Boost converter completely refuses to work at currents of about 50mA? - Page 1
Forum section: electronics / beginners

author: Esposch T. Tapir
author_rank: Contributor
timestamp: September 11, 2015, 11:14:10 am
content: Alright, so I'm just trying to make a simple boost converter to pump up 5V (from USB) to an arbitrary output between 5V and 12V. It doesn't need to be the most accurate thing in the world, and I have a micro that can pump out 100kHz square waves, so I thought I'd start by following the instructions here:https://learn.adafruit.com/diy-boost-calc/overviewAnd what I made is not working at all.I used a BS170 for the FET, a 100uH wire wound choke for the inductor, an electrolytic for C6 and a ceramic for C5.The diode was a 1.5A schottky from Jaycar.If I put 1k across the output, it functions fairly well (although struggles to go above the 12V point), but reducing that resistance too much causes the voltage to drop dramatically. With 100 Ohm in there, it refuses to go above about 5.5V, and increasing the duty cycle only causes the output voltage todecrease.Naturally, I double checked the circuit (three or four times!) and swapped from a USB port to a direct +5V connection (20A rated), but it all seems to do nothing.Swapping out the inductor for a 470uH seemed to help a little bit (could get to 18V@1k and 6.5V@100R), which I thought was interesting because the calculator in the tutorial said I'd only need about 30uH.Does anyone have any idea what exactly could be going on? I understand it won't be ideal, but surely performance this bad isn't normal?It probably goes without saying that I have extremely limited SMPS experience.Thanks,~Chris

author: bktemp
author_rank: Super Contributor
timestamp: September 11, 2015, 11:21:07 am
content: BS170 has an on resistance of a couple of ohms. That is too much for higher power levels. Try a different mosfet with a lower RDson.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 11, 2015, 06:34:02 pm
content: Remember a boost converter converts amps to volts. If you have a 100ohm load and want 12V output, you're asking it to deliver 0.12A. That means it's going to be drawing anaverageof (12/5)*.12 = 0.288A input.And that's only the average. Since the converter operates in pulses, thepeakcurrent through the inductor and FET is going to be a few times higher still.If your FET has too high a resistance, as  accurately suggested, this won't work. It also won't work if the peak current exceeds the rating of the inductor, in which case it saturates, and ceases to act as an inductor. You didn't mention the current rating of the inductors you tried, and that is significant.I'm sure there's maths to determine the peak current, but I'm not quite that well versed in switching power supplies, and find it easier just to simulate the circuit in SPICE to determine that.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 11, 2015, 06:45:45 pm
content: I have some "Super" torroids that out perform stuff I've seen lately. They were surplus and I grabbed them all. To use just any core will not work. I checked and the permeability of these is quite high. I can get fairly HV, 600V out of them, of course lower voltages with higher current. Why not do up an MC34063 with a FET and just twiddle a pot for output. Lots of info on that route.As suggested earlier, a low RDS and high permeability as well as a datasheet are your friends.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 11, 2015, 07:05:04 pm
content: I just noticed that Adafruit tutorial has a calculator on the sidebar, which will calculate various parameters - including peak current - for you. Nice.

author: Esposch T. Tapir
author_rank: Contributor
timestamp: September 12, 2015, 01:41:23 am
content: Thanks for the responses.I don't think it's the inductor; they're wire-wound ones with iron cores rated for 5A.I'll consider a more conductive FET and play around with things in SPICE.Failing that I'll go the IC and pot solution.

author: amyk
author_rank: Super Contributor
timestamp: September 12, 2015, 03:02:37 am
content: Quote from: Esposch T. Tapir on September 12, 2015, 01:41:23 am"iron cores" could mean a wide variety of things, not all of which are suitable for boost converters. Do you have a datasheet for the inductor, or failing that, a picture?

author: poorchava
author_rank: Super Contributor
timestamp: September 12, 2015, 04:58:26 am
content: You could try to use IRLML2505 mosfet. They are cheap and have about 50 miliohms of rdson.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 12, 2015, 05:21:03 am
content: Quote from: Esposch T. Tapir on September 12, 2015, 01:41:23 am's right, if you're using something like this:They're built with a series of laminated iron plates, which get magnetized and demagnetized with every cycle. It's a relatively slow process, and so this core material only works to a few Khz. You'll need something with a ferrite core.

author: rob77
author_rank: Super Contributor
timestamp: September 12, 2015, 06:31:47 am
content: BS170 is cheap a general purpose mosfet, so it's definitely not suitable for power switching applications. you can use pretty much any low voltage mosfet in TO220 package and it will work. but avoid the high voltage power mosfets - those usually have 1-2 ohms Rdson (so in your case useless as the bs170 is).in the past i built a crude step up converter (just for fun) - it was a IRFZ34N mosfet driven by a 555 timer and a lm393 comparator was resetting the 555 every time the output voltage exceeded the set voltage (simple zener as a ref). it was stepping up from 12V to 20-24V and was able to deliver 15-20W using a toroidal inductor rated for 3A (something like 150 or 220uH if a recall correctly).so don't be disappointed - just change the mosfet and probably the inductor as well and it will work like a charm

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 12, 2015, 10:31:34 am
content: If you are using something like that, I don't think that inductor will be very useful or efficient. What would be your inductance and efficiency at the frequency that your trying to switch at with a big old clunky iron core I and E inductor?No affiliation with the pictures provided, but inductors like these are more in line.http://www.g4jcp.freeserve.co.uk/pw/battpsub.jpghttp://shop.rabtron.co.za/catalog/images/Inductor,radial,6.8uH,2.90A,0.035%20Ohm.jpgBy the way I suggest you lookup Daves excellent video on the MC34063 in EEVBlog 110.Even if you don't go that route, it has a lot of info.Quote from: Chris C on September 12, 2015, 05:21:03 am

author: Circlotron
author_rank: Super Contributor
timestamp: September 13, 2015, 12:51:46 pm
content: I have used hundreds of these Jaycar inductors in boost regulators for 10V in, 16V at 4 amps out.http://www.jaycar.com.au/Passive-Components/Ferrites%2C-Inductors-%26-Suppression/Chokes/220uH-Prewound-Ferrite-Choke/p/LF1276Seeing most all of their mosfets are the same price you might as well use one of thesehttp://www.jaycar.com.au/Active-Components/Discrete/FETs/MOSFET-IRF1405-N-Channel-55V-169A-TO-220/p/ZT2468Way more powerful than you need but the same price as their other stuff.

author: rob77
author_rank: Super Contributor
timestamp: September 13, 2015, 01:10:54 pm
content: Quote from: Circlotron on September 13, 2015, 12:51:46 pmdoes everything have a pricetag $5.95 in single piece @ jaycar ?that mosfet is 1.89Eur single piece @ TME and other versions of IRF1405 are even cheaper - starting @ 1.06Eur single piece.for the inductor it's similar - same size and current rating inductors are 1.50 - 2.50 Eur in single piece.

author: Circlotron
author_rank: Super Contributor
timestamp: September 13, 2015, 01:28:23 pm
content: Quote from: rob77 on September 13, 2015, 01:10:54 pmI actually get the inductors through Electus, their wholesale division. About AUD$1.50 each or so for 1000 pcs IIRC.Jaycar is a retail shop on the main road, but you can just walk in and snuffle around to find what you want.Yeah, the mosfet is a bit over the top if you want to drive it from a micro port at 100KHz.