I'll put the cool part right up top: it works.
Everyone else I've seen who does a variant on this project uses a primary inductor made either of regular wire or copper tubing. I thought a PCB inductor would be more elegant. This inductor is two stacked turns on 0.6mm PCB, 100mm center span and 8mm wide. I measured the inductance to be about 1.9µH.
This was my first time using an actual solder stencil and hot-plate reflow rather than daubing on solder paste and using the hot air station. Turns out, using an actual stencil is way easier. Who'd have thought. Also turns out a scrap MTG card is a great paste spreader -- thanks to my gf for the improvised tool 🌈✨
Soldered up beautifully.
Since previous test coils had their 18ga wires badly overheat, I used bundles of 4x18ga to connect the board and test inductor. This is either a hacky kludge to get more surface cooling area, or a way of making bootleg litz wire -- take your pick.
Heatsinking for the mosfet is on the bottom side of the board -- you can see some of the orange fins peeking out. (There was an odd misadventure of the board flexing and making poor contact with the heatsink until I was able to melt down the kinda-thick phase-change thermal interface pad with the hot air station.)
I designed the board to use a duo of 0.1µF and 10µF capacitors for input power smoothing/decoupling... then realized during assembly that in my capacitor kit, those values aren't rated to enough voltage. 6.8nF was the highest value that fit in the 0603 footprint with a 50V rating. Fortunately the power input looked stable enough under load.
I spent quite a while futzing with the circuit and adjusting components trying to track down what looked like self-oscillation. Turns out nah I was just being a silly goose; the potentiometer was upside-down relative to v0.1 and I'd been turning it the wrong way, in spite of having triple-checked during board layout and marking the orientation on the board.
Clearly still needs some tuning but this is the basic waveform I expected. Yellow trace is gate (5V/div), purple is Drain (50V/div). Interestingly I was able to get strong oscillations and a running toroid even at 12V supply, when BackMacSci talked about 18V being the minimum for his setup. I'm hoping that adjusting Rg will reduce some of that higher-frequency ringing.
The PCB inductor still gets immediately and disconcertingly hot, even with much more dissipation area that the initial wire coil.
Here's another photo of the toroid:
You can see that the o-scope waveform is all wonky when the toroid is actually toroid-ing.
For these tests, I was using a Miniware MDP-XP power supply. I'm a big fan of that tool -- it's tiny, usability is great. It also runs off of USB-PD, and in tracking down a variety of odd behaviors I ascertained that the USB power brick I was using may have been rated to 20V and 5A... but not both of those at the same time. Full load was causing substantial voltage sag.
Next tests: a beefier bench PSU. Stay tuned!
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