1. PMOS cannot open with Vsg -0.7 where is my logic wrong?

What is "open"?
Garden gates open and close.
In electronics "open" means "no connection".
A MOSFET or transistor is either "on" or "off", or "conducting" or "non-conducting".
Please use those terms.

 

Hello,I am trying to understand the way values were chosen in this current limiting switch.
Q1 NPN ,Q3 is PNP Q2 is enchament mode P-channel mosfet.
given that Q1 is 2N3904 Q3 is 2N3906 Q2 is IRF5305,I assume that we want Q1in saturation when they are ON because VCE is very small and its the closest to short circuit.

1.Q1 is in saturation when Vce=0.2
12−I⋅50K−0.2−I⋅15K=0
I=0.182mA
given the datasheet how can I know that when Ic=0.182mA Vce will be 0.2V?

2.
V_e_q3=12V
V_c_q3 =12V-0.128mA*50Kohm=5.6V
Vec_q3=6.4V which is by far not a saturation state give such large Vec.
but basickally How can I know that Q3 is even open?
How can I know given the datasheet the base voltage of Q3 ?
2N3904
https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/4846/DS_497_2N3904.pdf
2N3906 PNP
https://www.onsemi.com/download/data-sheet/pdf/2n3906-d.pdf

 

hi yef,
You are over thinking the operation of that resistive divider.
Note the Vgate voltage to the MOSFET is the same for all 3 circuits, providing that the Ratio of R1 and R2 is the same value.

The MOSFET would be either switched On, conducting when Vgate = -9V or Off, not conducting when Vgate =0v.

Do you understand that this section of the circuit is only to control the On/Off state of the MOSFET?

E

 

How can I know that Q3 is even open?

Again what is "open"?

 

Hello Eric,Yes I understand that the ratio between them desides the state of NPN Vce
So We know the VCE of Q3 shown below.
before going into the mosfet I need to know what is the Base voltage of Q3?
Is there some mathematical intuition Which could help me find Vbase of Q3 shown below?

Hello crutschow , open means Vbe forward biased and npn has current flow from collector to emitter.

 

t I need to know what is the Base voltage of Q3?

Hi yef
Q3 Vbe = 0.5 * Load Current
E

 

Hello Eric, As you can see in the photo the load current is the end of the circuit will be 1.3A so vbe will be 0.65
when Vin=3.3V PMOS Vsg=11.35V-3V=8.35V
I was told to use enchasment mode PMOS which works with negative VSG
As you can see my logic made me an opposite result and Vsg is +8.35V.
Where did I go wrong?
Thanks.

 

hi yef,
For The MOSFET you have to use the +12V as the 'common' reference for your measurements.

When the voltage drop across the 0.5R exceeds 0.65V the Q3 transistor will start to conduct which causes a voltage drop on the Vgate voltage, so the MOSFET current will be limited to approx 1.3Amps
E

 

I was told to use enchasment mode PMOS which works with negative VSG
As you can see my logic made me an opposite result and Vsg is +8.35V.
Where did I go wrong?

You are absolutely right,
VSG (Voltage on Source relative to Gate) is +8.35 V.
But in electronics usually used other parameter: VGS.
VGS (Voltage on Gate relative to Source) is -8.35 V.

 

open means Vbe forward biased and npn has current flow from collector to emitter.

As I previously stated, in electronics open means no current flow, the opposite of what you are stating.
Please use the correct electrical terms if you are discussing electronics.

 

Hello,Yes I understand pmos in enchasment mode works at negative Vgs.
Two questions:
1. how did you know that when current will flow to the load Q3 will be conducting current from emitter to collector?
2.give Vin=0 how can I logically asses the situation for Q3?
Thanks.

 

As I previously stated, in electronics open means no current flow,

It depends.
For example, in Russia, in electronics:
Open (otkryt) means "opened way for current".
Close (zakryt) means "closed way for current".

 

It depends.
For example, in Russia, in electronics:
Open (otkryt) means "opened way for current".
Close (zakryt) means "closed way for current".

Well, this is an English only forum, so let's stick to that.

 

Hello , The intuitions are very important,could you help me with the questions in post 31#.
Thanks.

 

1. how did you know that when current will flow to the load Q3 will be conducting current from emitter to collector?
2.give Vin=0 how can I logically asses the situation for Q3?

hi yef,
Q3 will start to conduct when the voltage drop across the 0.5R exceeds -0.65V, which is Vbe turn On voltage
As Q3 conducts it effectively reduces the value of the R3 resistor, which in turn reduces the value of the Vgs voltage driving the MOS so the Load current limited.

...... is that what you are asking????

Where is Vin on the circuit? is it the 3.3V pulse??
E