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Why npn transistor is not used for high side switching?

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Muhammad Moiz khan

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Jul 24, 2015
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Hello, my question is that why npn transistor is not used for high side switching and why pnp transistor is being preferred over npn transistor for high side switching?

Sinking and Sourcing are terms used to define the control of direct current flow in a load. A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load.

Two types of input cards exist, those that 'sink' current (also known as positive logic) and those that 'source' current (also known as negative logic).

Most common in Europe is the 'sinking' type of output, these will be used with the PNP transistor. Less common nowadays are output cards that 'source', these were popular in Asia and require the NPN type of transistor in order to operate correctly

it is in the switching voltage required.
the voltage required for an npn is large compared to that for pnp.


imagine a NPN transistor for the high side switching. In the sipmlest way it is connected to the 5V supply.

Now to switch the bjt ON you need a voltage on the base of about 5.6V, But where to get it from, and how to control it?

With a PNP instead the base voltage needs to be 4.4V instead. So a simple resistor to GND switches the bjt ON. It could be controlled with a microcontroller port pin.

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There is also the issue of fault tolerance.

Most common type of short circuit in the distributed wiring of large systems are ground faults.
Low side switching has the advantage that a fault just causes a load to stay on, nothing is destroyed.
If you short the output of a high side switch to ground, it often kills the high side driver pretty quickly.
Current limiting is possible, but it adds complexity and cost.

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