I've wanted a nixie watch for some time, and have been collecting parts and learning how to build pieces of the circuit for a while. The next big hurdle is making a tiny boost converter that can go 3V (2xAAA)->120V@3-4mA or thereabouts. I understand the basic theory of boost converters, so now I'm playing with various hand-wound inductors and a microcontroller PWM peripheral, trying to get a comprehensive feel for what's going on before I build the real thing.
My test circuit is shown below. The GPIO/PWM is running at 8MHz/256 = 31.25kHz period, and I can set the pulse width from 1/256 to 255/256. I have a 40MHz, 2-ch analog 'scope, which I'm triggering off the GPIO pulse and looking at the inductor/FET connection. I have a cheapish LCR meter, but it doesn't seem to give reliable measurements of my inductors, so I'm not sure what my hand-wound inductors are. I also have a few commercial, marked inductors to test with.
With a 100uH inductor, I get boosted voltage between 13 and 40 with pulse widths between 1/256 and 30/256. (I think I can go higher, but my current filter cap is only 50V, and I'd rather not pop it.) On the 'scope, the FET pulls the low end of the inductor to ground and it rings while rising to ~0.5V or 1V as current increases. When the FET turns off, I get a big clipped ring between the output voltage and ground, decaying eventually to 12V before the next pulse starts.
With hand-wound inductor A, I see a similar trace, but after the inductor current rises to a certain level, the FET top voltage goes up rapidly ... I think the inductor is saturating, so I'm seeing the current through saturated-inductor / FET / 0.22R resistor. Is that guess reasonable?
With hand-wound inductor B (many turns, thin wire, possibly high-permeability toriod core) I see very little voltage rise during the "on" pulse, then the inductor output snaps up to the (low, ~13-15V) output voltage without much ring, then there's a low-frequency half-cycle dip or two towards the end of the PWM period, just before the next "on" pulse. My guess: (relatively) high inductance, not much energy stored in core during "on" time, and the ring frequency is much lower than with the 100uH inductor.
Another question: is the frequency of the ring I see when the FET switches related to the inductor value and the (parasitic) capacitance of the diode, FET, and other stuff at the scope node? Anything else affecting it?
And finally, I'm thinking of using a coupled / common-mode inductor to achieve a greater voltage boost, but I don't see how (physically) it would work. I'll add a schematic of how I think it should go together unless somebody says "won't work, don't bother!"
Thanks!
My test circuit is shown below. The GPIO/PWM is running at 8MHz/256 = 31.25kHz period, and I can set the pulse width from 1/256 to 255/256. I have a 40MHz, 2-ch analog 'scope, which I'm triggering off the GPIO pulse and looking at the inductor/FET connection. I have a cheapish LCR meter, but it doesn't seem to give reliable measurements of my inductors, so I'm not sure what my hand-wound inductors are. I also have a few commercial, marked inductors to test with.
With a 100uH inductor, I get boosted voltage between 13 and 40 with pulse widths between 1/256 and 30/256. (I think I can go higher, but my current filter cap is only 50V, and I'd rather not pop it.) On the 'scope, the FET pulls the low end of the inductor to ground and it rings while rising to ~0.5V or 1V as current increases. When the FET turns off, I get a big clipped ring between the output voltage and ground, decaying eventually to 12V before the next pulse starts.
With hand-wound inductor A, I see a similar trace, but after the inductor current rises to a certain level, the FET top voltage goes up rapidly ... I think the inductor is saturating, so I'm seeing the current through saturated-inductor / FET / 0.22R resistor. Is that guess reasonable?
With hand-wound inductor B (many turns, thin wire, possibly high-permeability toriod core) I see very little voltage rise during the "on" pulse, then the inductor output snaps up to the (low, ~13-15V) output voltage without much ring, then there's a low-frequency half-cycle dip or two towards the end of the PWM period, just before the next "on" pulse. My guess: (relatively) high inductance, not much energy stored in core during "on" time, and the ring frequency is much lower than with the 100uH inductor.
Another question: is the frequency of the ring I see when the FET switches related to the inductor value and the (parasitic) capacitance of the diode, FET, and other stuff at the scope node? Anything else affecting it?
And finally, I'm thinking of using a coupled / common-mode inductor to achieve a greater voltage boost, but I don't see how (physically) it would work. I'll add a schematic of how I think it should go together unless somebody says "won't work, don't bother!"
Thanks!