MOSFET Assisted soft switching of IGBTs
Avalanche, Fri Jul 21 2006, 04:46PMI came across this article (whilst searching for info about parallelling IGBTs) and found it quite interesting, so thought I would post a link. Might be a way to further increase the switching efficiency of DRSSTCs, particularly the 'mixed parallel operation'. Possibly a good approach for all these mini high frequency DRSSTCs that have been appearing?
Re: MOSFET Assisted soft switching of IGBTs
HV Enthusiast, Sat Jul 22 2006, 03:34AM
Its a good article, but i can't say I see any benefit with using it for DRSSTCs. With DRSSTCs, I have found that the extended turn off time due to tail current is essentially cancelled out using primary feedback. In otherwords, because the system is derived directly from the output primary current itself (which includes the tail current), any effects on the tail current is just fed back into the feedback to drive the system. Therefore, nearly perfect zero switching is achieved regardless of the tail current. I have measured and verified this in all my DRSSTC systems that utilize primary feedback.
HV Enthusiast, Sat Jul 22 2006, 03:34AM
Its a good article, but i can't say I see any benefit with using it for DRSSTCs. With DRSSTCs, I have found that the extended turn off time due to tail current is essentially cancelled out using primary feedback. In otherwords, because the system is derived directly from the output primary current itself (which includes the tail current), any effects on the tail current is just fed back into the feedback to drive the system. Therefore, nearly perfect zero switching is achieved regardless of the tail current. I have measured and verified this in all my DRSSTC systems that utilize primary feedback.
Re: MOSFET Assisted soft switching of IGBTs
Steve Conner, Sat Jul 22 2006, 09:36AM
Ya, I agree with EVR, for once :P The conditions in a DRSSTC with primary current feedback are already doing their best to help IGBTs switch as fast and as efficiently as they possibly could. You can't get away from the fact that IGBTs (like BJTs but unlike MOSFETs) stay On for a small time after you told them to turn Off. So if you want high efficiency, you have to arrange the circuit to take stress off them during this time.
I did look into adding an extra primary tank circuit tuned to the third harmonic, hoping it would flatten out the zero crossings of the current waveform and make the switching even softer, so slow IGBTs could be used at higher frequencies than in a regular DRSSTC. But I couldn't figure out how to make the phase of the third harmonic current stay right at all times.
Then I thought of using another inverter to generate the third harmonic current, but really that would just end up being the MOSFET assisted switching in another form.
Even with all these tricks, there's only so much you can do. I think I remember Richie Burnett tried an IR "Warp" IGBT in his 4MHz Class-E amplifier in place of the usual MOSFET. A properly tuned Class-E circuit gives the best soft switching you can hope for, but it was useless. The turn-off time was longer than a half cycle of the operating frequency, so the IGBT stayed more or less halfway on the whole time and went to a smouldering grave.
Steve Conner, Sat Jul 22 2006, 09:36AM
Ya, I agree with EVR, for once :P The conditions in a DRSSTC with primary current feedback are already doing their best to help IGBTs switch as fast and as efficiently as they possibly could. You can't get away from the fact that IGBTs (like BJTs but unlike MOSFETs) stay On for a small time after you told them to turn Off. So if you want high efficiency, you have to arrange the circuit to take stress off them during this time.
I did look into adding an extra primary tank circuit tuned to the third harmonic, hoping it would flatten out the zero crossings of the current waveform and make the switching even softer, so slow IGBTs could be used at higher frequencies than in a regular DRSSTC. But I couldn't figure out how to make the phase of the third harmonic current stay right at all times.
Then I thought of using another inverter to generate the third harmonic current, but really that would just end up being the MOSFET assisted switching in another form.
Even with all these tricks, there's only so much you can do. I think I remember Richie Burnett tried an IR "Warp" IGBT in his 4MHz Class-E amplifier in place of the usual MOSFET. A properly tuned Class-E circuit gives the best soft switching you can hope for, but it was useless. The turn-off time was longer than a half cycle of the operating frequency, so the IGBT stayed more or less halfway on the whole time and went to a smouldering grave.
Print this page