Problem with (multiphase) buck converter

Discussion in 'Power Electronics' started by entemomoh, Jul 14, 2016.

  1. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Hello everyone,

    I am building a bench power supply, that is realised by stepping down the 12V rail of a PSU. The pwm is done by a 555 timer and a digital pot and comparator feedback system. The problem is a voltage drop to 7 Volts at maximum pulsewidth when connected to just a 20 watt load. As a solution I want to use a multiphase buck converter topology, but I haven't found a way yet to delay the signal or alternate it between the two mosfets to split the current up evenly.

    I hope that someone has an idea of how to manipulate the signal like I described or if there is a completey different approach, just let me know. Thank you in advance :)

    PS I want to use no dedicated ICs and as little microcontrollers as possible
     
  2. crutschow

    Expert

    Mar 14, 2008
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    My crystal ball is in for maintenance so I can't really help you without a schematic of what you have.
     
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  3. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Allright, I will send a schematic as soon as I can
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Just post it on this forum by copying and pasting a screen shot or using the "Upload a File" tab at the bottom.
     
  5. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    upload_2016-7-15_20-48-5.png

    The "control pot" is representing the feedback system, but that is not important, I just want to control the output signal to switch two mosfets with an inductor each for multiphasing. I know that I could replace the diode with a mosfet, that runs of the inverted gate signal to make it synchronous, but that is not what I want, just multiphasing.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    It looks like you are using generic parts for the diodes and MOSFETs.
    You need to select some real part models to get a proper simulation.
    I know you are hell bent on going to a multiphase design but let's first understand your circuit operation before adding that extra complexity. ;)
    Post your .asc file.
     
  7. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Well, the schematic in ltspice is, as the name implies just a schematic :), I have built this curcuit in real life and it works quite well, and when i connect a 5 ohm load for example I get a drop from 12V to 7V even at maximum PWM. I have also tried to use an atmega for PWM but the same thing happens, but when I connect a little lightbulb, I can adjust the output voltage from 11.5V-1.8V, which is how it's supposed to work. Nevertheless when I'm on PC, I'm going to select real Parts and send the .asc file. And I already have a good understanding of normal buck converters;)
     
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    So why not just use a physically larger inductor and switching device? o_O
     
  9. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Because I want to use parts I already have if possible:D
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Then put two or three of them in parallel. :rolleyes:
     
  11. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    If you say so,I will try that. Unfortunately I'll have to wait a week before I can try, but it's interesting how there's nobody who actually tried that, everybody is always multi phasing. However, I will still except other possible solution, whatever works best. Thank you so far for your replies :)
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Who's everybody? o_O

    I've never multiphased SMPS's.:rolleyes:
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Multiphasing is mainly used to reduce ripple and requires complex timing. It is not readily done with a few added parts as you seem to think.
    If you want to use the parts you have, then follow tcmtech's suggestion.
     
  14. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Alright, I will try to parallel nmoses and inductors and tell you my results, as I said in the first post, I also accepted things other than multiphasing;) thanks crutschow and tcmtech for your help and quick replies:D
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Nobody who? Running multiple parallel inductors or HF transformers in power supplies is a very common design technique in applications where using one single unit would be too large or space constraint prohibitive.

    In most higher powered DC - DC power supplies like those used in larger power inverters and car audio amplifiers running 2 - 20+ smaller inductive devices paralleled to work as one large unit is standard design procedure. ;)
     
  16. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    It's all about how you phrase your google search:rolleyes:
     
  17. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In round numbers, single-phase buck regulators are perfectly fine for outputs up to around 100 W, thanks to large, relatively cheap power MOSFETs. Multi-phase buck regulators are *not* a simple extension of a basic buck circuit. Linear Technology has specialized controllers for this, and the sample schematics on the datasheet are an excellent indication of the overall circuit complexity.

    So far you have not mentioned the output voltage/current/power levels you are trying to achieve. Also, in the 20 W condition in post #1, what did the output voltage waveform look like? What was the ripple compared to a 10 W output. What were your calculations that led to the inductor size. Finally, what is the inductor part number, construction, and resistance. Each of these things individually can cause your problem, let alone combinations of characteristics.

    ak
     
  18. entemomoh

    Thread Starter New Member

    Jul 14, 2016
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    Well, first of all, due to 630uF main filter cap and 104 noise cap the ripple was within 100mV with both loads, not much difference there.

    Second(and please don't hate on me for that:)) I didn't calculate much for my inductor values, I don't even have a way of telling the inductance, since I salvaged them from an old PSU.

    My goal was to be able to get to like 11 Volts at maybe 10 amps, so close to 100W would be fine and as I mentioned multiphasing was only the apparent cure for all my problems, which is why I thought it might be a good idea(I don't think that way anymore;))

    As a last note, interestingly enough LTspice although only given a rough equivalent of my mosfet and the arbitrary size of 47uH for the main inductor gave me around 7 Volts with a bit less ripple than IRL, but still darn close for an estimate.

    hope that helps
     
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