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LM5180-Q1: Question about LM5180-Q1 design

Part Number: LM5180-Q1

Hi experts,

I have some questions about LM5180-Q1 Flyback design to check with you. Thank you.

1. Is there any need to add dummy load at the output? If yes, what value is suggested?

2. There are three modes of this device, FFM, DCM and BCM. How each mode affect EMI performance? In other words, to achieve better EMI performance, which mode should be designed as the dominant mode?

  • Hello

    It is my understanding that a minimum load is not required.

    However, I will ask the application engineer for this device and get back to 

    you as soon as possible.

    It is very difficult to say what mode will exhibit the best EMI performance.

    In my opinion any mode that keeps the frequency as constant as possible would

    be helpful.

    Thanks

  • Hello

    I have checked with the application engineer for this device.

    You dont need a minimum load, but you do need a Zener clamp on the

    output as shown in the application diagram.  This clamp will normally be 

    between 110% and 120% of your regulated output voltage.

    Please consult the data sheet and the component calculator on the product

    page for this device.

    Thanks

  • Hi Frank,

    Thank you very much for your reply. Is this Zener diode at the output mandatory? Why a Zener diode is needed here? I didn't find such kind of diode at the output of Buck or Boost converter. Is it spectial for Flyback? Is it used for over-voltage protection? But I think the output is stable, in which scenario will it becomes over voltage? Thank you.

  • Hello

    This device has a  min Fsw of 12kHz based on the requirement to sense the output voltage, so there is always some energy delivered to the output (Pout-min = 0.5*Lmag*0.3A2*12kHz).  So,  with no load on the output, the output voltage will rise. The Zener clamps the voltage during no-load

     Thanks