Patent Grant
8508328
1. Field of the Invention
This invention relates to an insertable polymer positive temperature coefficient (PPTC) over-current protection device, and more particularly to an insertable PPTC over-current protection device including a PTC polymer matrix formed with at least one hole.
2. Description of the Related Art
U.S. Pat. No. 4,238,812 discloses an over-current protection device that includes a thin film PTC element, two electrodes formed on two opposite surfaces of the PTC element, and two conductive lead pins attached to the electrodes, respectively. The PTC element can be made from a polymer material containing a conductive filler.
There is a need to enhance the electrical properties, such as the high-voltage endurability, of the over-current protection device so as to protect the over-current protection device from being damaged by an overvoltage. Although the high-voltage endurability can be enhanced by increasing the area or the thickness of the PTC element, the size of the over-current protection device is undesirably increased.
Therefore, an object of the present invention is to provide an insertable PPTC over-current protection device that can overcome the aforesaid drawback associated with the prior art.
According to this invention, there is provided an insertable PPTC over-current protection device that comprises: first and second electrodes, each of which has an outer surface and a peripheral edge; a solder material; conductive lead pins bonded to the first and second electrodes, respectively, each of the lead pins having a connecting segment extending along and bonded to the outer surface of the respective one of the first and second electrodes through the solder material, and a free segment extending outwardly from the connecting segment beyond the peripheral edge of the respective one of the first and second electrodes; and a PTC polymer matrix laminated between the first and second electrodes and having two opposite surfaces and a peripheral edge. The opposite surfaces of the PTC polymer matrix are in contact with the first and second electrodes, respectively. The PTC polymer matrix is formed with at least one hole that extends between the opposite surfaces of the PTC polymer matrix, that is spaced apart from and that is surrounded by the peripheral edge of the PTC polymer matrix, and that has an effective volume to accommodate thermal expansion of the PTC polymer matrix when temperature of the PTC polymer matrix is increased.
In drawings which illustrate embodiments of the invention,
Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure.
In this embodiment, the hole 20 in the PTC polymer matrix 2 is a through-hole extending through the opposite surfaces 21 of the PTC polymer matrix 2, has a circular periphery, and extends along a line (not shown) passing through a geometrical center of the PTC polymer matrix 2 and transverse to the opposite surfaces 21 of the PTC polymer matrix 2. Alternatively, the hole periphery of the hole 20 in the PTC polymer matrix 2 can be square, oval, triangular, or crisscross in shape.
Each of the first and second electrodes 3 is formed with an opening 31 in spatial communication and aligned with the hole 20 in the PTC polymer matrix 2.
The PTC polymer matrix 2 is made of a PTC composition that comprises a non-grafted olefin-based polymer, optionally an unsaturated carboxylic acid grafted olefin-based polymer, and a conductive filler dispersed therein. Preferably, the non-grafted olefin-based polymer is non-grafted high density polyethylene (HDPE), and the unsaturated carboxylic acid grafted olefin-based polymer is carboxylic acid anhydride grafted HDPE. Preferably, the conductive filler is carbon black, metal powders, or conductive ceramic powders.
The following examples and comparative examples are provided to illustrate the preferred embodiments of the invention, and should not be construed as limiting the scope of the invention.
10.925 grams of HDPE (polymer 1, purchased from Formosa Plastics Corp., catalog no.: HDPE9002), 10.925 grams of carboxylic acid anhydride grafted HDPE (G-HDPE, polymer 2, purchased from DuPont, catalog no.: MB100D), and 28.15 grams of carbon black powder (purchased from Columbian Chemicals Co., catalog no. Raven 430UB) were compounded in a Brabender mixer. The compounding temperature was 200° C., the stirring rate was 30 rpm, and the compounding time was 10 minutes. The compounded mixture was hot pressed in a mold so as to form a PTC polymer matrix having a thickness of 0.35 mm. The hot pressing temperature was 200° C., the hot pressing time was 4 minutes, and the hot pressing pressure was 80 kg/cm2. Two copper foil sheets were respectively attached to two opposite surfaces of the PTC polymer matrix and were hot pressed under 200° C. and 80 kg/cm2 for 4 minutes to form a sandwiched structure of a PTC laminate. The PTC laminate was cut into a plurality of chips with a chip size of 13.5 mm×13.5 mm. Each chip was irradiated with a Co-60 gamma ray for a total irradiating dose of 150 kGy, and was subsequently punched to form a through-hole (having a circular hole periphery with a diameter (d) of 1.5 mm and a hole area (πd2/4) of 1.77 mm2) in a central portion of the PTC polymer matrix and an opening in each of the copper foil sheets. A pair of conductive leads were then welded to the copper foil sheets of each chip, respectively, so as to form an insertable PPTC over-current protection chip-sized device. The resistance of each chip-sized device thus formed was measured, and an average resistance of the chip-sized devices thus formed was calculated (see Table 1).
The procedures and conditions in preparing the chip-sized devices of Examples 2-5 were similar to those of Example 1 except for the shape, the hole diameter (or the total hole area), and/or the number of the hole(s) in the PTC polymer matrix (see Table 1).
The procedures and conditions in preparing the chip-sized devices of Example 6 were similar to those of Example 1 except for the hole diameter or the total hole area (see Table 1). In addition, the hole in the PTC polymer matrix of Example 6 was a blind hole (having a depth of about 0.175 mm) and was formed by drilling.
The procedures and conditions in preparing the chip-sized devices of Example 7 were similar to those of Example 1 except for the hole diameter or the total hole area and the number of the hole (s) (see Table 1). In addition, the holes in the PTC polymer matrix of Example 7 were blind holes (having a depth of 0.175 mm) and were formed by drilling.
The procedures and conditions in preparing the chip-sized devices of Comparative Examples 1-3 were similar to those of Example 1, except that there is no hole formed in the PTC polymer matrix of each chip-sized device for Comparative Example 1, that there is a cured epoxy resin completely filling the hole (having a diameter of 1.5 mm and located at a position the same as that of the first preferred embodiment) in the PTC polymer matrix of each chip-sized device for Comparative Example 2, and that there are two semi-circular holes 23 formed in the PTC polymer matrix (as illustrated in
Performance Test
Switching Cycle Test
Ten chips prepared from each of Examples 1-7 and Comparative Examples 1-3 were subjected to a switching cycle test. The switching cycle test for each chip-sized device was conducted under a voltage of 90Vdc and a current of 120 A by switching on for 60 seconds and then off for 60 seconds for each cycle. The number (n) of the chip-sized devices for each of Examples 1-7 and Comparative Examples 1-3 passing 720 switching cycles was recorded, and a passing rate (n/10×100%) for each of Examples 1-7 and Comparative Examples 1-3 was calculated. The switching cycle test results are shown in Table 2.
Thermal Runaway Test
Ten chip-sized devices prepared from each of Examples 1-7 and Comparative Examples 1-3 were subjected to a thermal runaway test. The thermal runaway test for each chip-sized device was conducted by increasing stepwise the voltage applied to each chip-sized device from an initial voltage of 60Vdc to a breakdown voltage under a fixed current of 50A that is sufficient to enable each chip-sized device to trip at the initial voltage. The applied voltage was increased at an increment of 10Vdc per step and the duration time for each step was 2 minutes (i.e., each newly applied voltage lasted for two minutes). The breakdown voltages were recorded for the chip-sized devices of each of Examples 1-7 and Comparative Examples 1-3, and an average breakdown voltage of the chips of each of Examples 1-7 and Comparative Examples 1-3 was calculated based on the measured breakdown voltages of the chip-sized devices of each of Examples 1-7 and Comparative Examples 1-3. The thermal runaway test results are shown in Table 2.
The switching cycle test results and the thermal runaway test results demonstrate that formation of at least one hole in the PTC polymer matrix having an effective volume to accommodate expansion of the PTC polymer matrix can have an unexpected improvement in the high-voltage endurability of the insertable PPTC over-current protection device.
While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.
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