Flyback SMPS - getting noise at different loads

[narayana]

Hi,
I designed an SMPS using fly back configuration. it's in put is 230V ac supply.out put is 24V dc and 4.2 A.

We are getting good regulation with ripple voltage of 50mV around.
but i am getting noise at different loads.we are not getting any noise at full load.but from no load to 0.5 A noise is coming and also at 2.0A load also it is coming.

Can any body help me to sort out this problem?

 

[FvM]

Re: Flyback SMPS Noise

As a first point, you should understand how the SMPS controller handles load variations. There are different methods that
can result in very different behaviour. If it's a standard current mode controller (e.g. UC344x type) without special functions
for partial load, the loop gain variation between continuous (CCM) and discontinuous mode (DCM) must be expected
to cause instabilities. In this case, the loop compensation has to be adjusted.

 

[narayana]

Re: Flyback SMPS Noise

Hi thanks for your reply..
here it is operating in DCM only.it is not going in to continuous conduction mode only..i am using quasi resonant controller UCC28600 from texas instruments...

could suggest some thing for this loop compensation.

 

[dick_freebird]

Flyback SMPS Noise

It would help if you explained the nature of the noise.
Audible (magnetistriction) might come on if you fall into
subharmonic oscillation (changing your inaudibly-high
oscillator frequency to something N times lower). If your
discontinous-mode scheme is frequency-variable or a
pulse-skipping mode, same end result might be had. Output
voltage noise can result if the loop is unstable at light loads,
a not-uncommon thing (load pole moves in, when Zload
increases).

A 'scope picture would help a lot.

 

[FvM]

Re: Flyback SMPS Noise

For loads that are between approximately 30% and 10% full rated power, the converter operates in frequency foldback mode (FFM),
where the peak switch current is constant and the output voltage is regulated by modulating the switching frequency for a given and fixed
VIN. Effectively, operation in FFM results in the application of constant volt-seconds to the flyback transformer each switching cycle.
Voltage regulation in FFM is achieved by varying the switching frequency in the range from 130 kHz to 40 kHz. For extremely light loads
(below approximately 10% full rated power), the converter is controlled using bursts of 40-kHz pulses.

I would expect, that a loop compensation, that works for full load does not necessarily for 40-130 kHz. It may need adjustment.