Single Control Loop Design
Introduction
This tutorial is intended to guide you, step by step, to design a DC/DC converter with a single control loop. The selected converter for this example is the buck converter included in Figure 1.
Figure 1: Desired system
Buck converter design
1. Open your SmartCtrl Software.
2. To begin the design of a single control loop DC/DC converter, click on DC-DC converter – single loop Voltage Mode Control or ACMC. See Figure 2.
Figure 2: SmartCtrl initial window
It can also be accessed with:
a) Button
b) Select the corresponding option within the Design menu. See Figure 3.
Figure 3: Alternative access to design a Voltage controlled DC-DC converter
3. Select a buck converter and parametrize it as follows. See Figure 4 and Figure 5.
Figure 4: Defining the converter analog topology
As the design is controlled in an analog way, the checkbox “Digital” should be left unchecked. In case the design was digitally controlled, it should be checked at this stage or further changes will be lost.
Figure 5: Plant parameters
4. Configure the sensor. See Figure 6 and Figure 7.
Once the plant has been selected, depending on which magnitude is going to be controlled, the program will display the appropriate type of sensors.
Figure 6: Choosing the sensor
In the case of the voltage divider, the user should introduce the reference voltage and the program will automatically calculate the sensor gain. The sensor input data window is the following:
Figure 7: Defining the sensor
Click OK to continue.
Please, note that all the design processes will be carried out using this gain, and the resistor values to implement the voltage divider will be provided by the program together with the regulator components.
5. Select and configure the compensator. See Figure 8.
Figure 8: Choosing the compensator
As the design is analog controlled, only analog topologies are listed. To clarify this aspect a little bit, a comparison between Type 2 and Type 3 compensators has been done. This comparison is made by means of the solution map. See Figure 9.
Figure 9: Solution map quick comparison between type 2 and 3 controllers
Due to the fact that the plant is a second-order one, the best choice in order to obtain a proper phase margin and enough bandwidth is to use a Type 3 regulator as it provides a wider white area.
A remark should be done: the solution map window shows all the pairs Phase Margin – cross Over Frequency. All the ones contained in the white area will generate a perfectly stable controller. On the contrary, if chosen in the red area the controller will almost for sure be unstable.
In this tutorial, a Type 3 compensator has been chosen. So select the compensator type from the corresponding drop-down menu and parametrize it as done in Figure 10 and Figure 11.
Figure 10: Choosing Type 3 compensator
Figure 11: Configuring Type 3 compensator
The fields Vp, Vv, and tr are referred to the modulator and all of them defined the gain of the modulator.
Once the system has been defined, the crossover frequency and the phase margin of the open-loop must be selected.
SmartCtrl provides a fast shortcut to select the crossover frequency and the phase margin called solutions map. This window has been partially introduced in the previous point.
Each point within the white area corresponds to a combination of cross freq. and phase margin that leads to a stable solution. In addition, when a point is selected, the attenuation given by the sensor and the regulator at the switching frequency is provided. Note that not enough attenuation at the switching frequency could provoke high-frequency oscillations.
To carry out the selection just click on the “Set” button and SmartCtrl will display the solutions map. Then select a point within the white zone (left click) and click OK to continue. See Figure 12 and Figure 13.
Figure 12: Access to the solution map
Figure 13: Defining a point in the solution map
All the points in the white area will be stable what will vary will be the dynamic response of the controller. As a rule of thumb, a good point for controlling is about:
a) A phase margin of about 45 to 60 degrees
b) A cross over frequency = Switching frequency/10
Please, notice this rule of thumb is not always achievable, but the nearest point in the white area will be good enough.
The solutions map will be shown on the right side of the input data window. See Figure 14.
Figure 14: Converter definition window
6. Click OK to confirm the design and the program will automatically show the performance of the system in terms of frequency response, Nyquist plot, transient response, etc.
Additionally, optimization tools such as parametric sweep for sensibility analysis and control loop optimization algorithms, are provided. See the Figure 15.
Figure 15: SmartCtrl results window
SmartCtrl also provides the component values needed to implement the compensator, the voltage divider resistors, and the input parameters like the topology. To obtain those windows, click on the button of Figure 16 and Figure 17:
Figure 16: Report buttons
Figure 17: Input and output data reports
Design exportation to PSIM
To export to PSIM click on this button. Figure 18.
Figure 18: Exportation to PSIM button
And configure the exportation. See Figure 19.
Figure 19: Exportation to PSIM settings
Click OK and PSIM will be automatically opened and the simulation launched.
Design validation
In PSIM, the following system has been automatically created and configured by SmartCtrl. See Figure 20.
Figure 20: PSIM exported system
In this picture, all the different functional blocks have been highlighted. Ensure to configure the simulation time control as follows. Figure 21.
Figure 21: Clock configuration in PSIM
The result of the simulation is shown in Figure 22:
Figure 22: Result of PSIM simulation
As it can be seen, the output voltage is 3.3V, it is exactly what has been specified in SmartCtrl.
If the dynamic response of the system is to be analyzed, it would be necessary to introduce a step, for example, in the input voltage. See Figure 23.
Figure 23: Input voltage step in PSIM
The result of this new system can be seen in Figure 24.
Figure 24: New PSIM simulation where the dynamic response can be seen
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