From my experience, when the IGBT is conducting all time long, there are always some breakdown signals fed to the controller which however should not appear. Therefore, in real implementation I limited the duty ratio to 95% deliberately but with a limited output voltage. I am wondering if the IGBT can turn on 100% so that I can have enough output voltage. How about the heat problem if it works in 100% duty ratio just as a diode. I know that the IGBT cannot work in a very high frequency because of heat sink problems. What if it works in another extreme condition, like zero switching frequency, I suppose the heat produced should not be the minimum. I hope someone can help me solve this problem that puzzled me for a long time.
Dear Bing,
In the on state there will be the on voltage on the device and the on current. The power dissipated in the device in this case for continuous operation under this condition= Ion Von, this power is limited by the drainage of the heat from the device using heat think. This allowed maximum power dissipated is a characteristic of the power device given in data sheet and one would take this into consideration.
As for the absolute maximum rating , there is a maximum on current allowed in the device which is normally limited by fusing the metal electrode of the device or secondary breakdown.
The power dissipated in the device depend on the duty ratio as the duty ratio decreases the average power dissipation decreases proportionally.
Only the capacitive power losses is proportional to the frequency where the power dissipated in charging and discharging the input capacitor of the power device is
= f Cgs VDD^2
I would like that you refer to the data sheet of the device, you will find all the working conditions of the device including maximum and nominal ratings.
Best wishes
Abdelhalim abdelnaby Zekry
Dear Zekry,
Thanks for your help.
In my design, the current does not exceed the rated current of the selected IGBT. I refer to the datasheet, and I suppose the continuous DC collector current is the so-called rated current. I also have a secondary overcurrent protection unit as you said. Assume the heat sink can meet the thermal dispersion request, I think this gives me the confidence when the IGBT work in the ‘on’ state that may last 20ms. However, I have another concern about the breakdown signal from IGBT drivers (such as the concept module based driver). Some wrong signals always disturb the control system when duty ration reaches up to 100%, and sometimes I have to block this signal in the DSP for undisturbed operation. I understand that this may be a special case that happened in my past applications. And this is also why I have a worry about if the IGBT can work in continuous DC current mode in my current project.
Thanks again and Merry Christmas.
If you keep the gate of PMOSFET tied to ground or NMOSFET to Vdd, MOSFET shall be kept in ON always
R. Udaiyakumar
Thanks your help.
I am sorry not telling the application background. I want to know the if power IGBT/MOSFET can be kept on all the time. Because the Power IGBT usually works in a chopping mode where the power dissipation matters. I did not find any applications with IGBT in on state always. Since the IGBT in my application may works in the on state for 20ms, I want to know if it is feasible in practice.
The answer to your question is yes given you are able to descippate the heat generated because of conduction loss.
Since there is no switching, the miller capacitance will only charge once during startup which means switching loss is zero and only the conduction loss causes device heating. Conduction loss can be calculated easily using the Irms flowing through the device and using the conduction loss you can calculate the theoretical internal temperature of the device. If that is under the prescribed limit then there shouldn't be any problem.
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