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LM2587: Designing dual output flyback circuit.

ROBERT DELACRUZ
Prodigy 20 points
Part Number: LM2587

I’m trying to design a dual-output flyback circuit using the LM2587T-ADJ chip. Input would be 30 volts while outputs would be +/- 15 volts and each drawing 3 amps each. Figure 26 of the LM2587 datasheet seems to be a good starting point. Resistor and potentiometer will have to the added from FB (pin 2) to the (+) Vout side of the transformer secondary. Also, components to be used have to be sized up for the correct power requirement.

My question is, could I really use the circuit in Figure 26 and attain the outputs that I needed or am I completely off. If I am off, could you provide guidance?

Also, I am unsure as to how to accurately calculate the input power requirement of my circuit. When trying to calculate the input current requirement this is how I estimated it (assume efficiency is 70%):

(input power) * efficiency = output power

(30 volts * X amps) * 0.7 = 30 volts * 6 amps

X amps = (30 * 6) / (30 * 0.7) = 8.57 amps

Is the calculation correct? If it is, another question is, would the 8.57 amps be the switched current?

  • 0
    TI__Mastermind 34201 points

    Hi Robert,

    LM2587 minimum current limit value is 5A, which is not big enough for this application.

    You can take a look at the reference design: http://www.ti.com/tool/PMP20551

  • 0 in reply to Zack Liu
    Prodigy 20 points

    Hi Zack,

    Thank you for your reply. I really appreciate for taking the time to read my query and answering it.

    I do see that the LM2587 would be underpowered and won't be able to provide the particular output that I needed. Looking at the alternative design you suggested, I see how it could work but I would still need to add a negative output since the output requirement is +/- 15 volts each drawing 3 amps (max).

    I am considering of perhaps paralleling two (or three) LM2587 together to increase the load power. There is a literature put out by TI that provides hint as to how one might go about it. It is entitled, "Increasing Available Load Power in an LM2587 Boost Regulator (by Paralleling)". See attached file.

    In the sample circuit shown in the literature, it depicts two inductors and two (switch) diodes where each pair (inductor/diode) is connected to each of the SW pin of the LM2587 chips. Do you think this is necessary? In the Flyback configuration, that inductor would be the primary winding of the flyback transformer. I was hoping I could simply tie the SW pins together and connect them to the primary winding of the flyback transformer and its (switch) diode rather than coming up with a second flyback transformer.   

    - Robert

    snva552.pdf

  • 0 in reply to ROBERT DELACRUZ
    TI__Mastermind 34201 points

    Hi Robert,

    Sorry I have never tried to parallel two flyback circuits before and don't know what will happen if connecting two outputs. I'd suggest you optimize the reference design by adding a third winding to get the negative 15V Vout.