LiPo UPS schematic?

[konfu]

Hi,

for a project I need something like a small UPS: I have a circuit, powered by 3.3V. Estimated current draw about 500mA.

Since the location I want to use this circuit at has some kind of an unstable public power grid, I need to have a backup battery setup. I think a runtime of 4-6 hours on battery is enough. It just needs to cover shorter outages of an hour or several minutes but it should cover these outages uninterrupted.

So I instantly thought of: "Hey. What about a 2Ah LiPo? Should deliver enough power for the backup. It's easy to charge, it's small, it's something I know."

So I have three circuits that I need to connect together:

PSU1: a AC/DC power supply with an output of 5V, 1A DC (it will also supply power for the LiPo charger)
PSU2: a LiPo circuit (5V in, 5V out) with MCP73831 as LiPo charger and TPS61200 as 5V boost converter.
Load: Some circuit that will use 500mA at 5V ;-)


How do I throw these together? PSU1 will supply 5V to the Load and for PSU2 to charge the LiPo.

I can think of two designs:

1. 5V BUS: using diodes (or something like LM5050 with Mosfets) to have both PSUs feeding into the load all the time together. If PSU1 fails, PSU2 will cover the load. Some nice chunky cap in front of the load circuit should cover the time the TPS62100 need to adjust the load when it takes over the load. Since my PSU output is 5V and the Load circuits voltage is 3.3V anyway I can live with some diodes voltage drop. There's going to be a LDO to convert 5V to 3.3V in front of the load circuit anyway.

2. Real switchover: use some fancy IC like LTC4417 which allows for a nice switchover between different power sources. But that's occupying quite some space on the PCB and might just be overkill.

Are there any other solutions? Easier ones maybe? I'm open to any suggestions...

Something I am afraid of: I don't know what the TPS62100 will behave like when feeding into the "power bus" between PSU1, PSU2 and the Load. How can I influence that under normal conditions PSU1 takes all the load and PSU2 load will only jump in if PSU1 is gone?

[Paul Price]

First of all you cannot have 5V in and get exactly 5V out. There has to be some reason not to just  connect them together with a single wire.

If you have one backup supply and one AC mains supply, if they are both connected to the target circuit, then the one with the slightly higher voltage(just a few tens of millivolts higher mains powered) will dominate or more likely supply all the current, all the while the AC mains could be charging the backup supply.

As soon as the mains voltage supply shuts down from a power failure, the backup will take over automatically. No diodes needed.

First, you must figure why you have so many 5V supplies!

[NiHaoMike]

If both supplies are backfeed safe (as in you can connect another supply to the output with no adverse effects under all operating conditions), you can just tie them together and do away with diode losses. Non synchronous converters generally are just fine with being backfed, but synchronous converters must be designed with that in mind.

An alternative is to just supply 4.1V/cell (current limited) or so to the battery while drawing power from it. It's a common problem in every portable device that uses rechargeable batteries and there are lots of chips out there to solve exactly your problem.

[konfu]

I am looking for those chips already. I'm not sure what I am looking for...
I found these ideal diode ones like lm5050 ... not sure if that is the right kind of equipment to use.

[konfu]

Reading through several forums out there I think there might be another solution:

Why shouldn't I use the LiPo charger to power my circuit while charging the battery?

So I could use a 5V DC supply to feed into the LiPo charger. The output of the charger will feed into the "LiPo bus". attached to this bus is the LiPo battery itself and a boost converter to 5V for the output. So the boost converter will be fed by the bus which is being fed by the lipo battery and the charger.

If the charger would be able to provide more than 500mA of current I should be able to pull 500mA at the output while still charging the LiPo slowly. If AC power fails, the battery will provide the power for the boost converter. Is that a correct assumption?

Is there any alternative to the TPS62100 boost converter that will provide 500mA at 3.3V output? The TPS can only deliver 300mA or so which is not enough...