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DPA-Switch
™
DC-DC Forward
Converter Design Guide
Application Note AN-31
Introduction
The single-ended forward converter topology is usually the
best solution for DC-DC applications in industrial controls,
Telecom central office equipment, digital feature phones, and
systems that use distributed power architectures.
The feature set of DPA-Switch offers the following advantages
in DC-DC single-ended forward converter designs:
• Low component count
• High efficiency (typically >91% with synchronous
rectification)
• Built in soft-start to minimize stress and overshoot
• Built in accurate line under-voltage detection
• Built-in accurate line overvoltage shutdown protection
• Built in adjustable current limit
• Built-in overload and open loop fault protection
• Built-in thermal shutdown
• Programmable duty cycle reduction to limit duty cycle
excursion at high line and transient load conditions
• Very good light-load efficiency
• Selectable 300 kHz or 400 kHz operation
• Lossless integrated cycle-by-cycle current limit
The example circuits in this design guide illustrate the use of
these and other features of DPA-Switch.
Scope
This document gives guidance for the design of a single-
ended forward converter with DPA-Switch in applications
that require a single output voltage. It is intended for systems
engineers and circuit designers who wish to become familiar
with the capabilities and requirements of DPA-Switch in
DC-DC applications. This application note provides background
material that will assist users of the DPA-Switch DC-DC forward
converter design utility that is included in the software design
tool, PI Expert. Subsequent application notes will provide
comprehensive procedures for designs of greater complexity.
Designers are advised to check Power Integrationsʼ website at
www.powerint.com for the latest application information and
design tools.
U1
D L
S F
C
DC
INPUT
V
O
+
–
R
FC
C
BYPASS
R
UVLO
C
CLAMP
Drain
Clamp
Snubber
Power
Transformer
Output
Inductor
Bias
Voltage
Output
Capacitor
Input
Return
PI-2873-062204
FEEDBACK CIRCUIT
CONTROL
CONTROL
DPA-Switch
TL431 with
Frequency
Compensation
C
FC
+
+
Figure 1. Typical Configuration of DPA-Swtich in a Single-Ended DC-DC Forward Converter with One Output.
July 2004
