Intelligent Power Modules (IPMs) are widely regarded as the preferred solution for low-power motor drive applications, particularly where cost and space constraints are critical. Recent thermal studies of IPMs under different operating conditions provide valuable insights that help engineers accurately predict operating temperature, power dissipation, and PCB design requirements, enabling an optimal balance between reliability, cost, and system size.
Motor controllers used in home appliances and light industrial drives typically adopt Intelligent Power Modules (IPMs). These modules integrate:
- Gate drivers based on HVIC technology
- Power switching devices forming half-bridge or three-phase bridge structures
- Integrated protection circuits
An IPM connects directly between the motor and the processor running the motor control algorithm. Depending on system configuration, a single module can replace more than 30 discrete components.
As a highly integrated solution, IPMs offer several advantages:
- Simplified circuit design
- Reduced bill of materials (BOM) cost
- Lower PCB space requirements
- Improved system reliability
- Reduced electromagnetic interference (EMI)
In many applications, IPMs are designed to operate without an external heatsink, which further reduces system cost and simplifies assembly. However, careful thermal design is still required to ensure that the module maintains a safe steady-state temperature at maximum load, meeting long-term reliability targets.
For example, compact IPM solutions such as those used in inverter systems for HVAC equipment, fans, pumps, compressors, and variable-speed drives in the 150W–250W power range often rely on PQFN packages for efficient PCB-based heat dissipation. Modules from companies such as Infineon adopt optimized package structures that transfer heat through large electrical pads to the PCB copper layers.
The Role of PCB Copper Layout in Thermal Performance
The size and thickness of PCB copper traces significantly influence the module’s ability to dissipate heat to the surrounding environment, which directly determines the steady-state operating temperature.
If the copper layout is undersized, system reliability may be compromised due to excessive temperature. On the other hand, oversized copper areas may unnecessarily increase system cost and physical size.
To help designers achieve an optimal balance, engineers at SHYSEMI conducted a series of controlled experiments. By measuring steady-state temperatures across different power levels and PCB layouts, they established temperature–power relationship curves that serve as a practical design reference.
Using these curves, motor drive designers can:
- Optimize thermal design for specific applications
- Select the appropriate power rating
- Estimate the expected operating temperature of the IPM
Experimental Setup for IPM Thermal Measurement
The experiment used a controlled current injection method.
A known current was injected through the body diodes of two MOSFETs forming an inverter bridge arm. By adjusting the injected current, researchers could analyze the relationship between:
- PCB metallization (copper layout)
- Module operating temperature
- Power dissipation
The voltage drop across the two diodes corresponds to the voltage drop across the module along that path. By measuring this voltage, the power dissipation of the IPM can be calculated.
The simplified test circuit is shown in Figure 1.
Figure 1 Simplified circuit diagram for current injection testing.
Using this method instead of analyzing a full inverter driving a motor offers significant advantages. The experimental setup is simpler and easier to control, eliminating interference factors such as:
- Parasitic inductance and capacitance
- Voltage and current spikes
- Electrical noise
Because the experiment focuses on temperature changes caused by power dissipation, the DC current injection approach provides sufficiently accurate results.
PCB Copper Layout Configurations
Six PCB metallization patterns with different copper sizes and thicknesses were evaluated to assess thermal performance.
Table 1 summarizes the tested configurations.
Table 1 PCB copper traces used in the experiment had thicknesses of:
- 1 oz (35 μm)
- 2 oz (70 μm)
combined with three different copper area sizes.
Experimental Results
For each PCB design, the injected current through the inverter bridge diode was varied while recording:
- Test current
- Voltage drop
- Module case temperature
- Ambient temperature
From these measurements, the relationship between power dissipation, PCB design, and operating temperature was analyzed.
Figure 2 illustrates the relationship between power dissipation and the temperature rise from case to ambient (ΔTc-a) for the different PCB metallization patterns.
Because PQFN packages typically exhibit a very low junction-to-case thermal resistance (Rθjc) of approximately 2.2°C/W, it can be assumed that under steady-state conditions:
Tc ≈ Tj
In other words, the case temperature closely approximates the junction temperature. This characteristic allows IPM solutions from SHYSEMI to provide more accurate thermal performance predictions.
Figure 2 Case-to-ambient temperature rise versus power dissipation for different PCB copper patterns.
Impact of Forced Air Cooling
In applications such as fan control systems, the operating fan itself may provide additional airflow over the IPM surface. This cooling effect should also be considered during thermal design.
To evaluate such scenarios, the test board was placed in an enclosed chamber with airflow across the module surface ranging from 0.8 m/s to 1.2 m/s, measured using an anemometer.
Figure 3 compares the thermal performance of two PCB copper layouts under natural convection and forced-air cooling conditions.
Figure 3 Comparison of thermal performance with and without forced airflow in fan control applications.
Thermal Capacitance and Transient Behavior
In addition to steady-state thermal performance, it is often useful to predict how the system behaves during the warm-up phase from startup to thermal equilibrium.
This transient behavior can be modeled using a thermal resistance–thermal capacitance (RC) network. By calculating the system’s thermal time constant, engineers can estimate the module’s case temperature at any time during the startup process.
For designs using IPMs from SHYSEMI, accurate knowledge of these thermal parameters helps further optimize system reliability.
To measure thermal capacitance, the experiment used the PCB layout with the smallest copper area. A step current change was applied, and the module case temperature was recorded from the moment the step occurred until thermal equilibrium was reached.
Since the thermal resistance values at the initial and final temperatures were known, the thermal capacitance (Cth) could be calculated by measuring the system’s time constant (τ).
Figure 4 illustrates the thermal response of the system from startup to steady state.
Figure 4 Thermal response of the system during startup, showing a time constant on the order of several minutes.
Conclusion
Intelligent Power Modules used in many low-power motor drives adopt advanced packaging technologies that combine high thermal efficiency with compact size.
Because these IPMs are typically designed to operate without external heatsinks, the heat dissipation capability provided by PCB copper traces becomes a key factor determining power handling capability and long-term reliability.
Through experimental evaluation and modeling of steady-state thermal performance and thermal capacitance across various PCB layouts, engineers at SHYSEMI have developed a set of practical thermal characteristic curves.
These curves allow engineers to accurately predict system thermal behavior, helping them design more reliable and cost-effective IPM-based motor drive solutions for modern electronic systems.
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