Convergence Issues with Controllable Switch and Square Wave using MTK - iflifting

Context

I’m trying to simulate a switching circuit using an External Control Signal and a Controllable Switch. The Switch is designed as a non-ideal resistor where the resistance value varies from high to low in response to the Control Signal.

      +V  
        |  
       ┌┴┐     Control (Square Wave)
       │S│<─────●  
       └┬┘            
        |            
       [R]  ← Load resistor R  
        |            
       GND

Description

I tried a simple test circuit with two approaches for the Control Signal: a Sinusoidal Wave and a Square Wave. The Sinusoidal Wave works correctly and the system behaves as expected without any convergence issues. However, when the Square Wave is applied, there is a convergence problem stopping the execution very early (likely due to the nonlinearity induced by this signal). There isn't error messages prompted but two warnings :

┌ Warning: Rosenbrock methods on equations without differential states do not bound the error on interpolations.
└ @ OrdinaryDiffEqCore C:\Users\jnmor\.julia\packages\OrdinaryDiffEqCore\zs1s7\src\solve.jl:106

┌ Warning: At t=0.0, dt was forced below floating point epsilon 5.0e-324, and step error estimate = NaN. Aborting. There is either an error in your model specification or the true solution is unstable (or the true solution can not be represented in the precision of Float64).
└ @ SciMLBase C:\Users\jnmor\.julia\packages\SciMLBase\u2Ue2\src\integrator_interface.jl:623

As part of the suggestion to work around the issue, I tried to include the iflifting function, but when I include that into my code, I'm getting the error :

ERROR: BoundsError: attempt to access 0-element Vector{Int64} at index [1]

Expected behavior

Ideally, the model would converge, showing squared voltage on the Load resistor.
The main goal is to be able to model a Switching Circuit based on MTK.

Code

Here’s the code of the circuit model:

@mtkmodel R_Test begin
    @parameters begin
        RLoad_param = 10, [description = "Output Load (Ohm)"]
        Vin_param = 200, [description = "Vin Souce (Vdc)"]
        f = 100, [description = "Switch input Frequency (Hz)"]
    end

    @components begin
        switch_input = squareWave(frequency = f, amplitude = 1, offset = 0, dutyCycle = 0.3)
        #switch_input = Sine(frequency = f, amplitude = 0.99, offset = 1.0)
        voltage_input = Constant(k = Vin_param)
        switch = IdealSwitch(Ron = 1e-3, Roff = 1)
        source = Voltage()
        RLoad = Resistor(R = RLoad_param)
        ground = Ground()
    end

    @equations begin 
      connect(voltage_input.output, source.V)
      connect(switch_input.output, switch.conduction)
      connect(source.p, switch.p)
      connect(switch.n,RLoad.p)
      connect(source.n, RLoad.n, ground.g) 
    end
end

The Switch is defined as

@mtkmodel IdealSwitch begin
  @extend v, i = oneport = OnePort()  
  
  @parameters begin 
    Ron = 1e-3, [description = "On resistance magnitud"]
    Roff = 1e6, [description = "Off resistance magnitud"]
  end

  @components begin
    conduction = RealInput() 
  end

  @variables begin
    q(t), [description = "Control Signal "]
    R(t), [description = "Resistance "]
  end
  
  @equations begin
    q ~ conduction.u
    R ~  Ron + q * Roff
    v ~ i * R
  end
end

The Square Wave is defined as

@component function squareWave(;name, frequency,  amplitude = 1, offset = 0, dutyCycle = 0.5)
    @named output = RealOutput()
    pars = @parameters  amplitude=amplitude frequency=frequency dutyCycle=dutyCycle
    equation =  amplitude * mod1(t, 1/frequency) < dutyCycle/frequency
    eqs = output.u ~ equation
    compose(System(eqs, t, [], pars; name = name), [output])
end

and the solver configuration is

@named sys_dec = R_Test()
sys = structural_simplify(sys_dec, additional_passes = [IfLifting])
prob = ODEProblem(sys, Pair[], (0.0, 100e-3); guesses = [sys.switch.i => 5.0])
sol = solve(prob, Rodas5();
            reltol=1e-8,
            abstol=1e-10,
            dt=1e-7,        
            dtmax=1e-5,     
            saveat=1e-5)

Thanks for your help!

Start with the R_Test model, its IdealSwitch definition, and the squareWave component, then inspect structural_simplify with the IfLifting pass and the Rodas5 solve configuration. Reproduce the BoundsError and convergence warnings using the supplied code. Done means the switching model runs without the reported failure and produces the expected load-voltage behavior, or the issue is narrowed to a documented component or solver interaction.