Reverse polarity protection MOSFET not turning off - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: January 26, 2022, 03:49:43 am
content: I know similar questions about MOSFETs not turning off have been asked so many times, but I've tried everything I've read, without luck.I use a N-ch MOSFET (IRLML2502TRPBF) for both reverse polarity protection and as a power switch (through a slide switch).See schematics (I should have added a gate resistor, but forgot). The rest of the circuit is pulling about 215mA from this switch (both with the MOSFET in place, and with a bridge between pin 2 and 3). It should be powered with 4x AA batteries, but for testing I use a power supply set to 6V and limited to 300mA.I've tried with pull-downs from 1M, 100k, 47k, 10k and at the last attempt 1k. The only thing happening is the draw drops to about 170mA when I turn "off" the slide switch, then back on it draws the normal 215mA.I measure 5.9V on gate when slide switch is on, 0V when off (both relative to GND / source). If I measure relative to REV_POL_GND (drain), I see 5.9V when on, 0.65V when off. Just for the heck of it, I then tried to place the pull resistor between gate and drain, no luck there either.Did I miss anything?

author: georges80
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:11:43 am
content: Show an accurate schematic of what you have actually built.Typical n-chan fet reverse polarity would have the fet in the ground path. Reverse polarity with a fet typically has the fet wired 'backwards' so that the body diode conducts when power is correctly wired. How do you plan to turn the fet OFF since the body diode is conducting?Anyhow, provide the schematic.cheers,george.

author: okw
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:17:21 am
content: Hi George,I based the reverse polarity on this design, page 2, figure 2:https://www.ti.com/lit/an/slva139/slva139.pdfThe schematic is big (6 pages), and a lot of notes / garbage, so I hoped I wouldn't have to clean it up, merge into one page and post. Unless it's necessary?

author: georges80
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:23:26 am
content: Yes, the TI schematics show the typical n or p FET based reverse polarity protection.But, again, how do you plan to turn the FET OFF? Look closely at the body diode of the FET... You do understand how the FET operates in the reverse polarity schematics?cheers,george.

author: okw
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:43:25 am
content: OMG. That's why a second set of eyes is so valuableWouldn't switching pin 2 and 3 correct it? I tried but the PS maxed out at what i set the limit to (230mA).

author: georges80
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:53:56 am
content: If you flip D/S of the fet it will no longer function for polarity protection. Basically you need 2 fets in 'series', one does the reverse polarity protection (using the body diode to 'bootstrap' itself) and a second 'correctly' wired (D/S as expected) that you then control the gate to turn it 'on/off'. i.e. you can't do what you are thinking of doing with a single fet, sorry...cheers,george.

author: okw
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:20:06 pm
content: Would this be an effective way?