Overvoltage suppresion by clamping circuit in IGBT driver

[perprot]

I am designing clamping circuit for IGBT driver. The max voltage is 4800V. I was thinking about using STTH112A diode. In the datasheet, there is no information about thermal resistance - junction to ambient for SMB or SMBflat version. How can I assess the number of diodes I need to use and what is the maximal Vrrm of the diode which can be used in these circuit? Is STTH112A suitable for these design?

[DCISS.1]

Hi "Perpot", 

The Thermal resistance junction to ambient for SMB is available in datasheet  figure 6. For minimum footprint the Rthja = ~ 105°C/W (from figure 6). To withstand your maximum voltage 4800V, I advice you to use 5 diodes in series 6kV in total (with a derating of 80%). For each diode, you will need to put in parallel a resistor and a capacitance. Both are in parallel with the diode. R <= 1MOhm and C>= 200pF (C is defined for a didt = 100A/us, Qrr = 289nC, Irm = 3.8A at Tj = 125°C).

To reiterate the calculation, you case use this application note AN443.

Best regards, 

D.CISSE

[perprot]

Hi DCISS.1,

Thank you so much for your replay. I've missed this information.

If we will take Rthja = ~ 105°C/W, working temperature = 65°C and max operating temp = 175°C then max allowed power per diode is (175-65)/105 = 1.05W.

Vbr=4800V, inrush current 3.9A, inrush time 10us, inrush interval 10000us, we can calculate average Pd = 4800*3.9*10/10000 = 9.6W

so, we need 9.6/1.05 = 9.2 diodes with Vbr = 4800/9.2 = 524V

According to your advise we need 5 diodes with Vbr 1200V.

I am not sure which calculation can be used for the clamping circuit design. Any advise, please?

Kind regards,

perprot