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UCC21540-Q1: Phase-Shift-Full-Bridge Primary (HV-Current) waveform

Marc
Prodigy 10 points
Part Number: UCC21540-Q1
Other Parts Discussed in Thread: UCC28951, UCC28950

Hi,

in our Phase-Shift-Full-Bridge DC/DC we have the issue, that the primary (HV-Current) has not the expected waveform. We tried several things to get an idea why:

First we change the external leakage inductance between 4,7uH and 22uH but it has no big impact on this behavior. This leakage inductor is currently a metal powder-torroid which is not in saturation.

Then we changed the transformer from planar (with its higher parasitic prim.C= 312pF) to a regular one with lower parasitic capacitance (16pF), but also no big impact on this behavior. 

Somebody an idea where this could come from?

CH1 LV-Current (Load)

CH2 Switchnode T1/T2 (left Halfbridge)

CH3 Switchnode T3/T4 (right Halfbridge)

CH4 Primary Current (Transformer 14:1) 

 

  • 0
    TI__Expert 5921 points

    Hi Marc,

    Thanks for reaching out.

    I am working with some of my coworkers to come up with some ideas on this. In the meantime, would you be able to provide some more information:

    - What control scheme is being used by the PSFB? If you are using voltage mode control a DC blocking cap is needed to prevent eventual saturation due to flux imbalance.

    - Would you be able to provide a schematic for us to review?

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    Prodigy 40 points

    Hello Daniel,
    we are currently not using a control scheme. The measurements were taken with a fixed phase shift that we can manually set to achieve certain operating conditions.
    I'm not sure if we can provide schematics. Can you please specify what you want to find out?
    This is the power train architecture that we're using.

  • 0 in reply to Daniel Schweizer
    Prodigy 40 points

    Btw, there are quick free-wheeling SiC diodes placed between shim inductor and primary transformer winding, that are not shown in the diagram above.

  • 0 in reply to Daniel Schweizer
    TI__Expert 5921 points

    Daniel,

    Unfortunately, our experts have not seen this sort of transformer current before. Could you confirm where you are measuring the current, and what sort of probe you are using?

    Also, we have an application note that details the design of a PSFB: https://www.ti.com/lit/pdf/slua560

    Would you be able to look over the sample schematic included and confirm that it matches your application? This document may help you isolate the problem.

    Additionally, would you be willing to share what controller you are using?

    We will continue to discuss this internally.

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    Prodigy 10 points

    Hi Daniel,

    as my coworker Daniel Schweizer already described we are not using a control scheme right now. In fact saturation due to flux imbalance in the main transformer would be the same. We tried today 2uF/50V in series to the L_shim. We see that the prim current signal is more symmetrical now, but still we have the increasing/jumping current on the primary side if we are in the freewheeling phases. Our thoughts are going in the direction of reflecting current from the secondary side.

  • 0
    TI__Expert 5921 points

    Hi Marc,

    Thanks for confirming about your control system.

    Would you be able to check your shim inductor value with the calculations suggested on page 8 of this app note: https://www.ti.com/lit/pdf/slua560?

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    Prodigy 40 points

    Hi Daniel,
    here is our shim inductor calculation according to mentioned paper. We have calculated with the intended amount of FETs. But current measurements were taken with only half as much FETs and approx. 4.7µH to 10µH shim inductance. I also added the calculation via energy.
    Note: The switching frequency fSW is 100kHz

  • 0
    TI__Expert 5921 points

    Mark and Daniel,

    It seems that your design is acceptable, but something is off with the way the discrete solution is being driven.

    As a troubleshooting step, it may be useful to look at the voltage across your inductor as well as the current.

    Alternatively, you could add a controller to allow more precise control. TI has a couple controllers that could be ideal for this, either the UCC28950 or UCC28951. We even have PSFB evaluation modules you could order to compare to your design (https://www.ti.com/tool/UCC27714EVM-551). 

    I hope this is useful on continuing your troubleshooting.

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    Prodigy 40 points

    Hello there,
    we have a couple more measurements for you: all at the same load/input voltage conditions (phase shift 132° each). Currently we think that the high free-wheeling current is related to the dead-time on each half bridge. But so far changing the dead-time did not have an effect on the primary transformer current wave form. Do you have any other ideas what's wrong with it? Can you maybe provide similar measurements from your demo board?

    Transformer output current on CH1:

    SShim inductor voltage on CH1

    PPrimary transformer voltage on CH1

  • 0 in reply to Daniel Schweizer
    TI__Mastermind 22755 points

    Hello Daniel S,

    Daniel is out of the office at the moment but should respond to your latest questions shortly.

    Regards,

  • 0
    TI__Expert 5921 points

    Hi Marc, Daniel,

    I wanted to let you know that I am continuing to pursue this issue internally and will keep you updated as I have more information or questions.

    Regards,

    Daniel

  • 0
    TI__Expert 5921 points

    Marc, Daniel,

    As I have followed up on this it seems you are in email communication with others in TI on this issue. I believe it would be best to continue this conversation there as it will allow our experts to find solutions more directly.

    If I have misunderstood and this is not the case please let me know. 

    Regards,

    Daniel