MOSFET Question

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Sambuchi
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MOSFET Question

Post by Sambuchi » Thu Apr 09, 2009 12:32 pm

Ok... I've been playing with some hardware the past few months and I'm shocked at how much I forgot just by doing firmware over the years.

I need to run a 3.3V logic line to a P-Channel MOSFET. Its running to the gate and I have 5V at the Source. I will be drawing .08mA when active... I'm not switching this on at any fast rate.

I am running this model is switchercad and noticing different MOSFET's giving different results... mainly when trying to turn the fet off with 3.3V some can some wont and I cant seem to find what is the determining factor.

I would then like to get some MOSFET's from digikey.

Example: I run my simulation with...
Fairchild: FQB11P06,FDC5614P works! FDS9933A,FDS4953 doesnt

I am not sure if it has to deal with the Rds(on), Qgate ????
Well... thanks for your input in advance. :?

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philba
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Re: MOSFET Question

Post by philba » Thu Apr 09, 2009 12:42 pm

Your Vgs will vary between 5-Vhi and 5-Vlow (er, Vhi-5 and Vlow-5). It makes sense that some won't turn off, at least fully off. You might want to use an NPN with a pull up to drive the gate so that Vgs goes between 5 and 0.

Also, last time I looked SwitcherCAD had a pretty limited set of MOSFET models.

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Re: MOSFET Question

Post by Robert Reed » Thu Apr 09, 2009 4:56 pm

Pinch off voltage for mosfets are all over the place - the data sheet is just a generalized average. May have to pick and choose.

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Sambuchi
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Re: MOSFET Question

Post by Sambuchi » Thu Apr 09, 2009 7:37 pm

:D :D :D

OK I see what I was missing. In the Spice Model there is a variable called Vto.
I believe that deals w/ the threshold voltage.
As I pick certain FETs with lower Vto's they don't work.
I now feel comfortable buying some FETs now!
philba wrote:Also, last time I looked SwitcherCAD had a pretty limited set of MOSFET models.
I agree. There's maybe 20 P Channel MOSFETS mostly Fairchild. Wouldn't mind seeing some other brands.

Thanks philba, Robert... you made me stumble on some great reading material on web.
:D :D :D

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hmlittle59
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Re: MOSFET Question

Post by hmlittle59 » Sun Apr 12, 2009 8:55 am

Hello to All,

First time on this Forum, Your Question is Almost exactly what I'm doing. I'm getting back into electronics and will take any and all advice. I want to replace a momentary switch(2 -3 sec. push time) with an electronic switch(P-chan. Mosfet - or - Mouser 512-NZT605 NPN). I learned that I need a current limiting resistor in series from the BS2 I/O pin to the BASE of the NPN. Also a resistor from BASE to Grd. to shunt any false triggers to the NPN. It was also suggested that I could use the same resistors on the P-Chann(470 ohms in series and 470k ohms from Gate to Grd). I've also read in other places from Gate to Source(DATA sheet). I hope this helps, I'm searching for all the insight that I can get, I have boards on order(2 days LATE!!!) and all parts to build them (P-chan), I may now build one with the Fairchild (NPN) parts which I have.

Let me hear back from you with any in-sight. thank

hmlittle59

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Bob Scott
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Re: MOSFET Question

Post by Bob Scott » Sun Apr 12, 2009 9:39 am

Mr. Little,

Welcome.

Could you please supply more information. What is the device where "BS2" signal comes from?
Is it active high and active low? What high/low voltages?

What device are you driving?

The P channel FET is the opposite polarity of a NPN transistor or Darlington. The FET will be connected with Source terminal to power while a NPN transistor will have its Emitter to ground. That is, if the planned output is on the Drain or Collector. Did you plan to drive the high side or the low side of your output device?

A schematic diagram sketch would sure help.
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Sambuchi
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Re: MOSFET Question

Post by Sambuchi » Sun Apr 12, 2009 1:37 pm

Hello hmlittle welcome to the board. :D
hmlittle59 wrote:Let me hear back from you with any in-sight. thank
I put a small page together with my findings.

You can find it here. MOSFET LINK
Bob is right and you need to be sure what need N or P channel... two different polarities.

The above link has the following PNP MOSFET circuit with the output.
The program is switchercad from LT
Image


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Re: MOSFET Question

Post by hmlittle59 » Sun Apr 12, 2009 6:19 pm

Thanks for the quick reply,

My intent is to replace a push button switch(momentary) with a transistor or P-chann. From the BS2 5vdc would turn on the switch(2-3 sec.) simulating the push button switch. Both parts I looked at are rated at 5vdc 1.5 amp. When I talked to the makers of the Test unit all she knew was that it was 5vdc being shorted to Grd. with the pushing of the button. This activated a relay inside the unit and their electronics took care of the rest. All I'm doing is simulating the SWITCH. I just wanted to replace the $4.00 Reed Relay with a Sub-Dollar part(transistor/P-chan) that I was using. She also said the current was very low, less then an .5 Amp. of current. I could read the 5vdc on the back of the unit but every time I tried to read current the unit would activate an shut down like its designed to. I was just looking to just drop in the transistor with resistors on the base/grd. none on the Drain or Source but now I've been seeing other wise with all the different inputs from the Forums and sample circuits.
1)Can any one explain why the resistor is needed on either one of the locations?
2) Also can I use a 1/8 watt size SMT part or should it be 1/4 watt?

thanks again

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haklesup
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Re: MOSFET Question

Post by haklesup » Mon Apr 13, 2009 2:07 pm

Sounds like the gate was pulled up to VDD through a resistor and it is the gate you are pulling down through the momentary switch. The wattage of that resistor is just V squared over R but use a 2x safety margin. many resistors are smaller physically than they used to be. not much trouble to use 1/4W or even 1/2 W and still have a small package.

Your reference is good but if you want the complete picture, dig up the I-V characteristics for these 4 devices (P and N, enhancement and depletion). I find it much easier to explain function while looking at these graphs. This should be in their respective datasheets.

A resistor in series with the drain will set the overall current magnitude in the channel of the turned on FET when it is driven to saturation. When Id is =0A, voltage drop across this is 0V so the pulled up gate voltage is = to the drain voltage allowing the PMOS to be off, pulling the gate to 0V through a switch sets VGS to 0V turning it on. This simple circuit will not latch. A noisy momentary switch can cause multiple triggering. Furthermore, the PMOS gate can be damaged by ESD imparted by the user. The pullup resistor will only partially mitigate this threat and less so as its value increases. Almost any value can be used as a pullup unless speed is an issue (high value will take longer to charge the gate and low value will waste power)

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Re: MOSFET Question

Post by hmlittle59 » Mon Apr 13, 2009 7:38 pm

hello all,

I'm going to REPLACE the momentary switch with P-chann enhanced MosFet or the NPN Darlington. Which ever is cheaper and can handle the current less then 1 Amp. 5vdc. I will only activate this device 3 times within 90 sec.(Once every 30 sec.) for 2-3 sec max., this will simulates the pushing of the switch. When the switch is pushed now it shorts 5vdc to grd. and turns on the motor. From the BS2 I/O pin 5 vdc turns the device on thru the resistors then 0vdc turns it off. I've done this with a REED Relay and now I want to do it with a cheaper part. Switching speed is not a problem.

thanks

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Bob Scott
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Re: MOSFET Question

Post by Bob Scott » Tue Apr 14, 2009 8:43 am

GOTO TOP
END
EXIT
:smile:
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philba
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Re: MOSFET Question

Post by philba » Wed Apr 15, 2009 7:44 am

Bob, bwahahaha

Mr. Little, why do you focus on "P-chann enhanced MosFet or the NPN Darlington."?? What causes you to see those two as equivalent? I think bob is right, read the thread over again. I'll tell you now, the Darlington costs less than the MOSFET but will cost you more in time, cash and frustration. At least you might learn something.

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philba
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Re: MOSFET Question

Post by philba » Wed Apr 15, 2009 8:06 am

Sambuchi. Nice page on the MOSFET. One thing though, while Vgs(th) is interesting, you should also point out Rds(on) which usually shows the resistance of the device at specific Vgs values. In the case of the NTD2955, -4.5 yields an Rds of 500 mOhms for a drain current of 2A. This gives you good indication that the device would work ok in a 5V system. However, you need to calculate the impact of the half an ohm. Usually the datasheets also have a graph showing the relationship of Vgs to Rds. Very helpful for estimating the behavior of the device in your circuit.

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