• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 170 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5160A: Flybuck secondary output not regulating

Paul Zwart
Prodigy 30 points
Part Number: LM5160A
Other Parts Discussed in Thread: LM25183

We have a LM5160A configured in the flybuck configuration with an 8V primary voltage and aiming for a secondary output voltage of 7.5V. The following is some details on the setup:

  • 28V VIN
  • Max Iout on the secondary is 0.67A
  • 8V Vout primary
  • 7.5V Vout secondary

I also filled out the calculation spreadsheet which is attached. 

In the unloaded condition, the secondary's output voltage is about 21V. When loaded by 10kohm, it's about 12V. When loaded by 2kohm it is about 9V. When loaded by 200ohms, it is at the desired 7.5V and when it is loaded with 25ohms, it is at 7V. The primary is regulating fine, but we don't actually use the primary voltage at all. 

Any suggestions of what could be going wrong? Or is this expected behavior for this type of regulator?

Thanks,

Paul

Copy of LM5160FBKQuickStartCalculator - FinalVersion.xls

  • 0
    TI__Guru 74310 points

    Hi Paul,

    Thank you for designing with the LM4160A.  In a fly-buck topology the secondary side is cross regulated, which is derived by the well-regulated primary side voltage via transformer coupling, minus the secondary diode voltage drop and also some voltage losses in the transformer winding and PCB trace resistances.  KVL still applies here, basically:

    Vo_pri = (Ns/Np) * (Vo_sec + VF + Io2 * Rsec),  where Rsec include the secondary winding  PC resistances.  You can see that Vo will drop if Io2 increases.

    The output voltage would be more stable if the primary side has higher load.  At no load, there is still inductor current, and your secondary will be charged without being discharged, thus the output voltage can be high.  We recommend either add a dummy load or use a Zener clamp to prevent excessive secondary voltage.

    To get better regulation in the isolated output without using the primary output rail, I would suggest you to change to our LM25183 PSR flyback.  You may order an EVM to evaluate.  It will have much better output regulation, with only a small dummy load or Zener clamp.  You can choose Coilcraft MSD1048H-223 as the flyback transformer. Attached is a design by the quick start calculator for your reference. 

    proposal--LM(2)518x PSR flyback converter quickstart tool revB.xlsx

    Hope this helps.

    Best Regards,

    Youhao