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UCC256301: current control during soft start?

Goran Djogo
Prodigy 30 points
Part Number: UCC256301
Other Parts Discussed in Thread: TL431

We are considering an application of the resonant controller UCC256301 for a HV capacitor charger.

The capacitor charger will have LLC resonant converter topology.

 

My questions:

1) Can UCC256301 control primary current of the LLC converter during soft-start (by using hybrid hysteretic control)?

2) My understanding is that during soft start the feedback from the output capacitor voltage is not used by UCC256301 to change switching frequency.

Instead, voltage of the soft start capacitor (connected to the pin LL/SS) is used to change switching frequency (as long as soft start capacitor voltage is lower than voltage of FBreplica.

Is that correct?

3) Is there any way to model UCC256301 in SPICE? (Basically just the feature of hybrid hysteretic control)

 

Thanks,

Goran

  • 0
    TI__Guru 66065 points

    Hello,

    The UCC256301 does control the current during soft start.  However, it the opto isolator's photo diode sees excessive current during startup it will over ride soft start and demand too much current.  The ISNS pin will then go into over current protection and shut down.  As long as you use the recommended compensation from the excel design tool this should not occur.  You can find the excel design tool at the following link.  https://www.ti.com/lit/zip/sluc634 

    We do not have a Spice model for this device; however, we have a Simplis model that can be found at the following link.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    1) It seems from your answer that it is not true that soft start capacitor voltage is used instead of FBreplica voltage to determine frequency during soft start.

    Do I get this right?

    2) What can be done if feedback signal from the output capacitor does not exist, i.e. it is very impractical to realize it?

    Can capacitor charging by a LLC converter with UCC256301 controller still be done just by controlling primary current.

    Thanks,

    Goran

  • 0 in reply to Goran Djogo
    TI__Guru 66065 points

    Hello,

    If the IFB current becomes saturated it will over ride soft start.  The following application note has two circuits in figure 8 and 9 that can be used to prevent opto saturation and should fix your soft start issue.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    Can you, please, answer the second question from my previous message?

    Thanks,

    Goran

  • 0 in reply to Goran Djogo
    TI__Guru 66065 points

    Hello,

    I would not recommend trying to control the current from the primary. 

    To ensure the opto feedback works correctly.  I would recommend putting a 680 ohm resistor across the opto diode.  This ensures the TL431 has enough current to operate at lower output voltages.

    Regards,