Boost Converter Simulation

Discussion in 'General Electronics Chat' started by Chacabucogod, Oct 19, 2014.

  1. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    Hi I'm currently modeling a 5-12Vdc boost converter, and just out of nowhere decided to put a transistor between the output resistance and the diode. Now, this transistor is always connected to voltage so it shouldn't do anything at all, it's just a closed switch. What happens is that the transistor ends up eating a lot of the voltage. I don't really understand what's happening. Circuito.png
    The output voltage in the resistance should be something like 12 volts and it ends up being like 2.7 volts. and the current goes from 14 mA to -3mA. I guess that's where the problem lies. The trasnsistor is doing something to the current. Might it be that the maximum current this transistor can withstand is less than the one i'm giving to the resistance?

    Thank you.
     
  2. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    I've since changed the trasnsistor to a tip31c and the voltage on the resistor is now more, but still the transistor is eating up a lot of voltage. I forgot to mention that the DC voltage is oscillating from 13 to 9 volts, but I believe that has to do with the capacitor I chose.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Could the value of R1 be too large to allow Q2 to be driven into saturation?
    Being that Q2 is an emitter follower that might contribute to the problem.

    You could move Q2 to V- to gnd and it might saturate more easily.
     
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  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The voltage at Q2 emitter will be 5V minus the voltage drop across R1 (caused by the base current) minus Vbe, so it's not surprising you only get 2.7V. Put R5 in the collector path, not the emitter path. Also, reduce the value of R1.
     
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  5. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    It worked. Thank you. The thing is I'm trying to do an H bridge with 4 transistors. I was just putting one in order to see how it worked. I can't put the H-bridge after the resistance( In the collector path). What would you recommend me doing?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Use a PNP transistor for the top transistors in your H, and an NPN for the lower ones.
     
  7. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    So, after playing with the H-bridge. Nothing turned out. If I just connect 2 transistors an npn and a pnp it will work great (both of them connected to voltage), but when I introduce an npn and a pnp connected to ground everything goes Kaputt. I've attached an image of the circuit. The output voltage goes from 12 volts to an oscilation over 0 if I connect the other two tranisistors. Any idea what's going on? Thank you as always.
    [​IMG]
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Q26 is upside-down.
    Both Q26 and Q27 should have base resistors.
     
  9. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    Changed transistor's position and put resistors. Still not working. Any other idea?
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
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    Define "not working". Please post your revised schematic.
     
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