TOPXX Jitter

Hi SMPS077,

Thank you very much for considering Power Integrations product/s. With respect to your inquiry there is no way that the jitter frequency can be stopped.  The jitter function and frequency were included to basically address EMI requirement and EMI filter cost. Neither the jitter frequency can be changed. About the buzz sound, the TOPSwitch family doesn't switch below 30kHz as can be seen on the graph (kindly refer to page seven of TOPSwitch-HX datasheet and page 4 of TOPSwitch-JX datasheet). It is modulated at 240Hz typical but switching frequency is normally at 66kHz and 132kHz (with +/- 2.5kHz and +/- 5Khz respectively for jitter). If it is switching on the audible range then audible noise will be heard definitely. However that is not the case, switching is still far beyond the audible range. Am not pretty sure if that's what you mean about the buzz from a linear power supply but we hope it helps. Should you still have some inquiries please feel free to write us. In addition below are the links for the TOPSwitch family for your reference. Thank you very much.

https://ac-dc.power.com/sites/default/files/product-docs/topswitch-hx_family_datasheet.pdf

https://ac-dc.power.com/sites/default/files/product-docs/topswitch-jx_family_datasheet.pdf

Hi. Thanks very much for your reply. I believe that what is happening is different from what you are thinking.

Suppose for sake of discussion the switching is a sine wave instead of a pulse train. Your chip is applying 250Hz jitter to a 132kHz sine wave. This is Frequency Modulation (standard FM). This modulated carrier, obviously, exists on the secondaries at whatever amplitude is made by the transformer turns ratios. But, the topology of the rectification part of the typical design is similar to, but not exactly the same as, a number of standard FM detectors. This means that the 250Hz is essentially detected (though poorly) and appears at the output of the low pass filters in the PS.

This happens even with a pulse train, except that the detected 250Hz is uglier than what comes from a sine wave.

A diode rectified linear supply at 60Hz has its fundamental harmonic at 120Hz, from the capacitor charging pulses in the diodes. The second harmonic is 240Hz. Your 250Hz is nearly identical to this frequency and it sounds a lot like the buzz from a linear supply rectified from 60Hz and filtered with capacitors.

So, I had hoped to turn off the jitter (FM) to eliminate the buzz in the low audio spectrum.

But, I think I have to find another chip, although the supply works perfectly in all other respects.

Surely, you guys know about this? Or perhaps, my analysis is not correct in its conclusion that the jitter is responsible for 250Hz at the output of the supplies, independent of the actual switching frequency.

Thanks again.

Hi smps077,

We do understand that this is close to 240Hz. However, this was qualified carefully and accordingly. As you know there are parameters like ripple, transient, switching losses, and specifically EMI that were fully tested before such jitter frequency was chosen. How it was done and chosen however is a company confidential information which we cannot share or divulge. Now, turning off the jitter will again worsen your EMI result. Resulting in additional cost to your EMI solution. If we say the jitter frequency reflects on the secondary that is alright as long the requirements are being met. I am not sure why you specifically would like to turn off the jitter. Am assuming there is some failing parameters in your power supply design? say ripple maybe? Should you decide to use again our product we will be grateful to support you. Thank you very much.