Page 179 - Rashid, Power Electronics Handbook

168 Y.-S. Lee and M. Chow

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determination of the voltage and current ratings of high- large switching loss (which is equal to 0:5 CV f ). As a guide-
frequency recti®er diodes. line, a capacitor with 5 to 10 times the junction capacitance of
Assuming that the voltage and current ratings have been the recti®er may be used as a starting point for iterations. The
determined, diodes can be selected that meet the require- resistor chosen should be able to provide a slightly under-
ments. The following are some general guidelines on the damped operating condition.
selection of diodes:

For low-voltage applications, Schottky diodes should be
used because they have very fast switching speed and low 10.6.4 Precautions in Interpreting Simulation
forward voltage drop. If Schottky diodes cannot be used, Results
either because of their low reverse breakdown voltage or In using the simulated waveforms as references for design
because of their large leakage current (when reversely purposes, attention should be paid to the following:
biased), ultra-fast diodes should be used.
The reverse breakdown-voltage rating of the diode The voltage=current spikes that appear in those wave-
should be reasonably higher (e.g., 10% or 20% higher) forms measured under actual conditions may not appear
than the maximum reverse voltage the diode is expected in the simulated waveforms. This is due to the lack of a
to encounter under the worst-case condition. However, model in computer simulation that is able to simulate
an overly conservative design (using a diode with much unwanted coupling among the practical components.
higher breakdown voltage than necessary) would result Most of the computer models of diodes, including those
in a lower recti®er ef®ciency, because a diode having a used in the simulations given here, do not take into
higher reverse-voltage rating would normally have a account the effects of forward recovery time. (The
larger voltage drop when it is conducting. forward recovery time is not even mentioned in most
The current rating of the diode should be substantially manufacturers' data sheets.) However, it is also interest-
higher than the maximum current the diode is expected ing to note that in most cases the effect of forward
to carry during normal operation. Using a diode with a recovery time of a diode is masked by that of the effective
relatively large current rating has the following advan- inductance in series with the diode (e.g., the leakage
tages: inductance of a transformer).
(i) It reduces the possibility of damage due to transi-
ents caused by start-up, accidental short circuit, or
random turning on and off of the converter. References
(ii) It reduces the forward voltage drop because the
diode is operated in the lower current region of the 1. Recti®er Applications Handbook, 3rd ed., Phoenix, Ariz.: Motorola,
V-I characteristic. Inc., 1993.
In some of the ‘‘high-ef®ciency'' converter circuits, the 2. M. H. Rashid, Power Electronics: Circuits, Devices, and Applications,
2nd ed., Englewood Cliffs, NJ: Prentice Hall, Inc., 1993.
current rating of the output recti®er can be many times
3. Y.-S. Lee, Computer-Aided Analysis and Design of Switch-Mode Power
larger than the actual current expected in the recti®er. In
Supplies, New York: Marcel Dekker, Inc., 1993.
this way, higher ef®ciency is achieved at the expense of a larger
4. J. W. Nilsson, Introduction to PSpice Manual, Electric Circuits Using
silicon area. OrCAD Release 9.1, 4th ed., Upper Saddle River, NJ: Prentice Hall,
In the design of RC snubber circuits for recti®ers, it should Inc., 2000.
be understood that a larger C (and a smaller R) will give better 5. J. Keown, OrCAD PSice and Circuit Analysis, 4th ed., Upper Saddle
damping. However, a large C (and a small R) will result in a River, NJ: Prentice Hall, Inc., 2001.