I am making a circuit to supply power to a 12V fan. I am using a power supply and a battery with a MT3608 connected to it to step up to 12V.

I've designed the circuit in a way that when the power supply is connected, the MT3608 is disabled and when the fan is on, there is no charging on the battery (the charging is done by a TP5100.)

I saw this question that recommended using a P MOSFET. To prevent the charging of the battery, an extra Schottky diode had to be placed (because of the MOSFET body diode.) My situation is quite different because since I am turning the MT3608 off when the power supply is ON and I already have the D6 diode from the step up. Is the circuit below already adequate for use? By that I mean, am I accidentally charging the battery in some way or messing something up?

From what I know the D7 and D6 diodes are doing the diode auctioneering and the 8.4V of the battery is lower than the 12V from power supply, so all is fine.

There is no chance of the MT3608 having a higher voltage and damaging the power supply because it will never be ON while the power supply is connected.

Since the 8.4V from the battery is lower than the 12V from the power supply, it will not provide any current to the fan while the power supply is ON.

Am I missing something or am I good to go?


  • \$\begingroup\$ doby = body I guess?? \$\endgroup\$
    – Andy aka
    Aug 6 '21 at 12:19
  • \$\begingroup\$ @Andyaka correction made, I reviewed 3 times, but something always goes by unnoticed. Already changed to body, thanks! \$\endgroup\$ Aug 6 '21 at 12:24

am I accidentally charging the battery in some way or messing something up?

There's always a leakage current flowing through the diode when it's reverse-biased:

enter image description here

No details about the operating conditions are given, so it's not easy to estimate. For D6 in your case, it should be noted that if the diode's junction temperature exceeds 125°C by any chance (e.g. by external conditions) then it'll be possible to charge the battery from the supply voltage.

Am I missing something or am I good to go?

Depending on the potential risk mentioned above, you can decide if it's good to go.

  • \$\begingroup\$ Thanks for the answer Rohat! The circuit will be operating at ambient temperature and it will be low current, so I don't think we will have a temperature problem. If I keep the temperature in ambient, then it should run just fine without damaging the battery right? \$\endgroup\$ Aug 6 '21 at 14:48
  • 1
    \$\begingroup\$ @AugustoMattos Yes, that's correct. \$\endgroup\$ Aug 6 '21 at 15:03
  • \$\begingroup\$ Thank you very much Rohat for the help! \$\endgroup\$ Aug 6 '21 at 15:25
  • \$\begingroup\$ And @RohatKılıç if the temperature were a problem, a MOSFET between the MT3608 and the battery would solve the problem right? \$\endgroup\$ Aug 6 '21 at 15:58
  • 1
    \$\begingroup\$ @AugustoMattos a MOSFET has a body diode so the same things apply for MOSFETs as well. \$\endgroup\$ Aug 6 '21 at 16:22

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