Glyph said:
Very interesting project ya got going there. I've been trying to build power supplies myself (some work... and some don't) and find this project rather interesting for its unique requirements.
Correct me if i'm wrong but this what i believe you want:
1. low noise/ripple
2. small size/weight
Well, I ain't lookin to lug a 5kg box around- but keep in mind that I use car batteries. If it weighs less than that by a fair bit, I'm happy. Certainly, it will be smaller and more portable than a generator.
Glyph said:
That's more a matter of the design than anything that costs a lot. Just taking the time to put the right kind of controller and good safety options, and make sure I understand how they work. More on that in a bit.
Glyph said:
This is probably the most important requirement, other than being quiet.
Glyph said:
5. wide input range (3v-14v)
No, nowhere near. More like 8-14V, and I'm actually settling for 9.7V minimum. I intend to set the trip point at 11.2V, because below that you are damaging the battery; I'll have an override for it for those situations where it's important.
Glyph said:
6. excellent load regulation
Well, now, I'm interested in being able to plug 15A into it and have it not sag below 12V. The initial output is looking like about 13.9V, and I'm at 12.51V at 4.9A. That's about as high as I can take the current right now; I'm working out an efficiency problem I'll get into later.
Glyph said:
... good thing you didn't add CHEAP or you'd have defined the impossible.
What's cheap mean? How's $100US grab you? It's running (though as I say, I'm working bugs out of it), and I haven't spent that yet (at least not on parts for the supply- I had a couple other projects hanging around).
Glyph said:
I've been looking at your "low noise/ripple" requirement for your supply and the reasons why you choose a Cuk converter. I was wondering if you know the frequency at which your imaging system (CCD camera, tube, or whatever you're using) runs at.
It's a CCD camera, and it accumulates charges as the cells are struck by photons. It might be open for 10 minutes or more, collecting photons- it doesn't run at a "frequency" unless you wanna talk milliHertz.
I was familiar with the TV supply story; I've seen it before.
My real main goal in all of this is: when the battery gets to 12V, not all of its power is gone- there is considerably more power available, perhaps as much as 10% or more, but I can't use it because all my equipment needs at least 12V. Motors start sticking, drive controllers start getting crazy, and it is generally
bad. So if I can get at that power (and there's plenty of amp-hours left- perhaps enough to get me through the rest of the night, and almost certainly through this imaging session), and boost it to 12V somehow, then I'll be set. I'd like to be able to use the supply all the time, to smooth out glitches I've seen in the battery power, and generally give me good line regulation; the load regulation isn't very important except I want to stay above 12V. But the efficiency is a real problem; I'll lose far more time than I will gain back if I have to put up with efficiency that low.
Where things are at:
I have built the supply, and I went with the boostbuck rather than the Cuk. The main motivation was the feedback network; the design is easier with a positive rail rather than a negative one. Thinking back, that might have been a mistake; I might be able to see now how to get at it. But in any case, that's what I did. Luckily, I can undo it if I have to.
I ran into a problem: the tantalum cap I used for my energy transfer cap's leads started heating up! Surprised heck out of me, I jumped back after hitting the panic button expecting the usual red flames of a burning cap. After it had cooled off, I pulled it, and all of the problem was at the bottom- not up top, like it is if the cap itself starts to fail. I thought about it, and realized that it had been exposed to too much current- not exactly what you expect with a cap. I thought it over, and wound up with two series pairs in parallel- it's the same value, but twice the current handling capability. I'll address it more if it becomes a problem, but for now it looks like this solution was successful.
During the breadboarding, I realized that when the controller shut down due to current limit, it turned the switch in the boost converter off- but because I am using a ground-referenced drive and a P-channel MOSFET, it left the switch in the buck converter turned on, and this provided a direct low-resistance path to the load. Obviously, this is unacceptable from a safety standpoint, and in the event of a real short, the buck switch would be destroyed. I therefore added a second driver, so that I could detect current limit (luckily the current limit builds a voltage across an external cap, so I can detect that with a comparator and use the signal to control things) and now the system shuts down both switches, so the load sees zero current and zero voltage.
While I was at it, I added a reset switch (if you drain the limit voltage from the cap, it restarts), and a disable switch (just an AND gate that either permits or does not permit the signal from the controller output to reach the switches). I also added a comparator to watch the input voltage, and a housekeeping supply for all of the logic and the controller (it's a 723, so it browns out about 3V above the needed voltage- thus, the comparator shuts the system down when the input voltage gets to 3V above the brownout voltage- which is 9.7V). I initially had bad load regulation, but when I added the 723, it improved miraculously. I think the TL5001 likes a nice steady working environment.
The ripple is extremely low. I am running at 216kHz with a (now four) 22uF tantalum as the energy transfer cap, and I started with a 47uF tantalum as the filter- but went with a 100uF redcap instead. It improved the ripple spec and cut some of the switching noise out. The supply puts out 4.9A at 12.51V, so far with no problem except this:
Efficiency. I expected better than 85% in my initial design, and I'm only getting about 60%. That means that I'm drawing 100W, but only getting 60- the other 40 get dissipated in the supply, and that's kind of limiting me just at the moment. I'm looking the transistors over, because they seem to be the main culprits, according to my fingers (Ow! Yep, that one's hot). Not only is it creating problems that are limiting the load, but it's a waste of battery power.
The only other complaint I have is a certain amount of transient noise from the switches. I'm looking into some beeper caps for it now. We'll see what comes out the other end.