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UCC28911: Stops switching

Rajput Sumersingh
Prodigy 10 points
Part Number: UCC28911


Hello team,

We are going to use UCC 28911 in our design and prepared proto board on UCC28911EVM-718 board by some changes

We have some queries regarding the same please address

Q1: Is limit for Drain Over current protection [Id(ocp)] will get change w.r.t. Ripk resistor used ?

As per datasheet, If Ipk pin shorted to ground then Id(ocp) is 980mA (typ.)

But, If we use Ripk (>900 Ω) then is the Id(ocp) will remain same as 980mA or will get changed ?

 

Like, if I use Ripk = 1200 Ω then will Id(ocp) remains same as 980mA or it will get changed according to below formula

Id(ocp) = VCSTE_OCP / Ripk

 

In this case new modified limit of Id(ocp) will be = 885/1200 = 737mA

Is it correct ?

Please confirm…

Q2:  What is the significance of parameter Vcste (max) if I use Ripk = 1200 Ω (page 10 of datasheet) ?

Q3: Apart from above, we are facing start-up issue as described below

 Note: Design specification and circuit diagram attached below

 Design specification and circuit diagram.docx

Waveform details

Ch1: Drain voltage (Yellow) (100V/ division)

Ch2: Drain current (Green) (200mV/ division) (1mV=1mA)

Ch3: Voltage on VS pin (Blue) (5V/ division)

Ch4: VDD of UCC28911 (Pink) (5V/ division)

@ 36Vac, no load at Vout (with Pre-load 1K)

Zoom view

Why voltage on VS pin is continue increasing ?

It starts from 2V(at start of switching) and increasing till 5.1V and then stops

Please suggest some suitable actions to resolve the issue

Regards,

Sumersingh

  • 0
    TI__Guru* 86286 points

    Hello,

    Please see my comments below.

    Q1: Is limit for Drain Over current protection [Id(ocp)] will get change w.r.t. Ripk resistor used ?

    As per datasheet, If Ipk pin shorted to ground then Id(ocp) is 980mA (typ.)

    But, If we use Ripk (>900 Ω) then is the Id(ocp) will remain same as 980mA or will get changed ?

     > The cycle by cycle ILimit will be set by Ripk  You need the Vcste(max) to calculate the resistor value.

    >It is recommended on the UCC28911 to set this to 700 mA based on recommended operating conditions.

    Like, if I use Ripk = 1200 Ω then will Id(ocp) remains same as 980mA or it will get changed according to below formula

    Id(ocp) = VCSTE_OCP / Ripk

     

    In this case new modified limit of Id(ocp) will be = 885/1200 = 737mA

    Is it correct ?

    Yes I believe you are correct.

    Please confirm…

    Q2:  What is the significance of parameter Vcste (max) if I use Ripk = 1200 Ω (page 10 of datasheet) ?

     

    Vcste(max) is to set your Ripk to the nominal peak current.  Vcste_ocp and Vcste_ocp2 are used for calculating the OCP trip point.

    Q3: Apart from above, we are facing start-up issue as described below

     Note: Design specification and circuit diagram attached below

    Your design looks like it might be tripping due to an OVP fault.  You may need a little more pre-load to keep it from doing that.  The following link will bring to an application note that discusses OVP faults and how to remove them.  You may find this helpful. https://www.ti.com/lit/pdf/sluaac5

    When it comes to modifying the EVM for your specifications the following link will bring you to an Excel design tool that you may find helpful.

    https://www.ti.com/lit/zip/sluc624

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hi Thanks for quick response,

    Further inline to circuit diagram I have shared,

    For selection of RS1 & RS2, we have referred page no: 5 of datasheet

    Please find attached calculation sheet

    Feedback resistor calculation.xlsx

     

    As per calculation the resistor value should be RS1: 100K & RS2: 34K (33K used currently)

     If, we use RS2: 33K instead of 34K, then Vout will be 5.14V (which is <113.5% of Vout, OVP limit)

     As Vout is within limit then why switcher is sensing the output over voltage ?

     

    Regards,

    Sumersingh

  • 0 in reply to Rajput Sumersingh
    TI__Guru* 86286 points

    Hello,

    Your design is most likely shutting down to fault.  The UCC UC28740 uses primary side fault sensing.  The following link will bringing to an application note that discusses how primary fault sensing works, the most common issue and how to resolve these issues.  I believe you will find this information useful and helpful to resolve your startup issues.  https://www.ti.com/lit/pdf/sluaac5

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hello,

    Switcher starts switching but seen high spikes in between in Vout waveform

    Vout varies from 5.53V to 6.53V (can be seen from waveform)

    Ch3: Secondary winding output waveform (Blue) (10V/ division)

    Ch4: Vout (DC voltage) (Pink) (2V/ division)

    Also, When increasing the input voltage, it stops the switching again some where around 140V Vac and output swings in between 0V to 1.85V

    Zoom view

    Q1: Why high spikes seen in Vout at Vin=40Vac ?

    Q2: Why switcher stops switching above Vin =140Vac ? 

    Regards,

  • 0 in reply to Rajput Sumersingh
    TI__Guru* 86286 points

    Hello,

    The circuit is most likely shutting down due to OCP, OVP or input under voltage fault. The following link will bring you to an application note that describes what the issues with this fault sensing and how to resolve. 

    https://www.ti.com/lit/pdf/sluaac5

    Regards,