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Dual Power Source Selection using PMOS.

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dhruv_electro

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I want to build a circuit using which I can select the output power source using Arduino GPIO. I am trying this one:

Capture.JPG

In this circuit I am trying to switch a power supply to some load. Selection is between 14.8V external Power supply and 5V (Arduino's Power Supply). I want output to be selected when logic needs it.

Problems in this circuit / Help required:
1. If I turn on Q1, the 14.8V can go back to Arduino's 5V supply, because of the internal diode of FDD4141, which can damage my board. (Already damaged one board).
2. I have selected this FDD4141, because I have this one. Please suggest if I should use any other more suitable and cheaper part.
 

Hi,

there are dedicated power switches for this purpose.

Or:
You may use a bipolar PNP type transistor instead of the MOSFET. It doesn´t suffer from the internal diode.

Or:
you may use a series schottky diode with the 5V output path.

Klaus
 

Hi,

there are dedicated power switches for this purpose.

Or:
You may use a bipolar PNP type transistor instead of the MOSFET. It doesn´t suffer from the internal diode.

Or:
you may use a series schottky diode with the 5V output path.

Klaus

Thank you for the reply. Can you give me some example for switches / schottky diode / PNP type transistor? I want to have upto 3A current.
 

Hi,

There are many other parameters to choose devices than only voltage and current. Only you know these informations.

Every distributor and every manufacturer has interactive selection guides.
Thus it´s the best, fastest and easiest you try it on your own.

Klaus
 

I think in this application, I need to have Voltage, Current, On resistance and wattage rating of the component. I have only one PMOS FDD4141 that is why I have tried it. And it does have very low on resistance (10-15 mOhm) and 12A Current. What other parameter should I consider for this application?

For Power switches, if you can suggest any one, I will try to find out similar for my application.
 

Hi
What other parameter should I consider for this application?

Usually: availability, price, size, package, voltage drop....

Klaus
 

Load switches and hot swap controllers are likely candidates.
Many have the reverse leak-back protection you want. If
you go to mfrs' web sites there should be online selection
"choosers" that show you what to choose, based on.
 

Hi,

If you want to go to OnSemi and look for a PNP, NSS60600, it has equivalent RDSon of 50mOhm (I've yet to see one that is much lower than 44mOhm) and should be able to output about 3A at ~0.2VCE, it might work if that's they way you want to go. While there, do a parametric search of low VCE PNPs, maybe one will be more appropriate. You need a device that can dissipate maybe 0.6W to 0.75W upwards (without thinking about it too much, admittedly).
 

If you are always have 5V and 14.8V available and always want one or the other on, then just reverse the source and drain of the 5V MOSFET.
The way it will block the 14.8V but still allow the 5V to be turned on (since MOSFETs conduct equally well in either direction when on).
But note that the 5V will now come through the substrate diode when both voltages are switched off.

If that's a problem and you want to turn both supply outputs off at the same time, then use two MOSFETs in series connected source-to-source (with 100k from the sources to the gates) for the 5V supply. That will block in both directions since the substrate diodes are now back-to-back.

- - - Updated - - -
 
If you are always have 5V and 14.8V available and always want one or the other on, then just reverse the source and drain of the 5V MOSFET.
The way it will block the 14.8V but still allow the 5V to be turned on (since MOSFETs conduct equally well in either direction when on).
But note that the 5V will now come through the substrate diode when both voltages are switched off.

If that's a problem and you want to turn both supply outputs off at the same time, then use two MOSFETs in series connected source-to-source (with 100k from the sources to the gates) for the 5V supply. That will block in both directions since the substrate diodes are now back-to-back.

- - - Updated - - -

I will try this two MOSFET in series. I am using FDD4141. Can you suggest any cheaper MOSFET instead of this?
 

Hi,

Can you suggest any cheaper MOSFET instead of this?
Read posts#4 and #6.

The forum and it´s members are worldwide. A low cost device in Europe, Africa or USA isn´t necessarily cheap and available in your country.
A forum isn´t meant that others do your job. But the people will help you to do your job.


Klaus
 

Hi,


Read posts#4 and #6.

The forum and it´s members are worldwide. A low cost device in Europe, Africa or USA isn´t necessarily cheap and available in your country.
A forum isn´t meant that others do your job. But the people will help you to do your job.


Klaus

Klaus, I do not mean that. In any way if I sound like if "I want you to do my job" then I am sorry. I am only asking for the part number because I am not sure how to pick the right one and there are so many of them. As someone suggested, I have also ordered NSS60600 and the power switches like FPF2895 (The one you suggested in #2). I am trying to find out that if some of you experts have any widely acceptable option. Even this FDD4141 was suggested in EDA board forum.

With FDD4141, when Input source is 14.13V the output power is 13.17V and Current 1.11A. This is casing the problem.
 

I will try this two MOSFET in series. I am using FDD4141. Can you suggest any cheaper MOSFET instead of this?
Where do you live and who can you order parts from?
 
Where do you live and who can you order parts from?

His profile indicates India.

If you can tolerate a little extra voltage drop, the cheapest and easiest solution would use the extra schotky diode as suggested in another post.
 
Yes I am from India. I buy from Mouser, Element14 or local electronics distributors.

Is it possible that this problem occurs due to inability of Voltage source to supply required current or load drawing more current? or Is it a problem of MOSFET?:
With FDD4141, when Input source is 14.13V the output power is 13.17V and Current 1.11A. This is casing the problem.
 

This may mean most likely that the Mosfet is not fully saturated, due to insufficient Vgs.

Measure Vgs(on) right at the component terminals using a DMM
 

.........
Is it possible that this problem occurs due to inability of Voltage source to supply required current or load drawing more current? or Is it a problem of MOSFET?:
Anything's possible, but we can't tell without more info.
Measure the voltages at all the nodes (including the power supply and all three MOSFET terminals) and post them here.
 

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