Tyco Power Supply TX 75149 User Manual

Stability Analysis Tool  
User's Guide  
For use with Board Mounted  
DC-DC converters  
V1.3  
TYCO ELECTRONICS POWER SYSTEMS 3000 SKYLINE DRIVE MESQUITE, TX 75149  
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No oral or written information or advice given by Tyco Electronics or its  
distributors, agents or employees will operate to create any warranty or  
guarantee or vary any provision or information herein, and you may not rely on  
any such information or advice. Tyco Electronics reserves the right to change  
any portion of this data, and to change the Information, at any time without  
notice.  
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1
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 loads. From this plot, information 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 depends not only on the converter  
characteristics but also on the load characteristics and the output 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 application, 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 breadboard of the power-  
supply and the load network using a network analyzer.  
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 load network and avoids making measurements.  
System Requirements  
SAT v1.3 runs under the following systems:  
1) Windows 95, 98, 2000 or Windows NT version 4 operating systems  
2) Microsoft Excel 97 SR-2 or Excel 2000  
3) Intel Pentiumclass or above processor and at least 64 MB of RAM  
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Installation and Usage  
You should first read and accept the terms of the SAT usage.  
Download the file SAT_v1.3.xls from the Tyco Electronics Power Systems  
web site (given below) to a directory on your computer.  
Download one or more module data file(s) to the same directory.  
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 Analysis Toolpack and  
Analysis Toolpack – VBA.  
Technical Support  
For technical support, please contact us at  
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 Service and Support in other regions of  
the world  
Web:  
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2
Using SAT  
When you start SAT, you will see the window shown in Figure 1.  
Fig. 1. SAT Introduction and Module Selection page  
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There are two sheets, "Module Selection" and "Configuration Menu" shown  
initially at the bottom left-hand section of the Excel spreadsheet. Each session  
should start at the "Module Selection" sheet.  
Load Module Data File  
Click on the button  
This will bring the module data file  
import window as shown below in 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 downloaded before you can use the 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 as  
shown in Figure 3. You may want to save SAT_v1.3.xls file under a different  
name so that the original one that you downloaded remains unchanged.  
Fig. 2. Module data file import window  
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If you want to choose a different module data file, you can always go back to the  
Module Selection sheet to change it.  
The Configuration Menu lists five choices for representing the load circuit  
topology.  
The first configuration 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 capacitors are the same type (e.g. aluminum  
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  
SENSE  
DC/DC  
CONVERTER  
DC/DC  
CONVERTER  
CONFIGURATION  
1
CONFIGURATION  
2
RL  
RL  
SENSE  
DC/DC  
CONVERTER  
CONFIGURATION  
3
RL  
RL  
RL  
SENSE  
DC/DC  
CONVERTER  
CONFIGURATION  
4
SENSE  
DC/DC  
CONVERTER  
CONFIGURATION  
5
Fig. 3. Circuit configuration  
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model of the capacitor bank. The load is modeled by the resistance RL.  
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 situation commonly referred 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 applications, the power module is mounted on a  
separate printed circuit board and supplies a number of loads on different  
PCBs. In this case, it is common to have one capacitor bank on the power  
board and another one on the load board.  
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 lumped 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 corresponding to the other capacitor to zero.  
The second capacitor bank is typically placed as close as possible across the  
load terminals to improve dynamic response during load transients by minimizing the  
voltage overshoot and undershoot. The capability of modeling two capacitors with  
their ESR and ESL are provided to model different types and sizes of capacitors.  
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 (case 2), or across the module output.  
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 Figure 4.  
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 Ra1, through Ra3, and Rb1 through Rb3 are the ESRs of the capacitors  
and the inductances La1 through Lb3 are the ESLs.  
Enter all circuit parameters of your application. When you finish editing the  
parameter values, click on the  
button.  
Click to Calculate Stability  
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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 Cb2 is changed from 1000 µF to  
6800 µF.  
To investigate a different configuration, click on the Configuration Menu tab at  
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 analysis for another configuration.  
Stability Analysis Tool  
LS1  
RS1  
Ra1  
LS2  
RS2  
SENSE  
Magnitude of the Loop Gain  
60  
40  
20  
0
Ra2  
Ca2  
Ra3  
Ca3  
Rb1  
Rb2  
Cb2  
DC/DC  
CONVERTER  
Ca1  
La1  
Cb1  
Lb1  
RL  
La2  
Lb2  
La3  
-20  
-40  
Please enter the circuit parameters below  
Rload = 0.5  
ohm  
Rs1 = 1  
Rs2 = 5  
mohm  
nH  
10  
100  
1000  
10000  
100000  
Frequency (Hz)  
Ls1  
Ra1  
La1  
=
=
=
Ls2  
Rb1  
Lb1  
=
=
=
4
100  
10  
Ra2  
La2  
Ca2 = 10  
=
=
Ra3  
La3  
Ca3 = 1000  
=
=
Rb2  
Lb2  
Cb2 = 1000  
=
=
100  
1
100  
1
10  
1
10  
10  
mohm  
nH  
10  
Phase of the Loop Gain  
µ
F
Ca1 = 10  
Cb1 = 1000  
180  
120  
60  
Click to Calculate Stability  
0
Notepad Area for JFW050A Module  
-60  
-120  
-180  
10  
100  
1000  
Frequency (Hz)  
10000  
100000  
Hz  
degrees  
Gain crossover frequency:  
Phase Margin:  
Fig. 4. Page for entering and editing circuit parameters, shown for the case of  
configuration 4.  
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Stability Analysis Tool  
LS1  
RS1  
Ra1  
LS2  
RS2  
SENSE  
Magnitude of the Loop Gain  
40  
20  
Ra2  
Ca2  
Ra3  
Ca3  
Rb1  
Cb1  
Rb2  
Cb2  
DC/DC  
CONVERTER  
Ca1  
La1  
RL  
0
La2  
Lb2  
La3  
Lb1  
-20  
-40  
-60  
Please enter the circuit parameters below  
Rload = 0.5  
ohm  
Rs1  
Ls1 = 4  
Ra1  
=
Rs2  
Ls2 = 100  
Rb1  
=
1
5
mohm  
nH  
100  
1000  
10000  
100000  
Frequency (Hz)  
=
Ra2  
=
Ra3  
=
=
Rb2  
=
100  
100  
10  
10  
10  
mohm  
La1 = 1  
Ca1 = 10  
La2 = 1  
Ca2 = 10  
La3 = 1  
Ca3 = 1000  
Lb1 = 10  
Cb1 = 1000  
Lb2 = 10  
Cb2 = 1000  
nH  
µF  
Phase of the Loop Gain  
180  
120  
60  
Click to Calculate Stability  
0
Notepad Area for JFW050A Module  
-60  
-120  
-180  
100  
1000  
10000  
100000  
Frequency (Hz)  
696.6 Hz  
Gain crossover frequency:  
Phase Margin:  
52.06 degrees  
Fig. 5. Voltage-loop-response Bode plot with phase margin and gain crossover  
frequency calculated and displayed.  
Stability Analysis Tool  
LS1  
RS1  
Ra1  
LS2  
RS2  
SENSE  
Magnitude of the Loop Gain  
40  
20  
Ra2  
Ca2  
Ra3  
Ca3  
Rb1  
Rb2  
Cb2  
DC /DC  
CONVERTER  
Ca1  
La1  
Cb1  
Lb1  
RL  
0
La2  
Lb2  
La3  
-20  
-40  
-60  
Please enter the circuit parameters below  
Rload = 0.5  
ohm  
Rs1 = 1  
Rs2 = 5  
mohm  
nH  
100  
1000  
10000  
100000  
Frequency (Hz)  
Ls1 = 4  
Ra1 = 100  
La1 = 1  
Ls2 = 100  
Rb1 = 10  
Lb1 = 10  
Cb1 = 1000  
Ra2 = 100  
La2 = 1  
Ca2 = 10  
Ra3 = 10  
La3 = 1  
Ca3 = 1000  
Rb2 = 10  
Lb2 = 10  
Cb2 = 6800  
mohm  
nH  
µF  
Phase of the Loop Gain  
Ca1 = 10  
180  
120  
60  
Click to Calculate Stability  
0
Notepad Area for JFW050A Module  
-60  
-120  
-180  
100  
1000  
10000  
100000  
Frequency (Hz)  
399.5  
Hz  
Gain crossover frequency:  
Phase Margin:  
42.8 degrees  
Fig. 6. Voltage loop response Bode plot when Cb2=6800 µF  
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To analyze a different module, you should select the Module Selection tab at  
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  
LS1  
RS1  
Ra1  
SENSE  
Magnitude of the Loop Gain  
40  
20  
DC/DC  
CONVERTER  
Ca1  
RL  
0
La1  
-20  
-40  
-60  
-80  
Please enter the circuit parameters below  
Rload  
Rs1  
Ls1  
=
=
=
0.054 ohm  
1
mohm  
nH  
100  
1000  
10000  
100000  
Frequency (Hz)  
28  
Ra1 = 9  
mohm  
La1 = 0  
Ca1 = 10000  
nH  
µF  
Phase of the Loop Gain  
180  
120  
60  
Click to Calculate Stability  
0
Notepad Area for JFW050A Module  
-60  
-120  
-180  
100  
1000  
10000  
100000  
Frequency (Hz)  
229.1 Hz  
Gain crossover frequency:  
Phase Margin:  
52.93 degrees  
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 modules paralleled and the load, both of which are controlled by the  
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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 two 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 converter system as well  
This method assumes that the layout 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 Power Electronics Conference 2002, vol.1, pp. 367-373.  
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3
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 conduction mode (CCM). Therefore, make  
sure that RL is sufficiently low enough to guarantee a load current that leads to  
CCM operation. Modules with synchronous rectifier based output stages operate  
in CCM throughout the entire load current 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.  
3. For buck derived converter topologies, the sensitivity 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.  
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4
Troubleshooting  
If you encounter the following message when running SAT,  
there are two possibilities for the error message.  
1. Some of the parameters you entered for your network are nonnumeric.  
2. You have not enabled the Analysis Toolpack and Analysis Toolpack – VBA in  
your Excel environment as desribed in page 5 of this document under  
“Installation and Usage”. If you cannot see Analysis Toolpack and Analysis  
Toolpack – VBA under the Tools/AddIns tab in Excel, then you have to  
reinstall Excel with those options checked during installation.  
Make sure that you have the correct Excel settings and numeric data for the network  
parameters.  
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