Long Title: "Any ideas for DC power supply ORing and power source reverse polarity protection?"

Hi all,

For a current project of mine, I have to switch between two voltage sources (wall power supply or battery pack). When an external power source is present it should be used to power the device (with higher than the battery priority). For example there can be case, where the wall adapter voltage is 9V and the battery voltage is 12V, but also vice versa. My goal is a device powered from as many power sources as possible (laptop power adapters, etc., for the external power, and different battery power pack configurations). It will be good if there is some protection against connecting the external power source (or the battery) with reverse to the device circuity polarity.
The wall adapter or the battery should power a step-up regulator, where the peak power consumption of the device is around 2W (not all the time, usually the consumption should be less than 1W).

First I tough about using a bridge rectifier on both power source terminals (that would give to my device a protection against reverse polarity, and if it is present a protection to the other power source as well). Unfortunately there is voltage drop across th? diode bridge rectifiers, which I want to avoid for the cases, when the device is running only on battery. Of course I could use Schottky diodes for the diode bridge rectifier, but in that case there is still some voltage drop and there will be small current leakage as well. Also with that simple implementation I cannot guarantee, that with higher priority will be used the external power source and not the battery power pack (attached on the battery terminals).

Please, can anyone suggest a solution, that can cover my requirements.

Thanks in advance for your ideas and thoughts on that problem!

 

A switch over power socket on your device would be the simplest. When the mains power plug is plugged in it disconnects the battery

http://www.rapidonline.com/cables-connectors/2-1mm-chassis-mounting-switched-dc-power-socket-400552/

 

Here is a polarity protector that gives less voltage drop than a diode. It's shown for an electric guitar pedal, but perhaps the general principle would be useful to you?

 

First I tough about using a bridge rectifier on both power source terminals ...

If all your sources are DC, you don't need a full bridge, just ORing diodes. That would cut the diode drop in half, since a full bridge drops voltage across 2 diodes.

Since you're stepping up the voltage after it comes in anyway, why worry about a few tenths drop at the diodes? Yes, you'll lose a bit of power. But simple is good.

I think to control priority, you'll need at least a comparator to determine which sources are available, and then set the output using MOSFETs as switches. If setting the output state is too complex, you may need a microcontroller to handle all the permutations.

 

Thank you very much for the suggestions.
I found an IC - LTC4417 that could fit for this task, it has all the bells and whistles (all kind of protections, a high input voltage range, power path priority selection, etc.). Unfortunately in my country I could get that chip for a relatively high price ~11$ (the combined price of almost all my device components is relatively the same).
However I come to a simpler device by LT - LTC4412. There was an example in its datasheet ( http://cds.linear.com/docs/en/datasheet/4412fa.pdf) that almost covers my needs (On page 10 under Ideal Diode Control with a Microcontroller - see the attached Fig 4.png with snapshot of the schematic). There is a suggestion to control the power source selection via MCU on the CTL pin of LTC4412.
Here what is written in the data sheet:

With a logical low input on the CTL pin, the primary input supplies power to the load regardless of the auxiliary voltage. When CTL is switched high, the auxiliary input will power the load whether or not it is higher or lower than the primary power voltage. Once the auxiliary is on, the primary power can be removed and the auxiliary will continue to power the load. Only when the primary voltage is higher than the auxiliary voltage will taking CTL low switch back to the primary power, otherwise the auxiliary stays connected. When the primary power is disconnected and V
IN falls below V LOAD, it will turn on the auxiliary MOSFET if CTL is low, but V LOAD must stay up long enough for the MOSFET to turn on. At a minimum, C OUT capacitance must be sized to hold up V LOAD until the transistion between the sets of MOSFETs is complete. Sufficient capacitance on the load and low or no capacitance on V IN will help ensure this.

As I only need a priority set on the external power source, I decided to provide a high voltage to that CTL pin, when external source is present via strong pull-up/weak pull-down resistor configuration and diodes for reverse polarity protection of that pin. I also added LED to light up when the battery is connected in reverse polarity.
Please see the attached Power Selector.png for a schematic with my current idea.

However I'm not completely sure if that is the correct solution, I have certain concerns if there will be some issues like battery drainage by that UC, when the device is powered off. Also I have no idea how to implement the over-voltage and over-current protection of the external power source connection and it's positive and negative terminals.

I'll be glad on any opinion. I'm open to other schematic suggestions and UC's as well (If they are relatively cheap and does not involve a plethora of hard to find components).
Thanks in advance for your response.

 

Seems like a rather complicated solution.

Use a power jack with a switch to disable the battery when the power adapter is plugged in.
Use two diodes to OR the power.