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I am using IRF9540N P channel enhancement mode MOSFET for switching 3.3V to the power pin of the WiFi chip. The connection is as follows
Source --------- 3.3V
Drain ---------- WiFi chip power
Gate ---------- Ground

So now the Vgs is -3.3V which is lesser than the Vgs(th). If i connect the drain to WiFi chip power, the voltage is getting switched but my WiFi module is not getting ON. I tried connecting an 12V motor to just check whether the MOSFET is switching the power or not. Instead of switching 3.3V my MOSFET is switching only 2.7V and my motor is rotating very slowly.
My WiFi chip works fine with 2.5V also. I have no idea what could be the issue? Why is my WiFi chip not powering up?
If i connect directly without MOSFET then my WiFi works fine.
I dont think so the current should be the problem because the WiFi chip consumes only 300mA and the MOSFET can switch upto 30A. Any help would be appreciated

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  • \$\begingroup\$ To completely turn on a P-channel MOSFET configured as a high-side switch, you should connect the gate to ground. Are you doing that? \$\endgroup\$
    – tcrosley
    Commented May 23, 2015 at 8:52
  • \$\begingroup\$ Yes i am connecting the gate to the ground so that i get -3.3V but still its not switching \$\endgroup\$ Commented May 23, 2015 at 8:59
  • \$\begingroup\$ You mention 300 mA WiFi current. Is that the maximum and is that certain? - eg may it be less or much less. To do that with eg 0.1V drop you need Rdson = V/I = 0.1 V / 0.3A = < 333 milliOhms. A VERY undemanding spec. \$\endgroup\$
    – Russell McMahon
    Commented May 25, 2015 at 9:53
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    \$\begingroup\$ Vijay - While you could use a bipolar transistor as I noted, in your situation I'd use a MOSFET. A PChannel FET with Vgsth of about -1V and an Rdson of < 0.1 Ohm at say 1A should be easy enough to find. Even a SOT23 FET with s smattering of Cu PCB traces will do OK. | Do you have easy access to a range of parts? For instance if in eg Mumbai or Bangalore there are many suppliers. Elsewhere it varies. [Where are you located, for interest only]. Do you want some specific parts recommended or is the above good enough? \$\endgroup\$
    – Russell McMahon
    Commented May 25, 2015 at 10:59
  • \$\begingroup\$ @Russell My WiFi chip consumes 300mA while transmitting data and 80mA while receiving data. I am working in Delhi. Now i am looking for some P channel Enhancement mode Logic Level MOSFETs. I have few part numbers like IRLM 5103, FDS 6575, IRLM6302 etc but its not available in Delhi market. I am able to get it in bangalore and mumbai but they are taking long time for shipping say 2-3 weeks. Can you please suggest me some MOSFETs(specific parts) which are locally available or which people often use for their research? Thank you for understanding my problem. \$\endgroup\$ Commented May 26, 2015 at 9:03

2 Answers 2

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The IRF9540N is unsuitable for the task.
You may have misunderstood the parameter Vgsth To turn on Vs - Vg needs to be comfortably greater in magnitude than Vgsth.
Your available Vgs max is -3.3V.

The IRF9540N - data sheet here needs about -4.5V before it considers getting out of bed and would really like 7 V or more drive at higher currents.

While figs 1 and 2 suggest the 3.3V is too low, fig 3 suggests that if you extrapolate off the graph (and holdyour breath AND don't wonder if this seems unwise) then the 25C curve MAY get down to about 3V - but MAY curve down too steeply.

However, the graphs are for typical values.

On page 2 of the data sheet it says Vgsth is -2V min and -4V max.
When designing you MUST ALWAYS use the "WORST CASE values.
Here 'worst case' is -4v to get 250 uA and Vds = Vgs.
ie even if you had a best case FET it would have 2V Vds with 2V Vgs.
This would not be good.
If you pick through a bin of these you MAY find some that work somewhat better than others and some that MAY work well enough for you - but maybe not, and this cannot be guaranteed.

Best likelihood, based on all evidence and the data sheet, is that even though your load current needed is tiny - the Vgsth value is so far above your drive voltage that it just sits and grins at you.


... using darlington I got 1.2V drop and
then I used Sziklai Darlington(PNP) and I got 0.7v drop but was able to power up the WiFi module.

For low saturation voltage both Darlington pair and Sziklai pair are bad because they "steal their own drive" as they turn. With DP you get 1++ Vbe minimum drop and SP can be slightly lower but still > 1 x Vbe

Better is an independent NPN and PNP where the on voltage drop is the saturation voltage of the PNP (in this case) transistor. See below:

A logic level P Channel FET will drop into this circuit at a later date.
R4 then not needed but does no harm.

Q2 can be a single adequately rated PNP or several smaller ones in parallel.
A wide range of small bipolar PNPs will work. A very good small bipolar is the BC807-40/BC327-40 which has the current rating you need but beta (current gain) will drop somewhat at higher currents and saturation voltage rises with current.
BCxx7 data sheet here
See Fig 9. BCxx7-40 has a typical saturation voltage of about 0.07V at 100 mA rising to maybe 0.125V at 300 mA.

The -40 part has a beta of 400 typical (250-600 range). Lower beta transistors can be used with smaller values of R4. Ideally if N x Q2 are used then also equip N x R4 from Q1-c to each Q2 base BUT as shown will probably work OK. Resistor values are liable to be about right - play as required. R2 not required if input is always driven high or low and never floats. R3 is 'or safety' to ensure Q2 turns off. Lower values of R4 may be needed to get low enoughy saturation voltage. The data sheet saturation curve is at forced beta = 10 !!! ie 30 mA base drive for 300 mA Ic. A FET becomes attractive :-).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you Russell. I will try with some Logic Level MOSFETS \$\endgroup\$ Commented May 23, 2015 at 10:27
  • \$\begingroup\$ @VijayAnandChandrasekaran If current is not more than a few hundred mA then a small PNP bipolar may work well enough. \$\endgroup\$
    – Russell McMahon
    Commented May 23, 2015 at 16:40
  • \$\begingroup\$ But the voltage drop is around 0.7V for ordinary PNP transistor and i want to drive around 500mA. I tried darlington pair and again voltage drop was the issue \$\endgroup\$ Commented May 25, 2015 at 8:56
  • \$\begingroup\$ @VijayAnandChandrasekaran No. Saturation voltage of a bipolar transistor can be less or much less than the Vbe drop. Vbe can be say 0.6V or 0.7V while Vce is say 0.1V. What is your load current? This affects what is best to use. A MOSFET is a good choice but I mentioned the bipolar as probably OK. If you draw a Darlington cct you will see it is different. The falling poutput voltage "steals" drive from the input so Vc2 out can never be less than 1 x Vbe or more. So a Darlington may not do what you want but a single bipolar may. \$\endgroup\$
    – Russell McMahon
    Commented May 25, 2015 at 9:51
  • \$\begingroup\$ You are right, using darlington i got 1.2V drop and then i used Sziklai Darlington(PNP) and i got 0.7v drop but was able to power up the WiFi module. Then only i decided to go with MOSFETs and when i used IRF9540N i got stuck with the Vgs(th) problem, finally came up with logic level MOSFETs with proper Vgs(th) and now its not available in local market. My load current may vary from 0 to 500mA. This is the current consumed by my WiFi chip \$\endgroup\$ Commented May 26, 2015 at 9:13
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Your circuit seems correct in principle. The only problems are:

If you look at graph in the datasheet of current vs gate-source voltage (Fig 3) and extrapolate down to 3V you will see that almost no current will flow. Why are you using such a high power MOSFET for such a small current and voltage? All the circuit samples in the data sheet use 10V as the gate voltage. Search for 'logic level MOSFET' for one more suited to your case. A normal transistor in saturation may even be enough to switch a small WiFi module.

Also make sure you are using a static wrist strap and anti static bags. Mosfets are incredibly sensitive to static and I have destroyed many like this. Although they usually seem to fail on rather than off.

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  • \$\begingroup\$ A normal transistor would do and i experimented too. Normal transistors BC557 is switching only 100mA. I may need around 500mA for my WiFi. I tried using darlington pair but the voltage drop is around 1.2V, If i provide 4V to emitter then i will get 2.8 in collector. So i prefer MOSFET. Mine is a battery operated device so transistors may consume more current than MOSFET(though i am not sure about this point, its a guess because transistor is current operated) \$\endgroup\$ Commented May 23, 2015 at 9:05
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    \$\begingroup\$ You may also want to consider using an n-channel MOSFET as a low side switch. They are much more common, cheaper and usually have a lower on resistance than p-channel for the price. Here is one that seems will work with 3V sparkfun.com/products/10213 \$\endgroup\$ Commented May 23, 2015 at 9:21
  • \$\begingroup\$ you mean to say that instead of switching 3.3V i can switch ground using N channel MOSFET?? \$\endgroup\$ Commented May 23, 2015 at 9:24
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    \$\begingroup\$ I would recommend against switching the ground side on complex components like a WiFi module. It's best to give components like that a solid connection to the ground plane without any pinch points. \$\endgroup\$
    – Dan Laks
    Commented May 23, 2015 at 9:27
  • \$\begingroup\$ Thank you for the link. The problem is the WiFi module is designed such that i have given provision to switch the 3.3V only and the ground is given to the WiFi. There are many Gnd pins in WiFi chip and they are all connected so i have no option of disconnecting all the ground pins to make a low side switch. I have to go with high side switch only. If you know any P channel MOSFET link then please share it. Thank you very much \$\endgroup\$ Commented May 23, 2015 at 9:29

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