High voltage boost converter using UC3845. Need some assistance!

Discussion in 'The Projects Forum' started by daviddeakin, Jun 20, 2014.

  1. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    I am trying to design a boost converter using the UC3845. I want to step up from 19V (e.g. laptop PSU, but currently using a bench power supply) to about 250V. It would be nice to get 50mA out of it, but I would settle for less.

    I have a circuit that is sort of functioning, except its load regulation seems unusually poor. Here's the problem:
    Switching frequency ~39kHz
    296V into 100k load, duty cycle 20.2%
    250V into 25k load, duty cycle 29.5%
    242V into 20k load, duty cycle 31.8%

    Can anyone suggest why the output is so poorly regulated, and how to fix this?
    [​IMG]
     
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  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I would replace the inductor for a pulse transformer, to get better regulation, here is a circuit using a pulse transformer.
     
  3. Billy4184

    Member

    Jun 20, 2014
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    According to http://www.ti.com/lit/an/slva372c/slva372c.pdf your output capacitor is horribly low and the ripple could be around 3V for a 50mA current. What does the output waveform look like?
    I've only got experience with buck converters though.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    10uF is the minimum for the output capacitor
     
  5. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    I have only loaded it to about 12mA, so the ripple is not severe. Even so, would that really explain the poor overall regulation? Isn't ripple a separate issue from the average output voltage?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I could be wrong, but I think you need a much larger inductor if you are running at 39Khz. My guess is the inductor is saturating and the current sense shuts down the duty cycle as it should be in the 90's.
     
  7. Billy4184

    Member

    Jun 20, 2014
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    @daviddeakin, not necessarily, as it affects the feedback to the UC3845, which can only react within a certain timeframe set by the frequency, and when it does react, has to charge up the mosfet and wait for the inductor to respond as well. Did changing the cap change your results?
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    I modified a 12v/35v booster build with said chip so it can output nearly 100 volts.

    On the output side I had 100uF capacitor.

    Output regulation at high voltage level with only 100uF was very unstable, the digital display always jumping. I solved it by inlining one winding from an SMPS Transformer (as Inductor). Then I got the output voltage stable.
     
  9. Billy4184

    Member

    Jun 20, 2014
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    Also, I believe ronv is right, the inductor (according to the design sheet I linked to above) should be about 3500uH.
     
  10. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    With 100nF capacitor, output ripple is a triangle wave, 0.5Vpp at 2.3mA increasing to 3Vpp at 9mA. Increasing to 1uF did not alter the regulation, but it did reduce the ripple to 0.5Vpp at 9mA with some ringing at about 2.5Mhz.

    To answer Ronv, the UC3845 can only go up to a 50% duty cycle, so I'm trying to work within that, which explains the small inductor. The signal at the MOSFET drain is a fairly neat ramp up, with some ringing at switch off, so I don't think it is saturating...
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Hmm. That's a problem. You could use the 3843. Otherwise I don't think you can get there from here. Give Billy's link in post 3 a read.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The regulation occurs at the Vout + peak, so any output ripple hurts your regulation. Going much larger on C5 will help.

    You can also try a small cap bwetween wiper and gnd of the 50k trimpot, to force regulation to the average Vout not Vout peak. Be careful not to make that cap too big if you decide to try that technique.
     
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