Digital transistor for switching application

[Akshatha Venkatesh]

I found the below schematic in the application note of On semi. Can anyone please explain why two transistors are used , why is the npn transistor used to control the switching of the pnp transistor ? Why not use a single transistor ? Please help.

 

[Harald Kapp]

This is a high side switch (40 V) with integrated level shifter (control voltage ~5 V only).
You cannot control a switch on the 40 V rail (high side) directly from e.g. a microcontroller output pin (3.3 V or 5 V). The NPN transistor creates the base control signal for the PNP transistor while at the same time decoupling and protecting the microcontroller from the high voltage which would destroy the micorcontroller immediately.

 

[Akshatha Venkatesh]

Thank you so much . Can you explain the control voltage part please ?

 

[Harald Kapp]

Can you explain the control voltage part please ?

The voltage from the output of the microcontroller to control on/off of the transistor(s).

 

[Akshatha Venkatesh]

Hi ,

This is a high side switch (40 V) with integrated level shifter (control voltage ~5 V only).
You cannot control a switch on the 40 V rail (high side) directly from e.g. a microcontroller output pin (3.3 V or 5 V). The NPN transistor creates the base control signal for the PNP transistor while at the same time decoupling and protecting the microcontroller from the high voltage which would destroy the micorcontroller immediately.

I have another question, when the output from microcontroller is high , the npn is on and pulls the base of the pnp to ground,which turns the pnp on, right ? And when the microcontroller output is low , the npn is off , which in turn the pnp off .is my understanding right ?

 

[Harald Kapp]

right

 

[Akshatha Venkatesh]

This is a high side switch (40 V) with integrated level shifter (control voltage ~5 V only).
You cannot control a switch on the 40 V rail (high side) directly from e.g. a microcontroller output pin (3.3 V or 5 V). The NPN transistor creates the base control signal for the PNP transistor while at the same time decoupling and protecting the microcontroller from the high voltage which would destroy the micorcontroller immediately.

Hi, why haven't used the NPN transistor as the switch ?

 

[Alec_t]

You could do that, but it would be more complicated because the NPN base would need an input (control) voltage 0.6V or so above the voltage you are trying to switch. That, in turn, would need a second supply.