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What is the difference between Schottky and Fast Recovery Diode?

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.
 
E

Eeyore

Jan 1, 1970
0
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....

http://en.wikipedia.org/wiki/Schottky_diode


Graham
 
J

John Popelish

Jan 1, 1970
0
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.
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.
 
You'll discover that schottky diodes have rather limited reverse voltage
ratings. That should help aid your decision.

More on the physics here....

http://en.wikipedia.org/wiki/Schottky_diode

Graham

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?
 
B

Bob

Jan 1, 1970
0
John Popelish said:
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.

Great explanation, John. Thanks.

Bob
 
T

Tim Williams

Jan 1, 1970
0
John Popelish said:
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.

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
 
E

Eeyore

Jan 1, 1970
0
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.

Have you looked at synchronous rectification btw ?

Graham
 
F

Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :
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!

See onsemi's NIS6111
 
R

Ray King

Jan 1, 1970
0
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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