Hi
For a current project of mine, i have to switch between two voltage sources (wall power supply or battery pack). I have searched the internet and found some chips which implement a complete ORing power Switch solution. F.e. the LTC4370, which would have been my favourite, but it's very hard to source here in Austria and also very expensive.
There are various other devices from different suppliers too, but they are all too low voltage and current.
For my design, the power switch has to handle up to 12A and voltages up to 15V.
Because the wall adapter or battery is powering a stepup regulator,so if the input voltage decreases the input current increases. It's a 60W design, so at the lowest acceptable input voltage of 5V, there will be a current of 12A flowing, instead at 12V the current decreases to 5A. See attached file.
Using any kind of schottky diodes makes no sense, because of the high currents combined with the forward voltage drop. Even the most expensive high current schottky diode had a Vf of 0.4V, which will create 4.8W of heat, that's a huge waste of energy and battery life. Then also cooling measures have to be in place.
So the only way out is to use power MOSFETs. The only problem are the bulk diodes, which are present in every MOSFET. It would have been easy, if the input voltages are fixed but they aren't. There can be one case, where the wall adapter voltage is 5V and the battery voltage is 12V, but also vice versa. So in one of both cases, the body diode of one of the two MOSFETs will conduct, overheat and destroy the MOSFET.
After this insight, I searched the web for alternative solutions and found one on the EDN website.
http://www.edn.com/design/analog/4368307/Use-op-amps-to-make-automatic-ORing-power-selectorThey haven't solved the problem with the conducting bulk diode, they actually used it for the purpose. A OPAMP measures the voltage drop across the MOSFET. If there is a very small current flowing this current will actually flow across the body diode into the load. If the current increases, the voltage drop across the MOSFET also increases, and if it exceeds a predefined value, the OPAMP will switch on the MOSFET, so that the current stops flowing through the body diode and instead flows through the lower impedance channel of the MOSFET.
So far so good. It seems to work just fine.
But what happens when the OPAMP fails doing it's job? Then the same problem as before, the body diode will conduct the full current, overheat and destroy the MOSFET.
So my question is: Are there any other "safer" designs out there, or is there a solution which makes the EDN schematic almost "bullet proof"
Cheers Gregor