WO2010050973A1

WO2010050973A1 - Insulated conductive outer surface

Info

Publication number: WO2010050973A1
Application number: PCT/US2008/082042
Authority: WO
Inventors: Patrick R. Conway
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2008-10-31
Filing date: 2008-10-31
Publication date: 2010-05-06
Also published as: TW201021351A

Abstract

A method and apparatus for reducing unexpected sensations when a user contacts a conductive outer surface of a device are herein disclosed. In one embodiment, a computer system includes a conductive outer surface touchable by a user during computer system operation, a ground conductor, and a protection device. The protection device is coupled between the conductive outer surface and the ground conductor. The protection device conducts current from the conductive outer surface to the ground conductor when the voltage across the protection device exceeds a preselected threshold voltage, and insulates the outer surface from voltages within the computer system that are lower than the preselected threshold voltage.

Description

INSULATED CONDUCTIVE OUTER SURFACE

BACKGROUND

[0001] Computer manufacturers continually endeavor to make their products more appealing to consumers. Performance, features, and cost are attributes that often attract buyers to one or another computer system. [0002] Product aesthetics is also important to many consumers. Accordingly, computer manufacturers provide products in a wide variety of colors. Allowing a purchaser to select from a palette of available colors allows the customer to tailor the appearance of the system to suit his individual tastes. [0003] Durability is another product characteristic valued by computer purchasers. The ability to withstand the wear and tear of use, while especially important in harsh environments, is often attractive to users in less demanding environments as well.

[0004] Adding a layer of metal to a computer's outer surface is one means of making a computer both more aesthetically appealing and more durable. For example, an aluminum skin serving as the outer surface of a notebook computer is sometimes employed to make the device both more attractive and more robust. Unfortunately, adding a conductive surface to a computer's outer shell is not without its drawbacks. Users sometimes sense currents flowing into the metallic outer shell from within the computer. Such sensations can cause the user to erroneously conclude that the computer is defective. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which: [0006] Figure 1 shows a side view of a computer that isolates a conductive outer surface from an internal ground conductor in accordance with various embodiments;

[0007] Figure 2 shows a perspective view of a computer that isolates a conductive outer surface from an internal ground conductor in accordance with various embodiments;

[0008] Figure 3 shows an exemplary diagram of a system that uses varistors to isolate a computer's outer surface from internal currents, and to protect the computer from electrostatic discharge in accordance with various embodiments; and

[0009] Figure 4 shows a flow diagram for a method for safely isolating a conductive surface of an electronic device from internal currents in accordance with various embodiments.

NOTATION AND NOMENCLATURE

[0010] Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, computer companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to... ." Also, the term "couple" or "couples" is intended to mean either an indirect, direct, optical or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through an optical electrical connection, or through a wireless electrical connection.

DETAILED DESCRIPTION

[0011] The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.

[0012] Many portable electronic devices, and in particular, portable computer systems, such as notebook computers, tablet computers, etc., can be powered from either an internal battery, or external line power (e.g., 120 volt (V) 60 hertz (Hz) alternating current (AC)). When connected to line power, a power supply (an AC to direct current (DC) converter) is generally used to transform the AC power provided by the electrical mains to a DC voltage useable by the computer. For example, in some systems an AC/DC converter may convert 120 V AC power to 20 V DC power. The DC power can be used to charge the internal battery and, after further regulation, to power the various devices and circuitry comprising the computer. To help reduce the size of the computer, AC/DC conversion is generally performed in a module external to the computer. Such an AC/DC power supply can be connected to the computer and the power mains by appropriate cabling. The AC/DC power supply is generally called an AC adapter. [0013] A notebook computer, or other electronic device, may have a conductive outer surface, for example, a metallic skin, for aesthetic or other reasons. Such surfaces are accessible to a user, and may be connected to a ground conductor within the system, for safety reasons for example. Unfortunately, under some circumstances, alternating current can leak onto the system ground when the computer is powered from an AC adapter. The leakage currents are usually quite low, and therefore pose no danger to users. Many users are unable to detect the small currents. However, some users can detect the currents. Users who sense the currents sometimes report experiencing a tingling or vibratory sensation. Some such users incorrectly believe the sensation to result from a harmful electric shock, and therefore find the sensation to be objectionable. [0014] Embodiments of the present disclosure eliminate the objectionable sensations experience by users who touch a conductive outer surface of an electronic system, while also protecting the system's internal components from harmful electrostatic discharge ("ESD"). Embodiments inhibit current flow from within a computer to the computer's outer skin, and allow conduction of currents from the outer skin to an internal ground conductor when the voltage differential between the outer skin and the ground conductor exceeds a predetermined threshold.

[0015] Figure 1 shows a side view of an exemplary computer 100 that isolates a conductive outer surface 102 from an internal ground conductor 104 in accordance with various embodiments. Computer 100 comprises display 1 16, input devices (e.g., a keyboard), and various internal components 108, such as a processor, memory, input/output devices, and storage peripherals interconnected by buses, and battery. The computer 100 can be powered by the internal battery or by an AC adapter 110. The AC adapter 110 connects to a power main via, for example, AC connection cable 114. An AC/DC converter in the AC adapter 1 10 converts the AC line voltage to a DC voltage suitable for powering the computer 100 (e.g., 20 V DC). DC power is provided to the computer 100 through cable 1 12. The cable 1 12 preferably comprises at least a pair of conductors, one of which is a ground conductor. In some embodiments, the ground conductor of cable 1 12 is connected to the internal ground conductor 104 of the computer 100. [0016] Unfortunately, various leakage paths within the AC adapter 110 can allow for conduction of AC current onto the ground signal provided through cable 1 12. If the conducting outer surface 102 of the computer 100 is electrically connected to the ground conductor 104, a user who is exposed to the outer surface 102 may experience an objectionable sensation resulting in a perception that the computer 100 is faulty.

[0017] One possible solution is to retain a direct connection between the conducting surface 102 and the ground conductor 104, and to coat the conducting outer surface 102 with an insulator to prevent the user from making contact with the conducting surface 102. This solution requires extra manufacturing steps and results in potentially lower yield due to coating defects. [0018] Another possible solution provides no electrical connection between the conductive surface 102 from the ground conductor 104. This solution prevents current flow from the ground conductor 104 to the outer surface 102, thus forestalling user objections. Unfortunately, removing the connection between the conductive outer surface 102 and the ground conductor 104 also makes the computer 100 more susceptible to damage caused by electrostatic discharge. [0019] An electrostatic discharge ("ESD") can occur when a charged conductor, for example, the body of a user, which may be charged to several thousand volts, discharges into another conductor at a different potential. With regard to the computer 100, a charged user can discharge into the conducting outer surface 102 of the computer 100. If left floating, the charged outer surface 102 may thereafter discharge into the internal components 108 of the computer 100 damaging the components 108 and/or causing the computer 100 to malfunction. [0020] Embodiments of the present disclosure, neither directly connect the ground conductor 104 to the outer surface 102, nor provide a completely free- floating outer surface 102. Furthermore, embodiments of the present disclosure exclude an insulative coating over the conductive outer surface 102. Instead, embodiments of the present disclosure provide a path for current to flow from the outer surface 102 to the ground conductor 104 when the voltage between the outer surface 102 and the ground conductor 104 exceeds a predetermined threshold. Current flow is inhibited when the voltage between the outer surface 102 and the ground conductor 104 is below the predetermined threshold. [0021] In at least some embodiments, the functionality described above is provided by one or more protection devices 106 coupled between the ground conductor 104 and the outer surface 102. As shown in Figure 1 , the protection devices 106 preferably provide the only electrical connection between the conductive outer surface 102 and the ground conductor 104. Accordingly, the conductors of the DC power cable 112 are isolated from contact with the conductive outer surface 102, for example, by insulator 1 18 or other appropriate insulating means.

[0022] The protection devices 106 can comprise a variety of components, for example, a varistor (i.e., a voltage dependent resistor) or a transient voltage suppression diode. The protection device 106 is preferably selected to have a relatively high resistance (e.g., in the megohms) at voltages expected to be found between the outer surface 102 and the ground conductor 104 during system operation, thus leakage current is preferable kept relatively low. For example, if a voltage differential of up to 60 V is expected due to AC leakage onto the ground conductor 102, a protection device 106 that exhibits a high resistance at and below 60 V, and a low resistance at a somewhat higher voltage can be selected. Some embodiments may employ a WPSSAD0805-101 -01 surge suppressor provided by World Products Inc. having a threshold of approximately 1 15 V. However, embodiments are not limited to any particular protection device or any particular operational voltage threshold.

[0023] Additionally, the capacitance of the protection device 106 should preferably be relatively low, for example, less than about 3 pico-farads, to block any alternating current present on the ground conductor 104. [0024] Figure 2 shows a perspective view of the computer 100 that isolates a conductive outer surface 102 from an internal ground conductor in accordance with various embodiments. Figure 2 shows that the conductive outer surface 102 is a surface of the computer that is accessible to a user during normal operation of the computer. The user is likely to come into contact with the outer surface 102 while operating the computer 100.

[0025] Figure 3 shows an exemplary diagram of a system that uses varistors to isolate a computer's outer surface from internal currents, and to protect the computer from electrostatic discharge in accordance with various embodiments. An internal chassis 304 of the computer is grounded. The chassis 304 is preferably composed of conductive material (e.g., metal), and can provide mechanical support to the internal components 108. Embodiments include one or more varistors 306 to connect the chassis 304 to the conductive outer surface 102 of the computer. The varistors 306 exhibit a high resistance at low voltages and a low resistance at high voltages. The threshold voltage of the varistors 306 is selected to inhibit current from flowing through the varistors to the outer shell 102 under normal conditions (e.g., when voltage between the outer shell 102 and the chassis 304 is 60 V or less). However, current can flow from the outer shell 102 to the chassis 304 when a voltage develops across the varistors 306 that exceeds the threshold, for example and ESD voltage. Some embodiments employ other types of protection devices, for example, transient voltage suppression diodes, in place of or in addition to varistors 306. [0026] Figure 4 shows a flow diagram for a method for safely isolating a conductive surface 102 of an electronic device 100 from internal currents in accordance with various embodiments. Though depicted sequentially as a matter of convenience, at least some of the actions shown can be performed in a different order and/or performed in parallel. Additionally, some embodiments may perform only some of the actions shown. In block 402, currents flowing within the device are inhibited from flowing into a conductive outer surface 102 of the device 100. Blocking this current flow prevents a user at a different potential from sensing the voltage, when he touches the outer surface 102, and assuming, based on the sensation, that the device 100 is defective. Thus, various complaints and device 100 returns are avoided.

[0027] In block 404, the device 100 (e.g., via protection device(s) 106 or 306) determines whether a voltage between the conductive outer surface 102 and an internal ground reference (e.g., a ground conductor 104 voltage) exceeds a predetermined threshold. The threshold is selected to inhibit current flow when the voltage is below the threshold and to allow current flow when the voltage exceeds the threshold. For example, embodiments may employ a 100 V threshold if the voltage expected to develop between the outer surface 102 and the ground reference 104 is approximately 60 V.

[0028] In block 406, the protection device 106 coupled between the outer surface and the ground reference 104 is activated. Such activation automatically occurs in response to the determination of block 404 when the voltage between the outer surface 102 and the ground reference 104 exceed the predetermined threshold. In some embodiments, the protection device 106 can comprise a varistor, a transient voltage suppression diode, or other appropriate device. [0029] In block 408, current is conducted from the outer surface 102 to the ground conductor 104. The current can comprise current flow due to an electrostatic discharge created by charge transfer from a user to the conductive outer surface 102. Conducting current to ground prevents excessive current flow through sensitive system components 108. Thus, the device 100 is protected from damaging ESD, while insulating the conductive outer surface 102 from currents flowing in a ground conductor 104, and reducing the incidence of user complaints resulting from sensations of current flow.

[0030] The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, while embodiments are described primarily in terms of notebook computer systems, those skilled in the art will recognize that embodiments of the invention are applicable to a wide variety of electrical and electronic systems. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Claims

CLAIMS What is claimed is:

1. A computer system, comprising: a conductive outer surface touchable by a user during computer system operation; a ground conductor; and a protection device coupled between the conductive outer surface and the ground conductor; wherein the protection device conducts current from the conductive outer surface to the ground conductor when the voltage across the protection device exceeds a preselected threshold voltage, and insulates the outer surface from voltages within the computer system that are lower than the preselected threshold voltage.

2. The computer system of claim 1 , wherein the ground conductor comprises a conductive inner chassis, and the protection device is coupled to the conductive inner chassis.

3. The computer system of claim 1 , further comprising a plurality of protection devices coupled between a plurality of points on the conductive outer surface and a plurality of points on the ground conductor.

4. The computer system of claim 1 , wherein the protection device conducts only voltages greater than about 100 volts from the conductive outer surface to the system ground.

5. The computer system of claim 1 , wherein no insulative coating insulates the conductive surface from the user.

6. The computer system of claim 1 , wherein the protection device prevents a user from detecting current flowing into the conductive outer surface from within the computer system.

7. The computer system of claim 1 , wherein the protection device comprises one of a varistor and a transient voltage suppression diode.

8. The computer system of claim 1 , wherein the capacitance of the protection device is small enough to prevent passage of current alternating at power mains frequencies from within the computer to the conductive outer surface.

9. A method, comprising: determining whether a voltage between a conductive outer surface of a device and an ground conductor within a device exceeds a predetermined threshold; and conducting current from the conductive outer surface of the device to the ground conductor within the device when the voltage between the conductive outer surface and the ground conductor exceeds the predetermined threshold.

10. The method of claim 9, further comprising preventing current from flowing from within the device to the conductive outer surface of a device.

1 1. The method of claim 9, further comprising activating a protection device coupled between the conductive outer surface and the ground conductor to conduct current from the conductive outer surface to ground conductor.

12. An electrical device, comprising: a conductive outer shell; means for preventing a user from sensing a current flowing from within the device to the outer shell; and means for conducting current from the outer shell to a ground conductor within the device when the voltage between the outer shell and the ground conductor exceeds a predetermined threshold.

13. The electrical device of claim 12, further comprising means for insulating the outer shell from currents flowing within the device.

14. The electrical device of claim 12, further comprising means for determining whether the voltage across the outer shell and the ground conductor exceeds the predetermined threshold.

15. The electrical device of claim 12, further comprising means for protecting the internal components of the device from electrostatic discharge and isolating the conductive shell from line frequency alternating current.