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UCC28633: Constant Current Step-up Charger Advice

Callum Foster
Prodigy 50 points
Part Number: UCC28633
Other Parts Discussed in Thread: UCC28630, UCC24630, UCC24650

I'm trying to design a high voltage (~2kV) isolated capacitor charger based around the UCC28633 controller. I have chosen this device due to the PSR, good documentation, and apparent constant current limiting.

I am having trouble predicting how a circuit would behave in constant current mode for a sustained period of time. The primary would receive a signal stating that the desired voltage is reached via an optocoupler. I need this signal to immediately disable the charging.

A potential issue occurs if at low voltages. If the circuit feedback is designed for a constant voltage output of 2000V, but the external over-voltage circuit is set for 100V, the issue would occur. If running at constant current, when a capacitor reaches 100V, the output power of the PSU is a small fraction of the nominal power, and therefore I would expect the controller to be in low power mode. Therefore, the shutdown pin (Pin 2, for an NTC) is disabled, and I would have no way of turning off the output.

I think this wont be an issue, due to the full current flowing at the output, and therefore, large reflected currents occurring at the input. This would mean that the internal modulator is actually running at a higher demand level, and therefore, pin SD is still enabled.

My question is, am I correct? Does anyone have any advice they can offer on using this part, or recommendations for different converter topographies/controllers.

I realize this is a very open-ended question, but any assistance is greatly appreciated. Let me know if there's anything I can add in explaining what I am trying to do.

References: SLUSBW3C, SLIC015C, SLUC537D

  • 0
    TI__Intellectual 880 points

    Hey Callum,

    Thanks for choosing TI for your project!

    For the constant current question: in constant current mode the output will deliver the maximum current regardless of the voltage level. However, if VOUT droops, VDD tracks that voltage as well and could shut down the device due to the VDD voltage being below the turn off UVLO voltage.

    As for the shutdown pin: when the device is in its low power mode, the internal -pull up- is disabled, not the pin itself. You can still use a FET on this SD pin to ground to disable the device. However, note that the UCC28633 has a reset cycle and not a latching fault. Therefore, when the SD pin is below 2V, rather than shutting down the device will start UVLO cycling. If that is what you need then great, if you need a latching fault, then I would suggest going with the UCC28630 instead.

    Edit: fixed the typo, meant UCC28630

    Best regards,
    Davit

  • 0 in reply to DavitKhudaverdyan
    Prodigy 50 points
    Thanks Davit!

    I intended to power VDD both the auxiliary winding, and a bias supply that will also power a PFC stage and logic circuitry. But Ive only just thought, this may cause issues during the discharge and restart phase, as the VDD cap will never discharge. I should be able to mitigate this with a circuit that only keeps it powered when needed.

    For the shutdown pin, I have just found what you mean in Table 4 of the datasheet. This may pose an issue for me... I assume you meant the UCC28630? If so, then how would this work? The device has an overload timer. Wouldn't that mean that in constant current mode, the device would register overloaded, shutdown, and restart constantly? (my total charge period could be as long as 40 mins).

    If you were indeed referring to the UCC24630, than I am not following you.

    Thanks again Davit!
  • 0 in reply to Callum Foster
    TI__Intellectual 880 points
    Hey Callum,

    Yes, I did mean the UCC28630! Thanks for seeing that.

    Since the SD pin pull-up is disabled in low power mode, an external wake up circuit can be used to drive an alert signal and wake up the device again. This can be done by using the UCC24650, a dedicated 'wake up' device. Another method is to use an optocoupler to drive an external pullup circuit as shown in UCC28630 datasheet in section 8.3.19.

    As a side note, due to the holidays my responses may be delayed!

    Regards,
    Davit
  • 0 in reply to DavitKhudaverdyan
    Prodigy 50 points
    Hey Davit,

    I don't believe the UCC28630 is suitable due to the overload timer. It seems like the maximum I can get is 1000ms in CC mode, before the device shuts down. This is not suitable.

    I will look into adding a wake-up signal. The UCC24650 isn't suitable due to the voltages I am working at (similar design to PMP20899), unless I can put it on a smaller auxiliary winding, but I may be able to replicate the effect with a simple high voltage MOSFET.

    I think in the meantime, I will design/build the bias supply, which will also be based around the UCC28633. This will function as my development board, so I can get my head around some of the cool features of these controllers before I blow all my money on the full project.

    Thanks again, and don't apologize for slow replies! I have only recently become a massive TI fan. Between webench, reference designs, and now E2E, I have learned so much in just a couple months (Im just a student). This is great!

    Best Regards,
    Callum