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Low power PWM controller, possibly the 555 (flame war coming)

Discussion in 'Electronic Design' started by Klaus Kragelund, Aug 30, 2012.

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  1. Hi

    I am working on a small 100mW forward converter and I need a very low power PWM control.

    So the 555 comes up, along with some ultra low power opamps. But, isn't there a PWM chips out there with low operating current. Sofar I found the TPS5110 with sub mA current consumption:

    Anyone used one with lower current?


  2. Well, I have a Cortex M3 running at 4MHz, but the sw guys don't like
    me using that in a SW loop.

    I could generate a PWM to set the frequency, add F/F to set the
    output, and a comparator to reset the output (current mode control).
    Then add an voltage error amp to control the peak current level.

    Going further, I could use the internal comparator for the reset of
    the PWM for peak current control without CPU resources used, but the
    comparator has 2-4us propagation delay, seriously lowering the
    sensible frequency the converter can run at. I guess the delay is just
    a level shift of the peak current level and that is regulated back by
    the error amp anyway, so maybe it could work. Would however have a
    minimum duty cycle of 20% (for 100kHz and 2us propagation delay)

    For that solution I need to add a circuit to monitor the current and
    reset the CPU if it hangs. That takes some more parts in the wrong

    But, I would like to see if there is a PWM controller that has low
    enough current anyway.


  3. Yes, its a buck, and it has to large max duty cycle. Just an example of a low current PWM.

    My input voltage is from about 8V to 20V, output 3.3V. Transformer will probably be 1:1 so I can use off the shelf types approved for EN69650.


  4. Guest

    Well, a forward, isolated Buck. Sorry if that was not clear.
  5. Tim Williams

    Tim Williams Guest

    Do something like this, but configured for forward operation instead:
    obviously, replace the amp-gobbling TL431 with something more frugal.

    You can save the bias current wasted in the switching transistor by using
    a positive feedback winding, making the circuit a basic blocking
    oscillator instead.

  6. josephkk

    josephkk Guest

    You clearly missed the isolation requirement implied by EN69650.

  7. Guest

    Well, I originally wrote it was a Forward, and that is commonly known as an isolated buck.....


  8. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    The question being: why do you want a forward at this low power level?
  9. Very basic schematics here:

    Please note that it is simplified as I am not allowed to post details to the web.

    Single switch forward with extra winding reset. Chosen since the input voltage is not steady 20V, but can drop very fast, which would pose a problem with half-bridge solution (capacitor midpoint slewing slowly) and since access to the current is easy (sense resistor)

    I need high efficiency, so will probably convert this to ZVS topology lateron.
    The FET is controlled by microcontroller PWM. This PWM is setup as high duty cycle (45%), but peak current is detected by the comparator which in turnresets the PWM (input to PWM module) before reaching 45%.

    On the output I have classical TL431 regulation with optoisolated feedback to the primary side. (not shown). Larger caps needs to be placed on the output to ease the response time of the regulation loop (microcontroller measured error voltage)

    I have a need for an extra voltage on the primary side for internal supply,but have not yet found off-the-shelf magnetics that facilitates this.


  10. Sounds interesting, how can you make it much simpler? (notice that either the diodes or the FETs are mounted on the output, the FETs are for syncronous rectification)
    Yes, that would be simpler, but a flyback must be gapped, so no off-the-shelf magnetics in small sizes are available.


  11. Guest

    considered push-pull something like a MAX845 ?

    you must have a 3.3V available for the MCU, can you use that?
    probably a lot more to choose from at that voltage, 20V is a
    bit to high for most of the low power parts aimed at battery supply

  12. Guest

    it looks interesting. I have not been able to find any information on this exact topology, but it looks like a LLC converter without the high side switch. Do you have any more information on this?


  13. Guest

    Max845 takes up to 5 mA just for the chip and that's all my power budget and more to it.. Nice chip, though, I might find another one with lower current consumption. Anyway, how do they avoid flux walking..... The dead time they have inserted?

    I could run at a 3.3V which I o generate, but then I have two converters in series, and that's bad for efficiency. Also, at 3 volt the output diodes means I loose a lot of efficiency.


  14. Guest

    Yes, the compound you drew. It's a very nice circuit and avoids the need to drive a high Side FET and has zero voltage switching possibilities. Maybe a circuit added to monitor the ringing and turn on the FET at the right time to lower losses


  15. Tim Williams

    Tim Williams Guest

    The tubophiles have been calling it "parafeed" for years. I forget what
    RDH4 calls it. It's probably older still.

  16. Tim Williams

    Tim Williams Guest

    Except if you overdrive an SE amplifier. Now what? ;)

  17. Tim Williams

    Tim Williams Guest

    Well that was easy. You really are a terrible troll, you know that?

  18. Guest

    have you looked at linear? they have lot of sub mA micropower buck/

    or something like flyback but
    they suggest an number of trafos in the data sheet

  19. That looks like a very interesting part indeed, would almost match my needs. Only downside is the price, over 2USD in large quantities, but it saves many parts from my schematics :)


  20. Do you have a part number for that part?

    We use royer converters a lot, in designs that needs to be self sustained. Would probably be a good choice here too, thanks for the suggestion.


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