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LM2575HV: -40V Negative input, -24V negative output - Is it possible with LM2575HV-ADJ

Nir Katchinskiy
Prodigy 50 points
Part Number: LM2575HV
Other Parts Discussed in Thread: LM2575

Hi,

I have LM2575HV-ADJ chip from TI, and I would like to use it as a negative input , negative output buck converter.

My Vin is -40V , Vout is -24V.

I found a reference that says that it would not be possible: http://projectcircuit4u.blogspot.com/2011/07/negative-output-switching-regulator.html 

  • 0
    TI__Mastermind 25590 points

    Nir,

    That blog does not use a buck topology so their statements are correct but that doesn't mean it can't be done. You could achieve your spec by setting it up as a 40VIN to 16VOUT buck regulator. Then refer VIN of the buck to GND of the rest of the system. That means we map:

    • VIN from 40V --> 0V (GND of the rest of the system)
    • GND from 0V --> -40V
    • VOUT from 16V --> -24V

    Would that work for your system?

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 50 points
    Hi Sam,
    Thank you for the quick reply.
    Are you able to post a schematic? I am a bit confused ..
    I think I essentially tried something very similar to your suggestion -> treating my Vin as GND, and my GND / ON/OFF as -40V.
    When I tried that, it works when there is no load. But, as soon as I put a load on the power supply the voltage drops significantly.
    I have a suspicion that it is something to do with connecting the feedback in between R1 and R2, instead of doing some sort of level shift.
    Thank you again,
    Nir
  • 0 in reply to Nir Katchinskiy
    TI__Mastermind 25590 points

    Nir,

    Here's what I had in mind. The top picture is a regular buck. The bottom shows what happens to the relative voltages when you move the GND connection on the rest of the board from GND on the LM2575HV to VIN on the LM2575HV.

    Is this what you tried?

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 220 points

    The problem with that approach is that the regulator has no way to sink current from GND. So if you put a load on the -24V it will rise.

  • 0 in reply to Michael Biggs
    Prodigy 50 points
    I agree with Michael,
    This is exactly what I tried already and that it exactly what happens.
    Nir
  • 0 in reply to Nir Katchinskiy
    TI__Mastermind 25590 points
    Nir and Michael,

    You are absolutely correct, my suggestion would not work. I didn't think it all the way through. No wonder I haven't heard of it done this way.

    Please share a schematic of what you have and what you need. I'd like to know if the 40V supply is floating, already connected somewhere to the board... then we'll go from there.

    -Sam
  • 0 in reply to Samuel Jaffe
    Prodigy 50 points
    Hi Sam,
    I have a transformer that transforms 120v to +40v, GND, -40V (after the large 2200uF capacitors).
    I pass the signal through a rectifier, and then I want to produce a regulated voltage of +24V and -24V. I used the LM2575 to regulate +24V (that is simple).
    However, now, I am trying to use a similar topology to produce the -24V and it seems to not work with the LM2575HV. I could easily use a linear regulator, but it is not very efficient, it produces a lot of heat, and I am worried that it will not be reliable in the long run.
    I am wondering if I should just switch to a different regulator, or if I can use the LM2575HV.
    Nir
  • 0 in reply to Nir Katchinskiy
    TI__Mastermind 25590 points
    Nir,

    I believe you can use the method I described with a buck which can sink current. Look for a synchronous converter/controller that has a current sensing and reverse current limit (for protection).

    You could also use an isolated topology to convert from the 120VAC directly to +/-24V. Have you considered a flybuck (lower power) or a flyback (higher power)?

    -Sam
  • 0 in reply to Nir Katchinskiy
    Prodigy 50 points
    Hi Sam,
    Thanks for the direction.
    Seems like the LT8709 would do the trick.
    Do you have a suggestion on a similar chip that comes as through hole packaging instead?
    Thanks!
    Nir
  • 0 in reply to Nir Katchinskiy
    TI__Mastermind 25590 points
    Nir,

    I cannot assist with a design using an LT part since this is the TI forum.

    TI has not made through-hold packages in many years but you can search for this with the filters on digikey.

    -Sam