Hello !
I need some help for circuit.
I designed circuit such below, but that is not general.
I'm curious about role of R4 resistor in this circuit.


---------------------------------------------------------------------------
The operation of circuit :
** R1 is pull-down resistor
case 1
@ PWM = high -> N ch mosfet : on
@ Input signal = High -> NPN Transistor ON -> P ch mosfet : on
=> OUTPUT = Ground (by N ch mosfet)

case 2
@ PWM = Low -> N ch mosfet : off
@ Input signal = High -> NPN Transistor ON -> P ch mosfet : on
=> OUTPUT = Vdc * {R3/(R2+R3)}

case 3
@ PWM = high -> N ch mosfet : on
@ Input signal = Low -> NPN Transistor off -> P ch mosfet : off
=> OUTPUT = Ground (by N ch mosfet)

case 4
@ PWM = Low -> N ch mosfet : off
@ Input signal = Low -> NPN Transistor off -> P ch mosfet : off
=> OUTPUT = Vdc
---------------------------------------------------------------------------
When I simulate the circuit in the image, it works as intended.


However, if i remove the R4 resistor, case3/4 behaves strangely.


And let me know if there's anything else I should consider in this circuit.

 

Hi hyyee,
Welcome to AAC.

Is this a homework or college assignment.?

Moderation.

 

R4 is connected between two points. Consider the significance of those two points.

 

A MOSFET gate must never be left floating as it's voltage is then undefined.

 

Your circuit works, but not as you describe or intend.

Why do you need the NPN transistor AND the P-channel MOSFET to short the output to ground?

If R4 is null ohm = 0 ohm, then the P-Channel MOSFET is doing nothing useful.

If R2 is 0 ohm then you are shorting out your power supply, expect much magic smoke in the real world...

The simplicity of the simulation is lulling you into a false sense of security....

 

Hi hyyee,
Welcome to AAC.

Is this a homework or college assignment.?

Moderation.

I am HW engineer.
This is my seminar topic.

 

R4 is connected between two points. Consider the significance of those two points.

Thank you for your response. What I'm wondering is why it takes a peak (12V) and then drops to 2V.

 

A MOSFET gate must never be left floating as it's voltage is then undefined.

Thank you for your response. What I'm wondering is why it's taking a peak (12V) and then dropping to 2V, can I understand it as a strange behavior caused by the gate being in floating state?

 

Your circuit works, but not as you describe or intend.

Why do you need the NPN transistor AND the P-channel MOSFET to short the output to ground?

If R4 is null ohm = 0 ohm, then the P-Channel MOSFET is doing nothing useful.

If R2 is 0 ohm then you are shorting out your power supply, expect much magic smoke in the real world...

The simplicity of the simulation is lulling you into a false sense of security....

The resistors are not 0R, they are undetermined values. Sorry for the confusion.