LED Constant Current Boost circuit capping out at 200mA - why? - Page 1
Forum section: electronics / projects

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 02:35:12 am
content: Hello EEVBlog Community,No matter what I try, the current always seems to cap out at 250mA. I can set it to a constant current lower than 250mA and it will hold there fine. I've tried greatly varying the timing capacitor ranging from 470pF to 100nF with many steps in between, along with varying the inductor from 33uH to 470uH. No matter what I do it seems as though the current decides to stay fixed at around 200mA. I can just ever so gently bring it up to 250mA, but the issue still remains. When using this as a buck converter, I am also having a similar issue.My target current is 350mA, though I would like to be able to go higher.Some detail about the circuit:Vin = 5V; PSU rated 5V @ 10AVf of all LEDs = 8.6VOutput voltage is pretty clean for this sort of circuit (+/- 500mV ripple)The TIP-120 starts to get somewhat hot even with a heat sink on it and thermal putty - it climbs to about 42°C after 5 minutesHere my schematic:Any advice would be appreciated or help in the right direction, this circuit has been very frustrating!-Bear

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 02:39:50 am
content: Well thar's yer problem...No VCCUmm.... uhh, ewww? Why the heck is there a TIP120 there!?And why choose a bipolar part (that uses a darlington internally already!) for such a low supply? You're burning no less than 20% of your supply for no reason whatsoever, and that's without the TIP120!Reference:http://www.onsemi.com/pub_link/Collateral/NCP3066-D.PDFTim

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 03:04:31 am
content: Quote from: T3sl4co1l on August 07, 2014, 02:39:50 amI may have forgotten to throw in a VCC symbol in the schematic, but I clarified it in the original post that the input voltage is 5.0-Vdc (rated at 10A). I kept blowing up my NCP3066s due to too much peak current in the switch. I could use an N-channel MOSFET, but the TIP120 is just a high amp transistor I had lying around and it seems to work fine at my low (20kHz) frequency. I also realize I forgot to add the decoupling capacitors that are on my circuit to the schematic. I do have them there though.However, I do not understand what you mean by: "And why choose a bipolar part (that uses a darlington internally already!) for such a low supply?EDIT:I updated the circuit in the initial schematic. I'm using a TIP120 darlington transistor due to the notes on page 7 of a nearly identical driver's datasheet foundhere. This validates my high-peak current claim.I should also mention the end-game of this will be used to power 16 LEDs having a total forward voltage somewhere around 42 volts with an input voltage of either 32Vor48V if I can find a buck driver which can handle 48V input.

author: mij59
author_rank: Frequent Contributor
timestamp: August 07, 2014, 06:43:33 am
content: Hi,Connect pin1 of U1 to Ipk.Or Connect pin1 to LED+

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 08:47:13 am
content: Convince me your inductor isn't saturating...and yes, how come your schematic looks so little like the datasheet? IPk going somewhere odd. (Edit - no, I take it back, it looks OK. I blame your inductor!)Also, are you sure that chip can do what you want? Are you seriously aiming for 42V output, from 5V input? That's a lot harder than your eventual aim.None of the demo circuits get close to your spec.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 11:21:32 am
content: You don't just "grab whatever's on hand", a 10k base-emitter resistor won't magically convince a TIP120 to turn off in less than 10 microseconds. Even with an optimal value, you're looking at single digit microseconds for that part, still enough to discharge that inductor while depositing little energy in the output.MC33063 is a shitty little converter, best left forgotten. Was it good for its day? Dubious. Was it cheap? Oh hell yeah. Cheap means popular. Cheap or popular do not mean good. TIP120s are sometimes "popular" too... get the idea?You aren't using it correctly, in any case, if you're comparing against the '063. The latter has an internal "base current" input (the collector of the first output transistor), usually pulled up to VCC with a suitable current-limiting resistor. If you're using the NCP3066 in the same way, "C" should be pulled up to VCC with a resistor, so the TIP120 (or whatever) does the "output darlington" activity. This will at least get your saturation voltage down from 2-3V to maybe 0.7-1.2V, but turn-off isn't helped.I would not suggest using a MOSFET in this circuit. Even with "C" = VCC and a pull-down resistor on "E", the gate voltage will not go high enough to ensure saturation, let alone switching well enough to actually get any output from it.Tim

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 12:51:11 pm
content: Quote from: mij59 on August 07, 2014, 06:43:33 amI'm not sure how much difference that would make, but I will try it.Quote from: Precipice on August 07, 2014, 08:47:13 amI am aiming for 42V output with a 32V input. I believe that's pretty doable, though for now getting 5V to a constant current of 350mA at around 8.6V working would be nice.Quote from: T3sl4co1l on August 07, 2014, 11:21:32 amMy switching frequency is around 20kHz to 10kHz so I believe it should be able to switch quickly enough. I've also taken a look at the collector on my scope and it seems to be doing fine switching-wise. You could still be correct though, but I know the external transistor isn't the issue as my issue still occurred when I was using the internal darlington 'switch'. What would you suggest in substitute for the TIP120?

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 12:58:53 pm
content: 10-20KHz, 33uH, huh? That's an adventurous combination. I still think you're saturating it and it's turning into a piece of wire, rather than an inductor. Do you have access to an oscilloscope?I also really, really think you should stop messing around with your 5V input voltage. That's a bunch of trouble you just don't need, making this work at low voltage. Move to 12V input. There's almost nothing in common between what you're doing now and what you want to end up with.

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 01:03:38 pm
content: Quote from: Precipice on August 07, 2014, 12:58:53 pmI can't use a boost converter to have a 12V input and an 8.6V output, though I guess I could get more LEDs and step it up to 17.2V output needed.I've also been switching the inductor value from 33uH to 150uH, 300uH, 470uH, and 940uH to see the efffect. It seems that after about 150uH the increase in inductance does little to help, but 33uH to 150uH yields the best increase in desired output.I do have access to an oscilloscope, but not until I get home tonight. What should I probe?

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 02:04:36 pm
content: Since you have access to a convenient bias supply, the Darlington can be replaced with two separate bipolar transistors with the driver connected to the input supply so the voltage drop will only be one Vce saturation. Extra switching speed can be gained by adding a schottky diode as Baker clamp.If a power MOSFET is used, then powering the switching regulator or at least the MOSFET driver with the LED supply instead of the input supply will increase the gate drive voltage significantly.Check the inductor for saturation.

author: sporadic
author_rank: Regular Contributor
timestamp: August 07, 2014, 02:15:04 pm
content: Aside from their reference schematics and spreadsheet design, have you checked their sim tool -http://onsemi.transim.com/greenpoint/Pages/PartSelection.aspx?landingTab=led&lang=en-US?

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 02:22:19 pm
content: Quote from: sporadic on August 07, 2014, 02:15:04 pmI'm surprised I've never found this, though I may have ignored it due to requiring registration. I'll try it out and let you know, thanks!

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 03:55:32 pm
content: If you've actually built what your schematic says - why did you put your bulk decouplers on IPk, rather than VCC? That's going to really break things...Go on, take the time to re-draw the schematic so it makes sense. At the moment, it's just a confusing jumble. Well, it's confusing me, and I don't think it's helping you!

author: SittingBear
author_rank: Contributor
timestamp: August 07, 2014, 05:46:13 pm
content: Quote from: mij59 on August 07, 2014, 06:43:33 amAfter messing with the circuit it seems this fixed the issue, and I understand why. It could also be partly due to using a cheap multimeter for reading current at home whereas testing it at work on a lab grade multimeter showed a better value. I cannot confirm this until I get home and use the same 400mA circuit I have working at home and reading it with my multimeter.Quote from: Precipice on August 07, 2014, 03:55:32 pmYeah, you are right. It was late at night and I wanted to throw the question online so I could get some replies. In my rush I bodged the schematic completely. I'll fix it as soon as I get home and update my post.It also turns out that boosting low voltages (5V) to a relatively high current (>500mA) at a higher voltage (12V) is rather 'unstable', but not to the point of blowing anything up. It just simply does not work with this buck-boost IC.

author: SittingBear
author_rank: Contributor
timestamp: August 08, 2014, 12:37:44 am
content: Sorry for the double post, but I got it to work at the desired 350mA to 500mA!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2014, 08:03:36 am
content: So....many......jumpers....

author: komet
author_rank: Unknown
timestamp: August 08, 2014, 08:47:59 am
content: You didn't mention you were breadboarding it! That would have been useful information, because now it's a miracle it works at all. Switchmodes are considered PCB layout critical.

author: SittingBear
author_rank: Contributor
timestamp: August 08, 2014, 12:50:48 pm
content: Quote from: komet on August 08, 2014, 08:47:59 amI guessed as much with the possible interference coming in from all the jumpers, the possible contact issues, but most of all the resistance of the fine gauge long jumpers I used! I will be throwing it on a PCB once the design has proven to work!

author: mikerj
author_rank: Super Contributor
timestamp: August 08, 2014, 02:20:41 pm
content: Quote from: SittingBear on August 08, 2014, 12:50:48 pmYou have a rather paradoxical situation there.

author: dannyf
author_rank: Super Contributor
timestamp: August 08, 2014, 03:10:09 pm
content: QuoteDoes seem to suggest that it is rather important to follow the datasheet?

author: SittingBear
author_rank: Contributor
timestamp: August 08, 2014, 03:49:11 pm
content: Quote from: mikerj on August 08, 2014, 02:20:41 pmI was a bit vague. I meant to include the entire circuit design with a transformer, full-wave rectifier, and output cap built in as I've never done that before.

author: mrflibble
author_rank: Super Contributor
timestamp: August 08, 2014, 06:05:30 pm
content: Quote from: SittingBear on August 08, 2014, 12:50:48 pmResistance isn't the main issue in this case. It's more the parasitic inductance and capacitance that makes breadboarding switchers a fun learning experience.But hey, you got it to work and nobody died. All's well.