Is There Something Wrong With My Input Protection Circuit? - Page 1
Forum section: electronics / projects

author: silviustro
author_rank: Contributor
timestamp: July 27, 2017, 03:13:39 am
content: Hello everyone!Over the past couple of months I have been working on a device that requires an input protection and overcurrent protection circuit. It gets its power over either a micro USB port or a separate power input jack.The output voltage of the power supply is 5.1V but I've been having voltage drops down to 4.8V. The circuit requires at least 4.95 volts to function correctly. The device is connected to a Raspberry Pi 3 among other things, and this circuit was mainly inspired by reverse engineering the input protection on the PI itselfThis is the schematic:These are the components used:Main Mosfet: NXP PMV48XP datasheet:https://assets.nexperia.com/documents/data-sheet/PMV48XP.pdfPNP Array: NXP BCM856DS datasheet:https://assets.nexperia.com/documents/data-sheet/BCM856BS_BCM856DS.pdfPolifuse: BOURNS MF-MSMF250/16X-2 datasheet:http://www.mouser.com/ds/2/54/fmsmf-772347.pdfThe output is then connected to a microcontroller using another set of PMV48XP to power multiple devices (like fans to cool heatsinks down).I do not think the mosfet itself is the cause of the voltage drop. I wanted to know if there is any flaw with this circuit before starting to tinker around to try and find another possible causes for the voltage drop.Any help would be greatly appreciated!

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 27, 2017, 03:28:38 am
content: Are you using a decent USB cable?

author: silviustro
author_rank: Contributor
timestamp: July 27, 2017, 03:35:36 am
content: I do indeed think so.I intend to replace all the cables and use another power supply, but I would like to know if the circuit itself is A-OK first.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2017, 06:13:43 am
content: Fine enough. TVS should go at the input side, so as to sink large spikes rather than potentially blowing out the transistor. Since you have reverse protection, the TVS can be bidirectional (SMBxxxCA). Then it should withstand most any ESD and surge you could expect in service.If, as you say, the voltage drop is not the transistor, then it's not the transistor.If you're concerned about fractional volts on a USB supply, youreallyneed to be using something other than USB 2. (And, yes, this goes all the way back to the RPi's shitty power input requirements that caused so much trouble when it was released. And, I suppose, continues to.)Tim

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: July 27, 2017, 12:51:47 pm
content: What happens when you attach power to input jack and stick USB cable into a PC/phone?Leo

author: silviustro
author_rank: Contributor
timestamp: July 27, 2017, 05:28:48 pm
content: Hello Tim,Thanks for the reply, yeah, maybe I should change the position of the TVS, even tho the MOSFET can whitstand reverse polarity up to 12 volts (i think) so it wouldn't blow it up as I'd be working with voltages below 5.1V.What I meant by saying I don't think the MOSFET is the cause, is that it has a low RDSon of 0.048 ohm. Meaning the voltage drop shouldn't be significant. Yes, the RPi's power requirements are pretty shitty indeed, that's why I'm actually bypassing the internal protection and powering the Pi directly from the GPIO. I also have the option of using a DC jack instead of the Micro USB port, which, was never rated for such high currents in the first place, and i'm still confused as to why so many manufacturers use it for charging high current devices even tho we have USB C as an option.Is the MOSFET i'm using the right one? Could it be causing problems? i'm also using it further down the circuit to power the PI on the command of a microcontroller.

author: silviustro
author_rank: Contributor
timestamp: July 27, 2017, 05:30:05 pm
content: Hi Leo,You mean, the output to a phone? it recognizes the voltage and starts charging. I think the voltage range for charging an android phone is anywhere from 5.2 to 4.8 volts.