Interference suppression for semi-conducting switching devices

An electrical unit such as a lighting dimmer pack incorporating one or more semi-conducting switching devices such as triacs or thyristors, wherein the usual insulated mounting base (18) of the triac (30) is in turn mounted on the unit casing via an electrically insulating heat conductor, and the said mounting base is connected to the phase or neutral conductor (52), preferably the latter. In a preferred arrangement, a plurality of triacs are clamped by a printed circuit board which provides the connection from the triac bases to the neutral conductor.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
A typical internal construction of an insulated base triac is shown in FIG. 
1. The crystal 10 is mounted on a metal conductor 12 integral with the 
main terminal 14 and the whole is then further mounted on a ceramic 
insulator 16, then a metal mounting base 18. The other main terminal is 
referenced 20. This device exhibits good thermal conductivity from the 
heat-producing crystal 10 through to the metal base 18, which then allows 
the heat to be dissipated into the structure to which the base 18 is 
connected. It is to be noted that the construction leads to generation of 
a small but significant capacitance, shown as capacitor 40 in FIGS. 2, 3, 
and 5,; between terminal 14 and the metal base 18, which is typically of 
the order of 20-30 pF. 
In FIG. 2 is shown a circuit for a triac dimmer, including interference 
suppression components. The triac 30 has a suppression choke 32 connected 
in series and the triac/choke assembly is shunted by a suppression 
capacitor 34. The phase and neutral conductors, respectively 50, 52, have 
a suppression capacitor 36 between them, which capacitor serves to 
equalise and marginally reduce the interference of the pair, and a much 
smaller capacitor 38 is connected between neutral 52 and earth 54. For 
practical purposes, this capacitor 38 is limited to 4,700 pF due to the 
second aforementioned regulations. 
Interference can be measured by connecting an interference meter between 
earth and phase and between earth and neutral, both on the input side and 
on the load side. In spite of inclusion of the interference suppression 
components, interference exceeds limits set by the first aforementioned 
regulations. This interference problem is mainly caused by the parasitic 
capacitor 40 between the triac base 18 and terminal 14, coupling noise 
current from the output of the triac, before suppression by the choke 32, 
into earth, assuming the triac base 18 to be directly mounted on the 
earthed metal case (not shown) of the unit. 
A means of eliminating the noise current would be to reverse the connection 
of the triac and the circuit such that the parasitic capacitor 40 was 
instead connected to the incoming phase and therefore not a source of 
noise. However, the firing electronics 42 for the unit also inevitably 
carries an unavoidable intrinsic capacitance to earth, as indicated in 
broken line at 44, and would similarly cause a noise current to be induced 
to earth through the output of the triac. 
One possible solution to the problem, within the compass of the present 
invention, is for the insulated base 18 of the triac to be separated from 
the metal casing of the product by a further heat conducting insulator, 
and the base connected as shown by connection 19 in FIG. 6 to the phase 
conductor 50. This has a cost disadvantage in that each triac in a pack 
such as a dimmer pack has to be connected separately to its incoming phase 
and no common bussing structure is readily conceivable. 
A preferred solution in accordance with the invention is shown in FIG. 3, 
wherein similar reference numerals are employed to those used in FIG. 2. 
Here, the triac base 18 is connected through connection 2 to neutral 52, 
so that the noise current is effectively short-circuited through the large 
capacitor suppression 36. This has the further benefit that all the triacs 
in the multiple dimmer pack can be connected together via one common 
mounting assembly, with the need for only one neutral connection. 
Such a mounting assembly is shown in FIG. 4 and FIG. 5, in relation to one 
triac of the multiplicity thereof. The triac 30 is mounted on the casing 
60 via a heat conducting insulator 62. In order to ensure maximum thermal 
contact and to make a connection between the triac metal base 18 and 
neutral 52, the triac 30 is held clamped by a printed circuit board 64, 
with the copper laminate 66 on the underside held down against the triac 
base 18. The copper laminate 66 defining the printed circuit is etched so 
that clamping bolts 67 establish a requisite clearance 68 between the 
neutral conductor 52 and earth (casing 60), and so that said printed 
circuit connects all of the mounting bases 18 of the triacs held down by 
this method to neutral. The neutral connection to the printed circuit 
board 64, 66 is indicated at 70. 
The above-described embodiment utilises triacs for economy; however, the 
method is equally appropriate for the mounting of thyristors, thyristor 
packs or other semi-conductor switching device. The invention may also be 
employed, more generally, in an electrical unit incorporating only a 
single semi-conducting switching device such as a triac or thyristor, for 
the purpose of effecting interference suppression to the level necessary 
to meet the aforementioned regulations.