Selecting rectifier diodes for high frequency or SMPS supplies?

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What attributes and/or parameters are necessary for the rectifying diodes on the secondary of an SMPS?

Can some of these parameters be relaxed if the switching frequency is low, say below 100kHz? or below 50kHz?
Can some of these parameters be relaxed if the DC output voltage is higher, say greater than 15Vdc? or greater than 24Vdc?
Is it necessary to use a dual common anode/cathode type? Can a pair of single diodes be used?
 
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Single pair of diodes is fine.

You might want to look at the data sheets for these. These are typical general purpose devices that are (were) commonly used in SMPS and TV line output stages,

BY299/BY399

BA157

BA159

BY359

BYT77 glass passivated.

We always referred to them as soft recovery/high speed. The parameters can't be relaxed. The diode has a major (massive) influence on the efficiency and even the ability of the circuit to function.
 
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You might enjoy owning a copy of Christophe Basso's book (link), it covers a lot of ground and strikes a nice balance (IMHO) between theory and practice.

It may also be useful to download a copy of the ON Semiconductor handbook; excerpt attached.
 

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Thanks Mooly, I'll compare specs.
Thanks Trans, I have downloaded that pDF to add to the other ONsemi switching guide I have already.
Osv, why do Schottkys offer any advantage at higher voltages? Surely their reverse leakage is something that is only acceptable when their low Vf is a necessary requirement for good efficiency at low voltages.
That is why I asked the leading question: can parameters be relaxed at higher voltages?
 
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three times less current (at 3times the voltage) = 9times less heat and one third of the power loss.
At significantly higher voltages than 3V3 or 5V, I fail to see why low Vf with high reverse leakage, offers any advantage.
Show me how.

When I look inside SMPS I usually see different diodes on the different DC circuits.
There has to be a reason for the manufacturer choosing different diodes in one SMPS when commonality would save costs. There has to be "parameter choices" coming into the selection process.
 
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....why do Schottkys offer any advantage at higher voltages? Surely their reverse leakage is something that is only acceptable when their low Vf is a necessary requirement for good efficiency at low voltages.
That is why I asked the leading question: can parameters be relaxed at higher voltages?

Diffused junction devices clear the depletion zone by two mechanisms..sweeping the carriers out of the junction, and by recombination due to lattice defects formed by diffusion or ion implantation. The main problem with diffused junction was embedded in what metal they used to speed up the recombination. Platinum and gold were used, but gold (IIRC, it was 30 years ago) has a very bad trr vs temperature characteristic. It would slow down as it heated, so thermal runaway was a problem.

Schottky's do it by sweep. They are really just a capacitor, and total charge is the relevant factor. They don't really change speed at temp, but the leakage goes nuts.

Which device to choose is based on the forward dissipation vs reverse leakage/trr timeframe tradeoff. Reverse conduction losses during reverse recovery can run amperes at full rail voltage, so the trr is a big dissipator.

jn
 
As already mentioned, SMPS switch fast, so you need a fast diode. Silicon need not apply to this job. Schottky are increasingly popular for many reasons, now that higher PIV rating are available.

In general with almost any recitifer, the higher your desired PIV (reverse voltage) rating is, the higher the VF rating will be. A higher VF will give higher losses. Therefore, the designer chooses the lowest PIV that fits the design. Then the VF is usually lower giving better efficiency, and the cost is very often lower too. For cost reasons, the designer also usually chooses the lowest current rating that will fit the deisgn. That doesn't mean choosing one on the bleeding edge, but putting a big expensive 30 A device on a 4 A output makes no economic sense. Sometimes a higher maximum current rating device is chosen to give a lower VF at the operating current. Many variables = many devices.
 
As already mentioned, SMPS switch fast, so you need a fast diode. Silicon need not apply to this job. Schottky are increasingly popular for many reasons, now that higher PIV rating are available.

25 nSec Fast epitaxial product was easily obtainable back in '91, but it was limited to 200 volt PIV. 50 nSEc was available out to 600 volts.

Back in '82, I was working with a diffusion engineer on a 15 nSec fast epi, but I don't know what came of that product. (I made a TRR setup capable of quarter nanosecond measurement to assist him.)

45 and 100 volt shottky's were already out there in '86, but reverse leakages were in the milliamp ranges.


jn
 
When I can, I use Schotky in mains rectifiers, they provides lower voltage drop and lower looses, and lower temperature, and lower output impedances. And they are cost comparable to PN junctions.

The company I worked for used 800 volts as the minimum allowed reverse voltage for off mains rectification. Transients tended to kill the diodes otherwise.

If you mean after a transformer....nevermind...;)

jn
 
Thanks to you both.
Compromise seems to be the rule.

How do we select parameters to arrive at a reasonable compromise?
What parameters and attributes are required for HF rectifying?
What parameter requirments change as currents and voltages change?


Silicon is now available with fast and with soft ratings similar to schottkys.
I can understand for a 5V supply, that saving 300mV in the Vf is important to efficiency.
I can see that this 300mV saving is not as important when working with 24Vdc, or 50Vdc.
So what other factors are important in selecting rectifier diodes?

I do not believe the "only" important attribute is schottky !
 
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