Microchip TC648 Manual

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 2003 Microchip Technology Inc. DS91063A-page 1

TB063

INTRODUCTION

Less than six years ago, thermal cooling in the

electronics arena was mainly an issue for high-

performance, high-end applications, such as, military,

aerospace and large-scale industrial and medical

applications. Outside these sectors, thermal cooling

was just a premature notion. In the short span of a few

years, technological and other developments have

made thermal cooling necessary for many applications,

thus requiring the development of a new system

management category: thermal management.

This article discusses an integrated fan speed solution

that provides sophisticated speed control of brushless

DC fans, the most popular type of fan used in electronic

equipment, and helps designers get around problems

like acoustic noise, power consumption, mechanical

wear-out and fault detection.

TRENDS AND DRIVERS

On a board level, many more components need cooling

today than even six months ago; with the CPUs having

begun this trend some years ago. Memory chips never

needed cooling, but now SRAM and DRAM packages

require their own cooling solutions. Video card

processors (mostly the 3D type) and other add-in cards

also require cooling, as will the next generation models.

Hard drives, DVD players, CD players and chipsets are

now candidates for cooling as well. In general, board

speeds are becoming faster and boards are becoming

smaller and more heavily populated. Package

densities are increasing and performing more

functions, thus getting hotter.

New developments in digital chip architecture permit

higher system clock rates and additional on-chip

circuitry, causing the chips to run at higher tempera-

tures. A large portion of the power dissipation is the

capacitive charging and discharging during level transi-

tions. Since the power lost is related to the square of

the supply voltage, the trend to lower voltages reduces

power dissipation.

However, higher losses due to higher switching

speeds, significantly offset these savings. An example

is the 486 microprocessor, which was drawing 12 to 15

watts. As PCs moved into the first Pentium®

generation, the microprocessors started dissipating

25W. Today a Pentium® II dissipates about 40W, and

there have been reports that the forthcoming 64-bit

Merced microprocessor dissipates about 65W.

Thermal cooling is also in demand because of the

explosive growth of new embedded applications.

Telecommunications equipment, printers, household

“smart” appliances, and most importantly, networking

equipment (routers, switches and hubs) are only a few

of the products now driven by embedded CPUs. More

are being added each day, and with the complexity of

multiple functions, thermal cooling has become a

necessity.

WHY DO WE NEED COOLING?

The air immediately surrounding a chip initially cools its

surface. That air eventually warms and rises to the top

of the PC or other equipment’s chassis, where it

encounters more warm air. If not ventilated, this volume

of air becomes warmer and warmer, offering no avenue

of escape for the heat generated by the chips.

Typically, a PC chip designed for commercial use can

withstand up to 125°C – 150°C junction temperature,

although a safety margin of a few degrees might be

specified. Exceeding that limit will either cause the chip

to make errors in its calculations or fail completely. A

chip failure or malfunction impacts the entire system’s

operation. Additional cooling also extends component

life by limiting the maximum temperatures the

components are exposed to. In general, a 10°C

temperature reduction will provide a 2:1 increase in

MTBF (Mean Time Between Failure).

Author: George Paparrizos

Microchip Technology Inc.

An Integrated Fan Speed Control Solution Can Lower

System Costs, Reduce Acoustic Noise, Power Consumption

and Enhance System Reliability

TB063

DS91063A-page 2  2003 Microchip Technology Inc.

THERMAL COOLING METHODS

Consistent with the trend of increasing heat dissipation

in electronic systems, the thermal cooling methods

began moving from passive to active solutions. In the

days of the 486 microprocessor, the processor cooling

was implemented by a heat sink and an embedded fan

was only used to cool down the power supply. Since

entering the Pentium era, the processor now produces

enough heat to require a stream of cool air from a fan.

In addition, new 3D graphic cards generate substantial

heat and are also packaged with an embedded fan on

top of the graphic chip.

To date, there are three basic approaches to thermal

cooling: heat spreaders, heat sinks and fans:

1. Heat spreaders, which are made of a tungsten-

copper alloy and are placed directly over a chip,

have the effect of increasing the chip’s surface

area, allowing more heat to be vented upward.

Heat spreaders are frequently designed with a

specific chip in mind.

2. Heat sinks spread the heat upward through fins or

folds, which are vertical ridges or columns that

allow heat to be conducted in three dimensions -

length, width, and height - as opposed to the two-

dimensional length and width of heat spreaders.

Heat sinks maximize the amount of surface area

that can be air-cooled.

3. Fans simply focus the available air on a

concentrated space. The cooling capability of a fan

depends on the volume of air the fan moves, the

ambient temperature and the difference between

the chip temperature and the ambient tempera-

ture. While fans move volume of air, some PC

systems also require blowers to generate air

pressure.

BRUSHLESS DC FANS

Thermal complexity, thermal content and greater power

demands are boosting the popularity of brushless DC

(BDC) cooling fans (Figure 1). In addition to the

advantages of conventional fans, BDC fans have a

significantly higher mechanical reliability. They contain

no rotating commutator/brush assembly to shed dust

particles, wear out, or act as an ignition source.

Additionally, their magnetic coils are stationary and are

usually mounted within a rigid frame for superior

structural integrity and thermal dissipation. Finally, they

lack the rotating magnetic fields of AC motors and the

arching of conventional DC motors and, thus, are

electrically quiet.

While BDC fans adequately evacuate heat from the

system enclosure and are superior to conventional

fans, they add other problems. Such problems that

designers have to cope with in most applications are:

• Fan Mechanical Wear-out

• Acoustic Noise

• Power Consumption

• Fault Detection

The sections that follow provide an in depth summary

of the above mentioned problems and describe the

features, operation and advantages of Microchip’s

integrated fan speed control solution.

FIGURE 1: Brushless DC Fan Basics

Stator

Coil

Stator

Coil

Stator

Coil

Stator

Coil

Internal Control IC

Hall Effect

Sensor

Fan

Frame

Axle

Fan Hub

Center

Bearing Permanent

Magnets

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	TC648

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