Under the hood: The Outback FM80

Warning: Engineering porn ahead. All images are clickable to view in full resolution.

The Outback Power FM80 solar charge controller is a high performance MPPT controller which converts a solar array’s output (up to 150VDC, 64 amps) down to charge a 12, 24, 36, 48, or 60 volt DC battery string using a high efficiency switching buck converter and an extremely flexible microprocessor control system. It is field programmable from the front panel and can be linked to other system components using Outback’s communication buss and the MATE controllers for system logging and remote control.

It is extremely well built, and solid as a rock.

More photos below…

First off, here’s the back of the display board, and the large thick fin extending off the front cover which serves as a heatsink for the power switching transistors. Note that the entire metal casting which serves as the front of the unit is a big solid heatsink. The small cable running upwards goes to the plug for the user-replaceable cooling fan, which is inside a small removable enclosure at the top (not pictured here).

The entire board, showing the transistors; these are held to the heatsink/top cover with a copper heat spreader strip that secures with…  well, a lot of screws. The thermal design is excellent. The inductors appear to be wound with many parallel strands of nylon coated copper wire.

The board viewed from above. Note the size of the inductors! The small vertically mounted board contains the microprocessor and control logic.

The big heatsink fin. The gray pad is a compliant (squishy) phase-change (melty) thermal gasket.

MPPT charge controllers aren’t cheap, but with this one, you seriously get your money’s worth. I highly recommend the Outback controllers!

8 thoughts on “Under the hood: The Outback FM80”

  1. Good evening, Tom Morris! I’m impressed by your article: “Under the hood: The Outback FM80”. From the pictures I had figured out, that FM80 is based on two-channel dual phase synchronous buck-topology. It inspired me to build my own MPPT controller. Currently I’m carrying out experiments with buck-converter. But there’s a problem: I’m using 100V MOSFETS, but they burns, when the PV voltage is greater than 65V… The ratio between 65 and 100 is equal to 0.65. There must be safety operation under this conditions.I think they burnt because of voltage spikes or because of shoot-through during reverse recovery of synchronous MOSFET. Now look at FM80. They use there 150V FDP2532 MOSFETs and they say, that PV voltage shouldn’t exceed 140V. So the ratio beetwen 140 and 150V is 0.93, practically no safe operation under 140V. Please, help me, if you can: How in the world they supress voltage spikes on MOSFETs in FM80, if there is so low margin beetwen maximum operating voltage and allowable Vds of MOSFETs and their MOSFETs don’t burn? I saw on FM80’s PCB some tiny diodes and resistors and ceramic capacitors. Are they RCD-snubbers or something else? At last, have some more pictures of FM80 for example from bottom of PCB? Thanks a lot in advance!

    1. That’s a very interesting question… I don’t have access to this hardware anymore, but I am curious. My best guess would be that the diodes, resistors, and capacitors are for decoupling and protection if anything, they don’t appear to really be able to shift much power.

      My only guess is that the voltage seen at the MOSFETs is raised up by the battery voltage, so for instance, on a 48V battery system, the mosfets are seeing like PV – 52vdc….?

  2. Of course, battery voltage is subtracted from input PV voltage. But experiments had clearly shown that this doesn’t help much. Just admire Outback’s designers how they make efficient power systems. I’ll be experimenting with my buck again. Maybe luck will smile at me and at last I’ll build my own FM something like FM100! 🙂

  3. Dear i have faced same problem. i have used 200V mosfet but they blow up at about 135 panel voltage. i noticed that the spike at drain source of mosfet is about 200V. and my mosfet blown. so i used RC snubber and i have reduced the spikes. but there is power loss on the snubber. so i decided to lower down the panel voltage to about 90V and lower the value of snubbers. that was the solution. if you send me clear picture of FM then we can share their solution.

  4. I found this (relative dated) article. I hooked up a used flexmax 60 a month ago. 4 panels in series, 2 strings in parallel. Voc 29 volt x4 = 116 volt max. My 158 watt panels have Isc of 7.58 amp x2 in parallel is 16 amps max. I had put a 40 amp breaker (only one I had available, should have been a 20 amp breaker) in series with PV panels and a 64 amps breaker in series with the batteries (4 trojan L16G-AC)
    System has been working great until today. I found the 63 amp battery breaker tripped, no display. Turns out the PV input is shorted, unit does power up after resetting battery breaker. I took some pictures in case somebody wants to follow up on the design of outback: http://www2.dth.net/solar/outback/flexmax/

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