converting 12V to 24V - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 06:52:23 pm
content: I was asked today if I can convert 12V to 24V. We need to supply a fan that runs on 24V but the customers vehicle runs on 12V. So I'm thinking, for 24V@40A (worst case and over) is it feasible to run a H bridge and then voltage double rectify it to get 24V, being a Fan the exact voltage and regulation are of no concern. Or is a SMPS so much better ?

author: Christopher
author_rank: Frequent Contributor
timestamp: August 19, 2014, 06:57:52 pm
content: Problem with a boost converter is the switch current is much higher than the output current so it might be a bit of work to get a Switching regulator to bump up 12 to 24 at 40A.What kind of KW fan runs off DC?!

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2014, 06:59:42 pm
content: Boost converter will be better. If the fan is thermally controlled then you can use the SMPS control to do things like speed control ( low, med and high speed are easy with no switching letting 12V through, 18V and 24V to the fan) and on off switching as well.40A with a voltage doubler using capacitors means either massive capacitors or running switches in the MHz range with all the EMI and heat. Choke with big power MOSFET and diode ( here active rectification will be a definite power saving and make for a smaller heatsink) at 20kHz to 100kHz or so will be a lot smaller and cheaper.Of course a 12V fan is going to be a lot cheaper as well, and likely will be just as good and only cost 30 pounds or so, about the cost of the converter in any case.

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 07:00:11 pm
content: It's automotive, Alibaba have plenty of 12/24V units running 960W, Bosch make a very nice fan that runs 12V 48A, it's the dogs goolies in the business.

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 07:02:46 pm
content: Quote from: SeanB on August 19, 2014, 06:59:42 pmThe fan is brushless so I need to leave controls up to the fan. Apparently EMI is nor a problem. Yes switched mosfets for rectification will certainly be more efficient as in SMPS.

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2014, 07:04:13 pm
content: Look at your local automotive airconditioning suppliers, you can get a fan that will both fit, give the air flow and will run off 12V as well.Retrofitting a truck aircon unit to operate on a smaller vehicle Simon? Get 12V auxiliary aircon fans and use them instead with the standard block parts. Otherwise have a 24V battery and alternator in the engine compartment to run it all.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 19, 2014, 07:05:29 pm
content: For high-current loads like that, the traditional solution is to simply provide an additional 12V battery in series.Of course, you must make provision to charge the battery separately from the primary circuit.What on earth kind of fan takes 480 watts? That seems unreasonable to expect to power from the normal vehicle battery.

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 07:08:36 pm
content: I don't know what sort of vehicle it is but it's a new built as far as i know and the fans do need to be powerful sometimes, that 12V 48A fan has been used for years and was developed by bosch for audi initially, yes it can take of from the bench, I've never run one full power.Having a second battery means that either we need to charge it or a second alternator, both options are as complicated unless the duty on the fan is low so a low power converter keeping a series battery topped up can handle it. i will ask my colleague tomorrow.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: August 19, 2014, 07:18:55 pm
content: Whoa, that's one big fan... xD

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 07:23:10 pm
content: Engine cooling does not happen on it's own. I think many Audi's have a mechanical fan running permanently off the engine plus the powerful auxiliary fan. once you exhaust electric fans you turn to fans running on hydraulic motors that makes for a more efficient system and these days speed control and complicated electronic controls are being introduced to hydraulic systems because it still pays to keep it hydraulic but it needs regulating. I don't know why they want so much power in this case and are using an electric fan.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 19, 2014, 07:29:36 pm
content: Sure, probably most modern vehicles use electric fans for supplemental air-flow over the radiator.But they use fans designed for the primary vehicle battery voltage.Why are they deploying a 24V fan in a 12V vehicle?There appears to be a fundamental design flaw here.

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 07:31:50 pm
content: Once you above a certain power most fan manufacturers like to shift to 24V and assume that for the sort of power demands the vehicle will be running 24V I have no idea of the electrical background of the vehicle. apparently a 12v version is available but very hard to obtain.

author: mzzj
author_rank: Super Contributor
timestamp: August 19, 2014, 07:54:25 pm
content: Simon and his projects againI would choose smps with full bridge, 20 to 100khz operating freq and schottky diodes for rectification. Something like etd59 or EE 55 size ferrite core will handle the required power easily.Nothing that difficult for someone with enough experience with high current power supplies... Until Simon tells us that it has to potted, low outgassing for hard vacuum, has to withstand 5000g acceleration for 10 minutes and has to operate at liquid helium temperatures

author: Simon
author_rank: Unknown
timestamp: August 19, 2014, 09:17:13 pm
content: Quote from: mzzj on August 19, 2014, 07:54:25 pmFortunately they let me off on the g's and temperatures this time and i might even get to put it in front of the fan it's powering to keep it cool. I think it's all conjecture at the moment as the suggestion was an act of desperation in not being able to get a fan powerful enough at 12V but i'm certainly not going to let that get in the way of a good projectJust wondering if i can get away with this on a PCB using 4oz copper with tracks both sides in parallel

author: mzzj
author_rank: Super Contributor
timestamp: August 19, 2014, 09:34:17 pm
content: Quote from: Simon on August 19, 2014, 09:17:13 pm4 oz PCB should be workable with the required ~100A current but that is least of your worries in a project like this.This would get you pretty close:http://www.redarc.com.au/products/product/voltage-booster-12v-to-24v_c/

author: mzzj
author_rank: Super Contributor
timestamp: August 19, 2014, 09:37:51 pm
content: or 13 of these in parallelhttp://www.digikey.fi/product-detail/en/V28C24C100BL/1102-1309-ND/3001477not that cheap but if you need results...

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 05:46:37 am
content: Well obviously they will want it cheap......... The joys of working for a mechanical company that just see electronics as magic

author: Rasz
author_rank: Super Contributor
timestamp: August 20, 2014, 08:40:42 am
content: Quote from: Simon on August 19, 2014, 07:00:11 pm-put two batteries in the car-put 48v alternator in the car-put proper 12V fan-learn to just say 'no'-tell management 'sure, it will cost $1K'

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 08:48:40 am
content: Quote from: Rasz on August 20, 2014, 08:40:42 am- It's not my car and who said it's a car ?- Why 48V ?- As already mentioned at this power level 12V is rare for a good reason- What not get to spend time learning something new while being very essential as i can develop new products?- You can buy one off the shelf for lessHave you ever actually worked in engineering ?

author: firewalker
author_rank: Super Contributor
timestamp: August 20, 2014, 09:19:43 am
content: The battery should also be capable of providing ~100 amps plus any other consumption. What will be the duty cycle of the fun? Continuous operation?Alexander.

author: bktemp
author_rank: Super Contributor
timestamp: August 20, 2014, 09:22:13 am
content: It sounds like there is no need for a perfectly regulated output voltage. This makes everything a lot easier:All you need is a centre tapped transformer with 1:2 windings (or a bit more to compensate losses) and many paralleled mosfets that drives the thing. Have a look at big car audio amplifiers. Most of them use a similiar approach. There is basically no regulation of the output voltage, the output voltage follows the input voltage. The only purpose of the SMPS controller is to limit the no load voltage. Otherwise it runs at 100% duty cycle.Most of those power supplies do not have any kind of overcurrent protection. If something goes wrong, all mosfets explode.If you need to add some protection it can get quite complicated, 100A is a lot of current, especially at high frequencies.It might be easier to use 2 or 4 (phase shifted) converters in parallel.Using active rectification with this design can be as simple as adding auxiliary windings to the transformer for driving the gates.

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 10:16:03 am
content: Yes i have had a transformer suggestion, I would need to obtain one and transformers are not my area. No regulation is required so yes it just needs to double the input.I have tried to find out the duty of the fan but as usual we are non the wiser. I think they have now found a fan that does the power at 12V so maybe I won't need to try and make this or find something after all.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 20, 2014, 10:39:11 am
content: Quote from: bktemp on August 20, 2014, 09:22:13 amYou cannot make a switching converter (boost or push-pull) regulate in a CAR as the output power is proportional to the battery current that's always less that what is was when you attached the converter. (car audio manufactures no this very well so they use a fixed duty cycle and have active shut down using under voltage lockout when condition's doesn't satisfy)To the authorYou can use a simple TL494 some PID scheme of your choice with good magnetics fromhttp://www.arnoldmagnetics.com/that all you will struggle with.Those alibaba china converters should never be trusted in automotive environments such as a car, where environmental conditions are harsh and unpredictable, and peoples lives are at risk.

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 10:45:45 am
content: This is not for a car, apparently they have the required power available.

author: Rasz
author_rank: Super Contributor
timestamp: August 20, 2014, 02:21:14 pm
content: Quote from: Simon on August 20, 2014, 08:48:40 amyou did:>customers vehicle runs on 12Vunless you meant a bike, or an ATST walkerQuote24Quoteand yet someone found one alreadygood reason was probably 'gonna pick this one and make someone else worry about it'Quotewhy not use proper components instead of bodging?Quotewhole point of that line is to bounce your problem back up to the source, not to buy $1K box. Source of your problem was someone else arbitrarily deciding on a wrong component, and making your job unnecessarily difficult.QuoteAre you saying engineers job is to bodge shit to implement something unqualified people came up with?Even in case 12V fan couldnt be found, the easiest solution would be custom fan, or a bare fan + 12V motor.

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 04:18:32 pm
content: Vehicle does not mean car, it could be a bulldozer, an off road building site truck, a combine harvester, a tractor, nothing that we deal with at work is a car unless it's some stupid little thing.Manufacturers make what customers require. few customers require this power at 12V for the same good reasons fan manufacturers don't like making very powerful 12v fansWho's chosen any components yet ? whole idea of the thread.We live and work in a competitive market, you just don't turn round and say that it can't be done or quote so much money it's stupid, unless you want to be out of business.I'd work to find the best solution.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 20, 2014, 06:10:16 pm
content: Quote from: diyaudio on August 20, 2014, 10:39:11 amA properly designed switching regulator covers both input voltage variations and output load swings. Line voltage for universal power adapters can regulate across the 90-250VAC input range while maintaining output regulation usually better than 5%. All they need to do is sense output voltage and adjust duty cycle accordingly.If your step-up converter does not regulate, it is just a dumb inverter; a crude switcher operating in open-loop.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 20, 2014, 06:16:28 pm
content: Quote from: Simon on August 20, 2014, 04:18:32 pmAttempting to deliver demented projects is also a good way to go out of business.Learning to steer customers towards sane solutions is an important skill, especially if you can do it with tact, and then bid for the new solution.Often you'll have to tell the customer, then wait a while, after which the customer will, all on their own, come up with a new requirement which sounds suspiciously familiar. It's a bit of a surprise the first few times, but just go with it.

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2014, 07:40:32 pm
content: I agree with the high frequency inverter suggestion. This gets rid of the requirement for a massive inductor that a switching regulator would require.Flux balancing may be an issue however so I would use some variation of a saturable core inverter.

author: Simon
author_rank: Unknown
timestamp: August 20, 2014, 09:37:07 pm
content: Quote from: Precipice on August 20, 2014, 06:16:28 pmHave you ever heard of people who won't be told ? clearly the solution is that given the size of the vehicle and it's subsystems it should be 24V in the first place, They are obviously not that clever. I have already been recommending to find a fan that is 12V and have already told my colleague that boosting nearly 1KW is not simple or cheap. However I'm still interested. The market is flooded with similar converters so it's not exactly impossible........

author: DanielS
author_rank: Frequent Contributor
timestamp: August 20, 2014, 09:49:05 pm
content: Quote from: David Hess on August 20, 2014, 07:40:32 pmThat depends on the frequency. At 200kHz, a 40A output inductor would be fairly small. This also depends on maximum permissible output current ripple: if you do not care about output current and voltage ripple, you may even be able to forgo the output filter altogether aside from a small one for the PWM feedback loop if your load can safely withstand the transformer's raw secondary output voltage.Computer power supply manufacturers are cramming 12V rails with over 120A output capacity in ATX-sized power supplies (150 x 86 x 180 mm) and this includes the surge/EMI filter, AC-DC and active PFC front-end, galvanic isolation, the 5VSB supply, three other secondary output voltages and an output inductor large enough to handle over 120A with 1% regulation and less than 50mV ripple.Ripping out all the unnecessary stuff for a DC-DC converter would likely free up enough space to implement a 1kW unit as a pair of 500W modules to make primary-side currents more manageable.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 21, 2014, 01:27:34 am
content: Quote from: DanielS on August 20, 2014, 06:10:16 pmNOT when the source is a battery, you cannot make more energy OR maintain the same energy than what you started out with, that's my point, regulation is a battery thing the current source, explain how you keep 24 volts stable, if the current is depleting at a rate proportional to a heavy load. that's my point. not in relation to 90-250VAC input source but to a battery.Another misconception is that a these converters I spoke of isn't closed loop and are dumb (far from it) , how do you define close loop? their is a few compensation networks with a pole/zero at certain points that improve step response turn on, no outer loop exists from the output, that dynamically programs the controller to push the PWM to say another 5% duty cycle width to regulate. wont happen.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 02:29:31 am
content: Quote from: diyaudio on August 21, 2014, 01:27:34 amThe source being a battery makes no difference: tons of regulated switching voltage regulators operate off batteries in cellphones, tablets, laptops, calculators, remote controls, cameras, UPS, cars, LED flashlights, etc.Not sure where you take the "making more energy" bit from. If you want to drive a 40A 24V load from a 12V battery, the converter will need to draw 80A not counting converter losses regardless of whether it is a PWM, self-excited inverter or whatever else. No magically created energy there: V*A in = V*A out + losses.The way a DC-DC switching regulator compensates for battery (or whatever its input source might be) droop is exactly the same way an AC-DC switching regulator compensates for the 10-30V ripple on its AC input filter capacitor between input peaks: it modulates its duty cycle to compensate for changes in input voltage.If you want to design a double-forward switching regulator to step 10-14V battery input to 24V output, you need a transformer ratio of about 1:3 assuming you lose only 1V in both input and output components including wiring resistance. When the battery is fresh-off-the-charger at 14V surface charge, the PWM regulator would operate at about 32% duty-cycle per transformer half (64% for the whole transformer) and when the battery reaches 10V, the PWM should be at about 47% per half.

author: ConKbot
author_rank: Super Contributor
timestamp: August 21, 2014, 02:47:46 am
content: Gave the thread a quick skim may have missed the suggestion if someone put it out there already. Since this doesnt need to super well regulated, set up a push-pull converter (or full h-bridge). But, have it be a 1:1 (well, 1: 1:1 on the push-pull) so the converter spits out an isolated 12V. Put your isolated 12v in series with the existing +12v rail and your converter only needs to handle half the wattage as if it was outputting 24V.Could be as cheap and nasty as a transformer, oscillator of choice running at a fixed duty cycle, some hard-switched mosfets, with a good gate driver to try and switch fast, and some beefy diodes for rectifying. For a bit more refinement, you could do a current fed push-pull whichwould gain you regulation, and soft-switching of the FETs.

author: mzzj
author_rank: Super Contributor
timestamp: August 21, 2014, 04:47:45 am
content: Quote from: ConKbot on August 21, 2014, 02:47:46 amNot bad idea, i have used similar scheme with other power supplies. Output protection needs a little extra attention for short-circuits but i guess beefy diode and fuse can handle that.Your cheap-and-nasty has a possible problem of flux-walking, so i would recommend something a little bit more evolved with a pulse by pulse current limiting plus the necessary extra hardware.You can skip the filtering caps and output coil almost entirely in a push-pull converter if you run it unregulated and close to 100% duty cycle. I bet the required power supply BOM is less than 50 euros, but then you have spends thousands on design and build man many units.So i guess it alldepend does Simon Corporation need 5 or 50000 units.

author: Simon
author_rank: Unknown
timestamp: August 21, 2014, 06:07:24 am
content: Quote from: ConKbot on August 21, 2014, 02:47:46 amNot a bad idea, it won't entirely limit spike draws as I'll still need to draw in peaks of 80A but yes that would cut out a few parts and reduce costs an overall consumption

author: Simon
author_rank: Unknown
timestamp: August 21, 2014, 06:57:05 am
content: I suppose if an inductor does not cause too many losses a +12 / -12V DC/DC converter will do just fine as in series with +12V will give me 24V total. Although that might screw with the control system as it will put the fans ground at -12V although they could use an opto isolator to pass PWM speed signals.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 11:16:25 am
content: Quote from: ConKbot on August 21, 2014, 02:47:46 amGet out of here, that makes too much senseActually, your scheme can also regulate total output voltage: use a 1:2 transformer but instead of regulating its output as isolated, sense the whole battery + PWM output voltage so the PWM can cover the battery's droop with a ~650W design instead of 1kW. This makes things a little simpler since you do not need to isolate a floating output.

author: Simon
author_rank: Unknown
timestamp: August 21, 2014, 11:52:24 am
content: I'm actually thinking that that is exactly what most of the commercial units do, one that I saw only had 3 terminals so they are actually handling 1/2 the rated power.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2014, 04:29:28 pm
content: I didn't think voltage regulation was important, since this is driving a 24V fan which will be designed to be run off an unregulated supply. As long as line regulation, given the same input voltage, isn't too bad I don't see the problem.The idea of using -12V is good but you could also consider a 12V to 12V converter with the secondary connected in series with the input, giving 24VDC.A self-oscillating inverter is probably the simplest to build and will give decent efficiency.See the links below:http://www.seekic.com/circuit_diagram/A-D_D-A_Converter_Circuit/MOSFET_resonance_type_DC_DC_converter_circuit.htmlhttp://www.smps.us/inverters.htmlhttps://www.circuitlab.com/circuit/6f656p/mosfet-royer-like-oscillator-01/http://www.circuitsonline.net/forum/view/110947http://www.sciencezero.org/vasil/romtcb/groyer.htmlThe Schottky doides on the secondary can probably be replaced with MOSFETs to give synchronous rectification, if gate drive is provided by another winding.

author: Simon
author_rank: Unknown
timestamp: August 23, 2014, 04:33:35 pm
content: Well fortunately they have found a 12V fan but the idea was interesting. I was trying to stay away from a transformer unless they are readily available off the shelf. Seems that 4+ oz copper tracks both sides and possibly metal PCB base to carry away heat is the way to make it work on a PCB. But the PCB will then become a major component cost most likely. Apparently as much as 12oz is available but that is very special and costly.

author: Simon
author_rank: Unknown
timestamp: August 23, 2014, 04:48:42 pm
content: This is the most expensive SMPS transformer on digikey:http://www.digikey.co.uk/product-detail/en/750341145/732-4454-6-ND/3831276250W 200-700KHz and I still haven't worked out in what ratio they are offering it, according to the drawing there are options.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 23, 2014, 06:32:42 pm
content: Quote from: Simon on August 23, 2014, 04:33:35 pmHigh-power transformers are rarely available off-the-shelf since they usually have tons of application-specific requirements like secondary rails, auxiliary supplies, bias windings, duty cycle, operating frequency, etc.DigiKey's transformer selection is more typical of stuff you would need to build the PWM regulator manufacturers' reference designs and closely related variations.For a 1kVA transformer, your best bet would probably be a 1-1.5kVA UPS and you would need to rip the high-voltage secondary winding out to put your new primary in.BTW, you do not need 4oz copper if you wave-solder the board without solder mask on power traces and keep traces short or make it so all the high-current parts are on chunky copper pours instead of traces.

author: Simon
author_rank: Unknown
timestamp: August 23, 2014, 06:44:50 pm
content: Quote from: DanielS on August 23, 2014, 06:32:42 pmWe are talking about 40-80A and solder has 10 times the resistance of copper, yes it can be a great help to squeeze more amps out on marginal designs, I spoke to a PCB manufacturer and he was talking dual traces of 4oz (140um) with 35um plating at 15mm wide each side (30mm total) allowing for a 40C rise, there wasn't any plan to wave solder and it sounds more like an inconsistent bodge that may be difficult to do repeatedly and reliably in manufacturing.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 23, 2014, 07:50:19 pm
content: Quote from: Simon on August 23, 2014, 06:44:50 pmFew amps?Put a 0.5mm solder coat on top of those 15mm traces and you have just eliminated the need to add a second 4oz layer.Bodge or not, practically all computer power supplies and somewhat cost-conscious power products do this.

author: Simon
author_rank: Unknown
timestamp: August 23, 2014, 07:53:27 pm
content: Well i don't know if these would end up wave soldered, manually adding solder loading will just cost labor. The PCB's can also be heavily plated, 4oz copper is 140um usually you have another 35um of plating and more can be added on request, the PCB has to be manufactured and plated anyway.

author: mzzj
author_rank: Super Contributor
timestamp: August 23, 2014, 10:02:44 pm
content: How abouthttp://homemadecircuitsandschematics.blogspot.fi/2013/03/simple-dc-to-dc-high-current-voltage.htmlCapacitor ripple current requirements are going to make you cry but no need for magnetics.And both output terminals are swinging up and down.

author: Simon
author_rank: Unknown
timestamp: August 23, 2014, 10:11:04 pm
content: that sort of thing was my original plan, but diodes need replacing with switched mosfets

author: DanielS
author_rank: Frequent Contributor
timestamp: August 23, 2014, 11:18:05 pm
content: Then again, for such a relatively simple circuit as a 600W 12V DC-DC booster, it should not be too difficult to put together a layout where critical pins in high-current nodes can be bunched together and joined with solder blobs on copper islands instead of relying on PCB traces. The input filter might be a problem though... for a double-forward converter, the ripple would be in the neighborhood of 45A.For the charge-pump idea, you might still want to use inductors in line with those diodes to take some of those ripples off and soften turn-on edges... with about 40A RMS of ripple, each of the two capacitor banks would need about a dozen capacitors.

author: mzzj
author_rank: Super Contributor
timestamp: August 23, 2014, 11:50:16 pm
content: Quote from: Simon on August 23, 2014, 10:11:04 pmIf efficiency is not your main concern you get much easier results with the diode version.I was playing with the synchronous rectifier version on ltspice and getting >95% efficiency looked reasonably easy. Btw, the Synchronous rectifier version works both ways so if you swap the load and supply positions it will be voltage "halfer"

author: bktemp
author_rank: Super Contributor
timestamp: August 24, 2014, 06:21:02 am
content: I have built a completely synchronous voltage doubler/splitter for extending the voltage/current range of my small bench power supply. Since the voltage difference between the input and output side is always fixed (or changing slowly), you can use a single mosfet driver/oscillator for all mosfets and use ac copling for the ouput mosfets. This circuit works up to 2x50V 2A with low esr capacitors converting in all directions (positive to negative, negative to positive, positive doubling, negative doubling or splitting). The capacitors will be the main limitation when scaling the current up. Doubling 40A will require a massive capacitor with an ultra low esr of much less than 10mohms.I think a transformer will handle the current much easier than a capacitive doubler.

author: Simon
author_rank: Unknown
timestamp: August 24, 2014, 06:54:32 am
content: I think due to the fact that I'm just doubling the voltage it's touch and go as to if a transformer will be worth while. Due to not giving a toss about regulation etc I'm not too fussed about current spikes as you will get them either way. I think either way substantial capacitors will be needed.

author: mzzj
author_rank: Super Contributor
timestamp: August 24, 2014, 07:47:34 am
content: Quote from: Simon on August 24, 2014, 06:54:32 amWith transformer option you can get away with minimal capacitors if you keep it running very close to 100% duty cycle.----20v 560 uf or so solid electrolytic caps were rated for 5 A ripple current so 10 of those in parallei would do the trick in theory* And price for those was 1 eur per piece so caps wouldnt cost more than 20 euros. Can fit all the parts in smaller box than typical laptop power supply.*Solid electrolyte caps have nasty habit of short circuiting with high inrush currents so the startup sequence would need some thinking. Or use ordinary wet electrolytics if physical size is not critical.

author: Simon
author_rank: Unknown
timestamp: August 24, 2014, 07:56:50 am
content: I don't know how critical size is although as i said the potential for a project has gone away but still it's interesting to look at. Even a transformer based SMPS would surely need a decent input and output capacitance, of course no transformer means more capacitors, you can't have something for nothing.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 24, 2014, 06:27:35 pm
content: Quote from: Simon on August 24, 2014, 07:56:50 amThe input capacitors will have a rough day with somewhere in the neighborhood of 50A ripple but for the output ones, most of the ripple gets taken care of by the inductor and at 100kHz, you only need something like 10µH to get ripple to less than 3A RMS. Good low-ESR/ESL 25-35V 2700-4700uF caps can usually handle 3.5-4A RMS so that would be a dozen input caps, one output caps between HF transformer rectifiers and transformer center-tap and another capacitor across the whole output.For my own curiosity and as an LTSpice exercise, I decided to put together a crude proof-of-concept design based on an LTC3723 (first chip I spotted in LTSpice that looked promising) and got about 2.5A RMS ripple on output caps. The rise time takes a while due to the 100A input peak limit, transient response is untested and the component selection likely does not make sense either since I simply grabbed the first remotely plausible candidate in LTSpice's component list to get a non-ideal simulation. The PWM chip is wired to regulate the total output voltage. I did not bother implementing the synchronous rectifier part of the IC since simulated power does not cost anything, simulated components usually have no breakdown, the schematic was already getting too crowded and I had already spent far more time tweaking that doodle than I intended to.

author: mzzj
author_rank: Super Contributor
timestamp: August 24, 2014, 06:44:06 pm
content: Quote from: DanielS on August 24, 2014, 06:27:35 pmUse full-bridge or push-pull topology with nearly 100% duty cycle and your ripple current disappears next to nothing.Or if you insist on using boost converter use 2 of them and interleave the switch controls = near zero input and output capacitor ripple current with 50% duty cycle

author: DanielS
author_rank: Frequent Contributor
timestamp: August 24, 2014, 08:10:50 pm
content: Quote from: mzzj on August 24, 2014, 06:44:06 pmBut then you have no output regulation... the output voltage would be 3X battery voltage minus voltage drops.If you want input-output voltage regulation with a 100% transformer duty cycle, you need to use a current-fed design to regulate transformer current. You still end up with input capacitors seeing around 40A ripple though. A multi-phase PWM would help with that but this would start to become pretty fancy.

author: mzzj
author_rank: Super Contributor
timestamp: August 24, 2014, 08:31:12 pm
content: Quote from: DanielS on August 24, 2014, 08:10:50 pmWhat? 3x battery voltage? depends on transformer turns ratio.No regulation but i think Simon mentioned at some point that regulation is not necessary since load is only a big-ass fan.

author: SeanB
author_rank: Super Contributor
timestamp: August 25, 2014, 05:32:39 pm
content: With the push pull you can save a winding and use 2 diodes fed from the switching device collectors. The transformer action of the winding will give you 2x input less diode drop and saturation voltage very easily. Saves a winding and acts as a clamp at the same time. Diodes just have to be rated along with the winding for the full input DC current, but the diodes will run cooler with the pulsed operation.

author: mzzj
author_rank: Super Contributor
timestamp: August 25, 2014, 05:39:02 pm
content: Quote from: SeanB on August 25, 2014, 05:32:39 pmHa! Pretty clever, Didn't think of that!