Thermal Simulation of PTC Heaters

Thermal Simulation of PTC Heaters

The ROHM Solution Simulator, a web-based simulation tool, provides a number of simulation circuits (Solution Circuits). The ROHM Solution Simulator and Solution Circuits can be used simply by registering with MyROHM. You can use them to perform thermal simulations of PTC (Positive Temperature Coefficient) heaters.

This articles describe how to run thermal simulations using the simulation circuits.

This article is based on the “User’s Guide: PTC Heater Thermal Simulation”.
https://fscdn.rohm.com/en/products/databook/applinote/common/ptc_heater_thermal_simulation_ug-e.pdf

Thermal Simulation Circuits and Method for PTC Heaters

We begin with the first item, “Thermal Simulation Circuits and Method for PTC Heaters”.

A PTC (Positive Temperature Coefficient）heater is a self-regulating heater that uses the PTC characteristic according to which resistance increases as temperature rises. This heater has low power consumption because it stabilizes at low power consumption once the temperature reaches its upper limit. This article introduces a simulation environment that enables simultaneous electrical simulation of PTC heaters and temperature simulation of the built-in devices, and explains how to use it. By changing the parameters of the components, simulations can be performed under a variety of conditions.

Example of a Simulation Circuit

A simulation circuit for a PTC heater is shown below. The black and blue lines show the electrical simulation circuit and the red lines show the thermal simulation circuit. In the electrical simulation circuit, three insulated gate bipolar transistors (IGBTs), which serve as switches, are connected in parallel. One load resistor (heater) is connected to each IGBT and driven independently. The load current is adjusted in only three levels by turning the three IGBTs on and off, with no fine-tuning by switching. The circuit also provides overcurrent protection by detecting the total current of the three loads with a shunt resistor. The thermal simulation circuit calculates the temperature of the IGBTs and shunt resistor by modeling (ROM*1) the built-in device losses calculated by the electrical simulation and a typical PTC heater (including a water-cooled environment) for thermal simulation.

*1 ROM（Reduced Order Model）：A model using a method of reducing the dimensions of a model created by 3D-CAE to 1 dimension.

Simulation circuit

The four graphs on the right side of the figure above show the junction temperatures (Tj) of the three IGBTs and the shunt resistor. After startup, the junction temperatures of the IGBTs rise with time, and after about 4,334 seconds, they stabilize at approximately 125°C. The center graph shows the value of the current flowing through the shunt resistor RSHUNT (total current of the three loads), and the left graph shows the output voltage of the overcurrent protection circuit. The values in the center and left graphs are stable about 2,000 seconds after startup.

How to Run Simulation

Simulation Settings and Run

To run a thermal simulation, click the Run icon ▶ shown above. Simulation conditions can be set from the Simulation Settings icon. In the initial state, the simulation conditions have already been set and the simulation results are displayed. If you run a thermal simulation after changing the simulation conditions, the temperature graphs and other data will be updated.

Parameter Setting, Overcurrent Protection and Thermal Simulation Model

This section explains the second topic, “Parameter Setting, Overcurrent Protection and Thermal Simulation Model”.

Simulation Conditions

Definition of component parameters

The simulation time, convergence options, and other simulation conditions can be set with the Simulation Settings icon introduced in the above section. The table below shows the initial simulation conditions. If there is a problem with simulation convergence, you can change “Advanced Options” to fix it. Simulation temperatures and various parameters for electrical circuits can be defined in “Manual Options”.

Initial values for Simulation Settings

Parameter Setting

For the components shown in blue in the diagram above, parameters need to be set in the “Manual Options”. The table below shows the initial values of the parameters. Enter parameter values in the text box in “Manual Options”

Initial Values of Parameters

Definition of parameters

Overcurrent Protection

The overcurrent protection circuit is shown in the figure below. The load current is detected by the low-side sensing circuit using a shunt resistor and an op-amp. The total current flowing through the load (I_LOAD) generates a voltage of ΔV_SHUNT via the shunt resistor. This voltage is differentially amplified by the op-amp, and if it exceeds the threshold of the “Voltage to Digital” stage, the next stage switch is turned on to activate overcurrent protection. If the input offset voltage of the op-amp is ignored, the output V_O of the op-amp can be expressed as:

\(V_O = I_{LOAD} \times R_{SHUNT} \times \displaystyle \frac{R2}{R1} \quad [V]\)

In the default circuit, I_LOAD =30 A, R_SHUNT =1 mΩ, R1=2 kΩ, and R2=120 kΩ, resulting in an output of V_O =1.8V. The “Voltage to Digital” threshold is set to 2 V (overcurrent ≈ 33.3 A), thus overcurrent protection is not activated.

Overcurrent protection circuit

Thermal Simulation Model

The diagram below shows a PTC heater as a thermal simulation model. This model is a Reduced Order Model (ROM) that uses a method of reducing the dimensions of a model created by 3D CAE to one dimension. The terminals of the PTC heater model are also described in the table below.

Thermal simulation model

Terminal description of the thermal simulation model

• • The S_S_xxxx terminal can be used to monitor device temperature by providing the device losses.
• • The F_xxxx terminal is connected to “tc_amb” and is set to the temperature at the location.
• • The S_M_xxxx pin can be used to monitor the mold temperature of the IGBT or the lead temperature of the shunt resistor.

Components and Product Name List

This section concerns the third topic, “Components and Product Name List”.

Figure 1 shows a simulation circuit of a PTC heater. The main components used for thermal simulation, such as Q1 and RL1, are shown here. See Table 1 for the initial values of each component. Some of the components can be selected from a ‘Product name list’ that is pre-set with the available components. The components that can be selected from the Product name list are listed in Table 2. To change to a component in the Product name list, right-click on the component and select “Properties” to open the “Property Editor” as shown in Figure 2. Then, select the product name of the device you want to use from “Spicelib Part” in the “Property Editor”.

Figure１. Main components of a PTC heater simulation circuit

Table 1. Initial values for the components

Table 2. Components that can be selected from ‘Product name list’

Figure 2. How to change the component

・Links to related documents

• Product specifications:
  IGBT（TO247 package）
  Shunt resistors (PSR series)
  Ground sense operation amplifier
• Application note:
  Low-Side Current Sensing Circuit Design

3D Model for Thermal Simulation of PTC Heaters

This section explains the final topic, “3D Model for Thermal Simulation”.

Thermal simulation of PTC heaters employs a 3D model to create a Reduced Order Model (ROM) which is used as a thermal simulation model. The ROM is a model using a method of reducing the dimension of a 3D-CAE model to one dimension. The 3D model and the structural information of the PTC heater are shown below.

3D image of a PTC heater

PTC Heater Structure Information:

*For time efficient simulations, the thermal capacity of the aluminum housing is not taken into account.

This article is based on the following “User’s Guide: PTC Heater Thermal Simulation”.

https://fscdn.rohm.com/en/products/databook/applinote/common/ptc_heater_thermal_simulation_ug-e.pdf