High Voltage Forum
General electronics => Electronic Circuits => Topic started by: Phoenix on July 31, 2018, 05:37:45 PM

Hello :)
I would like to calculate the maximum frequency i can drive an IGBTBrick within its thermal limitation. I am using SKM200GB128D Bricks.
I am calculating the losses for 180A Hardswitching and a frequency of 20kHz. I need to find out the turn on/off energy first using the datasheet.
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And here is my question: When reading the energy of the chart, do i need to use the 180A of current or the change of current (dI/dt)? The change of current would only be around 22.6A/µs high for 180A at 20kHz, which would result in much lower turn on/off energies.
Greetings,
Phoenix

It is your peak current, that is how I calculated it for softswitching here: http://kaizerpowerelectronics.dk/teslacoils/drsstcdesignguide/igbts/
I guess you can use my example all the way through, just ignore the reduction factors for resonant switching that I use :)

Thank you for your reply :)
I have already followed your guide for the calculations but i am stuck at that point. In this application i am only switching an inductive load, which results in a triangular waveform. The IGBT's always switch at the peaks of this waveform (180A). There is no resonance circuit, so the IGBT's always switch at full current. Because of that, i think i have to read the chart at 180A of current. But if i read the chart at the operating current, i can't keep up with the manufacturers claim of 220A at 20kHz with a junction temperature of 150°C and a case temperature of 80°C. Here are my calculations:
As you see i am only able to switch at a maximum frequency of 8.462kHz but according to the datasheet the bricks should be able to switch 20kHz at 220A.
Is there something wrong with my calculations?
Edit: I just tried something new and i think i have found the problem: A square wave has a RMSFactor of 1, but a triangular wave only has a factor of 0,577. The maximum current rating in the datasheet is for DC, so there are no switching losses included. If i put in the numbers for 127A (220*0,577) into the equations, the result is 23,462 kHz and it should work for my inverter. Could this result be correct?
Greetings,
Phoenix

I was just about to mention that, just like I had to reduce for softswitching short pulses, as hardswitching is seen as a square wave, a triangle waveform would also have to use a reduction factor, but you already figured that out :)

I have now used the reduction factor for my 180A which gives me about 104A RMS. I used the RMS value for the CE Voltage and in the calculation for the conduction losses. But i have not used it for the turn on/off energy, because the IGBT still has to switch the 180A peak.
According to my calculations, if the junction has a temperature of 125°C and the Case has a temperature of 50°C, i can run my Inverter up to 20,6kHz. I also converted the equation to tell me the temperature difference between junction and case, at 20kHz the difference would be 73°C. I could now use a circuit which cuts of the supply voltage to the inverter using a relay, when the case temperature reachs 57°C, so the junction would never get hotter than 130°C :D
Greetings,
Phoenix

Just a thought, wouldnt average value instead of RMS be more accurate?
The loss is the integral of the current*voltage across the switching device during the switching event.

I only used the RMS Value for the calculation of the conduction losses during the ONState.
For the losses during the switching event, i used the full 180A, because the IGBT's switch at the current maximum.

Conduction losses for an IGBT should probably be done with the average (mean) current rather than RMS  they behave more like a diode than a resistor in forward conduction. RMS is appropriate for MOSFETs though, and it probably won't make a lot of difference anyway.