Hello rowand

Please post your schematic and ideally the datasheet of the inductor you are using and we'll take a look on both the noise and on/off questions.

You will need always need a pre-load on the output (>= 3mA) to maintain regulation - see post for explanation:

http://www.powerint.com/en/forum/ask-pi-engineer/lnk306-failed-open-loaded-problem

Regarding audible noise, no before the forum posts I had not heard of audible noise using discrete inductors and LinkSwitch-TN. I'm intrigued because maybe there are some inductors out there that are just not suitable for this application. It looks like the other user (zedman) with similar issues solved the issue during a redesign but the reason is not clear.

http://www.powerint.com/en/forum/ask-pi-engineer/lnk304-audible-noise

Cheers

PI-Chekov

Hi Rowand, we are trying to use LNK304 to provide 12V / 30mA and 3.3V / 30mA for powering an RF circuit and a relay. Unfortunatelly we still have this audible noise problem despite using the Tokin / CoilCraft 1mH inductors sent by Mr.Chekov. (thanks again for that) We also tried non wire wound small current (100mA) color coded inductors which should not have mechanical noise, but still have the noise. We also noticed that the noise gets lower while load is on. I managed to create a test PCB with through hole parts which IS silent. But I still cannot reproduce it. I think it's a kind of PCB design problem rather than inductor. I also cannot clearly say the noise is coming from the inductor. thanks, Zedman

Thanks for the reply and sorry for not replying sooner. I thought I would wait until I had built a smaller layout surface mount board before replying so I could see if that helped. Unfortunately the surface mount boards are noisy too. One design switches the LED load on and off, and another few designs have a constant 300mA load, but all of them have the noise.

The circuit I am using is the the same as the RDK-131 reference design circuit, except with a different inductor and equivalent transistors. RDK-131 reference design document (with circuit diagram):

RDR-131 Document

The transistors I am using are the On Semiconductor equivalent transistors:

Farnell MMBT3906WT1G
Farnell MMBT3904WT1G

I have tried two different inductors with the same problem. The first I used for prototyping was an Epcos 1000uH 0.33A inductor, found here:

Farnell Epcos B82464A4105K
Datasheet

The second type was a Coilcraft RFB0810-102L inductor, datasheet here:

Coilcraft RFB
Datasheet

Both circuits with different inductors have approximately the same amount of noise. However, I'm still not sure that the noise is coming from the inductor. It may be coming from the IC. It is hard to tell with all the components so close together.

Our PCB layout, while quite small, does not really attempt to minimise "the loop formed between the LinkSwitch-TN, inductor (L1), freewheeling diode (D1), and output capacitor (C2)" as stated in the LinkSwitch-TN datasheet. It was overlooked in the rush to get a PCB built. Could this contribute to some sort of noise problem? That's the only other thing I can think of.

Thanks.

Hi Zedman Would you be able to post a copy of your schematic and PCB layout file which you have built which does not experience the noise (ie the through-hole board you have built)?? If we start with this we may be able to work backwards to find the source of the problem. On this through hole board is the noise completely absent, or is it just less loud? We have an RDK-131 kit on order which should arrive next week, which we are told does not produce any noise. We are planning to use this to help us track down the problem. We'll let you know how this goes. Thanks! davidb

Sorry for the delay I am a bit busy now with my 2 weeks old daughter and I was a bit disappointed because just I had this noise problem, and I almost lost the hope but good to hear someone else has noticed it also. I'll post a photo of the silent test panel and also post the SMD pcb layout of ours soon. I don't know if it's really silent I just can't hear anything from that pcb.

Thanks for that Zedman, I'm interested to see your layout.

Here is the basic circuit and PCB layout for our prototype design. Note that some of the ideas, such as minimising loop sizes weren't really considered for this design. It would be interesting if anyone could tell me if the loop size is large enough that you think it would cause problems.

I also saw that the copper connected to the source pins should be minimised within thermal constraints, but since our boards will be on aluminium, the copper area has been kept very low. Would this be a problem?

The circuit is being fed with +110Vdc.

Here is the PCB pcb and the schematic, we also didn't take much care to that loop on this PCB because of size constraints. PCB shows only the psu part. I'll take photos of the test board when I get home today.

On our PCB the L1 D1 C2 loop is not too big altough I don't know what's called big in this context, it's max 15-20 mm. I can put them closer if it's needed. I'll check this on the test pcb.

Hi All,

I'm doing some measurements in the lab to confirm the source of noise (see attachment) and will report back later (hopefully today).

Cheers

PI-Chekov

Here are the results of measuring the audible noise of the RD-131 board.

With an LED load audible noise was very low, below 35 dBr A, our internal goal for audible noise (measured at 2 cm using the setup shown in the earlier posting). This is shown in figure 9 and 10.

Sweeping the load showed some conditions where the noise just exceeded this 35 dBr A level but it was not objectionable at a distance, basically inaudible at 1 m. This is shown in figure 7 and 8.

I'm now going to try to confirm that the inductor alone is the source.

Cheers

PI-Chekov

So here it is, tell me why it's silent?

Good news

It appears that the bulk of the noise is generated by the output capacitor C3 but only at light loads (when driving a LED load at 300 mA the design is silent).

Forum - RD-131 Audible Noise (6).jpg shows the no-load noise level comparison between ceramic and electrolytic cap.

Forum - RD-131 Audible Noise (7).jpg shows the swept load noise level comparison between ceramic and electrolytic cap.

Forum - RD-131 Audible Noise (8).jpg shows that the inductor can change the audible noise generated.

Cheers

PI-Chekov

Please try changing the output capacitor (C3) to an electrolytic or tantalum type and see if that solves your noise problem.

I've posted comparative plots of noise levels to illustrate towards the top of this topic.

Cheers

PI-Chekov

Wow, thanks a lot for those tests!
I'll see if changing the capacitor does anything, but if you say that the noise only appears at light load, then I'm not sure if it will completely solve the problem.

A few days ago I built the circuit in Figure 6 on page 11 of the Application Notes AN-37, "High-Side Buck-Boost Constant Current Output Configuration" (including the optional feedback diodes). This circuit seems to have much less noise at full load, and no noise at all at low load, even though the circuit is just a simple prototype built on veroboard.
The design uses electrolytic capacitors (and a single ceramic for the BP capacitor), so maybe that's part of the reason.

I plan to re-design the original circuit's layout and have both topology circuits made on PCB in a week or so, and I'll post results once I've built them.

I just noticed your posts on the top (this forum layout is a bit confusing), thanks for the throughout tests, I'll check C3 on our PCB and will post the results. Thanks, Zedman

So, I changed the output capacitor to a 100uF Tantalium one, but unfortunately it did not change anything. It's named Cp2 on our schematic. It was an electrolytic 100uF / 16V. The feedback capacitor is a 10uF / 35V electrolytic one. Should I change it also? Today I'll build a completely new test PCB to test if I can reproduce the silent board.

I've tried to rebuild the trough hole board, but had no success. Same board type, almost the same placement of the parts, same parts, also tried with Tantalium capacitors and Coilcraft 1mH inductors, but it has some noise. I achieved the lowest noise with 680uH non wire wound inductor and Tantalium caps. It would be nice to know how loud is that 35 dBr A Mr. Checkov mentoined as the limit. Because maybe I have a to sensitive ear and it would not be more silent.

We've done some testing of various components, and it appears that the noise is coming from two components.

Firstly, the output capacitor (C3, 2.2uF) is the noisiest component, but only when it is soldered to the board; its vibrations are amplified by the PCB when it is soldered on. The board is operating at relatively high current (almost the maximum of the LNK306), so it seems to be behaving differently to your audible noise tests. Removing it from the PCB and connecting it via wires makes it silent. We've tried the circuit without the capacitor connected at all, and the voltage ripple is fine for our application, so we are thinking about leaving it off completely.

Is there any reason we should not do this? We also plan to test several other capacitor types and values, so we shall see if that helps.

Secondly, we did the same thing with the inductor, removing it and connecting it with wire, and some noise is still there (in the inductor, not the rest of the circuit), but it is much more reduced. When the inductor is placed against the board, again, the noise is amplified. If the inductor is pressed hard against the board, the noise disappears, so it appears that the inductor is vibrating the PCB (either vibrating against the PCB, or shaking the whole PCB and making the noise). We've tried several different inductor brands, and all the ones we've tested have this noise.

With the inductor we're looking to:
1) test with more different types/brands
2) test with different values of inductance, and
3) change something mechanical about the PCB/inductor/mounting (not exactly sure what would work).

Any more suggestions would be appreciated.