I'm working on a design with two voltage inputs and I need to give priority to one of the two inputs when both are plugged simultaneously. Both must also work individually.
- the first input (V1) is from a battery (12VDC)
- the second input (V2) is from an AC/DC powersupply (12VDC)
When the battery and the power supply are connected simultaneously, I want the power supply to provide the energy so that the battery is not discharged (electrically disconnected). As soon as I disconnect the main power supply, the battery takes over and provide the energy.
To do this I thought of a circuit using a SPDT-NC relay: The non-priority input (battery) is connected to the NC input. The priority input (Main power supply) is connected to the NO terminal and I also use its voltage to switch the relay. The relay switch/latch from NC to NO as soon as the Main power supply is plugged.
It is about this last point that I have a doubt about the behavior...
I specify that it is V3 (voltage on COM terminal) which will then power the system (the load), so I cannot control the relay coil form a microcontroller since it will not be powered..
Have you ever had this experience, and does the attached circuit diagram seem functional?
how to improve the whole design to add reliability and protection ?
My load is around 10A@12V so 15/20A D2PAK schottky should be fine.
What's important in your solution is to check that the min voltage at full charge of the main PSU is always higher than the max voltage of the battery, to be sure that diode is indeed reversed and not conductive with both source simultaneously.
When battery is used without main PSU, the schottky continuously conduct. With a power dissipation of 10A*0.3V=3W it can quickly heat up... Standard Rja~35°C/W and Ta=30°C, Tj=135°C. It can require an external heatsink. My design is passive thermal cooling and cannot be ventilated.
the solution with the relay avoid to drive high current in a component (except the relay which is made, for I guess...) and prevent temperature rising in some conditions.
What I'm missing is how to drive the relay coil to make sure I don't damage it by switching it everytime I turn on the main PSU...