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UCC2818: Buck Controller with Average Current Mode Control and True 100% Duty Cycle

Ahmad Kamal
Prodigy 20 points
Part Number: UCC2818

Tool/software:

Hi team,

I'm looking for a buck controller that supports Average Current Mode Control (ACMC) and can also achieve a true 100% duty cycle. These two features seem difficult to find together in a single device.

Application context:
My primary goal is to slow down the system di/dt, and for that reason, I'm planning to use ACMC since it provides better control over the input di/dt compared to Peak Current Mode Control (PCMC). 

I came across the UCC2818A-Q1, and although I realize it's primarily intended for CCM PFC applications, I believe its internal architecture could be adapted for buck converter operation using its average current loop.

  1. Are there any major concerns or limitations when repurposing this device for buck operation?
  2. The product page claims support for 100% duty cycle, but the datasheet specifies a maximum of 99%. Can the device realistically operate at 99% duty, or are there internal limitations (such as gate drive timing or oscillator constraints) that prevent that in practice?
  3. Is there any known workaround to achieve closer to 100% duty cycle with this controller?
  4. If the UCC2818A-Q1 isn’t suitable, I’d appreciate any recommendations for TI controllers that support average current mode control and 100% duty cycle operation.

Lastly, if there are any alternative techniques (besides buck converter) that can help control input or load di/dt, I’d be very interested in hearing about them.

Thanks in advance for your guidance.

Best regards,
AK

  • 0
    TI__Mastermind 40350 points

    Hello AK, 

    Using the UCC2818A-Q1 in a buck configuration will pose some considerations:

    1. Its soft-start is designed for boost-PFC where the output voltage is already present as the peak of the input voltage before switching even starts.  Therefore there will be significant voltage across the inductor to demagnetize it as Vout rises to regulation.  In a buck configuration, Vout will start at 0V so there is initially no voltage to demagnetize the inductor.  Start-up duty-cycles must be limited to short duration for a long span of time to allow the inductor current to discharge into and charge up the output capacitor.  

    2. There is a minimum off time enforced in the oscillator block that prevents D > ~99%.  This off time has some variability, too. 

    3.  I don't know of any work-around for 100% D because most people prefer to avoid that condition.  However, I can suggest to add a turn-off delay to the MOSFET gated rive so that when the PWM output goes low, the drive to the MOSFET is delayed long enough to bridge the maximum off time.  Be aware that such a delay will also act during normal duty cycles.

    4.  I am not aware of any TI controller that inherently supports 100% D. 

    Regards,
    Ulrich