Thermal Management in Practice

Choosing a Power Amplifier Heatsink

My name is Takumi Musha and I work in Kikusui’s Solutions Development division. My surname means “warrior” in Japanese, but I am actually a regular, mild-mannered guy. This is my first article. My story involves a heatsink selection that didn’t go exactly to plan.

An assignment I was working on required me to find a heatsink for a rather large power amplifier which was rated 40 amps/400 watts. I was tasked with finding a heatsink capable of dissipating the 200 watts of heat generated by the amplifier.

The Specification

A finned, aluminum alloy heatsink was to be used. We were subject to constraints on where the heatsink could be attached, and the heatsink could not be any thicker than around 40 mm. The heatsink needed to dissipate the 200 watts of internal loss arising in the power amplifier using fan-assisted cooling. We also needed to keep the temperature of the amplifier case at below 70°C(158°F) when the ambient temperature was 25°C(77°F). The power amplifier chip was of the DIP (dual inline package) type with pins on the heat dissipation side and had a footprint of around 40 square millimeters. The chip’s slightly unusual shape made mounting it a headache.

Choosing a Heatsink

First, I calculated the thermal resistance of the heatsink based on the power consumption of the amplifier, the thermal resistance between the junction and the case, the thermal resistance between the case and the heatsink, the temperature of the amplifier junction, and the ambient temperature. As we would also using forced cooling, I also referred to data on the properties of cooling air when selecting a generic heatsink.

The heatsink I selected was 202 mm wide, 200 mm deep and 30 mm high, with a 5 mm base. (Figure 1)

Figure 1

I attached the power amp, positioned four fans on the exhaust side of the heatsink, and applied current. As I increased power, I found that the power amp casing hit a temperature of around 70°C(158°F) at only 100 watts. Deciding to boost airflow in order to increase the system’s static pressure, I added another four fans on the inlet side of the heatsink. This time I was able to increase power to about 120 watts before the temperature hit 70°C(158°F). I then looked at the temperature distribution of the heatsink (Figure 2). Only the area around the amplifier was hot, suggesting that heat was not being properly dissipated.

Figure 2

To help better disperse the heat, I used thermal grease to attach a highly thermally-conductive copper plate to the heatsink (Figure 3). The addition of the copper plate enabled power to be boosted by an additional 30 watts, but that was all. I had hit a brick wall.

Figure 3

Thickness is All Important

It was then that a colleague suggested that I use a heatsink with a thicker base. I selected a generic model with a 10-millimeter base, which was just within the height limit.
The heatsink I had selected was 154 mm thick, 200 mm deep, and 30 mm high, with a 10 mm base (Figure 4).

Figure 4

While I was worried that the smaller surface area might be a disadvantage, upon applying current to the circuit, I found that the amplifier could now be kept at 70°C(158°F) or cooler at a current of 200 watts. The thicker base had dispersed the heat.
This experience taught me that in applications where heat generating components are concentrated in a small area, it is more effective to use a thicker heatsink than a larger one. I had not for a moment imagined that such an improvement could be gained through a mere 5-millimeter increase in thickness.

Designs on paper can be different from reality. While I liked to think that I understood this, my experience with the heatsink brought the lesson home to me.

Thermal design is a particularly important part of an electronic device’s specification. It is dangerous to assume that a device will function based solely on its specifications on paper. I believe that pretesting devices is essential to make sure that they work.

Here are some photos of the device, for reference. (Photograph 1)

Photograph 1

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