day1 switches

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INTRODUCTION TO SWITCHES



POWER SWITCHING DIODE



•As shown in the fgure, there is heavi! "o#e"n$ su%strate with "o#ing eve o& '(')*+-. Thissu%strate &ors a cathode of the power diode.•On n$ su%strate, ight! "o#e" n/ e#ita0iaa!er is grown. This a!er is aso 1nown as drift

region. The "o#ing eve o& n/ a!er is a%out'('2*+-.•The the PN 3un+tion is &ore" %! "i4using aheavi! "o#e" #$ region. This #$ region &orsanode of the diode. The "o#ing eve o& #$region is a%out '(')*+-.

•The thi+1ness o& #$ region is '(5. Thethi+1ness o& n$ su%strate is 67(5.•The thi+1ness o& n/ "ri&t region "e#en"s u#onthe %rea1"own votage o& the "io"e.•The "ri&t region "eterines the reverse%rea1"own votage o& the "io"e.•Its &un+tion is to a%sor% the "e#etion a!er o&

the reverse %iase" #$n/ 3un+tion.•As it is ight! "o#e", it wi a"" signif+antohi+ resistan+e to the "io"e when it is &orwar"%iase".•9or higher %rea1"own votages, the "ri&t regionis wi"e.•The n/ "ri&t region is a%sent in ow #ower signa"io"es.



The most important feature of a high frequency rectifier is the reverserecovery characteristic. This affects S.M.P.S. performance in several ways.

These include

Increased diode switching loss,

higher peak turn-on current and dissipation in the power transistors

Increased generation of electro-magnetic interference e.m.i.! and voltage

transient oscillations in the outputs.

"learly, the rectifier must have optimum reverse recovery characteristics to

keep this catalogue of effects to a minimum.



In #ra+ti+e, the three a3or #araeters trr, :s an"Irr are a "e#en"ent u#on the o#erating +on"ition

o& the re+tifer. This is suarise" as &oows;/• Increasing the forward current, I#, increases trr, $s and Irrm.

• Increasing the dIF/dt rate by using a faster transistor and reducing

stray inductance, significantly decreases trr, but increases Qs and Irrm.

High dIF/dt rates occur in the high frequency square wave switching

found in S..!.S. a""lications. #$SF%&s can "roduce very small fall

times, resulting in very fast dIF/dt'.

• Increasing diode (unction tem"erature, &( increases all three.

• )educing the reverse voltage across the diode, *r , also slightly increasesall three.

9a+tors in<uen+ing reverse re+over!



aternating P an" N t!#e>n$#n/n$? a!ers.



aternating P an" N t!#e>n #n n ? a!ers.•The P t!#e i""e a!er is +ae" as %o"! o& =OS9ET. In this region , the+hanne is &ore" %etween sour+e an" "rain.•The n/ a!er is +ae" as "ri&t region,whi+h "eterines the %rea1"ownvotage o& the "evi+e. This n/ region is #resent on! in Power =OS9ETs not insigna eve =OS9ET.•The gate terina is isoate" &ro %o"! %! sii+on "io0i"e a!er.•When the #ositive gate votage is a##ie" with res#e+t to sour+e, the n/t!#e+hanne is &ore" %etween sour+e to "rain.•As shown in the fgure, there is a #arasiti+ n#n @T %etween sour+e an""rain.•To avoi" this @T turns on, the #/t!#e %o"! region is shorte" to sour+e region

%! overa##ing the sour+e etaiBation on to the # t!#e %o"!.The result is a parasitic diode which is formed %etween drain to source terminals. This

integral diode plays an important role in half and full %ridge converter circuits



Soe o& the &a+ts a%out Power =OS9ET is given %eow

'.Power =OS9ET has three terinas +ae" Drain,Sour+e an" Gate.6.It is a votage +ontroe" "evi+e.-.It is a uni#oar "evi+e so its o#eration "e#en"s u#onthe <ow o& a3orit! +arriers on!.2.The gate +ir+uit i#e"an+e ver! high.7.So the +ontro signa>gate votage? reuire" to turnon the =OS9ET is esser than reuire" +ontro signa&or the @T.

.There is no se+on" %rea1"own #ro%e in this "evi+e.F.=OS9ET has #ositive te#erature +oe+ient, sothera runawa! #ro%e wi not ha##en.



Sl No BJT MOSFET

1 It is a Bipolar Device It is majority carrier Device

2 Current control Device Voltage control Device.

3Output is controlled by

controlling base current

Output is controlled by

controlling gate voltage

4 egative temperature coe!!icient"ositive temperature coe!!icient

# $o paralleling o! B%& isdi!!icult.

$o paralleling o! t'is device iseasy.

(

Dive circuit is comple). It

s'ould provide constant

current*Base current+

Dive circuit is simple. It s'ould

provide constant voltage*gate

voltage+

, -osses are lo. -osses are 'ig'er t'an B%&s.

/$o used in 'ig' poerapplications.

0sed in lo poer applications.

B%&s 'ave 'ig' voltage and

current ratings.

&'ey 'ave less voltage and

current ratings.

1$itc'ing !reuency is loer

t'an O$56&.$itc'ing !reuency is 'ig'.

Suar! o& Co#arison %etween @T an" =OS9ET;



To select a MOSFET for a particular application,following parameters have to be consideredfrom the device datasheet

•Maximum Drain to Source voltage !DSS

"

•On#state drain to source resistance $DSO%"

•Drain &urrent 'D

•(ate to source !oltage !(S

•$everse recover) time Trr

•(ate charge *(

•+ower Dissipation +D

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