Help me with 12V to 48V DC-DC Converter - Page 1
Forum section: electronics / projects

author: laurianus
author_rank: Newbie
timestamp: December 05, 2010, 12:50:09 am
content: HiNeed help to design a StepUp convertor(or other sollution) with these features:Vin: LeadAcid batery 12VVout: 36 - 48V(unregulated)IOut: Up to 10A for maxim 400msCan i use MC34063 calculator and IC to do this convertor?Thanks

author: Bored@Work
author_rank: Super Contributor
timestamp: December 05, 2010, 09:33:27 am
content: Quote from: laurianus on December 05, 2010, 12:50:09 amNo, apparently you didn't even care to read the datasheet of the MC34063. And it looks very much that you in general lack the skills to build one.I recommend to buy one, they are mass products.

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2010, 09:44:28 am
content: Look in the RS Components, Farnel, Digikey etc. catalogues.

author: Psi
author_rank: Super Contributor
timestamp: December 05, 2010, 12:18:14 pm
content: Im a bit confused why you only need it for 400ms but...to get that output you're going to draw around 53A out of your SLA (assuming around 75% efficiency)So you're gunna need a decent size SLA.There are many ways to build want you need.One way you could do it is to build a DC-DC switchmode stepup using a toroid transformer.You can buy the toroid core and wind it yourself quite easily, and you learn alot from doing so.Just keep in mind it cant be any type, has to be a core intended for a switchmode powersupply.(faulty/damaged highpower car amps normally have some toroid cores you can reuse.)Youll have to play around trying different numbers of turns to work out which is the best.If using the toroid core from a car amp you can see how many turns they used on the primary.It will probably be a something like 10 turns with a tap.Center tap the primary and feed +12V into the tap, then ground each side of the primary through a fet. You can then drive each of the fets through a transistor in a push/pull arrangement from any of the wide range of swtichmode controllers available. The TL494 controller is one i've used before that maybe suitable.Centertap the secondary winding and add two high current schottky diodes + a large cap.(You want to use a center tapped secondary because it means you only need two diodes to create a fullwave rectifier.Without the centertap you'd need 4 diodes for fullwave and that eats into the efficiency of the design.)Dont forget to matchup/tune the switching frequency to get the best efficiency from the toroid you're using.That part is critical if you wind your own transformer, as you dont really know its exact characteristics.Start at low current and setup multimeters to watch input/output current/voltage and map out the efficiency over a range of switching frequencys. (Dave's video on switchmode design shows this i think)I've used this method to build a nice voltage stabiliser (10-15V input - 12V 10A output)I've even managed to drive this kind of circuit from a 555 timer + inverter logic gate to generate the push/pull output.There are lots of extra features you can add too, like a high side current sensor IC and feed that back into the switching controllers error amplifer so you have a current limit.If i remember ill post a circuit diagram tomorrow

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2010, 01:02:34 pm
content: I agree, the TL494 seems like a better choice because it's push-pull and using a sinlge ended boost topology is not practical for such a high current.http://datasheetoo.com/search/tl494+push-pullhttp://focus.ti.com.cn/cn/lit/an/slva001d/slva001d.pdfhttp://www.chipswinner.com/DS/TL494.pdfThere are also other (better ICs you can use:LM5030http://www.national.com/ds/LM/LM5030.pdfhttp://www.national.com/appinfo/power/files/LM5030_App_Intro.pdfhttp://www.national.com/nationaledge/dec03/article.htmlLTC3721-1http://cds.linear.com/docs/Datasheet/37211fs.pdfIf you're still stuck, buy a pre-made unit as mentioned above.

author: laurianus
author_rank: Newbie
timestamp: December 05, 2010, 09:20:10 pm
content: I have to be more specific1. I read the data sheet and I know that it's says there it can work on output with maximum 1.5A. But I also know that if you use right voltage, current, peak current on external components (mosfet, inductor, diode) u can theoretically (use formula) get infinite voltage and current. But practically, after a limit u will not find any external components with those characteristics. Another problem is that the energy loss (efficiency<100%) is transformed into heat. More power/voltage on output -> More ... more power on input and more heat. This and all StepUp controllers are actually a timer for switching, a comparator to regulate voltage and a short circuit/temperature protection, all put together in the same IC.2. I want this solution to be a custom design so that I can integrate it in a PCB. I want to find all the components(the inductor might be a problem) used for this solution on the market, not to be custom made. What I need can be started 1 minute before this single big spike (aprox 400 ms up to 10A - theoretically I need 12 spikes at 8A for 15ms with 5ms in between, meaning a total time of 180ms). During this spike voltage and current can drop(at the end of the spike it can be 36V and 5A). I know I can use more and bigger capacitors but it's not my preferred solution. I want to use maxim 2, but preferred 1, at 4700uF/50V. If I buy mass products of this StepUp, they can offer me something expensive, regulated and with continuous voltage and current. On the one hand if I buy the variant at 3-5A, it does not behave well(short circuit protection come's up) at the spike I need. On the other hand if I buy the variant at 8-9A with 10A spike, it is more expensive, bigger and with some features that are not useful to me.So, can some one come with a brilliant idea for what I need?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2010, 09:33:12 pm
content: I don't see what the problem is, your duty requirements are low so not much heat will be dissipated meaning you can derate the semiconductors accordingly. The only thing you can't derate is the magnetic cores since they'll still saturate.What do you want, someone to do it for you? How much do you want to pay for someone to design it for you?

author: TechGuy
author_rank: Regular Contributor
timestamp: December 06, 2010, 03:31:01 pm
content: Take a look at the LTC3721-1 Push-Pull Controller:http://cds.linear.com/docs/Datasheet/37211fs.pdfLast Page of the pdf has a 12V to 48V reference design for you. Although you will probably have to beef up the MOSFETS used and the transformer core used in the ref. design to handle a 10A burst @ 400ms. ~90% efficiency rating.I would also recommend doing a google search for "Push Pull Controller" to see if these are other options that suite your needs.For a Transformer core. I would go with either a P-core, PQ, or RM ferrite core to keep the PCB real-estate small. Toriods usually need a lot of board space and P,PQ,RM cores are much more easier to wind yourself. I doubt you will find a manufacture that provides an off the shelf 12-48V push pull transformer. For a production you can provide specs to a transformer winder vendor to run you a set of a few hundred custom transformers that meet your needs.QuoteIncreasing the switch speed can solve the saturation issue. Although you have to make sure the core material will operate at the higher switching speed. You can also increase the gap size in the transformer to extend the saturation point. But this lowers the inductance value and may need a higher switching speed to avoid excessive output noise and voltage drop under high load conditions.

author: Psi
author_rank: Super Contributor
timestamp: December 07, 2010, 11:22:39 am
content: As promised, in case anyone was interested, here is the circuit diagram for that 10-15V input 12V output @ 10A psu i built ages ago.I'm not 100% sure the values listed are what i ended up using.And i built it 10 years or so ago, so it's probably not the best design. but hey, it worked, and was around 85% at 10AIt would be quite easy to add more turns on the secondary and get 24 or 48 volts output.

author: shabaz
author_rank: Frequent Contributor
timestamp: March 17, 2011, 04:08:18 am
content: Funnily enough, I recently built a circuit to convert 12V to 48V to provide a constant 3A but the design can be modified for 5A constant. The link is here:http://badcafe.co.uk/2011/03/17/48v-150w-power-supply-12v-step-up/