explain the parallel operation of IGBT?

Question

Also, highlight the problem faced during parallel operation.

Answer 1

Parallel operation of IGBT:

Parallel operation of IGBT is done for obtaining high current rating. Paralleling of IGBT reduces conduction losses and thermal stress. IGBT combines the qualities of BJT and MOSFET. So, IGBT have both negative and positive temperature coefficient. Means for collector current up to about 70% of rated value IGBT shows negative temperature coefficient and after that IGBT shows positive temperature coefficient. For example, if IGBT is rated for 10A then for up to 7A the IGBT will show negative temperature coefficient and after that up to 10A IGBT will show positive temperature coefficient.

For successful parallel operation of IGBT

1.Each IGBT should have its own gate resistor.

2.The arrangement of the layout of the chips should be such that the current flow path should be symmetrical.

3.The IGBT should be thermally coupled by mounting them on the same heatsink.

Problem faced during parallel operation of IGBT:

• IGBT have static and dynamic current sharing problem.
  
• The static current sharing problem is related to magnitudes of individual collector current.
• The dynamic current sharing problem is related to turning on and  turning off times
• For better current sharing in parallel IGBT require gate-emitter threshold voltage and transconductance (g) which is the ratio of collector current (I_c) to gate voltage (V_GE) to match.
• The turning on and turning off time of IGBTs must be same.
  
• High frequency may cause dynamic unbalance problem.
• A drive circuit is needed for equal turn on and turn off time.
• IGBTs must share losses equally otherwise IGBT may get failure due to thermal differences.