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LM5161: Pre-load resistor in an isolated fly-buck topology

Stefano Infante
Expert 2320 points
Part Number: LM5161


Hello,

I am designing with LM5161 and for the various calculations I am using your Excel file (LM5161 FBK Quick Start Calculation). The schematics in the file uses a 2 kohm "pre-load" resistor; a note says it is used for regulation at no load, but I am not able to find references about this resistor on the part data sheet.

My design has four identical isolated lines, each consuming about 1.5 W. By using this resistor I have about 240 mW on each line, for a total extra power consumption of about 1 W over a total of 6 W (17% extra power), so it is relevant to the total power budget.

Can you please explain why I should use this resistor?

Thanks in advance.

Best regards,

Stefano Infante

P.S. sorry if this is not the correct forum, but when I select LM5161 as the part number I can only choose this forum...

  • 0
    TI__Mastermind 40340 points

    Hello Stefano,

     

    The pre-load resistor ensures that peak charging of the output caps remain minimal.  The 2k is a suggested value.  You can experiment with high value resistors, the performance or load regulation will depend on a number of factors, one more significantly is leakage inductance of the transformer and a no load condition on a particular rail.  If the rail is unloaded, this is when the regulation will be worse and the need for pre-load is more critical to maintain good regulation.  If you always have a load on the rail, then a pre-load resistor will not be needed.

    Hope this helps?

    David.

  • 0 in reply to David Baba
    Expert 2320 points

    Hi David,

    I have a somewhat constant load, but I also have to provide a way to disable it (by using a solid state relay).

    Hence, when the load is disabled I have no load and then, according to your suggestion, I need a pre-load resistor. But what if I use a double SSR in order to insert the pre-load resistor only when needed? I think I could arrange  a couple of SSRs in order to alternatively enable load or pre-load (see the attachment). What do you think about it? Could the operation of inserting and interrupting the lines generate unwanted pulses which can damage the circuit?

    Thanks,

    Stefano

  • 0 in reply to Stefano Infante
    TI__Mastermind 40340 points
    Hello Stefano,

    Understood. what is your regulation requirement? You can implement a pre-load using a Zener diode, in place of a Resistor, that way the Zener only Conducts when its Vz rating is exceeded at the output, without the need to switch anything in?

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Expert 2320 points

    Hi David, do you mean something like the attached circuit?

    As the transformer output will be about 22 V, I think I could use a 20 V Zener diode with a, say, 2k resistor in series. Do you think this should be ok?

    Thanks again

    Stefano

  • 0 in reply to Stefano Infante
    TI__Mastermind 40340 points
    Hello Stefano,

    No need for the Resistor in series with the Zener, the amount of clamping energy is very small at light loads, equal to approximately the energy as a result of the leakage inductance (0.5xI^2xLleak).

    hope this helps?

    David.