STP-2000-2115 Signal Transformer Inc., STP-2000-2115 Datasheet - Page 61

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

The following sections present a practical guide for the selection of power supply transformers, rectification schemes and
filter components. A number of basic assumptions have been made to allow focusing on the basics of the presentation.
For those interested in a rigorous theoretical analysis, there are a number of fine references available (a few are
listed further along). Additionally, circuit analysis using appropriate analytical software (SPICE or its equivalent) is
recommended in the uncommon instance where better understanding of a particularly complex application is needed,
or when it becomes necessary to optimize some secondary aspect of a design. Computer analysis has proven to be
particularly useful in understanding areas that are difficult to quantify using traditional circuit analysis methods; areas
such as capacitor RMS ripple current.
Specifying Power transformer parameters for a particular DC power Supply is something almost all designers confront
at some point. Proper specifications will vary and are subject to both the rectifier configuration and the filtering schemes
selected. Therefore, for the sake of clarity, it is useful to employ some simplifying assumptions. That said, the results
obtained from the simplified formulas that follow should still prove to be valid in 99% of mainstream applications. This
workup also presents some useful rules of thumb, biased toward ensuring conservative designs.
Filters
Choke input filters are very much out of favor due to the weight and cost of the chokes and the improved ability of
regulators to provide ripple reduction superior to that of the typical L-C network. For these reasons, this discussion deals
exclusively with capacitor input filters. Additionally, it should be noted that modern regulators excel at overcoming the
acknowledged poor output voltage regulation associated with capacitive input filters.
Another noteworthy disadvantage of the capacitive input filter is the discontinuous secondary current flow (high
peak-to-average ratio of forward diode current). Current is drawn in short, high amplitude pulses to replace the charge
of the filter’s input capacitor which discharges into the load during diode off-time. This results in higher effective RMS
values of transformer secondary current. However, the transformer’s average VA rating is the same as one feeding a
choke input filter because the higher DC output voltage obtained at the capacitor compensates for this effect. In addition,
except for some supplies handling very high currents, average semiconductor diodes can now easily meet the typical
peak or surge current requirements imposed by capacitive input filters
Rectifier Circuit
1.
2.
How to Specify Power Transformer & Filter Parameters
www.signaltransformer.com
Half Wave (single diode)
The only advantages of the half-wave rectifier are its simplicity and the savings in cost of one diode.
Its disadvantages are many:
1. Extremely high current spikes are drawn during the capacitor charging interval (only one current surge per cycle).
2. The unidirectional DC current in the transformer secondary biases the transformer core with a component of
The remaining single-phase rectifier circuits are of the “full-wave” type. Secondary current surges occur twice per
cycle so that they are of smaller magnitude and the fundamental ripple frequency is double the supply frequency
(i.e., 120 Hz rather than the 60 Hz of a half-wave circuit). All full-wave rectifiers apply the same basic rectified
waveform to the filter’s input capacitor.
Full-Wave Center-Tap
Uses 1/2 of secondary winding at a time Requires a center-tapped winding and the use of 2 diodes
This current is limited only by the effective transformer and rectifier series impedances, but it must not be too high
or it will result in rectifier damage. The short, once-per-cycle, current spike also results in very high secondary
RMS currents.
DC flux density. As a result, more “iron” is needed to avoid core saturation. About the only time it would pay to
consider using a half-wave rectifier is at very low DC power levels of about 1 watt or less. At these levels a power
transformer cannot be reduced very much in size (at reasonable cost) and a small filter capacitor will be large
enough for adequate DC smoothing.
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