You understand completely. Here is a a diode:
https://www.amazon.com/Amp-Volt-Stu...im_auto_1?ie=UTF8&refRID=1ST2YF163J4HZGP0NGRB Yep, it needs a mounting kit.
You can even use one diode of a bridge: **broken link removed**
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OK, that over with. As each person is different, each power supply is different and they decay at different rates, So, during the power up/ power down sequence of 3-4 sec as you described it, one or the other supply is bound to be higher or lower than the other.
So, each power up or power down cycle has the POTENTIAL of doing some damage.
50A is easy for a diode. If you can turn off the supply, then really only surge current is more important and the diode could probably be rated at 30 A rather than 50. If the voltage drops to zero on one supply and the load is resistive then the higher supply would only have to deliver 1/2 the current.
So, your right on that count. It does not have to be rated at the supply current with a safety margin.
A huge heat sink probably isn't necessary either especially if your going to turn the supply off.
There is a stud mount package and a large bolt on package (2 screws) that are available for high current and/or high voltage diodes.
Aside:
I caused quite some grief for a manufacturer that we had an excellent relationship with. A lamp power supply failed that needed to supply 40 Amps at 22 V. When I started to look for obvious things, I found the rectifier rated for only 10 AMPS. Naturally, I was furious.
Unfortunately for them, because of space reasons, they had to replace the transformer and the diode, They actually changed it to a full bridge, They did the modification for free.
We helped them a lot and we got lots of engineering changes for free, but they required factory returns.
Our help had to do with traceable spectrum calibration and adjustment of the light source.