74hc04 driving irf630n fet

Discussion in 'General Electronics Chat' started by Ramasule, Apr 6, 2010.

  1. Ramasule

    Thread Starter Member

    Nov 6, 2004
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    0
    Hello,
    I currently have a circuit that is the following setup.


    Sinking Pulse + Pullup Resistor -> 74HC04 (hex inverter) -> IRF630N

    There is no resistor on the gate.

    This circuit has been working for me and the and the fets are switching about 3 amps each.

    The 74HC04 sources and sinks current.

    Couple issues I think I may have;

    No resistor on the gate.

    Gate threshold voltage is stated 2 min 4 max and I am driving it at 5v.
    VGS(th) Gate Threshold Voltage 2.0 ––– 4.0 V VDS = VGS, ID = 250μA
    Gate voltage is.
    VGS Gate-to-Source Voltage ±20 V

    Datasheets:
    http://www.irf.com/product-info/datasheets/data/irf630n.pdf
    http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdf

    The fets are driving lighting loads (High power LED @ 12v)

    Thank you for your time,

    Derek L
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A small resistor might help with preserving the '04's output driver. However, that Vth marks the voltage at which the FET just begins to conduct. The FET will not be fully conducting until the Vgs is at least 10 volts above the source voltage

    You can toss the HC04 and use a CD4049 inverter instead. Use 12 volts for Vcc and the Vgs requirement is satisfied. Or use a 7405 with a pullup to +12 on the output. In either case, use a resistor of about 51 - 100 ohms in series with the gate.
     
  3. Ramasule

    Thread Starter Member

    Nov 6, 2004
    12
    0
    Thanks for the quick reply.
    http://pdf1.alldatasheet.com/datasheet-pdf/view/26882/TI/CD4049.html
    CD4049UB (inverting)

    My driver (to the cd4049UB) Luckily can handle up to 17 volts.
    The cd4049UB states that there is a current limit per input, does this mean I should limit it?
    It says 10ma is max per channel at any given time, my driver to the (cd4049UB) allows me to set any output I would like.

    Should I do as following.

    Driver (Set to 2ma Sink) + 10k Pullup @ 12v -> cd4049UB @12v -> IRF630 (nchannel)
    Really the variable I am unsure of right now is what to set my driver chip to.
    The driver btw is a TLC5940
    Datasheet:
    http://www.ti.com/lit/gpn/tlc5940

    Thank you,

    Derek L
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Are you using PWM on the gates?

    I don't know what your LED source voltage is, but the IRF630 MOSFETs have a max Vdss of 200v and Rds(on) of .4 Ohms, and a total gate charge of 43nC.

    You would be better off to use a logic level MOSFET with a Vdss rating that is a fairly close match to your voltage supply, as for a similarly-spec'ed MOSFET you'll wind up with a lower gate charge and lower Rds(on).

    Since your gate driver is quite anemic, you really need a lower gate charge.

    Why don't you post your circuit schematic? It will help us to help you more quickly and effectively.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
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  6. Ramasule

    Thread Starter Member

    Nov 6, 2004
    12
    0
    What about these two links.
    I just chose the IRF630N because I have a bunch of them lying around and this is what I was prototyping with. When I actually get a nice board made for myself I could go with a better suiting fet.

    http://cgi.ebay.com/STD17NF03LT4-N-...emQQptZLH_DefaultDomain_0?hash=item2306468381

    http://cgi.ebay.com/5-pcs-N-Channel...emQQptZLH_DefaultDomain_0?hash=item20acdcb291

    And here is the schematic.

    Top terminals are power.

    Side Terminals are for jumpers to Microcontroller

    Bottom Terminals are for the LED's

    Each Fet will be seeing up to 25w load (10 wattload LED + Room for another 10 Watt LED + safety + losses)
    LED run at 12v

    [​IMG]
     
    Last edited: Apr 6, 2010
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The STD17NF03LT4 would be a far better choice than the IRF630's.
    Gate charge is far lower, Rds(on) is far lower. Even running with logic levels on the gates, Rds(on) is 1/10 that of the IRF630.

    That is, if you can deal with the TO-252 SMT package, and provide some copper space for heat sinking.

    I wouldn't go with the 2nd MOSFETs.

    One problem you can get into with buying on auction sites is counterfeit components. Another is deceptive shipping/handling charges. If you want to avoid getting burned like that, use only authorized distributors.
    Some examples of authorized distributors are:
    Digikey.com
    Mouser.com
    Newark.com
    AvnetExpress.com
    Alliedelec.com

    If you buy from other than authorized distributors, you might think you're saving money - but you may very likely get burned, too.

    I cannot see any details in your schematic.

    Please export your schematic using 120 DPI resolution.

    If you are using Eagle version 4.16r2 or earlier, attach your .sch file. I can't read files created by later Eagle versions. I'm not interested in paying to upgrade my registered version.
     
  8. Ramasule

    Thread Starter Member

    Nov 6, 2004
    12
    0
    It was a good export, it's photbucket that wrecked it.

    Try this,

    ftp://lucksinger.ca/House Project/board v1.0.png

    It dosn't matter what fet I use. Thanks for the advice on wholesale ebay lots, I haven't had a bad experiance yet but I use ebay as well as digikey and mouser for a lot of my parts.

    Once again thank you for your time,

    Derek L
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, since you're not using the LED current limiter features of the TLC5940, you're going to have to somehow limit the peak LED current another way. Simply applying PWM from the TLC5940 won't cut the mustard.
     
  10. Ramasule

    Thread Starter Member

    Nov 6, 2004
    12
    0
    Im not worried about the load current.
    The current I was worried about was to the 4049 from the tlc5940. But I "think" I see now that the 4049 is a high impedance input's and does not need the current into the inputs limited.

    The 4049 will not behave like the 7404?

    If that dosnt work I guess I can just get the proper fets anyways which I plan to do and use the 7404 logic level voltage to drive the fets.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    STD17NF03LT4's can be driven by logic level devices.

    You should have a resistor between the gate and the device you use to drive it with. Place the resistor as close to the MOSFET as possible. This will help a great deal to prevent the gate from "ringing", which would lead to operation in the linear region and high power dissipation in the MOSFET.
     
  12. Ramasule

    Thread Starter Member

    Nov 6, 2004
    12
    0
    Yeah, I've had a fun experiance with ringing on some of my high powered devices. Funny thing about those Fire Emitting Transistors failing shorted.
     
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