Tyco Stability Analysis Tool User Manual

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StabilityAnalysis T ool

User's Guide

For use withBoardMounted DC-DCconverters

V1.3

TYCO ELECTRONICSPOWERSYST EMS3000SKYL INEDRIVEMESQU ITE,TX75149

*************************************************************************** * COPYRIGHT 2001, TYCO ELECTRONICS POWER SYSTEMS, INC. * * ALL INTERNATIONAL RIGHTS RESERVED * ***************************************************************************

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USER AGREEMENT

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Introduction

The Stability Analysis Tool (SAT) is a Microsoft Excel-based program that provides a Bode plot of the voltage loop gain of a DC-DC converter for a variety of external l oa ds. From this plot, informatio n such as crossover frequency and gain and phase margins can be obtained .

In classical control theory, the stability of a feedback system is assessed by evaluating its gain and phase margin which are read directly from the Bode plot of the loop gain. The loop gain of a DC-DC converter dependsnotonlyon the converter characteristics but also on the load characteristics and the o utput voltage sense location. This dependence of loop gain on load and sense location makes it impossible for the power converter manufacturer to determine stability margins in an actual applicatio n, since the load (especially the number, type and location of output bulk capacitors, parasitic inductances, etc.) and the output voltage sense location are unknown. Due to this, in the past if the customer wanted to determine stability margins, the loop gain would have to be measured using a breadboar d of the powersupply and the load networ k using a networkanaly z er.

The Stability Analysis Tool decouples the combined converter+load system into two subsystems; one representing the converter and the other representing the load. Tyco Electronics Power Systems provides converter data files on its web site. The customer can download the converter data file and import it into the SAT. SAT allows choosing one of five different configurations to best represent the combination of load representation and voltage sense location. The tool then allows assessment of the stability characteristics without the user having to build a prototype of the power supply and the loadnetwor k andavoidsmaking measureme nts.

System Requirements

SAT v1.3 runs under the followingsy stems:

1) Windows 95, 98, 2000 or Windows NT version4opera tingsystems

2) Microsoft Excel 97 SR-2 or Excel 2000

3) Intel Pentium class or above processorandatleast64MBofRAM

Installation and Usage

You should first read andaccep t theter mso f the SAT usag e.

 Download the file SAT_v1.3.xls from the Tyco Electronics Power Systems

web site (given below ) to a directoryon your computer .

 Download one or more module datafile(s )tothesam e director y.  You can start up the tool by following the usual procedures to open an

Excel Worksheet and follow the directions on the screen. Make sure that the Analysis Toolpak Add-ins are enabled in Excel. Go to Tools menu in Excel and click on Add-Ins. Check (tick) on A naly sis Toolpack and Analysis Toolpack – VBA.

Technical Support

For technical support,pleaseco nt actusat

USA Tyco Electronics Power Systems

3000 Skyline Drive Mesquite, TX 75149

USA

Phone - 800-526-7819 Fax - 888-315-5182

Outside U.S.A.: Refer to our Web site below for Serv ice and Suppor tinotherreg io nsof

the world

Web: http://power.tycoel ectr onics.co m/

Using SAT

When you start SAT, you willsee the windowsh ownin Figur e1.

Fig. 1. SATIntr oducti on and Modul eSelecti onpag e

 There are two sheets, "

Module Selection

" and "

Configuration Menu

initially at the bottom left-hand section of the Excel spreadsheet. Each session

" shown

should start at the "

 Click on the button This will bring the module data file

Module Selection

Load Module Data File

" sheet.

import window as shownbelowin Figure 2.

Note that all module data files have .pwr extensions. If you have not already downloaded the module data files for the particular modules you wish to analyze, they will need to be downloadedbe foreyoucanusethe tool .

 Choose the module that you want to use in your application by selecting the

corresponding module data file. In this guide, we will use the JFW050A module as an example.

 After highlighting the JFW050A.pwr file, click on the "OK" button or hit "ENTER".

SAT will load the module data file and move to the

Configuration Menu

shown in Figure 3. You may want to save SAT_v1.3.xls file under a different name so that the originalonethatyou downloaded remains unchanged.

Fig. 2. Module data file import window

 If you want to choose a different module data file, you can always go back to the

Module Selection

 The

Configuration Menu

sheet to change it.

lists five choices for repres enti ng the load circui t

topology.

 The first conf igur atio n is a simple one where the output voltage is sensed

directly at the output pins of the module. The capacitor is modeled as a series combination of equivalent series resistance (ESR) and equivalent series inductance (ESL). If all the capaci tor s are the same type (e .g . alumi nu m electrolytic) and capacitance value (e.g. 6800 µF), then this is a reasonable

Configuration Menu

Please click on the appropriate box to choose the configuration for your application

SENSE

DC/DC

CONVERTER

CONFIGURATION

SENSE

DC/DC

CONVERTER

CONFIGURATION

SENSE

DC/DC

CONVERTER

SENSE

DC/DC

CONVERTER

SENSE

DC/DC

CONVERTER

CONFIGURATION

Fig. 3. Circuit configuration

model of the capacitorbank.The load is modeled by the resistanceRL.

 The second configuration is similar to the first one except that the voltage sense

location is moved from the output pins of the module to the load. In this case, the voltage sensed and regulated is not the module output voltage but the voltage across the load,a situati oncommo nlyreferred to as "remote sensing".

 The remaining three configurations (3, 4 and 5) allow the inclusion of more

detailed models of the capacitor banks and parasitic elements between the module and the load. In some applicati ons, the power module is mounted on a separate printed circui t board and supplies a nu mber of loads on different PCBs. In this case, it is common to have one capacitor bank on the power board and anotheroneonthelo adboar d.

The first capacitor bank, which is a parallel combination of three capacitors, is usually located as close as possible to the module output pins. It conveniently allows for modeling different types of capacitors such as aluminum and tantalum electrolytics and ceramics. When the size and type of capacitors used in a bank are different, their individual frequency responses vary greatly from one another and hence they cannot be modeled as a lump ed capacitor as shown in the first two configurations. Note that if you only have one or two types of capacitors, you can still use the model by setting the values corr espon dingto the other capacito rto zero.

The second capacitor bank is typically placed as close as possible across the load terminals to improv e dynamic response during load transients by minimizi ng the voltage overshoot and undershoot. The capability of modeling two capacitors with their ESR and ESL are providedtomodeldi ffer entty pesandsiz es of capaci tors.

The three configurations also allow for modeling various sense points for the feedback voltage. Feedback voltage can be sensed across the load (case 3), across the capacitor bank (case2),oracrossth emodul eoutpu t.

 Click on the configuration picture suitable for your application. In this example, we

will choose Configuration 4. Clicking on the picture will bring that configuration's page as shown in Figure4.

 The top left section shows the configuration circuit with the feedback voltage

sense path depicted in red. Underneath the configuration circuit diagram, circuit parameters are listed with default values that can be edited in blue. The JFW050A module is a 5V, 10A output module and hence in this example, the load resistance is set at 0.5 ohms, indicating a 10A load. You should set the load resistance value corresponding to the actual load in your application. The resistors R and the inductances L

, through Ra3,andRb1through Rb3are the ESRs of the capacitors

through Lb3are the ESLs.

 Enter all circuit para meter s of y our application. When you finish editing the

parameter values, click on the button.

Click to Calculate Stability

SAT draws the Bode plot of the voltage loop gain on the right half of the page as

shown in Figure 5. The phase margin, which is a measure of stability, and the gain crossover frequency, which is a measure of fast transient response, are also calculated and presented below the Bode plot.

You can enter your notes under the "Notepad Area for JFW050A Module" section in the left lower corner of the page. You can also edit the circuit parameters and simulate various what-if scenarios and record the results on the same page. Figure 6 below compares two scenarios where the value of C

is changed from 1000 µFto

6800 µF.

 To investigate a different configuration, click on the

Configuration Menu

the bottom of the Excel worksheet which will take you back to the configuration-

selection page. If you click on the circuit diagram for Configuration 2, the page for

that configuration will appear in the window and the Configuration 4 page will

disappear (Figure 7). You can now repeat the analysis in a manner similar to the

previous case. There is no need to re-load the module data file for JFW050A to

perform an analysisfor another configurati on.

Stability Analysis Tool

DC/DC

CONVERTER

Ls1=

Ra1=

La1=

Ca1=

LS1R

SENSE

Please enter the circ uit parametersbelow

0.5 ohm

1 4 100 1 10

Ra2=

La2=

Ca2=

load

Ra3=

100 1 10

Click to Calculate Stability

La3=

Ca3=

10 1 1000

Notepad Area for JFW050A Module

LS2R

5 mohm 100 nH

10 10

1000

Rs2=

Ls2=

Lb1=

Rb2=

Lb2=

Cb2=

10 mohm 10 nH

1000

Gain (dB)

-20

-40 10 100 1000 10000 100000

180

120

Phase (deg)

-60

-120

-180 10 100 1000 10000 100000

Magnitude of the Loop Gain

Frequency (Hz)

Phase of the Loop Gain

Frequency (Hz)

tab at

Fig. 4. Page for entering and editing circuit parameters, shown for the case of configuration 4.

ain crossover frequency:

Phase Margin:

egrees

Stability Analysis Tool

DC/DC

CONVERTER

Ls1= Ra1= La1= Ca1=

LS1R

SENSE

Please enter the circuit parametersbelow

0.5 1 4 100 1 10

Ra2= La2= Ca2=

load

Ra3=

100 1 10

Click to Ca lcula te S tability

La3=

Ca3=

10 1 1000

Notepad Areafor JFW050A Module

LS2R

Rs2=

5 mohm

100 nH

Rb1=

Lb1=

Cb1=

1000

Rb2=

Cb2=

10 mohm

10 nH 1000

-20

Gain (dB)

-40

-60 100 1000 10000 100000

180

120

Phase (deg)

-60

-120

-180 100 1000 10000 100000

Gain crossover frequency:

Magnitude of the Loop Gain

Frequency (Hz)

Phase of the Loop Gain

Frequency (Hz)

Phase Margin:

96.6

2.06 degrees

Fig. 5. Voltage-loop-res pons eBodeplotw ithphasemarg inand gaincr ossov er frequency calculat edan d displ ayed.

SENSE

DC/ DC

CONVERTER

LS1R

LS2R

Please enter the circuitparameters below

=0.5

= 1 Rs2= 5

Ra1= 100 Ra2= 100 Ra3= 10 Rb1= 10

= 1 La2= 1 La3= 1 Lb1= 10

Ca1= 10 Ca2= 10 Ca3= 1000 Cb1= 1000

load

Click to Calculate Stability

Ls2=

100

otepad Areafor JFW050A Module

Stability Analysis Tool

mohm nH

= 10 = 10 = 6800

mohm nH

b2 b2 b2

-20

Gain (dB)

-40

-60 100 1000 10000 100000

180

120

Phase (deg)

-60

-120

-180 100 1000 10000 100000

ain crossover frequency:

Magnitude of the Loop Gain

Frequency (Hz)

Phase of the Loop Ga in

Frequency (Hz)

Phase Margin:

399.5

42.8

Hz degrees

Fig. 6. Voltage loop respons eBodeplot when Cb2=6800 µF

 To analyze a different module, you should select the

Module Selection

the bottom of the Excel Worksheet to go back to the first page and load the data file for the new module. Note that the new data file will overwrite the old one belonging to the JFW050A. If you want to keep your analysis results for the JFW050A module, you should save SAT_v1.3.xls under a different name, e.g. JFW050A.xls.

Stability Analysis Tool

LS1R

0.054 1 28

mohm nH mohm nH

-20

Gain (dB)

-40

-60

-80 100 1000 10000 100000

180

120

Phase (deg)

-60

-120

-180 100 1000 10000 100000

Magnitude of the Loop Gain

Frequency (Hz)

Phase of the Loop Gain

Frequency (Hz)

SENSE

DC/DC

CONVERTER

Please enter the circuit parameters below

load

Ls1= Ra1= 9 La1= 0 Ca1= 10000

Click to Ca lcula te Stability

Notepad Area for JFW050A Module

tab at

229.1

52.93

Hz degrees

Gain crossover frequency:

Phase Margin:

Fig. 7. Stability analysis of the JFW050A for Configuration 2.

Paralleled Modules

Paralleled modules can also be analyzed using SAT. Typically, in systems with paralleled modules, active current sharing is used to distribute the stresses evenly between modules. As described in [1], such a system can be analyzed separately for common-mode stability (which is solely dependent on the number of modules, the characteristics of the modules and the load), and differential-mode stability (which only depends on the current-sharing loop and is independent of the number of modules paralleled or the load). Since differential-mode stability is independent of the load, the manufacturer of the module is able to ensure that the current-sharing loop is stable. However, common-mode stability depends both on the number of modulesparall el edand theloa d,bot h of whichar e contr ol led by the

customer application.

SAT can be used to assess the common-mode stability characteristics of a paralleled converter system by analyzing an equivalent single-module system. The single-module equivalent is obtained by scaling the load by the number of paralleled modules. For example, in a tw o module parallel system, the load is scaled by a factor of two by doubling the load resistance, ESR’s and ESLs, and reducing the capacitances by half. The results from SAT are then valid for the paralleled convert ersy ste m as well

This method assumes that the l ay out is symmetrical in the sense that all parasitic impedances between the modules and the load are substantially equal and the sense locations of all modules are the same. If there is any discrepancy, then the single equivalent module approach will yield a different answer than the actual paralleled system.

References

[1] V. Joseph Thottuvelil, George C. Verghese, “Analysis and Control Design of Paralleled DC/DC Converters with Current Sharing”, IEEE Trans. On Power Electronics, July 1998, pp.635-644.

[2] Cahit Gezgin, Wayne C. Bowman, V. Joseph Thottuvelil, “A Stability Analysis Tool for DC-DC Converters”, IEEE Applied PowerElectronics Conference 2002, vol.1, pp. 367-373.

Limitations of SAT

Although SAT is a powerful tool, please keep in mind the following :

1. The loop gain Bode plot and phase margin predicted by SAT are valid when the module is operating in continuous c onducti on mode (CCM). Therefore, make sure that R CCM operation. Modules with synchronous rectifier based output stages operate in CCM throughout theentirel oadcur ren t range.

2. It is assumed that the module input voltage is at the nominal value, e.g. 48V for xWxxx, 24V for xCxx modules, etc. The loop response and phase margin will be different if the input voltage is not at the nominal value. However, for stability assessment purposes, since there are usually requirements for margins of >45° and 12dB, performing a stability analysis at nominal voltage of 48V is typically sufficient.

is sufficiently low enough to guarantee a load current that leads to

3. For buck derived converter topologies, the sensitiv ity of voltage loop response on load variations is small. Therefore, SAT captures the loop response over load variations, from CCM limit to full load, with reasonable accuracy. Future releases of SAT will address the effect of input voltage variations on the stability margins.

Troubleshooting

If you encounter the follow ingmessagewhen running SAT,

there are two possibilities for the error message.

1. Some of the parameters you enter edfor yournetwork are nonnumeric.

2. You have not enabled the Analysis Toolpack and Analysis Toolpack – VBA in

your Excel environme ntasdesri bedi npage5 of thisdocu mentunder “Installation and Usage” .If youcann ot seeAnal y sisToolpacka ndAnaly si s Toolpack – VBA under theTools/ Ad dInstabinEx cel , then youhav e to reinstall Excel with thoseoptio nscheck edduri n g installati on .

Make sure that you have the correct Excel setti ngsandnu mericdat afor thenetwor k parameters.

Tyco Stability Analysis Tool User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual