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LM51561-Q1: Input power limitation circuit

Part Number: LM51561-Q1
Other Parts Discussed in Thread: LM51561, LM5156

Dear *,

we want to use LM51561 in two possible configurations with input power limit circuit ( using SS or other pin) and adjustable output voltage.

Configuration 1)

LM51561 in non-isolated BOOST

Vin 5V 

Vout from 75V to 200V adjustable with external DAC on FB pin

Pin max 20W -> for Vout=200V -> Iout=0.1A, for Vout=100v -> Iout=0.2A...

q1a)

Please can you suggest the circuit for input power limitation?

Configuration 2)

LM51561 in isolated flyback ( PSR or optoc-oupler feedback regulation) 

Vin from 37V to 57V 

Vout from 75V to 200V adjustable with external DAC on FB pin

Pin max 20W -> for Vout=200V -> Iout=0.1A, for Vout=100v -> Iout=0.2A...

q2a)

Please can you suggest the circuit for input power limitation?

q2b)

Please can you suggest which topology is better for adjustable output voltage: primary side controlled flyback or flyback with opto-isolated FB?

q2c) 

is it possible to create isolated flyback with variable output voltage? if not is there a isolated boost topology with variable output voltage?

Best Regards,

David.

  • Hi d_zero,

    Thanks for reaching out with your questions and for considering the LM5156 in your design.

    For the first condition I suggest taking a look at the reference design below. This should be a good starting point for your application.

    https://www.ti.com/tool/PMP21781

    Regarding the power limiting, are you trying to limit the input current or the output current? 

    For the second use case I suggest using an opto-coupler to regulate the output voltage. This is going to provide better accuracy on the output voltage.

    Thanks,

    Garrett

  • Hi Garrett,

    thank for the replay.

    Regarding configuration 1)

    yes the ref design is a good starting point.

    i want to limit power for all scenarios to 20W max, so i think the best is to limit input current because the input voltage is fixed and the output voltage is variable, and for different output voltages we will have different output current to get max output power.

    Regarding configuration 2) 

    is it possible to adjust output voltage in configuration 2  like in configuration 1 if the converter is isolated flyback ( has fixed transformer) ?

    Br,

    David.

  • Hi Garrett,

    you can ignore configuration 2, we will go with configuration 1 BOOST topology.

    i tried to use Power Designer with following parameters but it could't create the design.

    Please can you tell is this configuration possible:

    Vin_min= 5V Vin_max=5V

    Vout_min = 12V (Iout = 1.25A), Vout max = 200V ( Iout = 75mA) (Vout Adjustable), Pout_max =15W


    Would you recommend any other boost controller for this application?

    Br,

    David.

  • Hello David,

    With 200V output at 5V input, the duty cycle calculation in Webench does not allow this design.

    I would recommend to use a flyback design for such a high voltage ratio.