A Comparison of Power-Electronics Simulation Tools

	ABOUT THE AUTHORS José Rodríguez has a Doktor Ingenieur degree in Electrical Engineering. He has worked for 25 years in power electronics doing research, teaching, and in industrial applications. Dr. Rodriguez has more than 100 publications in the power-electronics field. He is currently professor and head of the Department of Electronics at the Universidad Técnica Federico Santa María. Alejandro Weinstein is an electrical engineer and graduate student at the Department of Electronics at Universidad Técnica Federico Santa María. Pablo Lezana is a graduate student at Universidad Técnica Federico Santa María.

Modern power-electronic systems exhibit a strong interaction between voltage sources, loads, power semiconductors, and control circuits. This element interaction is complex, due to the nonlinear behavior of the power semiconductors and the different magnitudes of the circuit's time constants. Due to this complex interaction, simulation is almost the only way to study the behavior of power-electronic systems prior to prototyping.

Several simulators, each with different capabilities, are available. We assessed four types of commercial simulation programs designed to simulate power electronic systems on a single-phase boost rectifier circuit:

1. Equation solver: Matlab: Simulink Version 5.3.0
2. Circuit-equation solver: Matlab: Power System Blockset Version 1.1
3. General circuit solver: PSpice (Microsim 8)
4. Power-electronics-specific circuit solver: Simplorer 4.2

• A fast current-control loop that can be implemented with a Bang-Bang controller or a proportional-integrative (PI) controller plus pulse-width-modulation (PWM) modulator.
• A slow DC voltage-control loop.
• Use of fast-switching power semiconductors.

The purpose of the boost rectifier is to generate a controlled DC voltage (v_o) and a sinusoidal input current (I_s).

Figure 1:  Boost-rectifier power circuit (a) and control system (b).

These tasks are done by changing the conduction state of transistor T1, which operates in the switching mode (on/off). When T1 is on, the single-phase power supply is short-circuited through inductance L, increasing current I_L. When T1 is off, the inductor current flows through diode D, charging capacitor C. In this case, the inductor current decreases due the output voltage (v_o). This voltage should be at least 10% higher than the peak value of the source voltage (v_s) in order to assure good control of the current.

The equations describing this rectifier are:

Figure 1b shows the control system of the rectifier, which includes a PI controller to regulate the output DC voltage. The reference value I_Lref for the inner control loop is the product of the output of the PI controller and the normalized voltage v_f. A hysteresis controller provides a fast control for inductor current I_L.

Voltage Source	vf	220 V, 50 Hz
Inductor	L	6 mH
Resistor	R	50-100
Capacitor	C	1 mF
Voltage Reference	vo ref	380 V
Hystersis		1.2

Table 2:  Circuit values used in simulating the single-phase boost-rectifier power circuit

The simulation with Matlab-Simulink was performed with a fixed-step algorithm to avoid convergence problems. In PSpice, the voltage tolerance (VNtol) was 10^-6, while the maximum number of iterations used in Simplorer was 20.

^* Other algorithms are available, but this one is recommended

Table 3:  Simulation conditions for the single-phase boost-rectifier power circuit.

All the simulators in this comparison generated the same results, shown in Figures 5 and 6. Figure 5 shows the dynamic behavior of the output voltage v_o and inductance current I_L in response to step-changes in the load (from 100 to 50 and back to 100 ). Table 4 shows the time needed to simulate the rectifier during this operation. Figure 6 presents the steady-state behavior of input voltage and current.

Figure 5:  Dynamic behavior of inductor current (I_L) and output voltage (v_o) due to changes in load resistance.

Figure 6:  Steady-state behavior of input voltage (v_s) and input current (I_s).

Table 4:  A comparison of the four simulators shows substantial differences in run times, difficulty of use, and control-element availability. All simulations were done on a Pentium III 500 MHz computer with 64 Mbytes of memory. ^* Does not include display time.

The Matlab environment is well known for its very powerful control tools. Simplorer can use these tools through the Sim2Sim module to increase its control library—this capability lets Simplorer implement the standard controllers used in power electronics. Finally, PSpice's control features are limited and are more difficult to implement, making PSpice a poor simulation alternative. Table 4 shows that Matlab-Simulink is the most difficult to use, while Simplorer is the simplest.

Given the previously stated particularities of power-electronic systems and considering the results of this work, the authors conclude that a power-electronics-specific circuit simulator offers the most convenience. This type of program simulates the power and control parts of the circuit very well. In addition, these types of simulators have low execution times, are numerical robust, and are user friendly.