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UCC28950 syn rectifier operation during burst mode

ISO Chung
Prodigy 40 points

With  DCM Pin tied to ground forcing syn. rect.  to operate, will syn. rect. output, outE and outF be disabled by internal circuitry during burst mode operation?



  • 0
    TI__Mastermind 25570 points
    DCM is used to increase system efficiency at light loads where the switching losses in the Synchronous Rectifiers become greater than the conduction losses in the diodes. In DCM mode the OUTE and OUTF outputs will both be set to 0V. I'm traveling this week and won't have access to a lab until mid next week, I'll check system behaviour in burst mode with DCM turned off then but I would recommend that the DCM feature is used.
    Regards
    Colin Gillmor
  • 0
    Prodigy 40 points
    Colin,

    Thanks for your reply.

    I am working on a power supply with various output ranging from 1V to 8V. I wish to turn on the synchronous rectifiers at whatever output when the converter is running at CCM. That's why I take an approach to switch on synchronous rectifiers whenever the DC current reaches a predefined level at which the converter is running at CCM . If OutE and OutF are not disabled during the burst mode operation. Then the power supply may run at DCM mode which might be a problem.

    Regards,
    ISO Chung
  • 0 in reply to ISO Chung
    TI__Mastermind 25570 points
    Hello ISO
    DCM mode is described in the DS in the section entitled 'Dynamic SR ON/OFF Control (DCM Mode)' on page 22.

    If the DCM is tied to VREF the SR outputs, OUTE and OUTF are disabled, both in normal and in burst modes. Output rectification is always by diodes. The output inductor current may be continuous or dis-continuous depending on the load current because the diodes prevent the inductor current going negative.

    If the DCM is tied to GND the SR outputs, OUTE and OUTF are enabled ALWAYS. Output rectification is by SR and the output inductor current will always be continuous. The SRs will allow the inductor ripple current to alternately swing positive and negative even though the average inductor current may be close to zero. This means that the input to output transfer function does not change at very light loads and the burst mode of operation is not triggered. Remember that burst mode is triggered if the Ton falls below Tmin, ie the duty cycle becomes quite small - this can happen only if the inductor current goes discontinuous. This mode of operation is likely to be lossy because the SRs require some current to charge and discharge the gates and the negative going inductor current means that energy is being transferred from output back to input for part of the switching cycle.
    Having said that, this mode of operation isn't actually hazardous to the power train - unless the system enters DCM while the inductor current is negative. If this happens then the energy in the inductor will generate a significant voltage spike on the SRs (now acting as diodes) and may damage them.

    Normally, a designer chooses a current level at which the system should enter DCM - this will depend on the ripple current in the output inductor as a percentage of full load - ideally the system should enter DCM just as the minimum of the inductor ripple current reaches zero.

    Of course, you always have the option to use an external SR driver IC This may make sense for the very low output voltages.

    Regards
    Colin