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I'm trying to make an UPS for my Raspberry Pi 3.

Based on

https://nomadaselectronicos.wordpress.com/2015/05/22/fuentecargador-con-carga-compartida-load-sharing/

and

http://ficara.altervista.org/?p=2736

I ended up with this: enter image description here

In theory, it should provide 5v (output of the MT3608) when powered either from USB wall charger or from 18650 battery (through the TP4056). The Mosfet will do the switching between mains and battery.

When I power my RPi, it boots but then it enters on a boot-loop which is reported to be caused by low voltage (see: https://elinux.org/R-Pi_Troubleshooting#Raspberry_Pi_shuts_down_.28or_restarts.29_soon_after_booting_up).

I meassured the output voltage and it is stable when the RPi is off as well as when it is on (4.9v ~ 5.1v). The boot up sequence tipically consumes about 150-300mA on a Raspberry Pi B+ I use to prototype.

When the same wall charger is connected directly to the microUSB port on the RPi, everything works flawlessly.

18650 battery is a Samsung INR 18650 25R.

What could be causing the current drop? I'm considering replacing the MT3608 with an USB DC-DC Booster 0.9v-5v (https://www.ebay.com/itm/132487410767) but running out of ideas...

vazquezjm
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1 Answers1

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You are using IRFP9240. This is not suitable MOSFET for this task. This MOSFET requires higher Vgs than you are providing. You are going to have Vgs around 3.6-4.2V. IRFP9240 requires around 10V to fully open (or at least 5V to be usably open).
Try different MOSFET with much lower Vgs(th). For example something like PMV48XP (I just randomly googled, you can find plenty of suitable MOSFETs).

Chupacabras
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  • I removed the switching circuit (TP4056 to MT3608) and RPi boots! Now I need to find a suitable Mosfet as you suggested – vazquezjm Jun 24 '19 at 22:29
  • Do you think a Vgs of 10v will do the job? – vazquezjm Jun 24 '19 at 22:36
  • @vazquezjm you do not have 10V in your circuit. So you have to find mosfet with lower Vgs(the). – Chupacabras Jun 25 '19 at 05:18
  • It looks like IRF4905 will do the work: https://pdf1.alldatasheet.com/datasheet-pdf/view/68156/IRF/IRF4905.html – vazquezjm Jun 25 '19 at 16:06
  • @vazquezjm no, it has the same Vgs(th). Why are you looking for huge bulky power mosfet? I would recommend you to search for a mosfet in SOT-23 package (or the like). Small mosfets tend to have smaller Vgs(th). – Chupacabras Jun 25 '19 at 18:53
  • Package is not an issue, neiter power handling. I'm new to mosfets and probably that's the issue :) What Vgs(th) values should I look for? – vazquezjm Jun 26 '19 at 12:32
  • @vazquezjm I would recommend you max. value of Vgs(th) to be less then 2V. Typical value around 1-1.5V. Something like AO3400. You have to calculate with 3V in your circuit (lowest value of li-ion battery). At 3V are such mosfets fully open. – Chupacabras Jun 26 '19 at 12:48
  • So, basically any of these, right? https://www.digikey.com/short/phrcww – vazquezjm Jun 26 '19 at 13:10
  • @vazquezjm any that will be fully open at Vgs=3V. Read the datasheet for the particular mosfet, see graphs, tables, and so on. – Chupacabras Jun 26 '19 at 13:52
  • Changing the MOSFET to a AOD4185 did the trick. Thanks @chupacabras for your help! – vazquezjm Jul 16 '19 at 12:54
  • @vazquezjm You are welcome ;) Now you can click to accept the answer. – Chupacabras Jul 16 '19 at 14:44