Overview

Bode plots are an important tool used in the design of control loops—including control loops for switch mode power supplies (SMPS). To simplify the creation of Bode plots, LTspice 17.1 includes a new, easy-to-use frequency response analysis (FRA) component and directive, which can be used to provide the test signal and analysis, respectively. Specifically, this new feature[i] produces Bode plots using the voltage-only Middlebrook method[ii]—by injecting a series of test signals into the closed loop system to determine open-loop gain and phase. One advantage of this method is that the SMPS retains its closed loop, and its operating points. The steps for performing a transient frequency response analysis are enumerated here.  

To generate a Bode plot for a switch mode power supply …

• Place the fra component in your schematic:

The fra symbol produces the series of test signals at run time that are used in the frequency response analysis

a. Choose Edit > Component, or Press F2; The Select Component Symbol Dialog appears



b. Type “fra” in the search field
c. Select the fra symbol
d. Click OK
e. Place the fra component in the SMPS feedback loop in series with the feedback pin.

• Configure the fra symbol:

You set the test signal parameters, including frequency range and amplitudes. 

a. Right-click the fra component to edit its attributes. The Frequency Response Analyzer dialog appears.



b. Click Help Me Configure This for a Switching Regulator. The Configure FRA… dialog appears

c. Enter as much information as you know—rough approximations are fine.
d. Click Configure FRA. LTspice fills in reasonable analyzer parameters.
e. Review the Analysis Start Time—make sure this is long enough to allow the output voltage settle to its final value.

• Add a .fra spice analysis directive.

The .fra directive tells LTspice you would like to have it produce signals from the fra device and analyze the results. Remember to comment out any other spice analysis directives.

a. Choose Edit > Spice Analysis
b. Click the Transient Frequency Response tab



c. Add appropriate parameters (described in Help)
d. Comment out any other analysis directives.



f. Click Run. When run, the .fra analysis is applied to determine the open loop response of the test signals produced by the fra component, and a Bode plot is produced. 

• Review the Results:

Once the simulation is complete, a Bode plot appears, annotated with phase margin, crossover frequency, and gain margin (if found).

Further examples and documentation can be found in the educational examples (..\LTspiceIV\examples\Educational\FRA\) and LTspice Help.

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[i] This feature is available in LTspice version 17.1 and later.

[ii] International Journal of Electronics, Volume 38, Number 4, 1975