When should external diodes be used with a power supply?

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I've written a few of articles over time concerning the usage of external diodes with power provides (or FETS), one was again in 2007 regarding fault tolerance, one other was when driving DC motors and a third on operating power supplies in series.

Just lately two different functions got here up, simply once I thought I had the topic lined!

Redundant Operation

Connecting two power provides collectively for redundancy is broadly used.  (To not be confused with brute mode present sharing)

If PSU #1 fails, theoretically PSU #2 takes over, proper?  Not fairly….

Trying contained in the power provide, the output voltage is often monitored by an op amp and is then in comparison with an internal reference.  If the output voltage is just too high, then the comparator will make the management circuit decrease the output voltage by lowering the switching converter pulse width.  Likewise, if the output voltage is just too low, the switching pulse width will improve to make the output voltage rise.
Let’s say PSU #1 is at 24.0V and PSU #2 is ready at a barely greater voltage, say 24.1V.  PSU #1’s management circuit now “sees” 24.1V as an output voltage and can flip the switching converter off believing that its output voltage is just too high. 
Within the occasion of PSU #2 failing, the load demand will fall on PSU #1, which must flip the switching converter again on and should trigger a non permanent lack of voltage supplied to the load.
 
Including a diode in collection with every power provide output will cease the power provides from “seeing” the opposite’s output voltage, and though PSU #2 might present your complete load, if it fails PSU#1 will be lively, prepared to supply power, and be in a position to hold a voltage accessible to the load.

Battery Again-Up
On many low value, low wattage power provides, overvoltage protection is supplied by a Zener diode related throughout the output terminals of the power provide.  Within the occasion of a management circuit malfunction inflicting the output to rise, the Zener will fail brief circuit, forcing the power provide into overcurrent protection (“hiccup” kind present restrict mode should be used by the power provide designer).
If battery back-up is being used (or one other power provide), then present will circulate into the defective power provide and trigger overheating of the Zener and surrounding circuitry.
Once more, a diode in collection with the power provide will stop this.