Thursday Class

Modeling of switches +N

+N

+N

R -N

on

-N switch

Three types of switches permitted in SPICE are Voltage-controlled switches Current controlled switches Time-dependent switches

R

0 ff

Diode Model A

A R ++ + V

+

++

V _

D

s

K Static diode model with reverse biased condition (Diode surrent which depends on its voltage is represented by a current source. •

+ ++ V

D

R

C

D

D

_

_

s

s

C

D

R

R

K

D

•

Rs is the series resistance and is due to the resistance of semi conductor is depended on doping

+

V

R

s

D

-

The capacitance CD is a non linear function of the diode voltage and is equal to CD=dqd/ dvD where qd is the depletion –layer charge.

• PSPICE model statement for diode • .MODEL DNAME D(P1=V1 P2=V2 … v PN=VN) • For e.g., • Reverse breakdown voltage BV=1200 • Reverse breakdown current IBV=13mA • Instantaneous voltagev =1V iat =150A • Reverse recovery charge Qrr =194μC at IFM =200A F

F

F

 VD  n = − 1 I D I S  e V T    1  1(25.8*10^  −3) − 1 150 = I S  e    I S = 2.2E − 15 A

So if the diode name is DMOD, the transit time

τ

T

= Qrr/IFM= 194μC /200A=1μsec

Assuming CJO =2pf, VT is the thermal voltage. The PSPICE model statement is .MODEL DNAME D(P1=V1 P2=V2 … PN=VN)

.MODEL DMOD D (IS=2.22E-15 BV=1200 IBV=13E-3 CJO=2PF TT=1US)

• Switching elements can be implemented using a transistor and diode. Why? • How to realize semiconductor switches in general? • Switch implementation : • depends on the power processing function being performed.

Requirements of switches • Switches of Inverters and cycloconverters: • Require compicated implementations than those of dc-dc converters. • Behavior of switches: • Semi conductor power devices behave as single pole single throw switches.

How an ideal switch can be realized using semi conductor devices ? • • • • • • • • • • • •

It depends on the polarity of the voltage that the devices must block in the off state, On the polarity of the current that the devices must conduct in the on state. If the current and blocking voltage lie in a single quadrant of the plane , then the switch can be implemented using a transistor or a diode eg., switches in dc-dc converters. In inverter circuits, two-quadrant switches are required. The output current is ac, and hence some times +ve and some times –ve.The semi-conductor switch realization is more complicated. A two-quadrant SPST switch can be realized using a transistor and diode. There could be a case where the switch current is always +ve, but the blocking voltage is ac. The above arrangement can be constructed using a different arrangement of a transistor and diode. Cyclo converters require four-quadrant switches, which are capable of blocking ac voltages and conducting ac currents. The synchronous rectifier examined reveals: The need for reverse-conducting capability of the metal oxide semi conductor field effect transistor(MOSFET) allows it to be used where a diode would normally be required. Majority carrier devices including the Mosfet and Schottky diode, exhibit very fast switching times,and hence are preferred when the off state voltage levels are not too high. Minority carrier devices (BJT,IGBT) thyristors (MOS controlled thyristor )MCT exhibit high break down voltages with low forward voltage drops, at the expense of reduced switching speeds