: '>,1'

I ~ -' '_ ','. 4

,-Vs

- ....... --'

(~) Voltage ~a~eforms1'1 '.t 1'"

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Ideal Characteristics

The characteristics of an ideal switch are as follows:

1. In the on-state when the switch is on, it must have (a) the ability to carry aforward current IF,. tending to infinity; (b) a low on-state forward voltageVON, tending to zero; and (c) a low on-state resistance RON, tending to zero.RON causes low on-state power loss PaN' These symbols are normally referredunde-rde steady-state conditions.

2. In the off-state when the switch is off, it must have (a) the ability to withsthigh forward or reverse voltage VBR, tending to infinity; (b) a low off-stateage current [OFF, tending to zero; and (c) a high off-state resistance RoFF, ten ."...".<I'

to infinity. High ROFF cause low off-state powerloss POFF' These symbols are ...."'...._"..Mally referred to under de steady-state conditions.

3. During the turn-on and turn-off process-it must be completely turned on and o~1instantaneously so that the device can be operated at high frequencies. Thus, it<must have (a) a low delay time td, tending to zero; (b) a low rise time t., tendint~~to zero; (c) a low storage time t; tending to zero; and (d) a low fall time tf' tend-;ing to zero.

4. For turn-on and turn-off, it must require (a) a low gate-drive power PG, tendingiazero; (b) a low gate-drive voltage VG, tending to zero; and (c) a low gate-.driv.e'!

. ,r~

current IG, tending to zero. ;,:fBoth turn-on and turn-off must be controllable. Thus, it must turn on with a gatesignal (e.g., positive) and must turn off with another gate signal (e.g., zero ornegative).' ./For turning on and off, it should require a pulse signal only, that. is, a small pulse,with a very small width tw, tending to zero.It must have a high dvldt, tending to infinity. That is, the switch must be capable.of handling 'rapid changes of the voltage across it.

8. It must have a high dildt, tending to infinity. That is, the switch must be capable ofhandling a rapid rise of the current through it. .

- .9. It requires very low thermal impedance from the internal junction to the ambient

RJA, tending to zero so that it can transmit heat to the ambient easily.10. The ability to sustain any fault current for a long time is needed; that is, it must.

have a high value of i2t, tending to infinity. ~.

11. Negative temperature coefficient on the conducted current is required to resultin an equal current sharing when the devices are operated in parallel.

U. Low price is a very important consideration for reduced cost of the power elec-tronics equipment.

2 Characteristics of Practical Devices

During the tum-on and -off process, a practical switching device, shown in Figure 1.19a,requires a finite delay time (td), rise time (tr), storage time (ts), and fall time (tf). As thedevice current iSlO rises during tum-on, the voltage across the device Vsw falls. As the de-vice current falls during turn-off, the voltage across the device rises. The typical wave-forms of device voltages Vsw and currents isw areshown in Figure 1.10b. The turn-on

Vsw

VSW(sat)

o ~~----~----------~--------~------_. t. f--- ton --1 l--- toff ---1ISW-rl----~--------------~

I II II I

Isw8 ~~I ~I :_ _ _l _L__ L __ .:=:===.....tIsws

--- ........-Vcc

Vsw V G(sat)

+

switch

IIII

I I-~~----~--------------~

(a) Controlled switch

,.1I

o ~~------~------~-----------------.tf--:,__------- T = I1fs --------

o ~~----~--------~-- __ ----~------~t. ,Psw, ,

I,,,"

OL_J,--..L.....--- ......L__ -'----t

(b) Switch waveforms

FIGURE 1.10

Typical waveforms of device voltages and 'currents.

1~ cneprer I

ILIONPON = - pdt

Ts 0

time (t ) of a device is the sum of the delay time and the rise time, whereas theoff tim~n(toff) of a device is the sum of the storage time and the fall time. In contrastan ideal, lossless switch, a practical switching device dissipates som~ e~ergy whenducting and switching. Voltage drop across a conducting power device IS at least onorder of 1 V, but can often be higher, up to several volts. The goal of any new .to improve the limitations imposed by the switching parameters.

The average conduction power loss, PON is given by

where Ts denotes the conduction period and p is the instantaneous power loss (product of the voltage drop Vsw across- the switch and the conducted current isw)' _'--..,..._-"...

losses increase during turn-on and turn-off of the switch because during the transitifrom one conduction state to another state both the voltage and current have signifi-cant values. The resultant switching power loss Psw during the turn-on and turn-off pe-riods, is given by

(s = lITs is ~heswitching frequency; t, ts' and f f are the rise time, storage time, and fallnrne respectively Therefore, the power dissipation of a switching device is given by:

PD. PON + Psw + Pc

where Pc is the gate driver power.

(1.3)

4.3 Switch Specifications

1?e characte~istics of practical semiconductor devices differ from those of an ideal de-VIC:.;~~ de~Ice manufacturers supply data sheets describing the device parametersan . err ratmgs. There are many parameters that are important to the devices Themost important among these are: .

Voltage ratings: Forward and reverse re etitiv kforward voltage drop. p e pea voltages, and an on-state

Current ratings: Average, root-mean-square (rrns) ..peak, and off-state leakage currents. ms ,repetItIve peak, nonrepetitive

Switching speed or frequency: Transition fromconducting state (turn-on) and from a full a fu~ly nonconducting to a fullystate (turn-off) are very important ara y c.onductIn~ to ~ fully nonconductingquency fs are given by p meters. The SWItchIng period Ts and fre-

fs = _!_ = 1. Ts td + t, + ton + ts + tf + toff

where toff IS the off time during which the swi .e SWItch remains off

di/dt rating: The device needs a minimum amount of time before its 'whole coniducting surface comes into play in carrying the full current.If the current risesrapidly, the current flow may be concentrated to a certain area and the device maybe damaged. The di/dt of the current through the device is normally limited byconnecting a small inductor in series with the device, known as a: series snubber.dvldt rating: A semiconductor device has an internal junction capacitance C]. Ifthe voltage across the switch changes rapidly during turn-on, tum-off and alsowhile connecting the main supply the initial current, the current C] dvldt flowingthrough C] may be too high, thereby causing damage to the device. The dvldt ofthe voltage across the device is limited by connecting an RC circuit across the de-vice, known as a shunt snubber, or simply snubber.Switching losses: During turn-on the forward current rises before the forwardvoltage falls, and during turn-off the forward voltage rises before the current falls.Simultaneous existence of high voltage and current in the device representspower losses as shown in Figure 1.10b. Because of their repetitiveness, they rep-resent a significant part of the losses, and often exceed the on-state conductionlosses.Gate drive requirements: The gate-drive voltage and current are important pa-rameters to turn-on and -off a device. The gate-driver power and' the energy re-quirement are very important parts of the losses and total equipment cost. Withlarge and long current pulse requirements for turn-oh and turn-off, the gatedrive losses can be significant in relation to the total losses and the cost of thedriver circuit can be higher than the device itself.Safe operating area (SOA): The amount of heat generated in the device is pro-portional to the power loss, that is, the voltage-current product. For this productto be constant P = vi and equal to the maximum allowable value, the currentmust be inverse proportional to the voltage. This yields the SOA limit on the al-lowable steady-state operating points in the voltage-c~rrent coordinates.121 t for fusing: This parameter is needed for fuse selection. The I2t of the devicemust be less than that of the fuse so that the device is protected under fault cur-rent conditions.Temperatures: Maximum allowable junction, case and storage temperatures, usu-ally between 150°C .and 400°C for junction and case, and between - 50°C and175°C for storage.Thermal resistance: Junction-to-case thermal resistance, Q]C; case-to-sink ther-mal resistance, Qcs; and sink-ambient thermal resistance, QSA- Power dissipatio~must be rapidly removed from the internal wafer through the pac~age ~nd ulti-mately to the cooling medium. The size of semiconductor power ~wlt~besIS small,not exceeding 150 mm, and the thermal capacity of a bare device IS too low tosafely remove the heat generated by internal losses. Power devices are 8~neraUymounted on heat sinks. Thus, removing heat represents a high cost of equipment.

The bipolar transistor in Figure 4.7 is specified to have ~F in the.range of 8 to 40. The load resis.tance is Re = 11 fi. The de supply voltage is Vee = 200 V and the input voltage to tbe.basecir.euit is VB = .10 V. If VCE(sat) = 1.0 V and VBE(sat) = 1.5 V, find (a) the value of RB that resultsinsaturation with an ODF of 5, (b) the ~forced, and (e) the power loss Pr in the transistor.

SolutionVee = 200 V, ~min= 8, ~max= 40, Re = 11 fi, ODF = 5, VB = 10 V, VeE(sat) = 1.0 V, andVBE(sat) = 1.5 V. From Eq. (4.14), Ies = (200 - 1.0)/11 = 18.1 A. From Eq. (4.15), IBs =18.1I~min= 18.118= 2.2625 A. Equation (4.16) gives thebasecurrent fOIj an overdrive factorof 5, . . . .' . . . . ....

IB = 5 x 2.2625 = 11.3125A

a. Equation (4.9) givesthe required value of RB,

b. From Eq. (4.17), ~f~rced = 18.1111.3125 == 1.6.c. Equation (4.18)·yields the total powerloss.as

, ~ .. '. r :

Pr = 1.5 x 11.3125 + 1.0 x 18.~ = 16.97 + 18.1 = 35.07 W

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+Vee

(a) Circuit diagram o --..c.. ~ VBE(b) Input characteristics

I,Ie ~ I

Saturation: ,region I

, 1.', IBn> IBI >t,IBO

--------- IB4

-------IB3

------------ IB2

------------ IBI-__J"",,/-/-S-~t-~f-!r-=-/~p.-·fJ-~~/I~/-/~/~/""'""/--IB ' 0

O--~~~~~~~~~~~~~~-'VCE(C) Output ~haracteristics

FIGURE 4.4

Characteristics of NPN-transistors.

'----Active,-____.- :-+-; SaturationIIIIIII,II,,

,,,I,I,,

II, ,

VCE(saci L-_-__--_!.T_-_-_-_-_-_-_-_-::-_'-_-_-:--i. • IB

~Bs

..

0.5 VBE(sat)

.,.,'

FIGURE 4.5

Transfer characteristics.

Switching CharacteristicsWithout any gate signal, an enhancement-type MOSFET may be considered asdiodes connected back to back or as an NPN-transistor. The gate structure has para:

InRD

Rl G+ o-----..NV' 0

+ + +

~

+ VDDVOl

VDsVG VGs

+ +V VGsG

s(a) Circuit diagram

(b) Equivalent circuitFIGURE 4.21

Steady-state switching model of MOSFETs.

VGVI -----.------- _

I,/ /t', ,

FIGURE 4.24

Switching waveforms and times.

capacitances to the source, Cgs' and to the drain, Cgd. The npn-transistor has a reverse-biasjunction from the drain to the source and offers a capacitance, Cds., Figure 4.22a shows theequivalent circuit of. a parasitic bipolar transistor in parallel with a MOSFET. Thebase-to-emitter region of an NPN-tninsistor is shorted at the chip by metalizing thesource terminal and the resistance from the base to emitter due to bulk resistance:ofn-and p-regions, Rbe, is small. Hence, as MOSFET'may be considered as having an internal-diode and the equivalent circuit is shown in Figure 4.22b. The parasitic capacitances aredependent on their respective voltages. . --.,

The switching model of MOSFETs is shown in Figure ~~2'3)Jk~typical s'_"itch-ing waveforms and- times .are shown in Figure :.4·~i·~11te~-·:f.urn~'O~titielay-,;ja-tOri):.-'tStbe

.. .,.. , ..;."' .

time that is required to charge the input capacitance to threshold voltage level.Therise time t, is the gate-charging time from the threshold level to the full-gate voltageVGSP, which is required to drive the transistor into the linear region. The turn-offdelay time td(off) is the time.required for the input capacitance to discharge from theoverdrive gate voltage Vi to the pinch-off region. VGS must decrease significantly be-fore VDS begins to rise. The fall time tf is the time that is required for the input ca-pacitance to discharge from the pinch-off region to threshold voltage: If VGS =:; Vr,the transistor turns off.·' .

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REVERSE 'RECOVERY CHARACTER'STICS

The current in a forward-biased junction diode is due to the net effect of majority andminority carriers. Once a diode is in a forward conduction mode and then its forward cur-rent is reduced to zero (due to the natural behavior of the diode circuit or application ofa reverse voltage), the diode continues .to conduct due to minority carriers that remainstored in the pn-junction and the bulk semiconductor material. The minority carriers re-quire a certain time to recombine with 'opposite charges and to be neutralized. This time

36 Chapter 2 Power Semiconductor Diodes and Circuits

IIRR -------

~

(a) Soft recovery

FIGURE 2.3

Reverse recovery characteristics.

: \ ;

IRR -------

(b)J\brupt recovery, .

is called the reverse recovery time of the diode. Figure 2.3 shows two reverse recoverycharacteristics of junction diodes. The soft-recovery type is more common. The reverserecovery time is denoted as t., and is measured from the initial zero crossing of the diodecurrent to 25% of maximum (or peak) reverse currentIRR. The t., consists of two com-ponents, fa and tb' Variable t, is due to charge storage in the depletion region of the junc-tion and represents the time between the zero crossing and the peak reverse current IRR'

The tb is due to charge storage in the bulk semiconductor material. The ratio '(tito-isknown as the softness factor (SF). For practical purposes, one needs be concerned withthe total recovery time t., and the peak value of the reverse current IRR.,

t., = ta + tb

The peak reverse current can be expressed in reverse dildt as

diIRR = tadt

(2.5)

(2.6)

Reverse recovery time t.; may be defined as the time interval between the instant thecurrent passes through zero during the changeover from forward conduction to re-verse blocking condition an~ the mo~ent the reverse current has decayed to 25% of itspeak reverse value IRR· Vanable trr IS dependent on the junction temperature, rate offall of forward current, and forward current prior to commutation I

h ' F·.Reve~se recovery c a.rge '!RR, is the amount of charge carriers that flows acrossthe diode m the reverse direction due to changeover from f d d .. . . .. orwar con uction to re-verse blockmg condition. Its value IS determined from the area enclosed b h h ofthe reverse recovery current. y t e pat

!be stor~ge ch1arge,which is the area enclosed by the path of the recoverv cur-rent, IS approximate y "J

I\

or (2.7)

ting IRR in Eq. (2.6) to IRR in Eq. (2.8) gives

t t = 2QRRrr alii/lit (2.9)

be lb is negligible as compared to la, which is usually the case, t., ~ ta' and Eq. (2.9)ecomes

t., :::: 2QRRdildt (2.10)

(2.11)

It can be noticed from Eqs. (2.10) and (2.11) that the reverse recovery time t., and thepeak reverse recovery current IRR depend on the storage charge QRR and the reverse(or reapplied) dildt. The storage charge is dependent on the forward diode current iF.The peak reverse recovery current IRR, reverse charge QRR, and the SF are all of inter-est to the circuit designer, and these parameters are commonly included in the specifi-cation sheets of diodes. -

If a diode is in a reverse-biased condition, a leakage current flows due to the mi-nority carriers. Then the application of forward voltage would force the diode to carrycurrent in the forward direction. However, it requires a certain time known as forwardrecovery (or turn-on) time before all the majority carriers over the whole junction cancontribute to the current flow. If the rate of rise of the forward current is high and theforward current is concentrated to a small area of the junction, the diode may fail.Thus, the forward recovery time limits the rate of the rise of the forward current andthe switching speed.

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