what is a mosfet

Discussion in 'General Electronics Chat' started by electronis whiz, Jun 4, 2012.

  1. electronis whiz

    Thread Starter Well-Known Member

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    i have ran acros some parts when i google there model that come back as being power mosfets or mosfets. what is a mosfet though? i'm guessing htat a power mosfet is just a hevy duty mosfet for doing somthing with power. from what i have heared they sound identical to a transistor in what they do but then why call the msomthing else so i think i misunderstood it or the source was wrong.
     
  2. Papabravo

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    MOSFET is a compound word constructed from 2 acronyms.

    MOS = Metal Oxide Semiconductor
    FET = Field Effect Transistor

    It is basically a voltage controlled current device. That means an input voltage is used to control the current flowing in the output. It does this by using an electric field to control the thickness of a channel in the semiconductor material.

    It is a very different animal from from a Bipolar Junction Transistor (BJT) which is a current controlled current device.

    One more thing FCS use Google and Wikipedia, they should be your very good friends.

    http://en.wikipedia.org/wiki/MOSFET
    http://en.wikipedia.org/wiki/Power_MOSFET

     
    Last edited: Jun 5, 2012
  3. ErnieM

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  4. kubeek

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    Seems a bit odd that you need to ask that when you call yourself electronics whiz ;)
     
  5. Papabravo

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    Who knew you were so impertinent as to point that out! Shame!!;)
     
  6. kubeek

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    I am deeply sorry, but I just couldn´t help myself :)
     
  7. Ron H

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    Perhaps he's a work in progress.;)
     
  8. Papabravo

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    Your contrition is duly noted.:D
     
  9. electronis whiz

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    ok i think i gooled it before and didn't understand what ever i found on it so i came here. i have not yet figured out much more than transistors working on voltage regs and mosfets next. so a mosfet is what a dimer switch would use to dimm the lights?
     
  10. Evil Lurker

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    Aug 25, 2011
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    To summarize:

    Three terminal device. Most are NPN configuration, meaning the first pin (usually on the left hand side) is the gate. Applying a voltage to this pin (somewhere between 2-5v usually) will allow current to flow into the device via the drain pin (usually the middle pin) and out the source pin (usually the pin on the right). Your load is connected to the drain pin with the source pin generally connected to ground.

    The idea with a MOSFET is to use it as a switch, meaning in a fully on or fully off state and the goal is to turn it on and turn it off as quickly as possible for maximum efficiency. MOSFET's come in a variety of configurations depending on what reverse blocking voltage you need. Generally the higher the reverse blocking voltage, the higher the on state resistance, the less current the mosfet can handle and the more heat it generates.

    One thing to be aware of is that since MOSFETS gates are voltage controlled and conduct much current at all, the gates generally act like small picofarad value capacitors. Its not much of a problem at low frequencies, but when switching in the tens or hundreds of kilohertz range sometimes some sort of local pulldown circuit is needed, such as adding a PNP transistor like a 2907A, 1N4148 diode, and resistor to ground and depending on the driver IC sometimes an additional resistor is needed say 10-35ohms or thereabouts to protect the gate.

    Another thing to keep in mind is that some FETS also integrate a diode. If the reverse voltage goes too high, the diode goes into avalanche mode to protect the FET from being damaged. And some FETS even incorporate their own controller ICs and have additional pins for on/off control and feedback.

    Thats about as simple a description as I can give... FET theory gets much more complicated when you start digging around into the nitty gritty of things. I suggest getting a 555 timer and start playing around with them so you can see exactly how they behave in circuit.
     
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  11. panic mode

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    term transistor covers variety of products (BJT, UJT, FET or JFET, MOSFET, IGBT).

    MOSFET is a form of FET where gate is isolated by layer of glass (silicon oxide) and control is achieved by electric field rather than current (as in BJT). Lookup JFET and see how it works and then MOSFET will be no surprise.

    biasing allows to exploit different parts of transistor's characteristic (mode of operation).

    in DC dimmers, transistor is used as a switch that is turned on and off very fast - too fast for eye to notice individual transitions. by changing ratio of "ON time" and "OFF time" one can change brightness of the lamp.

    reason for using switching mode instead of linear/analogue is simple - far better efficiency. and if efficiency is high (losses are small) you don't need to worry much about cooling of the transistor.

    also it does not hurt that turning something on and off is particularly easy with digital controllers (MCUs for example).

    when transistors are operating in switching mode, they need to handle transients (unless load is pure resistive circuit). one notable form of transient occurs when turning off inductive load (relay, motor, inductor, transformer,...). this tends to generate large voltage spike (lookup Lenz's Law) which can destroy transistor and components around it. one way to fight this is to install suppressor diodes. in transistors designed for switching applications, diodes are usually integrated into transistor for it's protection.
     
  12. Wendy

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  13. Evil Lurker

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    An easier way to say it would be that FETs are used for PWM, or pulse width modulation. In my idea of PWM, the frequency stays the same, but the duty cycle changes. Frequency means the number of on/off cycles in a set period of time, i.e. on once and off once in a 10 second time period. Duty cycle means mean the time on and time off during that ten second period. For a 50% duty cycle the time on would be 5 seconds and time off would be 5 seconds, for a 30% duty cycle the time on would be 3 seconds and time on would be 7 seconds, so on and so forth.

    Of course this wouldn't be good for an LED since it would blink on and off so what needs to be done is increase the frequency, lets say we turn it on and off 1000 times a second, much faster than the eye can tell its being switched. In a perfect world with no losses and everything operates on a linear curve, lets say we kept the on time at 80% of that 1/1000th of a second period... the LED should be 80% of the brightness it would be at full power. Of course in real life it doesn't work that way, but basically that is how you would dim a LED, control a DC motor's speed, or whatever using a FET.
     
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  14. electronis whiz

    Thread Starter Well-Known Member

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    ok i have heared about pwm in motors controlers and other divice controls. so is the big 3 lead transistor thing on the primary side of a switching PSU be a mosfet? think i will try to see what i can do with them i have always wanted to make a transformer driver to work off DC and run normal and smps transformers. think a 555 timer and a power mosfet and some other componets to configure the 555. probley also a capacitor in series with a potintiometer to control the frequency.
     
  15. kubeek

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    Maybe, maybe not. You need to check the number on it to see what it is, for example alldatasheet.com is good site for that.
     
  16. Evil Lurker

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    That won't work. FET's are primarily designed for high speed switching applications. A "normal" transformer is made of iron plates wheras a SMPS transformer is made of ferrite... they each have vastly different magnetic properties and are designed to work at different switching frequencies.

    As far as the 555 timer goes, I use three potentiometers in my PWM circuit and two 1N4148 switching diodes. One is a 10k pot on the input from pin 8 to pin 7 to give fine control of the switching frequency, the other two provide independent control the charge/discharge times of the timing capacitor. Remember, the 555 timer works by turning the output high or low (on/off) by comparing the voltage of the timing capacitor with the input voltage going into the IC, when the voltage goes to either 1/3 or 2/3 of the input the IC changes its output state.

    And if your going to use any sort of transformer and your load is not fixed, then your going to blow something up unless you have some sort of IC that can adjust it's duty cycle based on output voltage.
     
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