EE Help: What is the purpose of the mosfet in this schematic?

[cirthix]

http://img519.imageshack.us/my...?image=max1535caw6.jpg

The mosfet "p4" is in series with p3, so I don't see the reason for it being there. Is there any harm in removing it? The schematic shows it as optional, but I don't see how it does anything other than waste power.

 

[The Boston Dangler]

maybe better luck in highly technical?

 

[darthsidious]

probably to prevent some sort of feedthrough from the diode in p3. If you notice, p4 is hooked up in such a way that the body diode is opposite to that on p3. without p4, the body diode of p3 would conduct if there was 0.7 V of potential or so across the diode, even when the switch was off, which you might not want. But the body diode of p4 prevents that path from being able to conduct. I think such back to back diodes are common in protection circuitry. But without knowing what this circuit is, or what the role of the PDS signal is, I can't give any more help

 

[cirthix]

Thanks

http://datasheets.maxim-ic.com/en/ds/MAX1535B.pdf is a link to the full datasheet

 

[darthsidious]

Yeah, that makes sense. In many charging applications, you dont't know which end of the mosfet will have the higher voltage, so just relying on one mosfet could lead to diode leakage when the polarity is reveresed. Therefore, you use back to back diodes to ensure that no matter what the state of the voltage on either end, the body diodes can never conducut.

P.S. May I ask what you're building this for?

 

[cirthix]

Originally posted by: darthsidious
Yeah, that makes sense. In many charging applications, you dont't know which end of the mosfet will have the higher voltage, so just relying on one mosfet could lead to diode leakage when the polarity is reveresed. Therefore, you use back to back diodes to ensure that no matter what the state of the voltage on either end, the body diodes can never conducut.

P.S. May I ask what you're building this for?

a custom umpc

http://img184.imageshack.us/my.php?image=63xgasf0.jpg

 

[darthsidious]

Then I think you should add it. The miniature hit in efficiency you'd take is negligible compared to frying your computer. If this was going to be a mass production part, and every penny of the BOM mattered, it might be worth a further investigation, but for a one off personal build, the cost/power increase is negligible.

Anyway, good luck with your build.I do circuit design, but it's been a while since I've worked with real components. My last lab class was 2 years ago, and now at work I just sit at work and simulate circuits . Well, when our part comes back from the fab, there'll be plenty of lab work.

 

[cirthix]

http://www.cryo-laboratory.com...es/cirthix/max1535.png

I used both

VERY compact for a 2 layer board. nearly all of the space is covered by components. A full 8a smart battery charger in a 5/8"*1.45" board. not bad

layout and routing done by hand. passes freedfm with no real problems (only a few insufficient soldermask errors, but those are automatically fixed)