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NPN vs PNP

Discussion in 'General Electronics Discussion' started by naam, Jun 7, 2012.

  1. naam

    naam

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    Jun 7, 2012
    Hey guys,

    I am new on the forums, this is my first post. Look forward to being an active member.

    I just had a question about a negative voltage power supply I am building using the buck boost topology. In order to switch the circuit I was initially using a NPN-BJT transistor, but the circuit did not quite work when I tested it in the real world (almost like the NPN would not switch at all!!). I was advised to swap my NPN for PNP and it magically worked. I dont understand why a PNP transistor could switch but not an NPN.
    The NPN transistor was not faulty.

    Any help would be great!!!

    Thanx :D
     

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    Last edited: Jun 7, 2012
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That circuit is designed to only use a PNP transistor. You can see this from the fact that the arrow on the emitter points TOWARDS the base. An NPN has the arrow pointing AWAY from the base. NPN and PNP transistors are NOT interchangeable! A circuit that is designed for one type will NOT work with the other type.
    There are two differences between NPNs and PNPs that will prevent the wrong type from working in this circuit. There isn't really much point going into details. An NPN transistor COULD be used in this type of circuit, but only with various changes to the design, and a PNP is much simpler to use in this application.
    If you were converting a negative voltage to a positive voltage, you would use the same circuit but with an NPN instead of a PNP. Also the diode would be reversed, and the polarities of all polarised capacitors would be reversed.
     
  3. naam

    naam

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    0
    Jun 7, 2012
    Thanx for the quick reply!

    When I model the circuit using an NPN transistor in pSpice, it works like a charm so I thought there wasn't anything fundamental that was stopping the circuit from working.

    I still don't understand why an NPN wouldnt work. For instance, in order to turn an NPN on, you only require Ve<Vb> Vc, which is quite achievable using a square wave from a signal generator, with an amplitude of 5V. Why is this circuit configuration not workable for an NPN in real life?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    With an NPN you need VG>Vi to open the transistor. But where will you get that voltage from?.
    Are you sure you modeled exactly the same circuit in SPICE? I wonder how this could have worked. Maybe in SPICE the emitter was connected to the inductor L, not to the battery? Then an NPN could have worked, but only if the control voltage to the base of the NPN switches between positive and negative. Because if the control voltzage switches only between 0 V and positive, the NPN will become conducting as soon as the emitter voltage becomes negative - which is the purpose of this circuit.
    Or maybe you connected the emitter to Vi, but used a bas control voltage higher than Vi?

    Harald
     
    Last edited: Jun 7, 2012
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