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UCC28780: About BUR Hysteresis Circuitry to Eliminate ABMAAM Transition (Audible Noise)

Part Number: UCC28780

I designed a 200W power supply using the UCC28780 for audio amplifiers.

I suffer from audible noise from transformer. 

This is varnished transformer.

Since it is used for audio, I want to reduce the noise generated from the transformer as much as possible.

I found the following attachment, is it valid?

Are there any side effects?

BUR hysteresis circuitry to Eliminate ABM-AAM transition (Audible Noise) .pdf

  • Hello Tak, 

    I think for a 200-W supply, even a varnished transformer may be susceptible to noise from the ABM-to-AAM transition, due to the higher peak currents (compared to the power level in the application note).  

    The hysteresis circuit described in the application note that you attached is valid. We have successfully applied this in several ACF designs. 
    The concept is to use a micro-power comparator with open-drain output to toggle a third BUR resistor in or out depending on the average voltage of the RUN pin. 

    When operating in AAM, RUN is always high so its average voltage is 5V.  The comparator inputs are polarized in a way that the output is low so that Rbur3 is normally in parallel with Rbur2.  This sets the ABM threshold voltage.   When the load drops to the ABM threshold set by Rbur1 and Rbur2||Rbur3, RUN starts to have low intervals which drops its average voltage.  This average is compared to a reference voltage either internal or external (such as resistor divider from REF).
    When the comparator removes Rbur3 from Rbur2, the BUR voltage increases and also increases the peak current which drives the loop deeper into ABM. This is positive hysteresis and helps to skip over the random transition region between AAM and ABM and avoids the associated noise.  

    The reverse happens as load increases and operation moves from ABM to AAM, again skipping over the transition region.

    The design example in the app-note is geared toward USB-PD applications, so the component values are probably not suited for your application.  However the design principle is the same so it should be straight forward to calculate your initial values.  Optimization may involve some iteration during prototype evaluation. 

    Regards,

    Ulrich

    .  

  • Thank you for great tips.
    I understood in detail.
    I'll evaluate this circuitry next step.

    B.R. Tak