Schottky seems to have much lower values of forward voltage drop and breakdown voltage then fast recovery diode. What is their difference other then this? I'm confused with which one I should choose as bootstrap diode for a MOSFET bridge driver. Thanks for helping.
You'll discover that schottky diodes have rather limited reverse voltage ratings. That should help aid your decision.
More on the physics here....
Graham
Schottky diodes are essentially half junctions, made of metal and P type semiconductor, instead of a PN junction.
As you say, this allows then to conduct forward current with about half the normal voltage drop (half the wasted power) than full, PN junctions. And also, as you say, they have low reverse voltage ratings (and high reverse leakage current. And the reverse leakage goes up very fast as temperature rises. Though they generally have high reverse biased capacitance than similarly current rated PN diodes, and it is a very nonlinear capacitance, with respect to reverse voltage, there is no real reverse recovery time that PN junctions have.
Reverse recovery refers to the fact that PN junctions, when forward biased, fill each side of the junction with minority carrier charges (i.e. the N side has holes injected into it, and the P side has electrons injected into it). And if the forward bias is changed to reverse bias, suddenly, most of those minority carriers have to get sucked out of the semiconductor, back across the junction, before the junction can develop an insulating, charge depleted layer that withstands reverse voltage. So, during this clean up process (the reverse recovery time), the diode looks pretty much like a short across the reverse bias. And the end of this process can result in a nasty snap from high reverse current to low current, and that snap can generate lots of high frequency energy. Schottky diodes are more graceful as they swing to reverse blocking, with only the changing junction capacitance carrying a little reverse current.
So, if you want higher efficiency during forward conduction, cleaner, faster switching to reverse blocking and can live with the low reverse voltage rating, the higher reverse bias leakage current (especially at high temperature), and the probably higher price for the Schottky device, then the Schottky is probably the best choice.
So is it correct to say, Schottky diode is always the best choice, particularly in low voltage application, and fast recovery type is used only when the required breakdown voltage is higher then what Schottky can provide? Which of them has higher recovery speed?
Great explanation, John. Thanks.
Bob
Yep, that's why you find them in switching power supplies (for 5V, but not for the 12V supply).
Right now I'm using two SBL3040gibberish, or something like that -- dual schottky in TO-247, anyway -- on a solid aluminum heatsink connected to a CT'd winding I put on an MOT. It's making about 6VDC and I'm drawing around
60A from the thing. I happen to have some FWB's, but they would entail four times the voltage drop and dissipation!Tim
-- Deep Fryer: A very philosophical monk. Website @
Have you looked at synchronous rectification btw ?
Graham
Tim Williams a écrit :
See onsemi's NIS6111
-- Thanks, Fred.
Hi, I have been following the schottky explanation but getting back to your application for the diode for the boot strap. The rule of thumb is the high side cap is 0.1 times the value of the non floating cap. Depending upon what frequency you are running and the size of the mosfet you are trying to drive. A 1 uf 20v floating cap and a 10 uf 20v ground cap will do most anything. The diode requirments for 100khz and below at 300v buss is a 0.5 to1 amp diode at 200ns trr. 100khz to 300khz a 100ns is ok. the most importent requirment for the diode is the voltage rating of 20% higher than the highest buss voltage. At buss voltages above 30 volts a schottky is not a good idea. Ray
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