Why are 2 diodes needed instead of 1 when diode-ORing two power supplies?

Question

I see many circuits online of dual power supplies connected in parallel through output diodes on their outputs, to provide redundancy in case the main one fails. Usually the primary supply is an AC-DC adapter and the secondary is a battery backup.

I'm designing a 12 V UPS for my router but am not sure why a diode is needed in series with the primary voltage source.

I understand that the diode in the secondary supply rail is needed to prevent the main 12 V supply feeding into it. In my case, the primary rail is a simple mains-to-12 V DC adapter. I don't see any issue in the battery backup "feeding" back to that if it loses power.

This is my design so far. It's missing some capacitors on the load output. Note that the diode I'll use won't be a 1N4148, that's just the default Circuitlab had when I drew it.

As I understand it, the primary 12 V supply will take priority because the voltage on both sides of the diode will be 12v. With 0 V across the diode, it won't conduct and the main supply will power the load. If the primary 12 V system loses power, then the voltage at that point drops and the diode begins to conduct, allowing the boosted-up backup battery to power the load. With a diode voltage drop of, say, 0.5 V this would give 11.5 V.

In this case it's also "backfeeding" into the main primary supply, but I don't see the danger this causes. I'm wondering if I can skip the diode on the primary voltage source in order to avoid the voltage drop that comes with it.

Answer 1

If you assume the power supply tolerates being back-fed, you might as well assume the boost regulator tolerates being back-fed too, and no diodes are necessary. But this is not the case.

You need two diodes because you can't back-feed voltage/current into a powered off-supply. Or you can, but the power supply may get destroyed.

You seem to assume a powered-off supply somehow disconnects itself from the output and does not care if something else keeps the output at 12V (or 11.5V) when mains input to power supply is off.

While some power supplies may tolerate that, some might even be especially designed for it, but some power supplies will not tolerate it and get damaged.

Some power supplies may even have a mechanism to try discharging the output voltage to have a faster and definite turn-off ramp instead of slowly decaying voltage.

You don't know beforehand if the power supply tolerates being back-fed or not.

Therefore you need to assume it does not handle being back-fed, just like you are assuming your battery powered boost converter does not tolerate being back-fed with 12V.

Answer 2

Short: They are not needed

Longer: If you look at the schematic of most common switching mode supplies there is a already a diode in the output, so backfeeding isn't even that big of an issue.

It's a question of what failure mode you want to protect from and what your primary power source is tolerating.

^{simulate this circuit – Schematic created using CircuitLab}

This is a simplified schematic of a typical buck converter commonly used in many power supplies. What you don't see is the implicit reverse Diode in the MOSFET M1.

So if you feed in Voltage at M1, U1 gets a signal on FB (which could prevent it from stating up again if it just has had a paower failure on the primary side). We're also backfeeding into Vin via the implicit reverse Diode in M1.

It can work, it can be no issue at all. It can be no iddue for a few times or under certein circumstances.

Backfeeding in a power supply is normally not a case that's defined in the datasheet. So the device can do whatever it wants if it happens, it's an undefined state.

So putting in that second diode in (it can be a shottky diode or an "ideal diode" if you don't want the voltage drop) is generally just the safe option.