7004 Datasheet, PDF (1/12 Pages) Bourns Electronic Solutions – Network Interface Device

Application Note 7004
June 2001
Power Converter Topology and MOSFET Selection for 48-V Telecom
Applications
Craig Varga
Introduction
With the recent proliferation of telecommunications equipment, there is more demand than ever
for voltage converters that are powered by the nominal 48V telecom supply. Depending on the
application and operating environment, the supply voltage range can vary widely. A typical
specification can range from a low of 36V to a high of 72- with a 48-V nominal. In some designs,
transients in excess of 100V need to be considered. Most of these designs will require input to
output isolation of up to 1500V.
Output voltages are frequently 5V and below with 3.3V probably the most common requirement,
and 2.5V gaining in popularity. If a processor is on the card, voltages as low as 1.3V are not
unlikely. One common approach is to regulate a distributed power bus, say the 5V rail, and then
use non-isolated DC/DC converters to generate lower voltages. With the tendency away from 5V,
the 3.3V rail is beginning to serve as the distributed bus, although, from the power supply
designer’s perspective, this is not the most of desirable situations.
Fairchild has recently introduced a family of high voltage MOSFETs ranging from 80- to 200-V
drain voltage specifications. This application note will provide information helpful in the proper
selection of FETs for primary side switches – available in various types of 48V power converters.
Basic Topologies
There are a nearly endless variety of power converter topologies that can be used for 48V
conversion. A large number of considerations will enter into making the final choice. Power level
is going to be the main determining factor, although output voltage and input/output isolation are
factors to consider.
Flyback Converters
Figure 1 shows a basic flyback design using the FDS3670, 100V MOSFET. The circuit
illustrated has the advantages of low parts count and simplicity, making it useful for relatively
low power levels. Output current is the major limiting factor in flyback designs. The RMS
currents in output rectifier(s), transformer secondaries, and output capacitors tend to be large
compared to the average output current (i.e., a high crest factor). As such, at high output
currents, the secondary side power components tend to get physically large and efficiency
suffers. The off-voltage of the primary side power switch is inherently unconstrained in this
topology. As this occurs peak voltage is limited by snubbers and/or clamps that generally
dissipate power and reduce efficiency. While non-dissipative active clamp schemes have been
developed, they are achieved at the expense of significant added complexity.
©2001 Fairchild Semiconductor Corporation
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Rev. A, June 2001