Howdy - I've been thinking alot about brushless motor control lately. I've been using some COTS brushless motor controllers and have been very unsatisfied with them - they've all been overly large, overly complicated, and overly costly. So I'm thinking about trying to build my own. I'm thinking about having it be something like this:25-75VDC supply voltage 10A continuous output hall, quadrature encoder, and analog position inputs small (in physical size) CAN connectivity ~25KHz switching speed or faster
The first thing that I'd tackle, of course, would be the hardware design. I'm looking for very precise control - so I want a good deal of built in smarts. My gut instinct says to go the FPGA route. Somehow that just strikes me as a nice clean way of handling things. I'm more experienced with MCUs - but I'm always happy to learn a new technology.
Next up - and the topic that I'm having the most trouble with - is the design of the half bridges. I'd want to use N-FETs throughout to maximize efficiency. Driving the high side FET gates is troubling. The only method of driving FETs like this is with a floating power supply grounded on the source of the FET, and then optotransistors switching the FET with that. There has to be a simpler solution. First of all - is there a clever way to combine the floating power supplies such that you only have one high side FET power supply?
This is the problem that I'm having the most trouble with. I mean - to keep from blowing through the gate of the FETs (by going over the max VGS) - you could put a zener across VGS on each high side FET (with a series resistor of course) - but then you're going to be draining the floating supply when you turn that FET on until the source voltage goes high enough to turn off the zener. That is why it's very attractive to have a separate floating supply on each FET's source - but yeah... size is important here.
Lastly - driving the FETs - the standard solution I seem to see for problems like this is to have some sort of a low side high voltage (well, high enough to handle the bus voltage) N-FET with the source grounded and a resistor between the gate supply and the FET's drain, and the gate of the half bridge high side FET connected to the FET's drain. This has the clear problem of having a huge trade off between FET turn on speed and power drop across that resistor. I've also seen optoisolators used for this, which takes care of that problem but also adds large parts and would seem to me to place a limit on the switching speed (as I feel like optos are typically substantially slower than normal transistors).
So I guess - my big questions here are:
- How to combine gate power supplies?
- How to drive high side FET gates?
Thanks so much!