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Old 11-21-2015, 03:27 PM   #2341 (permalink)
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Originally Posted by P-hack View Post
Actually I lied, the output boost diode bounces between roughly 300A and zero, 180 "degrees" out of phase with the bottom switch bouncing between 300A and zero, with a typical 30% inductor ripple anyway (i.e. 166uh here @ 20khz, ~%50 duty cycle).

It isn't a cop-out, it is just puzzling, why the form of a capacitor is dictating and severely limiting the boost circuit design. asthetics?!? do you see divinity in symmetry? Fibonacci's three phase controller? the boost switch/leg needs to be at least 3x larger for "hotrodding" purposes, sorry.
I would seriously like to improve the boost stage's capability. Right now, it would come at the expense of reducing the rest, because of the diameter constraint. I suppose I could increase the diameter and add those boost stage switches. At this point I'm not even sure I **want** a boost stage. However, the system advantages say it's worth at least playing with. If it doesn't work out, there's the opportunity to provide more output by symmetrically arrranging more phase leg switches.

The goal with this design is to provide as high a power/volume and power/weight controller as possible, in a "build it yourself" type format.

Therefore, options not considered were things like direct die switches, etc. I've seriously considered the stuff Toyota did for the Prius controller. I think Toyota did an amazing job with both the controller and the motor.

However, it would be impossible, within the design constraint of making this buildable, to use the direct die switches they used. In addition to that, they used a lot of tricks with the capacitance, cooling, etc. It's a very evolved, elegant design.

That said, I feel extremely happy that this design offers very close power/weight and power/volume numbers. One thing you cannot do when attempting this is leave a lot of cards on the table, like providing 3X headroom for "hotrodding" purposes. You will notice that you don't have anywhere near 3X headroom in the Toyota design. It would require a complete re-design to boost the power output that much. If you want that, I suggest a more normal design using IGBT "bricks" that can be added until you're happy.

Regarding symmetry and the capacitor - yes, there is divinity in symmetry. The capacitor is critical for the volume/weight reduction. The capacitor functions best when loaded as physically symmetrically as possible.

SBE, the capacitor's manufacturer, has a ton of info on their site about this. They strongly emphasize symmetry and a low inductance bus-plate system. This inverter design actually takes their recommendations a step further with complete symmetry. The benefits of symmetry and lower inductance provide lots of opportunity for higher performance. However, this is buried in the subtleties of SiC switches and high switching frequencies.

Thanks for the thought provoking questions; I'll see what I can do to improve the power output of the boost phase.

- E*clipse

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