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UCC28950 / basic question

S.Satoshi
Guru 21685 points
Other Parts Discussed in Thread: UCC28950

Hi,

Please let me ask about UCC28950. I listed questions from customer in attached. Can you confirm it and please ask if you need more information.

Best Regards,

Sonoki

UCC28950_Q&A list_20130402_E2E_1.xlsx
  • 0
    TI__Guru* 86266 points

    Hello Sonoki,

    The PDF attachments do not open in your excel file.

    Question1

    The current sense signal in current mode control acts as a PWM ramp.  It is level shifted within the IC by 0.85V please reference figure 3 in the data sheet.  The voltage amplifier controls this peak current limit to what ever duty cycle the controller demands.TABSET sets the turn on delay of FETs QA and QB to achieve ZVS.  Please refer to figure 3 of the data sheet for the A..F sequence based on duty cycle.

    Question 2

    Since Rt sets the charge and discharge current for the internal oscillator I believe this is based on a ratio and holds true for the entire frequency range. You might want to verify that on the bench to make sure.

    Question 3

    Slope compensation is added to the current sense signal after it is level shifted.

    Question 4

    I know the data sheet specifies that this device can be used in average current mode control.  However, it is generally used in voltage mode control or peak current mode control and not average current mode control. Average current mode control would require a separate current amplifier and would remove cycle by cycle current limiting and I would not recommend it.

    Question 5

    Figure 21 f the user's guide shows what the CS signal should like with respect to gate drive and transformer primary input voltage.

    http://www.ti.com/lit/htm/sluu421

    You/Customer might want to get a hold of one of the EVMs to evaluate it too see what certain waveforms look like.

    Regards,

     

     

  • 0 in reply to Mike O'
    Guru 21685 points

    Hi Mike-san,

    Sorry for my delayed response. I added attachment again for Q1 and Q5. Can you give your comments again for them?

    Q1,

    Please let me know the detection timing of CS voltage.
    I understand that UCC28950 set delay time like TABSET by detecting CS voltage. Then I would like to know the detection timing of CS voltage. For example if CS voltage like attached is applied continuously, does UCC28950 detect CS voltage as 1.0V, 0.2V or averaged value?

    2642.UCC28950 CS voltage.pdf

    Q5,

    Tha attached waveform looked that the peak current is detected during off-duty period. Please let me know the operation when the peak current is detected during off-duty period like this. Is this waveform correct?

    6014.Current detection timing.pdf

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Guru* 86266 points

     

    Hello Snookie,

    Q1>

    Hopefully the attached presentation with aid of the data sheet wills this question for you.

    Q5>

    During the freewheeling period the controller should not be doing anything with the current sense signal.

    The current sense signal of CT1 should figure 21 of the EVM User's guide http://www.ti.com/litv/pdf/sluu421a

    Regards,

    UCC28950 Basic Timing.ppt