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  • Cycling rarely used electronics question

    There was a thread a month or so back speaking about capicators going bad from sitting and popping when powered up after a long sit. My question is can I power up my Chinese plasma cutter, ibgt inverter tig welder and various vfds and not use them and have this count as a cycle?. If I only power them up and don't actually use them is this actually helping? Now no actual use would mean no load put on them. But I don't actually know how the capacitors actually function in the circuits.

    Now my second question is this. On my igbt inverter welder if it has 10 igbts that are rated at 25 amps each. If I set it at 25 amps is it using only one igbt to its max currant or all 10 at 2.5amps each? Always wondered that..

  • #2
    Generally, yes, just being powered up will do the trick. Leave powered up for a bit, and that should do what is needed.

    As for the IGBTs, very possibly all in parallel, possibly a different setup. There are various choices, depending on the circuit chosen.

    Parallel can be tricky, but is quite possible to do.
    CNC machines only go through the motions

    Comment


    • #3
      It is common in Low Voltage inverters that the IGBTs will be parallelled and sharing.
      Sharing is an attribute of IGBT compared to some other semiconductor switches.
      IGBT have consistent gate capacitances to help them all commutate simultaneously,
      also the on state resistances promote sharing.
      https://library.e.abb.com/public/642...8_25082013.pdf

      For information on electrolytic capacitor reliability, you could refer to a manufacturer's publications.
      eg
      http://jianghai-europe.com/wp-conten...2012-10-30.pdf

      At the end of this doc.
      http://nichicon-us.com/english/produ...f/aluminum.pdf

      I have seen misleading info given by well-meaning contributors about electro reforming on some of the camera forum ( flash caps) and machining forum ( drive caps etc)

      Comment


      • #4
        Originally posted by wombat2go View Post
        ......
        Sharing is an attribute of IGBT compared to some other semiconductor switches.
        IGBT have consistent gate capacitances to help them all commutate simultaneously,
        also the on state resistances promote sharing.
        ......
        Actually, that is the absolute opposite of true....

        An "IGBT" is an "Insulated Gate Bipolar Transistor". Much the same sort of device as any other transistor, aside from having a "gate" instead of the normal "base" of a power transistor. Bipolar transistors do NOT share well, their on-state (saturation) voltage goes DOWN with increasing temperature, so bipolar transistors tend to "hog" current, and the hottest one will steal current from others.

        For IGBTs, much has been done to improve the characteristics, but the underlying physics of operation is still less than ideal for paralleling.

        MOSFETS have characteristics that inherently DO promote sharing and paralleling. Their on-state resistance increases with temperature, so the ones carrying less current tend to take on more since they are the lesser resistance. That makes for good sharing of steady-state current. IGBTs are more popular because they are more easily available in high voltage high current ratings.

        Then you come to turn-on speed... That problem is present in both mosfets and IGBTs. The device with the fastest turn-on will tend to carry the full current until others turn on as well, which stresses the part and lowers reliability.

        That is best handled by matching devices for "effective gate capacitance", which is partly an actual capacitance, and partly another capacitance that is multiplied by device "gain". Devices can be manufactured to cause a fairly tight control of the capacitance and gain. But that is not an inherent characteristic of the IGBT, or the Mosfet, for that matter.

        The best solution in most cases is to use a higher current device in the first place, or to use a circuit type that does not need as high a current at the switching device.

        Sometimes paralleling cannot be avoided. Most high current IGBTs are made up of many smaller devices in parallel, all mounted inside one package. The manufacturer selects them, usually choosing chips that are from a particular area of the processed wafer. Normally they will then be as close as possible in characteristics. At a former employer, a high power 3 phase switching device (200kW) used a set of IGBTs that were each about the size of a paperback book.

        Usually manufacturer selection will be more successful and less expensive for the same performance than paralleling packaged devices. However, manufacturers who want the very lowest cost, may elect to parallel many devices, gambling that there will be enough to offset the worst effects of mis-matching. Often they get away with it. Not always.
        CNC machines only go through the motions

        Comment


        • #5
          Hi JT,
          Your info above about IGBT sharing temp coefficients is out of date?

          Refer to Vce_sat versus temp for this Misubishi IGBT
          http://www.mitsubishielectric.com/se...0dx-24t1_e.pdf

          and ( not an IGBT but a straight BJT) this GeneSic Silicon Carbide Junction Transistor that I am presently fiddling with
          http://www.genesicsemi.com/images/pr...10JT12-247.pdf

          You will see that they have positive temp co

          The info you posted is correct for 1970's junction transistors and 1980's IGBTs, but time has moved on.

          Comment


          • #6
            Originally posted by wombat2go View Post
            Hi JT,
            Your info above about IGBT sharing temp coefficients is out of date?

            Refer to Vce_sat versus temp for this Misubishi IGBT
            http://www.mitsubishielectric.com/se...0dx-24t1_e.pdf

            and ( not an IGBT but a straight BJT) this GeneSic Silicon Carbide Junction Transistor that I am presently fiddling with
            http://www.genesicsemi.com/images/pr...10JT12-247.pdf

            You will see that they have positive temp co

            The info you posted is correct for 1970's junction transistors and 1980's IGBTs, but time has moved on.
            It is valid for MUCH later devices than the 1970s LOL.... I'd have liked to use IGBTs back then, but even the MOSFET was not very common......

            The very recent devices (expensive, as well) can be different, but of course they are not the same as the bulk of silicon based IGBTs used in equipment even now. The standard silicon IGBT is still produced and used in large volumes. It is the most likely device to be found in a chinese welder.

            The fact that you can get parts like the GA10JT12-247 is nice, but the price is almost 10x the price of a similar standard device (same 1200V 25A). Not to be found in HF welders, most likely...... For every one Ferrari sold, there are a LOT of Chevys.
            CNC machines only go through the motions

            Comment


            • #7
              I've been repairing electronics for ages, and have never heard of or read an analysis of cycling to increase the life of capacitors. Heat is the most common cause of capacitor failure, which is why caps loaded next to CPU's on motherboards is such a common failure area. The other highest cause of failure is just poor quality in the construction of the cap. Following that, cutting corners on design parameters. Unfortunately you have most of that going for you in a Chinese designed and manufactured welder.

              Comment


              • #8
                Originally posted by J Tiers View Post
                It is valid for MUCH later devices than the 1970s LOL.... I'd have liked to use IGBTs back then,
                Block IGBTs were being parallelled in induction heating by USA Japan and EU manufacturers and I think in motor drives ( maybe even traction) since late 1980's
                The earliest 250 Amp one I recall were positive tempco at high currents ( where it matters) and there was a transition to negative at low levels.
                And remenber, those IGBTs on the chip are hundreds of parallel elements.

                Anyway it has been a long time since power electronic designers had to worry about the negative tempco by standing the switches on emitter resistors etc

                Edit: Here is an archive of Mitsubishi data from 1998 ( 20 years ago! ) Fig showing IGBT postive tempco at the upper currents. https://www.scribd.com/document/2432449/IGBT-Mitsubishi

                So I think AH's weldere has to be a good one!
                Last edited by wombat2go; 03-30-2018, 02:10 PM.

                Comment


                • #9
                  I just took a random sampling through Digikey.....

                  I found that they are mixed in currently available parts. Some are negative, some are weakly positive, some are a bit more positive.

                  The issue is that there are a number of parameters that affect on current. Matching can usually be accomplished only over one or two, unless they are linked. Not all are. So there will always be mismatches. A god strong positive tempco of resistance helps maintain good current sharing despite variations.

                  I'd say that "a long time since designers had to worry about it" may be too strong.

                  In any case, matching among discrete parts is not usually going to be nearly as good as matching between chips that were neighbors on the wafer. When you buy a high current device, the many internal chips are matched better than you will ever get consistently with discrete parts.

                  When an equipment manufacturer chooses to parallel discrete parts, they are not getting as good a match, and not getting as good a reliability, as one "part" that may have a number of internal chips.

                  You start out with the thermal linking on the baseplate, vs thermal linking through the individual part baseplates into the heatsink. Then you add in that the parts are selected to be wfer neighbors, and the result is very good. far better than you get any other way
                  CNC machines only go through the motions

                  Comment


                  • #10
                    Originally posted by wombat2go View Post

                    Anyway it has been a long time since power electronic designers had to worry about the negative tempco by standing the switches on emitter resistors etc
                    All good and sunshine until someone tries to parallel several mosfets for linear operation!
                    Location: Helsinki, Finland, Europe

                    Comment


                    • #11
                      Originally posted by MattiJ View Post
                      All good and sunshine until someone tries to parallel several mosfets for linear operation!
                      I've done that in a number of designs. Worked well... when we got graded parts from the manufacturer, and used ballast resistors. Up to 1600W amplifiers. Did 2500W with 2 IGBT paralleled, 4 total, switching, half bridge audio output
                      CNC machines only go through the motions

                      Comment


                      • #12
                        Originally posted by Forestgnome View Post
                        I've been repairing electronics for ages, and have never heard of or read an analysis of cycling to increase the life of capacitors. Heat is the most common cause of capacitor failure, which is why caps loaded next to CPU's on motherboards is such a common failure area. The other highest cause of failure is just poor quality in the construction of the cap. Following that, cutting corners on design parameters. Unfortunately you have most of that going for you in a Chinese designed and manufactured welder.
                        I'm in the same boat. Stuff at home and at work that was used regularly or left on the shelf for a decade all seemed to work or fail with equal frequency.

                        I've seen computers were the power caps on the motherboard went bad and it seemed to occur MORE often on computers left on than those used very infrequently and properly shut down for days at a time.
                        Chilliwack BC, Canada

                        Comment


                        • #13
                          Low ESR caps are mandatory where switching speeds are high. This doesn't mean they will survive for decades though, and heat is not the only factor in their early demise. Consider that the ripple currents flowing in and out of the power supply caps are happening with much greater frequency, the caps are usually on the small side, and there will be a heat build-up- especially when the cap begins to fail. In any event, there's going to be a surge current at initial power-up, before any load is placed on the machine. It's going to happen every time you power up. If you want to power up the device from time to time just to keep the circuitry from dying through lack of use, it would be good to know how much steady state current the thing should be drawing at no load so you can do a quick test each time you power up. We've gone through this before- an inline meter and a safety resistor (light bulb in series, with a bypass switch) would be handy to have. Use it for testing all manner of plug-in electronics, and create a chart to show what the 'idle current' is for each device you'd like to maintain.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                          Comment


                          • #14
                            Power supply high voltage capacitors are much better than they used to be.

                            Even so, over time, the dielectric layer degrades, and the capacitors lose voltage withstanding capability. They "leak" more and heat up as a result. If the leakage is enough, they may overheat, short, or just boil off their liquid. In the process they may literally explode.

                            By periodically turning them on, and leaving on for a while, this degradation is reversed, the insulation film is built up by an electro-chemical process to a level that will withstand the applied voltage. But the process involves electrical current flow, and if the applied voltage is too high for the insulation film, a lot of current may flow.

                            For devices that have been stored a long time, it is prudent to hold them at 25%, then 50%, 75% and finally full normal line voltage for a while to make sure not to exceed the possibly reduced voltage withstanding capability before it is restored by having voltage applied.
                            CNC machines only go through the motions

                            Comment


                            • #15
                              From the Mitsubishi document linked by wombat2go above:
                              NOTE:

                              It may be observed that Mitsubishi IGBTs have a negative temperature coefficient of saturation voltage over a wide range of collector currents. This is not a deterrent to parallel operation and, in fact, is an advantage as it yields lower conduction loss at high junction temperature. The homogeneous process characteristics of H-Series IGBTs produce VCE(sat) characteristics that track as a function of current and temperature such that, once a VCE(sat) rank is chosen, the parallel devices will share within the given derating factor.


                              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!
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

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