Peak Current Control with AC sweep model
SmartCtrl Tutorial
Peak Current Control with AC sweep
model
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Peak Current Control with AC sweep model
SmartCtrl1 is a general-purpose controller design software specifically for power electronics applications.
There are many predefined topologies, compensators and control types in SmartCtrl that allow a
straightforward design of the control loop. Moreover, if the model of the converter is not included into
the program database, SmartCtrl proposes an alternative: the design of the compensator considering
the frequency response of the plant directly introduced by the user. Thanks to this feature, the designer
is able to carry out the control loop design and optimization for almost any plant.
This tutorial is intended to guide you, step by step, to design the peak-current control of a DC/DC
converter using the SmartCtrl Software. The model of this converter is not included yet into the
database of the program, and therefore the first step is to achieve the frequency response of the
converter.
One of the alternatives to obtain the frequency response of a plant is to perform an AC sweep using an
electrical simulator like PSIM. The PSIM schematic of the proposed circuit is shown in Figure 1, including
the compensating ramp that avoids the sub-harmonic oscillation. This circuit can be found in one of the
examples provided by PSIM2.
Figure 1
The magnitude and phase of the frequency response achieved after the AC sweep of the mentioned
circuit are shown in Figure 2.
These results can be automatically exported to SmartCtrl by using the link between the two programs.
The PSIM main toolbar has a button (Figure 3) that automatically exports the frequency response to
SmartCtrl. It is necessary to indicate whether the transfer function is a “Voltage Transfer Function” or a
“Current Transfer Function”. In this particular example, the converter is a voltage controlled one and the
frequency response corresponds to the output voltage to duty cycle transfer function of the plant and
the modulator. Therefore the selected option is “Voltage Transfer Function” (Figure 4). Other required
parameters are the switching frequency and the output voltage. These values in the presented example
are 100 kHz and 15 V respectively.
1
SmartCtrl is copyright ©2009-2012 by Carlos III University of Madrid, GSEP Power Electronics Systems Group, Spain
2
“Using PSIM for Analyzing the Dynamic Behavior and Optimizing the Feedback Loop Design of Switchmode Power Supplies”. A
Powersys Training Course by Richard Redl (ELFI S.A.), copyright @2005, Powersys s.a.r.l.
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Peak Current Control with AC sweep model
Figure 2
Figure 3
Figure 4
Once the export parameters have been indicated, the SmartCtrl will open automatically. A new blank
design will be displayed. By clicking the button “Import transfer function (single loop)”, placed in the
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Peak Current Control with AC sweep model
SmartCtrl main toolbar (Figure 5), the design process of the control loop will start. A first window will be
displayed, showing the imported frequency response data, as well as the switching frequency and the
output voltage that were already provided to SmartCtrl (Figure 6).
Figure 5
Figure 6
The next step is to select the sensor to measure the output voltage. In this particular case the voltage
divider will be considered in the compensator design, and thus the “Regulator embedded voltage
divider” has been selected (Figure 7).
For a fixed voltage reference and a peak-current control, a good selection is the “Regulator embedded
voltage divider” and an “Unattenuated Type 2 regulator”. When introducing the data in the regulator
window (Figure 8) it is very important to modify the default data provided by SmartCtrl in order to get a
modulator gain equal to one. The reason is that the modulator behavior is already included in the
introduced frequency response data. In this case, these parameters have been modified in such a way
that the carrier signal is a ramp with a voltage ripple equal to one and a duty cycle also equal to one
(rising time equal to the switching period).
Once all the components of the control loop have been selected, the last step is to specify the dynamic
requirements. The solution map provides the boundaries of the design space of the control loop, in a
graphical plot that represent the margin phase versus the cross-over frequency. The parameters
selected in this example are 10 kHz (cross-over frequency) and 60 deg (margin phase), as depicted in
Figure 9 and Figure 10.
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Peak Current Control with AC sweep model
Figure 7
Figure 8
Figure 9
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Peak Current Control with AC sweep model
Figure 10
Considering the specified input data, the program will automatically show the dynamic performance of
the system by means of the Bode plots, the Nyquist plot and the transient response, depicted in the
graphic panels in Figure 11. The discrete components of the compensator as well as other output data
like the frequency of the poles and zeros, can be displayed by clicking on the button “View output data”,
in the main toolbar (Figure 12).
Figure 11
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Peak Current Control with AC sweep model
Figure 12
SmartCtrl has a link with PSIM that allows the exportation of the design compensator to any PSIM
schematic (Figure 13). In this example, the components of the regulator have been exported to a PSIM
schematic with the original plant in Figure 1. The exported circuit and the exported parameter file with
the values of the components are depicted in Figure 14. The designer has to properly connect the
compensator to the original power stage. In this case it is important to highlight that the modulator
proposed by SmartCtrl must not be considered, since the original design has its own modulator (Figure
15).
Figure 13
The PSIM transient simulation results are represented in Figure 16 (output voltage and current through
the inductor).
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Peak Current Control with AC sweep model
Figure 14
From output voltage
To modulator
Figure 15
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Peak Current Control with AC sweep model
Figure 16
Comparison of transient response using Simview
It is possible to perform a voltage step at the reference in order to compare the simulation results with
the transient response provided by SmartCtrl. To do this, follow the next steps:
1. Simulate the PSIM circuit including a voltage step at the reference equivalent to that provided in
SmartCtrl (0.1% of the voltage reference (Figure 17).
2. Export the SmartCtrl transient response (Figure 18). The reference step can be shifted in time. In
this particular example, according to the reference step in the simulation, the time shift is
0.004s (Figure 19). A text file is generated with the transient response.
3. Go to Simview and merge the simulation results with the text file generated by SmartCtrl. As
depicted in Figure 20, the simulation results (PSIM) and the theoretical ones (SmartCtrl) are in
very good agreement.
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Peak Current Control with AC sweep model
Figure 17
Figure 18
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Peak Current Control with AC sweep model
Figure 19
Figure 20
Comparison of Bode plots using SmartCtrl
For validation purposes, it is also interesting to measure the Bode plot of the open loop gain using PSIM.
In order to compare the Bode plots obtained by means of PSIM and those provided by SmartCtrl, an
option is to import the PSIM Bode plot with SmartCtrl. To do this, follow the next steps:
1. Perform an AC sweep analysis to obtain the open loop gain with PSIM (Figure 21).
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Peak Current Control with AC sweep model
2.
3.
4.
5.
Save the PSIM data (Figure 22) in a text file including the magnitude and phase of the Bode plot.
Click on the button to import the transfer function (main toolbar in SmartCtrl, Figure 23).
Once the window “Functions to be merged” is opened, click on “Add” (Figure 24).
Select the transfer function T(f), that corresponds to the open loop gain of the circuit (Figure
25).
6. Select the option text file and look for the text file generated with PSIMView.
7. The added function will be shown in the window “Functions to be merged” (Figure 26).
8. After finishing this process, the added function will be displayed in SmartCtrl. Simulation results
are extremely similar to SmartCtrl Bode plots (Figure 27).
Finally, the described design methodology of a peak current control loop using SmartCtrl can be
considered very satisfactory.
Figure 21
Figure 22
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Peak Current Control with AC sweep model
Figure 23
Figure 24
Figure 25
Figure 26
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Peak Current Control with AC sweep model
Figure 27
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