Top cooling or bottom cooling, which is better for high power step-down conversion?

Autonomous driving is a new and important trend facing all automotive OEMs in this era, and the number of electronic control units (ECUs) in vehicles has increased dramatically. This covers many applications, such as driver assistance cameras, data fusion ECUs, and their respective power management. Depending on the application and operating range, the output power of the pre-regulator ranges from a few watts for parking assistance ECUs to hundreds of watts for data fusion ECUs. This series of articles will explain the potential significance of using heat sinks to reduce thermal stress in electronic devices, and the correlation between system thermal performance and various factors, such as the location and size of the heat sink.

Previously, we shared an overview of the thermal evaluation test principles for high power buck conversion and compared the thermal performance of the board in three different configurations. Today, we will focus on the effect of using a heat sink when designing a high output power pre-regulator.

Comparison with and without heat sink

Figures 24 and 25 show the temperature variation of the MOSFET without and with the top heat sink. At higher currents, the low-side MOSFET without the heat sink runs hotter than the MOSFET with the top heat sink. The low-side MOSFET runs about 30°C cooler at 20.0A with the 60mm heat sink than without the heat sink. Similarly, the temperature of the MOSFET with the 25mm heat sink is 22°C lower than the MOSFET without the heat sink.

Figure 24. Low-side MOSFET temperature variation with and without top-side heat sink

The high-side MOSFET is approximately 33°C cooler with a 60mm heatsink at 20.0A than without a heatsink. Similarly, the MOSFET temperature is 26°C lower with a 25mm heatsink compared to the MOSFET without a heatsink.

Figure 25. High-side MOSFET temperature variation with and without top-side heat sink

Figures 26 and 27 show the temperature variation of the MOSFET without and with the bottom heat sink. At higher currents, the low-side MOSFET without the heat sink is hotter than the MOSFET with the bottom heat sink. The low-side MOSFET is approximately 29°C cooler at 20.0A with the 60mm heat sink than without the heat sink. Similarly, the temperature of the MOSFET with the 25mm heat sink is 23°C lower than the MOSFET without the heat sink.

Figure 26. Low-side MOSFET temperature variation with and without bottom heat sink

The high-side MOSFET runs about 31°C cooler with a 60mm heatsink at 20.0A than without one. Likewise, the MOSFET with a 25mm heatsink runs 25°C cooler than without one.

Figure 27. High-side MOSFET temperature variation with and without bottom heat sink

Effect of gap pads

For a 60mm heat sink, the following measurements were recorded using 3W/(m·K) and 6W/(m·K) gap pads at a load current of 20.0A to understand the effect of gap pad thickness on thermal performance. The two different gap pads are KERAFOL 86/300 SOFTTHERM and 86/600 SOFTTHERM, as shown in Table 2 at the beginning of this white paper.

When the gap pad was changed from 3W/(m·K) to 6W/(m·K) (approximately half the thermal resistance) using the top side heat sink, a 1.6% temperature reduction was observed for the low-side MOSFET and a 3.5% temperature reduction for the high-side MOSFET (Table 15).

Table 15. Clearance Pads with Top Heat Sink

When the bottom heat sink is used, the temperature of the low-side MOSFET is reduced by about 7.6%, and the temperature of the high-side MOSFET is reduced by about 6.6% (Table 16).

Table 16. Clearance Pads with Bottom Heat Sink

Top Exposed Pad vs Bottom Exposed Pad

As highlighted, the PCB is optimized for good thermal conductivity and heat dissipation, and can serve as a very effective heat sink for the MOSFET. This approach is often undesirable in real-world applications where there are multiple heat sources and the PCB has limited heat dissipation capabilities. The preferred method of heat dissipation is through the ECU case, which is thermally connected to the PCB. MOSFETs in “top-side cooled” packages achieve the lowest thermal resistance between the heat source (MOSFET) and the heat sink (case), allowing a direct thermal connection between the exposed pad of the top-side MOSFET and the heat sink, while minimizing heat flow into the PCB.

MOSFETs with the same die but different packages are needed to directly compare their thermal performance. All previous measurements used the NVMFS5C460NL, but this MOSFET is not available in a "top side cooled" package variant. So the NVMFS5C450N (SO-8FL bottom side exposed pad) and the NVMJST3D3N04C ("top side cooled" package, top side exposed pad) were chosen for the following measurements.

The NVMJST3D3N04C is only available as a standard-grade device, while the NVMFS5C460NL is a logic-grade device. In this application, the efficiency of the standard-grade device is expected to be slightly lower than that of the logic-grade device. Nevertheless, since the losses are not significant, only the differences in thermal performance, NVMFS5C450N and NVMJST3D3N04C can be compared.

Table 17. Package Overview

The plastic surface area of ​​the top surface of the NVMFS5C450N in the SO-8FL is 31.7mm2, which is slightly larger than the plastic surface of the bottom surface of the NVMJST3D3N04C in the LFPAK10 TC (27.0mm2). The exposed pad size of both devices is roughly the same.

A heat sink with a height of 25mm was used for the following measurements to avoid any restriction of the heat sink and to maximize thermal performance to optimize any differences in heating.

Bottom Side Exposed Pad MOSFET Measurement (NVMFS5C450N)

Tables 18 and 19 show the temperature of the high-side and low-side MOSFETs (NVMFS5C450N) with and without heat sinks. The heat sink is mounted on the top surface of the MOSFET (plastic case).

Table 18. NVMFS5C450N - No heat sink

Table 19. NVMFS5C450N - 25mm heat sink on top

Figure 28. NVMFS5C450N - Low-side MOSFET temperature with and without heat sink

Figure 29. NVMFS5C450N - High-Side MOSFET Temperature with and without Heatsink

Figures 28 and 29 show the improved heat dissipation of the low-side and high-side MOSFETs using a heat sink mounted on the plastic top surface of the MOSFET.

At a load current of 5.0A, the temperature of the low-side MOSFET is about 6°C lower than without a heat sink, and the temperature of the high-side MOSFET is about 8°C lower. At a load current of 20.0A, the temperature of the low-side MOSFET is about 40°C lower than without a heat sink, and the temperature of the high-side MOSFET is about 37°C lower.

The thermal performance for both measurements is within the expected range, and the heat sink reduces the MOSFET temperature significantly. In general, the NVMFS5C450N switches slower due to the higher gate charge, so the temperature is higher than the previous measurements with the NVMFS5C460NL. Even the on-resistance is slightly lower.

Top side exposed pad MOSFET measurement (NVMJST3D3N04C)

Tables 20 and 21 show the temperature of the high-side and low-side MOSFETs (NVMJST3D3N04C) with and without heat sinks. The heat sink is mounted on the top surface of the MOSFET (exposed pad).

Table 20. NVMJST3D3N04C - No Heatsink

Table 21. NVMJST3D3N04C - 25mm heat sink on top

Figure 30. NVMJST3D3N04C - Low-Side MOSFET Temperature with and without Heatsink

Figure 31. NVMJST3D3N04C - High-Side MOSFET Temperature with and without Heatsink

Figures 30 and 31 show how the heat dissipation of the low-side and high-side MOSFETs can be improved by using a heat sink mounted on the exposed pad on the top surface of the MOSFET.

At a load current of 5.0A, the temperature of the low-side MOSFET is about 8°C lower than without a heat sink, and the temperature of the high-side MOSFET is about 10°C lower. At a load current of 20.0A, the temperature of the low-side MOSFET is about 40°C lower than without a heat sink, and the temperature of the high-side MOSFET is about 37°C lower.

Also, in this measurement, the thermal performance is as expected. Since the NVMJST3D3N04C and NVMFS5C450N use the same die, the losses and heating are higher than in the previous measurements using the NVMFS5C460NL due to the higher switching losses caused by the higher gate charger.

Comparison between bottom and top exposed pad

Figure 32 compares the low-side MOSFET temperature with exposed pad on the bottom side (NVMFS5C450N) and top side (NVMJST3D3N04C), with heat sink on the top side.

Figure 32. NVMFS5C450N and NVMJST3D3N04C - Low-side MOSFET temperature (with heat sink)

Figure 33 compares the high-side MOSFET temperature with exposed pad on the bottom side (NVMFS5C450N) and top side (NVMJST3D3N04C), with heat sink on the top side.

Figure 33. NVMFS5C450N and NVMJST3D3N04C - High-Side MOSFET Temperature (With Heatsink)

In general, the thermal performance of this particular PCB and setup is very similar, independent of whether a MOSFET with a bottom-side or top-side exposed pad is used, and a heat sink on the top side of the MOSFET package. For low-side MOSFETs, the bottom-side exposed pad package performs slightly better than the top-side exposed pad, and vice versa for high-side MOSFETs.

For MOSFETs with bottom-side exposed pad, a lot of heat flows into the PCB, optimized as an effective heat sink. A heat sink on the plastic surface on the top side of the MOSFET also helps reduce the MOSFET temperature.

MOSFETs with exposed pads on the top side have relatively poor thermal coupling between the PCB and the bottom plastic surface. However, the leads soldered to the PCB also allow heat to flow into the PCB. The exposed pad on the top side of the MOSFET is connected to the heat sink and dissipates the heat effectively.

Both configurations dissipate heat through the bottom and top surfaces of the MOSFET package. For the bottom exposed package, the thermal resistance between the MOSFET and the PCB is lower than the thermal resistance between the MOSFET and the heat sink. For the top exposed package, the reverse is true; the thermal resistance between the MOSFET and the heat sink is lower. This results in similar thermal performance being achieved using completely different configurations, and effective heat dissipation can be implemented for both types of packages.