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A lot of H-bridge diagrams feature PNP transistors on the high side. I have not found a satisfying answer as to why they do that.

My reasoning for using a PNP on the high side was that I did not want the base current being affected by the load or by the other transistor downstream, since the current flowing into the base pin would flow down through the emitter pin, through the load, and do ground through the low side transistor.

That was the only real reason I could think of - are there any other good reasons?

enter image description here

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    \$\begingroup\$ A trade-off between simple driving and added losses (or cost). Have you looked at what's needed to drive the H-bridge in your schematic versus an all NPN one? \$\endgroup\$
    – winny
    Feb 8, 2022 at 15:29
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    \$\begingroup\$ I think the main difference is the voltage that is required at the base pins of the NPN transistor on the high side. You would need a voltage very close to VCC, right? \$\endgroup\$
    – RGB Engineer
    Feb 8, 2022 at 15:34
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    \$\begingroup\$ No need for a base drive voltage above the +ve supply rail. \$\endgroup\$
    – user16324
    Feb 8, 2022 at 15:40
  • \$\begingroup\$ @RGBEngineer That design you show isn't really a design. It's more of a behavioral picture -- a concept "cartoon." so to speak that strips away how all these BJTs are operated in conjunction with each other, let alone how. If you use google images for a circuit like this, you'll find countless implementations. Have you tried that, yet? \$\endgroup\$
    – jonk
    Feb 8, 2022 at 18:08

2 Answers 2

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The reason is for simplifying the driving circuitry of the H-bridge. In the circuit shown above the bases of Q1 and Q3 can be driven low with respect to the DC supply without much effort. Converseley if all transistors were all NPN or N-MOSFETS there would be a requirement for a more complex design since the gates of the high side transistors would have to be driven positive with respect to the DC supply rails (i.e. extra need for extra charge-pumps or converters)

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This scheme is simpliest voltage polarity change on load.

Q1, Q4 open, Q2,Q3 closed - one polarity. Q1, Q4 closed, Q2,Q3 open - other polarity. PNP in high side needed to source "+" to load. NPN in low side sinking to ground.

In case of DC motor this is rotation direction control.

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