Hat Coil Updates

Another safe avalanche-free winter at home with about half a dozen summits, but enough mountaineering, back to power electronics!

As promised, hatcoil is being brought back to life. Instead of repairing a questionable design, I decided to start over and make the whole thing better. This time it’ll be all on one heatsink and control board with a full bridge plus an over-current/inrush protected boost converter.

Since recently I haven’t been real into using TL494’s for moderate or high power boost converters, I just put a bang bang hysteresis controller on the board with voltage feedback. I also put a hall effect current sensor on the boost inductor, since I think inrush was killing my old boost. Basically this is what goes wrong without this protection:

When the battery pack is plugged in, the coil’s bus cap is uncharged, and will quickly charge through the boost inductor. There is no problem with this and it is normal behavior, except that if the boost logic is running during this phase, the boost FET might blow up since it has to take this current too. So to solve this, I just put a current sensor on the inductor, and at the start of every boost bang it checks with the sensor and makes sure inductor current is below a set threshold level (a few tens of amps at most, or probably even lower since this is a discontinuous boost design). This also has the pleasant side effect of making sure inductor current doesn’t build up and become continuous at the early stage of charging the bus-capacitor, where -di/dt during off time might be less than charging di/dt (leading to possible fault modes at a 50% duty cycle).

Both of these issues can be seen in the following simulations, the first without protection and the second with. Green is inductor current, and blue is transistor current.

I also did a bit of math on the boost inductor this time rather than just winding onto a random ferrite and crossing my fingers (it’s so tempting in power electronics, but it’s such an incredibly bad idea!)

I first considered conservation of energy in the inductor, which says that P_{desired} = \frac{f}{2}\,L\,I_{peak}^2 assuming discontinuous mode. I_{peak} can be replaced with \frac{V}{L}*\frac{D}{f} where D is duty cycle. This says that P_{desired} = \frac{1}{2}V^2 \frac{D^2}{L\,f} which is the equation you can use to figure out an appropriate inductor (ignoring any losses which could be added to account for less than 100% efficiency). Substitute battery pack voltage, desired duty cycle, desired frequency, and solve for inductance. Then all that is needed is to make sure the inductor won’t saturate. I came out with ~5uH for 50khz operation.

To etch (on dextrin paper, I can’t wait to try that magic!):

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Posted on January 31, 2012, in Uncategorized. Bookmark the permalink. Leave a comment.

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