Preface: I am designing a UPS such that if the AC main fail the device can continue to run for at least 10 days. My goal here is to make a design that can last years off a non-rechargeable battery. When starting this I originally thought to use a comparator, but after looking/googling I found that the diode approach was superior.
Design:
What: A 3.3 Volt .5 Amp UPS(Uninterrupted Power Supply)//I know you guys like numbers 🙂
Questions:
1) Use a diode or schottky diode?
schottky diode pros – I will enjoy the efficiency on the low voltage drop when operating from the battery
schottky diode cons – The higher reverse leakage current will decrease the lifespan of the battery which is my biggest design constraint.
2)Is this sufficient with regard to the battery not supplying current back towards the mains? Should I include a diode from the mains towards the load?
3) Would it make more sense for the back up power to come after a voltage regulator? I know that AC mains can fluctuate quiet a bit so this could remove any instance were the battery may engage when its not needed.
4) Are there cheap/simple alternatives to the diode approach that could provide superior results to my proposed method?
Thanks,
Josh
Best Answer
Go with the schottky if you can. In your design, the leakage current will actually charge the battery a bit. This prolongs battery life if the battery can handle this extra current. Example: if you have a 10amp-hour battery with 10 year life, its self discharge current is 114 microamperes; if your schottky has less leakage than that, you can use it.
It is sufficient, as long as your Vcc is always > (3.6 + Vdrop) volts
Your regulator will only decrease voltage, so putting the battery after regulator and keeping everything the same will not help. The advantage of putting battery after regulator is longer battery life: regulator drops at least 0.1-1.5 volts, so your load will not see full 3.6 volt if battery is before the regulator.
Diode is quite good. If you are really concerned about voltage drop, two mosfets and a special controller will be more efficient, but will make design significantly more complicated.