Implement thermal calculation functions

Pure functions for first-order thermal response calculations:
- Temperature derivative and update using Euler integration
- Case temperature with self-heating via θ_ca
- Junction temperature calculation via θ_jc
- Steady-state junction temperature helper

src/py_dvt_ate/simulation/physics/thermal.py

@@ -0,0 +1,193 @@
+"""Thermal calculation functions for physics simulation.
+
+Pure functions implementing first-order thermal response calculations
+for the coupled thermal-electrical simulation.
+
+All temperatures are in degrees Celsius.
+All time values are in seconds.
+All power values are in watts.
+All thermal resistances are in degrees Celsius per watt (°C/W).
+"""
+
+
+def calculate_temperature_derivative(
+    current_temperature: float,
+    target_temperature: float,
+    time_constant: float,
+) -> float:
+    """Calculate the rate of temperature change for first-order response.
+
+    Implements: dT/dt = (T_target - T_current) / τ
+
+    Args:
+        current_temperature: Current temperature in degrees Celsius.
+        target_temperature: Target temperature in degrees Celsius.
+        time_constant: Thermal time constant in seconds.
+
+    Returns:
+        Rate of temperature change in degrees Celsius per second.
+
+    Raises:
+        ValueError: If time_constant is not positive.
+    """
+    if time_constant <= 0:
+        msg = f"Time constant must be positive, got {time_constant}"
+        raise ValueError(msg)
+
+    return (target_temperature - current_temperature) / time_constant
+
+
+def update_temperature(
+    current_temperature: float,
+    target_temperature: float,
+    time_constant: float,
+    dt: float,
+) -> float:
+    """Calculate new temperature after a time step using Euler integration.
+
+    Args:
+        current_temperature: Current temperature in degrees Celsius.
+        target_temperature: Target temperature in degrees Celsius.
+        time_constant: Thermal time constant in seconds.
+        dt: Time step in seconds.
+
+    Returns:
+        New temperature in degrees Celsius after the time step.
+
+    Raises:
+        ValueError: If time_constant or dt is not positive.
+    """
+    if dt <= 0:
+        msg = f"Time step must be positive, got {dt}"
+        raise ValueError(msg)
+
+    derivative = calculate_temperature_derivative(
+        current_temperature, target_temperature, time_constant
+    )
+    return current_temperature + derivative * dt
+
+
+def calculate_case_temperature_derivative(
+    case_temperature: float,
+    ambient_temperature: float,
+    power_dissipation: float,
+    time_constant: float,
+    theta_ca: float,
+) -> float:
+    """Calculate rate of case temperature change including self-heating.
+
+    Implements: dT_case/dt = (T_ambient - T_case + P_diss × θ_ca) / τ_case
+
+    The case temperature is driven by:
+    - Convection/conduction to ambient (chamber) temperature
+    - Self-heating from power dissipation through case-to-ambient thermal resistance
+
+    Args:
+        case_temperature: Current case temperature in degrees Celsius.
+        ambient_temperature: Ambient (chamber) temperature in degrees Celsius.
+        power_dissipation: Power dissipated by the device in watts.
+        time_constant: Case thermal time constant in seconds.
+        theta_ca: Thermal resistance from case to ambient in °C/W.
+
+    Returns:
+        Rate of case temperature change in degrees Celsius per second.
+
+    Raises:
+        ValueError: If time_constant is not positive.
+    """
+    if time_constant <= 0:
+        msg = f"Time constant must be positive, got {time_constant}"
+        raise ValueError(msg)
+
+    # The effective target includes self-heating contribution
+    thermal_drive = ambient_temperature - case_temperature + power_dissipation * theta_ca
+    return thermal_drive / time_constant
+
+
+def update_case_temperature(
+    case_temperature: float,
+    ambient_temperature: float,
+    power_dissipation: float,
+    time_constant: float,
+    theta_ca: float,
+    dt: float,
+) -> float:
+    """Calculate new case temperature after a time step.
+
+    Args:
+        case_temperature: Current case temperature in degrees Celsius.
+        ambient_temperature: Ambient (chamber) temperature in degrees Celsius.
+        power_dissipation: Power dissipated by the device in watts.
+        time_constant: Case thermal time constant in seconds.
+        theta_ca: Thermal resistance from case to ambient in °C/W.
+        dt: Time step in seconds.
+
+    Returns:
+        New case temperature in degrees Celsius after the time step.
+
+    Raises:
+        ValueError: If time_constant or dt is not positive.
+    """
+    if dt <= 0:
+        msg = f"Time step must be positive, got {dt}"
+        raise ValueError(msg)
+
+    derivative = calculate_case_temperature_derivative(
+        case_temperature,
+        ambient_temperature,
+        power_dissipation,
+        time_constant,
+        theta_ca,
+    )
+    return case_temperature + derivative * dt
+
+
+def calculate_junction_temperature(
+    case_temperature: float,
+    power_dissipation: float,
+    theta_jc: float,
+) -> float:
+    """Calculate junction temperature from case temperature and power.
+
+    The junction temperature is assumed to respond instantaneously to
+    changes in case temperature and power dissipation (no thermal mass
+    at the die level for this simplified model).
+
+    Implements: T_junction = T_case + P_diss × θ_jc
+
+    Args:
+        case_temperature: Case temperature in degrees Celsius.
+        power_dissipation: Power dissipated by the device in watts.
+        theta_jc: Thermal resistance from junction to case in °C/W.
+
+    Returns:
+        Junction temperature in degrees Celsius.
+    """
+    return case_temperature + power_dissipation * theta_jc
+
+
+def calculate_steady_state_junction_temperature(
+    ambient_temperature: float,
+    power_dissipation: float,
+    theta_jc: float,
+    theta_ca: float,
+) -> float:
+    """Calculate the steady-state junction temperature.
+
+    At steady state, the case temperature reaches equilibrium where
+    the heat flow through θ_ca equals the power dissipation.
+
+    T_case_ss = T_ambient + P_diss × θ_ca
+    T_junction_ss = T_case_ss + P_diss × θ_jc
+                  = T_ambient + P_diss × (θ_jc + θ_ca)
+
+    Args:
+        ambient_temperature: Ambient (chamber) temperature in degrees Celsius.
+        power_dissipation: Power dissipated by the device in watts.
+        theta_jc: Thermal resistance from junction to case in °C/W.
+        theta_ca: Thermal resistance from case to ambient in °C/W.
+
+    Returns:
+        Steady-state junction temperature in degrees Celsius.
+    """
+    return ambient_temperature + power_dissipation * (theta_jc + theta_ca)