rhitee05
10 kW
I've been tossing around ideas in my head for a homebuilt controller for a while now. I'm an EE, it's what we do for fun. I finally got around to choosing some components and drawing the schematic. My plan is to design a board that will contain the FETs, drivers, and misc support circuitry (voltage regulators, current shunt, etc.). Instead of integrating a uP onto the board, I'll provide headers and connect to a ready-made logic board to serve as the brains. Since C code isn't my strong suit, I plan to use an FPGA board (probably these: http://www.knjn.com/FPGA-RS232.html), but a PIC/AVR/etc would work just as well.
There seems to be a fair amount of interest around here in tinkering with controllers and some home-brew, but I know from experience that not a lot of people have the skills needed to design the FET side of things (I've blown up my share). Since I'm already planning on making it a modular design, I was wondering if there was interest around here in something like this? I'm not an expert in controller design, but I have PCB design experience and have done enough work with FETs and power circuitry to be confident I can design something that will work. I won't attempt nor claim to make an optimal design, just one that works pretty well without destroying itself.
If there is some interest, perhaps we can work something out where I'll provide a few of these dev kits on a limited basis (bare board, populated, whatever). If not, perhaps at least we can have some discussion on the finer points of controller design for the benefit of all? I know there are others around with more controller experience and I. I'm interested in comments and suggestions from the community, and I'm happy to make the schematic and CAD files available when complete. I have no interest in making profit or going into production, just experimenting and perhaps contributing to the community.
Design Goals:
For my application, the design specs are 10s LiPo (42V hot) and roughly 75A peak phase (motor) current. The parts I've chosen will all be good to 60V, and that can be raised to 75V by making a couple of substitutions (FETs and properly rated caps). By making a slight change to the regulator, that could be upped again to 100V. I've done some calculations for power dissipation, and I think 100V/100A is probably doable with a bit more aggressive cooling, at least for bursts. Calculations are only an estimate until the real thing can be tested.
FETs, Driver, Caps
I've chosen to use IRF7749L2 FETs for my design. They're 60V FETs in a surface-mount can similar to those used in the RC ESCs, but a better part in a larger package. They have an impressive 1.1mOhm Rdson (less than 2 mOhms hot) and the can package gives them low inductance and good thermal properties. I'm not under tight size constraints, so I can also spread the FETs out to give them more copper area. I'm also planning on providing airflow. The board will be laid out for 24 FETs, although I think 12 will be sufficient for my needs. IR makes similar FETs for this package in 40, 60, 75, and 100V versions. The 100V version (IR7769L2) has a 3.5mOhm Rdson which compares well to the popular 4110s.
The driver will be the Linear Tech LTC4444. It has a strong gate drive and I like the built-in shoot-thru protection. One driver for each half-bridge and separate resistors to each gate.
By leaving space between each set of FETs, it's also possible to provide more distributed caps closer to where they're needed. It'll be a proper bypass array with 3-4 different values spread across decades, smaller caps very close to the FETs, etc. I think a well-designed (and well-placed) set of caps can perform well without needing 1000's of uF, but its an easy thing to leave a few unpopulated slots just in case more is needed.
Voltage Regulators
It'll add a couple bucks to the cost, but I decided to go with a switch-mode supply for this design to cut down the heat. A linear regulator would be something like 1/3 of the total dissipation under nominal design conditions! I like the LM5010, which can source up to 1A from supplies as high as 75V. It also only requires a couple of external parts, so it's not much more cost. The high output current means lets me use the 15V rail to power a fan, light or some other external gadgets. If there's interest in a 100V version, I can put in provision for a pre-regulator. I'll use a secondary linear regulator to provide 3.3V-5V as a logic supply to the control board.
Measurement Circuitry
The board will include a shunt and associated circuitry for measuring battery current and a circuit for measuring battery voltage. I've chosen pars fast enough to regulate current on a pulse-by-pulse basis. I'm also planning to include the phase voltage circuits necessary for sensorless operation, but I haven't decided yet if I want to measure each directly or just provide comparators (suggestions?). Since I plan to use an FPGA for control, the board will include one or two ADCs for these signals. I can provide separate analog headers for a uP or something with internal ADCs. I'm also going to include at least one digital temp sensor thermally coupled to the FETs so thermal protection can be implemented. All of this will probably reside on an SPI bus.
Since I'm planning for forced-air cooling, the board will also incorporate a couple of low-side FETs set up to drive fans off the 15V rail. The intent (eventually) is to provide PWM control to the fan(s) based on the temp sensor(s). It could also be used to drive a light or some other accessory.
Thanks to anyone who actually reads this through. Let me know what you think!
There seems to be a fair amount of interest around here in tinkering with controllers and some home-brew, but I know from experience that not a lot of people have the skills needed to design the FET side of things (I've blown up my share). Since I'm already planning on making it a modular design, I was wondering if there was interest around here in something like this? I'm not an expert in controller design, but I have PCB design experience and have done enough work with FETs and power circuitry to be confident I can design something that will work. I won't attempt nor claim to make an optimal design, just one that works pretty well without destroying itself.
If there is some interest, perhaps we can work something out where I'll provide a few of these dev kits on a limited basis (bare board, populated, whatever). If not, perhaps at least we can have some discussion on the finer points of controller design for the benefit of all? I know there are others around with more controller experience and I. I'm interested in comments and suggestions from the community, and I'm happy to make the schematic and CAD files available when complete. I have no interest in making profit or going into production, just experimenting and perhaps contributing to the community.
Design Goals:
For my application, the design specs are 10s LiPo (42V hot) and roughly 75A peak phase (motor) current. The parts I've chosen will all be good to 60V, and that can be raised to 75V by making a couple of substitutions (FETs and properly rated caps). By making a slight change to the regulator, that could be upped again to 100V. I've done some calculations for power dissipation, and I think 100V/100A is probably doable with a bit more aggressive cooling, at least for bursts. Calculations are only an estimate until the real thing can be tested.
FETs, Driver, Caps
I've chosen to use IRF7749L2 FETs for my design. They're 60V FETs in a surface-mount can similar to those used in the RC ESCs, but a better part in a larger package. They have an impressive 1.1mOhm Rdson (less than 2 mOhms hot) and the can package gives them low inductance and good thermal properties. I'm not under tight size constraints, so I can also spread the FETs out to give them more copper area. I'm also planning on providing airflow. The board will be laid out for 24 FETs, although I think 12 will be sufficient for my needs. IR makes similar FETs for this package in 40, 60, 75, and 100V versions. The 100V version (IR7769L2) has a 3.5mOhm Rdson which compares well to the popular 4110s.
The driver will be the Linear Tech LTC4444. It has a strong gate drive and I like the built-in shoot-thru protection. One driver for each half-bridge and separate resistors to each gate.
By leaving space between each set of FETs, it's also possible to provide more distributed caps closer to where they're needed. It'll be a proper bypass array with 3-4 different values spread across decades, smaller caps very close to the FETs, etc. I think a well-designed (and well-placed) set of caps can perform well without needing 1000's of uF, but its an easy thing to leave a few unpopulated slots just in case more is needed.
Voltage Regulators
It'll add a couple bucks to the cost, but I decided to go with a switch-mode supply for this design to cut down the heat. A linear regulator would be something like 1/3 of the total dissipation under nominal design conditions! I like the LM5010, which can source up to 1A from supplies as high as 75V. It also only requires a couple of external parts, so it's not much more cost. The high output current means lets me use the 15V rail to power a fan, light or some other external gadgets. If there's interest in a 100V version, I can put in provision for a pre-regulator. I'll use a secondary linear regulator to provide 3.3V-5V as a logic supply to the control board.
Measurement Circuitry
The board will include a shunt and associated circuitry for measuring battery current and a circuit for measuring battery voltage. I've chosen pars fast enough to regulate current on a pulse-by-pulse basis. I'm also planning to include the phase voltage circuits necessary for sensorless operation, but I haven't decided yet if I want to measure each directly or just provide comparators (suggestions?). Since I plan to use an FPGA for control, the board will include one or two ADCs for these signals. I can provide separate analog headers for a uP or something with internal ADCs. I'm also going to include at least one digital temp sensor thermally coupled to the FETs so thermal protection can be implemented. All of this will probably reside on an SPI bus.
Since I'm planning for forced-air cooling, the board will also incorporate a couple of low-side FETs set up to drive fans off the 15V rail. The intent (eventually) is to provide PWM control to the fan(s) based on the temp sensor(s). It could also be used to drive a light or some other accessory.
Thanks to anyone who actually reads this through. Let me know what you think!