Thermal Performance Optimization of High Power Density Buck Converters

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1 Introduction

For several years Quad-flat no-leads (QFN) package with thermal pad have been standard starting point for high current designs. Technology and trends pushed buck converters to get even smaller and more efficient. Therefore, optimized PCB designs need to be considered to dissipate the heat efficiently through the PCB and enable low operating temperatures.

Especially for thermally-limited designs, mitigating the heat effectively from the Integrated Circuit (IC) through the Printed Circuit Board (PCB) has become paramount. The PCB is the actual carrier and heatsink of ICs and therefore the inﬂuence of PCB on ICs thermal capability should be analyzed. This necessitates an optimal PCB layout design to ensure reliable operation of ICs over a wide range of temperature and different layouts can be realized depending on packaging technology and associated pinouts.

This application note discusses three different PCB design approaches and the cost trade-off for a high power density package. A brief comparison between simulated and measured results is subsequently explained. For the study, TI’s new TPS62866- 6 A buck converter in a 1.05-mm x 1.78-mm x 0.5-mm WCSP/DSBGA package, is considered. The package size is equal to die size in these packages for PCB space saving which makes heat dissipation from these packages more challenging.

2 Thermal Vias in Power PCB design

Usually, power PCB designs focus on improving thermal performances by increasing the copper area or adding ground planes in a multilayer boards. Thicker copper planes and wider traces resulting in low resistance and low parasitic inductance also helps to dissipate heat in the same size board area.

However, one of the most effective ways to propagate heat through the PCB is to use vias. Thermal vias are commonly used in PCBs as a mean to dissipate heat vertically from surface mount components to the PCB inner layers. Vias are simple mechanical drillings made to penetrate the PCB and thereby connect multiple layers in the PCB.

Figure 2-1 shows the different type of vias typically used in the PCB industry, they also differ in terms of use and costs.

Figure 2-1 Different use of vias

Through-hole vias are the most used and cheapest to manufacture. There are micro vias called buried vias: which can connect the internal layers without being exposed to outer air, and blind vias: which can connect the internal layers and are exposed on one side of the board. Both buried vias and blind vias provide only a partial connection between the PCB layers and add a degree of complexity to the design which results in a more expensive PCB design.