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finding transistor and Schottky Diodes replacement how to

Discussion in 'General Electronics Discussion' started by damasta86, Nov 5, 2012.

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  1. damasta86


    Oct 5, 2011
    Hi everyone, I need some advice, I normally look for the exact part but I dont know how to find an alternative.

    I need a replacement for a STPS16L45CT which is a schottky diode transistor.
    the data sheet is this

    I know that I need to find the something that meet or exceeds the original parameters.

    how like to know how do I read this and put it in a search engine

    IF(AV) 2x8 A
    VRRM 45 V
    TJ (max) 150 C
    VF (max) 0.45 V

    Im doing a search here but I cant understand the abreviation in the filter

    I read I got to match package/voltage/current, but what does it means?

    hopefully someone can guide me, I really love this hobby.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    OK, there are several important parameters.

    The most important is that the diode has an isolated tab, meaning that the metal tag at the top is not connected to the diodes.

    This means you need a similar part, or one with a "plastic" case.

    Searching on Digikey is generally what I do.

    I searched first for "schottky rectifier", then selected Diode arrays (or something similar). It was then a case of selecting parameters. Firstly I selected all forward voltage dropd 600mV or less (in retrospect, perhaps 400mV or less would have been better). Then reverse voltage (I selected 45V, 50V, and 60V), and finally I selected all of the TO-220 style packages (again, perhaps I could have spent more time here picking ones that were likely to be insulated). Oh, I also selected forward currents ranging from 8A to 15A.

    After doing this I get a list of 44 devices, and I picked one that looked OK.

    It is this one.

    OK, the forward drop is higher, perhaps you could sort on forward voltage drop and select something closer to what you want.

    Anyway, that's what I would do.

    On the onsemi search page you can hover the mouse over the headings to see what they mean. I couldn't find a suitable diode after a quick search.
  3. damasta86


    Oct 5, 2011

    Hey thanx for the info, Im starting to understand abit more now but I still got some question.

    the STPS16L45CT in the data sheet doesnt say the forward voltage drop so why do you choose 600mv?

    does 2 X 8 A mean 16 amps total?

    the reverse voltage I think is the vrrm which is 45 V right? also how much can I exceed here?

    you said you selected forward currents 8a-15a, would like to know how u got that from the datasheet, also how much can I exceed.
    also the isolated tab you mean ontop where u put the screw to a heatsink right?

    metal tag at the top is not connected to the diodes. didnt understand this

    Thank for giving me some pointers, sorry for all the question:)
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Mostly because I had to choose something. It seemed a reasonable value.

    As to why I thought that, and whether my conclusions are valid... that's another story. It also might be one we need to investigate further later on.

    Yes. Each individual diode is rated at 8A and they are used in parallel, so you can pass 16A through them in totsl.

    The actual value you require is hard to determine. However knowing that the part being replaced is rated for this, and knowing that this is a limit, beyond which the device may behave unexpectedly (or be destroyed), it is wise to select a replacement with the same or higher rated reverse voltage.

    In essence there is no limit as to how high you might go, however some characteristics of the device will suffer as you go in this direction. The one which is possibly most important here is the forward voltage drop. So if you go too far in reverse voltage rating you may find that the forward voltage drop (that you normally want to be as small as possible) starts to get too high.

    Again, this is a limit. The original part's datasheet specified this as 2 x 8A (so 8A per diode). As a limit, I can go higher, but not lower. And also as you go higher other characteristics may suffer (reverse voltage, forward voltage drop) so you don't want to go too far. I decided that up to about double was OK.


    If that heatsink is connected to ground, or if there is another device attached to it then the isolated tab means the cathodes of these diodes will not be shorted to them.

    Something I didn't check (but you should) is that the thermal resistance (junction to case) is similar for the devices. It is either 1.3 or 3.5 deg C per watt depending on the case style. Given that manufacturers don't put oversized heatsinks in power supplies, you need to get something with a similar rating so it doesn't risk overheating.

    In this case, with a higher Vf, the dissipation will be higher and it might be wise to determine if the junction temperature is going to be a limiting factor.

    Yes, and that can be a really big trap.

    Knowing it has a metal tab, I now know you have the TO-220AB version, not the TO-220FPAB version (I probably picked that up earlier though)

    Assuming these run at 16A, with a 0.5V drop, they are dissipating 8W. If the heatsink is 10 degC/w, then the junction rise will be around 8 * (10 + 1.3) = 90 degC. If ambient is 30 degC then the junction temp may be as high as 120C. (most of these figures come out of thin air). You would need to repeat these calculations with the old and the new device, with figures based on reality rather than guesswork to see if the replacement will operate at a safe temperature.

    As you can see, it's a non-trivial problem, so it is good that you ask questions.
  5. damasta86


    Oct 5, 2011
    (*steve*) I really appreciate your help. Thats valuable information
    I be doing more research to learn more.
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