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Trying to control 24v using MOSFET from propeller. — Parallax Forums

Trying to control 24v using MOSFET from propeller.

Tony_tsiTony_tsi Posts: 98
edited 2011-12-09 16:52 in Propeller 1
I need help. I am trying to control a MOSFET from my propeller micro controller. Here is a link to the MOSFET data sheet I am using http://www.fairchildsemi.com/ds/HU/HUF75344G3.pdf . On the MOSFET the source is connected to ground, the drain it hooked up to my 24V led matrix, the gate needs to be hooked up to my propeller chip. What protection devices do I need to use between the gate on the MOSFET and the micro controller. This is for my UV exposure unit more info here http://www.tonytronix.com/myprojects/uvexposure.html

Comments

  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 14:51
    To me it would seem that I could just connect them with no protection because the gate threshold voltage is 2v min 4v max. But I do not know how much current this will draw from the micro controller. Also I do not know how to find out if it is possible for the MOSFET to back feed the 24v into my micro controller.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-12-09 15:00
    Tony_tsi wrote: »
    To me it would seem that I could just connect them with no protection because the gate threshold voltage is 2v min 4v max. But I do not know how much current this will draw from the micro controller. Also I do not know how to find out if it is possible for the MOSFET to back feed the 24v into my micro controller.

    I think your gate to source leakage is on the order of 100 nano amps. And the only way the mosfet can "back feed" to your propeller is if the mosfet fails. I usually put about 300 ohms to maybe 1 K ohms on a prop pin controlling the gate of a mosfet just to be safe in the event of a mosfet frying out.
  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 15:04
    Thanks for your help. For this project I was planning to use a QFP but parallax is out can I use solder paste on a QFN?
  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-12-09 15:07
    Tony_tsi wrote: »
    Thanks for your help. For this project I was planning to use a QFP but parallax is out can I use solder paste on a QFN?

    I would post this as a separate question in order to catch the eye of somebody on the forum who does that sort of soldering.
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2011-12-09 15:29
    The HUF75344G3 is not suitable for direct drive from the Prop.
    The threshold voltage to even start turning on is 4 volts, see the spec. The Prop only outputs 3.3V, not enough to even turn it on.

    A much better choice would be the IRF3708 which has a threshold voltage of 2V and be strongly conducting at 3.3V.

    One limitation is the Drain to Source voltage rating is 30V. Well within your requirement of running on 24V.

    Duane
  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 15:39
    Duane C. Johnson wrote: »
    The HUF75344G3 is not suitable for direct drive from the Prop.
    The threshold voltage to even start turning on is 4 volts, see the spec. The Prop only outputs 3.3V, not enough to even turn it on.

    A much better choice would be the IRF3708 which has a threshold voltage of 2V and be strongly conducting at 3.3V.

    One limitation is the Drain to Source voltage rating is 30V. Well within your requirement of running on 24V.

    Duane

    Where can I find this info in the data sheet?
  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-12-09 15:42
    Tony_tsi wrote: »
    Where can I find this info in the data sheet?

    I'm guessing he was looking at Figure 8.
  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 15:46
    I put the 300 ohm resistor in and tested it it works great. I thought the table on page 2 told the voltage where it says Gate to Source Threshold Voltage 2v min 4v max. What is Gate to Source Threshold Voltage?
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2011-12-09 15:55
    Gate to Source Threshold Voltage is the voltage that the MOSFET starts conducting at. It is not precisely controlled, which is why it says 2v min, 4v max. So, depending on the lot of transistors, you could put 3.3v into the gate and it wouldn't turn on at all, while others would be very solid.
  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 16:15
    Circuitsoft wrote: »
    Gate to Source Threshold Voltage is the voltage that the MOSFET starts conducting at. It is not precisely controlled, which is why it says 2v min, 4v max. So, depending on the lot of transistors, you could put 3.3v into the gate and it wouldn't turn on at all, while others would be very solid.
    What if I used a a small pnp transistor to power the gate using a 5v supply. Would this work? I have about 100 of these MOSFETs so I would like to learn how to control them using a propeller if I can.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-12-09 16:17
    You would need two transistors: a PNP to drive the MOSFET, and an NPN to drive the base of the PNP from the Propeller. But, yes, that would work. I would use a gate supply of at least 6V with that MOSFET, though. 10V would be better still.

    -Phil
  • Tony_tsiTony_tsi Posts: 98
    edited 2011-12-09 16:28
    Phil Pilgrim (PhiPi) wrote: »
    You would need two transistors: a PNP to drive the MOSFET, and an NPN to drive the base of the PNP from the Propeller. But, yes, that would work. I would use a gate supply of at least 6V with that MOSFET, though. 10V would be better still.

    -Phil

    Why do I have to have the npn? Is it a hardware reason or is it just to avoid rewriting some of my code.
  • kwinnkwinn Posts: 8,697
    edited 2011-12-09 16:32
    Vgs is the voltage at which the transistor starts to conduct. See http://en.wikipedia.org/wiki/Threshold_voltage for a more detailed explanation.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-12-09 16:45
    You need the NPN, because you can't drive the PNP directly from the Propeller. The reason is that, even with the Prop output high, its 3.3V will still be more than 0.6V lower than the PNP's emitter voltage, so the transistor will always be turned on.

    -Phil
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2011-12-09 16:52
    Good engineering practice says that one should assume the worst values in the spec.
    In this case the threshold voltage says 2V to 4V. So for this application assume 4V for the HUF75344.

    The IRF3708 threshold is 0.6V to 2V.
    Clearly the Prop exceeds the2V threshold. However, one needs to drive it with more voltage to get sufficient current to flow.
    Rds(on) in the spec says that Vgs of 2.8V will have a resistance of 29mO. Note! I use 29mO instead of 14.5mO.

    Depending on the required current in the application this will define how much heat will be dissipated.

    Sure, most of these MOSFETs will have lower threshold and less resistance and with a bit of experimentation one may be happy with the operation on a single test case.
    But if you are doing a good design trust in the specification and study the worst case specs and you can be confident a circuit will work even if you have never even seen the part.

    How much current do you need to switch?
    How fast do you need to do the switching?

    Duane
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