H-bridge curiosity - why PNPs at top of h-bridge?

Discussion in 'General Electronics Chat' started by redbird_is, Jul 17, 2008.

  1. redbird_is

    Thread Starter Member

    Jan 13, 2008
    Hello everyone,

    I’m working on building a BJT-based H-bridge, similar to this one (although mine runs off a +/- 12 volt supply):


    My question is mostly out of curiosity - I notice that nearly all H-Bridge circuits use PNP transistors in the upper part, and NPNs in the lower part.

    Why is it never the other way around? (i.e. emitters connected together, rather than the collectors). Maybe it has something to do with regenerative current protection?

    Thanks for reading my post!
  2. Audioguru

    New Member

    Dec 20, 2007
    A PNP transistor saturates with a voltage loss of only 0.2V. An NPN used an an emitter-follower has a voltage loss of from 0.7V to 1.5V.

    Since your supply is +/-12V then level-shifting transistors are needed to convert the 0V-5V from the PIC to +/-12V to drive the H-bridge.
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    The transistors can be the other way around (emitter follower) but the maximum voltage to the motor will be lower by at least 1.5V due to the 2 base-emitter voltage drops which in a low voltage circuit may be too much to lose. A downside of the common emitter arrangement in the diagram is that for a brief moment both upper and lower transistor will be on at the same time which will cause large current spikes on the supply lines but there are better arrangements that avoid this happening.
  4. redbird_is

    Thread Starter Member

    Jan 13, 2008
    ok, that makes sense. Thanks for your replies!
  5. Tahmid

    Active Member

    Jul 2, 2008
    so that means that if the npn transistors are used as high side switches, no isolated supply is required?
    Say if i use all npn transistors at 310v line and drive these at 50hz, wont i get approximately 220vac at the bridge output?