DESIGN REVIEW REQUEST - Basic Power Filtering Circuit

[WaveformWonder]

Iv been designing for fun for about a year now, Iv read a few books on the subject. This is the power regulation and monitoring circuit for the wireless comm module for a drone.

Feel free to comment on any aspect of the design, however I am specifically interested in advice concerning the initial 12V/5V switching design.

Will this effectively disable the 12V 1A input when the 5V USB is connected? Is there a better, cheaper, simpler, ect. way to go about this?

Thank you all very much! Cheers!   :cheers:

[Ian.M]

SS8550 isn't a MOSFET, and if you try to use it as-if it is, Q1 would dump 12V onto USB Vbus as soon as you plug in the cable, probably killing the host, or at least blowing the host port!

Assuming you replace it with a P-MOSFET as shown by schematic symbol, it wouldn't be likely to cut off 12 V power, as to do so it would need a gate threshold voltage in excess of 7 V, and to be wired with its source  to the positive power source, otherwise its body diode would bypass it!

The next MOSFET, switched by MAIN_PWR, is also ineffective due to its body diode polarity. 

Q2 should function as designed. Note that CHG_OFF must swing rail-to-rail, i.e. about 12V at this point.  However the TP4057 charger IC has Abs.Max. Vin of 6.5 V so will fail if you enable charging with the 12V source supplying power, and may take the battery with it, explosively!

[WaveformWonder]

I have implemented the changes you suggested, thank you.

As you can see I still make simple mistakes like reversing the ICs.

Any other suggestions would be greatly appreciated.

Cheers!

[Ian.M]

It seems you also don't understand bipolar junction transistors (BJTs).

Hint: the emitter arrow in the symbol indicates the direction of current flow through the transistor, and the Base-Emitter junction is a forward biased diode in normal operation.

You have used the symbol for a NPN BJT for Q3, Q4, and Q6 but a SS8550 is a PNP BJT.   

The 12V power switch section cannot work as drawn.   The pass transistor Q7 is backwards (emitter and collector reversed) which both vastly reduces its gain and its breakdown voltage, probably below 12V.  Its also held on all the time by R15.   If Q6 was a PNP as indicated by its part number it would dump 12V straight onto USB Vbus with consequences described above.  If it was a NPN it would STILL not work as that arrangement is incapable of turning Q7 off.

The rest of the design has similar deficiencies.   I strongly suggest trying simple transistor circuits in a SPICE simulator (or if desperate in the online Falstad simulator) till you understand them well enough to get sensible results, with appropriate device currents and pin voltages, then trying them with real parts on breadboard to confirm the simulation is consistent with reality.  If you do it the other way around you will burn up a lot of parts and melt breadboards!

[WaveformWonder]

You are correct I do not understand many of the components as well as I could. I appreciated the suggested lessons.

I have updated the components to true MOSFETs as stated they serve better as switches, as you all have suggested.

Will this turn off the 12V power when the 5V is connected? Is there a better way to do this?

Thanks, cheers!

[Ian.M]

No.  The low gate threshold voltage of the AO3401A means that reducing Q7's gate voltage from -12V to -7V (both relative to its source) when USB Vbus 5V is present will not turn it off.   The  other AO3401A P-MOSFETs will switch correctly given rail to rail gate drive.   

A N-MOSFET with Vgs  threshold less than 2V, source to Vbus (your USB_5V_IN), drain to Q7 gate, and gate biased to +4V via a potential divider from 12V_IN, + a 100K gate pullup for Q7 would be the  easiest way to switch Q7.   When USB Vbus is present, the N-MOSFET's gate would be approx. 1V negative with respect to its source so it would not pull down Q7's gate to turn it on.  When Vbus is not present,  the N-MOSFET will have approx. 3.4V gate drive turning it on, and will thus pull down Q7's gate to turn it on.

[moffy]

The following link shows how to use a P channel MOSFET as a high side switch: https://www.refcircuit.com/articles/842-high-side-p-channel-load-switch-on-discrete-components.html