• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 648 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Charging Large Capacitor with UCC28950

Mark Walters
Prodigy 100 points
Other Parts Discussed in Thread: UCC28950, UCC28070

We are building a 2KW unit to charge a large capacitor bank to the 250-350V range. The system uses UCC28070 for PFC in the first stage followed by UCC28950. The design has about 3700uF recommended for COUT on the converter and the capacitor bank to charge is in the 15000uF to 25000uF range. We are also using standard rectifiers on the output vs. synchronous rectification.  A couple of our concerns are below:

1.  The large capacitor bank is going cause an excessive current spike on startup.  My thoughts are that the input current limiting on both of the stages will control this spike.  Is this an expected action by the system?

2.  The output capacitor bank will also likely exceed the recommended output capacitor ESR ratings.  Will this be acceptable or should I have an diode on the output to isolate the bank from the output components?  Other methods?

Thanks,

Mark.

  • 0
    TI__Mastermind 25570 points

    Hi Mark

    The soft start feature on both the UCC28950 (SS/EN pin) and UCC28070 (SS pin) should control the current during the start-up phase. The boost PFC stage has an inherent inrush current when the mains is connected and the controller cannot influence this - you need to put an inrush limiting circuit in place to prevent blown fuses - or worse - .

    Re: ESR - I think you are referring to the Cout calculations in sec 8.2.2.7 of the UCC28950 DS. This limit on ESR related to load transient response and to figuring out how much output capacitance should be used. Before the control loop can react - the added transient current comes from the output capacitors and there is a voltage drop across their ESR. To be honest, I'm not sure if you really need to worry about this - you have already set the Cout at a large value.  If this isn't clear, then please send me a sketch of your circuit and I'll look at it in more detail.

    Regards

    Colin