Power switch brainstorming, DC in with battery failover.

[hitech95]

A friend asked me to help him with a small project he is working on.

I've thinked of a couple of solution but are quite complicated and doesn't cover all the requirements at once:

• Two power busses
  - 12V battery in (standby)
  - 8-26V DC In (active)
  
• 12V@3.5A (max not continuous) regulated output
• DC IN must have reverse polarity protection, UVP, OVP (to protect switching converter)
• When DC IN fails, (no present, UV cut off, OV cut off) it must immediately switch over to 12V battery.
• When DC in come back switch over must be performed after DC IN has a power good signal for some time. (To prevent to switching when the PSU hiccups)
  The only solution for this that is coming to my mind is using an MCU.
• The OVP/UVP and the active/standby status must be capable to be monitored by and external MCU/CPU.
  My solutions fails to do this. In fact any additional load that i attach to the two voltage references to handle OVP/UVP will unbalance the gate network.
  Resulting in the P MOSFETs that I'm using as a switch are always open.
• And it must be Cheap!

I'm attaching my idea below, but as I said before it is convoluted and not properly working. (It is also missing the reverse polarity protection.)
In that simulator the voltage references component is not present, so i had to create them manually!
And it is missing the DC DC part for the 12V stabilized output, but that should be the easiest part.


Any idea is welcome!

[NiHaoMike]

Use a pair of Schottky diodes as the core of the switching circuit. On the main input, place an OVP protection circuit. On the backup input, place a LDO that's normally set for 7.5V or so. If the main input drops below 8V, then switch the LDO to 20V or so which effectively turns it into a bypass.

If the Schottky diodes would cause too much loss, use an "ideal diode OR" circuit instead.

[Peabody]

You might want to look at power mux chips such as the TI TPS2120, 2121 and 2124.  They won't quite get you to 26V, but pretty close.  This type of product usually lets you chose between automatically selecting the highest voltage source, or manually selecting a particular source to be the backup without regard to voltage.  Other manufacturers have similar products.

[hitech95]

Thanks for the replys.

Unfortunatly those chips are a bit limited in voltage ranges. 26V was a bit of a stretch on the lower side.
Going even lower is not possible.

Anyway I've seen some interesting documents from TI I'll have to take a deeper look into them.

Probably 2 diodes to an LDO to power a small micro (like an attiny) to control the flow would be the easy way.
To actually switch the load a proper power switch would be awesome but all depends on costs!