标签：
嵌入式
Anyone who has actually put an operating laptop computer on top of their lap knows why cooling is important for any computer. Microprocessors dissipate a lot of electric power, which turns into heat, which must be removed somehow. They do this despite the fact that the metal-oxide-semiconductor (MOS) field effect transistors (FETs) making up the bulk of the computer circuitry are theoretically very efficient when operating in switched mode (saturated on or cut off).
MOSFETs are effectively voltage-variable resistors. A voltage applied to the transistor’s insulated gate electrode modulates resistance between the source and drain electrodes. In an off state, no conduction path exists between the source and drain electrodes, hence no current flows, and no power is dissipated. Placing sufficient voltage of the correct polarity on the control gate creates a low-resistance inversion layer between source and drain. Current then can flow efficiently between those electrodes, meaning that, once again, very little power is dissipated.
Microprocessors generally incorporate millions of MOSFET transistors.
There are three factors that combine to foil this effect and burn a lot of electric power in MOSFET-based microprocessors.
First, today’s microprocessors operate at very high clock frequencies.
Second, substantial (in microscopic-transistor terms) current is needed to charge and discharge the interelectrode capacitance between the gate and ground each time a transistor switches. The higher the frequency, the more often charge and discharge cycles occur, and the more current flows.
Third, there are millions of transistors in microprocessor ICs. Combining millions of transistors switching at high speeds with substantial current needed to switch all these transistors leads to a lot of power dissipation — and a lot of heat. Heat management therefore becomes an important part of every computer design, including single-board computer (SBC) design for embedded control.
Your typical computer — whether an SBC or any other form factor — gets heat out of a microprocessor by pressing a great whacking head sink on top of it, and blowing lots of nice, cool air across it. That air has to come from somewhere and blow to somewhere else. Those places are outside the computer enclosure, and the air gets in and out through holes in the enclosure.
Air isn’t the only thing that can go in and out through those holes. Dust and fluids can get in, too. Dust and fluids are very bad for electronics and are especially problematical in the industrial environments where many embedded systems live and work.
A second issue with cooling microprocessors by blowing cool air across them is noise and vibration. Few computers can survive using only natural (unforced) convection for cooling. There’s too much heat to extract by just letting cooling air waft lazily by under natural convection. You need a substantial fan to push enough air through the enclosure. Fans make noise and vibrate. Any facility with a number of computers going at once becomes noisy and vibrates.
In response, a number of SBC vendors have introduced conduction-cooled products for embedded systems. These SBCs solve the dust-and-fluid intrusion problem by the simple expedient of closing up ventilation holes, and the vibration-and-noise problem by eliminating fans.
Completely closing SBC enclosures, however, leaves designers with the problem of how to get rid of heat. They have solved it by placing the microprocessor package in thermal contact with the aluminum enclosure walls.
Heat generated as the MOSFETs go clickety clack at multi-GHz speeds conducts through the IC package to the case. The case then acts as quite a large heat sink. Being aluminum, the case can efficiently conduct that heat to the system’s mechanical support structure, a large heat sink, or cooling fins on the outside where natural convection works better.
The result is an adequately cooled SBC with great tolerance for dusty, damp, wet, nasty environments. In addition, no forced-air cooling means no fan, so the SBC is virtually silent, and no holes means superior immunity to electromagnetic interference (EMI).
Yeah, I haven’t mentioned EMI, yet, have I? There are two types of EMI to consider: radiated and absorbed. The radiated type arises when the SBC acts like a UHF transmitter, interfering with anything around it capable of being interfered with. An SBC is also more or less susceptible to absorbing — and being interfered with by — UHF signals radiated by other equipment.
EM waves gradually lose amplitude while traversing a conducting wall. The thicker the wall, the more amplitude lost.
Metal enclosures act as Faraday cages, blocking (within limits) entry of external EMI fields, and containing (again within limits) radiated EMI. When an electromagnetic (EM) wave impinges on a conducting wall, it immediately begins losing amplitude. The energy lost out of the wave either reflects back out, or heats the material through resistive dissipation. In either case, what reaches through the wall is greatly diminished. How diminished depends on the wall’s thickness and conductivity. High conductivity leads to rapid amplitude decline. Thicker walls give provide more room for the wall to work on the wave, so it exits at much lower amplitude. Thick (several millimeters) walls made of highly conductive material can attenuate the wave by many orders of magnitude.
Things that limit an enclosure’s ability to block EMI include material conductivity, wall thickness, and openings.
The aluminum typically used for fanless SBC cases is an excellent conductor. Plastic enclosures often used for less rugged units generally have little or no EMI-blocking ability unless fortified with conductive fillers, such as carbon.
Holes have the effect you would expect. They let portions of the wave front pass through the wall without attenuation. The interaction between a conducting wall and an EM wave is more complex than simply blocking some energy and allowing some to pass through, but the net effect is to let significant radiated power in, and the larger the hole, the more EMI can pass through.
Conductively cooled SBCs also typically have thicker walls. There is less machining to do (no vent holes to cut), so designers need not thin the walls to reduce machining expense. Enclosure walls act as a much needed heat sink, which motivates designers to thicken them as well. Of course, thicker walls produce a more mechanically rugged package for industrial use.
In the end, the reason to choose conductively cooled SBCs for industrial embedded applications is for better environmental, EMI, and mechanical characteristics.
系统分类:
工控机 | 用户分类:
无分类 | 来源:
原创