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Continuing on with my power supply repairs. I hate it when I don't have a component and even with all of the junk parts I have I still have troubles finding things. So I want to buy some Schottky Diodes to have on hand. I am looking at

http://www.futurlec.com/DiodesGen.shtml#Shottky

They have some 1N58XX, BAT41-86, SBXXX, Along with lots of others. My question is: can I just buy the higher voltage and current rated Schottky diode and sub them for the lower rated ones. Is the main reason for getting the exact match the cost and ability to fit? Do I only need to match package/voltage/current? Are Schottky and fast recovery the same? Can I sub an ultra-fast for a fast recovery? And... at the moment I need a Schottky that is in the TO220 package. Can I use two axial leads and just wire the cathodes together. I know you can't do this for transistors but is the Schottky just two diodes in one package? Lots of questions and lots of thanks for any help with this. Russ

PS: Are those good prices?

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2 Answers

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Most of your questions can be answered "yes", but be careful with Schottky vs. (ultra) fast diodes:

"Can I just buy the higher voltage and current rated Schottky diode and sub them for the lower rated ones?" Should Work.

"Is the main reason for getting the exact match the cost and ability to fit?" Some issues like the diode's capacitance have an influence on switching speed and, as a consequence, switching losses, i.e. heat generated in the diode. Try to find at least a similar device. Also, pay attention to peak values for current spikes and the like. Average current is one thing, peak current and power handling capabilities are another.

"Do I only need to match package/voltage/current?" This is what designers do when looking for second source parts. In most cases, you will be good to go.

"Are Schottky and fast recovery the same?" Usually, fast recovery is a label for p-intrinsic-n diodes, i.e. Si diodes, that are trimmed towards blocking fast upon being reverse biased. Schottky diodes are, really, always as fast as can be and should not be replaced with slower Si diodes, even when they're labeled "fast". The general rule is: Don't replace Schottky with Si diodes.

"Can I sub an ultra-fast for a fast recovery?" Using an ultra-fast Si diode in place of a fast Si diode should work as far as switching losses go, but the faster switching action might cause worse electro-magnetic emissions.

"And... at the moment I need a Schottky that is in the TO220 package. Can I use two axial leads and just wire the cathodes together?" Diodes in three-pin TO220 packages are really just two diodes. They are, however, very similar. When connected in parallel, they will share the load really well. Different packages are also often a hint towards different peak current/power handling capabilities. And, of course, thermal properties will be different. A TO220 has its own little heat sink even when mounted in free air; axial diodes don't have this nice feature.

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I would agree with all of these points, and so I won't make my own answer. However, I would add to "Do I only need to match package/voltage/current" that Schottky's can also vary widely in reverse leakage current (from nanoamps to milliamps!), which may or may not be important. Also, you didn't answer the "PS" - Those are OK prices (for singles). However, they're all the cheaper, lower-power, lower current axial package. TO-220 will run about a buck for singles. – Kevin Vermeer Jul 26 '10 at 18:00
Oh, and in your discussion of Schottky vs. fast/ultra-fast vs. Si, some rule-of-thumb times might help: Schottky - <100 pS, ultra-fast 1-10nS, fast 10-100nS, Si/rectifier >500nS. – Kevin Vermeer Jul 26 '10 at 18:09
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Depending on your application you may also need to check the forward voltage, as low forward voltage is often the reason to choose a Schottky diode.

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