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UCC28950: SLUA560C change to Non-synchronous rectifiers

Satoshi
Guru 19645 points
Part Number: UCC28950
Other Parts Discussed in Thread: TINA-TI

Please let me know about two points below for SLUA560C change to Non-synchronous rectifiers. (Secondary side was change to Diode)

①Is there way to set ARELEF, OUTE, and OUTF pin? (For example; Open, GND, etc)

②Are D8 and D9 (SLUA560C page 2) need to connect on Non-synchronous rectifiers condition?

　If D8 and D9 are need, is there required diode spec?

Best regards,

Satoshi

  • 0
    TI__Mastermind 25570 points
    Hello Satoshi

    If you are not using the SRs then these pins should be left open circuit. It is also a good idea to connect the DCM pin to VREF so that the OUTE and OUTF signals are not generated by the UCC28950 controller. This has no bad effects on the controller operation and will reduce system noise by eliminating the high dv/dt edges on OUTE and OUTF

    I cannot find D8 and D9 - perhaps I'm looking at the wrong diagram. Can you send me a screenshot showing these parts

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Guru 19645 points

    Colin-san

    Thank you for quick reply,

    Sorry, I mistaken diode number(D8 and D9), correct is DB and DC.

    Screenshot is attached below;

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Mastermind 25570 points

    Hello Satoshi-san

    That's much clearer to me now - thanks.

    The answer is yes - the diodes will probably be needed if you use Diode rectification instead of SRs. The only condition where the diodes cannot be used if if the transformer leakage inductance is large enough so that a separate shim inductor is not needed.

    The diode spec is a little less clear - I would suggest that the customer model their schematic - the slum277 TINA-TI schematic would be a good starting point. It could be modified to use the actual values of shim inductor and transformer inductances and turns ratio relatively easily. The SRs in the model can be simply replaced with the diodes that the customer is going to use. The simulation will give peak and average currents for power dissipation calculations.

    Three things are clear -

    1/ The diode reverse voltage spec must be greater than the Vin voltage - I'd suggest 600V diodes on a 400V bus.

    2/ The diodes must be ultra fast parts - we have used 600V, 50ns MURS360 devices on our 600W EVM for example.

    3/ The diode current paths must be low inductance.

    Please let me know if you need any more information

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Guru 19645 points
    Colin-san

    Thank you for reply,

    About diode selection, is there additional condition for If(Forward current)?

    Best regards,
    Satoshi
  • 0 in reply to Satoshi
    TI__Mastermind 25570 points

    Hello Satoshi-san

    The peak current in the clamp diodes will be approximately the same as the peak primary current,  the average current in the diodes will be small because they carry current only for a short time during the switching cycle. To provide some margin the diodes should be sized to carry the full primary current . I'm afraid I cannot be more specific than this because the current depends on many details - the type of secondary (Centre Tapped or single winding), whether a blocking capacitor is used or not (this is needed in Voltage mode control), the value of the output inductance etc.

    My suggestion would be that the customer run a simulation with their own component values and measure the diode current in that way. The TI-TINA simulation at http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slum277&fileType=tsc  will allow them to do that (with appropriate component value modifications)

    Regards

    Colin