Patent Application
20140118871
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1. Field of the Invention
The present application relates to an over-current protection device, and more particularly to a surface-mountable over-current protection device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Over-current protection devices are used for protecting circuitries from damage caused by over-heat or over-current. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has positive temperature coefficient (PTC) characteristic that the resistance thereof remains extremely low at room temperature and instantaneously increases to thousand times when the temperature reaches a critical temperature or the circuit has over-current, so as to suppress over-current and protect the cell or the circuit device. When the resistive material gets back to the room temperature or over-current no longer exists, the over-current protection device returns to be of low resistance and as a consequence the circuitry again operate normally. In view of the reusable property, the PTC over-current protection devices can replace traditional fuses, and have been widely applied to high density circuits.
When a low-resistance and high-current PTC device is surface-mounted to a printed circuit board or a mother board of which the connecting line has different widths according to various designs. Most of the copper connecting lines of the mother board connecting to the PTC device have a larger circuit area, therefore the trip time of the PTC device may be out of the specification. Owing to small difference of abnormal current and hold current of the PTC device and more efficient heat dissipation of the connecting line with a larger circuit area, the heat accumulation rate of the PTC device is lowered and therefore the trip time of the PTC device will be delayed.
If abnormal over-current event is not removed for a long time, the PTC material may be deteriorated, burned, and/or a short circuit may occur between the upper and lower metal foils of the PTC material layer. As a result, the PTC device no longer provides over-current protection and therefore the circuits of the mother board or the circuit boards may be burned or damaged also.
The present application relates to an over-current protection device, and more particularly to a surface-mountable over-current protection device. The over-current protection device of the present application can trip timely to be in compliance with the specification regardless of the different connecting lines on the circuit board or the mother board.
According to an embodiment of the present application, a surface-mountable over-current protection device has opposite upper and lower surfaces. The surface-mountable over-current protection device comprises a PTC device, a first electrode, a second electrode, a first circuit and a second circuit. The PTC device comprises a PTC material layer, a first conductive layer and a second conductive layer. The PTC material layer is disposed between the first conductive layer and the second conductive layer and contains crystalline polymer and conductive filler, e.g., carbon black, metal or conductive ceramic powder, dispersed therein. The first electrode comprises a pair of first metal foils formed at the upper and lower surfaces of the device, and the second electrode comprises a pair of second metal foils formed at the upper and lower surfaces of the device. The first circuit is configured to electrically connect the first electrode and the first conductive layer, and comprises a first planar line extending horizontally and a first conductive connecting member extending vertically. The second circuit is configured to electrically connect the second electrode and the second conductive layer, and comprises a second planar line extending horizontally and a second conductive connecting member extending vertically. At least one of the first planar line and the second planar line has a thermal resistance sufficient to mitigate heat dissipation by which the over-current protection device undergoes a test at 25° C. and 8 amperes can trip within 60 seconds.
In an embodiment, if a short-circuit between the first and second conductive layers occurs, this event may be caused by the deterioration of the PTC material layer when over-current happens, at least one of the first and second planar lines is so narrow that it can be melted and broken to cut off the over-current.
In an embodiment, at least one of the first and second planar lines has a minimum width less than ⅔ of the width of the connected corresponding first electrode or second electrode.
In an embodiment, the ratio of the length along current flowing direction to the minimum width of the first and/or second planar line is greater than one.
According to another embodiment of the present application, a surface-mountable over-current protection device has opposite upper and lower surfaces. The surface-mountable over-current protection device comprises a PTC device, a first electrode, a second electrode, a first circuit and a second circuit. The PTC device comprises a PTC material layer, a first conductive layer and a second conductive layer. The PTC material layer is disposed between the first conductive layer and the second conductive layer and contains crystalline polymer and conductive filler dispersed therein. The first electrode comprises a pair of first metal foils formed at the upper and lower surfaces of the device, and the second electrode comprises a pair of second metal foils formed at the upper and lower surfaces of the device. The first circuit electrically connects to the first electrode and the first conductive layer, whereas the second circuit electrically connects to the second electrode and the second conductive layer. At least one of the first and second circuits comprises a fuse element, and the fuse element and the PTC device are in series connection between the first and second electrodes. If a short-circuit between the first and second conductive layers occurs due to the deterioration of the PTC material layer, the fuse element will be melted and broken to cut off short-circuit current. In an embodiment, the fuse element has a minimum width less than ⅔ of the width of the first or second electrode.
Because narrow planar circuits are employed, the device will not be affected by high heat dissipation to the external circuit to delay trip timing if abnormal current occurs. Therefore, the surface-mountable over-current protection device of the present application can meet the test requirement of the specification. More specifically, the planar line is so thin that it has sufficient thermal resistance to mitigate heat dissipation, and therefore the device can trip timely to meet the specification.
Moreover, if the over-current is not removed for a long time, the PTC material sustained in high-temperature trip state will be deteriorated, or carbonized, for example. As a consequence, the first and second conductive layers in open-circuit state when the device trips may change to be in short-circuit state. When short-circuit occurs, instantaneous large current can melt and break the planar line to cut off the current so as to provide fuse protection. Even if there is short-circuit in the over-current protection device, the device contains a circuit to be melted and broken and therefore the short-circuit will not damage the circuits of the mother board or the circuit board. By means of changing the internal or external line circuit of the device, the narrow planar line can be melted and broken first if short-circuit occurs between the upper and lower conductive layer of the PTC device, so as to prevent the circuits in the circuit board from damage or explosion.
The present application will be described according to the appended drawings in which:
The making and using of the presently preferred illustrative embodiments are discussed in detail below. It should be appreciated, however, that the present application provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific illustrative embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
In an embodiment, the first planar line 211 is formed at the upper surface 24 and connects to the first electrode 17. The first conductive connecting member 212 is formed at the first lateral surface 26, and connects to the first planar line 211 and the first conductive layer 13. The second planar line 221 is formed at the upper surface 24 and connects to the second electrode 18. The second conductive connecting member 222 is formed at the second lateral surface 27, which is opposite to the lateral surface 26, and connects to the second planar line 221 and the second conductive layer 14. The first and second conductive connecting member 212 and 222 may be conductive plated through holes (PTH) as shown in
In another embodiment, the lower surface 25 may be further provided with planar lines as those formed at the upper surface 24, and as a result it is not necessary to consider the device direction for soldering.
In yet another embodiment, a vertical conductive hole may be formed at the first end surface 28 to connect the upper and lower metal foils 171, and the vertical conductive hole is insulated from the first and second conductive layers 13 and 14. Likewise, another vertical conductive hole may be formed at the second end surface 29 to connect the upper and lower metal foils 181, and is insulated from the first and second conductive layers 13 and 14. As a result, the upper and lower metal foils 171 are electrically connected, and the upper and lower metal foils 181 are electrically connected. Therefore, even if there is no planar lines at the lower surface 25, it is not necessary to consider device direction for soldering either.
The first insulating layer 15 is formed on the first conductive layer 13, and the second insulating layer 16 is formed on the second conductive layer 14. The first and second metal foils 171 and 181 at the upper surface 24 are formed on the first insulating layer 15, and the first and second metal foils 171 and 181 at the lower surface 25 are formed on the second insulating layer 16. A solder mask 23 is formed on a surface of the insulating layer 15 between the first and second metal foils 171 and 181 at the upper surface 24, and overlays the planar lines 211 and 221. Similarly, the second insulating layer may be overlaid by a solder mask 23 as well for insulation.
In an embodiment, the first planar line 211 has a minimum width less than ⅔ of the first electrode 17, and/or the second planar line 221 has a minimum width less than ⅔ of the second electrode 18. For example, the minimum width of the first portion 241 is less than ⅔ of the width “W” of the first electrode 17, and the minimum width of the first portion 251 is less than ⅔ of the width “W” of the second electrode 18. In practice, the minimum width of the planar line may be less than ½ or ⅓ for increasing thermal resistance further. In another embodiment, the ratio of the length along current flowing direction to the minimum width of the first planar line 211 and/or second planar line 221 is greater than 1, or larger than 2, 3, 5, 7 or 10 in particular. The ratio usually is not greater than 20. When current goes through narrower first planar line 211 or second planar line 221 in comparison with the widths of the electrode 17 or 18, the higher electrical resistance of the planar lines 211 and 221 will generate heat. However, the smaller heat conduction area of the planar line 211 or 221 has higher thermal resistance. Therefore, the heat generated from the PTC material layer 12 cannot be conducted or dissipated effectively. In other words, the planar line has a thermal resistance sufficient to mitigate heat dissipation, so that the over-current protection device 10 can be trip timely to pass the specification test.
A first insulating layer 35 is formed on the first conductive layer 33, and the second insulating layer 36 is formed on the second conductive layer 34. The first and second metal foils 371 and 381 at the upper surface 24 are formed on the first insulating layer 35, and the first and second metal foils 371 and 381 at the lower surface 25 are formed on the second insulating layer 36. The first planar line 411 is formed at the upper surface 24, and the first conductive connecting member 412 penetrates through the first insulating layer 35 and connects to the first planar line 411 and the first conductive layer 33. The second planar line 421 is formed at the lower surface 25, and the second conductive connecting member 422 penetrates through the second insulating layer 36 and connects to the second planar line 421 and the second conductive layer 34. A solder mask 43 is formed on a surface of the insulating layer 35 between the first and second electrode 37 and 38, and overlays the planar line 411. Similarly, a second insulating layer 36 and the planar line 421 may be overlaid by a solder mask 43 as well for insulation. The planar line 411 and the metal foil 371 may be formed from a same metal foil by etching; therefore the planar line 411 and the metal foil 371 may have a same thickness. Accordingly, the solder mask 43 on top of the planar line 411 is very thin, and is not illustrative in
In practice, the first and second circuits 41 and 42 of the over-current protection device 30 need not exist simultaneously. The device can mitigate heat dissipation as long as one of the circuits 41 and 42 can provide sufficient thermal resistance. For example, the first circuit 41 may be as shown in
Likewise, the ratio of the minimum width of the planar line to the width of the electrode and the ratio of the length along current flowing direction to the minimum width of the planar line have to meet the requirement as mentioned in the first embodiment, so as to obtain sufficient thermal resistance to mitigate heat dissipation and avoid the delay of trip timing.
The planar lines for preventing heat dissipation of the surface-mountable over-current protection device of the above two embodiments are formed at the device surface, i.e. the planar lines are external circuits. The planar lines can be formed inside the device as internal circuit as well, as mentioned below.
Because the narrow planar line 53 or 54 has high electrical resistance, it will generate heat when electric current flows therethrough. Moreover, the smaller heat conduction area of the planar line 53 or 54 has higher thermal resistance. Therefore, the heat generated from the PTC material layer 32 will not be conducted or dissipated rapidly, thereby avoiding the delay of trip timing.
The PTC material layers 12 and 32 contain crystalline polymer and conductive filler and exhibit PTC characteristic. The crystalline polymer material layer may comprise high-density polyethylene (HDPE), medium-density polyethylene, low-density polyethylene (LDPE), polypropylene, polyvinyl chlorine, polyvinyl fluoride, copolymer of ethylene and acrylic acid, copolymer of ethylene and acrylic resin, copolymer of ethylene and vinyl alcohol, or the combination thereof. The conductive filler may be metal powder or conductive ceramic carbide powder of a resistivity less than 500 μΩ-cm. The conductive filler may comprise nickel, cobalt, copper, iron, tin, lead, silver, gold, platinum, tungsten carbide, vanadium carbide, titanium carbide, boron carbide, silicon carbide, germanium carbide, tantalum carbide, zirconium carbide, chromium carbide, molybdenum carbide or the mixture, alloy, solid solution or core-shell thereof. By using the conductive filler with low resistivity, the resistivity of the PTC material layer 12 or 32 is less than 0.2 Ω-cm.
The over-current protection devices each having a single PTC device are exemplified above. The present application can also be applied to a device structure having multiple PTC devices in series connection, such as the double-PTC device structure shown in U.S. Pat. No. 6,377,467.
Thermal resistance is equal to L/kA, where “L” is the length through which heat transfers, “A” is the cross-sectional area through which heat transfers, and “k” is the thermal conductivity of material. It can be seen from the equation that the thermal resistance is larger if “A” is smaller or “L” is larger. The larger the thermal resistance, the worse the heat dissipation is. In other words, the circuit having higher thermal resistance will obtain better prevention to heat dissipation.
Table 1 shows thermal resistances corresponding to the devices in terms of various form factors, planar line structures, and sizes. The material of the planar lines includes but not limited to copper. The thermal resistance L/kA corresponds to a single planar line. As said, if the thermal resistance of a single planar line is sufficient, the heat dissipation can be mitigated effectively.
Moreover, the devices of the above examples in Table 1 are subjected to tests by a circuit board with a line width of 60 mil. The ambient temperature of the tests are 25° C. The supply voltage is the maximum voltage of the specification of the devices, and the test current is 8 amperes. The voltage and current curves of the devices are recorded by an oscilloscope. The oscilloscope starts calculating the time when the current increases by 20%, and ends the calculation when the current decreases from the maximum current by 20%. The time period is the responding time or trip time of the device. It indicates that the device is tripped when the current decreases by 20%. In contrast, it indicates that the device does not trip if the current does not decrease.
It can be seen from Table 1 that the thermal resistances of the examples 5, 6, 13, 14 and 24 are less than 100 K/W, such small thermal resistance cannot effectively avoid heat dissipation. The thermal resistance of the devices is equal to or greater than 100 K/W, or greater than 200 K/W or 400 K/W in particular, so as to mitigate heat dissipation effectively. The devices of other examples can trip within 60 seconds when undertaking 8 amperes, in which the devices with larger thermal resistances can even trip within 5 seconds.
The PTC material may be burned due to the deterioration of the PTC material when the device had been tripped for a long time period. As a result, a short-circuit event may occur between the upper and lower conductive layers of the PTC device. When a short-circuit occurs, the first planar line and second planar line can serve as fuse elements if one or both of them are so narrow that it or they can be melted to change to be open-circuit. That is, the first planar line and the second planar line function as fuses, providing further over-current protection.
If the planar line is so narrow that it can be melted to be open-circuit when over-current occurs, the planar line is equivalent to a fuse. Accordingly, the equivalent circuit of the over-current protection device is shown in
Not only does the surface-mountable over-current protection device of the present application solve the issue that the device cannot trip timely under the test of the specification, but also it contains equivalent fuse elements to provide further protection to the circuit board mounted with the over-current protection device if the PTC device is dysfunctional.
The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 101140460 | Oct 2012 | TW | national |
