I provided some engineering assistance a couple years ago for a company designing photo-voltaic array (PVA) based power inverters. These are sometimes referred to as micro-inverters since they are rated at a few hundred to a few thousand watts (rather than tens or hundreds of kilowatts). At first glance, I assumed all the high technology was in the inverter. The PVA portion seemed pretty low-tech. I was mistaken.
In a PVA there are long series strings of solar cells. The individual solar cells are matched to the extent that is practical (output voltage vs. light intensity). Unfortunately, that's not good enough.
When a section of the panel gets shaded (by trees or clouds), its output drops. So you've lost the matching you tried to achieve. With the shaded section producing less voltage, current gets pushed through from the other cells. This results in heating of the darker cells. You're wasting the energy you're trying to collect. The fix has been to set up sections in the PVA and add bypass diodes like this:
That helps, but it's not perfect — it reduces the heating some and moves the location of the heat source. Here are some typical numbers: With 10A flowing through the panel, a standard silicon diode (forward diode drop of 0.6V) would waste 6W. Even a Schottky diode with a forward drop of 0.4V will waste 4W.
When I cranked through these numbers, I decided what we needed was some sort of electronic switch that acted like a diode but had active control circuitry. I thought of a BJT or an FET, but I would need a separate isolated power source for base current or gate drive for each device. What had seemed like a good idea for a few minutes quickly looked rather impractical.
Apparently, someone else went through the same analysis and concluded almost the same thing — except for the part about it being impractical. Someone came up with a simple way to put the needed power supply circuitry right where it was needed, like this:
This is the SM74611, a “smart bypass diode” from Texas Instruments. It's intended for use with PVAs. It has a low-power switched capacitor boost power supply plus an N-channel power FET. The power supply generates sufficient voltage to drive the gate and fully enhance the conduction channel. Compared to those two diodes cited above, this device drops 26mV at 8A forward current, so that's a power dissipation of just over 200mW. The devices come in a D2PAK much like the standard Schottky diodes. You would normally mount these on a small PC board with quick disconnect terminals, in a totally sealed junction box. Low power dissipation means low temperature rise. I'll leave it as an exercise for the student to estimate temperature rise in a sealed box with a 6W load inside.
I wish I'd had these two years ago.