Hat Coil

Hatcoil has been an on and off project lasting for a few months. During IAP (MIT’s January month-off) I redesigned the controller and boost converter, and having lots of free time at home spent a great deal of time in Eagle making a very reliable board structure to make it compact and easy to service that I am very pleased with.

The controller schematic can be seen here:

The input to this schematic is just a current sense transformer around the primary coil. I also added a parallel inductor of about 20uH to give the signal a little bit of phase lead to help compensate for time delays in the control circuit and switches.

The outputs drive a pair of gate drive transformers which drive an H-Bridge. Originally I was using somewhat slow IGBT’s (HGTG40N60B3D), but upon switching to the much faster FGH40N60SMD it gave extremely clean current and bridge waveforms.

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.

The second image appears to be skipping every other pulse, which is true in this point of the boost cycle. This is because the current is not dropping low enough for the next pulse’s threshold in time, so it skips the second one. This only occurs during the initial capacitor charge up since -dI/dt in the inductor is fairly low with a small voltage across it. Once it gets to a higher output voltage, this goes away and every cycle can be used.

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.

Here is the boost schematic:

Some pictures of the H-Bridge, Boost and controller (which is now all conveniently in one package)

This is the newer revision testing out a new polyphonic controller:

Here’s a video of the old version:

  1. Raffael Celestino Lima

    Hello, my name is Raffael. I am trying to make a musical tesla coil like yours, i would like to know if you have a complete tutorial on how to do the controller and the boost.

    Sorry for my bad english, thanks and I´ll be waiting.

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