Mosfet as switch calculation

Discussion in 'Embedded Systems and Microcontrollers' started by afonso, Dec 9, 2012.

  1. afonso

    Thread Starter New Member

    Mar 29, 2009
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    Hi people,

    I want use a n-Mosfet as switch with drain connected to 95 VDC, a Load resistance of 5 ohm connected to source and the other pin to gnd.

    The gate is operated by a PIC ( 5 V ) throught a NPN transistor.

    The Mosfet I intent to use is the IRF 3415, but I´m not sure it´s ok.

    Question 1 - How to verify if it´s the correct mosfet Using datasheet elements ?
    Question 2 - Assuming that VGS = 10 V ( when PIC output = LOW -> NPN transistor in cut-off region) the Source voltage should not be nearby 95VDC? If it's correct we have another issue related with VGS max = +/- 20V ( data sheet parameter ).

    Does any body help me ?

    Thanks

    Af
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That MOSFET wants zero to turn off, and at least 5 volts if not 10 volts to turn on.

    Experience taught us not to go above 10V unless you don't mind the occasional blow up which happened when we were driving them at 15V. No manufacturer actually tests this parameter 100%.

    I can't tell what you are doing with the NPN, but you may well be on the right track with your approach. Can you post or link to your schematic?

    Also,this is generally refereed to as a high side switch in case you wish to google it.

    Finally, this is more of a General Electronics Chat type of question. You would get many more answers had you posted there. Don't post it there now, but maybe if you ask the moderators they can move it for you. Perhaps the best way to do that is to hit the red triangle report post button.That opens another window where you can ask nicely.

    And welcome to the forums!
     
  3. John P

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    Oct 14, 2008
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    "drain connected to 95 VDC, a Load resistance of 5 ohm connected to source and the other pin to gnd."

    Come on, pal. Think about what you're doing and what you're describing. How many pins does this part have?
     
  4. afonso

    Thread Starter New Member

    Mar 29, 2009
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    Hello everybody,

    Thanks for the answers.

    John P,

    "Come on, pal. Think about what you're doing and what you're describing. How many pins does this part have? "

    Please check the image sent. The other pin was referring the connection between resistor and gnd.

    Ernie,

    The schematics is attached as requested.
    My doubt is if it´s the right mosfet to do this , and how to solve the issue, when switched ON and the Vsource is aprox 95 V ( less Rds*Id), meaning that VGS > 20 V, so it burns!

    Thanks again

    Af
     
  5. shortbus

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    Sep 30, 2009
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  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Thanks for the diagram, but what it shows is that your circuit can't possibly work. You'd need to get the gate 10V above the source, and how will you ever do that? A special driver of the kind Shortbus suggested would be the only way.

    Must the load be connected to Gnd? If you could connect the load between Vin and the MOSFET drain, and the MOSFET source to Gnd, then it's solved.

    Alternatively, you need a P-channel MOSFET and a pullup on its gate to Vin, then the control transistor needs to pull the gate low to turn it on. But "low" means no more than 20V below Vin, so you'd need an additional resistor to create a voltage divider on the gate. Also, the processor output would be inverted, so high = on, low = off.

    There aren't many P-channel FETs with the ratings to do this job. The IRF6218 seems usable. Its on-state resistance is 150mohm though, which seems high. Perhaps there's something better out there.

    Edited to say, if you want to pursue the P-channel FET option, the Fairchild FQA36P15 looks like a good candidate. It's only 0.09ohms, and it's cheaper.
     
    Last edited: Dec 10, 2012
  7. ErnieM

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    Apr 24, 2011
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    Off the top of my head something like this may work better:

    [​IMG]

    A P channel device needs a negative gate to turn on, with respect to source. So if gate < source it turns on, and hey, we can do that easily.

    Now the transistor is turning on a zener that makes the gate voltage, and the resistor gives a way to discharge the gate capacitance. As long as you don't run this too fast it should work. For speed you'd need a real driver.
     
  8. kubeek

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    Sep 20, 2005
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    Just don´t forget to use a 2W resistor for the 10K.
     
  9. John P

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    Oct 14, 2008
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    I can see the zener diode giving some psychlogical comfort, but in fact it won't conduct unless the power supply goes over 110V, or 132V if the diode has a 12V rating. (Because it gets 1/11 of the voltage).
     
  10. ErnieM

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    Apr 24, 2011
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    Making all the resistors in my previous post 10K would be an improvement. 100K may be even better.
     
  11. afonso

    Thread Starter New Member

    Mar 29, 2009
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    hello everybody,

    Thanks for the help.
    I still having a doubt. When we want use a mosfet as switch, shall we use it in saturation or ohmic region?

    I've been reading some notes about it, and I still confused.
    Another question is related with Id(on).
    It´s the drain current in datasheets for the Rds(on ) given?
    Can anybody show an example ?

    Thanks

    Af
     
  12. John P

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    Oct 14, 2008
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    You can't afford to use a transistor in anything but a full-on state. Your current is so high (~20Amp) that you need to get a component with the lowest possible on-state resistance, and make sure it's fully turned on. Also, if you need to switch the load at anything like a high frequency, you need to consider how much power will be dissipated during switching, and what that adds up to under worst-case conditions.

    Power lost in the transistor is equal to voltage drop times current, or resistance times current squared. Work out the numbers and select a transistor that works. I believe you'll find that if you want to do this with a P-channel FET, there are not many choices that are usable.
     
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