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Old 21st August 2008, 03:11 PM   #1
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Default fast versus schottky diodes for smps

i was looking at various articles about Schottky's versus fast recovery, ultra and very fast diodes.

it seems that the Schottky is the best for every application in the various areas of the smps, however, it seems that the capacitance can cause some problems with inductors.

also it seems to have low voltage drop but with higher currents can have higher loss and thermal runaway issues.


the fast variety has higher Vd but may handle higher currents.

so in the various areas of the smps, such as input bridge, switching area and rectification areas. what are your thoughts?


input bridge - standard recovery for 50/60hz seems the best

switching area- both or either

rectification- ???


trying to get the best application/efficiency

it does seem that the schottkys have come a long way in reverse voltage though.....
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Old 22nd August 2008, 11:24 AM   #2
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i think it's really application depend if that.
try substituting shottky for ultrafast and observe the differences. If the capacitance of a shottky diode causes something bad to happen i guess ultrafast is the go for that area.

also consider synchronous rectification and it's advantages and disadvantages.
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Old 22nd August 2008, 12:11 PM   #3
Eva is offline Eva  Spain
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Below 100V Schottky is likely to perform better due to the absence of reverse recovery and due to the fact that higher capacitance does not matter that much at low voltages.

Between 100V and 200V Ultrafast diodes perform quite good and their reverse recovery charge is not that high. There are Schottky diodes rated for more than 100V but in most cases they exhibit similar or higher forward voltage drop than ultrafast ones (threshold voltage is still lower, but internal resistance is higher). There are some exceptionally good performers like MBR40250, though.

Above 250V there are no suitable Schottky diodes but there is a relatively new (and expensive) type called SiC diode that does not exhibit reverse recovery either. It has its own drawbacks, though, like not very good surge capability, high forward voltage and positive temperature coefficient. This is not yet a mature technology. Check SDT12S60 (first generation from Infineon), IDT16S60C (second generation) and CSD20060D from CREE.

For 600V there are special Hyperfast diodes, like 15ETX06 or APT16DQ60K, that feature considerably lower Qrr than Ultrafast ones at the expense of higher forward voltage drop. These are intended for hard switching.

Above 600V, reverse recovery charge becomes higher and higher resulting in not very good switching performance. High voltage hard switching is not recommended at all (but it may be done efficiently by connecting lower voltage sections in series).
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Old 28th August 2008, 02:04 AM   #4
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thanks,

i guess that sort confirms my suspicions, the voltage ranges are interesting.
i am dealing with low voltages and i though that the lower Vd would be best......
i have thought about self biasing synchronous rectification. which would give less Vd in the bridge.

thanks again
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Old 28th August 2008, 07:57 PM   #5
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Are there any drawbacks in using ultrafast soft recovery diodes in output rectifiers? Like http://pdf.eicom.ru/datasheets/inter...02/10etf02.pdf

Does the soft recovery characteristic translate into less ringing during primary transistor turn-off?
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Old 30th August 2008, 12:29 PM   #6
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mmm.. Vf is not bad 1.2v but, how do you suppose the Trr of 142ns will limit switching speed?
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Old 30th August 2008, 03:41 PM   #7
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Quote:
Originally posted by zilog
Are there any drawbacks in using ultrafast soft recovery diodes in output rectifiers? Like http://pdf.eicom.ru/datasheets/inter...02/10etf02.pdf

Does the soft recovery characteristic translate into less ringing during primary transistor turn-off?

It's not just time but charge stored. Area under the curve.
These will be better:
http://www.vishay.com/docs/88963/88963.pdf
http://www.vishay.com/docs/88552/88552.pdf
http://www.vishay.com/docs/88964/88964.pdf

You can browse around at http://www.vishay.com/diodes/rectifi...00-v-lt-400-v/
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Old 3rd September 2008, 12:51 AM   #8
TechGuy is offline TechGuy  United States
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FWIW: Another option is to use Synchronous rectification for low voltage applications. At low voltage output and high current output (ie 5v 3.3v 1.8V, etc), Synchronous rectification is the best option because the power losses are significantly less at low voltages. A diode has a foward voltage drop. While at higher voltages the voltage drop is a small percentage, at low voltages it can be a significant percentage. Synchronous Rectification uses a set of Mosfet instead of diodes. The power loss is dependant on Rds and Ton/Toff.

There are also Trench Schottky Diodes with have a lower voltage drop than standard Schottky's. I believe the voltage drop of a Trench Schottky Diode can be as low as 0.3v (if the diode is running hot). These compete with synchronous rectification.
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Old 3rd September 2008, 11:42 PM   #9
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Default sync rect

ok so what are good resources for sync rectifier practical. the typical books just seem to brush over it.

what are the pros and cons of self controlled vs ones with a controller?
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