Boost converter high pitch audible noise - Page 1
Forum section: electronics / beginners

author: Veketti
author_rank: Regular Contributor
timestamp: December 25, 2023, 08:33:59 am
content: Dear All,So my first design of using boost converter - what could go wrong.I'm usingMCP16251and I tried replicating the recommended layout on page 18 of the datasheet, as good as possible, but was not able to do it exactly like it was there. Issue with this is audible high pitch whine which I assume is coming from the boost converter circuitry. With normal load, display off @7.5mA it's almost inaudible, but with display on ~20-50mA load I can clearly hear it. This converter should work with 500kHz of frequency so it shouldn't be audible so maybe I screw up something. Could anyone review the pictures and see if you can point out any obvious mistake I've made, I'd really appreciate it?OT. I've been wondering why haven't anyone invented camera which would work like thermal camera, except instead of heat, would pick soundwaves. How easy it would be when you could see the source of noise. It would be as useful as thermal camera, to pinpoint eg. car interior rattles which are super annoying, component noises etc.Thank you again

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 25, 2023, 09:34:12 am
content: Quote from: Veketti on December 25, 2023, 08:33:59 amNot all "recommended layout" are actually good. Some of them are laughably bad it isn't even funny, especially one from TI for some reasons....Quote from: Veketti on December 25, 2023, 08:33:59 amMost SMPS now days use PFM in low load condition. The switching frequency sometimes can go so low it's audible. However IMO it shouldn't be the case here. Try with different load and/or different input voltage and see if it changes. You can try touching the components one by one and figure out where the noise disappear or pitch changed.Quote from: Veketti on December 25, 2023, 08:33:59 amWith boost converter the most critical loop is the one involving lowside - highside/diode - Cout. Try to make that loop as small as possible and if possible don't switch layers. Your C3 trace from pin 5 could be improved here. It's way too long, thin and involves two vias.I don't know where you pick your FB point is, but it should be either after or at worstatthe output capacitor (C3). You want it to actually capture the output voltage waveform and not the switching waveform. The way you laid out the board gives me the impression that you may "intercept" the FB between the diode/highside and output cap, which isverybad.The inductor is also too far from the switch nodes for my comfort. Idk if it's possible with your margin of manufacturing, but you can try move input cap to straddle pin 1. That way you get direct pour connection for your input caps and short fat connection for inductor SW node.If you can't avoid using vias, use more than one vias in parallel for power traces.Quote from: Veketti on December 25, 2023, 08:33:59 amAcoustic cameras exist, it's still stupidly expensive right now, and still with resolution far worse than your average thermal camera.We use it to diagnose large industrial machines. Generators, bearings and such, gas pipe leakages. Good luck using it for something as dense as modern PCB (with current tech). Tbh I never try it for that purpose so

author: jonpaul
author_rank: Super Contributor
timestamp: December 25, 2023, 12:37:37 pm
content: Audible whine is Subharmonic Oscillation sub cycle oscillation, loop instability.Poor PCB layout prob not at fault.Research and debug the circuit and component choice.j

author: Buriedcode
author_rank: Super Contributor
timestamp: December 25, 2023, 07:24:49 pm
content: Well the sound either comes form the coil, or the output cap. Nice sized ceramic caps there, 1206, which should reduce the DC bias effect, but can still "sing".Got a scope so you can see the switching node?Maybe I'm blind but I dont' see a trace going to the FB pin - unless theres a via-in-pad?The closest device to those I've used were the older MCP1640. They were very sensitive to noise on the FB line so I reduced the value of the feedback resistors by a factor of 10 (I think the datasheet recommended >700k to 1M..) and kept the bottom resistor very close to the pin.Figure 2-17 on page 7 of the datasheet you linked shows the PFM to PWM threshold - so perhaps your load is a bit lower than you think and its still in PFM. The graph below gives an example of the burst with 3.3V out @1mA and shows a burst every ~800us.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 26, 2023, 08:50:30 am
content: The SMPS chips that are efficient at low current do so by operating in burst mode, so unless running at close to full rated output power will be generating bursts at a lower frequency than the switching oscillator - these chips are not suitable for audio as a result, and here you get to hear the L or C vibrating.See figure 2-18 in the datasheet you linked to...