EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: panoss on April 02, 2019, 01:55:54 pm
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I 've built a device which is powerεd by a 5V PSU.
The voltage firstly goes through an FQP27P06 (P-Channel MOSFET) for reverse polarity protection.
Then it goes to the 1117.
So my question is: do I need the MOSFET for reverse polarity protection?
Or the 1117 has it's own protection against reverse polarity?
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Hi,
Here is the simplified internal schematic. I would say it is not reverse polarity protected. However I cannot see any path to the reverse current to flow as all of the transistors are blocking that way.
I use a schottky diode for revesre polarity protection. I don't know which one is the better... I always see the least in TI's reference designs. They must have a reason.
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If a reverse polarity PSU gets connected the current will stop at the transistors at the bottom of the schematic?
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I think yes, they block the way. But the question is the voltage that those transistors can carry on their E-C without blowing.
When I get home I will try. :)
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My take on this is, unless the datasheet specifically indicates that it is reverse-polarity protected, and the reverse voltage withstanding level, I would assume that it is not protected.
For LDOs, I also prefer a PMOS rather than a schotky where extremely low dropouts are required.
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Agreed, it's an essential rule of product design - If it isn't in the datasheet then it doesn't exist. No amount of peering at simplified schematics changes that. Even if you get away with it on a specific sample, it can't be assumed that it will work for other production batches or second-sources.
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I have heard that 1117s from different manufacturers have significant differences.
As shown the reverse voltage cannot exceed 1 Vbe + 1 reverse Vbe so about 6 volts. I think all of the junction isolation substrate diodes go to the adjustment pin so the series resistance at that pin will limit current.
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Ok, thank you all. I 'll keep the MOSFET.
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There's a possibility the MOSFET wouldn't be needed at 5V. When I've tested this with a PNP transistor, I found that applying reverse polarity produced no current - up to 8V. So it acted like a Zener. You could test this with your meter. Apply reverse voltage through an ammeter, and see at what voltage current begins to flow.
Also, is there actually a risk of reverse polarity here? The risk usually comes with batteries, but you said 5V, which doesn't match most battery types.
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LM2937IMP-5.0 have reverse polarity protection and 60V transient protection.
Downside is they're not cheap
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The risk for reverse polarity is, indeed, very small because the 5V comes (from a PSU) from a standard connector like this one:
(https://images-na.ssl-images-amazon.com/images/I/21XVreSxZpL.jpg)
But for this 1 in a million case...