UCC28019A: Audible noise heard at no load

This is not a normal behavior . The burst rate is very high for no load and there must be somewhere this extra energy is going. Normally at no load you will get a very low duty cycle with a low repetition rate. For some cycle of the AC there will be no switching at all. Because the energy delivered at no load is so small there will be no audio noise. So I think something else is happening here.
Please check some other waveforms.
(1) Is the output voltage being regulated ?
(2) What does the input current look like?
Perhaps there is a mechanical issue with a component not soldered properly or a voltage rating on the capacitors or FET, diode.

Regards
John

Hi, 

Please zoom in the PWM pulse at no load, if the duty is not narrow, then add a 1.5Mohm resistor between VCC and ICOMP pin, to see if the audible noise goes away. If you can still hear audible noise, then reduce the resistance, but keep it greater than 1Mohm. 

Note the PWM on/off is called burst mode, this is normal because there is no load.

Regards,

Bosheng Sun

 

Hi Bosheng,

Thanks very much adding 1.5Mohm the audible noise disappear.

Will this solution changes my circuit charastric?

thanks!

This resistor will add some offset on ICOMP, it may affect the THD a little bit. However, 1.5Mohm is big value, the adding offset is very small, in most case, you won't see THD change.

Regards,

Bosheng

Bosheng,

Is it a general practice to add 1.5M ohm between VCC and ICOMP ?

And also how do you look at this, same PCB 2 pcs out of 250 pcs got this symptom, do you think the IC is damaged? or it is simply IC variation the IC itself still is a good one (if adding 1.5M ohm solve the problem)?

Thanks!