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  • #16
    Originally posted by PStechPaul View Post
    From the Mitsubishi document linked by wombat2go above:


    I didn't see a graph of Vce(sat) vs temperature, but there is a large effect of gate voltage. There is a chart showing Vce(sat) versus current, and at 125C it is a straight line, indicating mostly resistance, with a constant 1 volt minimum drop. The greatest difference is about 0.5 volts at 160 amps. The curves cross at 440amps, with Vce(sat)=2.8 volts, and at 800 amps it is 3.75 volts at 125C and 4.25 volts at 25C. But with 3400 watts dissipation, it won't stay 25C very long!
    [/SIZE]
    pretty much any negative variation is less than good.... it depends on how well you can keep them at the same temperature, and also on how MUCH the variation is.

    No, they will not stay at 25C that long.... the issue is what happens when they are heating up. If one heats a little more, due to any sort of variation in parameter, it can hog current and have more dissipation than any of the others. depending on how much that is, it may decrease reliability, or cause that part to exceed limits when others are comfortably below the limit. The heating is inherently unstable, because more heating means better conductivity, and more current flow, which heats the part more...... it is a positive feedback that can rapidly go to the worst case.

    If it is a weak negative variation, it may not be so bad, the variation may not end up changing the current a lot. The "worst case" may be tolerable. That depends on the more constant resistive portion, which can provide an effect similar to putting in "ballast" resistors as was always done with paralleled standard bipolar parts in linear operation.

    Another way to help avoid problems is to mount all the paralleled parts as directly as possible to a "heat plate", close together, without insulation. You can do that because they are all acting as one big composite part. Then you insulate that plate electrically as a whole from the heatsink.

    What that does is to put the whole set on a relatively constant temperature base. The thermal resistance from each to the plate is as low as you can get it, and any heating from whatever part will heat the plate up. That provides thermal feedback to the other parts, so that they are closer to the same temperature as the hottest part.

    If they are each on their own electrical insulation, then they are much less "connected" thermally, and can have considerably different temperatures. That allows any negative tempco to more directly affect the imbalance between parts.
    Last edited by J Tiers; 03-30-2018, 09:07 PM.
    CNC machines only go through the motions

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    • #17
      Found some solid information on electrolytic cap shelf life. Good info on pages 14-17.
      http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

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      • #18
        Forest,
        Thanks, that is a very good resource for those of with interest in old gear.

        I read in it that the little pcb mount electros are better sealed than the upright can power filter types.

        In 2016 I restored this old blade chassis amplifier from a Magnavox stereo
        https://app.box.com/s/f8wk5i9uvpfw6r4037oaqd9yv92t4srw

        I think it from late 1950's the transistors are germanium and the power transistors, on pressed steel
        heatsink, were by Bendix labelled "68P1K" and "6624N" but could find no internet ref.

        In this amp, all the small electros were open circuit and the photo shows that I replaced them with Nichicon.
        But the DC ripple was good at full power as was the square wave test on the amplifier,
        so I left the nice looking power supply filter capacitor in place ! (at upper right).

        An interesting item about this amplifier was that instead of a 'loudness button" that we see in later stereos,
        this one has a passive circuit that varies the frequency response toward more treble boost as the volume pot is
        turned down.
        Also the dual treble pot had (I think was intentionally built with) different resistance values for each channel.

        Here is the frequency response test showing different levels between the channels.
        https://app.box.com/s/gwqz4xv9u87n48bmt5pjlkstu2i7tq86
        I have since replaced the dual pots with new ones with equal values. ( Those old dual pots are expensive)

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        • #19
          my old haidenhain dro started giving up the ghost a few years back. well, after reading about the caps somewhere i left it on for a week. has been working ever since. a vfd i bought about 20 years ago died after a few years. i was the caps. now, about 15 years later it still works, maybe because i switch it on from time to time when not in use?

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          • #20
            If anyone's interested, there is a forum called "badcaps forum".

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            • #21
              Old mart. I used the search engine built into this forum and searched for Badcaps and did not find a forum with that word in the title. Perhaps you could post the address??

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              • #22
                I get the forum by googling it, www.badcaps.net

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                • #23
                  Originally posted by PStechPaul View Post
                  From the Mitsubishi document linked by wombat2go above:


                  I didn't see a graph of Vce(sat) vs temperature, but there is a large effect of gate voltage. There is a chart showing Vce(sat) versus current, and at 125C it is a straight line, indicating mostly resistance, with a constant 1 volt minimum drop. The greatest difference is about 0.5 volts at 160 amps. The curves cross at 440amps, with Vce(sat)=2.8 volts, and at 800 amps it is 3.75 volts at 125C and 4.25 volts at 25C. But with 3400 watts dissipation, it won't stay 25C very long!
                  [/SIZE]
                  Hi PSTech
                  From my experience with water cooling, , industrial silicon semiconductors paralleled will be sized for continuous duty ( 24/7)at a safety factor above the wet bulb temperature.
                  So for duty at ambients on the planet, you could assume 38 ~ 42 Celsius on the heatsink below the case, subject to better information.
                  In my experience the junction temperature ( silicon generally) should be around 80 to 95 C for active devices switching.
                  Regards

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