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Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I can see that a custom made sinusoidal inverter would be quieter (both electrically and acoustically) than a normal VFD, however I wouldn't have thought the extra complexity and cost would be worth it; efficiency is good in both, as one has IGBTs at a low frequency, and the other fancy GaN transistors that switch very quickly. High frequency switching is often problematic as stray inductance and capacitance stresses components with voltage spikes (this isn't too bad if lots of LC filtering is used), but feedback could be a challenge.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
DerAlbi wrote ...
A follow-up on the Audio-ClassD amp...
300W single chip soloution:
Wouldnt that be great?
no, because where is no need to go for special chip, Half bridge controller will do the same. Also i need at least 10 times more power, as well i need to produce it from 220V/3phase input
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Ok, i was a little confused by the LTC3722 abvoe which has comparable voltage rating to the Class-D. The Chip includes the mosfets so there would less work to do. Running from rectified 3-phase is another story again. Also your proposed Mosfets only handle 200V, so they dont fit either. You also stated something with 100V..
High input voltages decrease efficiency (look at the "Converter Efficiency"-graph in the LTC3722 datasheet, 1st page) at any high frequency for a given load. Just extrapolate how 300V++ would look like. Hmmh.
Look at and see which low pulse rate is deemed appropiate for a motor. I think if done correctly you should simply be able to use IGBTs. The question is frequency selection again: as low as possible would be perfect. Using 6 - 12 pules per half wave is a common tradeoff (in 50Hz motors afaik). If you go higher than that you favor eddy current losses in the motor if you dont use a filter in front of the motor. A filter however will strain the system with additional reactive power transfer.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
DerAlbi wrote ...
High input voltages decrease efficiency (look at the "Converter Efficiency"-graph in the LTC3722 datasheet, 1st page) at any high frequency for a given load. Just extrapolate how 300V++ would look like. Hmmh.
No, HV does not decrease efficiency. it usually adds efficiency, then we are using resistive type parts ( mosfet's )
Doubling voltage increase power for same mosfet by a factor of 4
You are mistaking same setup that has to work from 30 to 80V, with one that is designed for very narrow window of input voltage.
Nearly finished my design that will test single phase at 10-150W, will see how it goes, after that, can make full power setup with high power mosfet's running from PFC or rectified mains.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
From the MOSFET datasheets I've studied, (and I've spent too much time drooling over ultra low on-resistance FETs:) the power remains more or less constant, with a 2 * increase in voltage causing a 4 * increase in R(ds_on) This is provable from the basic theory; you can make the same size silicon piece wider and/or longer to increase it's conductivity, or you can make it thicker to increase breakdown voltage, at the cost of an exponentially higher R(ds_on). OK, that's oversimplified, but you get the picture.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
There is a point where you have more switching losses than resistive on-losses. Switching losses increase linear (turn off) and quadratically (turn on) with voltage and linear with freqency. You simply cannot [should not] build a high frequency _and_ high voltage buck converter. Thats why i keep focusing on lower speeds so much....
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
DerAlbi wrote ...
There is a point where you have more switching losses than resistive on-losses. Switching losses increase linear (turn off) and quadratically (turn on) with voltage and linear with freqency. You simply cannot [should not] build a high frequency _and_ high voltage buck converter. Thats why i keep focusing on lower speeds so much....
+1
The only way I can think of an application of high voltage and high frequency is at resonance: soft switching Even then there are problems.
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Better mosfet then eGaN possible ?
