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LM5022: Isolated Boost converter with simple transformer (only 2 winding primary/secondary), optoisolator, few components

Michele Bonollo
Prodigy 170 points
Part Number: LM5022
Other Parts Discussed in Thread: LM51551, LM5155, LM3481, TPS40210, LM3478, LM3488, LM5020, LM51551-Q1

Hi, i need and search the easier isolated boost converter like Flyback or Forward or others.

I have low voltage input 10 to 30 V about, and i need 55V output about... (with ratio transformer) not too much power, about 20/25 Watt 

what is the easier, few components DC/DC regulator, besides the LM5022  ??   there are others controllers ??

Thank you

  • 0
    TI__Guru* 75825 points

    Hi Michele,

    Thank you for posting.   Please consider the LM5155, or the LM51551.  The difference is mainly that the latter has the hiccup mode under overloading and the former does not.  You can also use the LM5022 but the LM5155 is our latest device which should have better performance for your applications.

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 170 points

    Hi Youhao and thank you very much for your help.

    Yes is correct i can analyze the newer LM5155 but it seem too much complex, too much components, the transformer with 3 winding! (i prefer two),

    I don't need many control, only UVLO, current limited on the external mosfet, Vin and Vcc may be the same easy!

    The my input voltage is fixed and robustness +12Vcc 2A, The Output is not critical it's for a PoE injector 54.75V to 57V  Target 55,5Volt (about)

    0,4A out (about) i don't need a perfect regulations...  With a 10 pin maximun of IC controller

    You have a large and nice choice of isolated Boost controllers (external switch), i think at least:   !!!!!:

    I have see the LM3481, LM5022  or others ???

    What is the last one easy controller IC of this, that i can find for the next 10 years about, not obsolete...  the Newest  ??

    Thank you

    LM5155DSS (snva866).pdfLM5022MM(tidrf08).pdfLM3481MM (snva761a).pdf

  • 0 in reply to Michele Bonollo
    TI__Guru* 75825 points

    Hi Michele,

    We also have TPS40210, LM3478, LM3488 and LM5020 for flyback.  

    Not sure where did you get an impression that the LM5155 is more complex than the LM3481.  Because of the number of pins, or because of the EVM schematic?  The LM5155 has two more pins, of which the PGOOD can be left floating, and SS will help you to program the soft-start for better performance.  The third winding on the transformer is optional and you don't have to have it, and two winding transforms also works. And every else is the same, and, the LM5155 is actually smaller in size.

    The reason for recommending the LM5155 is that it is our newly developed device which having performance improvements:  stronger gate driver,  lower current sense working voltage, etc.  However, if you prefer to use the older parts, any of our flyback controller, they should be able to do the work.  We do not have plan to obsolete any of these parts. 

    Thanks,

    Youhao 

  • 0 in reply to Youhao Xi
    Prodigy 170 points

    Hi Youhao,  you're right, maybe it's better to use the last chip LM51551-Q1 (LM51551DSSR)   I have only these doubts:

    1) I must have a flyback transformer with only two winding NOT Aux Vcc winding (Only primary & secondary, i think 5:1 ratio)

    2) I need flyback Optoisolated feedback regulations, VCC pin (N°2) must be connect to the BIAS pin ?? or left floating with by-pass capacitor only??

        I have +12 Vsupply input and +55V Out with 5:1 transformer coil ratio

     3) The New Hiccup mode is present only on the LM51551 version OK!  But there is difference from LM51551QDSS and LM51551DSS  ??

    I can use LM5155  without problem on the same PCB ??  the Hiccup mode is automatic and transparent  ?? Not need change components ??

    Many thanks

    Michele

    @Transformer_Wurth_750342957.pdf

    @Transformer_Coilcraft_NA5743-AL.pdf

  • 0 in reply to Michele Bonollo
    TI__Guru* 75825 points

    Hi Michele,

    1) Yes, you can use a transformer only having two windings.  

    2) The VCC should be left alone with its terminal bypass capacitor.  You should not connect VCC to the BIAS pin in your case because BIAS can see voltage that exceeds the VCC rating.  The primary to secondary turns ratio should be 1:5.

    3)  Other than the hiccup mode, the two devices are the same, and pin to pin compatible. You can swap them without any problem. Just not having the hiccup mode with the LM5155.

    I looked at both transform datasheet, and the primary inductance is too small and you will end up with a DCM flyback.   The smaller the inductance the deeper DCM it will be. Nothing wrong with it but the efficiency may be lower than a CCM.  

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 170 points

    Thank you very much for your valuable advice.  

    It's all pretty clear for the project. Yes i want find a better transformer but it is not easy! I not found other DC/DC Transformer for about 12V to 55V.

    This week I put the complete drawing on a related question.

    Best regards