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UCC25702: Current protection and soft-stop cycle

Baudouin Bout de Marnhac
Prodigy 60 points
Part Number: UCC25702

Dear Sir or Madam,

I'm currently working on a PFC using UCC25702 and we observe during tests behaviours that we do not manage to link to our understanding of the data-sheet. 

Please find hereafter some questions that will help us have a better understanding :

 

What is the reaction time on cycle by cycle current protection when Ilim reaches 0.2V ? Is there a blanking time before this protection is active ? (minimum duty cycle)

Is there a possibility for the controller to reach a current protection which activates a non-latched soft-stop cycle ?

When Ilim reaches 0.6V, should the OUT pin cut off instantly and remain like this or does this initiate a soft-stop cycle ?

Thanks for your answers,

Baudouin de Marnhac

  • 0
    TI__Mastermind 31580 points
    HI Baudouin

    I can see no delay in the block diagram on the cycle by cycle current limit, is there some waveforms you can share to show the issue?

    From the datasheet I can see the following.

    There is a COUNT feature in the controller which starts to integrate to 4V if there is successive voltage levels on the CS pin above 200mV. If the COUNT ramps to 4V then there is a latched soft stop cycle.

    An voltage level over 600mV on the ILIM pin results is a latched soft stop

    The information on the soft stop cycle is

    At High Line or Low Line fault conditions, the soft start capacitor is discharged with a controlled discharge current of about 500
    mA. During the discharge time, the duty cycle of the converter is gradually decreased to zero. This soft stop feature allows the synchronous rectifiers to gradually discharge the output LC filter. An abrupt shutoff can cause the LC filter to oscillate, producing unpredictable output voltage levels.
    All other fault conditions (UVLO, VREF Low, Over Current (0.6V on ILIM) or COUNT) will cause an immediate stop of the converter.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 60 points
    Thank you Peter for your answer,
    We understood our delay when cycle by cycle current protection is activated, we had a large filtering causing this delay.
    For the stop mode, I understand with your last quote from p.5 of the datasheet that COUNT or ILIM>0.6 fault condition should cause an immediate stop of the converter and these faults are latched, but what we see are soft stops and sometimes unlatched.
    Considering the latch, do you know what could unlatch this protection although we do not have change in VDD nor VFF ?

    Regards,
    Baudouin
  • 0 in reply to Baudouin Bout de Marnhac
    TI__Mastermind 31580 points
    Hi Baudouin,

    Looking at the block diagram in the datasheet there are two inputs that can reset the fault latch and its Vdd and Vff as you stated already.

    Have you verified that the COUNT is exceeding 4V or that ILIM is over 0.6V ? Are you testing the device in an actual power stage or in an open loop test board?

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 60 points
    We effectively verified that ILIM rises but couldn't precisely measure it (we make the measurement on a board in an stove). It is an actual power stage, we do not have dedicated measure points. We assume ILIM rised to 0.6V since the count pin voltage first rises and is then pulled down to zero before reaching 4V. We might have some noise on Vff pin that could make the converter restart but this voltage on our board was chosen constant equal to half Vref with resistor divider bridge next to the pins.
    Anyway, we had the converter stop although it shouldn't, now that we understand why we know that the converter works properly in every specified condition.

    Thanks Peter,
    Have a nice day !