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LM5026: Schematic Review

chentsu chung
Mastermind 6180 points
Part Number: LM5026
Other Parts Discussed in Thread: UCC28780

Hi,

After testing the prototype sample, we found that the waveform of Q5_Vds is abnormal, could you help to check that the schematic if anything needs to be modified. thank you.

6661.LM5026.pdf

 Vin: 12V

Vout: 12V/1A

  • 0
    TI__Mastermind 34465 points
    Hello Chentsu
    Your schematic does not have an inductor on the output.
    The LM5026 is an active clamp forward controller and this configuration requires an output inductor.
    Please review the design examples on this web page:
    www.ti.com/.../toolssoftware
    Regards
    John
  • 0 in reply to John Griffin1
    Mastermind 6180 points

    Hi John,

    Sorry for typo in schematic, the polar of X'mer is reversed, and we use LM5026 as Flyback converter.

    But LM5026 can be used as Flyback like PMP7895, am I right?

    Or not good for Flyback topology?

    If it's can be use as Flyback, could you please help ot check the schematic is there anything wrong? thank you.

    C.T.

  • 0 in reply to chentsu chung
    TI__Mastermind 34465 points
    Hi CT,
    The LM5026 is normally used as a forward converter controller.
    I have not seen any flyback applications with this part but the waveform makes a little more sense now that you are using a flyback converter.
    When the main FET turns off you will require the drain voltage to reflect the output voltage. In your application the auxiliary FET is turning on after a short delay and preventing the secondary winding from applying the correct voltage pattering to the output.
    If you wish to you an active clamp flyback controller I recommend that you use the UCC28780 . This is a dedicated high frequency active clamp flyback controller.
    There are no reference designs available for an active clamp flyback with the LM5026.

    Regards
    John
  • 0 in reply to John Griffin1
    Mastermind 6180 points
    Hi John,

    Understand, so may ask TI design PMP7895 is for what kind of application?
    Thank you!

    C.T.
  • 0 in reply to chentsu chung
    TI__Mastermind 34465 points
    Hi CT,
    I am trying to locate the engineer who did this reference design and he should be able to explain .
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Intellectual 2530 points

    PMP7895 is an active clamp flyback.  It operates in continuous conduction mode.  The design attached appears to operate in discontinuous conduction mode.  The handwritten comments on the schematic say the xfmr has 0.78uH of primary inductance and a 2:3 turns ratio...is this correct?  The peak/rms FET and diode currents get rather high with this value of inductance.  The converter should work with 0.78uH, but 2uH would lower the peak/rms currents.

    The clamp resonant frequency of the active clamp flyback is the clamp capacitor and the xfmr leakage inductance.  The note on the schematic says the leakage is 82nH.  The resonant clamp frequency needs to be equal to the swithcing frequency for proper operation between 25-75% duty cycle.  This means the clamp capacitor (C19) needs to be 2.8uF for the correct clamp resonant frequency.  Operating in DCM mode will also result in duty cycles less than 25%.  There will most likely be increased power dissipation below 25% duty cycle (light loads) because the clamp current will be flowing in the wrong direction when the main FET turns on.  With the lower input voltage and DCM operation I don't see much benefit to using the active clamp.  I think an RCD clamp will provide the same or better efficiency.  A 1-2A ultra-fast diode with the same voltage rating as the FET can be used.  A 0.1-0.22uF capacitor and 20-40k ohm resistor should be sufficient at this power level.

    I also don't see much benefit to using a sync rectifier with only 1A of output  current.  The schottky diode should provide similar efficiency.

    The dot is wrong on the xfmr secondary, but the previous email thread seems to say this was fixed.

    The R15 snubber resistor value seems low.  I usually use a 75-100 ohm resistor.

    The U3 opto should be the "A" rating with the CTR range limited to 80-160%.  The opto with no rating can have a CTR from 50-600%.  The loop gain can vary more than 20dB making loop compensation difficult.

    I'm not familar with the error amp used on the secondary so I can't comment on the operating point bias or loop compensation.

    Hope this helps.  Please respond if you have more questions.

    Thanks,

    David

  • 0 in reply to David Strasser
    Mastermind 6180 points
    Hi David,

    Understand, thanks for your detail explanation!