• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 5 answers
  • Subscribers 62 subscribers
  • Views 3089 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

UCC28951: 2KW OBC design - How to fix resonance issue during debugging

UCC28951: How to select or design shim inductor for PSFB topology

Prem Kumar G G
Prodigy 110 points
Part Number: UCC28951
Other Parts Discussed in Thread: PMP8740

Hi,

We are currently testing the PSFB board. We are almost realized the ZVS.

Now the heat dissipation on MOSFET is very minimum while we increasing the load.

But now our concern is the heat dissipation on shim inductor.

Shim inductor Part No: 2301-V-RC - 10uH 20A toroidal inductor

We have reduced number of turns to make it 2.7uH at which we are able to achieve ZVS.

In this condition the temperature of shim inductor is increasing above 75°C when 10Amps loaded in the output.

So we are not able to increase the load further because it may cause failure in shim inductor.

Please guide us how to design or select the shim inductor. Please suggest any suitable part in your view.

Please send if there is a design document for shim inductor designing.

  • 0
    TI__Guru 74235 points

    Hello,

    If the shim inductor is getting hot it is because it is saturating.  Please select a shim inductor who's saturating current is higher than the peak current in your design.

    Regards, 

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Sir,

    Thanks for the input.

    As per calculation the peak current in our design is 10A.

    So we are using a 20A toroidal core inductor which is higher than peak current. But still there is more heat in the inductor.

    But in our latest test we used a Ferrite core based inductor having 45A saturation current. This is very better than the previous inductor.

    It could be because of higher saturation current or due to the core property.

    But what we observe is that the saturation current need to be 5 or 6 times greater than the peak current.

    Now we are ok with shim inductor design.

    We have another query in the Primary current waveform. As per the datasheet the Transformer Primary current looks like a sinewave.

    We have captured below waveform at different load condition. Switching frequency in our design is 210kHz.

    Condition 1:

    Output voltage: 47V

    Load current: 4A

    Ch1 : Transformer primary current 3A/div

    Ch2: Voltage across Primary winding 200V/div

    Condition 2:

    Output voltage: 47V

    Load current: 11A

    Ch1 : Transformer primary current 5A/div

    Ch2: Voltage across Primary winding 200V/div

    Condition 3:

    Output voltage: 47V

    Load current: 15A

    Ch1 : Transformer primary current 5A/div

    Ch2: Voltage across Primary winding 200V/div

    In Condition 1, at light load, the Primary current waveform is almost looks like a sinewave.

    But in condition 2 and 3, when we are increasing the load, the current waveform is changing.

    Is this expected behavior? We really need your kind support to progress our testing.

    In the test report: PMP8740 also we did not found the Primary current waveform. So we want to confirm our waveform by your support.

  • 0 in reply to Prem Kumar G G
    TI__Guru 74235 points

    Hello,

    I cannot open the waveforms could you please resend them as attachments?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Sir,

    I have attached test report excel with waveform.

    Kindly check.

    ThanksPSFB_Load_testing_17_11_2020.xlsx

  • 0 in reply to Prem Kumar G G
    TI__Guru 74235 points

    Hello,

    In condition 1 your transformer primary current looks to be primary magnetizing and inductor output ripple current with a DC offset.  What this means is the output inductor is operating in critical conduction this alone is not a bad thing. This more like sawtooth waveform.

    In condition two the primary current of the transformer does not seem to be correct.  The following waveforms shows what the transformer primary current should look like.  I think that your output inductor might be saturating.  Could you double check this?  The other thing that may be happening is you are measuring the shim inductor current vs the primary transformer current.  Could you double check this as well? The more I study your waveforms it looks like you might be measuring the shim inductor current. 

    Regards,

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Sir,

    Thanks for your opinion on waveform.

    We ensured that we are measuring the Primary transformer current waveform only. We are not measuring shim inductor current.

    We are investigating further.

    We have a question about clamping diode at center point of shim inductor and transformer. I have highlighted the diodes in the attached picture.

    Question: In the ZVS topology explanation, there is no clamping diode.

    But while implementation only we are using this clamping diode. What is the significance for this clamping diode? How it works?

    We are using a schottky diode in the place. The Part No: IDH16G65C6XKSA1

    This diode has Qrr of 21.5nC.

    Can this diode have impact in the Primary current waveform?

    Changing this diode to Ultrafast recovery diode as suggested by TI can give better primary current waveform?

    Kindly give your opinion and also explain us the diode usage.

  • 0 in reply to Prem Kumar G G
    TI__Guru 74235 points

    Hello,

    The primary transformer current just does not look correct.  I wonder if the output inductor is saturating?

    Could you take the output inductor current waveform, primary transformer current waveform and SR FET drive waveforms with an oscilloscope to see if the output inductor is saturating?

    Regards 

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Sir,

    We have captured output inductor current.

    Load: 13A. Waveform below.

    Ch1: Output inductor current

    Ch2: Primary voltage

    We are using diode in the secondary as of now. Not MOSFETs.

    We see the output inductor is not saturating.

    Kindly give further support for solving the problem.

  • 0 in reply to Prem Kumar G G
    TI__Guru 74235 points

    Hello,

    Those waveforms look O.K.

    What seems to be off is your turn on delay.  I would adjust those to give you valley switching and ZVS depending on loading.

    The following application note will describe how to set the timing to achieve ZVS.  

    Regards,