Electric lamp and discharge devices: systems – With load device temperature modifier
Reexamination Certificate
1999-03-15
2001-10-30
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
With load device temperature modifier
C315S118000, C315S032000, C315S050000, C315S363000
Reexamination Certificate
active
06310439
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electric power controllers and more specifically relates to a novel structure and process for making a wall mounted light dimmer control having a reduced temperature rise.
BACKGROUND OF THE INVENTION
Electric power controls, particularly wall mounted dimmers are well known. U.S. Pat. Nos. 3,746,923 and 4,803,380, the disclosures of which are incorporated herein by reference, show typical light dimmer controls having a manually operable slide control. These devices conventionally have a controllable a-c current control device such as a triac mounted on a metal, typically aluminum, heat sink to dissipate heat generated by the triac during its operation. A control circuit which may be fixed to a suitable printed circuit board and a linearly adjustable potentiometer, which is connected to the slide control, is mounted atop, or adjacent, the triac. A rotary potentiometer may be used in place of the linearly adjustable potentiometer. The triac, potentiometer and control circuit are enclosed by a back cover. The external power conductors, such as insulated wires or stabs, which are connected to the interior triac, extend through the back cover for connecting the lighting load in series with an a-c source.
The control circuit is connected to the triac gate terminal to control the conduction phase of the triac, thus varying the power to the load. An on-off switch may also be provided in series with the triac to effect positive turn-off of power to the load when the slide or other control reaches an end position, or alternatively when a separate switch activator is operated.
The slide control is preferably mounted within an insulation adapter cover plate mounted on the surface of the heat sink plate opposite to the surface receiving the triac. A large number of other control schemes can also be used, including control by a microprocessor or coded radiation from a hand held radiation source.
Underwriters Laboratories, an independent approval agency, requires that the temperature rises measured at several strategic locations on the dimmer not exceed specified maximums. These temperature rises are measured over ambient temperature with the dimmer installed in a wall box in an insulated wall, with the dimmer carrying the maximum rated load. Further these temperature rises are the steady state temperature rise reached after the dimmer has been operating for an extended period of time. One requirement is that the back cover, which encloses all of the dimmer electronics, not exceed a 65° C. temperature rise. This requires the selection of a particular type of triac (or other semiconductor switch) which has a sufficiently low power dissipation at the highest rated load current. Generally, a heat sink with fins and/or channels for flow of air past the heat sink have been needed to sufficiently cool the triac and wall box when load current is increased. Such devices require a large and relatively unsightly “stand-off” of the face plate of the dimmer from a wall. It has not been possible to increase the rating of an otherwise well designed low profile dimmer from 1500 watts to 2000 watts because the back cover temperature rise will exceed 65° C. without a special heat sink. Thus, users of such low profile dimmers have to use plural 600 watt and/or 1500 watt units (without fins) to supply a 2000 watt load.
It would be very desirable to provide a 2000 watt dimmer which does not require exotic cooling techniques to achieve such power control without unduly increasing the temperature rise of the dimmer components such as the back cover and permitting a minimum stand-off of the dimmer face plate from the wall surface to which the dimmer is mounted.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, the heat sink of a dimmer control is better utilized to transfer heat from a semiconductor switching means, to produce a reduced peak temperature rise of the heat sink and back cover by distributing the input heat generated by a triac or other semiconductor switching device over a larger portion of a heat sink of given area. More specifically, the single triac or controllably conductive switching device of the prior art is replaced by a plurality of parallel connected similar switching devices which are relatively widely spaced (within the dimension or “footprint” of the back cover), over the surface of the heat sink. An identical function is carried out by both the single device or the plurality of identical devices. In other words, the plural parallel devices which replace a single device each carry a respective fraction of the total load current with identical instantaneous current magnitudes. That is, each of the parallel devices carry currents of substantially equal instantaneous magnitude as a function of time. Thus, a single triac may be replaced by a plurality of identically functioning triacs. This distinguishes from the prior art class of devices in which plural devices of different functions, for example, spaced unidirectional and back to back connected (antiparallel) devices (SCRs) may be spaced on a common heat sink as in U.S. Pat. No. 5,327,047 to replace a single triac. More specifically, U.S. Pat. No. 5,327,047 shows two discrete SCRs which respectively energize a lamp load during alternate half cycles of the AC voltage. The necessary separate placement of the SCRs produces lower heat sink temperature than a single equivalent triac at the cost of added components and complexity. While the two SCR's of the U.S. Pat. No. 5,327,047 patent may superficially be thought to be in parallel, they are really in an antiparallel arrangement since the two SCRs conduct current in different directions.
In accordance with the invention, a given amount of heat is injected into the heat sink over a much larger area than with a single triac which results in a reduced peak temperature of the heat sink. The spreading out of the heat sources also helps reduce the thermal gradient over the heat sink surface. As a further and unexpected benefit of using two or more parallel connected, but physically and thermally spaced devices of identical function in place of the single device, the actual total power dissipation of the plural devices was also found to be reduced. Thus, the total heat input to the plate was found to be also reduced by practice of the invention.
As a still further advantage of the invention, it has been found that the device with plural parallel triacs is more robust than single unit device, having a much higher fusing current I
2
t. This provides significant improvement in enabling the dimmer to survive lamp blow out.
Another further and unexpected result of using two or more parallel connected, but physically and thermally spaced devices in place of a single device is that the reliability of the dimmer is not decreased. One of ordinary skill would expect that a dimmer with a plurality of triacs would be less reliable than a dimmer with a single triac because the individual failure rates of the triac would ordinarily be multiplied. Applicants have found the reliability of the dimmer using plural triacs remains approximately the same as that of a dimmer using a single triac due to the significantly lower temperature of each of the plural parallel triacs caused by less heat being generated and by the heat sources being spread out.
Preferably the heat sink in the dimmer of the invention is a flat plate of thin high thermal conductivity material, preferably copper. It has been found that the use of copper in place of the conventional aluminum heat sink enables the parallel operation of the spaced triacs or equivalent devices, such as spaced pairs of simultaneously conducting SCRs; IGBTs; and diodes (and other bipolar devices) without the need for external current balancing circuits. That is, the parallel devices are sufficiently thermally coupled (even though spaced) that automatic current balance occurs.
As a further feature of the invention, the copper or other metal plate is also thermally coupled to the insulation adapter
Billington Scott A.
Jacoby, Jr. Elliot G.
Spira Joel
Weightman Russell
Lutron Electronics Company, Inc.
Vo Tuyet T.
Wong Don
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