GB2266020A - EMI suppression filter with transient suppression element - Google Patents

Abstract

An EMI suppression filter of the type having a ceramic capacitor 14 mounted around an inner, signal feedthrough conductor 10 within an outer metal housing 12, wherein an electronic chip component 26, such as a metal oxide varistor chip, capable of providing transient voltage protection is mounted within the metal housing 12 between the inner conductor 10 and the outer housing 12. The capacitor 14 is provided with mounting pads which diverge and maximise their area in the region where they connect the component 26 to provide optimum current flow under transient conditions. <IMAGE>

Description

DESCRIPTION EMI SUPPRESSION FILTER WITH TRANSIENT SUPPRESSION ELEMENT The present invention relates to electromagnetic interference (EMI) suppression filters having an associated means for the suppression of transients.

EMI Suppression filters incorporating ceramic feedthrough capacitors are commonly used to protect electronic equipment from the deleterious effects of conducted electro-magnetic interference (emi). The filter in its simplest form provides a capacitance between a conductor leading into the equipment and the case of the equipment, that is, ground. This forms a low pass filter, allowing d.c. and low frequency signals to pass, with high frequency interference signals becoming increasingly attenuated.

The cut-off frequency (3dB attenuation) of such devices can be as low as a few kHz, and high attenuation ( > 70dB) is often required up to microwave frequencies of 10GHz or more. Such filters are optimised for dealing with small signals of a few volts or less.

In addition to small signal electro-magnetic interference, electronic equipment can also be subjected to large scale conducted disturbances, commonly referred to as transients. These may originate from lightning strikes for instance, where a large transient voltage of hundreds of volts is induced onto a conductor connected to the equipment.

In general, conventional emi suppression filters are not optimised to attenuate transients; either the frequency content of the transient is less than the lower effective range of the filter, and no attenuation occurs or, the transient voltage exceeds the breakdown strength of the capacitor dielectric, leading to a damaged filter.

Additional electronic devices are utilised to protect equipment from transient voltages; examples of these are reverse breakdown silicon diodes, metal oxide varistors, and gas discharge tubes. These devices break down recoverably when a threshold voltage is exceeded, diverting the transient to ground, as a transient current. Such devices, however, are bulky and time consuming to mount, and occupy pcb area. Furthermore, the transient currents are thereby introduced into the equipment, leading to the risk of currents radiated into the enclosure.

It is an object of the present invention to provide an emi suppression filter which is associated with a transient voltage suppression element in an improved manner such as to take up substantially less space than devices known heretofore.

In accordance with the present invention in its broadest aspect there is provided an emi suppression filter of the type having a ceramic capacitor mounted around an inner, signal feedthrough conductor within an outer metal housing, wherein an electronic chip component capable of providing transient voltage protection is mounted within said metal housing between the inner conductor and the outer housing.

The chip component can be any suitable device which can provide the necessary transient suppression function whilst being small enough to be fitted in the abovedefined location. An example would be a metal oxide varistor chip.

Preferably, the outer housing is generally cylindrical and is mounted coaxially around the inner feedthrough conductor with an annular multi-layer discoidal capacitor disposed therebetween, said chip device being mounted radially adjacent to one end face of the discoidal capacitor.

Preferably, said one end face of the discoidal capacitor is formed with conductive mounting pads which are integral with, or connected to, respective peripheral terminations formed at the inner and outer edges of the annular discoidal capacitor, the terminations of chip device being attached to said mounting pads by a suitable means, e.g. soldering.

In a typical embodiment, the inner and outer terminations on the discoidal capacitor are of annular or cylindrical configuration, i.e. are effectively concentric rings of conductive material.

Advantageously, the mounting pads are profiled so as to merge with said inner and outer peripheral terminations on the capacitor in a divergent configuration which serves to minimise the electrical resistance and inductance presented by the mounting pads in connecting the chip components to said inner and outer terminations.

The invention is described further, by way of example only, with reference to the accompanying drawings, in which: Fig.la is a side elevation of a standard conventional emi suppression filter device in the form of a discoidal multilayer feedthrough capacitor; Fig.lb is an end view of the capacitor of Fig.la; Fig. it shows the equivalent electrical circuit of such a filter; Fig.2 is a longitudinal section through the capacitor of Figs. la and lb; Fig.3 is a view similar to Fig.2 but showing an electronic chip component mounted in accordance with one embodiment of the present invention; Fig.4 illustrates a conventional pad mounting arrangement for the attachment of chip devices to a p.c.b. or other circuit supporting member;; Fig.5a is a plan view illustrating a preferred component mounting means in accordance with the present invention; Fig.5b is a transverse section through the assembly of Fig.5a; and Fig.6 is a view similar to Fig.5a but showing how Fig.5a would be modified if conventional mounting pad techniques were adopted.

The conventional emi suppression filter of Figs.

1 and 2 comprises a signal conductor pin 10 which extends coaxially through an outer metal housing/body 12 with an annular discoidal multilayer capacitor 14 disposed therebetween. A reduced diameter portion 12a at one end of the outer housing is externally screwthreaded and receives a nut 16 and crinkle washer 18 for fixing the housing in a mounting aperture of an equipment casing or bulkhead (not shown) in a well known manner. Disposed around the feedthrough conductor pin 10 within a cylindrical insulating sleeve 20 of, for example, PTFE, is an annular ferrite bead 22. The opposite ends of the housing 12 are filled in with a suitable potting compound 24 such as an epoxy. In its operational position mounted on the (grounded) equipment enclosure or bulkhead, the outer housing 12 of the filter is thus held at ground potential.

The equivalent electrical circuit of such a device is shown in Fig.lc.

For transient protection when using a known leadthrough suppression filter of the above described type, the conventional practice has been to mount the protection device or devices, such as reverse breakdown silicon diodes, metal oxide varistors and gas discharge tubes, on entirely separate printed circuit boards mounted within or on the equipment casing or bulkhead. As mentioned hereinbefore, the main problem with such transient protection devices has been that the additional pcb area involved in their mounting to the equipment is bulky and requires extra assembly time. Furthermore, the necessity to use pcb for such devices, mounted actually within the equipment enclosure, leads to the risk of transient currents being introduced into the equipment enclosure and hence to the risk of currents being radiated into the interior of the enclosure.

Referring now to Figs. 3, 5a and 5b, there is shown an assembly in accordance with the present invention wherein a transient protection device 26 in the form of a chip device is mounted within the space defined by the outer housing 12 of the filter, between the filtered line formed by the axial pin 10 and the grounded housing 12. The device 26 is located over the discoidal capacitor 14 and enclosed by the epoxy potting compound 24.

As shown in Figs. 5a and 5b, the transient protection device 26 is mounted to solderable, electrically conductive pads 28a,28b, are in electrical connection with annular conductive areas 30a,30b, respectively, formed at radially inner and outer locations on one end surface of the capacitor 14 and connected to the outer and inner peripheral cylindrical terminations 32a,32b of the capacitor.

The conductive areas 28,30,32 are preferably formed using a metallisation ink (usually palladium-silver) which is applied during manufacture to the ceramic body of the capacitor and sintered, typically at 7009000C. The chip device 26 is typically soldered in place on the pads 28a,28b using a 60/40 Sn/Pb solder.

An advantageous aspect of the mounting arrangement for the transient protection chip 26 is as follows. As illustrated in Fig.4, when mounting electronic chip devices 26 to conductors 33a,33b on other devices 34 or mounting surfaces generally, the conventional practice is always on the conductors 33 to form mounting pads 32a,32b having pairs of parallel sides 36a,36b and 36c,36d. The problem with this usual configuration for the mounting pads is that it can restrict the current flow between the conductors 33 via the device 26. However, it is important that a minimum resistance and inductance should be introduced since peak pulse currents can exceed 500 amps with rates of rise as high as lkAns 1, so that any restriction to current flow can lead to pulse overshoot, with consequential risk to damage to the associated electronic equipment.

The application of the aforegoing conventional technique to the present situation would therefore give rise to the configuration shown in Fig.6, wherein parallel sided mounting pads 40a,40b would receive the chip component 26 and result in the disadvantages of high resistance and inductance explained above.

In order to reduce this problem and minimise resistance and inductance effects associated with the mounting pads, in a preferred structure in accordance with the present invention, the pads are profiled as shown in Fig.5a so that they diverge and maximise their area in the region where they connect the component 26 to the conductive areas 30a,30b (equivalent to the conductor lines 33a,33b in Fig.4).

This has been found to provide optimum current flow under transient conditions.

Thus, by virtue of the construction in accordance with the present invention, transient voltage suppression is provided within the same overall housing as the suppression filter itself, thereby eliminating the need for separate board mounted devices, and containing the diverted transient current close to the outside wall of the equipment enclosure.

Claims (8)

1. An emi suppression filter of the type having a ceramic capacitor mounted around an inner, signal feedthrough conductor within an outer metal housing, wherein an electronic chip component capable of providing transient voltage protection is mounted within said metal housing between the inner conductor and the outer housing.

2. An emi suppression filter as claimed in claim 1, wherein the outer housing is generally cylindrical and is mounted coaxially around the inner feedthrough conductor with an annular multi-layer discoidal capacitor disposed therebetween, said chip device being mounted radially, adjacent to one end face of the discoidal capacitor.

3. An emi suppression filter as claimed in claim 2, wherein said one end face of the discoidal capacitor is formed with conductive mounting pads which are integral with, or connected to, respective peripheral terminations formed at the inner and outer edges of the annular discoidal capacitor, the terminations of chip device being attached to said mounting pads.

4. An emi suppression filter as claimed in claim 3, wherein the terminations of the chip device are attached to said mounting pads by soldering.

5. An emi suppression filter as claimed in claim 3 or 4, wherein the inner and outer terminations on the discoidal capacitor are of annular or cylindrical configuration.

6. An emi suppression filter as claimed in claim 3, 4 or 5, wherein the mounting pads are profiled so as to merge with said inner and outer peripheral terminations on the capacitor in a divergent configuration which serves to minimise the electrical resistance and inductance presented by the mounting pads in connecting the chip components to said inner and outer terminations.

7. An emi suppression filter as claimed in any of claims 1 to 6, wherein the chip component is a metal oxide varistor chip.

8. An emi suppression filter, substantially as hereinbefore described with reference to and as illustrated in Figs. 3, 5a,5b or 6 of the accompanying drawings.