I have a switching power supply that operates at 4 V output. I want to have my GSM always powered when my primary voltage is removed. I use a simple Li-Ion battery 4.2 V and I want to use it as backup battery.

What is the best way to make a switch over? I'm looking for something robust. I don't want something that it could not work in some circumstances.

If my primary voltage rail was much above battery's maximum voltage I could use a simple comparator.

I was thinking that I could increase my primary voltage to 4.3 V and use a comparator but I do worry about GSM operation because it has maximum voltage 4.4 V.

Photo as a reference:

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  • \$\begingroup\$ Please, put a schematic and clarify what is primary voltage, if 4.4V is the absolute rating, maximum transient peak voltage or else, and in particular what is the minimum supply voltage for the GSM. Using a Schottky diode from the backup battery with a low enough forward voltage at the GSM absorbed current, you might avoid any switching. \$\endgroup\$
    – andrea
    Sep 1, 2017 at 17:26
  • \$\begingroup\$ I was thinking to set the primary power at 4.3V and go with comparators and mosfets. 4.3V will be always higher than battery's voltage but I'm worried about GSM's maximum operating voltage which is very close, 4.4V. I believe that during transmitions the power rail would have some ripple that may cause its voltage to go up and down a little. \$\endgroup\$
    – MrBit
    Sep 1, 2017 at 18:06
  • \$\begingroup\$ Would you be happy with a relay that is energised by the primary power source so that when power is lost, the relay switches over to the battery backup? You could use capacitors to "ride out" the switching time between sources. You haven't mentioned charging the battery but an off-the-shelf charge circuit could work here. \$\endgroup\$
    – scotty3785
    Nov 8, 2021 at 17:08

1 Answer 1


Essentially you want to make a UPS - Uninterruptible Power Supply for your module. This can't be solved by means of simple "comparators and MOSFETs", it would also need to manage the charge of the cell, and provide smooth power bypass.

You already have a solution, posted in your previous questions. It is one of PMIC from TI, BQ24xxx/BQ25xxx ICs. You just need to override internal defaults (watchdog timer, etc) using I2C interface.

If you don't have a MCU with I2C port, you need to look for a stand-alone circuit, something like this one, or similar, although it will be difficult to determine if these kind of designs will provide all functions you need.

Another alternative is to find a proper powerbank module which allows internal battery charging and external power delivery at the same time, which would be also a challenge to determine without buying samples first.

If designing the circuit from scratch, you might want to look into "Prioritized PowerPath Controllers" similar to this one, LTC4417.


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