How do I choose the correct Schottky diode for a project?

[JBaughb]

I am new to custom circuit designs and I was looking for information about what I should be looking for in regards to a diode for voltage protection with a circuit. They will be attached internally to the power rails of two USB lines. Its a single USB device attached internally via USB to a host computer, but when a second computer is plugged in via an external USB port the internal switch IC, when it detects 5V coming in from the USB cable, switches D+/D- from the first computer to the one that was just plugged in. The diodes will prevent issues with the two power rails being attached together from interfering with each other.

Obviously its 5V, the device uses less than 100mA under normal use but it can use up to 350mA when its under high load. I think the USB spec requires all devices to never go over 500mA. I would need a diode on each of the 5V rails coming from each computer before they join together so theres no power problems. If someone has an idea of what kind of surface mount diode would suffice, it would be much appreciated.

Even better would be if someone could explain what I would be looking for as far as data sheet specs go so I can learn how to pick one myself in the future. I have a basic knowledge of electronics but I get lost when I read about forward voltage vs reverse voltage, leaked current vs rectified, recovery time, etc and how it would all relate to this and future projects I am working on.

Also, someone had commented that a polyfuse would be a good idea and if anyone has info about what to look for with that, it would be helpful. Do i pick a fuse that blows when the current exceeds the 500mA max of the USB 2.0 spec or should I pick a value closer to the 350mA that the device would use under max load or am I completely off and I should be considering other factors about the setup? Thank you all.

[exmadscientist]

Even better would be if someone could explain what I would be looking for as far as data sheet specs go so I can learn how to pick one myself in the future. I have a basic knowledge of electronics but I get lost when I read about forward voltage vs reverse voltage, leaked current vs rectified, recovery time, etc and how it would all relate to this and future projects I am working on.

First you think through what the various properties are. Sometimes it helps to write it out on paper (or whatever), especially when it's a new field for you or a complex problem, so let's do that:

• Forward voltage drop: The voltage loss across the diode when it's conducting. Varies with current. The reason people like Schottky diodes is that this type has low forward voltage drops. An "ideal" diode would have zero forward voltage drop.
• Reverse voltage: the maximum voltage you can apply to the diode in reverse polarity before it lets out the magic smoke. 
• Leakage current: when a diode is reverse biased, some current leaks through anyway. The smaller, the better.
• Rectified current: how much current the diode can pass comfortably before the manufacturer is no longer prepared to guarantee the magic smoke will stay in the part.
• Recovery time: how fast the transition from reverse to forward biased can be.

Then you think about what's important in your application. For power-ORing diodes, recovery time is usually totally insignificant. Reverse current is also usually not very important. Reverse voltage needs to be above the value of the highest power supply in the system (i.e., if you're ORing 12V and 5V, all the diodes better be rated 12V or higher!). You also need a bit of headroom; it's never good to run parts at 100% of their rating if it can be avoided. Your rectified current, as you've identified, should be 500mA minimum. (I might choose the next size up, 1A, Just In Case™. Or I might not, if I'm size-constrained on the layout or it's a non-critical part.) And then you can just pick the part with the lowest forward voltage drop that meets your other criteria of price, package size, availability, &c.

(Also consider what else you could use this part for; often it is better to buy two identical parts, even if they're individually a bit more expensive or somewhat overspecified, than managing two separate part inventories for different parts in different circuits. BOM line item reduction makes everyone happy!)

I adore manufacturer catalogs that list all their parts and relevant specs, because it makes this game a lot easier. NXP has a great MOSFET and diode catalog, available here, and guess what? I pick NXP diodes a lot more often than other manufacturers'. (HINT FOR MANUFACTURERS: make my life easier and I'll buy your parts!) If you load that up and scroll to around page 36 (PDF page 20 because they paginated it bizarrely), you'll see a whole bunch of candidate parts. Many would probably work for your application. PMEG3005EJ looks like a decent starting point, but you may prefer to optimize for other things (smaller, higher power, better available, etc.) I usually pick a couple of favorites that look good, then search DigiKey, Mouser, or Octopart and see how available and expensive each is, and then just pick one.

Sometimes if I need something a little more exotic, availability will be more limited. (That is not the case with bog-standard diodes.) In that case I usually start with the distributors' sites and see what they've got first, rather than what's in the catalog and might or might not be actually purchasable.

[JBaughb]

Thank you for this. It was incredibly helpful. 

[JBaughb]

USB spec requires host port to supply at least 4.75V, there is no schottky diode that does 0.25V voltage drop at room temperature.

Thanks for bringing that up. In addition to the 5V in from USB, the device I'm powering has a separate power input that accepts 3.7-5.5V so I should be able to use that after taking the voltage drop across the diode into consideration. The IC that does the switching wants 4.3V ideally with 5.5V Max.

[macboy]

USB spec requires host port to supply at least 4.75V, there is no schottky diode that does 0.25V voltage drop at room temperature.

You are wrong. It took me 3 minutes on Digikey to find "RB496E". After filtering with Digikey's tools I only downloaded one data sheet, many other devices may work too. The RB496E drops about 250 mV at 100 mA (per diode, there are two in the package which you can parallel) at 25C. Vf drops with higher temperature of course.