themotorman said:
MOSFETs do parallel well as they have positive resistance slope so if one heats up too much the load is taken up by the others. I agree that for high amps IGBTs are a nice way to go. The main problem is switching transients due to the inductive part of the system. The down side of this is that when IGBTs or MOSFETs go they form a dead short so you go like a bat out of hell. With a brushless motor a dead shorted power device will usually mean a dead stop! Take your choice , I'd prefer the dead stop! Whatever system you use the ideal is a pulse by pulse current controlled system. The controlling system by using the current into the motor in the feedback loop will behave much like the accel pedal of a gas car..
Paralleling large numbers of FETs and then trying to switch them fast, for a high commutation rate controller, for example, is a bit fraught. You're right about the DC conditions, but this isn't the issue. High voltage (c.100V) FETs have a high gate charge, the really, really big problem is driving enough current evenly into the gates of a parallel array at speeds commensurate with maintaining low switching losses. Most multi FET controllers are pretty poor at doing this well, particularly at the budget end of the market.
The very best illustration of this is the user experience from the multi-FET controllers commonly used on big ebikes. As the number of paralleled FETs increases, the current capability of the controller doesn't increase in proportion. This is hardly surprising when you think that a typical 100V 36 FET BLDC controller will have gate drivers that are trying to shift around 1200nC of charge around evenly between 6 gates at a time - a tough challenge and one that needs RF-type circuit board layout around the gates, with equal length transmission lines matched to carefully sized gate resistors to minimise gate ringing. The Chinese multi-FET controllers don't attempt to drive the FETs fast and cleanly, they just slug the switching times right down to get rid of the problem and accept the high FET switching loss, which is the reason for the law of diminishing returns when it comes to increasing the number of FETs versus the power the controller will handle.
Using single big FETs, or IGBTs if the voltage is over around 120 to 150V, where IGBTs start to look better than FETs, is a much simpler proposition, as it removes at a stroke the difficult multiple gate drive challenge. It is far, far simpler to supply a single gate with a nice, clean and fast gate drive pulse, and the whole controller becomes a very much simpler thing to lay out.
I'm of the view that controller designers have thought too much about the DC issues, like Rdson losses and associated junction to ambient thermal conductivity problems, and have only paid lip service to the need for clean gate drive and proper RF type layout of the drive signal path. One look at the plethora of messy controller designs around, often with fairly grim gate drive layouts, tends to confirm this.
Jeremy