Insulated Gate Bipolar Transistors (IGBTs)

1 Lecture Notes Insulated gate Bipolar Transistors (IGBTs) William P. Robbins Professor, Dept. of Electrical and Computer Engineering University of Minnesota Outline Construction and I-V characteristics Physical operation Switching characteristics Limitations and safe operating area PSPICE simulation models IGBTs - 1. Robbins Multi-cell Structure of igbt igbt = Insulated gate Bipolar transistor . contact to source emitter diffusion conductor f ield oxide gate oxide gate width N+ N+ N+ N+. P. N- P. buffer layer N+ (not essential). P+ collector metallization gate conductor IGBTs - 2. Robbins Cross-section of igbt Cell gate emitter SiO. 2 + +. J N N. 3 P. Ls J. 2. N- +. N. P+. J - Unique feature of igbt collector 1 Buffer layer Parasitic thyristor (not essential). Cell structure similar to power MOSFET (VDMOS) cell.

2 P-region at collector end unique feature of igbt compared to MOSFET. Punch-through (PT) igbt - N+ buffer layer present. Non-punch-through (NPT) igbt - N+ buffer layer absent. IGBTs - 3. Robbins Cross-section of Trench- gate igbt Unit Cell Non-punch-thru igbt Punch-thru igbt IGBTs - 4. Robbins igbt I-V Characteristics and Circuit Symbols increasing V. i GE. C i v C. GE4. No Buffer Layer v GE3. VRM BV. CES. v v V GE. With Buffer Layer GE2 GE(th). v GE1. V 0. RM Transfer curve v CE. V BV. RM Output characteristics CES. drain collector gate N-channel igbt circuit symbols gate emitter source IGBTs - 5. Robbins Blocking (Off) State Operation of igbt gate emitter SiO. 2 + +. J N N. 3 P. Ls J. 2. N- +. N. P+. J - Unique feature of igbt collector Buffer layer 1 (not essential). Parasitic thyristor With N+ buffer layer, junction J1 has small breakdownvoltage and thus igbt .

3 Has little reverse blocking capability - anti-symmetric igbt . Buffer layer speeds up device turn-off IGBTs - 6. Robbins igbt On-state Operation gate emitter + +. N N. MOSFET section designed P to carry most of the igbt . collector current lateral (spreading). N- resistance +. N. + + + + P. + + + + + On-state VCE(on) =. VJ1 + Vdrift + ICRchannel collector emitter gate + + Hole injection into drift N N. region from J1 minimizes P Vdrift. N- +. N. P+. IGBTs - 7. collector Robbins Approximate Equivalent Circuits for IGBTs drift region r e s i s t a n ce V. J1. V. drift gate I R. C channel Approximate equivalent circuit for igbt valid for normal operating conditions. igbt equivalent circuit showing VCE(on) = VJ1 + Vdrift + IC Rchannel Transistors comprising the parasitic thyristor. IGBTs - 8. Robbins Static Latchup of IGBTs lateral (spreading).

4 Resistance gate J emitter 3. + +. N N. P. J2 N- +. N. J1. + + + P+ + + + +. collector Conduction paths causing lateral voltage drops and turn-on of parasitic thyristor if current in this path is too large Lateral voltage drops, if too large, will forward bias junction J3. Parasitic npn BJT will be turned on, thus completing turn-on of parasitic thyristor. Large power dissipation in latchup will destroy igbt unless terminated quickly. External circuit must terminate latchup - no gate control in latchup. IGBTs - 9. Robbins Dynamic Latchup Mechanism in IGBTs J emitter gate 3. + +. N N. J2 P. lateral (spreading) expansion of resistance N- depletion region +. N. J1. P+. collector MOSFET section turns off rapidly and depletion layer of junction J2 expands rapidly into N- layer, the base region of the pnp BJT. Expansion of depletion layer reduces base width of pnp BJT and its a increases.

5 More injected holes survive traversal of drift region and become collected at junction J2. Increased pnp BJT collector current increases lateral voltage drop in p-base of npn BJT and latchup soon occurs. Manufacturers usually specify maximum allowable drain current on basis of dynamic latchup. IGBTs - 10. Robbins Internal Capacitances Vs Spec Sheet Capacitances C gc C. bridge G C. +V - C ge C ce b E C gc Bridge balanced (Vb=0) Cbridge = C gc = C res G C. G C. C ies E. C oes C ies = C g e + C gc E. C oes = C gc + C ce IGBTs - 11. Robbins igbt Turn-on Waveforms v (t) V. Turn-on waveforms for GE GG+. igbt embedded in a t stepdown converter. Very similar to turn-on t waveforms of MOSFETs. d(on). Io Contributions to tvf2. i (t). C. Increase in Cge of t t ri MOSFET section at low collector-emitter voltages. V. V C E(on).

6 DD. Slower turn-on of pnp v (t). CE. BJT section. t fv1 t t fv2. IGBTs - 12. Robbins igbt Turn-off Waveforms Turn-off waveforms for igbt . embedded in a stepdown converter. Current tailing (tfi2) due to stored charge trapped in drift region (base of pnp BJT) by rapid turn-off of MOSFET section. Shorten tailing interval by either reducing carrier lifetime or by putting N+ buffer layer adjacent to injecting P+ layer at drain. Buffer layer acts as a sink for excess holes otherwise trapped in drift region becasue lifetime in buffer layer can be made small without effecting on-state losses - buffer layer thin compared to drift region. IGBTs - 13. Robbins igbt Safe Operating Area Maximum collector-emitter i voltages set by breakdown C. voltage of pnp transistor - 2500 v devices available. -5. 10 sec -4 Maximum collector current set 10 sec FBSOA by latchup considerations - 100.

7 A devices can conduct 1000 A. DC. v for 10 sec and still turn-off CE via gate control. dv i re-applied CE. C. dt 1000 V/ s Maximum junction temp. = 150 C. 2000 V/ s Manufacturer specifies a RBSOA 3000 V/ s maximum rate of increase of re-applied collector-emitter v voltage in order to avoid latchup. CE. IGBTs - 14. Robbins Development of PSpice igbt Model Cm gate source Coxs Coxd + +. N N. Cgdj P. Ccer Cdsj Drain-body or N- + Rb base-collector N. Cebj + Cebd depletion layer +. P. drain Reference - An Experimentally Verified Nonlinear capacitors Cdsj and Ccer due to N-P junction depletion layer. igbt Model Implemented in the Nonlinear capacitor Cebj + Cebd due to P+N+ junction SABER Circuit Simulator , Allen R. MOSFET and PNP BJT are intrinsic (no parasitics) devices Hefner, Jr. and Daniel M. Diebolt, IEEE Trans.

8 Nonlinear resistor Rb due to conductivity modulation of N- drain drift region of on Power Electronics, MOSFET portion. Vol. 9, No. 5, pp. 532-542, (Sept., 1994) Nonlinear capacitor Cgdj due to depletion region of drain-body junction (N-P junction). IGBTs - 15. Circuit model assumes that latchup does not occur and parasitic thyristor does not turn. Robbins Parameter Estimation for PSpice igbt Model Built-in igbt model requires nine parameter values. Parameters described in Help files of Parts utility program. Parts utility program guides users through parameter estimation process. igbt specification sheets provided by manufacturer provide sufficient informaiton for general purpose simulations. Detailed accurate simulations, for example device dissipation studies, may require the user to carefully characterize the selected IGBTs.

9 Drain Cebj +. Cgdj Cebd Built-in model does not model Ccer ultrafast IGBTs with buffer Coxd Rb layers (punch-through IGBTs) or gate Cdsj reverse free-wheeling diodes Cm +. Coxs Source IGBTs - 16. Robbins PSpice igbt - Simulation Vs Experiment IGBTs - 17. Robbins