Looking for low (audible) noise isolated DC-DC converters - Page 1
Forum section: electronics / projects

author: WolfK21
author_rank: Contributor
timestamp: November 28, 2022, 08:03:10 pm
content: Hello,at the moment I'm doing software development for a pimped version of Dave Ericksons PS-Loadhttp://djerickson.com/ps-load/.During all the software and hardware testing I became more and more annoyed by the hissing sound of the isolated DC-DC converter on the board.The sound is at a frequency of 12-13kHz (my Smartphone told me) and is horrible to listen at if you need to work next to it.To improve this sound I already tried some things:Adding some capacitors (MLCC) at the inputs and outputsPacking the DC-DC in some packing foam (didn't help at all)Mirror my test setup. Now I sit left instead of right of the board as my right ear seems to be less sensitive to this frequencyTo get finally rid of this hissing sound I ordered some other DC-DCs to try if I can find one that is also good and silent.Sadly It seems like I couldn't find a part that works. All DC-DCs I bought seem to have this issue at the one or the other frequency.Does anyone have a suggestion for a good DC-DC that can withstand my (maybe) too good ears?The Specs the DC-DC needs to fulfill:Galvanically isolatedInput Voltage: 12V, 15V or 24VOutput Voltage: +/- 12VOutput Current: +/- 35mA (Positive Rail draws 31mA, Negative Rail 15mA)Ideally: Low Coupling capacitance between primary and secondaryThese are the parts I tested:- Traco Power TMR 1223- Traco Power TMR3-1223- RECOM RB-2412D- Murata MEA1D1512SCCan anyone help with a "proven in use" part recommendation?Best regards,Wolfgang

author: Weston
author_rank: Regular Contributor
timestamp: November 29, 2022, 05:56:11 pm
content: Are you sure that the noise is coming from the isolated DC-DC converter and not somewhere else on the PCB?Typically noise comes from some unstable control loop / converter running in pulse skipping mode that excites some inductor or MLCC.These small isolated DC/DC modules are typically not regulated and operate open loop with some sort of push-pull converter at ~80kHz, so they should not be generating noise at these frequencies. I have used similar modules before and never had any issues with noise.If the load on the DC/DC converter is fluctuating there is some possibility the fluctuating load is generating noise in the isolated DC/DC module, but I would check for other sources. Can you see any supply fluctuations on an oscilloscope in the 12-13kHz range?

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 29, 2022, 08:16:38 pm
content: I agree with Weston. I have only used the various 0505 (5VDC to 5VDC) isolated ones, but even the cheap chinese ones have been silent thus far.In particular, the Recom RB-2412D datasheet (from Mouser) says its internal operating frequency is 50 kHz minimum 100 kHz typical, 105 kHz max; and is rated for 0% load.Have you run any of the DC-DC converters on their own, from a 24V lab supply, with various (half-watt) resistors and/or pots as loads, to verify the noise comes from the DC-DC converter?(I am an utter uncle bumblefuck when it comes to the actual electronics, so don't listen to me, but the only thing that comes to my mind after looking at the project schematics if is the opamps somehow start oscillating at that 12-13 kHz frequency. But I'm probably wrong.)

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2022, 08:27:04 pm
content: I've used various DC-DC converters and never encountered one yet that made audible noise. The sort I'm talking about are the little potted bricks in various shapes and sizes.

author: WolfK21
author_rank: Contributor
timestamp: November 30, 2022, 05:06:49 pm
content: Hello,I have to admit that I found the cause of the hissing sound. It isNOTthe DC-DCWhile thinking about what I wrote yesterday morning I got unsure about the cause, because the frequency of the hissing was always the same, regardless of the DC-DC I used. This and the fact that the datasheet of the RECOM explicitly talks about a switching frequency far beyond the audible range made me even more suspicious about the root cause.To make it short (and to blame it on me). The Problem was the chargepump connected to the output of the DC-DC that runs at 12-13kHz. After increasing the frequency the whole circuit stays silent.As we now know that my brain doesn't work while hissing sounds are nearby, I think we can close the topic.Best regardsWolfgang