STP-2000-2115 Signal Transformer Inc., STP-2000-2115 Datasheet - Page 67

• Manufacturer Part Number: STP-2000-2115
• Description: Power Transformers & High Frequency Magnetics Your One Source For All Your Magnetics Needs!
• Manufacturer: Signal Transformer Inc.

How to Specify Power Transformer & Filter Parameters
Diode Selection
METHOD OF DETERMINING SECONDARY CURRENT RATINGS
www.signaltransformer.com
True RMS value of the current flowing into a capacitor Input filter is 2 to 3 times the DC output current because the
current is delivered in short pulses. Assuming a full wave center-tap or bridge, this means that although each diode
is conducting only on alternate half cycles, it should be rated for at least the full output current. To ensure adequate
surge capability during turn-on, a diode rating of at least twice the output current is recommended, especially for
higher current supplies where the ratio of filter capacitance to output current is somewhat higher. Keep in mind that
axial lead diodes achieve most of their heat sinking through the leads. Short leads soldered to large area standoffs
or printed circuit pads are definitely recommended.
For “short circuit proof” IC regulated supplies using three-terminal regulators, an additional diode derating may have
to be used. Long-term output shorts do not harm the regulator, which goes into a current limit or thermal limit mode
to protect itself. The diodes, however, may experience a substantial current increase during the short.
Regulator data sheets should be consulted for current limit values, keeping in mind that current limit is a function of
input-output voltage differential. At high input voltages, the short circuit current of lC regulators is often less than full
load current, tending to alleviate this problem.
The secondary currents shown in the tables are RMS ratings. Depending upon rectifier circuit configurations, the
RMS secondary current is different from the DC output current. This is indicated in the chart below:
For example, in a F.W. Bridge circuit with a capacitive filter, if the load is 1 Amp DC, the RMS Secondary current is
1.6 to 1.8 Amp RMS.
Full-Wave
Center-Tap
Full-Wave
Center-Tap
Full-Wave
Bridge
Full-Wave
Bridge
Rectifier
Type
Capacitor Input
Capacitor Input
Choke Input
Choke Input
Filter Type
= 0.7 x
= 1 to 1.2 x
= DC Amps
= 1.6 to 1.8 x
Secondary
Current is
DC Amps
DC Amps
DC Amps
RMS
38
higher current supplies where the ratio of filter capacitanc
to output current is somewhat higher. Keep in mind that
axial lead diodes achieve most of their heat sinking throu
the leads. Short leads soldered to large area standoffs or
printed circuit pads are definitely recommended.
three-terminal regulators, an additional diode derating ma
have to be used. Long-term output shorts do not harm th
regulator, which goes into a current limit or thermal limit
mode to protect itself. The diodes, however, may ex-
perience a substantial current increase during the short.
Regulator data sheets should be consulted for current lim
values, keeping in mind that current limit is a function of
input-output voltage differential. At high input voltages, th
short circuit current of IC regulators is often less than full
load current, tending to alleviate this problem.
12
For “short circuit proof” IC regulated supplies using
Toll Free 866-239-5777 • Tel [phone] • Fax [phone]
[e-mail] • [e-mail]
©2008 Signal Transformer — Specification subject to change without notice. 07.08
500 Bayview Avenue, Inwood, NY 11096