Thermoelectric cooler assembly

Abstract: A thermoelectric cooler assembly (40) in which all connections between a thermistor (56) and the thermoelectric cooler (52) are attached to a cold side (54) of the thermoelectric cooler and avoid contact with a hot side (42) of the thermoelectric cooler which reduces the sensing of the ambient temperature, as well as the hot side of the thermoelectric cooler, by the thermistor, thereby providing a more accurate temperature control.

Description: This invention relates generally to thermoelectric coolers and temperature sensors. Thermoelectric coolers are used in a wide variety of electronic applications where a component must remain either at a constantly maintained temperature or a specific temperature either warmer or cooler then the ambient temperature to work correctly. Thermoelectric coolers have a cold side and a hot side. A device to be either heated or cooled is put in contact with the cold side of the thermoelectric cooler while the hot side of the thermoelectric cooler is put in contact with a heat sink. If the device is to be cooled, then the thermoelectric cooler removes heat from the device and adds heat to the heat sink. If the device is to be heated, then the thermoelectric cooler removes heat from the heat sink and adds heat to the device. Thermoelectric coolers are often used with thermistors, which are used to sense temperatures and enable control of the thermoelectric cooler, commonly as an input to a thermoelectric cooler driver. The thermoelectric cooler and the thermistor each have two wires that must be connected. The wires are fragile, difficult to handle, and susceptable to damage from handling because the wires can be as thin as 0.05mm (0.002 inches) in diameter. The thermistor is usually mounted on the cold side of the thermoelectric cooler or to a device being cooled. To properly regulate temperature, the thermistor must only sense the thermoelectric cooler cold side temperature or the device it is trying to cool, whichever it is mounted on. However, the electrical connection to the thermistor is through wires or pins at a substantially higher ambient temperature. As a result, the thermistor senses not only the cold side temperature of the thermoelectric cooler or device being cooled but also the higher ambient temperature through the thermal conduction of the wires or pins of the electrical connections. The thermistor as a result averages these two temperatures and sends a false feedback signal to the thermoelectric cooler driver. The overall result is an ambient dependent temperature control with a set point that varies according to changes in the ambient temperature. Tests of this phenomenon show this variation at between two degrees centigrade and seven degrees centigrade. In many cooling applications, variations of this magnitude are unacceptable. In laser printing applications, thermoelectric coolers are used to cool laser diodes which have a strong temperature dependent optical power output. Variations as large as those cited cause unacceptable fluctuations in the output power of the laser diode. The unacceptable output power fluctuations in the laser diode cause additional expensive circuitry to be added to the system to maintain a constant optical power output from the laser diode. EP-A-477,841 discloses a laser module and method for fabricating it, in which a thermister assembly is formed by fixedly attaching a thermister onto a base having good thermal conductivity, and the thermister assembly and a laser diode chip are fixedly attached onto a carrier arranged in contact with an electronic cooling element using brazing materials with different melting points. Accordingly, it is a primary aim of the invention to provide a thermoelectric cooler assembly which reduces the sensing of the ambient temperature by the thermistor thereby providing a more accurate temperature control. Accordingly, the present invention provides a thermoelectric cooler assembly as defined in the appended independent claims 1 and 6. Various embodiments of the invention are defined in the dependent claims. In one aspect of the present invention, there is provided a thermoelectric coolerassembly comprising: two spaced apart ceramic plates, one of the plates being a hot side and the other of the ceramic plates being a cold side, with thermoelectric cooler elements interposed between the ceramic plates. Two conductive pins, a thermistor input pin and a thermistor output pin, are attached to the cold side, the thermistor input and output pins having a given diameter and extending through said cold side. There are two openings in the hot side and the two conductive pins extend through two holes in said hot side. The holes in the hot side have a diameter larger then the given diameter of said two conductive pins attached to said cold side, so that the pins may pass through the holes without contacting the hot side and leaving a gap between the conductive pins and the hot side. In another aspect of the invention there is provided a thermoelectric cooler assembly according to claim 6 of the appended claims. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a thermoelectric cooler and thermistor assembly of the present invention, Figure 2 is a cross section of the thermoelectric cooler and thermistor assembly shown in Figure 1 taken through cross-section line 2-2, Figure 3 is a plan view of an alternate thermoelectric cooler and thermistor assembly and Figure 4 is a cross section of the thermoelectric cooler and thermistor assembly shown in Figure 1 and attached to a printed circuit board. Figure 1 shows a thermoelectric cooler and thermistor assembly 40. The thermistor assembly 40 shown in Figure 1 has a ceramic plate 42, solid metal pins 44, 46, thermoelectric cooler 52 with a cold side 54 and a thermistor 56 attached to the cold side 54 by lead wires 58. Thermoelectric cooler elements 53 are between the ceramic plate 42 and the cold side 54. Pins 48, 50 are used to connect the thermistor 56. In Figure 1, the pins 48, 50 are soldered to a metallized pad on the cold side 54 of the thermoelectric cooler 52. The pins 48, 50 then pass through the metallized ceramic plate 42 without touching it as shown in Figure 2. An insulating gap 60 is left between ceramic plate 42 and pins 48, 50. The insulating gap 60 substantially reduces the thermal shorting between the ceramic plate 42 and the pins 48, 50. The insulating gap 60 can be filled with any insulative material with a preference for a simple air gap as shown in Figure 2. This construction puts the lead wires 58 of the thermistor 56 in physical contact with the cold side 54 of the thermoelectric cooler 52. This effectively eliminates the problem of the thermistor 56 sensing not only the cold side temperature of the thermoelectric cooler 52 but also the higher ambient temperature through the thermal conduction of the pins 48, 50. Thermal loads from the ambient temperature through convection and radiation are still present, however, these thermal loads play a minor role compared to thermal conduction loads. Figure 3 shows an alternate embodiment of the same assembly using a surface mounted thermistor 56'. Parts in Figure 3 that are the same as parts in Figure 1 are given the same reference numeral with a prime designation (') appended. The structure in Figure 3 is very similar to the structure in Figure 1 including a ceramic plate 42' with solid metal pins 44', 46' extending through the ceramic plate 42', and a thermoelectric cooler 52' with a cold side 54'. Attached to the cold side 54' of the thermoelectric cooler 52' is a thermistor 56'. The pins 48, 50 shown in Figure 1 are no longer needed since the thermistor 56' is mounted directly on the cold side 54' of the thermoelectric cooler 52' via solder bumps 58'. Figure 4 shows a third embodiment which mounts the assembly of Figure 1 onto a printed circuit board or other device. Parts in Figure 4 that are the same as parts in Figure 1 are given the same reference numeral with a double prime designation (") appended. Figure 4 shows a thermoelectric cooler 52" with its cold side 54" exposed. Attached to the cold side 54" is a thermistor 56" with lead wires 58". The lead wires 58" are attached to pins 48", 50" (pin 50"not shown in Figure 4). The pins 48", 50" extend through ceramic plate 42" as well as a circuit board 70 and heat sinking plate 72. Holes 74 are left in heat sinking plate 72 as well as holes 76 which are left in circuit board 70 so that an appropriate heat sink or housing may later be attached. The entire assembly is attached to the circuit board 70 by four solder joints 78, two of which are shown, between the pins 44", 46", 48", 50" and the circuit board 70. Embodiments of the present invention provide a thermoelectric cooler and temperature sensor assembly which is easier to install, has fewer loose wires, better temperature maintenance and improved reliability over prior art devices.

Claims: A thermoelectric cooler assembly (40) comprising: a) two spaced apart ceramic plates (42,54), one (42) of said plates serving as a hot side and the other (54) serving as a cold side when the thermoelectric cooler is in use, b) thermoelectric cooler elements (53) interposed between said ceramic plates (42,54), c) two conductive pins (48,50), which serve as thermistor input and output pins, both of said pins attached to said cold side, said thermistor input pin and thermistor ouput pin having a given diameter and extending through said cold side, d) two openings (60) in said hot side, said openings being of a diameter larger than the given diameter of said thermistor input pin and thermistor output pin, and e) said two conductive pins also extending through said hot side, said two conductive pins and said two openings being constructed and arranged with relative dimensions such that each one of said two conductive pins passes through its respective opening without contacting said hot side and is spaced from and not contacting the hot side so that a gap is formed between the two conductive pins and the hot side ceramic plate. The thermoelectric cooler assembly of claim 1 further comprising a thermistor (56) having two lead wires (58), one lead wires being attached to said thermistor input pin and the other lead wire being attached to said thermistor output pin. The thermoelectric cooler assembly of claim 2 further comprising said thermistor being bonded to one of said two spaced apart ceramic plates. The thermoelectric cooler assembly of claim 3 where the assembly (40) is mounted on a heat sink (72) and a circuit board (70). The thermoelectric cooler assembly of claim 1 wherein the gaps between said two conductive pins are both filled with insulative material. A thermoelectric cooler assembly comprising: a) two spaced apart ceramic plates, one of said plates being a hot side and the other of said plates being a cold side, b) thermoelectric cooler elements interposed between said ceramic plates, c) two spaced apart electrically conductive areas directly attached to said cold side and d) a thermistor operatively connected to said cold side through said two spaced apart conductive areas.

Inventors: ESKANDARI JOSEPH, ESKANDARI, JOSEPH
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: H01S3
IPC codes: F25B21, H01L21, H01L35