Some of you may have followed my adventures here with trying to create a simple yet effective boost power supply giving 9 volts from 2 AA cells. (The last installment was this thread.) As things stood last, I was working with two competing designs: my own, consisting of a multivibrator turned into a boost converter, with the 2nd transistor acting as the switch; and a design of Mr Al's, an interesting 2-transistor switcher whose operation I still don't fully understand. (I just built Mr Al's version last night; so far, it's winning against my design, at least in terms of output.)
One of my goals was to see how much I could do with as little circuitry as possible; part of my motivation is to see if I can give the "Minty Boost" a run for its money without using a dedicated IC like everyone else seems to do). I want to see what can be done with just a few transistors.
However, it struck me last night that maybe I was going a little too far in that direction, with my multivibrator/switch combo. I may be simply asking too little to do too much. So I'm trying a different tactic. I still like the idea of basing this on a simple multivibrator, but it occurred to me, why not just use that to drive a MOSFET switch?
Here's the block diagram of the thing (your basic boost converter topology):
**broken link removed**
The circuit I'm currently simulating is posted below (Spice). After initially failing to work at all, it's giving pretty decent performance, at least in simulation. I can get nearly 6 volts at 600mA out of it (for comparison, the Minty Boost claims 5V @ 500mA), on a 2.75V power supply (partially-depleted AA cells).
Unlike many other circuits where component values aren't terribly important, everything here seems to depend on very small adjustments. In particular, the following seem critical:
I have some specific questions about this. Rather than go off on a dozen tangents, I'm really hoping that someone can focus on just these questions and help me understand things better. Once again, keep in mind that this is the farthest thing from any kind of commercial product idea. I'm doing this because I'm interested in small, efficient power supplies, but equally important, I'm trying to teach myself some basic electronic principles. If anyone else learns along the way, so much the better!
Here are my goals and criteria for this project:
So my questions:
1. While it performs reasonably well as-is, there are some things which bother me about it. The first on is the driving waveform. Look at this snippet:
**broken link removed**
The top trace is the multivibrator output, while the bottom is the MOSFET gate voltage. The gate is getting an extra serving of AC here in the form of ringing on the rising edge; what's up with that? After all, this is measured on either side of a resistor. I can only guess that the ringing has something to do with the inductor current somehow spilling over in the the gate.
In any case, I don't like the waveform much. Mushy; that rising edge should ideally be like a cliff face. Is it possible to get any better performance out of a lowly 2-transistor multivibrator? Something to shape that into a cleaner square wave? and get rid of that ringing, too. That can't be doing my efficiency any good.
2. I've never worked with MOSFETs below, and just grabbed one from LTspices's parts box that looked like it might work OK. Can someone suggest some devices that would be appropriate for this, based on my power requirements? Remember, I'm not after overkill here. Ideally this would be something small, in a TO-92 package, or similar, not a big FET requiring a heat sink. I'm using a N-channel; is that the right way to go here?
3. This oscillator runs a bit on the slow side (~11kHz currently). I'd like to get that up somewhere closer to the 30-40-60kHz neighborhood if possible, but I have problems with the simulation just not starting if I decrease the caps too much. There seems to be a fairly delicate balance running a multivibrator on such a low voltage. Any pointers on why that's so and how to work around it would be appreciated.
4. Obviously this circuit is lacking regulation (I'm trying to get the boost working first). In the interest of simplicity, I'd like to use my classic poor-man's regulator circuit:
**broken link removed**
In my other design, I simply connected the collector here to the base of one of the multivibrator transistors. Works surprisingly well. Any other ideas appreciated. (Simple ones.)
One of my goals was to see how much I could do with as little circuitry as possible; part of my motivation is to see if I can give the "Minty Boost" a run for its money without using a dedicated IC like everyone else seems to do). I want to see what can be done with just a few transistors.
However, it struck me last night that maybe I was going a little too far in that direction, with my multivibrator/switch combo. I may be simply asking too little to do too much. So I'm trying a different tactic. I still like the idea of basing this on a simple multivibrator, but it occurred to me, why not just use that to drive a MOSFET switch?
Here's the block diagram of the thing (your basic boost converter topology):
**broken link removed**
The circuit I'm currently simulating is posted below (Spice). After initially failing to work at all, it's giving pretty decent performance, at least in simulation. I can get nearly 6 volts at 600mA out of it (for comparison, the Minty Boost claims 5V @ 500mA), on a 2.75V power supply (partially-depleted AA cells).
Unlike many other circuits where component values aren't terribly important, everything here seems to depend on very small adjustments. In particular, the following seem critical:
- The oscillator frequency, obviously (set by the two capacitors and two base resistors), as well as the duty cycle. I've tried to lengthen the on time for the driver (signal "mv1") as much as possible by tweaking the capacitor values.
- The coil inductance (tied to frequency).
- Even changing the MOSFET gate resistor seems to have a big effect on the output.
I have some specific questions about this. Rather than go off on a dozen tangents, I'm really hoping that someone can focus on just these questions and help me understand things better. Once again, keep in mind that this is the farthest thing from any kind of commercial product idea. I'm doing this because I'm interested in small, efficient power supplies, but equally important, I'm trying to teach myself some basic electronic principles. If anyone else learns along the way, so much the better!
Here are my goals and criteria for this project:
- Small & compact; I currently build small projects in Altoids or similar tins, and would like this to fit in one (again like the Minty Boost).
- Run off cheap, readily-available power (AA cells, alkaline or rechargeable).
- Two target voltages:
5 volts, with enough current to run typical USB devices (my goal, as stated above, is to rival the Minty Boost's 500mA);
9 volts, with an arbitrary goal of supplying ~250mA, which should be plenty for most 9-volt battery-operated devices/circuits I'm interested in. - Well regulated, well-filtered supply that could be used with audio devices without hum, noise or other bad things.
- Suitable for most loads; reasonably non-reactive ones, at least. Only need to handle small to moderate inrush currents; no high starting current devices like DC motors.
So my questions:
1. While it performs reasonably well as-is, there are some things which bother me about it. The first on is the driving waveform. Look at this snippet:
**broken link removed**
The top trace is the multivibrator output, while the bottom is the MOSFET gate voltage. The gate is getting an extra serving of AC here in the form of ringing on the rising edge; what's up with that? After all, this is measured on either side of a resistor. I can only guess that the ringing has something to do with the inductor current somehow spilling over in the the gate.
In any case, I don't like the waveform much. Mushy; that rising edge should ideally be like a cliff face. Is it possible to get any better performance out of a lowly 2-transistor multivibrator? Something to shape that into a cleaner square wave? and get rid of that ringing, too. That can't be doing my efficiency any good.
2. I've never worked with MOSFETs below, and just grabbed one from LTspices's parts box that looked like it might work OK. Can someone suggest some devices that would be appropriate for this, based on my power requirements? Remember, I'm not after overkill here. Ideally this would be something small, in a TO-92 package, or similar, not a big FET requiring a heat sink. I'm using a N-channel; is that the right way to go here?
3. This oscillator runs a bit on the slow side (~11kHz currently). I'd like to get that up somewhere closer to the 30-40-60kHz neighborhood if possible, but I have problems with the simulation just not starting if I decrease the caps too much. There seems to be a fairly delicate balance running a multivibrator on such a low voltage. Any pointers on why that's so and how to work around it would be appreciated.
4. Obviously this circuit is lacking regulation (I'm trying to get the boost working first). In the interest of simplicity, I'd like to use my classic poor-man's regulator circuit:
**broken link removed**
In my other design, I simply connected the collector here to the base of one of the multivibrator transistors. Works surprisingly well. Any other ideas appreciated. (Simple ones.)