The simple explanation is that you can decide to use a spice engine to run a simulation that has been built using the PSIM schematic editor, but there is no co-simulation between the PSIM engine and SPICE. Here is a tutorial video to help you get started.
A basic example is that a resistor defined with a PSIM schematic element will be converted into a spice resistor element. There is a selection of PSIM elements that are SPICE compatible, like the resistor the conversion can be very simple, for other elements a subcircuit is used in SPICE to represent what was a single element in PSIM. There are PSIM elements that are not SPICE compatible, these will typically be motor, digital control, and code generation elements. You must build a simulation with 100% spice compatible elements if you want to be able to run it with SPICE.
What are the options to simulate a PSIM schematic? There are three, PSIM, LTspice and Hyperspice
You can use the PSIM engine, the PSIM solve algorithm is not SPICE based and is typically much faster with better convergence than SPICE solvers. Please watch this webinar to learn more. Hyperspice was included in PSIM in the 2022.3 release, this is the Altair SPICE engine. PSIM can also generate an LTspice compatible netlist which can be run in LTspice. If you set the LTspice executable path in PSIM, LTspice will be called at the command line and the results loaded back into PSIM providing a seamless experience. You can also generate a generic SPICE netlist to run on other SPICE engines but you will have to inspect the syntax to ensure it is compatible.
There is no way to automatically import a SPICE model and convert it into elements that PSIM can simulate. There are two main model types of interest, PWM control ICs and switch models, both need to be manually recreated within PSIM.
There is a tuning process to fit a PSIM level 2 or level 3 model to model a diode with reverse recovery, a MOSFET, or an IGBT. In most cases a PSIM model that closely resembles the SPICE model can be made.
For PWM control ICs, the functional block diagram can be recreated within PSIM which can represent the functionality of the SPICE device.
I will mention again that you cannot simulate digital control, motor drives, or c code with the SPICE link.
There is no denying that there are many elements and models created for SPICE, with PSIM we provide users with the ability to link a PSIM schematic element with a spice model allowing you to use PSIM for design and then switch to manufacturers SPICE models without needing to recreate everything in a new simulation tool. However, SPICE is known for convergence issues and slow simulation speeds of switching power converters. We have not fixed this, but hopefully being able to verify the operation in PSIM before switching to SPICE will make you SPICE simulation troubleshooting easier.
For complex topologies convergence times should mean that you use PSIM instead of SPICE, so you should be looking to model your entire system with level 2/3 PSIM devices to ensure trouble free and fast simulations.
If you are hoping to model losses and efficiency you should consider our thermal models