Advice for designing a switchmode power supply - Page 1
Forum section: electronics / projects

author: krish2487
author_rank: Frequent Contributor
timestamp: March 21, 2015, 07:37:06 pm
content: Hello.I have some doubts as to the (re)design of an isolated DC-DC converter. Not doubt per say, but more of a what-if question. We have a legacy design where, depending on the range of the DC Bus (40-140V , 120-240V , 220-420V), we use 3 identical DC-DC converters, the only difference being the transformer itself and a input tank elcap. The topology is a flyback converter. The outputs are 5x(0-30V) @ 1A. We use three seperate magnetics for three different operating ranges of DC. In a effort to reduce inventory variety, I wondered if the converter can be redesigned to operate from a very wide range input and still meet the specs. It will help reduce custom components and lead time for them. Topology not withstanding,is it at all practical to design a converter that performs at such a wide range. It is okay if the cost of the magnetics go up by a small percentage, as it becomes a standard reusable unit.There is a second question that is kinda on the same lines. Say, we stick with the same design, Instead of getting the transformer wound with all the five windings on the same bobbin,is it better to split up the transformer into 5 small transformers that only have one primary and one secondary. Even such a scenario reduces my lead time since ALL transformers have a constant ratio and only one primary and one secondary.Third question,if I do split up the transformer into small units. how critical would the PCB layout become. since instead of having a single switching element, we now have 5 elements, but all switching the same primary voltage. Noise, crosstalk all such factors get amplified in playing mischief.Has anyone here worked on such a design before?? If so, what were such other design issues that are likely to crop up and what you guys did to address them??Thanks in advance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2015, 10:43:35 pm
content: What about knock-on specs like efficiency, EMI, transistor size etc.?You can make a wide range converter, but the ratings of everything must go up. And that's not a small cost for up to 150W of output.Would a variable PCB be a smaller cost? Or perhaps soldered jumpers?Although I doubt they're good enough for whatever isolation you need, general-purpose transformers do exist, e.g.http://www.digikey.com/product-detail/en/VPH1-0190-R/513-1194-1-ND/754691Something like that could be made, and configured on the PCB using jumpers.Or you could try planar magnetics, which may require a more expensive PCB (4 layers would be something of a minimum), but you might save area in other places with the greater layer count. In that case, the board itself varies, or you can still wire it from jumpers.Tim

author: 37electrons
author_rank: Newbie
timestamp: March 21, 2015, 10:46:54 pm
content: Answering Q1: I dont think it's possible. Flybacks are known to work well at realtively narrow range of input voltages. Problems start beyond 3:1 range. There are appnotes on the web on how to make a so-called "wide input range flyback" but even those tricks allow to extend Vin range up to 6:1 max., AFAIK.So 40-420Vin flyback is something from SciFi area, I'm afraid.I'd say if you only need to change transformer and input caps to make another model then the the design is good enough already.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 22, 2015, 05:25:20 am
content: @TeslaCoilThe DC-DC converter provides power supply for the rest of the system. Efficiency is not a primary concern as the generally the systems are 10 KW and above. I am willing to trade off efficiency for a single magnetics design to a certain extent. Likewise with transistor size.Getting custom transformers wound is not an issue. The problem I face now is one of lead time, as integrating 5 outputs on a single ER 29 core/bobbin means a LOT of work to make the transformers and a certain minimum order quantity for my vendor to break even. Hence the next part of the question. If i can split up a single core with 5 outputs into 5 smaller transformers with only primary and secondary windings which will be faster to wind, consequently my lead time reduces. Unlike in the US or UK, custom magnetics are upto a certain level cheaper to manufacture here in India. :-) The biggest difference for us here is, materials are expensive and man power is cheap unlike the rest of the world where the reverse is true. So no exotic materials like 4 layer boards and planar magnetics.@37electronsI dont mind redoing the design in a forward topology or push pull or anything. As I ve said before, It is good enough but takes almost 45 days of lead time for me to get a small quantity of transformers (300 nos approx). I am more interested in reducing this time and the variety of custom items for our production.

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 06:12:24 am
content: You could change the frequency with the input voltage range ... makes better use of the magnetics.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 22, 2015, 06:26:57 am
content: @Marco.Thats an interesting approach. I ll run a couple of calculations to check if a change in frequency will yield any benifit in operating range.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 22, 2015, 12:40:46 pm
content: No other single topology has as wide an input voltage range as a flyback and it's a reasonable choice for the power level. To extend the voltage range without stocking multiple transformers you could instead redesign the transformer with multiple, identical primary windings that can then be wired in various combinations of series and parallel via jumpers on the board. The number of primary windings needs to be a power of 2 and you then get a geometric sequence of ratios. E.g. - with 4 primary windings you will get primary to secondary ratios of 4:n, 2:n and 1n. Note that this kind of approach is not common because it increases the leakage inductance of the transformer which may/will require a beefier snubber for both the primary switch and the secondary rectifier(s).Keep in mind that the switch still has to be sized for both the max current at ~40V input and the max voltage at ~420V input. Additionally, the current limit needs to be changed for each ratio.Changing the switching frequency is not a realistic approach to increasing the operating voltage range of an SMPS and screws up the feedback loop compensation, besides.Good luck; this is what military folk call "an opportunity to excel" (ie - a serious challenge).

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 01:59:09 pm
content: Quote from: MagicSmoker on March 22, 2015, 12:40:46 pmBang bang DCM flyback has basic to none compensation, with 5 secondaries tight regulation isn't going to be a huge design constraint ... there areICswhich change frequency (and thus Ipk) automatically to deal with low loads, how is it unrealistic?

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 22, 2015, 02:39:08 pm
content: krish2487Shouldn't you change your country flag to India?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2015, 03:20:58 pm
content: Quote from: Marco on March 22, 2015, 01:59:09 pmI've got some discrete circuits that operate that way, although I prefer to run them with proper compensation anyway.http://seventransistorlabs.com/Images/Discrete_Tube_Supply.pngTim

author: krish2487
author_rank: Frequent Contributor
timestamp: March 22, 2015, 06:13:34 pm
content: I will be coming back to the states sometime in the near future. I have been called back for some help in maintenance and production streamlining in my family owned company post a fairly serious business debacle in 2013-2014.QuoteThats sincerely a very good suggestion and seems most practical. Will go with this less travelled road see where it leads!!!.Thanks a ton.Quote from: MagicSmoker onYesterdayat 11:40:46 PM

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 22, 2015, 07:48:59 pm
content: Quote from: Marco on March 22, 2015, 01:59:09 pmFirstly, DCM flybacks are intended for very low power applications like USB chargers and the like that put out 10W or less and need to have a very low no-load power consumption; 150W pretty much requires CCM to keep peak currents reasonable. Secondly, to use a variable frequency controller IC the OP would have to totally redesign the board which also hardly qualifies as realistic. Thirdly, there is the problem of "reflected voltage"; that is the voltage which appears across the secondary diodes when the switch is on which equals Vin / (Np/Ns). In this case, the turns ratio will have to be less than 1 to be able to deliver 30V on the secondary with only 40V on the primary so the secondary diodes will have to be 600V parts... which is all kinds of crazy for 30V outputs.Possible to do? Sure. Practical/realistic? No F'in Way.

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 09:31:23 pm
content: Quote from: MagicSmoker on March 22, 2015, 07:48:59 pmI don't see why, that just sets the duty cycle and I don't see much point in designing for 50% duty cycle at an input voltage range extrema. The calculator someone else linked above picks the average voltage of the range for 50% duty cycle for instance.QuoteOf all the losses in this scheme some extra diode losses would be the least to worry about.