Table of Contents Page Chapter 5 Gate and Base Circuits and Protection ......................... 235 Objectives of Chapter ........................................................................ 235 5.1 Gate and Base Drive Circuits..................................................................... 235 Objectives ........................................................................................................................ 235 5.1.1 Introduction............................................................................................................ 235 5.1.2 MOSFET Gate Drive Circuits .............................................................................. 236 5.1.3 Bipolar Junction Transistor Drive Circuits ........................................................ 238 5.1.3.1 Switch-on Control ..................................................................................... 239 Activity 5.1 .............................................................................................................. 241 Suggested Answer for Activity 5.1 ......................................................................... 241 5.1.3.2 Switch-off Control ..................................................................................... 242 5.1.3.3 Proportional Base Control ....................................................................... 243 5.1.3.4 Anti-Saturation Control ........................................................................... 244 Activity 5.2 .............................................................................................................. 247 Suggested Answer for Activity 5.2 ......................................................................... 247 Test 5.1 .................................................................................................................... 247 Solution for Test 5.1 ............................................................................................... 248 5.1.4 Isolation of Gate and Base Drive Circuit ............................................................. 249 5.1.4.1 Pulse Transformer Method ...................................................................... 251 5.1.4.2 Optocoupler Isolation Method ................................................................. 251 Activity 5.3 .............................................................................................................. 252 Suggested Answer to Activity 5.3 ........................................................................... 252 5.1.5 Thyristor Drive Circuit ......................................................................................... 253 5.1.6 Unijunction Transistor .......................................................................................... 255 5.1.7 Programmable Unijunction Transistor ............................................................... 259 Activity 5.4 .............................................................................................................. 262 Suggested Answer to Activity 5.4 ........................................................................... 262 Test 5.2 .................................................................................................................... 263 Solution to Test 5.2 ................................................................................................. 263 5.1.8 Gate Drive Integrated Circuit .............................................................................. 264 5.1.9 Summary ................................................................................................................ 265 5.2 Device and Circuit Protection .................................................................... 265 Objectives ........................................................................................................................ 265 5.2.1 Introduction............................................................................................................ 265 5.2.2 Cooling and Heat Sinks ......................................................................................... 266 Activity 5.5 .............................................................................................................. 273 Suggested Answer to Activity 5.5 ........................................................................... 273 Test 5.3 .................................................................................................................... 274 Solution to Test 5.3 ................................................................................................. 274 5.2.3 Thermal Modeling of Power Switching Devices ................................................. 275 5.2.3.1 Electrical Equivalent Thermal Model ..................................................... 275 5.2.3.2 Mathematical Thermal Equivalent Circuit ............................................ 277 Activity 5.6 .............................................................................................................. 278 Suggested Answer to Activity 5.6 ........................................................................... 278 5.2.4 Reducing Loss with Snubber Circuits ................................................................. 278 - i - Table of Contents Page 5.2.5 Reverse Recovery Transients................................................................................ 280 5.2.6 Voltage Protection ................................................................................................. 283 Test 5.4 .................................................................................................................... 284 Solution to Test 5.4 ................................................................................................. 285 5.2.7 Current Protection................................................................................................. 285 5.2.8 Summary ................................................................................................................ 288 Summary of Chapter ........................................................................................ 288 Reference ............................................................................................................ 289 Glossary .............................................................................................................. 289 - ii - List of Figures Page Figure 5.1: Fast switch-on MOSFET switching circuit ........................................................ 237 Figure 5.2: Totem pole gate drive circuit with pulse-edge shaping ...................................... 238 Figure 5.3: The waveform of the base current for a BJT base drive .................................... 239 Figure 5.4: (a) Circuit for base current peaking during switch-on and switching-off conditions and (b) the input waveform .............................................................. 240 Figure 5.5: (a) Base current peaking circuit for different switch-on and switch-off requirements and (b) the input waveform .......................................................... 243 Figure 5.6: A circuit shown proportional base current control ............................................. 243 Figure 5.7: A Baker collector clamping circuit .................................................................... 245 Figure 5.8: Single phase H-bridge inverter (a) Circuit and (b) gate signals ......................... 250 Figure 5.9: Gate voltage between gate and ground .............................................................. 250 Figure 5.10: (a) A pulse transformer isolation base drive circuit and (b) pulse signal ............. 251 Figure 5.11: An optocoupler isolation circuit ........................................................................... 252 Figure 5.12: A photo-silicon control rectifier isolation circuit ............................................... 253 Figure 5.13: A pulse transformer isolation circuit for triggering SCR circuit ........................ 254 Figure 5.14: (a) Input pulse and (b) output pulse as SCR triggering voltage ......................... 254 Figure 5.15: Construction and symbol of unijunction transistor ............................................ 255 Figure 5.16: Equivalent circuit of unijunction transistor ........................................................ 255 Figure 5.17: Characteristic curve of UJT ................................................................................ 256 Figure 5.18: UJT circuit triggering an SCR circuit................................................................. 258 Figure 5.19: Load line of UJT triggering circuit..................................................................... 258 Figure 5.20: Waveform at the node E and B1 of UJT circuit shown in Fig. 5.18 ................... 259 Figure 5.21: Programmable unijunction transistor PUT ......................................................... 260 Figure 5.22: Biased circuit of PUT and its characteristic ....................................................... 260 Figure 5.23: PUT relaxation oscillator and its characteristic .................................................. 261 Figure 5.24: Illustration of gate drive circuit for (a) power MOSFET and IGBT .................. 264 Figure 5.25: Rate of flow of energy in a conductor of different end temperature .................. 266 Figure 5.26: Illustrating heat flows from device through different material to atmosphere ... 267 Figure 5.27: Electrical model of heat transfer from a