Cuk converter PWM controllers?

[Schneibster]

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

 

[FRIED]

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.

 

[Schneibster]

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.

You don't say how much current.

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

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.

 

[FRIED]

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.

 

[Schneibster]

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!

 

[Schneibster]

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!

 

[Russlk]

Getting a PWM supply to be quieter than a battery will be tough. Why not put a 10 amp charger on the battery and a linear 10 amp regulator on the output? Power dissapation in the pass transistor is only 18 watts.

 

[Schneibster]

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.

 

[RGBrainbow]

Hi,
maybe this can help You:
http://www.elektor-electronics.co.uk
search for cuk*
hth
regards
joachim

 

[evandude]

Sorry if this isn't what you're looking for, but I just assembled the second circuit on this page today:

https://www.4qdtec.com/pwmmod.html

simple voltage to PWM converter. I haven't messed with it much, but so far I have it running at about 80kHz and it looks at least reasonably clean... I doubt 100kHz would be much different. input voltage range is something like 0.67v to 1.74v to go from 0% to 100% duty so you'd have to design a simple op-amp biasing circuit for your particular application...

I'm currently working on making a simple boost converter followed by linear regulator, for running a 12v device in a car on the nominal 12 - 14v of the battery voltage, while also surviving the voltage droop due to the starter motor when the engine starts... so it sounds similar to what you're doing... I'd highly appreciate if you could provide any circuits, links, etc that you found useful in your design... a schematic of your Cuk converter would be much appreciated as well... mainly for component values of the main parts, as well as the accessory components you spoke of. In case you couldn't guess, yes, I did just start toying with DC-DC converters a couple days ago, so I'm still reading up on it.

edit: here's a quick screenshot of the output so far.
(don't mind the apparent DC offset of the signal, that's just because I have the display shifted up...)

**broken link removed**

 

[Schneibster]

RGBrainbow, thanks, but I already have the circuit designed pretty much, except for the PWM.

Evandude, yes! That is just what I am looking for! Good resource, thanks a bunch!

I'll let you have a diagram of the circuit once I have it breadboarded and tested up. Might be a couple weeks- there aren't any good electronics stores on the East Side, so I get everything online. So far I am going with a pair of 18uh coils for the two inductances, and a pair of tantalum film 'lytics for the energy transfer (22 uF) and output filter (4.7 uF) caps, damped with big cans (220 uF for the energy transfer and 47 uF for the output filter) in series with 1.8 ohm 2W damping resistors.

I went with an ST part for the MOSFET- an STP35NF10- and an IR 30CPQ100 schottkey diode pair.

My first test was with a MAX4420 decoupled with another 4.7 uF tantalum to drive the MOSFET, and a MAX636 controller- but it looks like the controller isn't working the way I'd like, and it blew a MOSFET up, apparently by keeping it on until it cooked. I tried to cobble up a PWM from some TL084s I had lying around, but apparently they don't quite have the oomph to give a really good pulse output. I may try them in that circuit you pointed me to. If they still don't work well, I'll probably break down and get a comparator- maybe I'm just tilting at windmills expecting a BIFET opamp to do the job of a comparator.

I should warn you that my capacitor values are liable to change; I haven't done the small-signal AC analysis yet. So don't buy parts based on this- I may wind up with different values, and I intend to go with a switching frequency that is suited to my coils- 60kHz appears to be a bit low, by my analysis, and I'm still working on exactly what I want.

And one more warning: 18 uH is a hell of a lot of coil, and 220 uF is a lot of capacitor- don't mess with this thing alone! Respect it- it could kill you. Make sure there is someone there to help if something goes wrong. I keep my wife informed of what's going on in my shop and bring her in to keep an eye on things when I'm running a test. Once it's buttoned up, it'll be safe enough, but during the breadboard phase, I take no chances. I wanna live. If you mess with switching converters, you assume all liability for any mishaps- be safe! They aren't toys.

 

[evandude]

Hope it helps you. Meanwhile I think i'll go start my own thread so I don't hijack yours asking questions about my design :lol:

 

[Schneibster]

Hee hee, OK. I just spent the evening getting the sawtooth generator working- and it's not bad, so far- but I'm going to have to do some tweaking, I think. Thanks again for the reference.

 

[Schneibster]

OK, for starters, I am considering using a boostbuck converter instead of a Cuk- I have identified a P-channel MOSFET from International Rectifier that would do the trick. I am, however, not entirely sanguine about either increasing the parts count, or decreasing the efficiency. The parts count issue is obvious- the efficiency issue comes up because it doesn't take zero energy to drive the gate, and each time you turn the gate off, you drain the energy from the gate to ground. The more gates you drive, the more energy you have to push into the gate, and then waste to ground. In addition, each transistor and each diode has a small but non-zero voltage drop- so you dissipate energy from each switch element while they are on. Granted Rds(on) for the MOSFETs is 0.035 ohms (for the ST component) to 0.06 ohms (for the IR component), and maximum forward voltage drop for the Schottkey diode is 0.86V, nevertheless, I will be pushing that resistance and dropping that voltage in two components instead of one.

The P-channel MOSFET is an IRF5210.

In addition, I have identified two Texas Instruments PWM controllers that are appropriate for this application. They are the TL5001 or TL5001A (the A part has a tighter reference tolerance), and the UC3572- there are also 1572 and 2572, which are essentially identical, except for their temperature ratings. The UC3572 is much simpler, and does not have some of the nice features of the TL5001A- duty cycle control, easy short-circuit protection. It also has a smaller frequency range, although it can go to a lower minimum frequency (10kHz vs. 20kHz minimum, but 200kHz vs. 500kHz maximum). Finally, the UC3572 draws 9mA, but the TL5001A only draws 2.1mA. This makes it more efficient, and efficiency is definitely a concern as above.

I am studying the data sheet preparatory to ordering the parts- and making my decision about whether to add the two extra parts and sacrifice the efficiency to get an all-around design. I'll post more when I have verified that the parts will work, and made my final decision.

It's worth noting that the boostbuck would also allow the addition of a power transformer and rectifier bridge to drop 120VAC to 12VDC with ripple- and this could be fed directly to the converter to yield a nice clean 12VDC output, so the unit could be provided with both a battery and an off-line input, making it much more general-purpose.

The final advantage of the boostbuck converter over the Cuk converter is that there aren't any ground-loop problems- which could be an advantage in an automotive application, because if you use conventional equipment and connect that equipment's ground/negative to the negative output of the converter, there will be a -12V difference between that and the auto ground- if they should touch, it would cause a short and could do damage or blow the converter up. In the off-line application, the transformer protects against this, but the transformer's neutral (that is, the line from the transformer that is in-phase with the neutral from the AC line input) could not be connected to ground, because of the potential for the similar problem, and this could pose a safety risk. I have enough coils, caps, N-channel MOSFETs, and Schottkey diodes for two supplies, and I have a dead supply that I'm tired of fooling with that still has a good transformer and rectifier, so I might make the boostbuck for that and dual-purpose it for the automotive application.

 

[Schneibster]

The saga continues. I have finally completed and verified the operation of a Windows calculator program for the Cuk converter (which is identical to the boostbuck converter, at least in this regard) and can now do iterative designs. I have determined that I have several options based on the available inductors that I could not see earlier due to the difficulty in completing the calculations for one iteration (it takes about an hour, and that's with my trusy TI-89 and several sheets of paper, so writing a program to do it was a priority). I still have not decided between the Cuk and boostbuck, but I am leaning strongly toward the Cuk; I don't have to decide yet, because the component decisions are the same either way save for whether or not to order extra diodes and the P-channel MOSFET.

I am now working my way through the canonical model and checking the minimum values of blocking capacitance for the damping networks for the energy transfer and filter caps, for each of my iterated designs. Once I have completed analysis of one, I will automate that as well, and improve my program a bit, and then crunch all the numbers to see what the final designs look like with an eye toward purchasing some components. I intend to post the final design here in the next few days.

 

[Schneibster]

I have settled some issues with the design, but have not fully decided on cap and coil values. Since I have 18uH and 22uH coils available, and all the caps I need, I also ordered a pair of 33uH coils and the needed cap values for them.

I decided to order the P-channel MOSFETs just in case- they aren't very expensive. I'll decide my design when I see the output on the 'scope.

I ordered 10 10-ohm 15-watt resistors to serve as a test load.

The LM339-based PWM is useful- but a third-generation PWM controller like the TL5001A is head-and-shoulders above it in terms of both performance and features. It has a soft-start timer, a voltage reference and error amp compensation. All of these are things that have to be added to the 339-based PWM, and all of them consume power. By the time I was done, I was eating nearly 25mA, and the TL5001A only eats 10mA. Every little bit helps!

During breadboarding, it was driven home that the gate of a power MOSFET is not a small load by any means. The 339 just wouldn't drive it- the highest frequency I could achieve 90% duty cycle at was about 40kHz. I added a totem-pole driver made from a 2N2222 and a 2N2907, with the collectors wired directly to power and ground, driving the MOSFET from the emitters in an emitter-follower configuration, and still couldn't get enough current to switch the MOSFET on quickly enough for anything above about 120kHz. The off time was much better- it was very fast- but getting the sucker on was a real problem. I have had experience with the MAX4420 MOSFET driver, and this really is the way to go.

The one problem with the 4420 is that it is very sensitive. I blew up both of the ones I had before I realized that soft-start is an absolute requirement. The oscillator needs to be stable before its output is applied to the driver, and the first cycles need to not be high-duty-cycle- they did some pretty special stuff in this chip, and it requires a nice stable environment or the special stuff blows its output. This also has the unfortunate effect of blowing the gate on the MOSFET, probably by exceeding VgsMAX and destroying the SiO2 layer between the gate and the body of the transistor. The blown chips still limp along, but they are clearly degraded; if you hear a "snap" and everything stops working, but you don't smell smoke or see any sign of problems on the chip or the transistor, pull the chip and test it with a PWM and pull the transistor and check the gate; most likely you have blown them both.

It looks like decoupling will be an absolute "must-" I saw a lot of ringing and a lot of noise on ground. I'm going to have to look into how to decouple the MOSFET- it's going to cause real problems otherwise. I suspect just following the decoupling instructions for the MAX4420 and the TL5001A will work for them; I may not need to decouple the outputs at all, just their power.

 

[evandude]

hantto posted this link a while ago:

**broken link removed**

I have been looking at it and it looks pretty good. especially knowing that it's a working design to start from is reassuring, and I can use a part from radio shack to wind my own pseudo-transformer.

I'm not a huge fan of the rather basic UC3842 that it is based on, since it doesn't have any of the fancy features of the more modern ones, like charge pump gate drive... but it's a starting point. if I get it working I'd like to revise it to use a fancier controller, and probably a gate driver IC as well. we'll see! wish I wasn't a poor college student and could afford to buy enough parts to build all the different designs I want to try out :lol:

 

[billgmdie]

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.

Why don't you just get one of those small petroleum-powered electricity generators? (something like this, for instance:
**broken link removed**)
You need only ~300W anyway.
With such machine you can automate things and prolong exposure even more.

 

[Schneibster]

Two reasons:
1. Space- my truck is full with my complete kit, which comprises a large mount (Losmandy G-11) for a Celestron C-9.25, and a smaller mount for a C-5 and a 300mm f/2.8 lens, plus several cases of goodies of one kind and another; a couple cameras, a case of Nagler eyepieces, etc.
2. Noise and pollution- star parties are not just about the sky, they are also about experiencing the night, and the sound and smell of a generator is unlikely to make one popular. At multi-day parties, generators are often banned until noon so that people can sleep, and they are almost always banned at night. Getting your batteries charged can be a challenge, to say the least.

The battery option is really the only viable one for me. But good thinking.

 

[billgmdie]

1. Space- my truck is full with my complete kit, which comprises a large

Doesn't seem like a valid concern to me. Those 950W generators are very small, and if you really wanted to, you could squeeze it in somewhere, I suppose (maybe on the roof even).

2. Noise and pollution- star parties are not just about the sky, they are also about experiencing the night, and the sound and smell of a generator is unlikely to make one popular. At multi-day parties, generators are often banned until noon so that people can sleep, and they are almost always banned at night. Getting your batteries charged can be a challenge, to say the least.

Hmm, I see... Never heard of star parties. Well, in this case, you could try to find an even smaller generator. Pollution is not a problem, IMHO, because those 63cm3 engines are quite efficient, and almost don't produce anything other than H2O and CO2 (and power, of course).
Noise pollution is actually a problem. This 950W generator produces > 60dB of noise at 7 meters, so you'd need to move it ~ 200m away for the noise to disappear.

You could make a hybrid system, too. During the day a few solar cells charge 2 batteries (the one you bring with you, and your truck's), and during the night the generator acts as a backup device - if voltage on the batteries drops too much, it'll kick in, and you won't loose the picture. Of course, you'll need to move the generator away, but I don't see why that would be a problem.

Also, you may get a gas-powered generator (if you can find one, of course). These are very quiet.