4 Designing for Optimal Thermal Performance

The R_θJA value is one metric to quantify a device’s thermal performance. The R_θJA value depends on the power module’s design, as well as the PCB’s design. Having external thermal pads under the package allows good thermal performance by allowing multiple GND vias to heat sink the device to the PCB’s multiple GND layers. Also, having a pin-out that allows large copper planes to connect to the device’s power pins (VIN, GND, VOUT) reduces the R_θJA value. Figure 4-1 shows that the TPSM82866A provides a large thermal pad, while Figure 4-2 shows that the pin-out allows an easy plane connection to the power pins.

Once the power module is designed for good thermal performance, the PCB must be designed to work effectively with the power module to remove the power loss. Thermal vias should be placed beneath the thermal pad to transfer the heat from the power module to the layers within the PCB. Placing multiple vias closely spaced to each other reduces the R_θJA value. However, once a few vias are placed on the thermal pad, the point of diminishing returns is reached and adding more vias does not usually reduce the value significantly. The number of vias shown in the data sheet’s layout example or in the package drawing is a good starting point for achieving good thermal performance. Thermal Performance Optimization of High-Power Density Buck Converters discusses the impact of vias on thermal performance in detail.

In addition to thermal vias, having ground planes on multiple PCB layers and increasing the copper area connected to the power pins of the device helps improve the thermal performance. Adding airflow greatly reduces the R_θJA value. Improving the Thermal Performance of a MicroSiP™ Power Module provides more details on improving thermal performance through thermal vias and additional PCB layers.