Switch Realization - Lesson (PDF)

lOMoARcPSD|17553410 Switch realization - lesson Civil Engeneering (Pangasinan State University) Scan to open on Studocu Studocu is not sponsored or endorsed by any college or university Downloaded by Adrian Paul Abella (a...

lOMoARcPSD|17553410 Switch realization - lesson Civil Engeneering (Pangasinan State University) Scan to open on Studocu Studocu is not sponsored or endorsed by any college or university Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Chapter 4. Switch Realization 4.1. Switch applications Single-, two-, and four-quadrant switches. Synchronous rectifiers 4.2. A brief survey of power semiconductor devices Power diodes, MOSFETs, BJTs, IGBTs, and thyristors 4.3. Switching loss Transistor switching with clamped inductive load. Diode recovered charge. Stray capacitances and inductances, and ringing. Efficiency vs. switching frequency. 4.4. Summary of key points Fundamentals of Power Electronics 1 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 SPST (single-pole single-throw) switches Buck converter SPST switch, with voltage and current with SPDT switch: polarities defined 1 L iL(t) + 1 2 Vg + C R V i – + v – – with two SPST switches: 0 iA A L iL(t) + vA – + – All power semiconductor Vg + vB B C R V – devices function as SPST + iB switches. – Fundamentals of Power Electronics 2 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Realization of SPDT switch using two SPST switches A nontrivial step: two SPST switches are not exactly equivalent to one SPDT switch It is possible for both SPST switches to be simultaneously ON or OFF Behavior of converter is then significantly modified —discontinuous conduction modes (chapter 5) Conducting state of SPST switch may depend on applied voltage or current —for example: diode Fundamentals of Power Electronics 3 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Quadrants of SPST switch operation 1 Switch i on state A single-quadrant + current switch example: v ON-state: i > 0 – OFF-state: v > 0 0 Switch off state voltage Fundamentals of Power Electronics 4 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Some basic switch applications switch switch on-state on-state Single- current Current- current quadrant bidirectional switch two-quadrant switch off-state voltage switch switch off-state voltage switch switch on-state on-state current current Voltage- Four- bidirectional quadrant two-quadrant switch switch off-state switch off-state switch voltage voltage Fundamentals of Power Electronics 5 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.1.1. Single-quadrant switches 1 Active switch: Switch state is controlled exclusively i by a third terminal (control terminal). + v Passive switch: Switch state is controlled by the applied current and/or voltage at terminals 1 and 2. – SCR: A special case — turn-on transition is active, 0 while turn-off transition is passive. Single-quadrant switch: on-state i(t) and off-state v(t) are unipolar. Fundamentals of Power Electronics 6 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 The diode A passive switch i Single-quadrant switch: 1 on can conduct positive on- + i state current off v v can block negative off- state voltage – provided that the intended 0 on-state and off-state operating points lie on the diode i-v characteristic, Symbol instantaneous i-v characteristic then switch can be realized using a diode Fundamentals of Power Electronics 7 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 The Bipolar Junction Transistor (BJT) and the Insulated Gate Bipolar Transistor (IGBT) 1 An active switch, controlled BJT i by terminal C i + C Single-quadrant switch: v on – can conduct positive on- off v 0 state current can block positive off-state IGBT 1 voltage i + C provided that the intended v on-state and off-state – operating points lie on the 0 instantaneous i-v characteristic transistor i-v characteristic, then switch can be realized using a BJT or IGBT Fundamentals of Power Electronics 8 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) An active switch, controlled by i terminal C Normally operated as single- 1 on quadrant switch: i + C off v can conduct positive on-state v current (can also conduct – negative current in some on (reverse conduction) circumstances) 0 can block positive off-state voltage Symbol instantaneous i-v characteristic provided that the intended on- state and off-state operating points lie on the MOSFET i-v characteristic, then switch can be realized using a MOSFET Fundamentals of Power Electronics 9 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Realization of switch using transistors and diodes Buck converter example iA A L iL(t) + vA – + – + vB B Vg C R V – + iB Switch A: transistor – Switch B: diode iA iB Switch A Switch B SPST switch on iL on iL operating points Switch A Switch B off off Vg vA –Vg vB Switch A Switch B Fundamentals of Power Electronics 10 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Realization of buck converter using single-quadrant switches iA vA L + – iL(t) + – vL(t) – Vg + vB – + iB iA iB Switch A Switch B on iL on iL Switch A Switch B off off Vg vA –Vg vB Fundamentals of Power Electronics 11 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.1.2. Current-bidirectional two-quadrant switches Usually an active switch, controlled by terminal C i 1 on Normally operated as two- (transistor conducts) i quadrant switch: + C off v can conduct positive or v negative on-state current – can block positive off-state on 0 (diode conducts) voltage provided that the intended on- state and off-state operating BJT / anti-parallel instantaneous i-v points lie on the composite i-v diode realization characteristic characteristic, then switch can be realized as shown Fundamentals of Power Electronics 12 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Two quadrant switches i switch on-state current on 1 (transistor conducts) i + off v v switch – off-state voltage 0 on (diode conducts) Fundamentals of Power Electronics 13 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET body diode i 1 on (transistor conducts) i + off v C v on – (diode conducts) 0 Power MOSFET Power MOSFET, Use of external diodes characteristics and its integral to prevent conduction body diode of body diode Fundamentals of Power Electronics 14 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 A simple inverter iA + Q1 Vg + D1 vA v0(t) = (2D – 1) Vg – – L iL + + Vg + D2 v C R v0 – B Q2 – – iB Fundamentals of Power Electronics 15 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Inverter: sinusoidal modulation of D v0(t) = (2D – 1) Vg Sinusoidal modulation to v0 produce ac output: Vg D(t) = 0.5 + Dm sin (ωt) D The resulting inductor 0 0.5 1 current variation is also sinusoidal: –Vg v0(t) Vg iL(t) = = (2D – 1) R R Hence, current-bidirectional two-quadrant switches are required. Fundamentals of Power Electronics 16 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 The dc-3øac voltage source inverter (VSI) ia Vg + – ib ic Switches must block dc input voltage, and conduct ac load current. Fundamentals of Power Electronics 17 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Bidirectional battery charger/discharger D1 L + + vbus Q1 D2 vbatt spacecraft main power bus Q2 – – vbus > vbatt A dc-dc converter with bidirectional power flow. Fundamentals of Power Electronics 18 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.1.3. Voltage-bidirectional two-quadrant switches Usually an active switch, controlled by terminal C 1 i Normally operated as two- i + on quadrant switch: can conduct positive on-state v v current C off off (diode (transistor can block positive or negative blocks voltage) blocks voltage) – off-state voltage 0 provided that the intended on- state and off-state operating points lie on the composite i-v BJT / series instantaneous i-v characteristic, then switch can diode realization characteristic be realized as shown The SCR is such a device, without controlled turn-off Fundamentals of Power Electronics 19 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Two-quadrant switches 1 i switch + i on-state current v on – 0 v switch 1 off off off-state (diode (transistor voltage i + blocks voltage) blocks voltage) v C – 0 Fundamentals of Power Electronics 20 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 A dc-3øac buck-boost inverter iL φa + vab(t) – φb – + Vg + vbc(t) – φc Requires voltage-bidirectional two-quadrant switches. Another example: boost-type inverter, or current-source inverter (CSI). Fundamentals of Power Electronics 21 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.1.4. Four-quadrant switches switch on-state current Usually an active switch, controlled by terminal C can conduct positive or negative on-state current switch off-state can block positive or negative voltage off-state voltage Fundamentals of Power Electronics 22 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Three ways to realize a four-quadrant switch 1 1 1 i i i 1 + + + i + v v v v – – – – 0 0 0 0 Fundamentals of Power Electronics 23 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 A 3øac-3øac matrix converter 3øac input 3øac output ia van(t) + – vbn(t) ib + – – vcn(t) + ic All voltages and currents are ac; hence, four-quadrant switches are required. Requires nine four-quadrant switches Fundamentals of Power Electronics 24 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.1.5. Synchronous rectifiers Replacement of diode with a backwards-connected MOSFET, to obtain reduced conduction loss i 1 1 1 on i i i + (reverse conduction) + + C off v v v v – – – on 0 0 0 ideal switch conventional MOSFET as instantaneous i-v diode rectifier synchronous characteristic rectifier Fundamentals of Power Electronics 25 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Buck converter with synchronous rectifier MOSFET Q2 is vA controlled to turn on iA L iL(t) + – when diode would Q1 normally conduct – Semiconductor + C Vg vB conduction loss can – C + be made arbitrarily Q2 iB small, by reduction of MOSFET on- resistances Useful in low-voltage high-current applications Fundamentals of Power Electronics 26 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.2. A brief survey of power semiconductor devices Power diodes Power MOSFETs Bipolar Junction Transistors (BJTs) Insulated Gate Bipolar Transistors (IGBTs) Thyristors (SCR, GTO, MCT) On resistance vs. breakdown voltage vs. switching times Minority carrier and majority carrier devices Fundamentals of Power Electronics 27 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.3.1. Transistor switching with clamped inductive load transistor waveforms iA vA iL(t) L Vg + – vA(t) iL physical iA(t) MOSFET – Vg + vB ideal – diode 0 0 + – gate + t iB iL DTs Ts driver diode waveforms iB(t) Buck converter example 0 0 t vB(t) vB(t) = vA(t) – Vg transistor turn-off i A(t) + iB(t) = iL transition –Vg pA(t) Vg iL = vA iA area Woff W off = 1 VgiL (t 2 – t 0) 2 t t0 t1 t2 Fundamentals of Power Electronics 28 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Switching loss induced by transistor turn-off transition Energy lost during transistor turn-off transition: W off = 1 VgiL (t 2 – t 0) 2 Similar result during transistor turn-on transition. Average power loss: Psw = 1 pA(t) dt = (W on + W off ) fs Ts switching transitions Fundamentals of Power Electronics 29 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.2.1. Power diodes A power diode, under reverse-biased conditions: v + – low doping concentration + p + + – { { E v depletion region, reverse-biased n- + n – – – Fundamentals of Power Electronics 30 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Typical diode switching waveforms v(t) t i(t) tr 0 t di dt area –Qr (1) (2) (3) (4) (5) (6) Fundamentals of Power Electronics 31 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Forward-biased power diode v i + – conductivity modulation + p + + { + + n- + n – – – minority carrier injection Fundamentals of Power Electronics 32 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Charge-controlled behavior of the diode v The diode equation: i + – q(t) = Q0 e λv(t) – 1 p n- n Charge control equation: + + dq(t) q(t) + + + = i(t) – τ + dt L + + With: λ= 1/(26 mV) at 300 K τL = minority carrier lifetime (above equations don’t include current that charges depletion region capacitance) Fundamentals of Power Electronics q(t) Q0 λv(t) i(t) = τ = τ e L L 33 } Total stored minority charge q In equilibrium: dq/dt = 0, and hence – 1 = I 0 e λv(t) – 1 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Charge-control in the diode: Discussion The familiar i–v curve of the diode is an equilibrium relationship that can be violated during transient conditions During the turn-on and turn-off switching transients, the current deviates substantially from the equilibrium i–v curve, because of change in the stored charge and change in the charge within the reverse-bias depletion region Under forward-biased conditions, the stored minority charge causes “conductivity modulation” of the resistance of the lightly-doped n– region, reducing the device on-resistance Fundamentals of Power Electronics 34 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Diode in OFF state: reversed-biased, blocking voltage v(t) v + – t p n– n E i(t) – v + 0 { Depletion region, reverse-biased t Diode is reverse-biased No stored minority charge: q = 0 (1) Depletion region blocks applied reverse voltage; charge is stored in capacitance of depletion region Fundamentals of Power Electronics 35 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Turn-on transient v(t) The current i(t) is determined by the converter circuit. This t current supplies: Diode conducts with low on-resistance charge to increase Diode is forward-biased. Supply minority charge to n– region to reduce on-resistance voltage across depletion region Charge depletion region charge needed to i(t) support the on-state On-state current determined by converter circuit current charge to reduce t on-resistance of n– (1) (2) region Fundamentals of Power Electronics 36 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Turn-off transient v i (< 0) + – p n- n + + + + + + + + } Removal of stored minority charge q Fundamentals of Power Electronics 37 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Diode turn-off transient continued v(t) t (4) Diode remains forward-biased. Remove stored charge in n– region (5) Diode is reverse-biased. i(t) Charge depletion region capacitance. tr 0 t di dt Area –Qr (1) (2) (3) (4) (5) (6) Fundamentals of Power Electronics 38 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 The diode switching transients induce switching loss in the transistor iA vA L iA(t) see Section 4.3.2 + – iL(t) transistor waveforms Qr fast transistor Vg – Vg + vB silicon iL – diode vA(t) + – + 0 0 iB t iB(t) diode waveforms iL vB(t) Diode recovered stored charge 0 0 Qr flows through transistor t area during transistor turn-on –Qr –Vg transition, inducing switching loss tr Qr depends on diode on-state pA(t) forward current, and on the = vA iA rate-of-change of diode current area ~QrVg during diode turn-off transition area ~iLVgtr t0 t1 t2 t Fundamentals of Power Electronics 39 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Switching loss calculation iA(t) Energy lost in transistor: transistor waveforms Qr Soft-recovery Vg diode: vA(t) iL WD = vA(t) i A(t) dt 0 0 (t2 – t1) >> (t1 – t0) switching transition t Abrupt-recovery iB(t) With abrupt-recovery diode: diode waveforms iL diode: vB(t) 0 0 (t2 – t1) 0 0 Negative inductor current removes diode area t – Qr stored charge Qr vB(t) When diode becomes reverse-biased, t 0 negative inductor current flows through capacitor C. –V2 Ringing of L-C network is damped by t1 t2 t3 parasitic losses. Ringing energy is lost. Fundamentals of Power Electronics 44 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Energy associated with ringing t3 vi(t) V1 Recovered charge is Qr = – iL(t) dt t2 t 0 Energy stored in inductor during interval –V2 t2 ≤ t ≤ t3 : t3 WL = vL(t) iL(t) dt iL(t) t2 Applied inductor voltage during interval t2 ≤ t ≤ t3 : 0 di (t) area t vL(t) = L L = – V2 dt – Qr Hence, t3 t3 vB(t) diL(t) t WL = L i (t) dt = ( – V2) iL(t) dt 0 t2 dt L t2 –V2 W L = 12 L i 2L(t 3) = V2 Qr t1 t2 t3 Fundamentals of Power Electronics 45 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.2.2. The Power MOSFET Source Gate lengths Gate approaching one micron Consists of many n n n n small enhancement- p p mode parallel- connected MOSFET cells, covering the n- surface of the silicon wafer n Vertical current flow n-channel device is shown Drain Fundamentals of Power Electronics 46 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET: Off state source – p-n- junction is reverse-biased off-state voltage n n n n appears across n- p p region depletion region n- n drain + Fundamentals of Power Electronics 47 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET: on state source p-n- junction is slightly reverse- biased positive gate voltage n n n n induces conducting p p channel drain current flows channel through n- region n- and conducting channel n on resistance = total resistances of n- region, conducting drain drain current channel, source and drain contacts, etc. Fundamentals of Power Electronics 48 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET body diode source p-n- junction forms an effective diode, in parallel with the channel n p n n n negative drain-to- p source voltage can forward-bias the Body diode body diode n- diode can conduct the full MOSFET rated current n diode switching speed not optimized drain —body diode is slow, Qr is large Fundamentals of Power Electronics 49 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Typical MOSFET characteristics Off state: VGS < Vth 00V 0V On state: VGS >> Vth =1 =2 10A V DS =2 DS V MOSFET can V V DS ID conduct peak currents well in on state excess of average 5A 1V current rating V DS = —characteristics are unchanged off V DS = 0.5V state on-resistance has positive temperature 0A coefficient, hence 0V 5V 10V 15V easy to parallel VGS Fundamentals of Power Electronics 50 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 A simple MOSFET equivalent circuit D Cgs : large, essentially constant Cgd : small, highly nonlinear Cgd Cds : intermediate in value, highly G nonlinear Cds switching times determined by rate Cgs at which gate driver charges/discharges Cgs and Cgd S C0 V0 C '0 Cds(vds) = Cds(vds) ≈ C0 vds = vds v 1 + ds V0 Fundamentals of Power Electronics 51 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Switching loss caused by semiconductor output capacitances Buck converter example Cds Vg + Cj – + – Energy lost during MOSFET turn-on transition (assuming linear capacitances): W C = 12 (Cds + C j) V 2g Fundamentals of Power Electronics 52 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET nonlinear Cds Approximate dependence of incremental Cds on vds : V0 C '0 Cds(vds) ≈ C0 vds = vds Energy stored in Cds at vds = VDS : V DS W Cds = vds i C dt = vds C ds(vds) dvds 0 V DS W Cds = C '0(vds) vds dvds = 23 Cds(VDS) V 2DS 0 4 — same energy loss as linear capacitor having value 3 Cds(VDS) Fundamentals of Power Electronics 53 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Characteristics of several commercial power MOSFETs Part number Rated max voltage Rated avg current R on Qg (typical) IRFZ48 60V 50A 0.018Ω 110nC IRF510 100V 5.6A 0.54Ω 8.3nC IRF540 100V 28A 0.077Ω 72nC APT10M25BNR 100V 75A 0.025Ω 171nC IRF740 400V 10A 0.55Ω 63nC MTM15N40E 400V 15A 0.3Ω 110nC APT5025BN 500V 23A 0.25Ω 83nC APT1001RBNR 1000V 11A 1.0Ω 150nC Fundamentals of Power Electronics 54 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 MOSFET: conclusions A majority-carrier device: fast switching speed Typical switching frequencies: tens and hundreds of kHz On-resistance increases rapidly with rated blocking voltage Easy to drive The device of choice for blocking voltages less than 500V 1000V devices are available, but are useful only at low power levels (100W) Part number is selected on the basis of on-resistance rather than current rating Fundamentals of Power Electronics 55 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 4.2.3. Bipolar Junction Transistor (BJT) Base Emitter Interdigitated base and emitter contacts Vertical current flow n n n npn device is shown p minority carrier device n- on-state: base-emitter and collector-base junctions are both n forward-biased on-state: substantial minority charge in p and n- regions, conductivity Collector modulation Fundamentals of Power Electronics 56 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 BJT switching times vs(t) Vs2 –Vs1 VCC vBE(t) 0.7V RL –Vs1 iC(t) + iB(t) iB(t) RB IB1 vCE(t) + 0 vBE(t) – –IB2 vs(t) + – – vCE(t) VCC IConRon iC(t) ICon 0 t (1) (2) (3) (4) (5) (6) (7) (8) (9) Fundamentals of Power Electronics 57 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Ideal base current waveform iB(t) IB1 IBon 0 t –IB2 Fundamentals of Power Electronics 58 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Current crowding due to excessive IB2 Base Emitter –IB2 – n – can lead to p – – + + – – p formation of hot n- spots and device failure n Collector Fundamentals of Power Electronics 59 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 BJT characteristics Off state: IB = 0 IC n V CE = 200V e regio On state: IB > IC /β v V = 20V 10A acti CE ation a s i - satur V CE = 5V Current gain β decreases q u rapidly at high current. Device should not be operated at slope saturation region =β instantaneous currents VCE = 0.5V 5A exceeding the rated value cutoff VCE = 0.2V 0A 0V 5V 10V 15V IB Fundamentals of Power Electronics 60 Chapter 4: Switch realization Downloaded by Adrian Paul Abella ([email protected])  lOMoARcPSD|17553410 Breakdown voltages IC BVCBO: avalanche breakdown voltage of base-collector increasing IB junction, with the emitter open-circuited BVCEO: collector-emitter breakdown voltage with zero IB = 0 base current open emitter BVsus: breakdown voltage observed with positive base BVsus BVCEO BVCBO VCE current