Chapter 3 Power Electronics 6/13/2009. Electronic Circuit II Chap 3 Power Electronics Silicon Controlled Rectifier

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Dorcas Byrd
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1 Electronic Circuit Chap 3 ower Electronics nstructor: jay umar adel athmandu Engineering College Course Homepage Electronic Circuit 1 ntroduction Thyristor Family SCR Triac TO MCT CT Thyristors have been the traditional workhorses for bulk power conversion and control lin industry The term thyristor came from its gas tube equivalent, thyratron Electronic Circuit 2 nstructor: jay umar adel 1

2 (SCR) Belongs to a thyristor family of semiconductors The is the most popular of the thyristor family of four layer regenerative devices so it is used synonymously with thyristor Compared to transistors Have lower on state conduction losses & handle higher power Have slower switching speeds and higher h switching losses Electronic Circuit 3 SCR ntroduction thyistor is a four layered semiconductor device of p n p n structure with three p n junction t has 3 p n junctions and 3 terminals t is normally turned on by the application of a gate pulse when a forward bias is present at the main terminals. However, being regenerative or 'latching', it cannot be turned off via the gate terminals node () J1 J2 ate () J3 Cathode () Structure of SCR Electronic Circuit 4 nstructor: jay umar adel 2

3 SCR Characteristics When V =positive (anode made more positive) JunctionJ J 1 and J 3 areforward biased Junction J 2 s reverse biased Small flows from anode to cathode ( D = off state ) blocking or off state condition ate () node () Cathode () J1 J2 J3 Structure of SCR Electronic Circuit 5 SCR Characteristics contd When V is increased sufficiently Such that J2 breaks down (avalanche) V= VBO (forward ) Large forward (Device O) Typical drop =1V (Ohmic drop) node limited by external resistance node must exceed latching Works as a normal diode once turned O ate () node () Cathode () Structure of SCR Electronic Circuit 6 nstructor: jay umar adel 3

4 SCR Characteristics contd.. When V =negative Junction J1 and J3 are reverse biased Junction J2 s forward biased Small flows from cathode to anode (R= reverse ) blocking state + p n p n J 1 J 2 J 3 V S R L - Electronic Circuit 7 When V is greater than or equal to V BO Device turned O node > L in order to maintain i the amount of carrier flow across the junction Otherwise the device reverts back to off state Latching Current Minimum anode to maintain the thyristor in on state immediately after it has been turned O and gating signal has been removed SCR Characteristics contd.. Latching Holding Electronic 8 Circuit T L H volt drop (Conducting) ate triggered V- Characteristics V BO V nstructor: jay umar adel 4

5 SCR Characteristics contd.. Once the device is O it operates a normal diode and there is no control over the device The device conducts as if there is an absence of the depletion region n O state if the anode is lowered below the Holding, depletion region forms due to reduced no. of carriers and switches to blocking state Latching Holding T L H volt drop (Conducting) ate triggered V- Characteristics Electronic 9 Circuit V BO V SCR Characteristics contd.. Holding Current Minimum anode to maintain the thyristor in O state Switching thyristor by making V >V BO leads to destructive turn O Thyristor can be turned O with V <V BO and applying positivegate Once conducting the gate signal can be removed if the anode > holding Latching Holding T L H volt drop (Conducting) ate triggered V- Characteristics Electronic 10Circuit V BO V nstructor: jay umar adel 5

6 Two Transistor model of thyristor Can be separated into pnp (Q1) & npn (Q2) T p J 1 n J 2 p = α + n C α C1 C2 C E = E = α C1 CBO = common base gain 1 = α = 2 + CBO1 + + C2 + 1 CBO1 CBO2 = α CBO2 1 ( α + α ) α 2 CBO1 For a gating, + α = CBO2 Q 1 Q 2 n p J 2 J 3 = T B1 = α C2 1 Q 1 C1 Q 2 α2 B2 Electronic Circuit 11 Two transistor model contd + + α = CBO1 CBO2 2 1 α + α 1 2 Thedeviceisinoffstatewhen α 1 + α 2 <<1. This happens for lower value of E (or, ). n this case isequal to the sum of the s. When α 1 + α 2 is nearly equal to 1, the increases without limit and the device is in the O state Electronic Circuit 12 nstructor: jay umar adel 6

7 Regenerative Feedback in SCR f C1 increases, B2 increases α 1 which increases C2 = T ( C2 =ββ B2 ). This increases B1 B1 = C2 ( B1 = C2 ) which then further Q 1 increases C1 ( C1 =β B1 ) C1 B2 α 2 Q 2 C1 B2 C2 B1 C1 Electronic Circuit 13 Two transistor transient model Electronic Circuit 14 nstructor: jay umar adel 7

8 umerical The capacitance of reverse biased junction J2 in a thyristor is C J2 =20 pf and can be assumed to be independent of the off state. The limiting value of the charging g toturnonthethyristor is 16 m. Determine the critical value of dv/dt. (ns: 800V/µs) Electronic Circuit 15 Methods to turn on the SCR Thermal High temperature increase in electron-hole pairs increase in increase in α 1 and α 2 Regenerative action leads to (α 1 + α 2 )=unity turning O the device Thermal runaway must be avoided Light Light striking the junction may increase electron hole pairs High Voltage When V > V BO Maybe destructive TUR O Electronic Circuit 16 nstructor: jay umar adel 8

9 Thyristor Turn on Methods contd. When the thyristor is forward biased njection of gate applying li +ve between gate and cathode ncreasing gate increases gating reduces forward blocking ating signal should be removed after turn O to decrease power loss Electronic Circuit 17 Thyristor turn on arameters Device turns O after time delay t O t O =delay time (t d ) + rise time (t r ) t O time interval between 10% gate to 90% steady state t d time interval between 10% gate to 10% O state t r time interval between 10% O to 90% O state T T i T 0.1 T 0 t Electronic Circuit 18 i T t t d t O t r nstructor: jay umar adel 9

10 ssignment 1 Write short notes on Thyristor Commutation Techniques, DC and TRC Deadline: Tomorrow 2:00 M Electronic Circuit 19 SCR Half Wave Rectifier Same a normal diode rectifier The average output however differs Conduction is defined by the phase of gating signal f v = V m sinθ is the input signal verage output is, 1 V V θ θ α 2 m av = V m sin 2 d = V V ( cosα ) 2 m av = 1+ ac R L [ cos θ ] α R Electronic Circuit 20 nstructor: jay umar adel 10

11 SCR Full wave Rectifier Same a normal diode rectifier The average output however differs Conduction is defined by the phase of gating signal ac R L R R f v = V m sinθ is the input signal verage output is, 2 V V θ θ m av = V m sin d = 2 α V V m av = ( 1+ cosα ) [ cos θ ] α v 0 t i T t 0 α Electronic Circuit 21 nstructor: jay umar adel 11

UNIT I POWER SEMI-CONDUCTOR DEVICES

UNIT I POWER SEMI-CONDUCTOR DEVICES SUBJECT CODE SUBJECT NAME STAFF NAME : EE6503 : Power Electronics : Ms.M.Uma Maheswari 1 SEMICONDUCTOR DEVICES POWER DIODE POWER TRANSISTORS POWER BJT POWER MOSFET IGBT