WO2004073373A2

WO2004073373A2 - Pseudo isolated power conditioner

Info

Publication number: WO2004073373A2
Application number: PCT/EP2004/001528
Authority: WO
Inventors: Alex Odey
Original assignee: Pepperl + Fuchs Gmbh
Priority date: 2003-02-19
Filing date: 2004-02-18
Publication date: 2004-09-02
Also published as: WO2004073373A3

Abstract

The invention relates to a pseudo isolated power conditioner, like a common mode inductor (1), or system with similar function, for field bus systems. The common mode inductor (1) or system with similar function, sited upstream of each participating segment (3) power conditioner (2), is utilized to provide, or assist, adequate segment (3) impedance and/or cross talk and/or adequate high frequency interference attenuation on and/or between participating segments (3), where participating segments (3) comprise adequate power conditioning (2) with each participating segment (3) unbalance within the constraints outlined within IEC61158-2.

Description

Pseudo Isolated Power Conditioner

Technical area:

The invention relates to a pseudo isolated power conditioner, like a common mode inductor, or system with similar function, for field bus systems.

State of the art:

The Physical Layer of IEC 61158-2 has been an international standard for some years now, and is widely used by Foundation Fieldbus (FF), WorldFIP and Profibus PA. The Physical Layer of IEC 61158-2 is the field bus standard for use in industrial control systems.

Background of the invention:

It is inferred, by IEC61158-2 clause 12 [formerly clause 22], that galvanic isolation should be provided for each segment. This isolation will provide toleration to parasitic cable cross talk and/or maintain adequate power source impedance should one bus conductor or the other of the two bus conductors fail to ground. Parasitic cross talk may also transpire where a bus conductor fault, described above, occurs on two discretely conditioned segments that do not have galvanic segregation from each other. However, ideal galvanic isolation for practical modern power converters is never attained. This is due to component parasitic influences and the requirement for adequate common mode and/or differential mode noise suppression. Never the less, modern power supplies are perfectly adequate for dealing with in-band frequency influences and frequency influences below the inband frequency, given the recommendatory network constraints outlined within IEC61158-2 clause 12. For redundant topologies, segment isolation presents a cost, size and efficiency penalty, progressively increasing with an increase in output power. Also, for modern power supplies, frequencies increasing above the in-band frequencies become proportionally problematic. The reasons are not instantly obvious, and this description will explain the_/ phenomenon, as well as a solution, which again, is not instantly obvious, that will negate the requirement for galvanic isolation. Technical objective

The invention is based on the objective of providing a common mode inductor, or system with similar function, which will negate the requirement for galvanic isolation in a conventional power supply conditioner for a fieldbus network.

Disclosure of the invention and its advantages

The objective is achieved according to the invention in that a common mode inductor, or system with similar function, is sited upstream of each participating segment's power conditioner, utilized to provide, or assist, adequate segment impedance and/or cross talk and/or adequate high frequency interference attenuation on, or between, participating segments in accordance with IEC61158- 2 or other similar telemetry standards.

This common mode inductor has the advantage that it is created a pseudo isolated power conditioner which will negate the requirement for galvanic isolation in a conventional power supply conditioner for a fieldbus network.

The participating segments power conditioner can also comprise adequate power conditioning, which participate an isolated or non-isolated common power supply, with each participating segment unbalance within the constraints outlined within IEC61158-2, or similar telemetry standards, with, or without, one of many participating segment's one of two poles in resistive and/or capacitive and/or inductive, of any magnitude, contact to ground and/or with one of many participating segment's one of two poles in resistive and/or capacitive and/or inductive, of any magnitude, contact to another of many participating segment's one of two poles and/or said provision with toleration to one terminator deficit from one, or more, participating segments.

The common mode inductor, or system with similar function, can be provided with optional additional measures, such as earth fault detection/alarm and/or electrically conductive component mechanical protection, to annunciate and/or prevent activities as described in previous description. The common mode inductor, or system with similar function, also can be provided with an optional addition of spur specific bi-polar current limiting to prevent inductor saturation during activities.

The common mode inductor is intended to provide pseudo galvanic isolation as an alternative and/or addition to galvanicaly isolated power supplies.

Brief description of the drawings in which the following is shown: Figure 1 a conventional power supply conditioner, where adequate impedance from a low impedance power source 5 is provided Figue 2a an inter segment 3a to 3n capacitive coupling with resulting capacitive unbalance Figure 2b an segment coupling to ground E with resulting capacitive unbalance Figure 3 unbalance between segments caused by multiple ground E faults

Figure 4 an AC model, for generic applicability, with two identical segments, connected to one common simplex power supply or one common redundant power supply and

Figure 5 another embodiment of the invention illustrating balanced non-linear current limiters, or non linear current limit circuit sensing, positioned in each pole.

Description of the invention:

The embodiment of this invention will now be described by way of example where figure 1 illustrates, in part, a conventional power supply conditioner, where adequate impedance from a low impedance power source 5 is provided, by use of a differential mode inductor 2, for a fieldbus network or segment or trunk 3 with end termination 4. The differential mode inductor 2 circuit may also comprise additional localized components to curtail network 3 resonance. The differential mode inductor 2 may comprise differing formats, however, for the purpose of this description, it is described and illustrated as a simplex differential mode inductor 2. The deviation to the conventional power conditioner, and the embodiment of this invention, is the inclusion a common mode inductor 1. The common mode inductor 1 may comprise, or be substituted by, discrete inductors or components, which perform the same or a similar function. However, for the purpose of this description, the common mode inductor 1 is described and illustrated as a simplex common mode inductor 1. The common mode inductor 1 circuit may also comprise additional localized components to curtail network resonance for example, each winding may be shunted and/or partially shunted, by one or more resistors and/or one or more capacitors and/or one or more inductors, arranged in any combination, and/or the two windings may be connected, upstream and/or downstream and/or at any point between, by one or more resistors and/or one or more capacitors and/or one or more inductors, arranged in any combination. Corrections for component tolerance will not be required for this description although they may be required for practical application. The function of the common mode inductor 1 will become palpable when analyzing an equivalent AC model, however, to realize the model, a description the factors that contribute to the model's rationale must first be described by way of example. It is assumed that the principle of network unbalance is understood. However, the AC model, which is covered later in this description, will ameliorate the perception of network unbalance and its consequence. There are many contributors to network unbalance:

Figure 2a illustrates inter segment 3a to 3n capacitive coupling with resulting capacitive unbalance 7c. Inter segment 3a to 3n capacitive coupling, and resulting capacitive unbalance 7c, may be attributed to the cable's manufacturing irregularities.

Figure 2b illustrates segment 3a to 3n coupling to ground E with resulting capacitive unbalance 7ca to 7cn. Segment 3a to 3n ground E Capacitive coupling, and resulting capacitive unbalance 7ca to 7cn, may be attributed to the cable, and/or the devices and/or the device couplers. This type of unbalance is similar, in one respect, to the unbalance illustrated in figure 2a.

Figure 3 illustrates unbalance between segments 3a to 3n caused by multiple ground E faults 7a to 7n, where a fault may comprise a resistance and/or inductance and/or capacitance and/or conductance for example, the cable terminals may be partially submerged in sea water.

For figures 2a, 2b and 3, the unbalance or fault may occur from any pole to any pole or from either pole to ground. Other parasitic factors may also be present but for the purpose of this description, they will not be considered. The result of any unbalance is parasitic crosstalk between two, or more, segments. To curtail the parasitic influence, it is recommended that galvanic isolated power supplies be used for each segment however, as described previously, ideal galvanic isolation is never attained.

The AC model, illustrated in figure 4 will now be described by way of example where, for generic applicability, two identical segments 3a and 3n are both connected to one common simplex power supply 5 or one common redundant power supply 5, or each segment 3a and 3n are each connected to one discrete simplex power supply 5 or one discrete redundant power supply 5. The system may comprise two or more segments 3a to 3n where segment 3n is one or more segments attached to the common points 11a and 11 b with orthogonal inter- segmental coupling 7c to 7co accordingly. However, for this description, only segments 3a and segment 3n are considered. The power supply filter capacitance 8a to 8n for discrete simplex power supplies or discrete redundant power supplies are coupled to ground E for interference reduction to be effective. For discrete redundant power supplies, filter capacitance 8a or 8n is the summation of each power supply's capacitance for example, if segment 3a comprises a galvanic redundant power supply each with a two nano-Farad bypass filter capacitor, then the total capacitance 8a will be four nano-Farads. Ground E path impedance 10 may tend towards zero ohms for discrete power supply topologies, or equal the summated filter capacitance of the common power supply topology. The ground E path impedance 10 may be capacitive and/or inductive and/or resistive. The ground E path impedance 10 may be resultant of many factors for example, a common power supply may have its negative rail connected to ground E. The filter capacitance 8a to 8n will tend towards zero ohms for the common power supply topology. The unbalance impedance 7ca to 7cn to ground E will tend towards zero ohms for multiple ground E short circuits. The cable-to-cable unbalance impedance 7c will tend towards zero ohms for pole- to-pole short circuits. Unbalance 7ca to 7cn and 7c may comprise capacitive and/or inductive and/or resistive and/or conductive components. The differential mode inductor 2aa and 2ab on segment 3a and differential mode inductor 2na and 2nb on segment 3n will, for the AC model, form a Pi filter with resultant mutual attenuation between segment 3a and segment 3n, with an attenuation magnitude proportionally dependant or partly proportionally dependant upon the terminators 4a and/or 4n and/or the common mode inductor 1a and 1 n and/or the Ground E path impedance 10 and/or fault unbalance 7c and/or 7ca and/or 7cn and/or the filter capacitors 8a and/or 8n. Mutual attenuation resulting from component impedance will also be proportionally dependant or partly proportionally dependant upon the harmonic content or frequency of the telemetry passing between segment 3a and segment 3n. This invention cited the omission of segment galvanic isolation, whereby this would effectively substitute the filter capacitors 8a and 8n with a zero impedance path. At this juncture, omission of the common mode inductor 1a and 1 n, with ground E path impedance set to infinity Ohms, and an unbalance which sets both faults 7ca and 7cn to a zero ohm resistive fault for example, one pole on each segment 3a and 3n is in short circuit to ground E, would result in excessive cross talk between segment 3a and segment 3n, with consequential telemetry distortion and ultimate communication failure. Taking the ground E impedance 10 down towards zero ohms will curtail the crosstalk but only where ground E faults 7a and 7n and/or capacitive unbalance 7ca and 7cn occurs, and not where pole to pole faults or capacitive unbalance 7c occur in isolation. Never the less, the ground E impedance 10 will proportionally affect the impedance of each power conditioner accordingly, to a point where the impedance constraints outlined within IEC61158-2 clause 12 are breached.

It may be noted at this point that the original draft specification SP50 allowed power conditioners to be grounded. In this case, it is apparent that, for a conventional power conditioner arrangement, a pole to ground fault would instantly divide the impedance by two under worst-case conditions. Therefore, grounding the power conditioner is not recommended unless there is suitable intersecting impedance available for example, impedance provided by the common mode inductor 1. This also applies, in part, to grounded terminators. On the other hand, ground E impedance 10, comprising one or more resistors and/or one or more capacitors and/or one or more inductors, arranged in any combination may be designed and utilized, in conjunction with the common mode inductor 1 if required, to provide, or assist, adequate attenuation and/or adequate impedance under said unbalance and/or fault condition and/or provide nonspecific segment injection utilities and/or segment reception utilities. All components 1a 8a, 10, 8n, 1 n, 7ca, 7cn, 7c and 2ab or 2nb will, in combination, affect the resultant impedance of 2nb or 2ab respectively therefore, components should be sized in view of maintaining adequate segment impedance with one or more combinational faults. One noticeable omission is the critical damping resistance of fifty ohms, which is normally placed in series with the differential mode inductor 2. As this said component offers no practical use other than for critical damping of second order resonating systems, it will not be included for this description, and forms no part of this invention. Never the less, there are other systems that will rectify parasitic network resonance, and the AC model may be utilized to include these for further analysis. As many of these said rectifiers are commercially sensitive, they will not be further considered for this description. It can now be observed how cross talk may occur. Cross talk from one segment 3a to the other segment 3n will occur where device 9a may be in transmit mode, and device 9n may be in receive mode whereby the receive amplitude will be subject to said attenuated magnitude. Up to a point, the crosstalk, at the said attenuated magnitude, may be tolerated. Beyond that point, any communication attempts to the receive device 9n on segment 3n will result in a communication failure due to excessive telemetry distortion or, the receive device 9n on segment 3n may successfully, but undesirably, receive data from the transmitting device 9a on another segment 3a. Many systems that are currently available utilize segment galvanic isolation, where the filter capacitors 8a to 8n offer adequate crosstalk attenuation. These said systems do not offer common mode inductor attenuation or at least adequate attenuation to curtail crosstalk where the common mode inductors are considered in isolation. Common mode inductors are sometimes utilized to curtail high frequency interference and not the in-band frequencies of interest for this description, and they are generally only used to filter out noise generated by the switching circuits of switch mode power supply converters or isolators. For this invention, the common mode inductors 1 are designed and utilized to provide adequate impedance to curtail inter-segmental 3a to 3n cross talk to an acceptable level and/or to provide adequate segment 3 impedance under fault conditions where galvanic isolation between segments 3a to 3n is not utilized, for example, and with reference to the AC model, where the filter capacitor impedances 8a to 8n tend towards zero ohms and the ground E impedance 10 is maintained at an acceptably high impedance such that the combination of all downstream components are favorable to adequate segment 3 impedance even with a direct short circuit fault for example, unbalance 7c may be replaced by a shorting wire, and that cross talk from one segment 3a to 3n is attenuated to an adequately low level even with said shorting wire.

The said adequately low level may be applicable with one terminator 4 deficit from one, or more, segments 3a to 3n. The essential similarity between the common power supply topology 5 with a common mode inductor 1 and the discretely isolated power supply topology 5 without a common mode inductor 1 is that, with an adequate inductance, the common mode inductor's 1 impedance will equal the impedance of the filter capacitor 8, ignoring, at this point, the common power supply topology 5 filter capacitor 10, at a given frequency. Above the said given frequency, the common mode inductor's 1 impedance will surpass the impedance of the filter capacitor 8 whereby high frequency interference suppression or attenuation on misbalanced segments 3a and/or 3n will be superior. Realizing the common power supply topology 5 filter capacitor 10, in shunt topology, will assist in further cross talk attenuation but selected not to significantly influence the power conditioner impedance under above said conditions. Of course, common power supply topology 5 filter capacitor 10, will only procure attenuation through ground E bourn influence 7ca and/or 7a and/or 7cn and/or 7n. The common mode inductor 1 will possess lower impedance below said given frequency. However, lower frequency induction and/or cross talk requires comparably higher energy levels and/or longer exposed lengths of cable core. Never the less, provided that the cross talk amplitude of said lower frequency follows the frequency toleration constraints outlined within IEC61158-2 clause 12 figure 67, then any communication failure will not occur. This said frequency toleration constraints forms a curve that the common mode inductor 1 should follow quite closely, as the attenuation order of the common mode inductor 1 will result in an attenuation of six decibels per octave or twenty decibels per decade.

For many common mode inductors of a given size, magnetic saturation can occur at very low levels of current unbalance or current differentials created by faults such as differing segment ground faults. Common mode inductors work most effectively where the current in each winding is equal and opposite and they are often referred to as current compensated chokes. Multiple Ground faults typically found, or occur, around each spur or device, can unbalance the current in each common mode inductor such that progressive saturation will progressively render the function of the common mode inductor ineffective or inadequate in curtailing parasitic crosstalk. Measures can be taken to prevent ground faults and these could comprise insulated covers for exposed wire connectors or even a simple plug and socket arrangement such that removal of wire/s will not make contact to ground. Other measures may comprise annunciation of a single, but tolerable, ground fault such that this can be effectively cleared before further ground faults, and consequential common mode inductor saturation, occur.

However, a further embodiment of this invention may comprise, in addition to said measures, a less obvious technique of providing balanced spur short circuit current limit for each pole of each spur. Figure 5 illustrates balanced non-linear current limiters, or non linear current limit circuit sensing, 13aa to 13nb positioned in each pole such that any spur 12a to 12n fault of any pole to ground Ea to En or any cross spur fault 14x from any pole on one spur 12a to any pole on another spur 12n will not, for segments 3a to 3n that share the same power source 5, permit current unbalance through each participating common mode inductor 1 to reach a level that would render the function of the common mode inductor 1 ineffective or inadequate in curtailing parasitic crosstalk between segments 3a to 3n. Of course, each segment 3a to 3n may comprise one or more spurs or splices 12a or 12n each with, or without current limit 13aa to 13nb as required.

Commercial applicability

The invention is especially commercially useful for a fieldbus network. In an advantageous manner the invention is designed and utilized to provide adequate impedance to curtail inter-segmental cross talk to an acceptable level and/or to provide adequate segment impedance under fault conditions.

Claims

CLAIMS:

1. A pseudo isolated power conditioner, like a common mode inductor, or system with similar function, for field bus systems, sited upstream of each participating segment's power conditioner, utilized to provide, or assist, adequate segment impedance and/or cross talk and/or adequate high frequency interference attenuation on, or between, participating segments in accordance with IEC61158- 2 or other similar telemetry standards.

2. Participating segments power conditioner as claimed in claim 1 , also comprising adequate power conditioning, which participate an isolated or non- isolated common power supply, with each participating segment unbalance within the constraints outlined within IEC61158-2, or similar telemetry standards, with, or without, one of many participating segment's one of two poles in resistive and/or capacitive and/or inductive, of any magnitude, contact to ground and/or with one of many participating segment's one of two poles in resistive and/or capacitive and/or inductive, of any magnitude, contact to another of many participating segment's one of two poles and/or said provision with toleration to one terminator deficit from one, or more, participating segments.

3. A common mode inductor, or system with similar function, as claimed in claim 1 and 2 with optional additional measures, such as earth fault detection/alarm and/or electrically conductive component mechanical protection, to annunciate and/or prevent activities as described in claim 1 and in claim 2.

4. The optional addition of spur specific bi-polar current limiting to prevent inductor saturation during activities as described in claim 2.

5. A common mode inductor as claimed in claim 1 to provide pseudo galvanic isolation as an alternative and/or addition to galvanically isolated power supplies.