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Wide Output Fast Transient SMPS Design

IP.com Disclosure Number: IPCOM000030194D
Publication Date: 2004-Aug-02
Document File: 2 page(s) / 77K

Publishing Venue

The IP.com Prior Art Database

Abstract

A simple design approach for an isolated switched mode power supply with wide output adjustable range and fast dynamic load requirement.

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WIDE OUTPUT FAST TRANSIENT SMPS DESIGN

MC Wong

April 2003

ABSTRACT                                                  

A simple design approach for an isolated switched mode power supply with wide output adjustable range and fast dynamic load requirement.

2.         Introduction

The output adjustable range of the switched mode power supply is directly related to the range of operational duty ratio of the PWM circuit.  Wider output adjustable range means wider variation requirement on the PWM duty cycle. 

A switching mode power supply with a fast dynamic load capability needs a low impedance output filter circuit and sufficient variable range for the PWM duty cycle to facilitate steep ramps in the output current. 

Therefore, there is a basic conflict between requirements for a wide output adjustable range and high di/dt dynamic load capability making it difficult to achieve both in a power supply design.  Also, if the operating duty cycle range is too wide, it will affect the output filter design resulting in higher output ripple or a more expensive filtering network.

3.         Design approach

Currently the most common design approach for an isolated converter with fast dynamic capability is a two-stage conversion approach.  The first stage is an isolated converter which handles input voltage variations while stepping the voltage down to a lower level between 5V to 12V.  The specific output voltage level of the first stage will depend on the output range requirement of the second stage.  To maximize the dynamic load capability, the second stage is normally a multi-phase buck converter, as shown in Figure 1 below. 

Figure 1  Common two stages converter.

This approach is adequate for most of the current microprocessor applications since the maximum to minimum output voltage ratio is less than 2:1.   The maximum operating duty cycle of multiphase PWM controllers currently available on the market is close to 75% which is sufficient for the buck output stage used in those applications.

However, if a maximum to...