Lets Talk About H-bridge 1: Why PNP-NPN? why not NPN only?

Dc motor can rotate in clockwise and counter-clockwise, just by switching the voltage polarity applied to the motor, and this is what a H-bridge is for.

I've written a few unworthy post 6 months ago, H-bridge 1, H-bridge 2, H-bridge 3. Looking back at the posts, my writting sucks and I made bad conclusions. I leave the review of my previous H-bridge posts to another day.

Have you ever wonder why h-bridge is made up of a pair of PNPs and NPNs? turning off a PNP can be quite troublesome, especially when the MCU voltage is at 5V, you'll need to disconnect the PNP base or apply 12v to it so that Emitter - Base voltage is no more than 0.7V.

so why don't we use an all NPN transistors H-bridge, as seen below?

You'll just need to apply a 0.7 voltage at each base to turn it ON, a MCU can easily handle it.
On the surface, it looks simple to feasible. However n closer inspection you'll notice that the 0.7 voltage is referring to the Base-Emitter voltage.

There will not be a problem for the bottom 2 transistors, as Vbe is the voltage from base to gnd (emitter connected to gnd).The problem arises when you try to turn on the top transistors. The Vbe needs to be 0.7V, but to achieve that, the voltage needed to the base, in respect to gnd would be

Voltage_Base= Vbe + voltage across motor + Vce of bottom transistor.
Assuming Vbe = 0.7 , M0tor voltage = 10, Vce = 0.8
Voltage_Base = 0.7 + 10 +0.8
= 11.5V

Essentially , you'll need to apply a freaking 11.7v to turn the top NPN transistors on.

Comparing NPN and PNP H-bridge.
With the PNP-NPN design, you'll have the option of disconnecting the PNP base to turn it off; NPN only design, you have to apply 11.5v to the base to turn it on, making it a little troublesome to implement.

Hence PNP-NPN design will be easier design! Wait, this is a rather incomplete conclusion to be made. Because the H bridge presented above isn't complete to be used together with MCU.

Turning on a PNP isn't hard, you just need to bring it to ground, but disconnecting it you'll need something extra. As seen in the diagram below, 2 additional NPN transistors are needed. Turning on NPN Q7, will bring PNP Q1's base to ground , and turning it on; while PNP Q1 will disconnect when, NPN Q7 is turned off.


For an all NPN H-bridge, you also just need 2 more NPN transistors. When NPN Q7 is off, NPN Q1 will be on (see red arrow); turning on NPN Q7 will bring Q1 base to gnd, shutting it off. Q7 and Q1 are like inverter, one on the other will be off, vise verse.

Either PNP-NPN or all NPN H-bridge they seem equal ( all using 6 transistors) So, why all NPN h-bridge aren't really common? Could it be that, it is common, just that I didn't do enough research??? For now it is still a mystery!


** All these are just theoretical ideas, based to diagrams drawn, no practical experiments were conducted. So Any mistakes, you are welcome to tell me!