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TPS53313: forced ccm operation

VICTOR VICTOR
Intellectual 290 points
Part Number: TPS53313
Other Parts Discussed in Thread: TINA-TI

hello!  i want to set TPS53313 to operate in forced ccm mode.  i have some questions regarding this mode:

1) is there any minimum load requirements?

2) the current of the inductor starts at zero for each cycle no matter what the load is?

3)at light load or even no load ,  if the Duty cylce remains the same as for heavy load as i understand, where do the energy goes if the output capacitor is already charged ?

4) can you show the signals of gate switch and inductor current at high and low load ?


thank you very much for your help !

  • 0
    TI__Guru**** 191445 points

    Victor,

    1. For FCCM, there is no minimum load requirement.

    2.  The inductor current does not start at zero except in the case that the load current is equal to one half the peak to peak inductor current.  For each cycle the inductor current starts at Iload minus one half the peak to peak inductor current.  The peak to peak inductor current is;

    ILp-p = (Vout/Vin)*(Vin-Vout)/(Lout*Fsw)

    3. The duty cycle ideally stays constant,  It does vary somewhat with load due to higher losses at higher loads.  The currents just circulate and are dissipated as RMS losses.

    4. I do not have any waveforms available.  TPS53313 has integrated internal FETs, so the gate drive signal is not accessible.  I would recommend you download the TINA-TI or PSPICE transient models available here:

    You can examine the waveforms under different load conditions.  If I have an EVM available, I may be able to take some, but it will not be until next week.  In any case, I think you may find using the simulation models to be educational.

  • 0 in reply to JohnTucker
    Intellectual 290 points

    Hey john, thank you for your quick reply.

    When the load is less than half peak to peak of the inductor current, in other words , at "dcm" mode, the switching should decrease the duty cycle, but at the forced ccm, the duty cycle remains the same( for constant vin) . So, the output capcitor will be charged more and more ..how does it possible?

  • 0 in reply to JohnTucker
    Intellectual 290 points

    Hey john, thank you for your quick reply.

    When the load is less than half peak to peak of the inductor current, in other words , at "dcm" mode, the switching should decrease the duty cycle, but at the forced ccm, the duty cycle remains the same( for constant vin) . So, the output capcitor will be charged more and more ..how does it possible?

  • 0 in reply to VICTOR VICTOR
    TI__Guru**** 191445 points

    Victor,

    During the high side on time, when the load current is zero, the inductor current is negative for half of the on  time and positive for half of the on time.  When the current is negative it is gong into the SW node and of the VIN node, charging up the input capacitor, when the current reverses polarity, that same amount of charge is supplied by the input capacitor.  The net DC effect is zero, but you will see the AC ripple voltage from the charge and discharge.