M
Michael
- Jan 1, 1970
- 0
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:
1. FPGA/MCU?
2. How to combine gate power supplies?
3. How to drive high side FET gates?
Thanks so much!
-Michael
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:
1. FPGA/MCU?
2. How to combine gate power supplies?
3. How to drive high side FET gates?
Thanks so much!
-Michael