psbook.pdf

Personal Computer

Circuit Design

Tools

Power Specialist’s App Note Book

P.O.Box 710

San Pedro, Ca. 90733-0710

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POWER SPECIALIST’S APP NOTE BOOK

Edited by Charles E. Hymowitz

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rev. 98/11

Acknowledgments

Authors

Christophe BASSO, consultant, Sinard, France

Charles Hymowitz, Intusoft, San Pedro, CA USA, charles@intusoft.com

Lawrence Meares, Intusoft, San Pedro, CA USA

Harry H. Dill, Deep Creek Technologies, Inc. Annapolis, MD USA, Testdesigner@compuserve.com

A. F. Petrie, Independent Consultant, 7 W. Lillian Ave., Arlington Heights, IL USA

Mike Penberth, Technology Sources, NewMarket Suffolk, U.K.

Martin O’Hara, Motorola Automotive and Industrial Electronics Group, Stotfold Hitchin Herts. U.K.

Kyle Bratton, Naval Aviation Depot, Cherry Point NC USA

Chris Sparr, Naval Aviation Depot, Cherry Point NC USA

Lloyd Pitzen, CCI Incorporated Arlington, VA USA

Editors

Charles Hymowitz

Bill Halal

Table Of Contents

Switched Mode Power Supply Design

Average simulations of FLYBACK converters with SPICE3

A Tutorial Introduction to Simulating Current Mode Power Stages

Write your own generic SPICE Power Supplies controller models

Keep your Switch Mode Supply stable with a Critical-Mode Controller

Exploring SMPS Designs Using IsSpice

Average Models For Switching Converter Design

Simulating SMPS Designs

SSDI Diode Rectifier Models

High Efficiency Step-Down Converter

IsSpice4 Scripting Gives You More Power

Magnetics Design and Modeling

Designing a 12.5W 50kHz Flyback Transformer

Designing a 50W Forward Converter Transformer With Magnetics Designer

Signal Generators

IsSpice4 introduces a dead-time in your bridge simulations

Three Phase Generator

Simulating Pulse Code Modulation

Modeling For Power Electronics

A Spice Model For TRIACs

A Spice Model For IGBTs

SPICE 3 Models Constant Power Loads

Simulating Circuits With SCRs

Power Schottky and Soft Recovery Diodes

Spark Gap Modeling

103

SPICE Simulates A Fluorescent Lamp

108

Sidactor Modeling

112

Transformer and Saturable Core Modeling

114

Modeling Nonlinear Magnetics

120

Current Limited Power Supply

126

Macro Modeling Low Power DC-DC Converters

127

Modeling Non-Ideal Inductors in SPICE

131

Motor and Relay Modeling

Modeling A Relay

136

New SPICE Features Aid Motor Simulation

141

Test Program Development and Failure Analysis for SMPS

Automating Analog Test Design

145

Simulation measurements vs. Real world test equipment

150

New Techniques for Fault Diagnosis and Isolation of Switched Mode Power Supplies

152

Application of Analog & Mixed Signal Simulation Techniques to the Synthesis

and Sequencing of Diagnostic Tests

163

SMPS Design

Switched Mode Power

Supply Design

Authors

Christophe BASSO, consultant, Sinard, France

Charles Hymowitz, Intusoft

Larry Meares, Intusoft

Scott Frankel, Analytical Engineering Services

Steve Sandler, Analytical Engineering Services

Power Specialist’s

Average simulations of FLYBACK converters with SPICE3

Christophe BASSO

May 1996

Within the wide family of Switch Mode Power Sup-

plies (SMPS), the Flyback converter represents the preferred

structure for use in small and medium power applications

such as wall adapters, off-line battery chargers, fax machines,

etc. The calculations involved in the design of a Flyback

converter, especially one which operates in discontinuous

mode, are not overly complex. However, the analysis of the

impact of the environment upon the system may require a

lengthy period of time: ESR variations due to temperature

cycles, capacitor aging, load conditions, load and line tran-

sients, the effects of the filter stage, etc. must be considered.

A SPICE simulator can help the designer to quickly

implement his designs and show how they react to real world

constraints. The simulation market constantly releases SMPS

models, and the designer can rapidly lose himself in the eclec-

ticism of the offer. This article will show how you can ben-

efit from these new investigation tools.

practically impossible to evaluate the AC transfer function

of the simulated circuit due to the switch.

Average models do not contain the switching com-

ponents. They contain a unique state equation which de-

scribes the average behavior of the system: in a switching

system, a set of equations describe the circuit’s electrical

characteristics for the two stable positions of the switch/ (es),

ON or OFF. The “state-space-averaging” technique consists

of smoothing the discontinuity associated with the transi-

tions of the switch/ (es) between these two states. The result

is a set of continuous non-linear equations in which the state

equation coefficients now depend upon the duty cycles D

and D

(1-D). A linearization process will finally lead to a

set of continuous linear equations. An in-depth description

of these methods is contained in D. M. Mitchell’s book, “DC-

DC Switching Regulators Analysis”, distributed by e/j

BLOOM Associates.

Simulating SMPS with SPICE is not a new topic

In 1976, R. D. Middlebrook settled the mathemati-

cal basis for modeling switching regulators [1]. Middlebrook

showed how any boost, buck, or buck-boost converter may

be described by a canonical model whose element values

can be easily derived. In 1978, R. Keller was the first to

apply the Middlebrook theory to a SPICE simulator [2]. At

that time, the models developed by R. Keller required manual

parameter computation in order to provide the simulator with

key information such as the DC operating point. Also, the

simulation was only valid for small signal variations and

continuous conduction mode.

Two years later, Dr. Vincent Bello published a se-

ries of papers in which he introduced his SPICE models [3].

These models had the capacity to automatically calculate

DC operating points, and allowed the simulated circuit to

operate in both conduction modes, regardless of the analy-

sis type (AC, DC or TRAN). Although these models are 15

years old, other models have been introduced since then,

our example circuits which have been based upon them will

demonstrate how well they still behave.

The general simulation architecture

The key to understanding the simulation of SMPS

with a SPICE simulator is to first experiment with very simple

structures. Figure 1 shows the basic way to simulate an av-

erage voltage-mode Flyback converter with its associated

components. As a starting point, simply draw a minimum

part count schematic: simple resistive load, output capaci-

tor with its ESR, perfect transformer (XFMR symbol), no

input filter, no error amplifier etc.

FLYBACK converter model

Output transformer

Output voltage

Out+

In+

COUT

RLOAD

Out-

Duty Cycle

In-

Input voltage

ESR

Duty cycle

input

Figure 1

Switching or average models ?

Switching models will exhibit the behavior of an

electrical circuit exactly as if it were built on a breadboard

with all of its nonlinearities. The semiconductor models, the

transformer and its associated leakage elements, and the

peripheral elements are normally included. In this case, the

time variable t is of utmost importance since it controls the

overall circuit operation and performance, including semi-

conductor losses and ringing spikes which are due to para-

sitic elements. Because SMPS circuits usually operate at high

frequencies and have response times on the order of milli-

seconds, analysis times may be very long. Furthermore, it is

By clicking on the average Flyback model symbol

or simply filling in the netlist file, the working parameters

will be entered, i.e. the operating switching frequency, the

value of the primary power coil, etc. Some recent models

require the loop propagation delays or overall efficiency.

The parameters for the remaining components are obvious,

except for the duty cycle input source. This source will di-

rectly pilot the duty cycle of the selected model. By varying

the source from 0 to 1V, the corresponding duty cycle will

sweep between 0 and 100%. For the first simulation, with-

out an error amplifier, you will have to adjust this source

such that the output matches the desired value. This value

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