Thermal Resistance Interpretation of Power Semiconductors and Thermal Design Guidelines
Thermal Resistance Interpretation of Power Semiconductors and Thermal Design Guidelines
Thermal resistance is a core parameter for evaluating power semiconductor heat dissipation performance. It describes the temperature rise generated per unit power consumption inside the device. Many designers only focus on the rated current and voltage of MOSFETs and diodes, ignoring thermal resistance derating. Excessive junction temperature will accelerate parameter drift, trigger thermal shutdown and cause permanent device failure.
Comparison of Thermal Resistance Parameters
| Parameter Symbol | Definition | Application Remarks |
| Rth(jc) | Junction to Case Thermal Resistance | Determined by chip packaging, cannot be changed externally |
| Rth(cs) | Case to Sink Thermal Resistance | Affected by thermal interface material and fastening pressure |
| Rth(sa) | Heat Sink to Ambient Thermal Resistance | Depends on radiator size, air flow and structure |
| Rth(ja) | Junction to Ambient Thermal Resistance | For surface mount devices without heat sink |
Common Causes of Over-Temperature Failure
Insufficient derating: continuous power loss is too close to the maximum power limit of the component
Poor thermal interface: air gaps between the device and heat sink increase thermal resistance sharply
Inadequate copper area: SMD devices rely on PCB copper for heat dissipation, small pads lead to heat accumulation
Local hot spot stacking: multiple power components are placed closely and heat interferes with each other
Degradation Mechanism under High Junction Temperature
High junction temperature raises the on-resistance of MOSFETs and forward voltage of diodes, creating a positive thermal feedback loop. Long-term operation above the rated temperature will degrade the packaging epoxy and bonding wire, increasing the risk of open circuit. Temperature drift also disturbs internal reference sources and causes control loop instability.
Heat Dissipation Design Principles
Calculate the total power loss and estimate junction temperature before schematic finalization. Choose suitable thermal grease or thermal pad to fill contact gaps. For SMD power devices, design large thermal pads and connect multiple thermal vias to inner ground planes. Reserve enough spacing between heating components to avoid thermal coupling.
Test and Validation Tips
Thermal imaging cameras are useful to capture hot spots under full load and maximum ambient temperature. Do not judge thermal status only by case temperature; the junction temperature is always higher. In sealed enclosure applications, the internal air temperature rises significantly, so extra thermal margin must be reserved in the design.