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  • Solar Panel Bypass Diodes

    I just discovered a product from Texas Instruments called SM74611. This is an integrated circuit that is specially built for the purpose of replacing solar panel bypass diodes. It contains a large power MOSFET switch in parallel with a conventional diode. The MOSFET is on when the diode is conducting, leading to much lower voltage drop and much less heat than would be with a conventional diode. It is rated for 30V reverse voltage and 24A forward current.
    https://www.ti.com/product/SM74611

    Does anyone have any knowledge of this product or experience with it? It sounds like a good idea, especially if you have to service an existing panel and replace the diode. Why not go with something better than what was there? It isn't as easy to connect as an axial-lead diode, but would run much cooler.
    7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

  • #2
    That device needs to get power to run itself by periodically releasing the short it
    applies to minimize dissipation. Those release spikes must develop radio
    interference, so I would not use them. The benefit I guess is a fraction of a volt
    more out of an array string when it is partly shadowed. If there are any other
    strings in parallel, the shadowed loss will shift this to a much lower current point
    on its curve anyway.

    Maybe it works best with micro inverters. Seems like a lot of complication just
    to make a slight improvement on a diode, for an infrequent event. Bruce Roe

    Comment


    • #3
      The "Active Bypass" diodes save 2 ways, a tiny bit more power from the panel that is not lost in the diode Vf. And with dissipating heat, at roughly half the wattage, it's easier to manage 12W of heat than 24watts. Running cooler, they should last longer. Here's an older article about them:

      https://www.digikey.ca/en/articles/a...nd-performance
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        If manufacturers start integrating them into the panel itself (as suggested), it
        will make it impossible to repair a panel with a bypass failure. Bruce Roe

        Comment


        • #5
          If the reason for diodes blowing is over-heating, then using active devices with "10x reduction in power dissipation" would eliminate the need to replace diodes. (Quote from the article that Mike shared.)
          7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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          • #6
            I don't understand where the problem is. Bypass diodes should only see significant current with heavy shading. The best practice for arrays with a lot of shading is MLPE. Using a string inverter when parts of the string see heavy shading is a bad idea.

            Comment


            • #7
              Ralph is right that there are ways to minimize the time that bypass diodes are on. But solar infrastructure needs to be engineered for very long life and very high reliability. In solar panels, bypass diodes are often the weak link and under rare but expected conditions, they do fail. There is ample statistics to show that bypass diode failure has been a real problem in the past, even with microinverters. It is possible that solar panel makers have switched to larger diodes and greatly reduced the problem, but even today, I hear of people with microinverter arrays that have a bad panel, and trace the panel problem to a bad bypass diode.

              At first, I was skeptical. I got the data sheet for a common bypass diode and a common panel. If we assume that a dark cloud passes and shades part of the panel on a sunny day, then there will be many minutes when one part of the panel is producing 5 amps or more and the other part of the panel is bypassed, resulting in 5 amps through the diode.

              At 5 amps, the diode voltage drop can be greater than 0.5 volt for a Schottky and greater than 1 volt for a conventional diode. That means that the small plastic-encapsulated diode can dissipate over 5 watts. The plastic package is rated roughly 15 degrees C per watt thermal resistance in free air, slightly worse in a small sealed plastic box, and even worse with other diodes in the same plastic box. With 5 watts of dissipation, that raises diode temperature 75 C above air temperature. It may not exceed 150 C but it will still be hot enough that the life of the diode is greatly reduced.
              7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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              • #8
                You make a valid point about partial shading of a single panel. I had never considered bypass diode failure as a serious concern, but you have my curiosity piqued. I may do some testing on some CSI and LONGi panels in order to characterize the bypass diodes. I have some PSUs that can easily output 10A, so I could do some high current testing too. I don't expect to encounter problems for a couple reasons:
                1) the bus bars that run through the middle of the panel connecting the bypass diodes should act as good heat sinks
                2) EV charging and high-efficiency server power supplies have lead to development of low-Vf, high power schottky diodes, which I expect modern 72-cell high-power panels would be using

                Comment


                • #9
                  Originally posted by bob-n View Post
                  If the reason for diodes blowing is over-heating, then using active devices with "10x reduction in power dissipation" would eliminate the need to replace diodes. (Quote from the article that Mike shared.)
                  It is a reason, but not the only reason. I have been in electronics long enough,
                  to see vacuum tubes continue functioning when semiconductors have failed
                  due to moisture, static/lightning spikes, and other things. Early solid state
                  computers get new ICs here every week. Bruce Roe

                  Comment


                  • #10
                    Yes, it is very possible that modern diodes are better than older ones. I sure hope so!
                    I don't know about modern panel construction, but on older panels that I have taken apart, diodes were in the small plastic junction box, not on the panel itself. The only thing drawing away heat was little metal clips, like in the picture below (image credit: Tesla Motors Club):
                    images?q=tbn:ANd9GcR7KvH6DeSxBay8QYQlJop1Lc7iGpRdCBcbog&usqp=CAU.jpg
                    7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

                    Comment


                    • #11
                      Bruce is right. Heat from high current is not the only failure mode for diodes.
                      I'm a bit focused on diodes because I've seen so many fail, yet I don't see individual solar cells themselves fail, and they are much more silicon area.
                      Yes, it's complicated. These are not apples-to-apples comparisons.
                      7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

                      Comment


                      • #12
                        I've repaired panels with bypass diodes blown from reverse polarity. Diodes all split in half with a hairline crack
                        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                        solar: http://tinyurl.com/LMR-Solar
                        gen: http://tinyurl.com/LMR-Lister

                        Comment


                        • #13
                          The specs on these are impressive, but I don't think any mainstream panel manufacturers used them due to the high cost.
                          https://www.microsemi.com/existing-parts/parts/136636

                          And here's a similar product from TI:
                          https://www.ti.com/product/SM74611

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