Heat Sink Technology
Heat Tunnels, Thermal Gradients and More
By John Roberts
Over the last several years we've seen a number of power amp ads purporting to educate the market about heat sink technology. More often than not, the chosen approach is neither novel nor superior. The advertisement is more creative than the design being pitched. The following description of Peavey's new heat sink approach will describe a real advance on the state of the art.
Forced Air and Heat Tunnels
Above a fairly modest power level, passive or convection cooling is inadequate to deal with the heat generated. While spreading out the heat is desirable in passive cooling, once you move up to forced air, you often need to bring the heat sources closer together to better control the cooling. A classic technique is known as a "heat tunnel". The general configuration of a classic heat tunnel is a fan pulling cool air from outside the chassis into a passage. The cool air inside this channel flows over a series of heat sink fins. While the air begins at ambient temperature, it extracts heat from the heat sink fins with every collision. By the time it exits the heat tunnel, it is tens of degrees hotter than when it entered.
I find it a little amusing that one competing approach uses a center-driven heat tunnel, essentially two short tunnels driven by a single fan. The remarkable claim is that because the heat tunnel is short there is no thermal gradient (temperature difference) along the length of it. This claim is obviously questionable because if there were no thermal gradient, the air would have to exit at the same temperature it came in; consequently, there would be no heat transferred to the cooling air.
A classic problem when using forced air to cool a multiplicity of devices arrayed along a heat tunnel is to equalize the device temperatures along the length of the heat tunnel. In high-power amplifiers you routinely use several power devices in parallel. If one receives less cooling than the others, it will run hotter. Like the weak link in a chain, it will reach its maximum temperature and fail first. This technique can make a huge difference in total work output available from the amplifier because you are limited by that one, hottest device. Thermal protection must shut down the entire amp to protect that one hottest device.
Even if all of the power devices have the exact same access to the cooling air- flow, the air picks up heat as it passes over preceding devices. This hotter air can't absorb heat as readily as the cool air. The classic solution for equalizing the thermal gradient in a heat tunnel is to tune the length of the heat sink fingers. Just look inside the industry standard CS800X, The heat sink at the hot, exit end of the heat tunnel has much more aluminum and longer fingers than the cool inlet end.
Multi-stage, graduated-heat tunnels are complex and difficult to manufacture. While you could just ignore the heat rise phenomenon and invest in advertising, we chose to apply our 30+ years of experience design power amps to find a better way.
I apologize for the name but these days we need to cut through the marketing clutter. Our new heat sink approach is deceptively simple. By angling two heat sink extrusions toward each other, forming a truncated "V", the heat transfer is significantly increased at the narrow end.
The cool ambient air enters at the wide end, top of the "V." As the cool air moves through the heat tunnel gaining heat, the walls move closer together increasing collisions with the air and helping the hotter air extract the same amount of heat the cooler air did in the previous section. By the time the hottest air gets to the end of the tunnel, the heat transfer is at it's maximum. The net result of tuning the heat sink for air temperature, the devices all operate at close to the same temperature and the amplifier will be able to do more work before the hottest power device gets too hot.
Note: Turbo V cooling is currently being used in the GPS 1500 and GPS 900 power amplifiers. This approach is novel and a US patent has been applied for