Choosing Igbt Current Rating

[n s k]

Hi, Now i am designing PWM base Voltage source inverter for 7.5KW induction motor. The estimated maximum motor input current for given load specification is 31A. Now if i want to use 6 Discrete IGBTs for my inverter what should be my IGBTs collector current (Ic) rating? Should i choose the IGBT collector current rating is equal to Maximum motor input current (31A) or should i choose the IGBT collector current rating is equal to PEAK of Maximum motor input current ( 1.414 X 31A = 43.84A).

[marke]

Hello nsk

 

The IGBT must be able to carry the peak current for the duration of that peak current and at the maximum operating junction temperature.

If you select on max current only, you will have problems with overheating Die under certain conditions.

 

You need to look at the power dissipation under worst case operating conditions and calculate the maximum junction temperature under those conditions. You may need to oversize the IGBTs just to get a low enough thermal resistance or power dissipation.

Power dissipation calculations must include switching losses.

 

Best regards,

[n s k]

Hello Marke,

 

Thanks for ur explanation. i will choose IGBTs with current rating well above 43.84A. i have one more question, in some IGBT datasheet i found derating curve (IGBT collector current Vs Switching frequency), but for Ultra fast IGBTs i could not find any derating curve. so can u suggest what will be the percentage of derating in collector current for ultrafast IGBT, if i switch IGBTs at 2 to 12KHz. (Example: IRG4PC50FPbF with derating curve (fig. 1 in datasheet), IRGPS60B120KD witout derating curve)

 

with regards

 

 

[marke]

Hello nsk

 

There are two sources of losses thqt you must consider with the IGBT (or any other switching device)

You have static loss while the device is conducting, and this is a function of the voltage across the device and the current through it. You also have switching losses and this is a function of the current, supply voltage and the switching time, and the switching frequency.

 

The static losses will give rise to a junction temperature rise that can easily be calculated and is dependent on the heatsink used. The switching losses increase the junction temperature rise further and once again can be calculated.

You can calculate the switching losses for a given device and application at different switching frequencies and determine the resultant temperature rise. If the temperature rise of the juction is too high, you will have to reduce the current. From this, you can derive your own curves.

 

Best regards,