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TPS54A20: Impact of Series Capacitor in TPS54A20 and Its Related Timing Analysis

J
Expert 1065 points
Part Number: TPS54A20

Tool/software:

Dear TI experts,

I have several questions I would like to ask,

please see my below questions,

  1. Does the capacitance value of the Series Capacitor in the TPS54A20 affect the circuit during load transients?

  2. Does the size of the Series Capacitor affect its own voltage ripple, and does the ripple size on the capacitor have a significant impact on the circuit?

  3. The size of the Series Capacitor is directly proportional to the pre-charge time. Generally, how should the pre-charge time be set for the circuit to operate normally?

  4. What are the effects of having a pre-charge time that is too long or too short on the circuit?

  5. What is the timing sequence of pre-charge and circuit operation?
  • 0
    TI__Expert 5328 points

    Hi J,

    Series cap also know as SCAP for simplicity was implemented in this design to help with energy transfer on both phases. 

    1) Yes, because it serves as storage of energy for output. Bigger SCAP means more energy to help with sudden transients, smaller SCAP means less help stabilizing output during load transient.     

    2) Yes the value of SCAP affects it's own ripple due to the changes of energy between phases. It doesn't directly show at output ripple but it can act similarly as having different cap value in the sense that the average voltage across SCAP will be read differently based on how much ripple there is therefor affecting switching of device. We recommend using low ESR for SCAP to mitigate ripple issues.

    3) Bigger SCAP means longer charge time. Device will want to charge SCAP based on VIN, it will start soft start once SCAP reaches up to half of VIN. The device has thresholds to trigger faults if voltage monitor reads voltage to be outside 35% and 65% of VIN triggering shutdown until values come back inside thresholds. If you're inside those parameters, device is most likely operating normally unless other conditions is triggered.

    4) I don't think short of pre-charge time would be an issue as device waits to do soft start until SCAP is 1/2Vin charged. Now if pre-charge is too long it will delay start up which if you're ok with that isn't an issue there. The real issue will be at steady state if you have too large of SCAP, it can slow down the system specially at transients.

    5) Vin ramps up, once it's above UVLO, EN pin goes up, SCAP precharges to 1/2 VIN, then soft start begins, and VOUT ramps up and PGOOD goes high.

    Delay due to SCAP precharge is found equation 1 of DS:

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Expert 1065 points

    Hi Eileen,

     Thank you for your explanation. I have a few follow-up questions:

    1. I tried using simulation software such as SIMPLIS to simulate the effect of changing the SCAP size on the circuit's transient response. However, I found that whether the SCAP is large or small, the transient behavior remains almost the same (for example, when the load steps from 50% to 100%). Is this possible? This made me wonder what the actual impact of SCAP size is on transient performance.

    2. According to the datasheet, the pre-charge current is approximately 10mA. Could you please explain where this current comes from? Is it possible to control this current externally? I ask because if I want to use a larger SCAP but still want a faster pre-charge time, I would need a higher pre-charge current.

  • 0 in reply to J
    TI__Expert 5328 points

    Hi J,

    1) Where did you get SIMPLIS model from?

    2) What is the application you are needing this part for?

    3) Please attach schematic

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Expert 1065 points

    Hi Eileen

    • I built the simulation myself, as it seems that TI does not provide a model for this part. However, I would like to better understand how the IC operates and how to design around it, so I used SIMPLIS for verification. That said, does TI provide any official model that I can use to validate the IC’s functionality?

    • This is for an application related to communications equipment.

    • The schematic is still under development. Currently, I’m conducting a preliminary evaluation and selecting components.

    • May I kindly ask if you could help answer the two questions I mentioned earlier?

  • 0 in reply to J
    TI__Expert 5328 points

    Hi J,

    Need to do some digging. Will reply Monday/Tuesday.

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Expert 1065 points

    Hi Eileen,

    OK, thanks for your help.

  • 0 in reply to J
    TI__Expert 5328 points

    Hi J,

    We don't have SIMPLIS sim for this device. Is possible that the design you created may not be a direct replica of device behavior. Developing this models that are accurate is very complex. Best would be for you to test it in bench.

    SCAP charging current is internal and can't be changed. Not sure on the mechanisms but most likely it has some mechanism connected to VIN reaching a specific level.

    Regards,

    Eileen