P channel MOSFET or 2N3906 PNP project

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
I happen to have a bunch of tri-color LED's with common cathode.
I happen to have a tri-color LED controller with controlled drains.
The controller output is PWM and sinks 12V through a common ANODE type tri-color LED
I don't have common Anode tri-color LED's. I have common Cathode tri-color LED's.
I would like to use them just as a hobby curiosity to show the grand children.

I'm not sure what would be better. I'm looking at this circuit (below) and would like advice on which would be better. P channel MOSFET or PNP BJT's. The Vf's are (red) 2.1, (green) 2.8 and (blue) 2.9. Using a common resistor will make little difference in the brightness, so I'm not worried about that so much. Target current is around 10mA. I'm not locked into any current levels so feel free to make suggestions on that aspect as well. But for now I would like to know which approach is better.

[edited diagram]

1702572932989.png
 
Last edited:

crutschow

Joined Mar 14, 2008
34,464
Your MOSFET symbol is for an N-type, not P.

The main disadvantage of using a BJT is that it requires a base resistor for proper operation, whereas the MOSFET does not require a gate resistor.
Otherwise it's a tossup as to which to use.
Cost and availability may determine that.
 
Last edited:

dl324

Joined Mar 30, 2015
16,943
I'm not sure what would be better.
If you have to buy the MOSFETs, I'd go with 2N3906 just for the cost. 2N3906 cost a nickel or so, but you can't buy a through hole P channel MOSFET for that price. And you don't need to worry about ESD.

The MOSFETs in your schematic are backwards and you'll want pull-up resistors on the gates to make sure they're off if the gates are floating (if you care about that).
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
OK, these are the corrected STOCK diagrams I made some time ago. Maybe the wrong drawings are why I had it backwards. But I want to be sure; are these correct?
1702573122947.png
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
you'll want pull-up resistors on the gates to make sure they're off if the gates are floating (if you care about that).
That's important information I completely forgot about. By now (fairly quickly) it should be clear I'm no MOSFET expert. Thanks.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
Correct now ? ? ?
1702573629980.png
To be honest, this doesn't make sense to me. Wouldn't the body diode conduct when as a source follower? Wouldn't that negate the purpose of the P-FET (shortened)
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
Maybe I'm not understanding the term "Body Diode". I'm assuming (probably incorrectly) it's the right most diode in the diagram.
 

crutschow

Joined Mar 14, 2008
34,464
To be honest, this doesn't make sense to me. Wouldn't the body diode conduct when as a source follower? Wouldn't that negate the purpose of the P-FET (shortened)
No.
The P-MOSFET's normal operation is for the drain voltage to be more negative than the source voltage so the body diode is reverse biased.
I thought I had this the right way around in the beginning. No?
No.
Look at the body diode.
It is forward biased.
You need to turn the MOSFET upside down so the source is on the top and the drain is the output.
Maybe I'm not understanding the term "Body Diode". I'm assuming (probably incorrectly) it's the right most diode in the diagram.
That is correct.
 

dl324

Joined Mar 30, 2015
16,943
Correct now ? ? ?
Yes.

The diodes are the result of how the device is manufactured. There are parasitic diodes on the source and drain, but the one on the source is shorted during packaging (unless you're lucky enough to find the rare 4 terminal devices) because you want the substrate to be connected to the most positive voltage.

From Intel 1984 Memory Components Databook:
PMOSFET-crossDiodes.jpg
If you're lucky enough to find 4 terminal devices, you can alter device behavior by controlling body bias.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
At long last - is this correct?
1702574824778.png
Not that I'll be using P-FET's. Seems I got my answer, using BJT's instead. And I haven't begun working on that part of the drawing yet. Still, thanks all for the education. Hope it sticks. I'll keep these drawings as a reference in the future.
 

Jony130

Joined Feb 17, 2009
5,488
You can't change the direction of a MOSFET body diode.
1aaa.PNG

As you can see, this diode is now forward-biased. And it will conduct current regardless of the voltage on the gate.

The proper way is this way

1aab.PNG

The body diode is reversed-biased and the gate voltage will control the MOSFET state.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,905
Now, as I look at this and think it through: When the controller wants the RED LED to be on and the G & B off, I won't get the Red, I'll get the Green/Blue. Now I need to invert the signal. Before I do that I probably should scope the outputs just to make sure when all are off ALL are OFF and not vice versa. There's a bit I don't yet know about the controller. But we'll get there.
1702576070163.png
 
Top