Boost converter vs transformer for an inverter - Page 1
Forum section: electronics / projects

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 06:11:42 pm
content: Hi everyoneSo, i'm going on a school trip with my friends and since we all bring our laptops, phones and tablets and that the trip is 12 hours longwe need more that our small batteries that will get drained in second... So, I decided to build a power box.What i've done is buyed a couple of standard lithium-ion battery (that kind used on laptop batteries), build a pack that is 14.8v at 20000mah and build 3 buck converters for 5v (usb), 12v and 19v (Laptops)The only thing I have to do next, is build an inverter to get 230v to power anything that does not run on 5, 12 or 19 volts. This is when I thought about using a boost converter.So, this is when I need your help ! Is it a good idea to use a boost converter instead of a transformer then modulate the 230 volts into a sine wave or, is it better touse the conventional transformer technique ?(sorry for the bad english)

author: mariush
author_rank: Super Contributor
timestamp: November 09, 2016, 07:11:28 pm
content: edit: technically, you'd need a boost converter to produce 18.5-19v to power laptops from 14.5v battery.You don't need to build an inverter, you could just buy one of those designed to work on 12v .. 24v, plenty of inverters designed to work from car batteries / cigarette lighters etcHere's an example:https://www.amazon.co.uk/Whitenergy-06580-24V-AC-Power-Inverter/dp/B00B33VZUW/ref=sr_1_1?ie=UTF8&qid=1478718214&sr=8-1&keywords=24v+inverterYou could have bought existing batteries, there's stores like HobbyKing which sell huge packs, and they also have chargers which are smart enough to balance the battery cells so that your battery will last a long time.Here's a couple of good chargers :up to 6S :https://hobbyking.com/en_us/imax-b6-ac-v2-charger-discharger-1-6-cells-genuine-au-plug.htmlsame but cheaper, only DC :https://hobbyking.com/en_us/turnigy-b6-pro-50w-6a-balance-charger-white.htmlhere's lots of batteries :https://hobbyking.com/en_us/batteries/lipoly-all-brands.htmlFor example 120$ for a 16000 mAh 22v battery which works with the chargers above and the 24v inverter :https://hobbyking.com/en_us/multistar-high-capacity-6s-16000mah-multi-rotor-lipo-pack.html

author: 3141592
author_rank: Regular Contributor
timestamp: November 09, 2016, 07:30:08 pm
content: Quote from: whatsthegeek on November 09, 2016, 06:11:42 pmSo you have 4 18650 cells in series, and at least 6 of these in parallel? I'm not sure that this is a good idea. How do you plan to charge them? Is it fused? Are the cells protected ones? Are they from a reputable manufacturer and source, or from ebay/ali etc?

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 07:57:39 pm
content: yes, I have 5 groups of 4 batteries in series each of them are 4000 mahit's these one:http://www.banggood.com/fr/4PCS-MECO-3_7v-4000mAh-Protected-Rechargeable-18650-Li-ion-Battery-p-992723.html?rmmds=searchI plan on using a simple 20 amp car fuseWhy do you think it's not a good idea ?to charge them, I want to build my own charger. LI-Ion seem quit easy to chargeAnd yes, I though about using lipo batteries, but They are quit dangerouse compared to li-ion. I have some for my rc planes so I already have a chargerSo, thanks for your answer,

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 08:16:04 pm
content: here is my schematic for now, I still have to add the charger:

author: Siwastaja
author_rank: Super Contributor
timestamp: November 09, 2016, 08:21:41 pm
content: Those are scam cells, don't buy them. At best, they are about 2000 mAh "Chinese technology" (typically between 1800 and 2400 mAh) cells with somewhat lacking safety. At worst, they are horribly unsafe. One typical scenario is that they are filled with corn starch or sand. At least they are rather safe in that case. 4000mAh 18650 cells do not exist.Get some proper brands such as LG, Samsung, Panasonic, Sony, etc. There still is a risk that they are fakes, but the risk is not 100% as it is with the product you linked. Get the exact part number and google for the datasheet. Google for sellers that are known to sell genuine cells in your area.Hearing the phrase "protected cell" most likely means unsafe scam product, and is a hint that should discredit anyone suggesting such products for this purpose.Google a bit more about li-ion technology and how to construct a pack using 18650s. They are meant to be used for this kind of purpose, but terms and conditions do apply.

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 08:26:28 pm
content: ok, tanks for the info. and thanks for saving me 55 dollard!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 09, 2016, 08:29:10 pm
content: Your software-defined DC/DC topology is lacking current sensing, which is pretty much essential. At startup or transition conditions, the caps at output will act as short circuits, causing the inductor core to saturate, killing the fets as short circuits. A pulse-by-pulse (or nearly so) current limit is absolutely necessary; it doesn't need to be precise. You could try to do without with huge amounts of overrating of the FETs, software soft start kludges etc., but it won't be very robust. The ATMega328's ADC is most likely an order of magnitude too slow to implement this protection, especially when you multiplex several channels.Software-defined DC/DC is an interesting learning opportunity, but to build a simple DC/DC voltage regulator for this task, I'd suggest using some of the standard controller circuits - they are very well integrated, some can work with almost no extenal components, and they can measure the current and provide pulse-by-pulse limit by using the FETs on resistance as the measurement shunt. For example, you could start with TI's WEBENCH tool; it will spit out a full schematic and several suggestions based on simple input parameters.

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 08:33:28 pm
content: I wanted to use my own dc to dc to learn more how they worked and to try to get the most ou of the batteries. Are you sure an irf1405 (169 amps peak current) will overheat at startup ?I will take a look at Ti's solutionOh, and just for fun:https://www.amazon.fr/Hipzop-4x3-7V-9800mAh-Rechargeable-batterie/dp/B01LWPOSB0/ref=sr_1_3?ie=UTF8&qid=1478723325&sr=8-3&keywords=batterie+li-ionxDDDDDDD

author: Siwastaja
author_rank: Super Contributor
timestamp: November 09, 2016, 08:46:49 pm
content: Building your own DC/DC controller is possible, but WAY more difficult. It's possible to do with a modern fast ARM microcontroller (or similar), but you'll also need current shunt, current measurement amplifier, FET driver and some understanding how to utilize the analog peripherals of the more complex MCU.

author: mariush
author_rank: Super Contributor
timestamp: November 09, 2016, 08:47:46 pm
content: It would be much cheaper and safer to buy ready made dc-dc converters from eBay for your 5v and 12v.Here's an example:http://www.ebay.com/itm/DC-DC-Power-Module-Buck-Converter-Step-Down-Module-4-75V-23V-to-1V-17V-Mini-360-/131992495036?hash=item1ebb5da3bc:g:uGcAAOSwA3dYGtIjWorks with up to 23v, up to 96% efficient (more efficient if input voltage - output voltage difference is smaller)They can do up to 3A but shouldn't be used for more than 1.5A continuously, which is enough for phone chargers and some 12v output.Just replace the trim pot with a proper resistor after configuring the output voltage to the right value and you're done.Here's a less efficient, but potentially capable of slightly more current continuously and can handle higher input voltages (up to 40v) :http://www.ebay.com/itm/DC-DC-LM2596-power-Supply-Buck-Converter-step-down-module-GM-/262136732989?hash=item3d08911d3d:g:XoYAAOSw6dNWSu3oFor that 18.5-19v, you'd do best with a SEPIC dc regulator (can do step up or step down depending on input voltage), here's an example:http://www.ebay.com/itm/DC-DC-Auto-Boost-Buck-SEPIC-Converter-LM2577-/332020522590?hash=item4d4df71e5e:g:KqsAAOSw2GlXKCU~

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 08:55:57 pm
content: Quote from: Siwastaja on November 09, 2016, 08:46:49 pmNot impossibleThank you for all you repliesI'm going to make my own buck and boost converters, I think I will learn a lot on them by doing so ! I have some already made 5 amp buck-boost converters, but just putting them in a box and say I made it myself is not realy fun, i'll post the final schematic and board design when I'll be done.

author: 3141592
author_rank: Regular Contributor
timestamp: November 09, 2016, 09:28:38 pm
content: Quote from: whatsthegeek on November 09, 2016, 07:57:39 pmThe 4000 mAh capacity is an obvious lie. The thing is that with these no name brands from china you just don't know what you get. Some of them are simply just used junk cells from laptops rewrapped. Best case is that they are low quality, low capacity cells. Either way they should not be paralleled, because they will be poorly matched. They shouldn't be bought tbh.Charging Li-ion packs is also not that easy, you shouldn't charge cells that have gone under 2.5V, you need to balance the charge etc... Overvoltage (faulty charger), or a short circuit could easily cause an unprotected pack to explode.I don't think this is a good idea, because Li-ion is still quite dangerous if you don't do it correctly.Quote from: Siwastaja on November 09, 2016, 08:21:41 pmTo my knowledge it simply means that the cell has an added PCB for overvoltage, overdischarge etc. protection. If you buy unprotected cells, you still should implement this separately.

author: rob77
author_rank: Super Contributor
timestamp: November 09, 2016, 10:25:55 pm
content: i would suggest to go for some good brand batteries, definitely don't cheap out on the batteries... you can't go wrong with panasonic or samsung ( but i personally wouldn't trust samsung after their galaxy note 7)TME has the 3350mAh panasonics 18650 for less than 5Eur + VAT in single piece quantities.http://www.tme.eu/gb/details/accu-ncr18650b/rechargeable-batteries/panasonic/ncr18650b/basically anything higher than 3500mAh is either bullshit or marketing trickery

author: rob77
author_rank: Super Contributor
timestamp: November 09, 2016, 10:33:30 pm
content: Quote from: whatsthegeek on November 09, 2016, 08:33:28 pmactually that's a real dealit says 4 x 3.7V 9800mAh - so it's 9800mAh for all 4 cells together... so you'll get 4x 2450mAhit's a nice example of marketing trickery

author: whatsthegeek
author_rank: Contributor
timestamp: November 09, 2016, 10:47:18 pm
content: Oh, lol, wouldn't have expected that xD

author: Someone
author_rank: Super Contributor
timestamp: November 09, 2016, 11:24:38 pm
content: Quote from: whatsthegeek on November 09, 2016, 08:55:57 pmYou're scaling up to larger power without already having a grasp of the basics, the schematic presented is a quick and messy sketch missing so many parts of the system its hard to draw any understanding from it (other than knowing it won't work). If you're really set on building a software driven regulator then you'll want to design the control system in a simulation before designing the circuit, so you know what you will need to measure. Thats before getting to the challenges of driving the mosfets, they won't drive hard from the IO pins of a micro controller, you might get a few watts of power out of such a convertor but you won't get the several dozen watts modern devices demand. Also the gate drive won't be sufficient for the buck regulators.

author: rob77
author_rank: Super Contributor
timestamp: November 10, 2016, 10:24:57 am
content: i just noticed the schematic... you can forget that right away1. for 12V you can't use a buck converter... you need a buck-boost2. you can't do 3 switchers and 3 feedback loops with one AVR reliably... (if even possible)3. you have no current sensing at all4. zero safety - what will happen if someone shorts the output of the boost converter ?

author: whatsthegeek
author_rank: Contributor
timestamp: November 10, 2016, 03:01:41 pm
content: Quote from: rob77 on November 10, 2016, 10:24:57 amWell, I guess I'm going to go with the TI soluton instead. Do you know where I can buy these components in europe, the site where I usualy buy my electronic components does not sell them :/

author: Siwastaja
author_rank: Super Contributor
timestamp: November 10, 2016, 04:05:49 pm
content: Farnell, for example.

author: whatsthegeek
author_rank: Contributor
timestamp: November 10, 2016, 04:23:31 pm
content: ThanksJust realized farnell is in the town I live in

author: Seekonk
author_rank: Super Contributor
timestamp: November 10, 2016, 04:36:36 pm
content: One interesting thing about wall warts is that many of 100-240V wall warts will work on only 60V DC. I can power and charge my laptop with my solar DC buss at my camp without problem. Simplifies things if a simple boost converter is used. Some are polarity sensitive. Use at your own risk........