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Posted on 01 November 2019

PLECS – The User-friendly Simulation Program for Power Electronics

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A new approach to simulating power electronic circuits together with their controllers

System simulators allow the simple creation of controllers but the power electronics they control must then be described by circuit equations. On the other hand, circuit simulators allow simple circuit creation but the interface to the controller is generally more complex. PLECS however allows the intuitive creation of the circuit on screen, couples it to its controller and achieves fast and stable simulations of the complete system.

By Orhan Toker, PLEXIM GmbH and Eric Carroll, EIC Consultancy

 

The Status Quo in Simulation

Power Electronics (PE) is today so ubiquitous that practically every industry has its own design centers to exploit it: the automobile, aeronautics, medical, domestic-appliance, energy management, renewable energy industries, etc., which has led to the need to develop and evaluate new circuits quickly and efficiently. This, in turn, generalizes the need for PE simulation to the point at which it becomes a “desktop” application as common as Excel or Word – ideally with the same ease of use. Traditionally this has not been the case. A PE circuit, apart from its potential complexity (for example, a snubbered 3-level inverter has over 20 branches, see Figure 1) includes events with a wide span of time-constants ranging from that of microseconds for semiconductor switching events to that of seconds for motor responses to that of minutes for thermal stabilization. This has invariably led to compromises between small integration steps (for good stability and convergence) and short computation time (for analysis and iteration). In general, the complexity of such simulations resulted in the establishment of simulation specialists with design engineers bringing them their problems to process – akin to taking one’s report to the typing pool.

Complete snubbered 3-level inverter with clamp circuit

A New Approach

In order to develop a software tool, which can be used to simulate PE problems ranging from simple circuits to the complex systems of Figure 1, the problem of the disparate time constants must be solved. In PE, the principle cause of short time constants is the fast switching of semiconductor devices such as thyristors, which occurs in the submicrosecond range. In PLECS, this problem is eliminated by considering all semiconductor switches as ideal – a concept which anyone familiar with power semiconductors will, at first, find strange! This is made possible by considering a new set of circuit equations at each switching event, whereby a switching event is deemed “ideal” and hence instantaneous (obviating the need to know how quickly it happens). To achieve this, PLECS creates a “switch manager” from the circuit directly entered by the user and this function oversees the boundary conditions (e.g. “Is the diode current negative?”) invoking the computation of the next set of equations resulting from the new circuit configuration. It is clear that in the circuit of Fig. 1, the precise behavior of the semiconductor switch has little impact on the overall system performance though it may, for instance, determine whether the semiconductor survives the event or fails due to over-voltage. This detailed analysis is best carried out in a separate, open-loop simulation on a single switching cell such as that of Fig. 2. Indeed, an important part of the inverter design is to evaluate the interaction of the semiconductor’s dynamic characteristics with parasitic parameters such as buss-bar inductances.

Detail of switching cell of Fig. 1 in which the unknown stray can be simulated in “open-loop”

The PLECS Environment

PLECS (standing for Piece-wise Linear Electrical Circuit Simulation) operates within Simulink®. Simulink® is an environment for multidomain simulation and Model-Based Design providing an interactive graphical environment and a set of block libraries that allows the simulation of a variety of time-varying systems, including communications, controls, signal processing, etc. Simulink® can also compute electrical circuit responses, provided that the circuits are described by their mathematical equations – a time consuming exercise which requires specialized knowledge. PLECS however, allows the arbitrary composition of circuits from a library of components, voltage and current sources, switches, meters, motors, etc. Simulink® provides the controller inputs, displays the output waveforms from the “meters” and receives whatever feedback signals are required from the PLECS-based circuit.

Simulating Semiconductors

Needless to say, semiconductor switches, which are at the heart of any PE system, are far from ideal and thus designers frequently clamor for a “model” for their prospective semiconductors in the hope of foreseeing all the circuit constraints which will arise from a particular choice of device. It must first of all be recognized that the true simulation of a semiconductor can only be based on physical models requiring not only sophisticated software but also knowledge of device structures and doping profiles – which the device manufacturers are not about to divulge. Therefore, only “behavioral” or “empirical” models should be expected, which will mimic the devices or, at least, part of their data-sheets. It must be remembered, that all device parameters are subject to process distributions. A manufacturer will therefore, either specify the minimal and maximal values of each parameter, or simply specify the “worst” value (e.g. the highest switching loss). In many cases, a datasheet parameter may only be ascribed a “typical” value with no indication of how “good” or “bad” that parameter might be in practice. Semiconductors require many parameter specifications: an IGBT, for instance, is described by some 20 parameters, most of which have voltage, current and temperature dependencies. For this reason, many semiconductor manufacturers are reluctant to offer models for fear that they might not describe the devices in quite the same way as their datasheets do, thus leading to greater, rather than less, frustration on the part of the users. To cap all this, the worst value of one parameter (say turn-off loss) will correspond to the “best” value of another parameter (e.g. longest fall-time) resulting in an optimistically low switching over-voltage.

Since most simulation programs require finite transition speeds for their switches, the demand for potentially misleading models, is understandable and the semiconductor suppliers generally try to oblige. A further reason for wanting a model is that the thermal constraints on the semiconductor can simultaneously be determined during simulation and hence the cooling requirements, calculated. PLECS however, requires no dynamic model for stable operation and does not waste computing time using inaccurate models to calculate losses. The computed currents and voltages of the ideal switches are used to read data from a one, two or three dimensional look-up table containing the datasheet losses of the considered device as illustrated by Figure 3.

2-D Look-up Table taken directly from a semiconductor’s datasheet conduction loss curves

Returning to the circuit of Fig. 2, the open-loop simulation of the switching event does, nevertheless, require a behavioral model and the wise user will exploit it knowingly. First of all, the parasitic components (LS1 – 3) are, at the design stage, just as unknown (but just as important) as the semiconductor’s switching speeds. Thus, the model will be used only to ascertain the interaction of the unknowns and to fix allowable limits for them, which can then be used as a basis for selecting or specifying the devices and optimizing the mechanical construction. PLECS recognizes this requirement and currently offers generic semiconductor models for diode and thyristor reverse recovery as well as MOSFET and IGBT turn-on.

3-D Look-up table derived from a semiconductor’s datasheet switching losses using switching loss curves

Graphical User Interface

The PLECS graphical user interface allows the designer to compose his circuit intuitively by “drag-and-drop” of circuit components, sources, “ammeters” and “voltmeters”, selected from a library. A typical PLECS circuit “window” can be seen in Fig. 4 (yellow background). The switches and meters are then interfaced to signal generators and oscilloscopes respectively, within Simulink® and the resultant waveforms observed. The controls are simulated under Simulink®, which then treats the graphically composed converter circuit under PLECS, as a separate Simulink® subsystem seen as the yellow block in the white Simulink® window.

The PLECS graphically composed circuit window and circuit block

Figure 4 illustrates the main parts of a PLECS simulation: the power converter PLECS Graphic User Interface and the Simulink®control blocks and output “oscillograms”. Meters are added to observe voltages and currents (shown in green) and connected to “Outports” which export the signal for Simulink®to display. A controller is selected from the Simulink®library and connected via an “Inport” to a switch gate (shown in brown).

Profile

PLECS is developed by PLEXIM; a Zurich based company founded in 2002 with the mission to develop and market fast and user-friendly simulation software for the power electronics industry. PLECS is used in over 30 countries around the world both in Industry and in Academia for its speed, stability and ease of use resulting in efficient prototyping and shortened time-to-market. PLECS licenses are available as single, concurrent or classroom versions.

 

 

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2 Responses

  1. avatar sargati says:

    im amazed of the software. how can i have a the full version of the software?

     

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    • avatar PowerDahl says:

      I suggest visiting the Plexim website (www.plexim.com). There is purchase information there and an option for a 30 day trial license for the software free of charge.

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