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Preventing UCC25600 from entering burst mode

John Stevens
Expert 7395 points
Other Parts Discussed in Thread: UCC25600

Team,

Is there a way to prevent UCC25600 from entering burst mode, even at low loads? Can I adjust the RT to some min/max value to prevent it. I'm considering this device for an audio power amp supply design but can't use it if it enters burst mode.

Regards,

John

  • 0
    TI__Mastermind 25570 points

    Hello John

    Section 7.4.1 of the Data Sheet advises that burst mode can be avoided by limiting the maximum switching frequency to less than 350kHz.. This may be done - as you note - by choice of the resistors at the RT pin. A resistor to ground sets the minimum frequency, a resistor in series with the feedback optocoupler limits the maximum frequency. Figure 2 shows the frequency vs RT current characteristics.

    Note that the reason the UCC25600 is provided with a burst mode is to prevent the output voltage increasing at light / no load. If you defeat burst mode then you may find that Vo increases at light / no load. There are some countermeasures you can take to minimise this effect - design the resonant tank for a minimum gain at light/no load using the excel calculator. Increase the dead time you provide. These should reduce the load level at which Vout would start to increase but you may have difficulty at no load - if this is so then adding a preload is probably the only solution.

    Light / No load management is always difficult and burst mode operation is the usual solution.

    The gain / frequency equation has been corrected in the latest version of the datasheet - please double the version you are using is the one currently on the TI website - SLUS846C – SEPTEMBER 2008 – REVISED JUNE 2015. the Excel Calculator for this product uses the corrected equation too. More information is also available at https://www.ti.com/seclit/ml/slup263/slup263.pdf

    Please note that all the above papers use a First Harmonic Approximation method to calculate the gain / frequency characteristic. FHI is a good approximation at and near to LLC resonance but becomes more inaccurate away from resonance - and light /no load operation will be well away from resonance. An alternative approach which is more accurate can be found in http://www.ti.com/lit/an/slua733/slua733.pdf  (be aware that the font used doesn't clearly distinguish I from L - sorry)

    Let me know if you need any more information

    Colin