MOSFET Characteristics

D Faust

What do MOSFETS do?· He is what I·gathered/deduced, please tell me if this is incorrect.· MOSFETs are transistors with a varying output (not just on/off) depending on the voltage applied to the gate.· The gate doesn't have to remain the same for the output to be the same (can be left unconnected and resistance will not change).· Is the voltage applied to the gate let into the output somehow or is the output only from the collector (where the non-changing voltage goes in)?· Please let me know if any of this terminology or info·is wrong.· Sorry for all the questions, but I want to get things straight before I try to make a projet with them. (use the PWM command of a BASIC Stamp to control voltage to a motor.)· Is it possible to over saturate the gate?· Thank You in advance for any help!!

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D Faust

D Faust

What about the difference between a MOSFET and a PNP/NPN transistor?

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D Faust

stamptrol

You can get more detailed info by Googling or looking at Wikipedia.

As they relate to Stamps and micros in general, the MOSFET is controlled by a voltage on the gate, a NPN/PNP is controlled by a small current flowing in the base circuit.

Typically MOSFETS should have the gate pulled down to 0 volts with 10K-20K resistor to prevent unwanted turn-ons. When purchasing MOSFET's, check the gate voltage required for full turn-on, it can be as high as 18-20 volts which causes design pain with 5 volt devices.

There many MOSFETS specifically designed for use with 5 volt logic levels on their gates. IRL510 and IRL520 are very stable and I've used hundreds with Stamp circuits.

A side benefit for MOSFETS is that they will easily and safely share current properly when connected in parallel. Bipolars require good design to share current in parallel.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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D Faust

Thank You for the tips for MOSFET usage, but the thing that I really want to know is: If left unconnected will the resistace of the MOSFET change?· Also what happens if the fully on threshold is surpassed?· I tried googling and wikipedia and have not found a good answer to my question.

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D Faust

stamptrol

As alluded to in needing the pull down resistor, if you leave the gate floating, the device can drift in and out of conduction mode, usually by capacitive coupling with nearby signals. So technically, the D-S resistance will vary all over the place. The device can go into its linear region and will likely begin heating, unless the circuit was specifically designed to operate linearly.

Related to general use with the Stamp, we usually want full on or full off. That is, either 0 volts on the gate or some high (5 volts) value which turns the device full on. Up to the device safe maximum, there is no harm in having a gate voltage higher than required for minimum Rdson. That value will vary from device to device.

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Tom Sisk

http://www.siskconsult.com
·

D Faust

Good that is what I was looking for. Are there any transistors that can be set to a certain resistance and remain that way until it recievs another signal. Would a D/A converter and a MOSFET combination do this?

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D Faust

Rob7

You can find all the info. ,you want about the MOSFET you need at www.IRF.com. You can also look at design tips ! Part No. and any revelent info you ever wanted to know !
Rob7

Paul Baker

Transitors don't operate as you are thinking, they are not a electrically controlled potentiometer. The resistance seen across the Drain and Source is not just a function of the Gate to Source voltage (Vgs) but also the Drain to Source voltage (Vds). MOSFETs have several regions of operations and each has thier own characteristic resistance profile, cutoff (sometimes also refered to as subthreshold) has miniscule amounts of current flowing through that are exponentialy related to the Vgs, at conduction (Vgs > Vthreshold) it enters into one of two states: linear or saturation. Which state it is in is dependant on the voltage potential between the drain and source. If that potential is low, it's in the linear region, if the potential is high its in saturation. Each of these regions have a different effective resistance seen between the drain and source. While there is some variation of the effective resistance in each of these regions as determined by the applied Vgs, its variation is not·several orders of magnitude. The linear region is more effectivly controlled by Vgs as far as resistance is concerned, but you always have to be careful you dont switch the mosfet into the saturation region.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 6/7/2007 5:54:08 PM GMT

D Faust

First, thank you for your explaination that is exactly what I wanted!! Second, I have one more question. Is the voltage at the source equal to Vds + Vgs? Drain is where current/voltage is input to (like positive end of LED), right? Sorry for being so inquisitive.

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D Faust

Paul Baker

D Faust said...
Is the voltage at the source equal to Vds + Vgs? Drain is where current/voltage is input to (like positive end of LED), right? Sorry for being so inquisitive.

No, the source is the reference point, voltage is always measured against a reference and in a mosfet that is the source node. Vds and Vgs are independent from each other. The orientation of the drain and source is dependent upon which type of mosfet used, a N-type or NMOS has the source node at the more negative potential, the gate voltage is some positive potential above that and so is the drain. When used in a circuit a NMOS transistor is sometimes called a low side driver because it controls the load's negative terminal (in this configuration the source is·frequently tied to ground and the drain is tied to the - terminal of the load). A P-type or PMOS has the source node at the more positive potential, the gate voltage is some potential below that (thats why Vth is listed as a negative number) and so is the drain. When used in a circuit a PMOS transistor is sometimes called a high side driver because it controls the load's positive terminal (in this configuration the source is·frequently tied to the positive terminbal of the supply·and the drain is tied to the·+ terminal of the load). The use of MOSFETs is much more diverse than switching power to a load but that is one of the most common applications.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

metron9

Check out the video lecture #9 from a professor at MIT.

MOSFET Amplifier Large Signal Analysis

http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-002Circuits-and-ElectronicsFall2000/VideoLectures/index.htm

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

D Faust

Thanks, I think that I got it now.

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D Faust