IGBT

Electronics

1. IGBT (Insulated Gate Bipolar Transistor ) Prepared by, Mr. A. Johny Renoald M.E., Ph.D.,

2. Symbol and Structure The insulated gate bipolar transistor is a three terminal semiconductor device Gate, Emitter and Collector Emitter and Collector-Associated with a conductance path Gate terminal is associated with its control

3. IGBT has MOSFET like input characteristics and Power BJT like output characteristics So its called as Voltage Controlled Device

4. Structure of IGBT

5. Working When collector is at positive potential with respect to emitter and gate also at sufficient positive potential. With no voltage between gate and emitter, no current flow from collector to emitter and device is in off state. When gate is made positive with respect to emitter by voltage Vg, a channel is formed below the gate. Now the collector region injects electrons into n- region, and more number of electrons flow makes the device conduction( I c ) The collector current I c in IGBT constitutes of two components I c = I e + I h I e – Hole current due to injected holes from collector to drift region I h – Electron current due to injected electrons from collector to drift region

6. Characteristics of IGBT

7. Transfer Characteristics The IGBT is in ON-state only after V GE is greater than a threshold value V GET .

8. Switching Characteristics of IGBT

9. Turn on time t on is composed of two components as usual, delay time ( t dn ) and rise time ( t r ) Delay Time Delay time is defined as the time in which collector current rises from leakage current I CE to 0.1 I C (final collector current) and collector emitter voltage falls from V CE to 0.9V CE Rise Time Rise time is defined as the time in which collector current rises from 0.1 I C to I C and collector emitter voltage falls from 0.9V CE to 0.1 V CE . The turn off time t off consists of three components, delay time ( t df ), initial fall time (t f1 ) and final fall time (t f2 ) Turn off Delay Time Delay time is defined as time when collector current falls from I C to 0.9 I C and V CE begins to rise Initial fall time Initial fall time is the time during which collector current falls from 0.9 I C to 0.2 I C and collector emitter voltage rises to 0.1 V CE Final fall time The final fall time is defined as time during which collector current falls from 0.2 I C to 0.1 I C and 0.1V CE rises to final value V CE

10. Series and Parallel Connection of SCR Nowadays, SCRs are available of ratings up to 10 KV and 3 KA. But sometimes we face demand, more than these ratings. In this case combination of more than one SCRs is used. Series connection of SCRs meets high voltage demand and parallel connection of SCRs meets high current demand. These series and parallel connection of SCR or Thyristor will work efficiently if all SCRs are fully utilized. Although all SCRs in a string are of same rating, their V-I characteristics differ from one another. This leads to unequal voltage or current division among them. Hence every SCR is not fully utilized. So the efficiency of string is always less than 100% according to the given expression

11. Series Operation of SCR When the operating voltage is more than the rating of one SCR the multiple SCRs of same ratings are used in series SCR’s having same rating, may have different I-V characteristic, so unequal voltage division is takes place For example if two SCRs in series that is capable of blocking 5 KV individually, then the string should block 10 KV. But practically this does not happen.

12. Voltage across SCR 1 is V 1 but that across SCR 2 is V 2 V 2 is much less than V 1 . So, SCR 2 is not fully utilized. Hence the string can block V 1 + V 2 = 8 KV, rather than 10 KV The string efficiency = 80% To improve the efficiency a resistor in parallel with every SCR is used. The value of these resistances are such that the equivalent resistance of each SCR and resistor pair will be same. Ensures equal voltage division across each SCR One value of resistance to get optimum result which is given by n = no. of SCR in the string V bm = Voltage blocked by the SCR having minimum leakage current. Δ I b = Difference between maximum and minimum leakage current flowing through SCRs V s = Voltage across the string

13. Parallel Operation of SCR

14. Static current sharing Resistors are used in case of static current sharing. When resistances are used in series, the losses may become high. Dynamic current sharing For dynamic current sharing, inductors are also used in addition to the resistors. In case of inductors (magnetically coupled), if current through the thyristor T1 increases, an opposite polarity voltage would be induced (as of series coil of T1) in the series coil of thyristor T2

15. Thyristor Protection or SCR Protection Protection of a device is an important aspect for its reliable and efficient operation SCR may face different types of threats during its operation due to over voltages, over currents etc. Different types of thyristor protection schemes available for satisfactory operation 1.Over voltage protection 2. Over current protection 3. High dv / dt protection 4. Thermal protection

16. Over Voltage Protection Most important protection scheme w. r. t. others as thyristors are very sensitive to over voltages. Maximum time thyristor failures happen due to over-voltage Internal Over-Voltages After commutation of a thyristor reverse recovery current decays abruptly that can exceed the rated break-over voltage and the device may be damaged External Over-Voltages Due to various reasons in the supply line like lightning, surge conditions (abnormal voltage spike) etc External over voltage may cause different types of problem in thyristor operation like increase in leakage current, permanent breakdown of junctions, unwanted turn-on of devices etc.

17. Over Current Protection Over current mainly occurs due to different types of faults in the circuit Due to over current loss will increase and high generation of heat may take place that can exceed the permissible limit and burn the device Protective Measures SCR can be protected from over current by using CB and fast acting current limiting fuses CB are used for protection of thyristor against continuous overloads or against surge currents of long duration as a CB has long tripping time

18. High dv / dt Protection When a thyristor is in forward blocking state then only J 2 junction is reverse biased which acts as a capacitor having constant capacitance value C j (junction capacitance) Current through capacitor follows the relation Hence leakage current through the J 2 junction which is nothing but the leakage current through the device will increase which damaged the device and it should be avoided

19. Thermal or Temperature Protection With the increase in the temperature of the junction, insulation may get failed. So we have to take proper measures to limit the temperature rise