12V -> 100V @ 100mA - what topology/architecture - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: March 02, 2015, 08:35:00 pm
content: I'm being asked to design a DC/DC converter with following specsVin: 12V +/- 10%Vout: 110VIout: 0.11AThat wouldn't be much of a problem, but space is very limited, and so it the height. There are only a handful of dedicated boost converters that will go into that duty cycle range (like 94% or something) and those that would work, require huge output caps (eg. 2x33uF/200V). The circuit can be 10mm high at most.I'm thinking about some transformer-based topology, but I only have experience with flyback converters when it comes to transformer based designs.Do you know any particularily compact solution to this problem? I was thinking about a forward converter with some low-profile pot core about 1:5 turns ration and full bridge drive to utilize the core power capacity as much as possible.

author: void_error
author_rank: Frequent Contributor
timestamp: March 03, 2015, 02:06:32 am
content: UC384x flyback converter? It doesn't require many extra parts. 10mm is pretty damn flat though.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 03, 2015, 02:32:47 am
content: Sounds like you've already been through TI's web bench designer, since that's the one that gives you the 2 x 33uf@250VI just did it.Ti Web Bench suggest the TPS40210. It's not bad, a simple boost converter.For the output caps, you might need to try to find 6x10uF , or 4x15uF .. (if you have the room). These are 7-10mm diameter through-hole and you can bend them over, and lay them down (again if you have the area). You can also make cutouts for the caps (while laid down).There are also some 10.5 and 11mm tall aluminum polymers, 15uF @ 250V, which are SMD. You can put 4 of them on your board, and they will take up less area then 4 radial lead devices. But at a height at higher than 10mm, you maybe can't use them.The design below is better than 92% efficiency, suggests 2x 33uF @ 250V output caps, and uses the Vishay IHLP6767GZER101M11 inductor. It's 7mm high.EDIT: The previous design was running at about 300kHz. If you go up in frequency to about 540khz you can use a simpler design with the LM3478 and a smaller and cheaper inductor, the Coilcraft mss1210-473meb. It's 12x12x10mm It's also about $1.50, vs. $8.50 for the Vishay-Dale inductor above. This design is about 93% efficient and uses 7 x 22uF caps at the output. You can optimize the caps as you need to fit your height, and area.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2015, 05:11:23 am
content: Sounds like something I did, don't think I have a good picture of it offhand but it's visible in the lower left in this picture.http://seventransistorlabs.com/Images/12BY7A_1.jpgThat's 75-150VDC, 300mA. Flyback, common ground. The transformer is a core and bobbin I have stock of, so, I don't much care that it's relatively tall.For your height, planar would seem to be the way to go. Some of the SMT transformer packages are possible, too: EQ, Planar ER and EFD types can be pretty low.I would absolutely recommend against using raw boost -- the ratio is just too awful. You can go with flyback, or however you want to call it, boosted flyback let's say -- simply, boost with an extended tap on the inductor. Depending on transformer type, probably 200-500kHz is reasonable; even for planar designs, pushing much above 500kHz without a resonant or quasi-resonant topology seems unlikely. The capacitance on the high voltage end of the winding adds up very quickly, so you don't want to be too aggressive with it.Tim

author: poorchava
author_rank: Super Contributor
timestamp: March 03, 2015, 07:02:03 am
content: I think my current approach will be a raw boost, uing TPS40210 chip at 800kHz with a leaded capacitor laying flat.PCB transformers are not an option I'm afraid - i failed to mention that the board must be single-side load only, as it will be glued to a flat surface. 2 capacitor leads sticking out on the other side might be ok though. They can be trimmed flush or a small recess might be drilled in the other surface.I'm also thinking about using one of many available 2MHz boost/flyback controllers with beefy gate driver added (eg ZXGD3003) to implement a simple boost topology, but I'm worried about EMI.