If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
A straight buck converter only needs switch voltage rating of Vin + 1 diode.
Buck can't be used because it can't correct power factor. So it's a tradeoff for having higher switch voltage; but it's not a special problem if I only need relatively low (<100V output voltages).
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
SEPIC converters require a coupling capacitor between the first choke and the second choke/diode that has to carry a lot of current. PP or ceramic is required for this type.
I like SEPICs but that should be qualified by saying I haven't played with buck boost or cuk very much.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The standard way of doing it seems to be a boost PFC converter that steps up to a 400V DC bus, and then a forward converter that converts 400V to 100V.
I'm not sure why. One reason, I think, is that the PFC will always have poor transient response and a lot of 100Hz ripple, because it's constrained to draw a pure sine wave current from the mains. So they like to put a big bulk capacitor on the 400V bus. You can easily store loads of energy at 400V.
Another reason is that the forward converter can give you an output isolated from the mains.
Yet another reason is that you can buy an off-the-shelf boost PFC controller chip to drive it, I think it's the L4981, or just borrow a complete PFC front end from another SMPS to avoid all the heavy maths
Speaking of heavy maths, IIRC the Cuk and SEPIC both have right half-plane zeros, but the forward converter doesn't, so its transient response can be better. This is probably one more reason why they use the two-stage approach.
The boost converter has a RHP zero too, so it might be similar enough that you could hook a L4981 up to a SEPIC or Cuk converter, stick loads of capacitance on the 100V output, and hope for the best! If I was doing this, though, I'd just grab a SMPS from my junk pile and start trying to rewind the transformer for 100V out.
I know Richie Burnett lurks around here, so hopefully he could comment on this... :|
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
SEPIC converters require a coupling capacitor between the first choke and the second choke/diode that has to carry a lot of current. PP or ceramic is required for this type.
I like SEPICs but that should be qualified by saying I haven't played with buck boost or cuk very much.
I used a rather large lityc there, like 50% of my output filter on low voltages sepic converters. Seeing this It appears I don't need but a rather small cap over there.... Low-ESR litycs (and tantalums for low voltages) aren't a real problem today... but if I needed really lots of power on mains I could probably go for a non polar cap. I haven't seen any mains SEPIC applications yet, though.
Steve:
I don't have any special wish yet apart from to learn something, but I liked the idea of being able to power a push-pull royer converter from 230V mains, and use it, for example, to drive and dim fluorescent lamps. Or I might just build a royer and stick a HV transformer onto it, because it's way most efficient circuit to deal with poor power factor loading I know.
The hard switched converter would step down the mains voltage to around 100V and down to zero, allow me to regulate the input to royer and, what would be neat, correct power factor if possible.
Or, in completely other case, I might want to grab a 1200V IGBT and build 0-600V power supply.
Switching from a buck to buck-boost converter costs nothing but some switch voltage which I anyway have, and gives PFC in return.
Still I read about all those other weird types of converters but I don't know much about them.
What is really special about Ćuk, apart from having a low side switch? I don't see how would it make worthy to use over buck-boost in majority of cases. What's about the claimed lower ripple? (Does that mean lower output ripple Vs. component cost?)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
This is where I found about the possibility of using buck-boost as PFC
I would use normal buck boost, not flyback with transformer isolation.
The standard way of doing it seems to be a boost PFC converter that steps up to a 400V DC bus, and then a forward converter that converts 400V to 100V.
For an example of fluorescent lamp driver/ballast:
But wouldn't this cause lower efficiency (+all the complexity) than a single converter?
And if I was going to do that I would still really prefer not to use a transformer since it's big, lossy and expensive. The only good thing I see from a dual converter arrangement, is that I could change the output voltage at any wanted rate; if I'm powering a royer converter I would need no cap and only a single inductor between it and buck stage, making it effectively a regulated current source.
Then I could even audio-modulate it if I wanted.
But when I look at all that complexity I think that i could actually gain efficiency by removing the royer and transformer fully and using a hard switched halfbridge instead of it.
I have big priority of keeping the complexity of everything down. But that's getting off the thread topic.
My problem is that I don't understand lots of things Steve is talking about.
I have no clue what planes (RHPZ, LHPZ) are.
What does ''stick loads of capacitance on the 100V output'' mean? Does that mean that I need more energy or the same energy as I would have with 400V cap?
From my understanding any converter that can operate from 0 to peak mains voltage would correct PF equally well.
And my initial question, what's the difference between Ćuk, SEPIC and buck boost? I understand SEPIC gives positive output voltage, but that's it.
Why is Ćuk used at all over buck-boost when it costs much more (power inductor and cap) and has no obvious advantages?
Can somebody explain the Ćuk converter better? What does this mean:
The advantage of the CUK converter is that the input and output inductors create a smooth current at both sides of the converter while the buck, boost and buck-boost have at least one side with pulsed current.
So does this mean I have 2x less ripple, or what?
PS. To add... I absolutely don't need isolation from mains anywhere here, as I don't want to touch any of these voltages anyway. So I want to avoid any transformers unless they are really needed.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Interesting, I didn't imagine the buck+boost topology as such... but very good and easy read for me...
They didn't cover other converters like buck-boost and Ćuk though - I expected buck-boost to be at least given as a possibility (especially because they are on 115V input voltage) but isn't even mentioned...
One thing I noticed, why are there recovery losses on the input rectifier? Shouldn't the decoupling cap take care of that (at cost of little worse power factor)..?
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Sepic vs. ćuk vs. buck-boost; converters
Seeing this 
