Tech Talk
Thermal Analysis of Mark Levinson Amplifier Heat Sinks
Joe Jagenow
Principal Engineer - HARMAN Luxury Audio
All electronic devices generate heat, especially the output transistor devices utilized in a Mark Levinson power amplifier, which have the capacity to generate considerable amounts of heat due to their high output power. The output transistors must be mounted to a heat sink to dissipate the heat from the devices and allow the devices to operate at a safe temperature, even at full power output. There are also considerations for the temperature of the top surface of the amplifier, which the customer can touch, so the enclosure should only heat up to an acceptable level.
In the Mark Levinson power amplifiers, an extruded aluminum heat sink contacts an output transistor’s hot junction surface. Aluminum has the significant advantage for heat sink design in that it can be easily formed by extrusion, making complex cross-sections possible. The large mass of the aluminum heat sink has a much greater heat capacity than the semiconductor and will transfer thermal energy (heat) from the higher temperature semiconductor to the lower temperature heat sink. The heat sink will reduce the semiconductor’s temperature via increased thermal mass and heat dissipation by conduction and convection. This rapid transfer of thermal energy quickly brings the semiconductor into thermal equilibrium with the heat sink, lowering the operating temperature of the transistor. Efficient function of a heat sink relies on rapid transfer of thermal energy from the semiconductor to the heat sink, and the heat sink to the semiconductor.
Figure 2 - Mark Levinson Amp Output Devices and Heat Sink
The industrial design of the Mark Levinson 600 Series of products included a novel T-shaped heat sink, which gives the amplifier a very rugged appearance to the customer while harkening back to previous ML amplifiers such as the Nº33H. The heat sinks are integrated into the amplifier chassis design, with external fins that are exposed to ambient air. All the cooling is achieved via natural convection of the heat sinks. No fans are utilized in the amplifier design since they would add noise in the listening room.
Figure 3 - №632 Amplifier
When we first considered the industrial design of the amplifier, we wanted to examine the design of the heat sinks to maximize the ability of the heatsinks to dissipate the heat generated by the output transistors. We were able to use computational fluid dynamics to examine the heat generated by the output devices, and to ensure that the design of the heat sinks would dissipate sufficient heat from the devices during use.
We performed the heat sink thermal analysis at 1/3 rated output power. Alternative heat sink geometries were analyzed, varying the spacing of the heat sink fins to determine the effect on the heat dissipation of the components.
Figure 4 - No632 Amp Channel and Heatsink - 10 mm Fin Spacing - 83.33 W (1/3 power)
First, we performed a thermal analysis using 10mm fin spacing. We can set the material parameters for the output transistors, thermal gap filler material, and heatsink. We chose to use 6063 aluminum alloy for the heat sinks, due to this material’s excellent ability to dissipate heat, in addition to its attractive appearance when anodized. Note that anodizing the heat sinks black increases the emissivity of the heat sink, meaning it radiates heat more effectively.
The color bars on the left show the maximum temperatures that are reached on the contact surface of the output transistors (50.44 °C), as well as the maximum temperature that is reached on the top surface of the heat sink (48.35 °C).
Figure 5 - No632 Amp Channel and Heatsink - 15 mm Fin Spacing - 83.33 W (1/3 power)
Next, we performed a thermal analysis using 15mm fin spacing. Reducing the number of fins will reduce the amount of aluminum that is required to extrude the heat sink profile. However, fewer fins also reduce the amount of surface area on the heat sink. The color bars on the left show the maximum temperatures that are reached on the contact surface of the output transistors (50.89 °C), as well as the maximum temperature that is reached on the top surface of the heat sink (48.79 °C).
Figure 6 - No632 Amp Channel and Heatsink - 20 mm Fin Spacing - 83.33 W (1/3 power)
Finally, we performed a thermal analysis using 20mm fin spacing. The color bars on the left show the maximum temperatures that are reached on the contact surface of the output transistors (51.91 °C), as well as the maximum temperature that is reached on the top surface of the heat sink (49.75 °C).
As a result of this analysis, we determined that the best fin spacing from our testing parameters was 10mm, prior to kicking off an extrusion tool for the heat sink.