Here is the block diagram of my UPS.
A old laptop 18V adaptor is used as the main power. The voltage is choosen to be just high enough for recharging the back up battery. The output of the adaptor and the battery is wired together with Schottky Or'ing diodes for their low drop and fast switching. Since the battery voltage is always lower than the supply, the diode on that branch is reverse biased. During a power outage, the power is routed from the battery. As the voltage from the AC adaptor drops below the battery, the diode connected to the battery starts to conduct and the battery picks up the slack. The switching is smooth as the bulk decoupling capacitors supplies the current during the transition.
The battery charger keeps the battery charged. NiCd and Lead Acid batteries are suitable for this type of application as they can handle long term trickle charge. Initially I used a NiCd pack, but switch to Lead Acid as I needed the higher battery capacity for the higher power electronics. UC2906 has temperature compensation and a 2 steps float charge for extending the battery life and is far superior than your regular float charger.
Some of the things that could be improved upon is to add a microcontroller to monitor the voltage, current usage, run time, a low battery alert. Since the battery life depends on the charger, I would still use a charger controller instead of rolling my own in firmware.
An under-voltage shutoff circuit protects the battery from over discharge. As you drop the load, the discharged battery voltage would climb back up. A large hysteresis is used to make sure that the output power would turn on after the AC power has been restored.
A Bargraph Display with integrated LM3914 driver was used as a voltage monitor. The part is now obsoleted. Inexpensive 3 digits Voltage display from China can be used as an upgrade/substitute.
My older design with integrated DC/DC. Everything were soldered down. This makes upgrading a soldering/desoldering exercise.
DC/DC Converters are used to convert the 18V down to the 5V/12V levels. After a couple of PCB revisions of trying to tailor to the changing needs of my electronics, I have decided a modular approach for the DC/DC modules. I used the old 78XX pinout for the modules. You can also modules that uses the same pinout and in similar form factor. My "SMPS replacement for 7805" was part of the module collection.
My new modular approach uses a 3-row connector (trimmed down VME connector) for the plug-in power supply modules. A breakout area with connectors are added to make it easier for changing the outputs. A1..A2 are Auxillary outputs that are have no backups. B1..B4 are the backup from modules and there are 2 outputs that bypass the modules.
A number of the routers can handle the higher voltages as they have internal buck converters. Some of them are designed with a wider range of input voltages as they may be using a non regulated transformers or non-regulated switch mode supplies (as they reduce the cost for $0.30). Those can be used with a jumper bypassing the module. There are some oddball modems such as my old DSL modem that requires 26V supply. For those, I have designed a boost converter module.
My ATA (VoIP) and my cordless phone both have decided to cut cost and both of them omitted the transformer to the POTS connection. This results in an undesirable DC current going through the supply and phone line. I had to make an non-regulated isolated supply for the phone.
Additional backup batteries can be added as it is a simple Or'ing diode connection. With a 8AHr battery from an old UPS and a home made float charger (show above), I got about 4 hours of backup power.
Looking at the list of WiFi hotspot from my router before and during a blackout, I can tell that about half of them has UPS, a large portions of them drops off after 2 hours. Not too bad for what I built from parts, free samples and from recycled parts.
- Backup Battery Charger: ...