CE Voltage of low side IGBT in a fulll bridge converter

[rahul91]

hi,
I am designing a full bridge dc -ac converter using IGBTs... I am using IR2110 as gate drivers..the gate emitter voltages of all the four IGBTs are around 11V...At 300V dc supply voltage, Vce of low side IGBTs are 16V and 26 V....What could be the reason for this....? The collector current is just around 500mA...

Thanks in advance...:smile:

 

[iop95]

IGBT are not fully on switched. Driver voltage / curent are not enough or there are not a very low impedance circuit between driver IC and IGBT. What kind of IGBT use?

 

[rahul91]

FGA25N120 from Fairchild..Its an NPT trench IGBT...It needs 160nC gate charge when Vge=12V and Vcc=400V.


Today first I reduced the H bridge power supply from 300V to 210V. Also reduced the collector current from 500mA to 149mA. The collector emitter voltage got reduced from 26 to 20V.
Then I increased the gate drive current from 1A to 1.6A. The collector emitter voltage then got reduced from 20V to 19.05V.
After that I changed the driver power supply from 12V to 20V. The total gate charge required for the IGBT was around 260nC at 20V VGE ( by extrapolating the gate charge v/s VGE characteristics). I used a 1uF bootstrap capacitor @ 100KHZ switching frequency. Also increased the gate drive current from 1.6A to 2.75A... Still the collector emitter voltage is 18V.

 

[goldsmith]

gate emitter voltages of all the four IGBTs are around 11V

Hi Rahul
GE voltage of IGBT must be exactly about 15 volt to be fully turned on ( unlike mosfets that can be turned on with 12 volts too ! )


Best Wishes
Goldsmith

 

[rahul91]

Hi Rahul
GE voltage of IGBT must be exactly about 15 volt to be fully turned on ( unlike mosfets that can be turned on with 12 volts too ! ]



Best Wishes
Goldsmith

Is there any demand of a minimum collector current for the IGBT to be turned on...Because I've tried the gate emitter voltage upto 20V...Its not turning on...Or is the gate emitter voltage need to be exactly 15V,not more than that..? The datasheet specifies Vge of upto 20V(absolute maximum)...

 

[FvM]

Is there any demand of a minimum collector current for the IGBT to be turned on...

Surely not. All your questions are answered by the FGA25N120 datasheet, at low Ic currents, Vce is about 1 V, even with lower Vge drive.

Your observations are far from being plausible, we can just guess that there's a serious problem either in your circuit or with your measurement method. Without detailed informations about it we can't help.

 

[goldsmith]

Is there any demand of a minimum collector current for the IGBT to be turned on...Because I've tried the gate emitter voltage upto 20V...Its not turning on...Or is the gate emitter voltage need to be exactly 15V,not more than that..? The datasheet specifies Vge of upto 20V(absolute maximum)...

Hi again
FvM has answered he first part of your query . about the 2nd part , as datasheet mentioned , it's maximum allowed voltage . nobody will cross nearby the red boundaries !

For more help more information is required .

Best Lucks
Goldsmith

 

[rahul91]

Surely not. All your questions are answered by the FGA25N120 datasheet, at low Ic currents, Vce is about 1 V, even with lower Vge drive.

Your observations are far from being plausible, we can just guess that there's a serious problem either in your circuit or with your measurement method. Without detailed informations about it we can't help.

I have attached the schematics..Please go through it...I am using Nichrome wire as the load for testing...

- - - Updated - - -

Hi again
FvM has answered he first part of your query . about the 2nd part , as datasheet mentioned , it's maximum allowed voltage . nobody will cross nearby the red boundaries !

For more help more information is required .



Best Lucks
Goldsmith

Yeah crossing the gate voltage maximum can breakdown the gate oxide insulation...I have uploaded the schematics ...Please check it...

 

[FvM]

By nature of the circuit (output transformer), Vce can be only observed as a dynamic (AC) quantity. Wrong probe calibration would be a simple explanation.