Boost Converter Feedback Loop Troubles

Discussion in 'The Projects Forum' started by bitrex, Dec 27, 2009.

  1. bitrex

    Thread Starter Member

    Dec 13, 2009
    79
    4
    Hi all - I'm using one section of an LM324 quad comparator to regulate the output of a boost converter circuit by comparing the output to a reference voltage. The output of the comparator is connected to the "reset" pin on a 555, so when the voltage reaches the correct level the drive pulses are shut down. My problem is that the comparator just doesn't seem to swing high enough to fully turn on the 555 when it's supposed to, and doesn't swing low enough to shut it down. I think part of the problem is that when the circuit I have is used with a 9 volt battery, the supply voltage plunges to 5 volts. The output of the comparator never rises higher than about 2.5 volts, and never falls lower than about 0.8 volts.

    Would using a switching transistor connected between the comparator output and the reset pin of the 555 help matters? The circuit seems to work properly when the reset pin is shorted to the supply or ground, so apparently that pin needs to really be pulled up or down hard. Or maybe there's something else happening? I thought I'd ask before I go desoldering things. :D
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You know that an LM324 is a quad operational amplifier, not a comparator, right?

    And that it's outputs are not rail to rail? You'll be lucky if it can get within a couple of volts of the positive rail.

    Instead of turning the timer on and off, have a look at using one 555 timer as an astable multivibrator to trigger a monostable 555 timer. Use pin 5 of the astable multivibrator to control its' frequency.

    A 9v "transistor" battery is a poor choice for a boost circuit. It won't last long at all.
     
  3. bitrex

    Thread Starter Member

    Dec 13, 2009
    79
    4
    I didn't realize the 324 had limited output swing until after I soldered up a prototype circuit. :( Beyond just that problem, I must have made an error when soldering it up because the circuit isn't boosting correctly and was acting erratically even with the 324 removed from its socket. So I'm going to scrap the board and try again. I guess that's what I get for trying to build a prototype before I'm positive the circuit works on solderless breadboard. This time I'll try using a proper comparator IC like the 393.
     
  4. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    Keep in mind that SMPS can easily turn into nightmares. So dont be surprised if it takes a few attempts:D
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you post your circuit design before you build it so that we can help you to evaluate it?

    I suggest that you take a look at this page:
    http://www.dos4ever.com/flyback/flyback.html
    Ronald Dekker did a great job on his site. This is a very helpful resource for switching supply n00bs.

    Keep in mind that there are high voltages involved in those projects. If you use capacitors to stabilize the high voltage portion, only use small caps so that only a small amount of energy is stored.
     
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