I've been working on a power supply for antique Teletype selector magnets. These need 120VDC 60mA for the first
2ms or so of each bit time, but the sustain voltage needed is only 3-4V. So I've built an isolated boost supply and interface board, all powered from a USB port. The whole project is on Github:where there's an explanation of how it works which doesn't require downloading the whole KiCAD project.
Schematic as image:
LTSpice model:
I've had a board made, and it produced 120VDC briefly, until the IRLB8721 MOSFET blew out. I just used that because it's a common MOSFET. Not surprising - the circuit shows 40V across source and drain, and that's a 30V part.
So what to put in there? I've been trying various parts in LTSpice. The AP9465GEM works great in LTSpice, but it's a surface mount part and only 40V, so there's no headroom on voltage. Almost nothing with a rating of 50V or better works in simulation. Turn-off is too slow, or capacitance is too high, and the resulting output voltage peak on the primary is too low. Try loading up the LTSpice model and using different MOSFETs to see this.
I don't really know what I'm doing here. I'm aware that this MOSFET application depends strongly on the transient properties of the device. Turning off that transformer primary to get the inductive kick is tricky. I'm trying to struggle through the Art of Electronics chapter on MOSFETs, while hammering on the problem with LTSpice.
(I'd like to use a TO-220 form factor, or at least a through-hole part, and get this first board working. After that, I can get a new board made.)
(I was asking previously about low duty cycle
555 timer circuits. That part is working fine now. Thanks.)John Nagle