Looking for DC-DC 36VDC in 8,5VDC out 0,4A eff >85% anyone? - Page 1
Forum section: electronics / beginners

author: Kjelt
author_rank: Super Contributor
timestamp: August 07, 2015, 09:54:21 pm
content: I am not an electronics beginner but coming to building my own SMPS DC-DC converter, yes I am a beginnerI am looking for a DC-DC converter with the following specs:Vin = 36VDC from a SMPSVout = 8,5VDCoutIout = 0,2 tot 0,4A max.Efficiency > 85%So I looked at the ready to buy DC-DC blocks from Traco, EP and the sorts and it seems that 36VDC is a magic border between low voltage (12-36V) and high voltage (36-72V) input which makes it low in the efficiency (70%) for my parameters.Then I started to look at TI, Linear Technology, Maxim and the sorts for their dc-dc converter ic solutions but man do they have zillions of chips out there, I really got lost there (dutch saying: through the large amount of trees I can no longer see the forest).So I guess I am going to ask some help from someone on this forum who has already built something like that or at least knows what he is doingfor some guidance.What modern high efficiency chips would you recommend and can I use without too much foreseeable problems (like EMC or huge ripple) and has a datasheet with a trustworthy pcb layout ?Hope someone can give me some guidance in this field. ThanksOh yeah last requirement (or additional circuit) failure of the dc-dc converter shall not result in that the input voltage will get to the output voltage (or the damage will be desastrous). Probably some additional circuit needed there to blow a fuse with overvoltage, or built in overvoltage protection in the ic itself.

author: Miles Teg
author_rank: Regular Contributor
timestamp: August 07, 2015, 09:59:50 pm
content: For'a good starting guidance you should have a look on the Texas instrument webench calculator. Following your needs it will propose lot of solutions but. You can peek the first one coming or the one you prefer. It will design a schematic with detailed chosen components around. And with the specs oh the chip will certainly come strong advice on layout with example of an evaluation board. You can find also chip from other manufacturers near the specs the ti tool will advise you.

author: indole
author_rank: Unknown
timestamp: August 07, 2015, 10:16:30 pm
content: Also recommend the TI Webench Designer. Just plugging in the specs you stated gives several options to choose from depending on space requirements, efficiency or BOM. The highest efficiency one is 91%. Here's an image from the report just to give you an idea.Link to design

author: Kjelt
author_rank: Super Contributor
timestamp: August 08, 2015, 12:05:51 pm
content: Thanks guys,I saw this tool but also had suspisions about it.I mean how stable are the automated designs and how ready to go is it?Do you guys have experience with this tool, do you use it for product design or does it take a lot of tweaking afterwards to get it stable ?Since most smps ic's are smd and need a groundplane I need to design the board and let it manufacture.Some of the ic's might be extremely difficult to order in small quantities?So before I take the plunge I probably need to hear some success stories but also fail stories , and why it failed?thanks.

author: Miles Teg
author_rank: Regular Contributor
timestamp: August 08, 2015, 03:14:59 pm
content: On 2 years, I made maybe around 10 supplies using this tool. 3-4 of them are on produced devices, 1x150pcs, 3 on a device ~20k pcs.I like also not getting everything from TI. So I use webench to get a rough idea of the chip type needed, and try sometime to find equivalent function in another brands.And that's mainly motivated by the stock or deal the factory I will work with have with some brands.I get only one problem with a booster stage where I used on my first prototype wrong main diode and I struggle to find a correct one.It doesn't take a lot of tweaking. The part taking me the longest time is the inducotr. When you need to find the smallest and cheapest one.For layout you will get advice in datasheets, that will tell you the loop between Switch, self, diode dans ground is the most critical. Surely a ground plane around and under will help. And everywhere keep low impedance to the ground.If you are designing a device that will need to go through regulation, EMC will be your main concern. And for this on design you will never be enough maniac on attention to pay to keep EMI low.For supply in low quantity, I mainly shorten the choice from webench by the chip I could get daily from Farnell. And sometimes I request samples from big distributor.Get mostly no problem to gest some pieces for prototype.Buck or boost chip are freaking easy to use compared to design a power supply from the main. (which I never did yet)So give it a try.

author: Kjelt
author_rank: Super Contributor
timestamp: August 08, 2015, 07:40:28 pm
content: Quote from: Miles Teg on August 08, 2015, 03:14:59 pmOk thanks Miles Teg, that is good to hear that you have positive proven experience with the tool. Also thanks for the other tips.I will definitely try this tool (after my holidays) and get an account with TI .Also thanks to Indole for showing it is possible to get a postive result within the parameters.Quote from: Miles TegSince this is not for production and small quantities that is not a problem. The only problem I fear is that the solution needs an exotic part that can't be getten anywhere or from Farnell (that has a 50 euro minimum order value , go order three inductors for 0,30 a piece and then try to figure out what stuff you actually don't need for another 49 euros)But I will try this next month when I return from holidayThanks all!