Non-ideal and Thermal Switch Models Simulation in PSIM

Albert_Dunford
Albert_Dunford
Altair Employee
edited June 2023 in Altair HyperWorks

Ideal switches are great for most simulations. When details are required you need a non-ideal switch model that will actually converge.
You don’t always need a realistic switch transition for your design work, but voltage overshoot, electromagnetic interference – EMI filter design, and other transient interactions can be the difference between a working product and a blown-up prototype in the garbage. A further complication is when we need to understanding the switching and conduction losses of devices. Simulating a high fidelity switching transition can be very computationally intensive PSIM has thermal models to speed this up, providing accurate loss calculations with efficient simulation.

What can we learn with non-ideal switch models?

  • Voltage overshoot
  • Electromagnetic Interference – EMI
  • Efficiency
  • Gate drive requirements
  • Interactions in long cables - radiated EMI
  • Other high-frequency interactions with parasitic inductors and capacitors

When you do include a non-ideal switch in your simulations, you need to ensure that the results resemble reality. Also, the addition of many small L and C values combined with high-frequency ringing can cause numerical instability so you need to ensure that you use a robust solve engine like PSIM.

image

Meaningful results without stability issues

A non-ideal switch model should simulate a device transition similar to how the real hardware will work. However, the model should not be so complex that the simulation takes forever to complete with questionable numerical stability. We have models that provide meaningful results without simulation stability issues.

Non-Ideal Switch Simulation

We recommend the use of ideal switches for most simulations. However, the detail of the actual switch transition can make or break your power converter, literally. You need to observe voltage overshoot, coupling through the miller capacitance, snubber design, switching losses, and interactions with parasitic inductors and capacitors throughout the circuit. A non-ideal switch also has a realistic dv/dt & di/dt during turn on/turn off; this is critical to properly analyze transients and radiated emissions in cables and conducted EMI.

We provide several options to model a non-ideal transition with different levels of precision. PSIM has a level 2&3 switch model which will provide a switch transition that is comparable to a SPICE model. The PSIM level 2&3 model requires a proper gate drive circuit and a model is available for Si MOSFETs, SiC & GaN FETS, & IGBTs. PSIM also has a level 2 diode which models reverse recovery. In addition, you can link from PSIM to LTspice and use the manufacturers SPICE models directly. The advantage of the PSIM model is it’s convergence and numerical stability.

image

Switching and Conduction Losses


An important part of any converter will always be efficiency and the main source of losses will be the switches changing state. The two loss types for switches are conduction losses and switching losses. Current, voltage, switching speed, junction temperature, and gate drive are all factors that contribute to the losses. You need to be able to:

  • ensure safe operating limits for the device is respected
  • compare losses between devices and operating conditions
  • determine the heatsink requirements
  • calculate efficiency


Switching losses can be a tough problem for simulation tools to solve. To simulate switching losses properly an accurate non-ideal switch transition is required, which can require a nanosecond or smaller timestep to properly resolve. This very small timestep is not very practical if you want to look at losses for one cycle of the fundamental at 60Hz for a PFC; the simulation would be very long to solve.

 

PSIM has special thermal models that treat the switch transition as ideal and then compute the losses from a lookup table while considering: voltage across the switch, current through the switch, junction temp, and gate drive circuit. These thermal switches provide excellent correlation with the real world losses with a fraction of the computation requirements of a more realistic switch transition.

PSIM thermal models: IGBT, MOSFET, Wide bandgap (SiC & GaN) FETs, Diode. Also, inductors and with core and winding losses

image

Electromagnetic Interference and Filter Design

Real products need to pass EMI certification. EMI certification has two components: radiated and conducted emissions. We can help you study and mitigate your conducted emissions. Studying conducted EMI requires having the necessary parasitic components for differential and common mode noise and a realistic switch transition.

PSIM level 2 models can provide the realistic switch transitions and parasitic elements can be added to the circuit. However, parasitic capacitors to ground, wire inductance values, and similar parasitic elements are typically very small, these small values can cause difficult and very slow simulations. The strength and robustness of PSIM’s solve engine is put on full display in these simulations as PSIM is able to work through the simulation without numerical or convergence issues.

 

Make use of our EMI Design Suite to:

  • measure common mode and differential noise
  • overlay and compare with the appropriate FCC, CSPR, MIL, or other standards
  • automatically generate a filter to mitigate the noise

Learn More:

Tagged:

Categories