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UCD3138PSFBEVM-027: Behaviour of gate signal waveforms not matching

Part Number: UCD3138PSFBEVM-027


I am testing the UCD3138PSFBEVM-027 and have few queries regarding it:

1. The delta between no load and full load waveforms given in SLUUAK4  (Figure 9 and Figure 10) is very less.

Also we are unable to see proper phase shift on the DSO with the change of load, as can be seen in the operational waveform of PSFB (Figure 24).

2. The behavior of tested gate signals of SR mos (red and green) seems different at no load or very light load as compared to the waveforms given in SLUUAK4  (Figure 9)


  • The most important question is - is it regulating?  Also, what do the signals actually look like?  What is your full load current?  

    I'm assuming that the scope shot you show is your signals.  It looks like the sync FETs are disabled?

  • Hello,

    1. sync FETs are disabled on lower loads and turns on or starts working at higher load around 5A (Red and Green signal). 

    2. The full load capability of EVM is 30A. We have tested it till 28A. The evm is working/regulating (voltage, current and power) fine in terms of values but we wanted to verify the waveforms and understand its working so that we can modify it according to our requirement.

  • Hello,

    If the SR keep turn on at light load, the converter work with CCM current. The delta between no load and full load will be very less. If the SR turn off at light load, the converter work with DCM current, the change of duty will be obvious.  Both are workable.

  • Hello,

    I have used sluc614 firmware provided for UCD3138PSFBEVM-027 from TI. Based on our observation and the above explanation, It is going to DCM mode during light load and CCM mode on higher load.

    But according to the waveforms given in SLUUAK4  (Figure 9 and Figure 10), device is throughout in the CCM mode (in light or no load as well as full load)

    Kindly help me out with justification of this observation.


  • Hello,

    Whether the SR enable or disable depends on what the threshold configured by firmware. The SLUUAK4 had been released in 2013. The code you downloaded may have been refreshed since then.