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UCC28950: CS pin calculate

Chao Fu Chen
Intellectual 1170 points
Part Number: UCC28950

Hi Team,

My customer have several question about CS pint action as below ;

There are several occasions where UCC28950PWR Pin15 (i.e. CS) is detected for control purpose.

(1) DCM

 > For DCM determination, Vcs (peak value) < Vdcm, DCM is triggered. Correct?

(2) Dead time between OUTA & OUTB, OUTC & OUTD

  > What is the time instant of Vcs signal is detected in this scenario?

(3) Delay time between OUTA & OUTF, OUTB & OUTE

  > What is the time instant of Vcs signal is detected in this scenario?

For the question2 &3 have further wonder to ask

> E.g. In equation (3) shown at the bottom of this mail, there exists a term called CS (Vcs from Pin15).

> Vcs is a continue signal input , right ?

> And equation (3) CS is take which moment of Vcs value ( peak or, average etc) to judge and calculate value of Tabset.

Eddie

  • 0
    TI__Guru* 86266 points

    Interesting questions I will research and get back to you

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    Please see my answers below.

    (1) DCM

     > For DCM determination, Vcs (peak value) < Vdcm, DCM is triggered. Correct?

    * Triggered on peak, refer to figure 38 and section 7.3.12 for a description of the DCM function.

    (2) Dead time between OUTA & OUTB, OUTC & OUTD

      > What is the time instant of Vcs signal is detected in this scenario?

    * This will be determined by the COMP voltage, reflected output current across the transformer and transformer primary magnetizing current.

    (3) Delay time between OUTA & OUTF, OUTB & OUTE

      > What is the time instant of Vcs signal is detected in this scenario?

    * This will be determined by the COMP voltage, reflected output current across the transformer and transformer primary magnetizing current.

    * The following link will bring you to an application not that describes how the UCC28950 works in more detail.  You may find this useful.  www.ti.com/.../slua560

    For the question2 &3 have further wonder to ask

    > E.g. In equation (3) shown at the bottom of this mail, there exists a term called CS (Vcs from Pin15).

    > Vcs is a continue signal input , right ?

    You will only see a CS signal when the transformer is delivering current from the primary to secondary.

    Continues Conduction refers to output inductor current.  On the CS pin this will be represented as a DD offset.

    > And equation (3) CS is take which moment of Vcs value ( peak or, average etc) to judge and calculate value of Tabset.

    Most designers choose to use a fix delay approach as mentioned in the SLUA560C application note.   However, this calculation is based on peak if you choose to use adaptive delay.

    Regards,

  • 0 in reply to Mike O'
    TI__Intellectual 1170 points

    Hi Mike'O,

    Thanks your support in here, My customer still have some help in question3 comment as below ;

    > Using peak value of Vcs to calculate Tabset & Tafset. Is this correct?

    > Any reason why choose to use fixed delay approach for most designers?

    Eddie

  • 0 in reply to Chao Fu Chen
    TI__Guru* 86266 points

    Hello,

    It is correct that Tabset Tafset are set with the CS peak current.

    Most phase shifted full bridge converters follow a PFC stage.  If the input voltage is fixed the FET Coss and switch node capacitance is fixed.  The inductances really don't move much so the tank frequency at the switch node is fixed.  This means that adaptive delay is not required to achieve ZVS.

    95% of the customer use a fixed delay approach.  However, the adaptive delay can be used to remove body diode conduction on the H Bridge.  How ever this improvement is generally small compared to the improvement from achieving ZVS.  This current is derived from leakage and shim inductance and the energy being dissipated is generally quite small.

    Regards,