5v PSU Spiky issue - Page 1
Forum section: electronics / projects

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 23, 2013, 09:16:49 pm
content: Hi,I've got a 5v switchmode PSU based on a MCP16322 and it's not outputting a very clean supply.I've tried various options (increasing Electro cap, adding extra Ceramic cap to react faster etc) but would like your (sensible) opinion as to what is the cause of the spikes.cheers

author: madires
author_rank: Super Contributor
timestamp: October 23, 2013, 09:46:00 pm
content: Please try a ripple filter, an inductor of about 10-33µH and a cap around 100-220µF

author: Nerull
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:00:45 pm
content: Many switching PSUs need some load for the feedback circuit to work properly, are you accounting for that if needed?

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:05:08 pm
content: Yeah I've put a 8ohm resistor on the line. Nice and toasty.So Madires, the LC filter seems to have done the trick. I didn't have 33uH in stock, Tried with 68uH and a 220uF Cap.What gets me is why the recommended design out of the datasheet is so flawed!The other concern now is the amperage through the inductor.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:18:06 pm
content: How much current are you pushing through that 8ohm load?

author: madires
author_rank: Super Contributor
timestamp: October 23, 2013, 10:23:23 pm
content: Quote from: minime72706 on October 23, 2013, 10:18:06 pm5V / 8Ohms = 0.625A :-)

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:27:09 pm
content: Well if we are going to be perdantic.609.756mA :pIt's a 8.2ohm resistor

author: madires
author_rank: Super Contributor
timestamp: October 23, 2013, 10:38:20 pm
content: Quote from: johnnyfp on October 23, 2013, 10:05:08 pmDC-DC converters always produce some amount of noise and ripple and a bad PCB layout could make things worse.QuoteShould match the maximum current output of the PSU ;-)

author: minime72706
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:57:04 pm
content: Heh... I must have missed the fact that the supply was putting out 5VDC.EDIT: I basically wanted to see how much power you were asking from the thing during that test --- that thought became moot when I realized that the scale was 1V/division ...

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:59:05 pm
content: I'm now wondering if I've actually blow this chip.Same circuit, same layout, but lower voltage 3.3v. This is not producing this rippling effect.I tried to tweak the the voltage Feedback pin (Was outputing a little high) and it's drop the output all the way down to 1v.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 24, 2013, 08:24:41 pm
content: So I had to replace the Switch mode controller, For some reason a USB power switch IC decided to give up the ghost and fail shorted. Which in turn blew up the Switch mode, which has me worried that these things, even though they say they protect themselves, don't.Anyway, I put a new set of chips in and with madires LC circuit, it's sort of cleaner, but am still getting the attached.Yellow is the 5v after the LC and purple is before and blue is from the Supply (switchmode of course )Ive included a close up and as you can see the switchmode is outputting a saw tooth that the LC is cleaning.So now it looks like the USB device I've attached is causing this nasty little ripple

author: madires
author_rank: Super Contributor
timestamp: October 24, 2013, 08:53:37 pm
content: Quote from: johnnyfp on October 24, 2013, 08:24:41 pmBy using the 8.2 Ohms resistor as load instead of the USB device you could verify that.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 24, 2013, 08:56:58 pm
content: Yes.Smooth as a babies bottom. The USB device is a Display that has a LED back light at 50% PWM duty.Which sorta matches the pulses in the image. So maybe another cap in the display?

author: madires
author_rank: Super Contributor
timestamp: October 24, 2013, 09:24:33 pm
content: Quote from: johnnyfp on October 24, 2013, 08:56:58 pmThat should help.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 24, 2013, 09:46:30 pm
content: Nah didn't help.

author: madires
author_rank: Super Contributor
timestamp: October 24, 2013, 09:59:21 pm
content: That's a pity! How much current does the USB device draw?BTW, could you please post a picture of the PSU.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 24, 2013, 10:11:01 pm
content: Full circuit and details atsharedcircuits.comI've attached a close up of the before modifications and the hack job getting it smooth. (Ugly I know, but this is a prototype board!).Am I being too worried about a 300-400mv ripple?The display is drawing 300ma.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: October 24, 2013, 10:47:21 pm
content: Mmmh, I've just burnt out the inductor!

author: madires
author_rank: Super Contributor
timestamp: October 24, 2013, 10:51:58 pm
content: Quote from: johnnyfp on October 24, 2013, 10:11:01 pmI've checked the datasheet again and looked for load change responses. Figure 2.30 shows the response for a change from 100 to 600mA at 3.3V and vice versa. The ripple is around 350-400mV which would match what you see. So it seems being within specs. To decrease the ripple you could try to soften the load changes by increasing the values of the LC filter (a few mH?).