STP-2000-2115 Signal Transformer Inc., STP-2000-2115 Datasheet - Page 66

• 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
Summary
References:
COMMENTS ON CAPACITOR & DIODE SELECTION
Capacitor Selection
Source 1
Reuben Lee
Electronic Transformers and Circuits
1947, John Wiley & Sons
www.signaltransformer.com
The preceding has been an attempt to provide a simple, practical method of specifying transformers for main stream
applications. Certain simplifying assumptions were made so this work-up does not represent a rigorous academic
analysis. For those who wish to utilize more detail in their analyses, materials are readily available in the literature
(see below).
For more detailed theoretical analysis the following sources are recommended:
∆
This yields 2000µ F/amp for 3V p-p ripple. At DC currents below l amp, capacitor heating is usually not a problem
and peak-to-peak ripple voltage is the determining factor in capacitor size. At higher values of capacitance, where
the ratio of capacitor outside surface area to volume is significantly lower, internal heating becomes a problem.
Ripple current rating may be a determining factor in capacitor selection, rather than ripple voltage. In many cases,
capacitor size will have to be increased to prevent excessive internal heating. Manufacturers’ data sheets should be
consulted (after an initial selection is made) to ensure that capacitor ripple current ratings are met. Remember that
the RMS ripple current ratings shown on capacitor data sheets are not the same as DC load current. RMS ripple
current in a capacitor input filter is 2 to 3 times the load current. In addition, the time-to-failure used to rate capacitors
on data sheets is often 10,000 hours. For five-year life (40,000 hours), ambient temperature may have to be derated
30°C from the data sheet rating. Capacitor life roughly doubles for each 15°C reduction in operating temperature.
The following calculations illustrate a typical design example:
Assume 1L = 3A, AV = 4V p-p,
VDC = 12V
C = (6 x l0
Manufacturer’s rating on a 4,600µ F/20V capacitor @ TA = 65°C is 3.1 A RMS. Dividing by 2.5 to convert from RMS
ripple current to output current yields a maximum DC load current of 1.24 amps. Obviously either a large capacitor is
required or ambient temperature must be reduced.
As a final note, be sure to check whether the data sheet ratings are for still or forced air. Computer grade
capacitors are often rated only for forced air. Other types may be rated for still air and are, therefore, actually more
conservatively rated.
Remember that capacitors are the number one cause of power supply failure. Don’t let your supplies dominate the
statistics column!
For low current supplies ( I OUT<IA) capacitor selection is relatively straight-forward, Capacitance is found by the
simple formula:
C = 1L x 6 x 10
Where: 1 L = DC load current
V = peak-to-peak ripple voltage ripple frequency = 120 Hz.
∆
V
4
-3
V
) (3A) = 4,500µ F
-3
Source 2
EE Staff - MIT
Magnetic Circuits and Transformers
1943, John Wiley & Sons
11
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©2008 Signal Transformer — Specification subject to change without notice. 07.08
Source 3
O.H. Schade
Proc. IRE, Volume 31, page 356
1943
500 Bayview Avenue, Inwood, NY 11096