DC/DC Converter That's Efficient? - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: January 12, 2015, 10:50:52 pm
content: Hello,Are there DC/DC converters that are efficient? I need to convert 30-36V (16A max) solar panels to 12V motor, however I was told there would be a lot of power loss if I go this route since the current will still be at 16A max.Is there a device that can preserve the amount of power after converting the voltage? In other words, lower the voltage but increase the max current.

author: katzohki
author_rank: Frequent Contributor
timestamp: January 12, 2015, 11:00:51 pm
content: There are a lot of ways to do this I guess. Some are more efficient than others. DC to AC, then AC to lower AC, then AC to DC again is fairly straightforward. Here's the question though: do you need the solar panels to run the motor at the same time? In other words, what is your application?

author: digsys
author_rank: Unknown
timestamp: January 12, 2015, 11:02:06 pm
content: If you're coming off Solar panel, the best choice by far is to use an MPPT controller charging a SLA type battery. There are MANY MPPT controllers out there,and many have efficiencies of 95%+ . Our solar vehicle runs 98.5+% flat. Chose a battery capacity that's app 4x the power level of the Solar panels.Higher is always better, but that's something you can play with depending on circumstances. ie IS the Solar panel always connected ? Is the motor alwaysrunning ? etc etc. Running a motor directly off a Solar panel is a HUGE waste of energy !! You have to have intermediate storage.

author: geratheg
author_rank: Regular Contributor
timestamp: January 12, 2015, 11:33:00 pm
content: Hey thanks for all the suggestions!Well this is for a competition, so it's just a temporary project. We'll be building a solar boat that will be operated by an electric motor. There will be 3 races, in none of the races are we allowed to charge the battery. The motor is 12V. The battery is 12V. Solar panels are 30V at max power. One race will use purely battery. The other two races will use purely solar. Hence the need to convert the voltage.Thanks!

author: digsys
author_rank: Unknown
timestamp: January 12, 2015, 11:51:03 pm
content: Ahhhh had a lot to do with model solar cars / boats. They almost exclusively use MMPTs and many design their own. Yes you need to squeeze every bit ofefficiency as you are using the energy live !! MAKE SURE you cater for cloudy / dull days. We've had many event days that were dull and the teams whoONLY designed for max light did poorly, even though many had great designs. Good luck. Note: A lot of students who start out on Model solar go on tojoin Solar Electric race teams !! Then it gets real serious ! Good luck.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: January 12, 2015, 11:52:39 pm
content: Any dc-dc converter, even if it isn't the most efficient is going to yield better results than the motor running directly off the panels. How many watts is the motor? How many watts of solar do you have? I'm not aware of any mppt devices that will output 12v directly (as opposed to charging a battery).Meanwell do some dc-dc converters that might do the job, I think the input is 18-36 volt with adjustable output voltage so you could tweak the motor voltage to get more voltage / speed.It also comes down to the budget you have. How big is the boat? How long does it need to run for?I hope it is a bright sunny day, otherwise you might need to use the panel as a sail

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 12:28:38 am
content: This isn't a mini model boat. It's a full sized boat. 235 watt panels, each can output 7.89A and 29.8V at max power. Wanted a parallel panel configuration.Budget is about $100-150.How much should I expect to pay for a decent DC/DC converter?Thanks for all the input guys!!!!!

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 13, 2015, 12:42:49 am
content: You don't just need a DC/DC converter, you need an MPPT. Look up the I/V curve for a solar panel. It's non-linear...at either no load or short circuit they deliver 0 power. Maximum power occurs somewhere in the middle. Where, exactly, depends entirely on solar irradiance, temperature, etc. If you just stick a DC/DC converter on a panel's output, as soon as the motor draws more power than the panel can provide (cloud passes by, etc), the DC/DC converter will drop its input impedance to compensate for the voltage drop, and you'll fall off of the edge of the panel's I/V curve and get essentially zero power, indefinitely.An MPPT always runs the panel at its maximum power point (the MPP in MPPT, the T stands for Tracking). Whether or not you have a battery, you must have an MPPT, and at the power levels you're talking about it's not very likely you'll be able to hit your budget.BTW - most MPPTs include DC/DC conversion. Most are designed to charge a battery, but you might be able to directly drive the motor instead. You should talk to the MPPT manufacturer about that, some are designed such that a battery is necessary and they can behave erratically without one.You may want to look into designing your own MPPT controller using a voltage monitor, current monitor, uC, and a variable output switching DC/DC converter. The uC monitors the panel voltage and current and adjusts the DC/DC converter's output voltage (and therefore it's input impedance, since you're driving essentially a fixed resistance) to keep the panel at it's MPP. This won't be an off the shelf solution, you'll need to do some board design and embedded programming.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 13, 2015, 01:17:43 am
content: Quote from: digsys on January 12, 2015, 11:02:06 pmImpressive! That did not come easy.

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 01:39:53 am
content: Quote from: gerathegEven better. MMPTs are a dime a dozen these days, and most work fine. Not knowing where you are, just do a search of your local electronics stores.In OZ, even our local Jaycar has a couple of reasonable ones. 20-30+ is not unreasonable and priced app $150-250 AUSBottom line, even a cheap one at a lowly 95% is a LOT better than just a DC-DC converter. You should just make that budget.In my experience, schools (etc) fix a budget where they know it is doable and not too high so that teams with lots of $$ can have an advantage !!

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 01:46:39 am
content: Quote from: rx8pilotwww.worldsolarchallenge.org,https://www.facebook.com/WorldSolarChallengeSome teams spend $1-3 million an event, so you need to spend a LOT on R+D to compete !! We can't afford to have ANYTHING on the car below ~98% efficient,including the motor. Our power engineer specialist is pretty awesome :-)

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 02:07:38 am
content: Thanks for the responses!!This is sooo hard, school projects and limited budgetsI guess I'll have to see if an MPPT controller can be used to power a load. In that case I won't need a DC/DC converter, correct?Quote from: suicidaleggroll on January 13, 2015, 12:42:49 amThis sounds really cool, but I'm new to any type of designing and don't think I have the skills to even do embedded programming yet.

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 02:20:01 am
content: Quote from: gerathegWell, it's designed that way. The point of projects is to make you THINK !! Find solutions to problems !! Not be spoon fed :-) Some Instructors / mentors give MOREpoints for failure !! (with reasons why of course) than a perfect outcome.A MMPT *IS a DC-DC converter anyway, just smarter. You DON'T use both. And many will drive a varying load JUST fine !! (albeit some losses, but everyone in yourcompetition is in the same boat !! (pun)). In RW, YES we do go via a battery pack first, but that varies up to 100% ie 100-200VDC in our case. Plus the load canvary many KWs. MPPTs are designed to cope with ALL that !!

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 02:28:05 am
content: A few questions:Do I plug the load in directly into the MPPT battery terminals?Does the MPPT adjust voltage according to what the motor takes using smart methods, or does it have a pre-configured output voltage?Edit: Figured out some MPPT controllers have load terminals.. I guess the battery terminals won't be used.

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 02:33:11 am
content: Quote from: gerathegI'd have a Polyfuse directly out of the MPPT +ve, then as much capacitance as allowed (maybe 10-100uF or more), plus 0.1uF 100V+ noise caps,then straight on to the motor. Just a bit of filtering to try to quieten down the motors current reflections.

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 02:42:37 am
content: Given my limited knowledge, I don't know much about noise caps.Also where would the MPPT +ve be located? The positive battery terminal?I found this controller:http://www.windynation.com/Charge-Controllers/WindyNation/TrakMax40-40-Amp-MPPT-Solar-Charge-Controller/-/182?p=YzE9MTc=It has load terminals.Will that be enough? Is that a reasonable price?Thanks!

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 03:05:06 am
content: Quote from: geratheghmmmm The unit looks fine, specs and price are good, BUT it MAY be too smart !! ie auto senses 12/24V !! At this point you seriously need project assistance.I can't believe they haven't set at least 3-4 people per group? You're not doing this alone, surely? There must be assistance available ?It's really not feasible to be able to answer / evaluate everything on-line, sorry. Maybe others here can continue, but at some point, you'll have to work it out.

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 03:13:38 am
content: Oh yes they did assign multiple people in my group, however they are still learning the very basics and haven't put a circuit together using basic switches and just wiring before. I decided to teach them some basics, which is still in progress. In addition to that, I also have to move ahead on my own time since I don't want to fall behind in the project. So when they are able to put the basic circuit together, I can just add on the converter to the schematic.Thanks for any assistance you have given me so far.I just learned that buck converters are pretty much MPPT type converters that bring the voltage down while increasing the current from Google. Didn't find any that can handle high current yet.

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 03:24:11 am
content: Quote from: gerathegOK, so not as bad as I thought. As far as your statement goes .. NOOOPE ! Look up MPPT and what gives it that name !! A good MMPT can switch DOWN as well as UPin some cases. Technically they're all DC-DC converters, but it's the smart code in an MPPT that makes it special !! You have a varying VI input and a varying VI output,even if there was a battery. Your good news is that MPPTs are usually high current +, so that helps. Keep reading :-)

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 03:46:01 am
content: Ahh I see. MPPT also varies output for max power instead of just converting it.Just curious, in an MPPT controller, can the load be wired to the battery terminals or does it have to be wired to the load terminals on the controller if we are not using it to charge a battery?In the specifications for the TrakMax40 that I linked it says it handles 40A of charging current and 15A of load control. Hmm, 15A is not enough.

author: digsys
author_rank: Unknown
timestamp: January 13, 2015, 04:35:58 am
content: Quote from: gerathegCorrect. They usually also have a much greater working range - Input and Output.Quote from: gerathegThis is slightly tricky to answer. An MPPT is PRIMARILY designed as a Battery charger, and has pretty tight specs on the chemistry is is suitable for, and the "expected" voltageit has to deal with. With all battery chargers though, there is a major problem they need to TRY to address. That is - HOW does it work out how much of the energy is forthe battery itself, and how much is for the load (which can vary greatly). eg Assume no load and the battery is nearly full at 1A. Suddenly the load comes on at 10A,but the charger has no idea if the battery suddenly went flat or some % of charge / load happened. You have to try and figure that out with code.They're saying the MPPT can charge at 40A and still cope with a 15A load as well, so a total somewhere around 55A max.There are tricks you can use - ie feed the load current back into the MPPT algorithm, etc etc.In your case, there is NO battery, so the algorithm may not be able to cope. Fortunately, any good MPPT has extensive monitoring and programming capabilities !!I would download the user / service manual of a make / model you want to consider and see what configuration options it has. That will tell you if you can run it okwithout a battery. Here also, is where you should be able to FIX an output voltage ie get rid of the auto-sense stuff.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 13, 2015, 01:28:31 pm
content: Quote from: geratheg on January 13, 2015, 03:46:01 amYou really need to contact the manufacturers with these questions. As digsys said, some of them might be TOO smart for your application. As soon as you deviate from the intended use of the product, you need to ask the manufacturer how the device will react and design your system accordingly. Unless somebody here has used that EXACT model in that EXACT way, none of us are going to be able to tell you what will happen.I can tell you that in one of my systems that uses Genasun controllers, I had to add a separate supercap circuit so the MPPTs would work correctly when the main battery was disconnected. This is because they have 3 modes...low voltage trickle-charge mode below ~7v, bulk charge mode from 7-14.2v, and float mode at 14.2. Without the battery connected, it would jump straight to float mode, and if any kind of load was applied while in that state the voltage would plummet and the entire system would reset. That's because it only tests the load current every ~300ms when in float mode, and applying a big load without a battery there to buffer it would just drain the system. The supercaps I added give it enough buffer capacity to keep the load going long enough for the MPPTs to sense the change and switch from float mode to bulk mode to keep the load running. The Genasun tech support was very helpful in figuring all of this out and coming up with a solution.

author: Seekonk
author_rank: Super Contributor
timestamp: January 13, 2015, 03:18:18 pm
content: I've seen the solar boat RA many times on its travels, anyone that takes a 40 foot 10hp boat out in the ocean has guts! I think what you really want is a Linear Current Booster, google search these. These are used for remote water wells that don't have a battery. The basic operation is MPPT, keeping input voltage of the panels fixed at the power point and converting that to higher current for the motor. You don't really need to track. The panels power point is solely determined by temperature. One chip manufacturer that claimed solar tracking did it by putting a temp sensor on the panel. Anyway, I do this and it only takes something as simple as a UNO and about 10 cheap FET. At 490 HZ PWM it is a simple build and as little as ten lines of code. Isn't the object to learn something. And don't put panels in parallel, it will only prove you don't know what you are doing.

author: geratheg
author_rank: Regular Contributor
timestamp: January 13, 2015, 06:50:42 pm
content: Quote from: Seekonk on January 13, 2015, 03:18:18 pmThanks for the suggestions on the Linear Current Booster.As for putting panels in parallel, I'm not sure what you mean there? I've seen parallel setups with solar panels all over youtube??Thanks for the input.

author: Seekonk
author_rank: Super Contributor
timestamp: January 14, 2015, 01:01:02 pm
content: Science marches forward, installations are left to the Labridoodles to do. Most solar systems are hack jobs that were not thought out in the beginning and just added on to any way possible by people with good intentions but not a lot of knowledge. A quick read of the issues of serial and parallel strings is in the following articlehttp://www.egr.unlv.edu/~eebag/Solar%20Cell%20Shading.pdfIt is easy enough and cost effective now to have inverters for each panel. google micro inverters, believe one company is Enphase. While these aren't applicable for you, the concept is. For only about $3 an Arduino mini could control four strings of panels, charging and speed control through linear current boosting.I don't know the metrics of your project but batteries should only play a small part for emergency situations. Solar should go directly to the motor and any excess should go to charging. You never get out what you put into a battery. If you just buy off the shelf, it is old technology and you have not learned anything. The secret to solar is effective control.

author: geratheg
author_rank: Regular Contributor
timestamp: January 19, 2015, 12:02:46 am
content: Thanks for the link, I actually learned something from it.Now I know what those diodes inside of the box on the back of the solar panels are.Given a small budget and the fact that this need not be 100% reliable since this is only a temporary project, I might just settle for something that works. I found this on ebay:http://www.ebay.com/itm/DC-DC-Converter-Voltage-Reducer-Regulator-24V-Step-Down-to-12V-20A-240W-DC-to-DC-/120966517136?pt=US_Power_Inverters_&hash=item1c2a2a9190I'm thinking of buying 2 of those, 1 for each panel. And then wiring the two converters directly in parallel. I don't know if that's a good or bad thing to do.What do you think?

author: Circlotron
author_rank: Super Contributor
timestamp: January 19, 2015, 10:42:59 am
content: Quote from: geratheg on January 12, 2015, 10:50:52 pmYes, it is called a buck converter. ->http://en.wikipedia.org/wiki/Buck_converterBuy one, 13 bucks->http://www.dx.com/p/produino-dc-5-32v-to-dc-1-5-32v-converter-buck-adjustable-electronic-power-regulator-module-289944

author: Seekonk
author_rank: Super Contributor
timestamp: January 19, 2015, 04:15:26 pm
content: That would not be a good choice. Doesn't anyone in the group have any electronic experience?

author: geratheg
author_rank: Regular Contributor
timestamp: January 19, 2015, 06:06:21 pm
content: Quote from: Seekonk on January 19, 2015, 04:15:26 pmI appreciate all the advice you give. I learn something from good electronics practices.For this project I'm just willing to compromise a little bit to fit the budget.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 19, 2015, 06:26:46 pm
content: That's not a small compromise,a regular buck converter will not work correctly off of a solar panel. Please read what I posted earlier again:QuoteIt's not just a matter of losing some efficiency, if you don't use SOME kind of power tracker intended for use with a solar panel, there's a good chance things will work ok for a while, and then just stop. It'll get stuck in an infinite reset cycle, maybe overheat itself, who knows. BEST case scenario is that doesn't happen, but you get SIGNIFICANTLY (read: half, third, etc) less power than the panel is capable of providing.A regular DC/DC converter expects a CV (constant voltage) supply, a solar panel is a CC (constant current) supply. You can't stick a DC/DC converter that's designed for a CV supply on a CC supply and expect it to work correctly.

author: Seekonk
author_rank: Super Contributor
timestamp: January 19, 2015, 06:38:21 pm
content: It is not about being cheap. I have over 1000W of panels that are controlled with a $6 UNO and simple electronics built from a couple old UPS. The world is your oyster, there is so much electronics out there that can be had for free. It just seems that if a group is going to do a project the members should bring something to the table or at least learn something. I learned electronics by reading. I had my first job repairing electronic organs at 15. My brother had to drive me around on service calls. You have the internet, far more out there than I had. You want to start with projects and not learn the basics.Basic buck converters can easily be modified to operate at power point, but you need a little electronic knowledge to do it. This is a great fun project. Too bad you don't want to put any effort into it.

author: geratheg
author_rank: Regular Contributor
timestamp: January 19, 2015, 07:45:58 pm
content: Quote from: Seekonk on January 19, 2015, 06:38:21 pmThat's very intriguing and sure does look fun, just a lot of effort with college work already taking up a lot of my time and I try my best to learn slowly about my hobbies (I actually have many, not just electronics). For this project I'm probably going a little too slow.Quote from: Seekonk on January 19, 2015, 06:38:21 pmOkay okay, you winAnd certainly, I appreciate all replies to this thread. I've gained some knowledgeThanks!