IRLZ44 MOSFET, saturation..is this enough?

Discussion in 'General Electronics Chat' started by Nicholas, Dec 26, 2012.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Hi all

    I'm very happy with all the advice I have received so far! Thanks!

    I have a controller that outputs +5V with 30mA max. Is this enough to
    saturate a IRLZ44 MOSFET, which I understand should be a logic-level
    device?

    The IRLZ44 is supposed to switch 24V and about 2.5A in my application.

    How does this sound? If anyone could make simple drawing of a schematic,
    that would be awesome

    Thanks!

    Nicholas
     
  2. kubeek

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    Sep 20, 2005
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    5V is plenty. At what frequency do you plan it to work?
     
  3. Nicholas

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    Mar 24, 2005
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    Hi!

    It's a solenoid that will only need to kick once in a while. Like with 10 secs or more
    (maybe minutes) between energizing. The duration will be about half a second.

    So 30mA will be enough?

    Thanks!
     
  4. kubeek

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    Sep 20, 2005
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    30mA will be enough, you will also need a flyback catch diode on the solenoid and a discharge resistor on the gate of the mosfet.
     
  5. tracecom

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    Apr 16, 2010
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    I have used that MOSFET in this configuration and it worked well.

    I was switching a resistive load, but you will need a diode across the coil of the solenoid.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    This might be a picky point, but I don't think MOSFETS can be saturated. This only happens in bipolar transistors. In a MOSFET the channel can be open, or on, or conducting. The opposite is pinched-off or not conducting.
     
  7. JMac3108

    Active Member

    Aug 16, 2010
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    MOSFET datasheets are pretty easy to read if you're just going to use it as a switch as in your application. See the attached doc which is just a snapshot of the top part of the first page of the datasheet. You can see in the product summary right at the top of the page that the IRLZ44 has 28milliohms of On Resistance with 5V on the gate. That should answer your question :)
     
  8. MKCheruvu

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    Nov 20, 2012
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    As the MOSFET has built in quenching diode across Source/drain, this will take care of inductive load spikes.
     
  9. JMac3108

    Active Member

    Aug 16, 2010
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    No, not really. What you see in the datasheet is the "intrinsic diode" which all mosfets have due to their physical construction. In general you should not use this diode due to its poor parameters compared to an external diode. And anyway, its not in the right place to be used for inductive spikes.
     
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  10. kubeek

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    Ther was a discussion about this in some other thread, and what I remebered from it is that mosfets in fact do have saturation, kind-of similar to bilpolar transistors. See the Vds to Id graph for example here http://en.wikipedia.org/wiki/MOSFET#Modes_of_operation
     
  11. SPQR

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    Nov 4, 2011
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    This has always been a problem for me. I've read datasheet after datasheet, and I find, more often than not, that they are cryptic.

    I wish there were a line that said "This device will turn on with an input voltage between X and Y, and requires an input current of A to B".

    So do I have to do the I = 5V/0.028 Ohms (178 amps?) for all devices like this?

    Or is there a "newbie" section to each datasheet?:D
     
  12. John P

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    Oct 14, 2008
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    Well, if you use the components much, you know what to look for in the data sheets. I see that Vishay's page for the IRLZ44 has a "Product Summary" where they list "RDS(on) (Ω) VGS = 5.0 V 0.028". Maybe that, plus "VDS (V) 60" is all you need. But you might care about gate capacitance or switching time, and then you'd have to look further. One thing to beware of here--the IRLZ44 has a max 10V gate voltage. For many MOSFETS that's 20, so if you were substituting one part for another, you'd need to know that.
     
  13. kubeek

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    Sep 20, 2005
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    They don´t teach the very basics in datasheets. Mosfet don´t work like what I think you calculated there. The mosfet has maximum drain current of 47A, and if you have 5V between gate and source, the mosfet has 0.025Ω between drain and source. When it has 10V between gate and source, then the resistance is a little less than that - 0.022Ω. When the Vgs voltage gets too close to Vgsth (the threshold voltage, in this case it may be anywhere between 1V and 2V) the transistor gets into the linear region and has some appreciable resistance, so for on-off applications you want to avoid staying there.

    Mosfets gate is basically a capacitor, in this case 3.3nF, so it doesn´t need ANY gate current to stay on. The only time it needs current flowing into/out of the gate is when the state changes - you need to apply current to charge/discharge the gate capacitance. If it takes too long, the mosfet stays in the linear region for too long and there will be lots of power dissipated in it, so it might heat up and blow.

    You said you have 30mA available, and this should charge the capacitance in less than 1μs so you should be safe as far as the switching times go.
     
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  14. Nicholas

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    Mar 24, 2005
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    Hey!

    Thanks for all the feedback!

    tracecom, thanks for the drawing. Can you tell me why you have both the
    resistor on the gate and also a resistor from gate to gnd?

    I'm nervous of something going back into my controller and killing it, so would
    a diode on the gate be a good idea? Just to be sure butterfingers(me) won't jump
    something and destroy the uC (great slang)

    Nicholas
     
  15. kubeek

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    Sep 20, 2005
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    If your controller puts out 5V you dont need a diode on the gate. The resistor is there to make sure the mosfet turns off when you disconnect the gate, because the controller may only be swithching between the 5V and "nothing", so it can´t hurt to have it there. I would rather use 10k instead of the 100K to have a better turn-off time.
    In the drawing tracecom posted you need to add a diode between pins 1 and 2 of the load connector, pointing from 2 to 1 - the same direction as in the mosfet.
     
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  16. Nicholas

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    Mar 24, 2005
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    Thanks, I'll make a drawing later that you guys can scrutinize :)

    I do have a diode on the coil already:)
     
  17. Nicholas

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    Mar 24, 2005
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    Well, I made the drawing! I hope you will comment on it!

    [​IMG]
     
  18. tracecom

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    You need a 10k pull down resistor from the gate to ground.
     
  19. Nicholas

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    Mar 24, 2005
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    Ok then! Thanks!

    I still don't understand what that does?
     
  20. tracecom

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    It keeps the gate of the MOSFET at 0 V until there is a high signal from the μc.
     
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