PSIM switch models: Choose Wisely!

NikosDimitrakopoulos · October 2023

Learning how to navigate through the PSIM switch model options

One of PSIM's main advantages, aside from its speed and robustness, is its flexibility. This flexibility allows you to reuse the same schematic to run simulations with different levels of fidelity and/or varying application goals.

Your main allies in this process are the different switch model options available for your power semiconductors. Take the MOSFET model, for example. By default, when you add a MOSFET to your schematic, the "Ideal" model level is selected. However, there are several other options depending on your specific application needs:

As seen above, four major categories of switch models are offered in PSIM.

A similar approach applies to the IGBTs. However, for IGBTs, the LvL1 and LvL3 options are omitted; you have the "Ideal" model and the higher-order "LvL2" model, along with just one thermal option. The MOSFET has two thermal options, but the most complete and recommended one is the "Thermal (MOSFET (Eon))". The "Thermal (MOSFET)" option (without Eon) should be used only when switching loss data is unavailable.

Let's break these options down in a simple slide:

1. The first model category is the ideal (maybe LvL1 for the Mosfet) switches. This is the fastest option in PSIM as it uses lower-order modeling. It is ideal for analog or digital control design, component and filter sizing, and large system-level simulations where the focus is on the overall system rather than detailed switching behavior.

2. The second model category involves thermal switches and is the recommended option for loss/efficiency calculations. These models provide both switching and conduction losses for all semiconductor paths (active path and reverse diode) and also calculate the temperature of the switch. To ensure accurate temperature calculations, you must also model the cooling correctly using either a 1D RC thermal equivalent or go 3D to your custom air or liquid cooling model via the PSIM-Simlab workflow.

The thermal models offer an efficient way to compute losses without capturing every detail of the switching events. Instead, they use a lookup table approach in the background and a hybrid of the ideal switching method in the foreground. The model relies on datasheet data related to losses. You can import these data as show here and then the model will calculate the losses by interpolation based on voltage, current, gate resistance, and case temperature at each simulation point. Additionally, it can update the MOSFET's Rds(on) resistance based on temperature, adding a "thermal feedback" mechanism to the ideal switching that is used in the foreground. This is why I call it a hybrid of the ideal switching method.

This hybrid approach ensures fast simulation while maintaining high accuracy, as demonstrated in this hardware proof of concept:

[Hardware Results] Thermal Analysis and Design of Inverters - The Myway Case

3. The third model category includes the LvL2 and LvL3 switches, which I call the "heavy lifters". These are higher-order PSIM models (not SPICE) designed to approach SPICE-level modeling while still using the PSIM engine to maintain speed and robustness. They aim to capture the full details of switching and gate-driving phenomena.

To achieve this fidelity, you may need to use a smaller time step compared to the ideal switch model. Additionally, these models require the use of higher-order gate driver models or the "On/Off Controllers" in PSIM. Refer to the article "How to Use the PSIM On-Off Controllers" for more details.

Since they capture complete switching and ripple effects, these models are suitable for conducted or radiated EMI estimation workflows. However, note that using these models with a smaller time step will significantly increase simulation time. Still, it will be much faster than SPICE and most importantly with no loss of robustness - even when small parasitic elements are added. This makes them a good option for users facing slow speeds and crashing issues with SPICE EMC simulations.

Please keep in mind that no model will magically solve your EMC issues. The goal is to identify potential problems early to drive actionable design decisions. An example proof of concept versus real hardware that demonstrates this is shown below:

Time Domain Common-Mode Current Results

Frequency Domain Common-Mode Current Results

4. The fourth model category includes the SPICE model option. The important clarification here is that SPICE models do not run with the PSIM engine. They are high-order, high-fidelity, and maybe the most representative option around as they directly come from the switch manufacturer. However, they are spice-based and the PSIM engine won't work with spice format.

To clarify, PSIM offers three engine options:

a. PSIM engine

b. HyperSpice engine. Altair's spice solver that comes built-in with PSIM. Mostly compatible with LTspice formats.

c. LTspice engine. If you have LTspice installed in your computer already (doesn't come built -in with PSIM), then you will need to specify its installation path in PSIM. Then PSIM will be able to call LTspice, acting just like the middle-man, and have it solve your schematic.

Please remember that PSIM is just calling LTspice & HyperSpice so non-spice compatible blocks won't run. You can identify which blocks are compatible by enabling the "Show image next to elements that can be used with SPICE" option:

There is a relevant article on what you can and what can't do with PSIM and LTspice link.

The SPICE switch models can only run with HyperSpice or LTspice, not with PSIM. Consequently, you won’t benefit from PSIM’s full speed and robustness when using SPICE switch models. But SPICE models offer the highest fidelity, right?

- Well they should, but is it worth it?

There's no simple yes or no answer. It depends on your simulation goals, system complexity, and personal patience.

Ideally, the dream would be to "tune" a PSIM LvL2 model to match the performance of a SPICE model, combining high fidelity with PSIM's speed and robustness. To assist with this, we've included the Double Pulse Test circuit examples, which you can run with either PSIM or SPICE and then compare results.

But I want to be clear: achieving a 100% match may not be always possible!

Let’s discuss two examples:

First, consider the WolfSpeed C2M0280120D SiC MOSFET device.

As shown, there is not a 100% alignment between SPICE and PSIM results. However, the switching waveform derivatives are mostly similar, indicating that CM and DM EMI results should be comparable.

Although some peaks and ripples are missing, the improved response - especially in systems with multiple switches rather than just a half-bridge - can be significantly valuable in certain situations:

A blazing 100x in simulation speed and convergence with parasitics can be a significant advantage. In this case, we have a moderately complex two-level inverter, and SPICE is already struggling. While SPICE might offer higher fidelity results, it can be a pain to use it for simulations involving many switches. That being said, using SPICE vs LvL2 models is always a topic that requires the right amount of compromise.

There are instances, such as with the Infineon CoolSiC IMW120R040M1H device, where results can match 100%. However, I used the Wolfspeed example to avoid giving the impression that this is always the case:

The PSIM LvL2 devices do include higher order modeling parameters like the parasitic capacitance and inductances. LvL3 also account for reverse path details:

Some higher-order parameters might not be available in the datasheet. In such cases, more Altair tools can be leveraged in the process of identifying those parameters and optimizing their selection to have the LvL2/3 models match spice performance.

For instance, you can use "Parasitic Extraction of PCB in Altair Simlab" to estimate additional parasitic inductances around the switch, which affect switching ripple and Vds max voltage:

Additionally, you can leverage Altair HyperStudy [Getting Started with PSIM & HyperStudy] and its optimization along with design exploration features, to come up with a semi-automatic way of tuning the LvL2 device parameters vs a spice benchmark or oscilloscope benchmark. For example, to optimize the “ON” transition, you could follow these steps:

Summary

From control design to thermal efficiency analysis, EMC workflows and SPICE models, PSIM offers a versatile solution for various aspects of power electronics simulations. You can maintain the same schematic while switching between different model levels and solver options.

The key takeaway is to choose the appropriate switch model based on the application and task. Some models may be more optimized for specific jobs, which can significantly impact your simulation experience.

A classic example is loss calculation. Users with a SPICE background might try estimating losses using LvL2 or SPICE models and math formulas. This approach can be much slower and sometimes problematic, if the switching transient is not perfectly captured. Even when the results are close, the speed difference will always be evident:

Useful Links

Ideal Switch Model