Bridge rectifiers and a diode's reverse recovery time?

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Ed: I have corrected your quote above to remove ambiguities. Please confirm if I've read it correctly, otherwise I'm not quite sure what you've said.

Is it a forward converter? What is the typical duty cycle? Why such a low frequency?

They do make SiC schottkies up to 800V, but they're way more expensive than needed at a paltry 25kHz.

Tim

"Tim Williams" "hrh1818"

Hmmmmmm.....

... Phil

How much current are you putting through them.

The UF4007 is ultra fast and will handle 1 amp the UF5408 will handle three amps and both diodes are rated to 1000 volts.

Shaun

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th

nfirm

Sorry I do not understand the meaning of you editing marks. Hence I am unable to you have read what I wrote correctly.

a low

No this is a push pull isolated output boost converter. The duty cycle is approximately 50%. A 25 kHz frequency is typical for SMPS.

an

"hrh1818"

Nearest you get is " fast recovery " silicon diode pairs, in common cathode and common anode types.

Using one of each makes a high speed, high current bridge rectifier.

.... Phil

Reverse recovery will create losses, period. Use of half wave geometry will not necessarily guarantee less loss.

...depends if the General is on LOA.

Hello Howard,

Reverse recovery time in Si rectifiers (or charge when you integrate the reverse current peak versus time) depends on actual current before reversing voltage, dI/dt , maximum reverse current and temperature.

High junction temperature, and high dI/dt, increases reverse recovery time. Several manufacturers specify reverse recovery time at different temperature and dI/dt. They also can cheat you by taking the maximum reverse recovery current far higher then the forward current. This results in less recovery time in the spec, but doesn't change the recovery charge.

When your SMPS output has low output impedance at reversing polarity (for example a full bridge circuit, or a push-pull fed from a solid supply), you will get high reverse current that will result in high switching loss in the switches (probably more loss then in the rectifiers).

A transformer's leakage inductance or just a series inductance can limit dI/dt, hence the reverse current peak. This inductance can also be used to create zero voltage switching.

Regarding simulation. The standard diode model isn't very good. Try to simulate the reverse recovery setup as used by the manufacturer and tweak with the parameters (transit time) to have a good match between datasheet and simulation.

A push pull circuit can be used with a full bridge rectifier or center tapped transformer with dual diode rectifier (with very good efficiency). I used series inductance to reduce dI/dt. When diode speed is really an issue, you might consider expensive SiC (Silicon Carbide Schottky) rectifiers. These diodes have low reverse recovery, but still you have to charge and discharge the diode capacitance. This may also result in high current peaks in a hard switched application (hence losses in your switches).

Best regards,

Wim PA3DJS

I agree. After I wrote "Are designers forced to use 1/2 wave rectifiers in this application?" I have since learned I was all wet when I asked this question.

Howard

You're waisting time even considering doing such a thing. 60Hz bridge rectifiers usually don't even specify a reverse recovery time because at

Even at 25kHz you'll want some soft recovery diodes (trr

Reverse recovery is not a loss mechanism associated with the diode itself. The reverse voltage across the diode does not rise significantly until after the peak reverse current is reached, when the voltage assumes a roughly capacitive charging relationship that depends again on diode characteristic "snap".

During the early period of recovery, full circuit voltage, load current and recovery current are carried by series elements; switch/source, resistive and inductive components or strays.

"Snap" is a term that you will find in few specifications but is basically the relationship between the slopes of rising and falling reverse currents - the falling reverse current slope being a function of doping chemistry and profile.

The total charge is dependant on the diode chemistry and doping profile, physical size ( 1/^ ), current density (^), temperature (^) and externally enforced di/dt (^) during the early switching period.

This is non-linear, and not easily spiced, even when a range of relevant diode response characteristics have been recorded. Usually the work is simplified by choosing one known circuit situation that is relevant - worst case or otherwise critical to specific performance requirements at environmental extremes, and by splitting the switching period into two-part approximations that are more easily linearized.

In any event, demonstrated performance cannot be ignored or avoided, though a comparison of printed spec characteristics can be used (with a grain of salt) to forcast the relative performance of two or mpre devices in a practical application.

RL

Actually, substitution of larger devices of similar technology (in an otherwise unchanged circuit) can offer switching loss improvements in some power applications through the reduction in current density, prior to turn-off. Their use may also allow the achievement of lower junction temperatures that can also be beneficial in this regard.

Specs are often misleading, in that they do not often directly compare performance under derated conditions.

RL