Q: Schottky Diode substitution 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?

A: 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.

A: 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.

A: In a very few cases, a higher voltage rating can be worse.
eg the 1N581x series of Schottky diodes has commonly used versions rated 20, 30 and 40 V.  All are 1A rated.
Many small solar powered systems use 1N581x diodes to isolate the PV panel from the battery. In many cases the voltage drop is unimportant, as the panel is run in close to constant current mode with excess voltage available in most cases.
In systems where voltage losses do matter - eg MPPT systems that seek to optimise energy extraction from the PV panel a gain of even 0.1V may be useful. eg at 3V, a 0.1V reduction in diode voltage drop provides an increase in efficiency of about 3%.
Depending on manufacturer and depending on how true datasheets are, a 1N5819 (40V) diode may have between 0.1V to 0.2V extra voltage drop at 1A compared to a lower voltage 1N5817. Notionally also, a 5819 should be (slightly) dearer than a 5817, but in practice, things like availability and demand affect prices enough for there to usually be no difference in cost.  
