PWM Mdimming of switch mode led drivers gives whining sound

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Flyback

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Hello,
Have just been running switch mode LED driver demo boards from ti.com and linear.com. I usually do Analog Dimming, so am not used to listening out for “whining” during PWM dimming.

However, all of these boards “whine” when PWM dimmed….the whining sound varies as the PWM dimming frequency is varied (kind of as you’d expect).

The feedback loop is stable in all cases.
The chips are LM3405,LT3756,LTM8040.


(LM3405 is hystetetic so is intrinsically unstable but you know what I mean)

LTM8040:
https://cds.linear.com/docs/en/datasheet/8040fa.pdf
LM3405:
https://www.ti.com/product/lm3405
LT3756:
https://cds.linear.com/docs/en/datasheet/375612fb.pdf


I’ve removed the shielded, surface mount power inductors, and connected them back into the circuit with twisted pair, so that they cannot resonate against the circuit board, but the whining is still there. (i didnt remove the inductor on the LTM8040, becuase its integrated into the chip)

Do you think it could be the ceramic caps?

I wonder if I change to unshielded through hole inductors it will get rid of the whining.

In a way, I’m not jolly well surprised that the circuit is whining, the current is being slammed on and off at an audio frequency. (that’s PWM dimming)
Any ideas to get rid of this whining?
 
You must have good hearing as the switching frequencies using the suggested component values in the data sheets are 650Khz, 1.6Mhz and 375 Khz Even when I was young I could not hear above 20 Khz. Are you sure the noise is not coming from the power supply that you are using to power them. This may happen if the input decoupling capacitor is too low in value or has a high ESR
 
Design a circuit that operates at 20kHz and produces a HIGH that varies from 2% to 100%. The operating frequency will always be above audible listening level.
 
If you are running the chips at a frequency of 200 hz to 20 khz then you need to re calculate the inductor value. As the inductor in the LTM8040 you cannot use that device at such a low frequency. As the frequency is in the audible range it is quite likely you will hear noise from the inductor.
 
It would be most helpful of Flyback would post the circuit he is using. I deleted my earlier post because of that uncertainty. I suspect he may be using this alternative circuit to get the low PWM frequencies he reports:



John
 
sorry no we are not using the external transformer, just giving a pwm signal input...BTW, the LTM8040 is switching at 500khz
 
I'm curious as to why the PWM frequency, and not (apparently) the PWM duty cycle, is being varied to dim the LEDs?
 
we're not changing the pwm dimming frequency, but the duty cycle of it. We have been doing it at different frequencies though, from 100hz to 35khz all told.....obviously less resolution available at 20khz plus, unfortunately.
 
Why are you modulating the PWM pulses (their duty cycle) at an audio frequency?? Then the PWM pulses go wide, narrow, wide, narrow at an audio frequency which is not dimming. Dimming is using constantly narrow pulses and to make the light bright you use constantly wide pulses.
 
thanks, I believe I agree with you....some led drivers cant do it at abve audio frequencies
 
Flyback said:
thanks, I believe I agree with you....some led drivers cant do it at abve audio frequencies
Click to expand...
What are you talking about, every LED driver can produce PWM at ultrasonic frequencies that nobody can hear.
PWM is a continuous ultrasonic frequency that turns the LED on then off then on then off at a frequency so high that it is quiet. Continuous wide pulses make the LED appear bright and continuous narrow pulses make the LED appear dim. You can slowly ramp up or down the pulse widths then the LED slowly brightens or slowly dims.

It sounds like you are not using PWM instead you are turning the LED on and off at an audio frequency so that it is turned on for half the time and appears half dimmed.
 
thanks, as you know, not all drivers can do pwm dim at >20khz, because eg in boost mode, with no "error amp control voltage hold" facility etc, you just cant get "brick-wall" current pulses at >20khz...they have too long rise and fall time
 
Show me only one PWM LED driver IC that boosts the voltage and cannot do PWM at >20kHz. Maybe you are using a boost converter IC to boost the voltage and are using it to PWM instead of using a boost converter IC plus a PWM IC?
Why not use the correct voltage in the first place and a normally fast PWM IC?
 
Hi,

From reading the other posts i can see that this question or whatever it is may not be that hard to answer.

To start, if any frequency is roughly 20kHz or below you may be able to hear it. It doesnt matter where it comes from: inductor, resistor, transformer, capacitor, light bulb, speaker, driveway rock. So if you can hear it, it is probably 20kHz or below.

So if you hear it, you know it is there, and that means the circuit is generating it SOMEHOW. Either you are using too low of a frequency or the circuit is using too low of a frequency or the circuit feedback allows sub harmonic regulation. Sub harmonic regulation can occur if a circuit operates at a high frequency but the load or associated circuitry has the ability to resonant, especially if at a lower frequency.

I would think you could look at the output with a scope and see what is going on. If it is not you generating the lower frequency (with your own signals) then it is the circuit, and you'll need to find out what is causing it within that circuit or load. A scope will help with this naturally.
 
thanks, we are gernerating the pwm dimming signal at audio frequency, and I suppose its no surprise that the components whine.
 
Hi,

Ha ha, well when you generate an audio frequency, you get an audio frequency

I would think it would be the inductor that creates the sound energy so replacing it with a better unit might help. Usually it's the magnetics that make the noise, especially transformers.
 
Thanks, I took the inductor off and connected it back at the end of a long twisted pair...then I listened to the board and the inductor separately..it was the board, ie the ceramic caps that were making the majority of the noise..the inductor was just a quiet swishing noise, the ceramic caps were whining like bumble bees...when I change the frequency of the dimming signal, I can play a little wee tune!
Amazing isn't it that these led driver datasheets tell nothing of this problem. of audio noise.
 
Did you ever hear the PWM frequency of an electric wheelchair, or electric car, bus or truck? Of course not because it is above 20kHz.
I hear the PWM frequency of my electric radio controlled model airplanes loudly, maybe the designer is deaf.
 
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