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Generate -100V DC

Discussion in 'Electronic Basics' started by Denis Gleeson, May 31, 2004.

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  1. Hello All

    I placed a similar question some time back but I didn’t give
    sufficiently detailed requirements. Im hoping that more details may
    solicit some responses.

    I need to generate somewhere –55 to –100V DC from a 12V
    source. I need to supply 100mA and cant use a transformer. I also cant
    waste a lot of power in doing this so I need high efficiency.

    I attempted a design with the LM3578 from Nat Semi but found I was
    generating a lot of heat in the pass transistor (I was consuming 130mA
    with no load) and not getting below about –25V.

    Ive looked at some devices from Maxim and will test them soon.


    Any suggestions for circuits or devices would be appreciated. And If
    anyone can guess why I was consuming 130mA with no load on my test
    cct, that would be interesting.

    Regards

    Denis
     
  2. The LM3578, should do what you want fine. However there are some major
    'caveats', which apply to _all_ such circuits. Firstly, the very high
    instantaneous currents, and gain in some parts of the circuit, really do
    make it very unlikely to work, unless built on a PCB, or hand wired, with
    good solid tracks. If you use things like plug-in 'prototype' boards, such
    circuits will generally not work properly. You may have had a problem of
    this sort. Secondly, such circuits generally do require a little load. Your
    system may have been driving just one cycle, then overvoltaging the output,
    and then sitting for a long time to recover, giving a 'silly' apparent
    voltage. Your output transistor, may have been unsuitable for the task.
    Again the currents involved can be high, and chosing the right transistor
    makes a huge difference to the overall efficiency.
    Look instead at the LM2577. This has it's own transistor built-in.
    Your high heat with no load, and failure to get above 25v (? you say
    below?), would probably suggest the inductor/transistor was not correctly
    selected.

    Best Wishes
     
  3. Denis Gleeson wrote...
    You want to generate a negative voltage from a positive one,
    without using a transformer, right? The LM3578 can do this in
    its "inverting regulator" mode, see datasheet fig 20. But you
    won't be able to have more than -1V output (see note 2) unless
    you add a HV transistor wired in cascode mode, see fig 21. If
    you use a 120V-rated (or higher) transistor you'll be able to
    get up to -100V output.

    Note, to provide 100mA at 100V, you'll draw 833mA average from
    your 12V supply. This translates to an inductor current ramp
    with a 1.66A peak. Or actually 1.89A peak, taking into account
    the 88% duty cycle with a 12:100V voltage ratio.

    There's an efficiency issue with the LM3578 used as an inverting
    regulator, due to its high Emitter Saturation Voltage, over 1.2V,
    wasting 10% of your power right up front. So you'll need over 2A
    peak capability. Oops! Sorry, the LM3578 is limited to 750mA.


    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  4. (snip)

    I don't understand this requirement, when the correct transformer can
    solve so many of your problems. Can you elaborate on this
    requirement?
     
  5. Winfield Hill wrote...
    This assumes discontinuous inductor-current operation, but if
    you work at a high frequency, so the inductor only partially
    discharges each cycle, you can get more power from an LM3578.
    For example, if the current increases from 650 to 750mA during
    the 88% on time, or 700mA average, you may be able to deliver
    about 75mA to your -100V load.

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  6. Joel Kolstad

    Joel Kolstad Guest

    I'm also curious about his 'no transformer' requirement in that one can
    argue that with a flyback supply what you really have is a 'tapped inductor'
    rather than a transformer. I.e., if for some odd reason he has a design
    document stating transformers aren't allowed, he can just call it what it
    really is and be OK.

    I suspect his real issue is that he wants to be able to purchase his parts
    off-the-shelf, and of course that's much more doable with just inductors
    than transformers.
     
  7. Dave VanHorn

    Dave VanHorn Guest

    A royer converter into a 6V or 12V center tapped transfirmer, should get him
    where he wants to go.
     
  8. Ken Smith

    Ken Smith Guest


    I'd be tempted to try something like the LT1070 from linear in a circuit
    like this:
    --------------------------!!------- etc
    !
    !-----------------!!---------
    ! !
    -----))))))------------!!-----))))))-----!<--------))))))-----!<-- etc
    ! ! !
    LT1070's / V ---
    Switch ! --- ---
    GND ! !
    GND GND

    By making the output a tripler, you make the cicuit place less demands on
    the diodes and inductors but require 3 and 4 of each. This may be a good
    trade off. At 33V a Schottky diode can be used. At 100V use have to use
    a SiC Schottky or a fast silicon diode.
     


  9. Greetings Win.

    Are we looking at the same figure 21 of the same datasheet?

    When I look at this one:

    http://cache.national.com/ds/LM/LM1578A.pdf

    I notice the external transistor Q1 of figure 21 is a PNP device with it's
    base being driven by the internal NPN device in the LM3578. It seems to me
    in this configuration the saturation voltage and maximum current capability
    of the NPN device aren't very important.

    Of course this circuit arrangement isn't especially great at efficiency
    since both the PNP saturation voltage and base drive current is all waste
    which is unfortunate.

    But it does seem like it should work provided the right parts selection and
    barring any other unforeseen problems.
     
  10. Fritz Schlunder wrote...
    You are right, I glanced too quickly and assumed it was a cascode.
    So my whole analysis goes out the window. Sheesh!

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  11. Genome

    Genome Guest

    | Hello All
    |
    | I placed a similar question some time back but I didn't give
    | sufficiently detailed requirements. Im hoping that more details may
    | solicit some responses.
    |
    | I need to generate somewhere -55 to -100V DC from a 12V
    | source. I need to supply 100mA and cant use a transformer. I also cant
    | waste a lot of power in doing this so I need high efficiency.
    |
    | I attempted a design with the LM3578 from Nat Semi but found I was
    | generating a lot of heat in the pass transistor (I was consuming 130mA
    | with no load) and not getting below about -25V.
    |
    | Ive looked at some devices from Maxim and will test them soon.
    |
    |
    | Any suggestions for circuits or devices would be appreciated. And If
    | anyone can guess why I was consuming 130mA with no load on my test
    | cct, that would be interesting.
    |
    | Regards
    |
    | Denis

    There's a voltage mode CUK in the binaries group under the title
    "Voltage Mode CUK". It's not going to be a minimum cost solution but it
    does the job.

    DNA
     
  12. Hi DNA

    Whats the binaries group?

    I know a stupid question for you but
    please point a blind man in the right direction.

    Denis
     
  13. Genome

    Genome Guest

    | Hi DNA
    |
    | Whats the binaries group?
    |
    | I know a stupid question for you but
    | please point a blind man in the right direction.
    |
    | Denis
    |
    |


    If you don't have access then call again.

    Oh you'll need LTspice if you want to have a play.

    http://www.linear.com/software/

    DNA
     
  14. Genome

    Genome Guest

    | | | Hi DNA
    | |
    | | Whats the binaries group?
    | |
    | | I know a stupid question for you but
    | | please point a blind man in the right direction.
    | |
    | | Denis
    | |
    | |
    |
    | |
    | If you don't have access then call again.
    |
    | Oh you'll need LTspice if you want to have a play.
    |
    | http://www.linear.com/software/
    |
    | DNA
    |
    |

    I meant to say....


    DNA
     
  15. Genome

    Genome Guest

    |
    | | | | | | Hi DNA
    | | |
    | | | Whats the binaries group?
    | | |
    | | | I know a stupid question for you but
    | | | please point a blind man in the right direction.
    | | |
    | | | Denis
    | | |
    | | |
    | |
    | | | |
    | | If you don't have access then call again.
    | |
    | | Oh you'll need LTspice if you want to have a play.
    | |
    | | http://www.linear.com/software/
    | |
    | | DNA
    | |
    | |
    |
    | I meant to say....
    |
    | |
    | DNA
    |
    |

    No..... ! I meant to say


    DNA
     
  16. Genome

    Genome Guest

    |
    | | |
    | | | | | | | | | Hi DNA
    | | | |
    | | | | Whats the binaries group?
    | | | |
    | | | | I know a stupid question for you but
    | | | | please point a blind man in the right direction.
    | | | |
    | | | | Denis
    | | | |
    | | | |
    | | |
    | | | | | |
    | | | If you don't have access then call again.
    | | |
    | | | Oh you'll need LTspice if you want to have a play.
    | | |
    | | | http://www.linear.com/software/
    | | |
    | | | DNA
    | | |
    | | |
    | |
    | | I meant to say....
    | |
    | | | |
    | | DNA
    | |
    | |
    |
    | No..... ! I meant to say
    |
    | |
    | DNA
    |
    |

    Let's try


    DNA
     
  17. Genome

    Genome Guest

    |
    | | |
    | | | | |
    | | | | | | | | | | | | Hi DNA
    | | | | |
    | | | | | Whats the binaries group?
    | | | | |
    | | | | | I know a stupid question for you but
    | | | | | please point a blind man in the right direction.
    | | | | |
    | | | | | Denis
    | | | | |
    | | | | |
    | | | |
    | | | | | | | |
    | | | | If you don't have access then call again.
    | | | |
    | | | | Oh you'll need LTspice if you want to have a play.
    | | | |
    | | | | http://www.linear.com/software/
    | | | |
    | | | | DNA
    | | | |
    | | | |
    | | |
    | | | I meant to say....
    | | |
    | | | | | |
    | | | DNA
    | | |
    | | |
    | |
    | | No..... ! I meant to say
    | |
    | | | |
    | | DNA
    | |
    | |
    |
    | Let's try
    |
    | |
    | DNA
    |
    |

    Or


    DNA
     
  18. Genome

    Genome Guest

    |
    | | |
    | | | | |
    | | | | | | |
    | | | | | | | | | | | | | | | Hi DNA
    | | | | | |
    | | | | | | Whats the binaries group?
    | | | | | |
    | | | | | | I know a stupid question for you but
    | | | | | | please point a blind man in the right direction.
    | | | | | |
    | | | | | | Denis
    | | | | | |
    | | | | | |
    | | | | |
    | | | | | | | | | |
    | | | | | If you don't have access then call again.
    | | | | |
    | | | | | Oh you'll need LTspice if you want to have a play.
    | | | | |
    | | | | | http://www.linear.com/software/
    | | | | |
    | | | | | DNA
    | | | | |
    | | | | |
    | | | |
    | | | | I meant to say....
    | | | |
    | | | | | | | |
    | | | | DNA
    | | | |
    | | | |
    | | |
    | | | No..... ! I meant to say
    | | |
    | | | | | |
    | | | DNA
    | | |
    | | |
    | |
    | | Let's try
    | |
    | | | |
    | | DNA
    | |
    | |
    |
    | Or
    |
    | |
    | DNA
    |
    |

    Hooray, if your news server has the group then that last one should take
    you to it.

    DNA
     
  19. Genome wrote...
    a.b.s.e. is alt.binaries.schematics.electronic

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  20. Fred Bloggs

    Fred Bloggs Guest

    You shouldn't have! If he couldn't get to square one with that simplest
    of simplest and totally self-contained 8-pin 3578 where all he has to do
    is paint by the numbers- or nomographs- then no chance in hell
    whatsoever with that Cuk. The OP is most likely a troll- no mention
    whatsoever of a single part number or operating parameter for his
    "prototype"- and a 2:1 range in acceptable output voltage?- all at same
    maximum current?
     
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