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+12V \ -12V, 0.5A - 0.8A Power supply

Shmuel Davis
Expert 2920 points
Other Parts Discussed in Thread: LM51551, LM5155, LMV431

I am looking for a design that will give me the smallest possible total PCB area with the following requirements : Input=+5V, Output=+12V \ -12V, 0.6A - 0.8A Power supply. What does TI have available. Please could you also provide specific components so that I can evaluate the actual PCB area 

  • 0
    TI__Guru 68495 points

    Hi Shmuel,

    Thank  you for reaching out.  You can consider a non-isolated flyback topology to get the +12V/-12V with two secondary windings on the transformer. Recommend the use of the LM5155 or LM51551, depending whether or not you need the hiccup mode of overloading protection.  Please refer to the following App Notes for the design of a single output flyback for 12V and 1.6A, then you can add the secondary winding to split the power into +12/-12V rails. The App node is for isolated flyback, but you don't need the isolation so you can save to opto coupler as well as the LMV431 circuits, and just use the FB pin of the LM5155 to directly sense the output. 

    www.ti.com/.../snva866

    Thanks,

    Youhao Xi, Applications Engineering. 

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao Xi,

    Thank you for getting back to me ! 

    It appears that the design that you are suggesting would require 2 transformers Wurth Elektronik 750317933 with 13.97mm x 18.2mm in area. There is no way that size design would fit in my limited PCB area ! Currently I am using a design from Linear \ Analog that fits in approx 22mm x 22mm on Top of PCB and 6mm x 8 mm on the Bottom of the PCB. Does TI have anything that can match this ?

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 68495 points

    Hi Shmuel,

    Oh no, it does not need two transformers.  It just need two secondary windings on the same transformer, one for +12V and the other for -12V.  If you currently has a design with a different IC, you can simply replace that IC with the LM5155. These ICs are simply PWM controllers and the LM5155 will work and offers better performance:  strong 1.5A driver, efficient current sense with up to 100mV working voltage, and low Iq, etc.

    The LM5155 is TI's newest controller, having many performance improvements. Please note that there are two devices in the same family (pin-to-pin drop in compatible), the LM5155 and the LM51551.  The latter has OCP hiccup mode protection, and the former does not.  In OCP, the former will continuing operation with limited peak current.  Choose the one that you prefer.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao,

    Thanks for getting back to me. 

    Would the design that you are suggesting require a (single) transformer similar to Wurth Elektronik 750317933 with 13.97mm x 18.2mm in area - as found in the design e.g. ? There is no way that this design could fit into our limited PCB space ?! 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 68495 points

    Hi Shmuel, 

    Your power level is up about 20W, so you should reserve adequate space for a 20W converter. You should contact the magnetic vendors like Wurth and ask for a smaller footprint design.  If you area is not enough, you may consider two boards to stack up with interconnections.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao,

    Thanks for your reply. Currently I have a design that (I am pretty sure that it) fits into my limited PCB space that gives me 2 supplies 12V @ 0.8A and -12V@0.55A. I was wondering if TI has a design that might be better (either same power smaller area of same area and more power ?)

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 68495 points

    Hi Shmuel,

    Can you share your current design and we may propose something in accordance?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao,

    I took it from the Linear LT8582 Data sheet TYPICAL APPLICATIONS : 1.5MHz, 5V to ±12V Boost and Inverting Converter and the parts that they outline there ! 

    Best Regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 68495 points

    Hi Shnuel,

    It is a converter (integrate FETs) and it would be difficult to support 0.8A load at  +/-12Vout.  The flyback solution for +/-12V output can be competitive in solution size and overall cost.   Can you tell us how big area can be used for this dc-dc on your board?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao,

    In fact even if the design could only give me 550mA from both +/-12V this would Probably be enough (which they claim is the Max current on the -12V !) however if I can get a design with a smaller in area or same size but higher Max current I might consider moving over to that design. 

    So far from my PRELIMINARY layout the

    TOTAL APPROX. PCB Area on the TOP = 18mm x 18mm 

    TOTAL APPROX. PCB Area on the BOTTOM  = 5mm x 8mm 

    If you have something that can better this please provide me all the details especially ALL the required external components ! 

    Thank you in advanced 

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Guru 68495 points

    Hi Shmuel,

    Sorry for the delay: I am currently on a business trip.

    I don't have a immediate design but if your area is 18mm x 18mm on both side, it should be doable.   If your back side only has 5mm x 8mm, I am afraid the space is very tight to place a ~15W transformer with two output rails,  two rectifier diodes, one MOSFETs, the controller, both input and output capacitor, and some resistors and capacitors, and via holes to connect signals on both sides of the PCB.  See if you can get more space on the back side.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 2920 points

    Dear Youhao,

    Thank you for all your help 

    Best regards

    Shmuel