5V to 3.3V converter - Page 1
Forum section: electronics / projects

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 06:11:01 am
content: Hello everybody,I need some theory on a switching converter:Input: 2.5W (5V, max500mA) (USB powered)Output: ~2.5W (3.3V, 757mA (theory) -600mA(practice)).What type of converter should I use and any practical considerations I should take into account?Also, if you have any websites with extensive theory that would be awesome.Any help is appreciated.--------------------------------------------------Ideas so far (see below for context):* flyback converter with synchronous rectification (researching)* linear regulators (out of the question - they cannot supply 600mA from an 500mA USB port)* Buck regulator (works just fine with input filtering, see posts on page 2)--------------------------------------------------Thank you for your comments.

author: Psi
author_rank: Super Contributor
timestamp: September 09, 2014, 06:17:12 am
content: Do you actually want to build it?Because you can buy modules to do that quite cheaply already.$7.50 for this one (and i have seen cheaper)http://www.digikey.com/product-detail/en/R-783.3-1.0/945-1036-ND/2256216

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 09, 2014, 06:23:08 am
content: Is galvanic isolation required and if so to what level?Is a linear regulator out of the question?If switchmode I would probably opt for a flyback converter due to simplicity and low output power.The efficiency won't be great due to the low output voltage unless you opt for synchronous rectification, but that increases the complexity.Very often at these low power levels it's best to simply purchase a PCB mount unit, depending upon volumes.Dick

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 06:29:22 am
content: Quote from: Psi on September 09, 2014, 06:17:12 amI'm looking to fail building it and learn from the mistakes I make, buying it won't teach me a thing about power electronics.

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 06:37:23 am
content: Quote from: Richard Head on September 09, 2014, 06:23:08 amIt is not required but I don't think I can do without it.Quote from: Richard Head on September 09, 2014, 06:23:08 amYes, I need more than 500mA at the output.Quote from: Richard Head on September 09, 2014, 06:23:08 amCan you point me to a place where I can read more about these things (including the local library)?

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2014, 07:25:08 am
content: One thing to watch out for: USB devices are supposed to operate down to 4.4 volts. That is the nominal 5 V supply with the worst case variation and the worse case resistive drop in the cable. At 500 milliamp that is only 2.2 watt, so if you want it to be bulletproof and work in the worst case voltage without drawing more than the port rating, you don't have a lot of overhead. If you only need to work in the common case, you have a bit more overhead.If you don't need galvanic isolation, a buck converter is about the simplest thing you can do. You can go read the wikipedia page on it, and pay attention to the figures. Dave also has a video on building a simple buck regulator. As I recall it was mostly about calculating components from the data sheet and testing rather than the theory of operation, but you should definitely watch that. Even if you use a different controller chip, watching him go through the process of calculating the parameters is useful.The big advantage of using a non-isolated converter is that they only require an inductor, not a transformer. Inductors are easier to find in exactly the ratio you need. Also, the feedback is much simpler, as it does not require an opto-isolator.

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 09:54:06 am
content: Quote from: ejeffrey on September 09, 2014, 07:25:08 amI will definitely keep this in mind, thank you.Quote from: ejeffrey on September 09, 2014, 07:25:08 amThe problem with the Buck converter is that it cannot supply an output current of 600mA as the inductor must first draw the 600mA (+ a ripple current) fromsomewhere, and thatsomewhere(the USB port) cannot supply that current... The Buck converter was the perfect answer if I was drawing less than 500mA with my load.The way I see it (please correct me if I'm wrong), the only way to transform 2.5W (5V,500mA) input power to 2.5W (3.3V,757mA) output power is by using a "power shaper circuit" and that means I have to use a transformer - my worst nightmare. Someone should tell Dave to make a Fundamental's Friday about them.Right now I'm researching Richard Head's advice,Quote from: Richard Head on September 09, 2014, 06:23:08 ambut further ideas are always welcome.

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 09, 2014, 10:10:27 am
content: the only way to transform 2.5W (5V, 500mA) input power to 2.5W (3.3V, 757mA)You are asking for a converter with 100% efficiency! The output power will always be a little lower than the input power. What's more, 3.3V output converters are generally not high efficiency units. I think you need to be a little more realistic with the efficiency. A buck converter will be more efficient than a flyback so it's the topology of choice. A drawback with the buck is that if the series power switch fails short circuit (It'll never fail open circuit!) it places the 5V on the 3V3 line.As far as literature is concerned, there is plenty. A good introduction is the one by Marty Brown. Quite old now but the fundamentals don't change.There was also a set of Philips application notes that was my bible on SMPS design.There's another by George Chryssis (Spelling?) that is not too bad.Sorry can't remember all the titles and I'm at work so can't check up.

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 10:17:57 am
content: Quote from: Richard Head on September 09, 2014, 10:10:27 amNo, no, no. Definitely no. I was asking "in theory". First post asks for 80% efficiency. I am well aware of that.

author: Kjelt
author_rank: Super Contributor
timestamp: September 09, 2014, 10:21:52 am
content: Linear are plenty of choices, look at Farnell. For instance LD1117S33

author: Anand
author_rank: Contributor
timestamp: September 09, 2014, 10:29:49 am
content: Quote from: Richard Head on September 09, 2014, 10:10:27 amAre these the books that you're referring to?http://www.amazon.com/Supply-Cookbook-Second-Edition-Engineers/dp/075067329Xhttp://www.amazon.co.uk/High-Frequency-Switching-Power-Supplies/dp/0070109516/Quote from: Kjelt on September 09, 2014, 10:21:52 amHow can I use a linear regulator to supply 3.3V with 600mA when I can only draw a maximum of 500mA from my USB port?

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 09, 2014, 10:34:05 am
content: The books look like the correct ones. My books look different but then they were printed 20+ years ago.

author: Kjelt
author_rank: Super Contributor
timestamp: September 09, 2014, 10:37:21 am
content: ah sorry replied to quickly.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2014, 03:39:40 pm
content: Quote from: Anand on September 09, 2014, 09:54:06 amNo, you need to go reread about buck converters. In a buck converter, some of the current comes from the source, and some of it comes "from ground" via the catch diode. You lower the voltage and increase the current.QuoteThat is not correct. You can do this without a transformer.

author: mariush
author_rank: Super Contributor
timestamp: September 09, 2014, 04:09:44 pm
content: Go to Digikey or Newark/Farnell and go to "switching regulators" or "switching controllers" and then filter by your requirements ... select buck regulators or sepic, should work with at least 4.5v, should work with up to 6v (some phone chargers with usb jack output that much), should output at least 500mA but ideally you want 1A or more.Once you filtered down that list, you can look at the datasheets for those chips and read the information in those datasheets. Some manufacturers have lots of information, others have less.You can look for regulators working at higher frequencies, which means you'll be able to use smaller components (inductors and capacitors) in connection with the actual regulator chip but the efficiency may be slightly less, at around 75-85%.There are also switching regulator chips that are designed from the start to output only 3.3v and some even have built in diodes so you only need an inductor and a capacitor to make them work, which makes the whole footprint very small.Also, when you pick a regulator, pay close attention to minimum voltage required to output 3.3v properly. For example, you may see some regulators listed as working from as low as 4v, but that only means they'll output something with 4v at input, for example 1v..2.5v, not necessarily something as high as 3.3vAnyway, with the best buck regulators and with ideal components and layout, you're probably looking at about 95% efficiency but you're not going to have that, so expect at best 90-92% efficiency. Then keep in mind that you're not going to have 5v all the time from USB, from the start it's permitted to have 4.75v-5.25v due to the standard and then you can also have some voltage drop on the cable at high currents, at 500mA even if the source gives perfect 5v, you may not have 5v but 4.9v instead.So expect to have as little as 4.5v from usb, and choose a switching regulator accordingly.Here's one that I think should work for your needs :http://www.digikey.com/product-detail/en/LM2651MTC-3.3%2FNOPB/LM2651MTC-3.3%2FNOPB-ND/363752(datasheet here:http://www.ti.com/lit/ds/symlink/lm2651.pdf)Here's one that wouldn't work, even though the specs hint that it would :http://www.digikey.com/product-detail/en/MIC4680-3.3YM/576-1220-ND/771689- even though it says minimum voltage 4v, the datasheet doesn't really show anything under 5-5.5v input voltage. Maybe it can output less than 500mA with 4.5-5v but not 1A.

author: Sebastian
author_rank: Regular Contributor
timestamp: September 09, 2014, 04:23:28 pm
content: I just put your requirements into Texas Instrument's Webench Designer and set it for highest efficiency.This is the result:

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2014, 04:32:55 pm
content: Quote from: Sebastian on September 09, 2014, 04:23:28 pmNice.

author: Rufus
author_rank: Super Contributor
timestamp: September 09, 2014, 04:36:26 pm
content: Quote from: ejeffrey on September 09, 2014, 03:39:40 pmThe full load current (+some) is passed by the converter switch while it is on. I think the OP isn't getting that some of that current comes from a capacitor on the input side of the switch the average supply current being less than the load current.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2014, 07:24:53 pm
content: Good point. It is easy to forget about the capacitor's role in a SMPS, but it is just as critical as the inductor. You need both for it to work properly.

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2014, 08:06:17 pm
content: Quote from: Anand on September 09, 2014, 09:54:06 amThis is not how buck converters work.Ignoring various losses like forward voltage drop in any switches, the duty cycle of the buck converter will be about 66% to convert 5 volts to 3.3 volts. Since the input switch is only connected 66% of the time, the average input current for a 600 milliamp output will only be about 400 milliamps.An ideal flyback converter will have the same input and output currents.

author: Anand
author_rank: Contributor
timestamp: September 10, 2014, 02:11:40 am
content: You guys are horrible teachers.I've spent 3 hours trying to understand your comments but I finally got what was wrong.Here is a Buck design I whipped out.The problem here is that even though the AVERAGE current drawn from the source is 400mA, the peak is 600mA (66.7% of the period) which is bad as it is above the 500mA maximum rated current of an USB port.And, after re-reading your comments, Rufus was right, I didn't understand the input side of the converter.I've added a filter to the input side and BAM, it's perfect.Thanks guys and girls.