DC - DC convert High Pitch Whine - Page 1
Forum section: electronics / general

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 03:04:24 am
content: Hi,I've put together a DC 2 DC converter based on the MC34063http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF, using the circuit details in the datasheet on page 7.I've got it connected to a MCU and all it's doing is flashing a low powered LED on and off. However when ever the LED is on I can hear a High pitch whine coming from the Inductor coil on the DC 2 DC converter.Is there anyway to stop that from happening? Is it a faulty coil or something?CheersJohnnyfp.

author: IanB
author_rank: Super Contributor
timestamp: May 08, 2012, 03:09:08 am
content: If the device is operating at an audible frequency you will hear it. Human hearing is incredibly sensitive.You need to adjust the operating frequency to a value outside the audible range. Check the datasheet for component values that may achieve that.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 03:49:23 am
content: According to the DS, I think, Based on my calculations (But the maths in this DS seems a bit weird). My Frequency is 45Khz. This is based on a 470pf Osc Capacitor. Humans can only hear at 22khz, 30khz max. Must be something else causing this.I suppose I could always listen to really load Music and destroy my Ears so I can't hear this anymore.

author: IanB
author_rank: Super Contributor
timestamp: May 08, 2012, 04:04:37 am
content: You might be exciting a lower frequency resonance. You could try some slightly different component values and see if it gets quieter.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 08, 2012, 04:07:59 am
content: At low loads, a lot of switching supplies would "skip periods" and operate at an effectively lowered switching frequency.

author: Psi
author_rank: Super Contributor
timestamp: May 08, 2012, 04:22:43 am
content: Yeah, if it was a pure 45khz sine wave you wouldn't hear it.It's because the 45khz is really a distorted square wave and skips some cycles, so it will contain various other frequencies some of which will be within human hearing.Shifting the frequency a little (+/- 5khz) may shift the audible frequency enough so you don't hear it any more without affecting the circuit too much.You could also try some hot glue on the inductor. It may stop it from turning the rest of the PCB into a speaker plate. (or it may make it worse, its a bit of trial and error sometimes)

author: amspire
author_rank: Super Contributor
timestamp: May 08, 2012, 05:58:07 am
content: Quote from: johnnyfp on May 08, 2012, 03:49:23 amIf the oscillator is actually switching at 45KHz, then any audible noise will be due to instability in the control loop for the regulator. An unstable control loop is something you do not really want, but quite often the regulator will continue to function adequately even with instability. For a production item, you want to design it out. For a one off design, you may be able to live with the instability, as long as your components can take the consequences - higher ripple current, higher peak inductor and transistor/mosfet current.Is it switching at 45KHz though? My recollection of the MC34063 is that it has a fixed ON time and a variable off time. So if it is not fully loaded, the oscillator frequency could slow down to an audible frequency.Richard.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 07:13:20 am
content: Well the circuit was only going to drive under 50ma or less. But I wanted it to be able to convert from 24v but still be super efficient. I didn't like the idea that an ldo would dump the voltage difference from 24 to 5v all as heat.Maybe I've choosen the wrong thing.Jfp

author: G7PSK
author_rank: Super Contributor
timestamp: May 08, 2012, 07:29:35 am
content: It square wave and square wave's are full of harmonics try replacing the 470 pf capacitor with a trimmer cap you can then tune it until it is quiet then measure the trimmer cap so as to replace it with a fixed cap. Fastest way I know to get the job done.

author: amspire
author_rank: Super Contributor
timestamp: May 08, 2012, 07:38:44 am
content: Quote from: johnnyfp on May 08, 2012, 07:13:20 amA switching regulator is a good choice, but part choice is important as well. Does the 470pF cap mean a 10uS ON time ? - that just sounds familiar. If so, the shortest off time will 10uS x 24/5 = about 40uS, so the highest frequency will be about 50uS or 20KHz. Now at 50mA output current, the frequency will be less then 20KHz if the inductor is less then about 2mH. At 10mA output current, the frequency will be less then 20KHz if the inductor is less then 10mH. So with this particular regulator, you need to look for fairly large inductors for low currents, otherwise the switching frequency does drop into the audible range.There are many different regulators that have a constant switching frequency and variable and shorter ON time, so they will not have this problem, and they can probably use smaller inductors. The UC3842 is a very common example of such a regulator, but I would look to one of the modern chips with the inbuilt fixed oscillator, preset 5V output and minimal part count.Richard.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 08:00:51 am
content: What would changing the cap do in terms of efficiency? If I change the cap to say 10000pf butkerp the inductor at 220mh would I get more line noise?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 08, 2012, 08:28:24 am
content: If you can move up to a few hundred kHz the problem will be reduced quite a bit, even if you are forced to operate in pulse skipping mode. The first advantage is that the passive components become smaller, and therefore they simply are less effective acoustic radiators (similar to the way a tweeter is not efficient at reproducing bass frequencies). The other benefit is that the minimum pulse width will be reduced, which reduces the size/energy of the transient. A 50 microsecond pulse at 5 kHz is a lot more audible than a 5 microsecond pulse at the same frequency.Another way to improve the situation (at considerable cost in complexity) is to use a transformer instead of an inductor. Then you can get the bulk of the voltage transformation from the turns ratio and operate at a duty cycle closer to 50%. If you are operating in pulse skipping/PFM mode, this will translate to a higher pulse frequency for a given load.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 11:12:20 am
content: Mmh transformers are bulky and require ac which I would have to regenerate. But Like the idea. Of smaller components. Currently I have a 470uf cap on the output which is huge compared to the rest of the circuit..

author: amspire
author_rank: Super Contributor
timestamp: May 08, 2012, 12:52:31 pm
content: Quote from: johnnyfp on May 08, 2012, 08:00:51 amIt is unlikely to improve efficiency much, and it is really a bad idea to lower the switching frequency even further - it will just lower the audible frequency and make it more annoying.The only way to get well above audible frequencies is to have a regulator with a much shorter minimum ON period then the MC34063 is capable of. The problem with the MC34063 is that it uses a fixed ON period, and a fixed frequency ( On + OFF periods combined) would be much better for you.The MC34063 is a much better regulator for an output voltage closer to the input voltage. The input to output voltage ratio of almost 5 is the real problem for the MC34063.A higher frequency regulator eliminates the audible noise totally, lets you use smaller inductors (ie - cheaper) and capacitors, and efficiency can still be over 90%.Richard.

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: May 08, 2012, 03:12:47 pm
content: Hey, this is a lesson from a old school er..we used to have transformer , coils etc that was noisy, we dipped the coil in some clear lacquer, ex for floorboards etc.. let it dry, and do it again if it doesn't help first time. That will dampen the oscillation in the coil.. moves the mechanical resonance frequency to hopefully a standstill... f=0

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 08, 2012, 08:24:21 pm
content: Hi,So what would you recommend as an alternative, I just picked this one because it had a nice simple circuit and seemed cheap? 24v-36v in, 5v out, up to 400ma. To drive a Microchip Pic based processor.(As you can tell this is the first time I've played around with a switch mode power source).CheersJfp

author: ejeffrey
author_rank: Super Contributor
timestamp: May 08, 2012, 09:35:46 pm
content: Quote from: johnnyfp on May 08, 2012, 11:12:20 amNot a low frequency transformer on the input, a transformer coupled switch-mode supply. You just replace the inductor in the SMPS with a high frequency transformer. Your switch drives the primary and you take the output from the secondary. The types you would look at are a forward converter -- the transformer coupled version of the buck regulator, or a flyback converter which is the transformer coupled version of the boost regulator.

author: amspire
author_rank: Super Contributor
timestamp: May 09, 2012, 12:00:42 am
content: Quote from: johnnyfp on May 08, 2012, 08:24:21 pmThere are a huge number of choices.You could go to TI's Webench, enter your parameters and it will list a whole lot of options and design the supply for you.You can go to Digikey. Mouser. etc and do a parametric search.I do not know your price-efficiency-simplicity tradeoff. For example, if you are prepared to spend a few dollars for the regulator, a chip like this is superb:http://cds.linear.com/docs/Datasheet/3642fc.pdfJust 4 parts in total - extremely good efficiency from 1mA output up - extremely low standby current - seperate enable input - very cheap inductor.Devices like this are cheaper and the documentation makes design very simple:http://www.onsemi.com/pub_link/Collateral/LM2574.PDFIf you wanted a good reliable regulator that has been around forever and has multiple manufacturers, something like the UC3842 family is fine, but more parts are needed, and the design is more complicatedhttp://www.ti.com/lit/an/slua143/slua143.pdf- see figure 30.The venerable TL494 could probably be used too, but using one of the purpose designed buck converter chips is much easier.Many solutions will have low efficiency - 40% to 70%, but this may not be a problem at only 50mA of current and it is still better then the 21% efficiency of a linear regulator.Richard.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 09, 2012, 01:17:27 am
content: Oh wow. There is a huge price difference. The creme de la creme LTC3642 is $7 NZD compared to the LM2574 at $3.5 while the MC34063 is only $1 (Based on just buying one from all the usual companies).But the LM2574 looks a really nice package. Has 500ma output and can cope with a wide range input, has several less components then my current setup. Oh what to do!But what will it do when my MCU goes into Standby and only draws uAmps. I really need to figure out a better way to prototype all these components. How I forgot how much R&D actually cost.Thanks for your help and now to test out this new and improved circuit.Jfp

author: amspire
author_rank: Super Contributor
timestamp: May 09, 2012, 02:19:28 am
content: Quote from: johnnyfp on May 09, 2012, 01:17:27 amThis is where the right choice of regulator and components helps.Some regulators have a low power mode of operation specifically designed for a microprocessor in a low power mode.If not, you want to look at the power actually drawn by the regulator chip itself. That will be the minimum load on the power supply.Now how does the regulator cope with microamps of load? This comes down to getting the inductor and output cap to work well together. Say your regulator has a minimum ON time of 1uS and you have a 150uH inductor. Then if the regulator turns on for 1 minimum pulse, That will be a peak current of 1uS * 19V/ 150uH = 126mA. That is an energy of 1/2 I2L = 1.2 uJIf you have a 100uF cap on the output at 5V, its stored energy is 1/2 C V2= 1.2mJ. To charge the cap to 5.05V would need an additional 26uJ of energy.So what this means is that if the regulator turns on for 20 cycles at minimum pulse width and then turns off until the output volts drops below 5V again, the output volts will only rise by less then 50mV when the micro is only drawing microamps. The principal here is all the energy in the inductor always ends up in ther the load or the output capacitor. If the micro is drawing essentially zero load, then all the inductor energy is added to the capacitor.The number of cycles the regulator will turn on for will be limited by the speed of the feedback circuit - the slower the speed, the more cycles it will turn on for. The point is as long as you have a regulator that never turns on when the output voltage is over 5V, then the regulator will be able to supply a 5V supply at microamps without a problem. I think most regulators will work fine like this, and so as long as the regulators quiescent current is low, they will put very little drain on the 24V.If you pick a lower speed regulator, you can try repeating these calculations to see what might happen.In choosing the regulator, if it is a one off design, the Linear Technology regulator may be a good choice as it performs absolutely brilliantly and it will just work first time. If you are looking for a design for manufacturing, then it is worth doing the parametric searches and doing the hard slog through all the possible chips to find one that has the combination of high input volts, very low quiescent current, minimum part count and great price. With enough looking, you will probably find something around $1 that is a far better choice then the MC34063. The danger with the parametric searches is that IC's is that it is hard to tie all IC's down to simple parameters, so a parametric search filter can easily accidentally eliminate good IC's.Richard.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 09, 2012, 04:08:08 am
content: Looking at the LM2574 DS, I can see that it tells me that I need to design for Heavy Load or for Light load, but doesn't seem to indicate a mix of Heavy and Light load.I would Ideally like to make a circuit that could adapt depending on needs, So for example the same circuit is used as a input control which would at most require 5ma of juice. But in another instance the circuit would be an output type system where it would need to power 300ma.So to make this more efficient it seems that I would need two Switch Modes circuits designed into it so that if there is a high load then I would use one size Inductor and when there is light load I would use a different sized inductor.Seems a bit of an overkill really.As for the Parameter Selection option, Unfortunately this lead me into my current predicament. But I will try again, I've often noticed that the data with these parameter selection option sometimes does not match what's in the DS.Jfp

author: JimmyM
author_rank: Regular Contributor
timestamp: May 09, 2012, 04:45:00 am
content: Before you go re-designing anything. Check the peak current in your inductor. I was building a boost converter and it squealed like crazy under load. I was using a "best guess" inductor from my parts bin and it was saturating. Once I got the right inductor in the mail, it shut up and things also ran a LOT cooler (1.25A output current). Your 50mA current may not be enough to notice a dT, but double check your inductor calculations. Your switcher chip is a proven chip and should easily handle your demands. It is most likely a compensation issue or inductor selection that's causing noise.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 09, 2012, 04:50:49 am
content: I'm using a Panasonic 220uH Choke rated at 410ma with a Resonant Frequency of 100kHzhttp://www.farnell.com/datasheets/1504547.pdfBeen using this make for my LED driving circuit and had not had any issues with them until now.Jfp

author: amspire
author_rank: Super Contributor
timestamp: May 09, 2012, 05:16:03 am
content: Quote from: johnnyfp on May 09, 2012, 04:50:49 am100KHz is the test frequency - not the resonant frequency. At 220uH and 50mA peak output, the minimum switching frequency I would like to use is 150KHz and I would prefer to use 500KHz or more. It sounds far too low for the MC34063 or the LM2574. You are going to need 680uH or more if you use the LM2574. Also, the LM2574 will probably draw at least 5mA when the micro is in microamp mode. Are you concerned about low power efficiency at all?Richard.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 09, 2012, 05:34:40 am
content: I am, very much so. If I have 80 of these all drawing 5ma (is that at 24v or 5v?) 0.005*5*80 that's 2W at 5v or 9.6W at 24v.That's potentially 230W/day * 365 = 84units of electricity. Which means I am wasting $17.64 a Year on a PSU doing nothing!!!Ok. I admit, I am fussing over not very much here, but it's the principle of the thing! I want to use that $17.64 to buy a pack of beer! (Which BTW I would owe you one as well).Joking aside, the import thing is to eliminate the whining, as these would be in bedrooms, more than the efficiency, but not to eliminate efficiency so badly to start looking at LDO's again (Which BTW until I started doing all this I never knew were quite so in efficient. I'm going to have to redesign some of my other circuits now as well because of that fact).Jfp

author: amspire
author_rank: Super Contributor
timestamp: May 09, 2012, 06:08:08 am
content: Quote from: johnnyfp on May 09, 2012, 05:34:40 am5mA or more at 24V. The specs are a bit vague.There definitely are regulators that have low power modes, but the majority of these will be lower input voltage regulators. 24V input greatly reduces the choices.Look at the lt3970 (http://cds.linear.com/docs/Datasheet/3970fb.pdf) or the LT3990.These regulators can drop down to microamps of current at very low powers. And they will work fine with your 220uH inductors.This is good valuehttp://ww1.microchip.com/downloads/en/DeviceDoc/25004A.pdfRichard

author: johnnyfp
author_rank: Frequent Contributor
timestamp: May 23, 2012, 11:58:34 pm
content: Ok I went for the LM2574. Now it's all quite and working great. And on top of that I have one less component than before.It just goes to show that putting a circuit together is more complicated than just slapping a few bit together and pray.Anyway thank you for your help.Jfp.