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UCC256403: Limiting the max switching frequency and only have Burst Mode for no-load regulation

Stephen Oberholzer
Prodigy 135 points

Part Number: UCC256403
Other Parts Discussed in Thread: UCC25600,

Good morning,

1. How simple is it to limit the maximum switching frequency to 300kHz? This is not mentioned in the datasheet.

2. How simple is it to limit the minimum switching frequency to keep it away from the capacitive area of the gain curve?

2. How do I simply set it up to only have burst mode at no-load to prevent the output voltage from running away?

This is not clear from the datasheet.

3. Can I disable the high voltage start-up by providing the BLK pin with 5V from an auxiliary power supply?

4. Just a comment: the UCC25600 is a much simpler device to use and works very well.

I know you will say that the UCC256403 gives much better transient response.

Thank you.

  • 0
    TI__Genius 16155 points

    Hi Stephen,

    Please see my responses below:

    1. 

    Stephen Oberholzer said:
    How simple is it to limit the maximum switching frequency to 300kHz? This is not mentioned in the datasheet.

    Please see the following thread:

    (+) UCC256403: Maximum frequency setting at burst mode disable (option 7) - Power management forum - Power management - TI E2E support forums

    2.

    Stephen Oberholzer said:
    How simple is it to limit the minimum switching frequency to keep it away from the capacitive area of the gain curve?

    (1) UCC256404: min and max resonant frequency limitation - Power management forum - Power management - TI E2E support forums

    3.

    Stephen Oberholzer said:
    How do I simply set it up to only have burst mode at no-load to prevent the output voltage from running away?

    If you want to avoid burst mode, I suggest first depopulating the resistor between RVCC and LL/SS. Next, you will want to decrease the bottom VCR capacitance. What this will do is make the peak to peak VCR voltage larger. To avoid burst mode, you want the peak to peak VCR voltage to be greater than the burst threshold (BMT=0.2V when the resistor between RVCC and LL/SS is depopulated).

    4.

    Stephen Oberholzer said:
    Can I disable the high voltage start-up by providing the BLK pin with 5V from an auxiliary power supply?

    There is no HV startup in UCC256403. So, you need to connect the HV pin to ground.

    You need to connect an auxiliary supply at the VCC pin.

    BLK pin needs to be connected to LLC DC input voltage through resistor divider. Please refer the EVM schematic: https://www.ti.com/lit/pdf/sluubx3

    5. 

    Stephen Oberholzer said:

    Just a comment: the UCC25600 is a much simpler device to use and works very well.

    I know you will say that the UCC256403 gives much better transient response.

    In addition with the better transient response, UCC25640x has internal gate drivers and ZCS protection and needs only Type 2 compensator.

    So more reliable, better performance, Less BOM can be achieved compared to UCC25600.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 135 points

    Hi Manikanta,

    Thank you very much for your detailed answers!

    I understand now how it protects itself from entering the capacitive region.

    I always prefer to set a minimum frequency (in the inductive region) to prevent the frequency from becoming too low (due to some fault condition) which will increase the flux density in the transformer and could even saturate it. I note that it can be done by placing a resistor between the FB pin and ground, as Ben Lough suggested. Maybe this is not necessary since the UCC256403 will detect an overcurrent condition and prevent the switching frequency from becoming too low?

    I would still need burst mode to prevent the output voltage from running away at no load. Similar to how the UCC25600 works. I can therefore not do away with burst mode. Burst mode should only happen at no-load, or a very small load.

    I apologise, I did not mean to say high voltage start-up in my question. I meant to say Bulk-voltage start threshold. Can I disable the bulk voltage start-up by applying 5V to the BLK-pin? Similar to the UCC25600 which does not have a bulk voltage start-up.

    Regards,

    Stephen

  • 0 in reply to Stephen Oberholzer
    TI__Genius 16155 points

    Hi Stephen,

    Stephen Oberholzer said:
    I always prefer to set a minimum frequency (in the inductive region) to prevent the frequency from becoming too low (due to some fault condition) which will increase the flux density in the transformer and could even saturate it. I note that it can be done by placing a resistor between the FB pin and ground, as Ben Lough suggested. Maybe this is not necessary since the UCC256403 will detect an overcurrent condition and prevent the switching frequency from becoming too low?

    Yes, Thats correct.

    Stephen Oberholzer said:
    I meant to say Bulk-voltage start threshold. Can I disable the bulk voltage start-up by applying 5V to the BLK-pin? Similar to the UCC25600 which does not have a bulk voltage start-up.

    The bulk voltage startup threshold will help the converter to avoid operation when the input voltage of the LLC is low. So, by using this protection, you will be directly limiting the minimum frequency of operation.

    If you always maintain a voltage on the BLK pin that is higher than the startup threshold, you are always relying on over current protection.

    Could you let me know why you want to disable the under-voltage protection?

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 135 points

    Good day Manikanta,

    I do not like this high voltage startup whilst testing the power supply for the first time, since I want to increase the input voltage slowly from 0 to its maximum. Once I am sure it works correctly, I will enable the high voltage startup.

    Therefore I will do what Ben Lough suggested by limiting the minimum frequency with a resistor from the FB to ground. He does not however give an indication of the value necessary for a certain frequency. Do you have an idea what value to use for a given frequency?

    This is why I like the UCC25600 so much. It is so easy to add external drivers or pulse transformers to the UCC25600. Yes, I know the UCC256403 will give a better transient response.

    Nevertheless, I have laid out my new PCB for the UCC256403 and will try it out. Maybe I am surprised.

  • 0 in reply to Stephen Oberholzer
    TI__Genius 16155 points

    Hi Stephen,

    I would recommend testing the power supply in open loop where you can set the frequency that you want the converter to operate by connecting a resistor at the FB pin.

    Follow the below procedure. 

    Running UCC25640x open loop would require some modifications to the VCR circuitry as well as the FB pin to sink a constant current out of the FB pin. Since the FB pin is loosely regulated to ~5.6V, connecting a resistor (RFB) from FB to ground sinks a fixed current (somewhere between 0uA and 82uA). The amount of current would determine the switching thresholds for VCR. And then depopulate the top VCR capacitance so that charge control is completely disabled. Now the switching frequency is only dependent on the internal 2mA ramp current and the lower VCR capacitance. 

       

    Where IFBpin is the amount of current sunk out of the FB pin.

    Let me know if you have any further questions.

    Regards

    Manikanta P

  • +1 in reply to Manikanta Pallantla
    Prodigy 135 points

    Good morning Manikanta,

    Thank you very much indeed.

    Regards,

    Stephen.