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Thread: Which MOSFET for DCSSR

  1. #11
    Join Date
    Jan 2008
    Location
    Bristol, UK
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    27
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    Default Re: Which MOSFET for DCSSR

    Okay, Thanks for your help guys, I think I'll go with Lightup's suggestion of try a tested design and see how I get on. I've got 11 months to do it.

    Happy Christmas everybody.
    192 + 12 Channels, P4 1.8GHz, Grinch & Parallel12, Win XP Sp3, Vixen 2.0
    [URL]http://tyzzerslights.org.uk/[/URL]

  2. #12
    Join Date
    Nov 2009
    Location
    Morrisburg, On, Canada
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    2,540
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    Default Re: Which MOSFET for DCSSR

    Some members here use the Asian source of parts - http://www.taydaelectronics.com/t-tr...s-mosfets.html

  3. #13
    Join Date
    May 2007
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    5,341
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    Default Re: Which MOSFET for DCSSR

    This comment will be somewhat conservative and over-simplified. The conservative element here is that I think that the circuit should be designed using the worst-case characteristics of the part (not the typical characteristics), and should be designed for the worst case situations. In this case, I'm assuming that the controller firmware locks up with a 100% duty-cycle and maximum load at a temperature of 40 C (104F).

    First, for that part in a TO-220 package the maximum allowable power dissipation should be (175 - 40)/(80C/W) = 1.6875W in order to keep the junction temperature within the 175 maximum temperature. This is not conservative at 40C, but is quite conservative at 0C. At 0C the allowable power dissipation is 2.1875W.

    Second, looking at Figure 7 in the datasheet (from mouser), the voltage drop from drain to source is about 0.4V (just an eyeball estimate) when the current is 6A (at almost any gate voltage above 4V). This equates to a power dissipation of about 2.4W (a little different from what I wrote above, as my eyeball estimate is different this second time around).

    If you want to live dangerously, go without a heatsink, and hope that the software doesn't lock up sometime at night or when you are away for several hours. Otherwise, either use a heatsink or limit the current to a much smaller value than 6A, or move to the North Slope of Alaska or the South Pole. Reducing the current to about 4A should decrease the maximum power dissipation to about 1.2W, which would be fairly safe (for the part, but not for your skin) to operate without a heat sink,
    Phil

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