Electromagnetically Shielded Electrical Converter and an Improved Electromagnetic Shield Therefor

This invention relates generally to an improved electromagnetically shielded electrical converter and to a special type of electromagnetic shield which has been found to be especially useful in such a converter.

This application is related to a copending commonly owned application Ser. No. 555,424 of Mr. John P. Walden and myself filed concurrently herewith and describing an improvement over the invention described herein below. In addition, this application is somewhat related to earlier filed copending commonly owned applications relating to line cord power supply units and various features thereof since the present invention is especially intended for use in such line cord supply units although those in the art will recognize that this invention has other more general application as well. Some of such earlier copending commonly owned applications are Ser. No. 406,162 filed Oct. 15, 1973 now abandoned; Ser. No. 416,115 filed Nov. 15, 1973; Ser. No. 434,580 filed Jan. 18, 1974, now U.S. Pat. No. 3,909,700 and Ser. No. 474,669 filed May 30, 1974, now U.S. Pat. No. 3,880,491.

The exemplary embodiment of this invention includes some features of an electrical converter or power supply unit utilizing a relatively high speed inverter circuit and transformer windings, etc. to isolate input and output voltages and to produce desired output electrical power from available input electrical power. To facilitate the exemplary explanation of this invention, its application to this type of electrical converter circuit will be described, although, as noted above, those in the art will recognize other applications for this invention.

In a typical line cord supply unit, for instance, normal household alternating current at 115 volts is converted to a lower d.c. output voltage. In order to decrease the size of the unit, increase its efficiency, etc., such power supply units usually rectify the available a.c. supply voltage to provide a d.c. supply which, is, in turn, used to drive an inverter circuit. Such inverter circuits typically utilize low loss fast switching solid state devices for alternately switching the polarity of the d.c. supply voltage to the primary winding of a transformer. To reduce the size of the magnetic circuit in the transformer among other reasons, this inverter normally operates at a fairly high frequency such as, for instance, 18kHz. The transformer of such a unit also includes a secondary winding which is magnetically coupled to the primary winding. This arrangement, of course, results in induced alternating voltages in the secondary of the transformer at the same high frequency at which the inverter circuit operates. These high frequency secondary induced voltages are then rectified in an output rectifier to produce the desired low voltage d.c. output.

As will be recognized by those in the art, the rectifying, inverting and transforming circuitry inherently involved in such a converter can produce considerably electromagnetic interference (EMI) especially since the inverter is operating at a fairly high repetition rate and since the inverter normally produces substantially square wave shaped alternating voltages which are rich in high frequency harmonics.

Unless suitably suppressed, these undesirably EMI signals will emanate from the power supply unit and are of sufficient magnitude to cause interference with nearby susceptible electronic circuits such as radio frequency receivers. Such EMI may leave its source within the power supply unit by various means. It may be conducted on the connecting wires to the input and output terminals and it may emanate from the power supply in the form of radiated electromagnetic fields.

Of course, the general problem of suppressing unwanted EMI has been encountered before in the art. However, normal engineering design solutions to these EMI problems are not practical for use in a line cord power supply, for instance, which has severe constraints related to the limited size of the unit and to the necessity of electrically isolating the input and output terminals of the unit due to safety reasons since such a unit will be utilized by the general public.

A straightforward application of the usual EMI suppression techniques would involve enclosing the converter circuitry within a metallic shielding and with electrical filtering circuits connected to the shielding and to the converter circuitry at both the input and output terminals. However, such conventional filtering circuit may well involve more discrete components than are possible within a given size constraint. Furthermore, and perhaps even more importantly, such conventional techniques do not provide the necessary electrical isolation between the input and output terminals required for safety reasons. In addition, the conventional techniques suppose access to an electrical ground potential having substantially the same electrical potential as the earth. However, in applications such as the line cord power supply unit, such ideal circumstances cannot always be anticipated since many existing house-wiring systems do not include earth ground connections. Thus, the line cord power supply unit, for instance, must be capable of properly performing using only the typical two-wire connection where one never knows for sure which side of the conventional 115V 60 Hz circuit may be connected to a given input terminal of the unit. Furthermore, neither of the conventional two-wire connections is actually an earth ground reference in a radio frequency sense.

It has now been discovered that substantial reduction in unwanted EMI can be obtained without relying on an actual earth reference potential and while maintaining the necessary isolation between input and output terminals.

In the basic power supply circuitry, it is a transformer which provides the primary electrical isolation between input and output terminals. This invention maintains that necessary isolation by, in effect, separately shielding the various elements related to the input or primary winding side of the transformer and the remaining components related to the output or secondary winding side of the transformer. Furthermore, these two separate systems of EMI shields are separately connected to independent and separate electrical reference points which are electrically associated only with the input and output electrical circuits respectively.

Others have attempted to electrically separate the shielding of transformer primary and secondary circuits, for instance, as shown in U.S. Pat. No. 2,896,096, to Schwarzer. However, Schwarzer, for example, relies upon the availability of at least one true earth reference potential and, furthermore, is not actually directed to the suppression of radio frequency EMI but, rather, to the suppression of alternating current hum and power line voltage fluctuations.

Besides electrically referencing the various EMI shields to separate circuit portions in the converter circuitry, this invention provides for connections to at least some of those shields at special null or "center tap" points on the shield surfaces. A given EMI shield comprises an electrically conductive surface defining the boundary of a substantially enclosed volume for containing an EMI source therewithin. According to this invention, this conductive surface is connected to a predetermined electrical reference point via a center tap connection which comprises a predetermined location or null point on the conductive surface where the voltages induced in the surface by the EMI signals therewithin are substantially balanced out such that unwanted injection of EMI voltages to the electrical reference point are substantially reduced. Of course, the importance and value of such center tap connections are especially noticeable where the electrical reference point itself is not absolutely tied to an RF electrical ground or earth potential. Thus, especially in a line cord power supply unit where no such earth reference potential is necessarily available, such a center tap connection to the EMI shield is especially valuable. Without it, significant EMI signals would actually be injected at the selected electrical reference point which is itself, of course, not absolutely fixed with respect to a RF reference potential in the exemplary embodiment.

In the exemplary embodiment to be described in detail below, such center tapped EMI shields are separately provided for the primary and secondary of the transformer. Furthermore, according to one feature of the exemplary embodiment, the secondary of the transformer is actually provided with a second enveloping center tapped EMI shield. In addition, the exemplary embodiment provides an outer guard shield which envelopes the secondary winding shield and is electrically connected to one of the input leads. Conventional low pass filtering is also used at the input and output leads of the exemplary embodiment to control direct conduction of unwanted EMI.

A more detailed and complete understanding of this invention will be had by reading the following detailed description taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a schematic block diagram of a prior art EMI shielding technique for an electrical power supply unit;

FIG. 2 is a schematic block diagram of an electomagnetically shielded electrical converter according to this invention and also depicting electromagnetic interference shields per se, that are especially useful in such a converter; and

FIGS. 3, 4 and 5 are sectional views of a typical transformer with electromagnetic interference shields according to this invention.

A typical approach to prior art EMI shielding techniques of circuits such as inverter powered power supplies is shown in FIG. 1. The power supply circuitry itself comprises an input rectifier 10 which takes an alternating input voltage E.sub.in and produces a d.c. supply voltage at 12 for driving an inverter 14. Typically, the inverter will operate at a fairly high frequency compared to the normal line frequency of 60 Hz. For instance, the inverter 14 may operate at 18kHz or at even higher frequencies. In this manner, the d.c. supply voltage at 12 is converted into an approximately square wave shaped alternating high frequency voltage and current at 16 which is then passed through the primary winding of a transformer 18. The primary winding of the transformer is magnetically coupled to a conventional secondary winding which then produces approximately square wave shaped alternating voltage at the inverter frequency but at a voltage determined by the transformer turns ratio at secondary output terminals 20. Since, in the exemplary embodiment, the desired output voltage E.sub.o is a low d.c. voltage, the a.c. voltage at 20 is rectified in an output rectifier 22 which produces the desired d.c. output voltage E.sub.o therefrom. The rectifier, inverter and transformer components may be conventional components that should be well-known to those in the art. Alternatively, special exemplary embodiments for the details of newer types of such components are shown, for instance, in the above cited copending commonly owned related applications.

According to conventional EMI suppression techniques, the circuitry just described in FIG. 1 might typically be enclosed in metallic shielding 24 with lumped parameter low pass filters comprising inductance and capacitance units being associated with both the input and output leads and the metallic shield 24 itself as shown, for instance, in FIG. 1. Additional EMI suppression may be achieved using a typical prior art installation which would provide for an RF ground connection 26 as also shown in FIG. 1. As should now be appreciated, the isolation between input and output terminals has been considerably reduced by such prior art techniques. Furthermore, in applications such as the line cord power supply unit, there will be no available RF ground reference connection. Furthermore, the number of discrete components involved presents a problem when severe size restraints are imposed as in a line cord power supply unit application.

An inverter powered a.c./d.c. power supply unit constructed in accordance with this invention is shown in an exemplary embodiment at FIG. 2. Although an RF ground reference potential such as 30 shown in FIG. 2 is not normally available within a line cord power supply unit per se, such a reference potential has been shown in FIG. 2 so as to more clearly illustrate the various EMI noise voltages that exist at the input and output terminals of such a unit. There is the usual a.c. input voltage E.sub.in and the d.c. output voltage E.sub.o. Superimposed across the input terminals A and B is a noise or EMI voltage component E.sub.N(in). In addition, there is a similar EMI noise voltage across the output terminals C and D labeled E.sub.N(o) in FIG. 2. In addition, at the input terminals A and B, there are potentially separate and independent EMI signals with respect to an RF ground potential labeled E.sub. N(a) and E.sub.N(b). At the output terminals, the common mode EMI noise component for output terminals C and D is shown as E.sub.N(cm).

The EMI components directly between terminals A and B and terminals C and D are controlled by normal line filtering with lumped parameter low pass filters comprising the pi filter networks using the lumped parameter inductances and capacitances shown in FIG. 2. Furthermore, such control of the EMI between input terminals A and B also constitutes substantial control of the EMI voltages between these terminals and an RF earth ground since there is a fairly low finite RF impedance from either side of the usual 115V 60 Hz supply to RF each ground.

However, since there is no natural low RF impedance path from the output terminals C and D to an RF earth ground potential, the common mode output EMI voltage must be attenuated to acceptable levels by other techniques. As shown in FIG. 2, these other techniques according to this invention involve several special features which will now be described in detail.

First of all, there is electrostatic shielding provided for the input rectifier 32, the inverter circuitry 34 and the transformer primary winding 36. As shown in FIG. 2, such shielding includes a primary winding shield 38 and an inverter shield 40, both of which are electrically connected to the common or return voltage connection 42 of the d.c. supply voltage provided by the input rectifier 32.

In a typical power supply circuit, the voltage differential between the primary winding 36 and the primary winding shield 38 is relatively high compared to the electrical impedance therebetween, i.e., because of the capacitive coupling therebetween. To minimize the impedance between the shield 38 and the common connection 42, a current return lead 44 is separately provided for the primary winding shield 38. Furthermore, the connection of the current return lead 44 to the primary winding shield is especially selected at 46 to be at the point where the primary winding leads enter the primary winding shield 38 at substantially half of the circumference of the coil from the primary winding shield gap (a gap in the electrical conductivity to prevent the winding shield from acting as a shorted electrical circuit with respect to voltages induced therein by the normally desired magnetic transformer flux). Another way of stating it is to say that the current return lead 44 is connected approximately one-half way around the coil circumference substantially opposite the normal gap in electrical conductivity. This special "center tap" connection point on the primary winding shield 38 is especially selected for balance purposes. The induced EMI voltages in the shield 38 are of such magnitudes that an arbitrary connection of the current return lead 44 thereto might well inject an EMI signal level to the common return node 42 which would, in due course, produce an E.sub.N(o) signal exceeding permissible levels. In the preferred embodiment, the center tap connection to the shield is maintained approximately within .+-. 1% dimensional precision of the desired point exactly one-half way around the coil or, in other words, opposite from the gap in electrical conductivity of the shield.

As will be described more fully below with respect to the description of FIGS. 3, 4 and 5, the electromagnetic shield 38 includes a gap in electrical conductivity along at least part of the electrically conductive surface boundary where such a gap is normally provided to prevent the shield itself from becoming a shorted electrical circuit (i.e., a one-turn transformer winding) with respect to voltages induced therein by desired magnetic flux fields and the electrical center tap connection is located substantially equidistant over the conductive surface along at least two opposed directions from the gap. An illustration of such a center tap connection is shown, for example, in the cross-section in FIG. 4.

The exemplary embodiment shown in FIG. 2 also includes an outer guard shield 47 comprising guard shield portions 47a, 47b and 47c with electrically conductive surfaces. Shield portions 47a and 47b substantially enclose the input and output circuitry, respectively, in separate conductively enclosed volumes.

Outer guard shield portion 47c in one exemplary embodiment containing outer guard shield surface 47' shields the primary transformer winding 36 and shield 38 which are part of the input circuit from the secondary transformer winding and shield 48 which are part of the output circuit. As may be seen in FIG. 2, outer guard shield portion 47a has surfaces including 47' and 47c which completely enclose the input rectifier and inverter and primary windings of the transformer as well as the input line filtering components. The outer guard shield portion 47b has surfaces also including 47' and 47c which completely enclose the output circuit including the secondary of the transformer, the output rectifiers and the output line filtering components. The guard shield portion 47c, enclosed within the guard shield portions 47a and 47b, actually separates the primary and secondary windings of the transformer. As illustrated in FIG. 2, outer guard shield 47 is referenced to one of the two input leads A and B at 58 on lead B. It should be noted that there is no such connection to the output leads C and D.

Selection of the center tap connection guard shield portion 47c to shield section 47', common to both shield portions 47a and 47b, is at points 54 and 56. Point 56 on the guard shield portion 47c is selected for balance purposes in the same manner as that described above for primary winding shield 38 and its center tap connection point 46. The selection of the connection point 54 on shield section 47' is a matter of design choice that varies with the specific leakage flux parameters of the transformer and capacitive elements in the input circuitry. In general, the connection point 54 is selected to provide a minimum voltage difference between point 54 and the common point 58 on lead B.

Enclosed within the shield portion 47c is a secondary winding shield 48 which has a center tapped shield connection 50, which in the exemplary embodiment, is electrically referenced to the center tap 52 of the balanced center tapped secondary transformer winding. As with the center tapped shield connections 54, shield connection point 50 is selected in the same manner as that described for the primary winding shield 38 and tap 46.

FIG. 3, a plan view of a typical transformer useful in the practice of the present invention, illustrates a metallic bobbin 60 havig a gap 62 therein, which gap completely separates one leg of the bobbin. The bobbin 60, illustrated more clearly in the sectional view of FIG. 4, has a primary winding 64 and a secondary winding 66. Each winding is wound on a portion of the bobbin 60 but insulated therefrom by an insulator 68. The primary and secondary windings are shielded from each other by shields 70 and 72, respectively, which completely surround each winding, but for gaps 74 and 76 which prevent the shields from electrically short-circuiting the primary and secondary windings 64 and 66.

FIG. 4 also illustrates connections 78 and 80 to the primary and secondary winding shields, which connections are especially selected at the point on the shields approximately one-half way around the coil circumference substantially opposite the normal gap in electrical conductivity. More specifically, connections 78 and 80 are selected in the same manner outlined above with respect to the selection of connection 46 to primary winding 36 of FIG. 2.

FIG. 5, a view taken along lines 5--5 of FIG. 4, illustrates the gap 62 and connection 80 to the secondary winding 66 on the bobbin 60.

As should be noted, there is thus provided a system of EMI shields for the various components of the power supply unit wherein at least a portion of the input components are shielded and referenced to a first reference point and wherein at least a part of the output components are shielded and referenced to a separate electrically isolated reference point, namely, the center tap connection of the secondary output winding from the transformer. Furthermore, the EMI shields that are actually referenced to circuit connection points within the power supply circuitry itself that are not necessarily RF ground potentials are referenced thereto via center tap shield connections which comprise null points on the conductive surfaces of the EMI shields whereat voltages induced by EMI sources contained therewithin are substantially balanced out such that injection of unwanted induced EMI voltages to such floating reference points are substantially reduced.

Although one exemplary embodiment of this invention has been described in detail, those skilled in the art will recognize that there are many possible modifications and variants of the exemplary embodiments which will include the improved and novel features described above. Accordingly, all such modifications and variations are intended to be included within the scope of this invention as defined in the appended claims.