# How to remedy audible noise from boost driver



## snarfer (Jul 30, 2008)

Well this is kind of a long shot, but I've been working on some unusually high voltage LED drivers. 72 volt out at 1000 mA actually. So far they work excellent. I measure 93.3% efficiency and they don't seem to get too hot. 

However I'm having a little problem with noise. Whenever I try to apply any kind of PWM dimming signal I get this annoying high pitched hissing sound. The drivers run at 162 kHz right now, and I've tried PWM signal ranging from 15 kHz all the way to 2 MHz. I've even tried dithering the frequency, which actually helped a little bit. However the noise is still there. When I try analog dimming there is no noise at all, but that doesn't do me much good, as I need to control these things with an MCU. 

I wonder if anyone has encountered a problem like this before and what some general approaches one would take to solve it would be.


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## tonycollinet2 (Jul 30, 2008)

The noise is almost certainly coming from your inductor. Can you pot it - or at a pinch gunk it up with hot melt glue?


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## snarfer (Jul 30, 2008)

Really? I tried the extremely low-tech and not recommended technique of sticking my finger on the inductor and that didn't seem to make any difference at all. At least I found out that it is not getting very hot.

I guess potting would be a lot more effective though. I will try hot glue when I have a chance to get some. Potting it is something I might do in future, but it would kind of make it hard to keep working on it right now.

The other obvious thing to do would be to eliminate the PWM and just use a digital potentiometer instead. Seems like the response is nonlinear though so it might not work very well. And 1024 step digital potentiometers are expensive.


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## VanIsleDSM (Jul 30, 2008)

Potting will help make the noise less audible, but it's not going to fix the inductor resonating.

When using PWM the switching frequency of the driver should be at least 10X that of the PWM fequency.. so 2Mhz for the PWM is way to high. 15Khz is about right.. but I'd try dropping it down a little more even to see what happens.

More details about the circuit would help. 93% efficiency is pretty low for a high voltage driver with a low switching frequency, what is the supply voltage for the driver?


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## snarfer (Jul 31, 2008)

Supply voltage is 24 volt. Based on National application note AN-1696. The reference design is only 90% efficient at 300 kHz, so I think 93% was pretty good at 162 kHz. Mostly the loss is in the very small inductor.

I wish National would publish a Spice model for the LM5022 part. It would really help a lot. I think 95 or even 96% is possible if I could do an optimization in ASCO. Right now I am just going by the reference design numbers for slope compensation and so forth, but ASCO usually finds better solutions. 

I will try reducing PWM speed to 10 kHz but it means I have to change my setup a bit. For some reason the MCU I'm using, the dsPIC30F1010, doesn't seem to want to output PWM when I reduce the oscillator frequency. As it is set up now the max period is 66 uS which equates to 15 kHz.

I was actually thinking that the noise is not coming from the inductor, but from the capacitors. There are five ceramic capacitors, three on input, two on output, 4.7 uF/100v each. All of the capacitors are ceramic actually. I was just reading this:

http://www.kemet.com/kemet/web/homepage/kfbk3.nsf/vaFeedbackFAQ/242F5F2E69DCEC7485256EDF004CA495

This is kind of a discouraging note about the piezoelectric effect and ceramic capacitors. It says piezoelectric noise might occur at 3 - 30 kHz, which seems a lot like what I'm hearing. Tantalum is way too expensive in these values, and with aluminum it would be a whole different design.

After some more experimentation with my thumb on the inductor, I think I might have been wrong in my earlier post. There might be some capacitor noise, but there is certainly inductor noise as well, if not predominantly.


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## snarfer (Jul 31, 2008)

Another theory I have is that maybe the PWM signal is interfering with the slope compensation loop. Dimming by increasing the gain of the current sense circuit doesn't introduce noise because it doesn't change the frequency of the converter, unlike adding a PWM signal.


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## SemiMan (Jul 31, 2008)

Why are you trying to PWM at such high frequencies? 1-2KHz is more than adequate such that your eyes will not see any blinking from the PWM. Given you have a 160KHz switching frequency and I am assuming have at least a bit of a soft-start, the switcher is unlikely to be able to respond to anything near a 15KHz PWM properly. Your ears are not overly sensitive to 15KHz, but you can hear it a bit. Who know what potential subharmonics are happening though. It may not even be properly responding to every PWM pulse. I would work to get your PWM frequency down into the 1-2KHz range. You can have short pulses, but keep the actual frequency low. If you want a high PWM frequency, then you may want to get your switching frequency up.

93% is relatively good for a 24V in, 72 out is quite good. Is this battery operated?

Semiman


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## SemiMan (Jul 31, 2008)

Keep in mind you always have a analog DAC if you have a PWM output from a microntroller. All you need to do is run it into and RC and you have your variable analog output which you could work into your feedback loop.

Semiman


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## snarfer (Jul 31, 2008)

Yes I tried using PWM through an RC for a bias voltage to the feedback circuit. I even tried using PWM fed to a small transistor and resistor and creating the bias voltage that way. Same awful noise.

I thought 93% efficiency was pretty good too. I don't know of any commercially available product that is over 90% efficient, or that could even supply 72 volts at 1 amp from a 24 volt input. And this is less than 4 square inches of PCB too. It should work all the way from 12-48 volts input actually, although I haven't fully tested it yet.

Actually I have some ideas that could make it even more efficient. Some of my models were showing 95 and 96 percent efficiency. I may also transition to a different controller, from a company that does supply simulation models... (I'm addicted to simulation models now)

This is for supplying current to a high power LED array for use on or next to a motion picture camera. So it needs to be able to run off of battery or a power supply equally well. And it needs to quiet down too!!

I really don't want to use PWM in the 1000 Hz range because maybe a camera running at high speed with minimal shutter angle might see some flicker. Also the LEDs I intend to run with it have minimal color shift with analog dimming anyway. So I am just trying to control the current as much as possible.

I am pretty much resigned to putting a digital potentiometer in line with the current sense amplification circuit at this point. Looks like the ceramic capacitors I'm using are some tricky little buggers so better just do whatever keeps them happy and quiet.

Anyway using a digital pot will be nice because now the MCU won't have to generate any PWM at all, just signals on the I2C bus. So it's an extra 2 dollar part. The thing is already expensive!

By the way the genesis of this project was a suggestion a few months ago in this forum to look up the previously mentioned application note. So thanks everyone who responded, couldn't have gotten this far without your help!


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## snarfer (Jul 31, 2008)

Looks like I was correct about the noise coming from the capacitors. I received the following information from National support desk:

"MLCC capacitors are known to have a piezoelectric effect that causes mechanical deformation within the capacitor that may create vertical vibration on the printed circuit board. To avoid this effect, you can replace the capacitor with a non-MLCC ceramic capacitor or Tantalum. "

Unfortunately in this case there is no non-MLCC alternative with similar capacitance and ESR, not to mention case size and price. Optimizing the loop compensation and using a digital pot are the only solution I can see.


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## SemiMan (Jul 31, 2008)

Well if National is like any of the other semi vendors (I am not pointing them out), then the response you just received is just slightly better than the one you would have received from a trained monkey.... ;-)

Can you qualify the level of noise? ...any idea what the frequency is?...actually what would be highly useful would be scope traces on the output.

Did you do a custom circuit board for this? Can you post a picture?

When you do PWM, does the dimming at least work?

Given you are using a movie camera, PWM is likely not best. It is difficult to do at those speeds with a switcher. I just realized you said "big capacitors"..... you can never ever PWM fast with big output capacitors... it does not work. The caps will never discharge fast enough, i.e. you are never really turning on the LEDS. Maybe the caps are "singing" ..would need a nasty waveform to get a reasonable audible waveform.

Semiman


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## Oznog (Jul 31, 2008)

Most likely you have designed your circuit wrong... post full schematic plz.

What part is the controller?
Did you configure for fixed on-time or constant freq? Fixed on-time is REQUIRED when buck ratio is over 50% or it will become unstable.
What is your ripple factor?

The HV9910 for example requires a ripple of about 15% or it will skip some cycles unless you use a 300ns RC filter on the CS line. This can drop down the working freq into the audible range and trying to up the freq will have no effect.


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## snarfer (Jul 31, 2008)

Oznog: This is interesting information regarding the HV9910. I have never used that part before. However this is not a buck circuit. It is a boost circuit, so I'm not sure how to interpret your comment. Maybe you did not completely read the thread before you posted. I have been guilty of this myself on a number of occasions too.

Semiman: Yes it's a custom PCB, but very very similar in design to the PCB in the National app note 1696. 2 oz. copper, 1.6mm FR4. LM5022 controller.

It is difficult to quantify the level of noise or the frequency. A couple of scope shots with and without noise:

http://picasaweb.google.com/stealing.thunder/ScopeShots?authkey=4TJynlB61lE

The one with the spike is actually the one with no audible noise. Note that the voltages all need to be multiplied by ten.

To me it looks like the noise actually is caused by the slope compensation getting out of wack. 

I reconfigured the circuit for analog dimming and the noise is almost completely gone, although I am still a little concerned about some of the high frequency transients. For the first shot, with no audible noise, I waited for a nice spike to show up before I hit the stop button. Notice that although there is that big spike, there is generally a lot less ripple.

The PWM dimming does actually work, and, strangely enough, it also frequency dependent. The lower the frequency, the dimmer it gets.

Total output capacitance is 9.4 uF. I would add a 100nF 1206, but I accidentally put the 0805 footprint in there instead so it will have to wait until I order the next PCBs.

Well anyway I am satisfied with my digital potentiometer solution for now. 

For the production model I will have two drivers and an MCU with DMX/RDM and analog inputs on one PCB. 

Also I have been trying to find some alternative inductors. I am currently using two 12mm toroids that I wound by hand, but I would like to see if I could get better performance from ferrite with a flat coil, or even planar.


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## VanIsleDSM (Jul 31, 2008)

93% is good for a boost circuit upping voltage that much. I thought you were using a buck driver.. in which case you could achieve ~98% with the right input voltage.


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