Electronic Circuits and Projects Forum

Hi, I'm new to this board but not to the 'Net.

I have been working desultorily on a Cuk converter design for a while, and I have done a design based on the Bill Behen design laminate. I have a pretty workable set of Ls and Cs, and picked out a suitable N-Channel power MOSFET and a good Schottkey diode. I have damped both of my tantalum-film 'lytics with nice big cans and series resistances, and everything looks great.

Now comes the rub. I had originally thought that I could use a particular Maxim chip to drive it; unfortunately, I apparently had a brain anomaly and it won't work (luckily I only blew up one MOSFET finding that out). I tried to make a PWM using a TL084 biFET op-amp with a relaxation oscillator, integrator, triangle-to-pulse converter, and comparator, but I can only get about 20kHz before the waveform goes to crud, and that's not enough for my design.

I've been looking for several days, and have yet to come across a really good looking substitute PWM or other type of controller. The big problem is that everyone wants to make custom ICs for computer power supplies, and nobody has been paying attention to Maxwell's theory and understands why Behen is right, and the Cuk/boostbuck is optimal. (It really is, particularly from an RFI/EMI standpoint- which is important for my application, I am doing CCD astrophotography. Obviously, Behen has me totally convinced ) If I just wanted 5V, I'd be all set, but this is a 12V supply for my telescope equipment, and I need 10A, and serious quiet. And I need something that will control the switch on a Cuk converter; yes, I'm pretty set on it being a Cuk converter, Behen has infected me and I also have Cuk and Middlebrook's Advances in Switched-Mode Power Conversion, both volumes. (All three- whatever, I guess they made the first two into a single one when they released the third one).

I have a leftover Maxim non-inverting gate drive chip, too, and it's all wired in. I'm ready to rock-n-roll if I just had a good source of voltage-controlled duty cycle to run the switch with, at like maybe 100 kHz or so. One other parameter to consider: I've found that big lead-acid batteries "glitch" every so often, and it's a real problem when you're trying to take a half-hour exposure and you don't find out until the end. What I want this thing to do is handle the 13.6 standard battery output and give me 12V, and keep giving me 12V come heck or high water until Vin goes down to, say, 11V or maybe even 9V (yes, yes, I know it will trash the battery, but I can always buy another one whereas a really clear night with excellent seeing is beyond price). I can do my own shutdown circuit if I have to; but of course the ideal controller would take care of all that for me.

So my question is, does anybody know of a really good controller for that? I've been looking at the UC2841 and UCC35702, and eyeing the MAX1651, which I think might actually work, but I would like to know if anyone out there has done this research before me and I am re-inventing the wheel.

Thanks in advance!
-Da Schneib

You need a sepic design. This is similar to a cap coupled cuk. You don't say how much current. Linear has some chips that will do this.

Originally Posted by FRIED

You need a sepic design. This is similar to a cap coupled cuk.

Too noisy. I need quiet power, as I said, so that it doesn't interfere with my CCD imaging equipment, and I don't want to be permuting the design putting huge, expensive caps into it to try to quiet it down. After that, not only would it be huge and heavy, but it also would be much more expensive than the already essentially built Cuk converter. As I said, it's all hooked up; all I need is a PWM controller appropriate for its control function.

Originally Posted by FRIED

You don't say how much current.

Actually, I did, quite specifically, but I'll repeat it: 12V, 10A.

Originally Posted by FRIED

Linear has some chips that will do this.

That's nice. Do they have any that will work for a Cuk converter? Would you care to suggest one in particular, as I did, or discuss the ones I did mention? That would be most helpful.

Sorry, missed the 10A. Linear's chips dont go that high. The UC3842 should do. Has a driver and cutoff if Vs <10V or>16V.

The UC3843 might be a bit more appropriate; my input will be a lead-acid 13.6V battery, and my desired output 12VDC, so the >16V lockout would be a problem. I will of course have to jimmy it to get voltage-mode control rather than current-mode; current-mode control is undesirable (and unnecessary) in a Cuk converter.

Thank you!

Quick question, did you show that picture because all six converters have the same control function? Or if not, can you tell me how the control functions are related to one another?

TIA!

I've found that the power from a lead-acid battery fluctuates noticeably, on a regular basis. You don't even want to think about what this does to the electronics of my CCD camera; it's ugly, and I've tracked it down to this. As far as noise goes, I expect far less than 0.1% ripple, and line-to-load regulation at least that good if not better. According to my evil experiments, this should be far more than sufficiently quiet for my purposes.

A linear regulator is far too wasteful; I need every watt I can get; I have to carry the battery with me to mile-high mountains, since I like living closer to the city than is entirely consistent with good dark skies. I hope to get pretty high efficiency out of this, but I might have to tweak on the design a bit to get everything just right. Certainly I expect to achieve far better efficiency than any linear regulator will give me, if not quite the quiet that I might get with one.

Last but not least, if I find myself in a situation where I can't get more battery or more power, and I need just a bit more to make a long drive to the mountains worthwhile, I want the option to drain the battery further than would normally be possible. I know that this will damage it, but $100 for a battery is small potatoes compared to $1000 for a trek to a really good spot, and making it worthwhile might be worth that $100 to me. Not only that but I use deep cycle marine batteries for exactly that reason; problem is, once they get below 11V the extra capacity is useless to me.