GB2322500A

GB2322500A - Heat dissipator using acoustically generated airflow

Info

Publication number: GB2322500A
Application number: GB9703454A
Authority: GB
Inventors: Stefan Lichterfeld; Alexander Schaefer; Michael Schottle
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1997-02-19
Filing date: 1997-02-19
Publication date: 1998-08-26
Also published as: GB9703454D0

Abstract

A cooling arrangement where an acoustically generated airflow is used to dissipate heat. The arrangement is particularly applicable to portable communication units 10 where heat is generated by for example an audio amplifier, power amplifier, signal processor or battery 16. The arrangement preferably comprises a vibrating member, such as a loudspeaker 14, that produces the cooling airflow. In one embodiment, a duct 18 is positioned adjacent to the vibrating member 14 to direct the airflow to the heat generator 16. The duct has an open end with a circular diaphragm 20 such that circular ring turbulences are produced by accelerating air pulses (Figure 3). To reduce noise, the duct may be designed for a particular air volume such that the resonant frequency is below the sensitive audio range. In another embodiment (Figure 2) the duct is provided within an antenna to facilitates airflow from within the communication unit 10. Temperature monitoring (Figure 5) may also be used to initiate the acoustic cooling operation once a threshold temperature is exceeded.

Description

EAT DISSIPATION ARRANGEMENT AND METHOD OF OPERATION Field of the Invention This invention relates to heat dissipation techniques. The invention is applicable to, but not limited to, heat dissipation in portable communication units.

Background of the Invention Certain electronic equipment, and communication transmitters in particular, produce heat that needs to be dissipated and transferred away from the equipment. In the case of communication transmitters, the heat generation is caused by use power amplifiers to generate the desired output signal in medium and/or high power applications. The power amplifier is often a high-current consumption power device which transforms the high current supplied to generate a high power communication signal. In doing so, the power device generates a lot of heat that needs to be removed from the surface of the device to prevent "burn-out". Hence, when using medium and high power devices to transmit communication signals, a heat sinking arrangement is used to dissipate the thermal energy produced by the amplifying device.

Heat sinking is typically achieved by either thermal radiation, using a dark colour surface on the power device to radiate the heat, or by thermal convection, by attaching the amplifying device to a metallic surface and providing air flow around the amplifying device to transport heat away from the device. Often a chassis with suitable thermal conductivity for heatsinking has to be provided. An airflow along the chassis would improve the heatsinking performance considerably.

A problem associated with the heat sinking of power amplifving devices, is the provision of an adequate convection air flow, in particular within the confined space and volume available, for example, in a mobile communication in a car. To improve the convection air flow a fan can be included to facilitate air flow away from the amplifying device. This adds additional cost to the communication unit and requires a comparable increase in space.

This invention seeks to alleviate some problems with heat sinking techniques and in particular to provide a more efficient arrangement for transferring heat away from power generating devices in mobile and portable communication units.

Summarv of the Invention According to a first aspect of the present invention a heat dissipation arrangement is provided including a heat generator and means for acoustically generating an air-flow to the heat generator to dissipate heat from the heat generator by means of the air-flow.

Preferably the means for acoustically generating an air-flow comprises a vibrating member, for example a loudspeaker of a communication unit, such that vibration of the vibrating member acoustically generates the air-flow to dissipate heat from the heat generator, for example an audio amplifier, a power amplifier or a signal processor. In the preferred embodiment of the first aspect of the invention, the means for acoustically generating an air-flow further includes a duct positioned substantially adjacent to the vibrating member, and within a closed box, to direct air flow to the heat generator. The box volume has an acoustical resonant frequency substantially below the sensitive audio range of the human ear. In this manner any tones generated by the resonant frequency of the duct due to air-flow will not be loudly heard by the human ear, particularly when the heat dissipater is used in a communication unit. Advantageously, the duct may have at least one end closed by a flexible diaphragm. At the open end, the inner diameter of the window of the flexible diaphragm is less than a diameter of the duct to create circular ring turbulences through the window by accelerating the air-stream pulses, that may be better understood as longitudinal waves, through this narrowed part of the duct. The rings contain and carry its air molecules from the duct exit, out of the duct and remain stable for a short period of time.

Advantageously, traditional heat dissipation requirements of fans and bulky heat-sinking arrangements are removed, or at least mitigated, depending upon the amount of heat dissipation required. Furthermore, the re-use of a built-in loudspeaker, as available in handportable communications and telephones or compact mobile transceivers, can provide the heat dissipation benefit at negligible additional cost. The airflow/ tone generation is easily implemented in, for example, the communication software.

According to a second aspect of the present invention a communication unit having the heat dissipation arrangement previously described is provided. Preferably the communication unit is a portable communication.

According to a third aspect of the present invention an antenna is provided having a duct with at least one open end, positioned substantially in parallel with and internal to the antenna for facilitating air-flow from the portable communications unit out of the at least one open end of the duct.

According to a fourth aspect of the present invention, a method of dissipating heat from a heat generator is provided. The method includes the steps of generating an acoustic wave of air flow and directing the acoustic wave of air flow to the heat generator to remove heat therefrom.

Preferably the step of generating an acoustic wave includes vibrating a vibrating member, for example a loudspeaker in a communication unit, to generate an electromagnetically driven acoustic wave, with air-flow being directed along a duct within the communication unit to the heat generator.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 shows a portable communication unit with a heat dissipation arrangement according to the preferred embodiment of the invention.

FIG. 2 shows a portable communication unit with a heat dissipation arrangement of an alternative embodiment of the invention.

FIG. 3a and FIG. 3b show a detailed diagram of the air-flow effect to dissipate heat in accordance with the preferred embodiment of the invention.

FIG. 4a and FIG. 4b show a portable communication unit with a heat dissipation arrangement of a further alternative embodiment of the invention.

FIG. 5 shows a flow chart of the operation of the heat dissipation arrangement according to the preferred embodiment of the invention.

Detailed Deseription of the Drawings Referring first to FIG. 1, a portable communication unit with a heat dissipation arrangement is shown in accordance with a preferred embodiment of the invention. The heat dissipation arrangement uses an acoustic velocity transformation of a volume of air, similar to the Helmholtz Resonator principle. FIG. 1 shows a portable communication unit 10, having a power dissipating device 16, for example an audio amplifier, a power amplifier or a signal processor, and main body 12, a portion of which is a loudspeaker 14 of the communication unit 10. In the preferred embodiment of the invention, a tube (or duct) 18 is positioned within the main body 12 and acoustically coupled to the loudspeaker 14.

The tube has a firm diaphragm 20 at an open end of the tube 18, coupling the tube 18 to air passing externally to the communication unit 10.

In operation, vibrations of a flexible member of the loudspeaker 14 cause air movement within the tube 18. The amount of air movement is controlled by the dimensions of the tube 18, the box volume and the driver frequency. The tube 18 and box size are designed to have dimensions such that a resonant frequency is generated when the loudspeaker vibrates. The communication unit has an inner volume V, a tube tunnel T with length I as a single-opening channel. The resonant frequency is defined by resonation formulas, for example mass-spring resonators, as shown in equation 1.

At the resonant frequency For at the open end of the tube 18 having an area S, the airspeed is high and air pressure is low, whilst inside the unit, in particular at the loudspeaker 14 the relationship between airspeed and air pressure is the reverse. This results in low loudspeaker vibrations and hence low drive power of the air-flow. Thus designing the dimensions of the tube 18 to have a resonant frequency Fo at the open end of the tube, causes the airspeed to be a maximum at this point, thereby providing maximum air-flow out of the tube 18 and communication unit 10, with the air-flow passing by the heat generator 16. Colder air external to the tube 18, is then sucked into the space vacated by the air-flow leaving the tube.

For improved thermal coupling the inner surface of the tube is optimised by adding suitable heat sinking fingers or a grid-like arrangement.

To avoid an uncomfortable loud tone coming out of the unit the resonant frequency is set to values lower than the sensitive audio range of the human ear, for example at, or below, the sub-audio range of 50 Hz, by designing the tube 18 for a particular air volume. Preferably, but not essentially, the acoustically generated air-flow consists of electromagnetic acoustic waves.

It is within the contemplation of the invention, that alternative designs to achieve such airflow rectification can be used, for example other mechanical valve-type arrangements. One such alternative arrangement could include the tube being positioned external to the communication unit, but still passing by, and being acoustically coupled to, heat sources.

The performance of such arrangements depends upon the mass of moving parts and mechanical limitations at medium and higher tone frequencies.

Additionally the required loudspeaker vibration may be high, which would result in an unacceptable loud tone emanating from the communication unit.

Referring next to FIG. 2, an alternative embodiment of a portable communication unit 30 having such a heat dissipation arrangement is shown. The portable communication unit 30, has a tube 18 positioned within an antenna shaft 32, which may be, but is preferably not, extendable. If an extendable antenna is used the length of the tube and corresponding antenna are designed for optimum frequency of operation of the communication unit 30 and also optimum acoustical velocity of air inside the tube when fully extended. The body of the tube again is acoustically coupled to a loudspeaker 14 and at an open end of the antenna 19, to air passing externally to the communication unit 30.

In this manner, the design of the portable communication unit 30 does not have to be modified as much, as any antenna coils can be wound around the tube 18 to provide both a circular helical coil antenna and an air-flow tube for removing heat from the communication unit.

To generate ring turbulences 22 leaving the duct window, and therefore to achieve an airflow being blown out of the tube, the airspeed has to be high. This is achieved by the resonance transformation, as previously described, and enhanced by the design of the diaphragm 20 at the tube open end. In such a manner, ring turbulences 22 (RT) at each peak airflow are generated, as shown in FIG's 3a and 3b. The end of the tube is designed such that a circular diaphragm D 20 has an inner diameter S, substantially between the ranges (0.4 to 0.8) *D. In the preferred embodiment of the invention, the inner diameter of the duct S is approximately 60% of the tube diameter D, and as such forces the pulsed airflow through the hole, producing desired ring turbulences 22, similar to smoke rings, coming out of the diaphragm. The tube 18 passes by, and is acoustically coupled to, a power dissipating device 16.

The turbulences remain stable for a short time, and then fade away as the ring turbulences move away from the tube end, carrying an amount of air from the inner surface of the tube. In the preferred embodiment of the invention, the air removes heat away from the power dissipating device 16 as it is pulsed out of the diaphragm 20 past the power dissipating device 16, as shown in FIG. 3a.

In this manner, the tube 18 allows fresh, cold air 21 from the side of the communication unit 10, to be pulsed back through the tube providing a cooling effect as shown in FIG. 3b. Hence, there is a constant movement of air-flow past the power dissipating device 16.

The sub-audio resonance is at: Fres =I/Tres (2) which can be split into two half-wave sections: 0 < tl < Tres /2 (3) where positive internal acoustic pressure 26 causes longitudinal waves to run out of the tube and hence, blow out a ring turhulence, or Tres /2 < t2 < Tres (4) where negative internal acoustic pressure 28 causes longitudinal waves to run into the tube and inhaling cold air 21 from the sides while prior turbulence rings 22 are forced away. The rigidity of the acoustic wave generator, for example the loudspeaker, was designed to be not too rigid to allow a sufficient drive at Fres. A typical resonance may be in the region of 300 Hz, as per a standard miniature speaker.

In the preferred embodiment of the invention, a baffle member such as a thin elasticated foil, is placed at the inner end of the tube to avoid humidity and dust being blown into the communication unit 10. To achieve low damping of the sub-audio resonance, the surface area of the elasticated foil is arranged to be substantially twice the tube diameter. For optimum cooling efficiency the tube 18 is arranged in the chassis such that the tube 18 runs through the hot spot(s) of the power dissipating device 16, for example an audio or transmit power amplifier. The loudspeaker is driven with a sinewave signal to avoid harmonics appearing in the audio frequency range of the human ear.

For a given unit volume of a tested portable communication housing (4*16*10 cm) V = 640 cm3 with a 15 mm diameter tube and length I = 10 cm, a suitable operating frequency of 40 to 55 Hz was used. With these dimensions, and an input power P to the loudspeaker 14 of Pin 0.3 Watts, an airflow speed of about 30 cm/second was measured. The cooling effect was observed with a heated resistor coupled via an aluminium tube to a heat sensor. Furthermore, when a Radio Frequency (RF) Power transistor of Pout = 25 Watt was used with an internal temperature of 90 degrees Centigrade and a 0.5 Watt speaker operating at 55 Hz, the temperature at the tube end was found to decrease in excess of 10 degrees Centigrade due to the air-flow transformation of the heat.

Referring now to FIG. 4a and 4b a portable communication unit with a heat dissipation arrangement is shown, according to a further alternative embodiment of the invention. FIG. 4a shows a front view of a portable communication device 10 with a battery 32 being positioned adjacent to a duct 18 having an open end 20 and in a substantially horizontal plane of the portable communication device 10. In this arrangement, additional circuitry is included in the form of a thermal sensor 34 connected to the battery 32. FIG. 4b shows a side view of the portable communication device 10, highlighting the acoustic coupling between for example a loudspeaker 14 and the duct 18.

In operation, the heat dissipation functions as previously described with regard to the acoustic expulsion of heat from a battery 32. However, the further alternative heat dissipation arrangement is particularly useful for use with communication units having Lithium batteries, where excess heat is sometimes more of a problem. This is especially true during a charging cycle of the communication unit when the communication unit is not in an operational mode, but the communication unit is still becoming hot due to the battery charging. When a threshold temperature is exceeded on the battery, a suitable signal is generated by the communication unit to activate an acoustical velocity transformation arrangement, for example as described with reference to FIG. 3a and FIG. 3b, which cools the communication unit during the charging cycle.

It is within the contemplation of the invention, that the tube or duct 18 need not be of a straight design, but for example, a serpentine design.

This is particularly useful in the design of the heat dissipation arrangement in order to achieve a specific length of tube, given volume constraints of the communication unit.

Referring now to FIG. 5 a flow chart of the operation of the heat dissipation arrangement is shown, according to the preferred embodiment of the invention. The communication unit is in a standard operating mode, as shown in step 40. If the temperature of the communication unit is cold, for example below a certain temperature threshold, as in step 42, the unit remains in the standard operating mode, as shown in step 40.

However, if the temperature of the communication unit is hot, for example above a certain temperature threshold, as in step 42, the communication unit activates a sub-audio driver signal 44 at resonance Fres to remove excess heat from any internal heat sources via the tube 18. In this manner, as the preferred option, the operation of the acoustic velocity transformation of air-flow to cool the communication unit is controlled by means of determining whether a threshold temperature of the communication unit is exceeded.

It is also within the contemplation of the invention that alternative arrangements for determining whether to operate in the acoustic transformation mode may be used, for example based on duty cycle information of a power amplifier, type of transmissions taking place that result in additional use of a digital signal processor, transmission periods etc.

Thus a heat dissipation arrangement and method of operation are provided that alleviates some problems associated with heat sinking techniques and in particular to provide a more efficient arrangement for transferring heat away from power generating devices in mobile and portable communication units.

Claims

1. A heat dissipation arrangement comprising: a heat generator; and means for acoustically generating an air-flow to the heat generator to dissipate heat from the heat generator by means of the air-flow.

2. A heat dissipation arrangement according to claim 1, wherein the means for acoustically generating an air-flow comprises a vibrating member such that vibration of the vibrating member acoustically generates the air-flow to dissipate heat from the heat generator.

3. A heat dissipation arrangement according to claims 2, wherein the means for acoustically generating an air-flow generates electromagnetic acoustic waves.

4. A heat dissipation arrangement according to any one of claims 2 or 3, wherein the means for acoustically generating an air-flow further comprises a duct positioned substantially adjacent to the vibrating member, to direct air flow to the heat generator.

5. A heat dissipation arrangement according to claim 4, wherein the duct has a resonant frequency substantially below the audio range of the human ear.

6. A heat dissipation arrangement according to claims 4 or 5, further comprising a baffle member positioned substantially at one end of the duct for prevention of particles entering the duct.

7. A heat dissipation arrangement according to any one of the preceding claims wherein an inner surface of the duct includes a heat sinking structure.

8. A heat dissipation arrangement according to any one of the preceding claims wherein the generation of an air-flow is performed within an antenna of a communications unit.

9. A heat dissipation arrangement according to any one of claims 2 to 8, wherein the flexible member is an audio enunciator.

10. A heat dissipation arrangement according to claim 9, wherein the audio enunciator is a loudspeaker.

11. A heat dissipation arrangement according to any one of claims 4 to 10, wherein'the duct has at least one end closed by a flexible diaphragm.

12. A heat dissipation arrangement according to claim 11, wherein the inner diameter of the flexible diaphragm is less than a diameter of the duct thereby creating a longitudinal wave of air-flow.

13. A heat dissipation arrangement according to claim 12, wherein the inner diameter of the diaphragm is substantially between the ranges (0.4 to 0.8) *D, the diameter of the duct.

14. A heat dissipation arrangement according to any of the preceding claims wherein the heat generator is at least one of the following: an audio amplifier, a power amplifier, a signal processor, a battery unit.

15. A communication unit comprising a heat dissipation arrangement according to any of the preceding claims.

16. A communication unit according to claim 15, wherein the communication unit is a portable communication unit.

17. A communication unit according to claims 15 or 16, wherein a duct for facilitatitng air-flow is positioned substantially external to the communication unit.

18. An antenna for a portable communications unit comprising a duct with at least one open end, positioned substantially in parallel with and internal to the antenna for facilitating air-flow from the portable communications unit out of the at least one open end of the duct.

19. A method of dissipating heat from a heat generator comprising the steps of: generating an acoustic wave of air flow; and directing the acoustic wave of air flow to the heat generator to remove heat therefrom.

20. A method of dissipating heat according to claim 19, wherein the step of generating an acoustic wave includes the step of: vibrating a vibrating member to generate an electromagnetic acoustic wave.

21. A method of dissipating heat according to claims 19 or 20, further comprising the steps of: monitoring a temperature of at least one heat generator; and initiating an acoustic air flow cooling operation when the temperature exceeds a threshold level.

22. A method of dissipating heat according to claims 19 to 21, wherein the vibrating member is a loudspeaker in a communication unit and the heat generator is at least one of the following, an audio amplifier, a power amplifier, a signal processor, such that air-flow is directed along a duct substantially adjacent to or through the heat generator.

23. A communication unit substantially as hereinbefore described with reference to, or as illustrated by FIG. 1, FIG. 2 or FIG. 4 of the drawings.

24. A method of dissipating heat substantially as hereinbefore described with reference to, or as illustrated by FIG. 3 or FIG. 5 of the drawings.