Ideas for DC step down design - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: October 11, 2013, 06:07:47 pm
content: I am trying to convert a DC into a lower DC without spending too much power. Outside of the obvious choice of a switched mode converter, eg LM2679ADJ type-of-IC, and without using coils, can you suggest another way of doing this? I have been experimenting with a 555 (control frequency or pulse width modulation) providing a waveform to control the charging of a switched capacitor, but could not get results (ie no power wastage).Any suggestions?

author: gregallenwarner
author_rank: Regular Contributor
timestamp: October 11, 2013, 06:12:54 pm
content: There's always going to be some power wastage (there's no such thing as a 100% efficient system), so what kind of efficiency were you trying to shoot for? Typically, a buck converter is the most efficient switched mode DC to DC step down converter, and that uses an inductor (coil). At any rate, that's the most common design that I've seen. Any particular reason you wish to avoid using a coil?

author: nickm
author_rank: Regular Contributor
timestamp: October 12, 2013, 01:14:45 am
content: Whats the reason of avoiding switch-mode? You can get them in a 5pin TO-220 and just add the L+C output filter along with decoupling and the resistor divider. Barely any bigger than the 78xx solutions with all the efficiency advantages.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 12, 2013, 03:38:07 am
content: To give any options we need some expectations. Each design has inherent limitations and strengths. We need thing like:VinVoutIoutEfficiencyVqEstimations are fine it is just to figure out what ballpark you are aiming for.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 11:21:06 am
content: The reason I wanted to avoid using a coil is that I have read a lot of negative comments about PCB layout, stray EMI and so on. Also, the LM2679 IC costs quite a lot, £4-£8 per IC, considering I paid just £5 for a 70W 19V/3.5A power supply!Nevertheless I have ought on ebay 20 of those ICs, for a much better price than Farnell, and will experiment when they arrive.I have tinkered with various switched capacitor circuits to produce an inverse DC, or to double the DC. Based on the same "switched capacitor" philosophy I was contemplating a way to reduce the DC without much power loss. For example one idea would be the opposite of the switched capacitor doubler circuit: instead of charging the caps in parallel and then applying them on the load in series, I could do the opposite, charge them in series and apply them in parallel, thus halving the DC.The real application, once I manage to get something working, is a battery charger and a bench PSU. My current battery charger charges from 1.2V to 13V up to 3 A. But it spends as much as 50W on the pass MOSFETs (or transistors) because it is fed by a DC 19V/. My bench PSU is up to 25V and 5A and again I have used a large heatsink to absorb the heat on the pass transistors. If I had a way to reduce the DC more efficiently then I could perhaps construct the bench PSU in a much smaller package and with much less heat, and the same for the battery charger.

author: dannyf
author_rank: Super Contributor
timestamp: October 12, 2013, 12:29:46 pm
content: QuoteThe simplest would be a linear regulator. Without knowing how much is "too much", and what exactly you are designing for, it is hard to tell.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 12, 2013, 12:46:22 pm
content: With your requirements you will require a SMPS to get better efficiency than your linear power supplies.There are several topologies but all contain an inductive element/s.The simplest SMPS topology. For your task is called a buck/step-down converter.Here are some tips for your first one.Use a chip that gives you a preferred layout for you to copy.Research the cost of all the parts in it's typical application cct.Find one with a reasonable total cost of parts obtainable in small quantities. Sometimes you will find a cheep chip requiring an expensive inductor, or an expensive chip capable of using low cost inductors.There is also the option of getting a prebuilt converter module and control it by your own circuitry.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 01:17:40 pm
content: I have already done it linearly. In the case of the battery charger the wastage could be over 50W expended as heat on the pass transistors [(19V - 1.5V) * 3A]. On the bench PSU it could be over 130W as heat [ (>28V - 1.5V) * 5A ].In my experiments, the "switches" are actually pass transistors which are either fully on or fully off. In both cases they should not expend too much energy, and it is only during the transitions where we spend energy (that's what I was hoping at least). I am using a square wave to drive them, and typically the charged caps are from 10uF to 100uF and the frequency is from 10KHz to 60KHz. But I cannot make a satisfactory design for halving, or for reducing the DC.I attach the idea of caps in series and then parallel.

author: mariush
author_rank: Super Contributor
timestamp: October 12, 2013, 01:21:47 pm
content: Quote from: akis on October 12, 2013, 11:21:06 ameBay is full of parts that didn't pass all the quality tests, or parts made some company in Asia instead of the official manufacturer, or parts they scraped off the original text to print something else. They could very well get a LM25** and print on it LM2679 and the majority of people won't know the difference.If your Vin-Vout or Current is small, it may be worth going for a linear regulator. Otherwise, there are plenty of buck switching regulators out there, you don't have to go with LM2679.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 12, 2013, 01:34:26 pm
content: Quote from: akis on October 12, 2013, 01:17:40 pmYes a method of capacitor banking is possible but relatively difficult to control. Not to mention the cost of the caps required for a 3 amp load would quickly surpass the cost of a buck converter.

author: dannyf
author_rank: Super Contributor
timestamp: October 12, 2013, 02:02:56 pm
content: Since you are using it to charge a battery where you don't need exact DC, one way to do it is to use a switch to periodically connect the power to the battery. A low-side switch would be sufficient. Controlling the duty cycle on the switch will control the energy being sent to the battery.For this purpose, you can think of the battery as a resistive load.

author: Marco
author_rank: Super Contributor
timestamp: October 12, 2013, 02:22:49 pm
content: Quote from: akis on October 12, 2013, 01:17:40 pmYou can't charge a capacitor losslessly from a DC voltage without an inductor.Charge pumps only become efficient once the output voltage is near their designed output voltage (always an integer fraction/multiple of the input voltage BTW). During the time it's reaching that point (which in this case would take a long time because of the battery) they are actually quite inefficient.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 03:42:43 pm
content: Quote from: mariush on October 12, 2013, 01:21:47 pmThis guy had a lot of good feedback, I hope I have not bought dudsHere is the item:http://www.ebay.co.uk/itm/121189619595?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 03:57:58 pm
content: I thought I might chime in: Remind me again what your input/output requirements are. It also wouldn't hurt to explain what your power source is and what you intend to power. I have lots of parts lying around and I might be willing to just give you something if I think it'll fit your requirements better.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 04:11:29 pm
content: Project 1 is battery charger. Planning to power it from a laptop power adaptor that gives out 19V/3-4A. The output is from 1.2V to 13V @ 2-3 A. Currently designed completely linearly, using 2 power mosfets to control current and voltage. Depending on the conditions the current limit MOSFET has to drop many volts (17-18V) and maybe at 2-3A - that is over 50W of heat.Project 2 is my bench PSU. Currently using a linear regulator (LM317 and LM337) with pass transistors (MJL3281A and MJL1302A) to produce 1.2V - 25V @ 5A.Want to make NEW of both to utilise the idea of down converting the input DC without generating heat.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 04:14:16 pm
content: I found the post that had the important tidbits, so nevermind! --- actually you summarized better the second time, so I appreciate that!The way good bench power supplies achieve decent efficiency is usually by having either a multi-tap transformer and relays to switch out the secondary depending on the requested output voltage, or a switch-mode tracking pre-regulator driving a linear regulator. For the latter you have the output of the tracking pre-regulator be equal to or greater than the drop-out voltage of the linear regulator it's feeding. You'll of course want to give yourself some margin. The benefit of doing it this way is that you get some of the efficiency of a switch-mode supply while having to deal with less severe ripple and switching noise.I rather like hearing the "click" of relays when turning the knobs on my bench PSU, but I've always wanted to design a bench supply that uses a tracking pre-regulator.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 04:47:43 pm
content: Though lots of people do it, using the LM2596 or similar devices as adjustable regulators is kind of abuse. "Adjustable" means that the user can set an arbitrary FIXED output voltage within the acceptable range, not that the output should be adjusted in operation. The components around the regulator should be tailored to the application at-hand. In the very least, you'll end up with mixed performance over the output range, especially in terms of efficiency. Again, I'm not saying you shouldn't do it. Just don't think of it as the switch-mode equivalent of an adjustable LM317/LT3080/etc. supply.I might be talking out of my ass here, so someone either back me up or shut me down.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 04:58:40 pm
content: It's just a resistor divider to adjust the output voltage, surely we can adjust it at run-time ? I did not see anything in the calculations regarding the other components.However from what I have read, and not my experience i should say, I do have concerns over the "fragility" of these circuits, how often they go wrong, how they emit radiation all over, how the PCB layout is crucial and whatever else we simply do not have with linear regulation.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 05:02:39 pm
content: The capacitors and the inductor, among other things, are designed for the application to give the best performance.

author: akis
author_rank: Frequent Contributor
timestamp: October 12, 2013, 05:31:09 pm
content: Yes , but I assume that even if we achieve as "low" an efficiency as 85% (typical is well over 90%) we are still miles ahead of the linear regulator case ?Here I attach a simplified circuit which inverses the DC, works quite well on breadboard. And another even simpler circuit that produces +/- V/2 (half V) from a DC V. The square wave source is the output of a 555, buffered or not.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 05:43:42 pm
content: Do you expect to get any appreciable power out of that circuit?

author: dannyf
author_rank: Super Contributor
timestamp: October 12, 2013, 06:04:39 pm
content: QuoteThe 2nd one wouldn't work - it is plain wrong.Both lack any meaningful current capabilities.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 12, 2013, 06:08:14 pm
content: I just realized what I was looking at in the first schematic. It's a "don't try this at home" capacitive power supply. They're terribly inefficient, though I imagine a higher frequency helps a bit. I made one to convert 120VAC to ~12VDC for a space-constrained situation where I just wanted to drive a relay. It worked, but it was inefficient and potentially unsafe.

author: Marco
author_rank: Super Contributor
timestamp: October 12, 2013, 08:35:16 pm
content: You can not make a high efficiency variable output voltage and input voltage independent charge pump DC DC converter, at least not without a ridiculous amount of switches and capacitors ... it is because how charge transfer to and from capacitors works, it's only efficient if the transfer is to/from roughly the same voltage. Otherwise ESR absorbs a large amount of energy (regardless of it's value).Here'san EDN article about it with a small bit of math.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 13, 2013, 03:41:52 am
content: Quote from: Marco on October 12, 2013, 08:35:16 pmThis is what I was talking about. Sure, you probably still beat linear regulation under all circumstances, but it's still not a good design.I have two rather thick books on SMPS design ... it's complex stuff.I'm not saying that you shouldn't go forward with making a little lab supply for yourself based on a SMPS, but really the only good way I can think of to make a decent lab supply without being an expert in the field is using a multi-tap transformer. I've personally not had a ton of luck finding what I'd be looking for on digikey or mouser - maybe someone has an idea. Usually I've only seen 2-3 taps.Performance with most SMPS designs is just going to vary A LOT over a range of output voltages, primarily efficiency. Still, I wouldn't expect anything worse than 60% efficiency or so.

author: akis
author_rank: Frequent Contributor
timestamp: October 13, 2013, 10:04:09 am
content: For my existing bench PSU I have wound a few turns to get a 5V DC independent of the main sources to be used for the voltage and current meters (they are old style and pull considerable power). I imagine I could unwind half of it, tap it, and wind it back on so as to get a mid-range tap on the secondary. The wire is thick however and it would require a lot of patience.The switched caps scenario need not be as bad as it sounds. The caps will not be discharged fully on every cycle. After the initial charge cycles the state of the switching caps will depend on the behaviour of the load. And like half-way/full wave rectification, we can add more switching capacitors to charge on the opposite side of the wave. I am working on a schematic, I will post it once it works.

author: dannyf
author_rank: Super Contributor
timestamp: October 13, 2013, 10:12:27 am
content: QuoteA switched cap design has low current capability. smps has low efficiency under light load. So if your application is one where a switched cap power supply can potentially work and you care about simplicity, a linear regulator is the way and maybe the only way to go.You are confusing two cases here: you used the battery charger example to justify the use of a smps when you are really designing for a low load situation.

author: akis
author_rank: Frequent Contributor
timestamp: October 13, 2013, 10:13:28 am
content: Quote from: dannyf on October 12, 2013, 06:04:39 pmIt works really well. Both circuits are academic shown here just to discuss ideas. It can be used to power op-amps, I only need 2-5mA standby and 5-20mA at full load. If you want a quick -Ve from a single supply, eg say a 9V battery, the second circuit is quite handy. Depending on your current requirements you can power it straight out of the 555 IC, otherwise I have added a simple pass transistor or a push pull pass. The diodes and caps are there to shift the base voltages away from each other (the PNP base sees a double voltage and the NPN sees a negative voltage) so that the overlap between the two transistors is minimal otherwise they conduct at the same time and draw a lot of power.

author: dannyf
author_rank: Super Contributor
timestamp: October 13, 2013, 11:34:10 am
content: QuoteIn the 2nd circuit, the transistors aren't even biased.

author: dannyf
author_rank: Super Contributor
timestamp: October 13, 2013, 11:35:54 am
content: QuoteA to92 linear regulator will do more than fine in such a case. You are wasting brain cells trying to get a smps to work within your constraints here.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 13, 2013, 05:18:57 pm
content: There is not a best topology for every possible power supply.Apparently you need an ever increasing number of power supplies.Pick one of them, make it the go onto the next.Asking about several at the same time will confuse things.At least name the supplies so others can keep the issues straight.

author: silent
author_rank: Contributor
timestamp: October 13, 2013, 06:38:34 pm
content: Hello.What is an acceptable efficiency for You?I have faced a similar problem some time ago. I needed a 12V to 5V converter with 3A output. In my previous project I used LM2576 converter for 1,5A load which did the job, but required the radiator (4x4x1.2cm) and when the load reached 2,5-3A, it was able to fry an egg. The second problem was the required inductor - 100uH/3A - hard to find in SMD.I have found a nice alternative - ST1S10. It is a good and cheap (4,5PLN in Poland - about $1,5) step-down switching regulator with 3A output. IC is available in tiny DFN8 and SO8 package with heat pad on the bottom, so the radiator is not required - the PCB is a radiator, but 2-sided board is required (recommended layout is available in datasheet). The high current-rated inductor is no longer a problem, because due to high switching frequency, 2-5uH is enough. Output voltage is set by two resistors, but maybe it could be set dynamically, i.e by a microcontroller and a DAC. Heat problem is also gone - efficiency reaches 90%. The circuit has also a thermal protection (I have tested it with thermal pad not soldered - it automatically switches off) and survives 20V input voltage (I have accidentally plugged the device to the second bench power supply (wrong hole! ;-) ) set to 20V).Over the past 12 months, I have assembled about 100 devices with this converter, which are running mostly 24/7. There was no problems with it so far.I think the switching converter is a very good option.

author: akis
author_rank: Frequent Contributor
timestamp: October 18, 2013, 07:49:55 am
content: Quote from: AcHmed99 on October 13, 2013, 12:42:19 pmNo, I imagined we could take ideas, schematics, and then scale appropriately. For example, if had a circuit to do DC 20V to DC 5V at some low current, what changes would I then need to make to increase the current capacity? Or if someone looked at DC Inverter circuit I posted above, what changes would need to be made to increase the output current capacity?Acceptable efficiency, off the top of my head, would be no more than a couple of watts on the switching transistors, when the linear regulator wastes over 20W to do the same job, ie 10% of the equivalent loss on a linear regulator.

author: Zbig
author_rank: Frequent Contributor
timestamp: October 18, 2013, 11:09:32 am
content: Quote from: akis on October 18, 2013, 07:49:55 amI can't help but think you somehow feel that few of your random ideas followed by some quick brainstorming on the forum could lead to some new, groundbreaking, 100%-efficient power conversion method, perhaps by smoothing PWM with just right configuration of some capacitors. Well, it's not going to happen so you can stop now. Ever heard of NASA improving the performance of their rocket engine after receiving an e-mail from some midnight hacker? Yeah, me neither. There's decades of engineering gone into modern, highly efficient switching power converters, there are people doing it for a living. Your time spent trying to reinvent the wheel will be the time wasted, a time you could spend by building something genuinely new and useful. But who am I to judge.