STCMB1: transformer makes noise

[AndresGG]

Hola, ¿Me podéis ayudar con mi problema?
Cuando conecto la realimentación del optoacoplador a RFmax el transformador hace ruido, se regula correctamente la tensión pero el transformador hace ruido. He variado los valores de Rfmin y Rfmax, también los valores de los elementos ISEN_HB pero no tengo ninguna mejora.

Translated to English

Hello, can you help me with my problem?

When I connect the feedback of the optocoupler to RFmax the transformer makes noise, the voltage is correctly regulated but the transformer makes noise. I have varied the values of Rfmin and Rfmax, also the values of the ISEN_HB elements but I do not have any improvement.

[Cristiana SCARAMEL]

Hello @AndresGG and welcome to the ST Community.

For a better support, can you share more details about your power supply topology?
Are you using a PFC pre-regulator, or the Resonant stage is supplied directly?

Here below some considerations about your question:

1. In a single optocoupler configuration, RFMAX sets, in brief, the frequency at which burst mode starts.
   Unluckily, there is not a strong relationship between RFMAX and output current at burst mode threshold.
2. RFMIN sets the minimum operating frequency since it sets the minimum current always sunk out form RFMIN pin.
   Its value must usually satisfy the max output current at minimum input voltage that is the DC Brown Out.
3. At the same corner, max output current / min input voltage, R_isen must be set to have V_isen <= OCP1 threshold.
   R_isen is the sensing resistor and its value also depends on the capacitive divider used to sink the resonant current.

Previous points are not connected to the trafo noise apart from the burst mode threshold, in the sense that follows.
When in burst mode, STCMB1 does not have any feature to control the burst mode period (= packet + idle).
This means that burst mode period can be in the audible bandwidth itself or that it can stimulate a noisy frequency of the trafo.
So, the first point to be considered is if noise happens in continuous switching, in burst mode or both.

If it happens in continuous switching, then most likely the problem is in the transformer.
If it is during burst mode, then how it changes with load must be analyzed.

In case it is something related to burst mode operation, as above mentioned, there is not a specific feature to avoid it.
Feedback and trafo must be tuned, but it not usually possible to remove the audible noise (remove = cancel completely).

[AndresGG]

Hi, thanks a lot for your reply.
I managed to get the power supply to work in normal mode by increasing Rs to 40 Ohms. I use LED matrices as a load and when I increase the load the output voltage becomes unstable and the LEDs flicker.