Bode 100 Application Note
直流模块电源环路稳定性分析
DC/DC Converter Stability Measurement
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Table of Contents
1
2
Introduction ................................................................................................................................3
Measurement Setup ...................................................................................................................4
2.1 The Circuit under Test ...........................................................................................................4
2.2 Selecting the Injection Point ..................................................................................................5
2.3 Connecting the Bode 100 ......................................................................................................6
2.4 Phase Margin and Gain Margin .............................................................................................8
Device Configuration .................................................................................................................9
Measurement & Results ...........................................................................................................10
4.1 Calibration ...........................................................................................................................10
4.2 Shaped Level ......................................................................................................................12
4.3 Injection Level .....................................................................................................................14
4.4 Supply Voltage Influence .....................................................................................................15
4.5 Load Current Influence ........................................................................................................15
Conclusion ...............................................................................................................................16
3
4
5
DC/DC Converter Stability Measurement
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1
Introduction
In this application note we show how to analyze the stability respectively the control loop behavior of a
step-down DC/DC converter.
To minimize the influence of supply voltage variations and load changes on the output voltage of a
DC/DC converter, a compensating controller is necessary. The quality of this control circuit
determines the stability and dynamic response of the entire DC/DC converter system. The following
pages show you how you can measure the loop response of such control systems using the
Bode 100
vector network analyzer in combination with the B-WIT 100 wideband injection transformer.
For the characterization of the loop we measure the open loop gain by using the voltage injection
method. This method is commonly used to analyze the stability of voltage regulators such as switched
mode power supplies.
The following points are discussed in this document:
How to choose the correct injection point to measure the loop gain
Determining gain margin and phase margin from the frequency response
How do supply voltage and load current influence the systems dynamics
Using the shaped level feature of the Bode 100 to improve the measurement results
DC/DC Converter Stability Measurement
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2 Measurement Setup
2.1 The Circuit under Test
The demo board 481A is a step-down converter featuring the LT1976. The output is optimized for
at a load current of
. The following figure shows the schematics of the demo board 481A.
Detailed information on the demo circuit can be found at
http://www.linear.com.
Figure 1: LT 481A demo board schematics
DC/DC Converter Stability Measurement
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2.2 Selecting the Injection Point
In order to measure the loop gain of a voltage feedback loop we need to break the loop at a suitable
point and inject a disturbance signal at this point. The disturbance signal will be distributed around the
loop and depending on the loop gain the signal will be amplified or attenuated and shifted in phase.
The Bode 100 output will provide the disturbance signal whereas the inputs will measure the transfer
function of the loop.
To ensure that the measured loop gain equals the real loop gain we need to find a point where the
loop is restricted to one single path and where the impedance looking in the direction of the loop is
much bigger than the impedance looking backwards.
The following figure shows the feedback loop of the circuit and indicates the suitable injection point.
The impedance looking backwards equals the output impedance of the converter which is very low (in
the range of several
). The impedance looking in direction of the loop is formed by the
compensator and the voltage divider and is in the range of several
.
Figure 2: Feedback loop and injection point
DC/DC Converter Stability Measurement
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