Sunday, August 19, 2012

FFv1.2s: First Test

I got the boards for FFv1.2s, a slightly smaller version of my latest line of motor controllers:

The boards are from OSH Park (formerly DorkbotPDX PCB Order, you might be able to tell by the color scheme). It's a terrific deal for small boards: $5/in^2 for three boards. For example, this board was 3in^2, so I got six for $30. The total time from order to delivery was a bit under three weeks.

These boards have exactly the same logic section as v1.1, but an entirely redesigned, smaller power section. Instead of massive D2Pak-7 FETs, it uses Super SO-8s. These are extremely popular in RC ESCs because of their low cost, relatively high power density, and thin package (more compact and easier to heat sink). I'll be trying two different FETs: the Infineon BSC016N04LS G (40V, 1.6mΩ) and BSC028N06LS3 G (60V, 2.8mΩ).

One difficulty with using Super SO-8 FETs is that, although it's possible to solder them from the sides, good thermal performance will probably only be possible if the drain pad under the chip is reflowed. The DRV8301 magic chip needs its ground pad reflowed anyway, so this doesn't add any extra steps for me.

It's just one trip to the bagel toaster.
Even though v1.2s is only reduced to 2/3 of the area footprint of v1.1, there's also a significant reduction in height thanks to the low-profile FETs:

The left-most board is a v1.1 without any wires or external capacitors. The middle board is v1.2s with XBee headers fitted. The right-most board is v1.2s with no XBee headers. In the smallest configuration, it's 7mm in thickness. The tallest components are now the three pushbuttons, and even those are lower profile versions of the ones in v1.1.

Here's what it looks like wired up:

The total weight with 16AWG wire and connectors is 35g, compared to 66g for FFv1.1 with 14AWG wire. Most of the weight is in the wire and connectors. The FETs are nowhere near as beefy as the D2Pak-7s, but I think the 40V/1.6mΩ version might still be able to handle 20A continuous and 40A peak current. (Compared to ~40A continuous and 75A peak for v1.1.)

The first assembled board survived power-up without exploding, so therefore it was flightworthy.

Poor Talon quad - it really has become a test bed for just about every piece of hardware. It has so much stuff attached to it now in such a disorganized and heavy way. I put a single FFv1.2s on, loaded with identical firmware as the other three v1.1's. Since they're running closed-loop RPM control now, the slight mismatch shouldn't matter much. Here it is ready for its first test flight with the Chair of Safety still in place:

After a few successful indoor test flights, I also took it outside a few times with the data collection running. The data is from the v1.2s, which was fitted with the XBee. The other three v1.1's are running blind. Here are a couple data segments showing climbs and fast maneuvers:

RPM tracking looks good and the average current is around 5A, peak 10A. The FETs are room temperature and I suspect I won't be able to even get them warm until I try it out on the CineStar...

One more motor controller done. Time to start a new one. Here's the teaser:


  1. babbytroller! is that using one of the IXYS 3-phase FET bricks from the 3PH series? so small!

    1. Nope, it's a little smaller than that:

  2. You should make a 100v tolerant controller, so we can run motors from half rectified line voltage. Or a 200v one, so we can run full rectified.

    Put all the mosfets on a daughtercard attached via standoffs that pass the main power bus up to it, so it can be built with variable current capacity / cost.

    1. At some point when I have money and space I would like to make a full-size EV, at which point I'll need at least a 144V power stage. Not sure if I would use FETs or IGBTs. I'll get back to you after my 200V FET-based Tesla coil explodes or doesn't. :P

      For rectified line voltage, there are some really nice Intelligent Power Modules that have integrated gate drive and transistors. Here's one that I've been looking at:

      Not as efficient as a custom FET board would be at 170V, I think, but still pretty high power density.

    2. You'll need a 400v power stage. Tesla S's pack runs up about that range. And their inverter can output 600A continuous, 1200A burst, supposedly. And it's packaged cylindrically! I really want to take one apart to see how it's done.

      But I think the hardest part for an EV is going to be sourcing a motor with reasonable power density. Most utility induction motors have terrible power density ... but I think that's because they are all run at 60Hz, so maybe you could get much more with higher frequency drive?

    3. I don't think I can compete with Tesla's power systems engineering...

      If somebody told me to build a full-scale EV right now for not much money I would probably use independent RWD with two smaller AC induction motors similar to the AC-35 kits from here:

      Although it would also be tempting to use even lighter and cheaper 40kW-peak brushless motors:

      Two of those could certainly propel a small car. And I could directly port my PMSM controller-building knowledge to that system.

  3. Is the power dissipation in the BSC016N04LS G's body diodes at the MOSFETs' maximum rated current level reasonable enough for you to use them as catch diodes when proper heatsinking is applied, or did you opt to use external diodes in your design?

    If not, which external diodes did you use?

    1. I use synchronous rectification on each phase, so the diode losses are pretty small. I would guess that they could be used as freewheeling diodes at up to 10-15A continuous, much higher peaks. That would give about 10W dissipation in the diode, which seems like a reasonable amount given the not-so-good heat sinking of this board.