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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, so the question is: For high power (>300W) off-line half-bridge driver, which are more suited? MOSFETs or IGBTs? Has anyone tried IGBTs in half-bridge to drive flybacks at high power? So far I have run half-bridge drivers only with FETs, mainly IRF740 (400V, 10A, 550mOhm). The problem is, they heat noticably @300W, even when conduction losses should be no more than 2W. Do you think, will there be a big improvement using IGBTs (such as the IRG4PC40W, which I can get for the same price as IRFP460)? My goal is to go over 500W to drive two similar flybacks in parallel. Any help on this is appreciated!
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I always use mosfets. I think IGBTs are better suited for low frequency/ high current applications, given their massive gate capacitance. 500W from rectified 230V mains is only 1.5A, so an IRF740 will do. Also driving mosfets is a piece of cake, so I would use a mosfet. If you decide to use IGBTs then remember the extra gate drive requirements, I seem to remember people talking about 30nF gate capacitances!
About the heating, are you sure the flyback core is not saturating? I was running 2 paralleled IRFP450s in my offline half-bridge driver, and they were still getting hot after short runs. (10 seconds ) I solved the problem by increasing the frequency to the point where the core no longer saturated, and suddenly I could use a single IRFP450 and run the driver at full power for as long as I wanted to! So make sure the core is not saturating! Be sure that the flyback cores can take 500W.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
IGBTs usually don't have too high gate capacitances. The last ones I worked with, IRG4BC20UDs, are around 500pF IIRC. 30nF was propably some big brick IGBT.
Driving IGBTs is quite a bit like driving MOSFETs.
The main difference is on the switching side. IGBTs are not as fast as MOSFETs, because their operation is based on minority carriers, which are, inherently, slow. This means slower switching and larger switching losses. When it comes to conduction losses, IGBTs are better for high current, as the conduction losses in an IGBT are propotional to the current. The conduction losses in MOSFETs go up with the square of the current.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Mosfets, and really purely for economical reasons. You can surely find an IGBT that is small, and fast, but it will likely cost more than a well suited MOSFET.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for all replies!
Uzzors wrote ...
About the heating, are you sure the flyback core is not saturating? ... I solved the problem by increasing the frequency to the point where the core no longer saturated, and suddenly I could use a single IRFP450 and run the driver at full power for as long as I wanted to! So make sure the core is not saturating! Be sure that the flyback cores can take 500W.
I tried to increase the frequency, but this results in decreased output power, so I have to remove some turns to get to the same power level again... The fets were a bit colder, but I think increasing the frequency too much will result in too high switching losses (though I can and will try to do that ) The 500W figure was meant for 2 parallel flybacks.
As to the IGBT speed problem, I think with modern fast IGBT it's no longer a big problem (in the datasheet of my selected IGBT, they state it is suited up to 150kHz (hard-switching), and this is when switching tens of amps. In my driver they will see only a fraction of that). I think I'll buy the IGBTs (they're not all that expensive) and tell you how they do in my driver.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I think I got to the trick. Previously I always set the running frequency just above resonant frequency, because this way I was getting higher voltages, and I thought also longer arcs. Of course not! When I set the running frequency right between two resonant frequencies, surprisingly I got longer arcs and cooler fets!
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
I think I got to the trick. Previously I always set the running frequency just above resonant frequency, because this way I was getting higher voltages, and I thought also longer arcs. Of course not! When I set the running frequency right between two resonant frequencies, surprisingly I got longer arcs and cooler fets!
Two resonant frequencies? I thought there's only one, how it is?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
jmartis wrote ...
I think I got to the trick. Previously I always set the running frequency just above resonant frequency, because this way I was getting higher voltages, and I thought also longer arcs. Of course not! When I set the running frequency right between two resonant frequencies, surprisingly I got longer arcs and cooler fets!
Two resonant frequencies? I thought there's only one, how it is?
ok maybe I was not clear on that, I meant harmonics of the fundamental resonant frequency
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just out of curiosity I tried replacing one FET in my half bridge by a small IGBT (IRG4BC20U) I found in my junk drawer, and it heats about the same as the FET. So I think FETs and IGBTs are quite "on par" in this type of circuit.
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Half-Bridge Flyback Driver: MOSFETs vs. IGBTs
) I solved the problem by increasing the frequency to the point where the core no longer saturated, and suddenly I could use a single IRFP450 and run the driver at full power for as long as I wanted to! So make sure the core is not saturating! Be sure that the flyback cores can take 500W.
)
