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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 1062
Keywords: PWM, flyback converter, telecom, DC/DC, DC-DC, high-frequency DC-DC converter, current
mode
APPLICATION NOTE 1062
Designing Compact Telecom Power Supplies
Jul 17, 2002
Abstract: This application note describes a compact 5W flyback, high-frequency, switching converter for
Telecom applications. Most aspects of the design process are discussed: power-stage selection and
design, transformer design, core size calculations, RCD snubber design, MOSFET selection, input and
output filter designs, frequency compensation, power-loss and efficiency calculation, and layout and
safety guidelines.
Telecom power supplies are specified for operation over a wide input-voltage range (36V to 75V) but
with circuit performance optimized at 48V. Such circuit designs should be compact, efficient, and have a
low profile to comply with the tight spacing between cards. This app note discusses a 5W flyback
converter for telecom applications, based on the MAX5021 IC—a universal offline power-supply
controller.
Telecom systems include numerous line cards. Connected in parallel to the high-power backplane, each
has its own input-filter capacitor and low-voltage power converter. The large number of input-filter
capacitors in parallel limits the value of each to a few microfarads, making the power-supply design fairly
difficult.
The MAX5021 IC is a high-frequency, current-mode PWM controller suitable for wide-input-range,
isolated telecom power supplies. It enables the design of small, efficient, power-converter circuits. A
fixed-switching frequency of 262kHz controls switching losses while allowing moderately small power
components. The IC includes undervoltage lockout capability with large hysteresis and a low startup
current. This results in low-loss designs for power supplies that feature a wide input-voltage range and
low output power. Cycle-by-cycle current limiting (achieved with a fastinternal comparator) reduces
overdesign in the MOSFET and transformer. Other features include maximum-duty-cycle limiting and
high-peak capability for the source and sink-drive currents. A reference design (Figure 1) illustrates the
5W flyback converter with an input-voltage range of 36V to 72V.
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