WO2003098788A1

WO2003098788A1 - A passive common mode noise reduction circuit

Info

Publication number: WO2003098788A1
Application number: PCT/NZ2003/000092
Authority: WO
Inventors: Dennis Alan Chapman
Original assignee: Enatel Limited
Priority date: 2002-05-15
Filing date: 2003-05-15
Publication date: 2003-11-27
Also published as: NZ518966A; AU2003228171A1

Abstract

A passive common mode noise reduction circuit (1) includes a coupling device, and an auxiliary winding (6) connected in series. A signal generator is configured to produce a signal substantially in anti-phase with, and of a comparative magnitude to, the noise signal to be reduced.

Description

A PASSIVE COMMON MODE NOISE REDUCTION CIRCUIT

TECHNICAL FIELD

This invention relates to a passive common mode noise reduction circuit.

In particular, this invention relates to a passive common mode noise reduction circuit for use in power supplies or circuits that contain switching devices.

Reference throughout this specification shall now be made to use of the present invention with relation to power supplies and/or circuits containing transformers and / or switches (in particular field effect transistors [FETs]).

However, this should not be seen to be a limitation on the present invention in any way as the present invention may be used with any circuit that contains a common mode noise generating element.

BACKGROUND ART

The problems associated with common mode noise are well known within the field of electronics, having particular relevance to the field of power electronics such as switching power supplies, niotbr drives and uninterruptible power supplies etc.

Common mode noise produces an unwanted current that can result in electromagnetic radiation being emitted (conducted or radiated) from a circuit.

This is internationally recognised as a significant problem as the emitted radiation can interfere with the operation of other circuits in close proximity to the one emitting the radiation and can also cause intermittent power supply problems to manifest themselves. In order to limit and/or control the level of radio frequency interference (RFI) a number of international standards for the level of acceptable radiated emissions have been introduced, these include CISPR22 (EN55022) and FCC part 15.

Electromagnetic compatibility (EMC) is becoming ever increasingly important in the design and construction of electronic circuits, particularly those for such applications that will require them to be in the proximity of magnetically sensitive equipment e.g.: computer hard drives, DAT drives, digital data storage (DDS systems), etc.

Interference from radiated common mode noise can not only interfere with the operation of some other circuits but can even corrupt information that is stored or transceived by some equipment, which can result in the loss of important information which could in some applications have a disastrous effect.

Due to the ever increasing complexity of electronic circuits the standards outlining acceptable levels of common mode noise are constantly under review with the limits becoming lower and lower, therefore making the circuit designers job more difficult in order to comply with the standards.

Clever circuit design and layout can help reduce the level of radiated emissions however currently forms of "shielding" and filtering are mainly used to try to keep the level of emissions to an acceptable level.

The use of shielding not only increases the overall weight of the circuit but also constrains its design, complexity and. size. Some active techniques of common mode noise cancellation have been implemented however these are generally very complex circuits which, similar to the shielding, limits the layout and size of the overall circuit and also adds significant cost to its manufacture and operation.

With the current trend in electronics being for ever more complex and economical circuits being in an overall smaller package there is a need for a means of reducing the level of common mode noise and hence the level of unwanted electromagnetic radiation generated by a circuit in a manner that does not overly restrict the complexity or size of the circuit to which it will be connected and that does not add a significant additional cost to the finished product.

With the ever tightening standards for conducted and radiated emissions, a method of significantly reducing these emissions from within a circuit would significantly improve the performance of the equipment to which it is connected.

US Patent No. 6249876 discloses a quite complex circuit for varying the switching frequency of a power supply.

EMI emission at any frequency is reduced by jittering the switching frequency of the switch mode power supply by using an oscillator with a control input for varying the oscillator switching frequency. This generates a jittered clock signal. In one embodiment of the oscillator is connected to a counter as it is clocked input, the counter drives a digital to analogue converter whose output is connected to the control input of the oscillator used for varying the oscillation frequency of the power supply.

In another embodiment the oscillator is connected to a low frequency oscillator wherein the low frequency output is used to supplement the output of the oscillator for jittering the switching frequency.

This invention therefore deviates "or jitters" the switching frequency of the switch mode power supply oscillator within a narrow range to reduce the EMI noise at any one frequency by spreading the energy over a wider frequency range.

Not only is this a complex and costly circuit but it only broadens the frequency range through which the EMI is omitted and does not actually remove the EMI signal. European Patent No. 1184961 operates in a similar fashion in that is uses a digitally controlled frequency clock system to reduce the noise level in a switch mode power supply by providing a clock signal that has a spread of frequencies over a predetermined frequency band and therefore spreads the noise signal over a band of frequencies which has the effect of reducing the overall peak at any one frequency.

Although these last two circuits are good improvements over the shielding and filtering techniques used previously they still have some serious problems associated with these, not only the high costs and added complexity of these circuits but they also do not remove common mode noise but merely spread it across a range of frequencies.

There have also been a number of patents filed for a number of noise filters or chokes which will generally smooth out and reduce the noise generated within the switching power supply.

With regard to the foregoing it is clear that each of these methods of circuits have some significant drawbacks as they are generally costly and do not fully remove the common mode noise generated within a noisy circuit.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided a passive common mode noise reduction circuit for an electrical circuit that contains a noise signal, including

a coupling device, and

an auxiliary winding that are connected in series

characterised in that

the passive common mode noise reduction circuit includes a second signal generator which is configured to produce a signal substantially in anti-phase with, and of a comparative magnitude to, the noise signal to be reduced.

It should be appreciated that throughout the specification the term "coupling device" should be understood to include a capacitor. However,, this should not be seen to be a limitation on the present invention in any way as in circuits where the primary and secondary circuits do not need to be isolated from the chassis the coupling device can be other components such as a resistor, or network or resistors, or any other suitable component.

It is known by those skilled in the art that in most circuits where common mode noise is generated, the primary and secondary circuits (i.e. the circuits either side of a transformer) must be isolated from the chassis in order to improve the safety of the equipment and to ensure that an operator of the equipment will not be subject to an electrical shock if they touch the chassis.

It is noted however, that as stated previously, there are some circuits where this isolation is not required and therefore components that do not provide this level of isolation or insulation can be used between the circuits and the chassis.

Also throughout the present specification the term "auxiliary winding" should be understood to mean an additional winding or an existing winding that has the correct phasing.

In preferred embodiments of the present invention the auxiliary winding will be at least one additional winding to any windings that may be used in the normal operation of the circuit to which the present invention is attached.

It should be appreciated that this auxiliary winding can be an independent winding such as a coil or it may in fact be an additional winding on a transformer, the other windings of which are used in the normal operation of the circuit that is suffering from common mode noise.

In some preferred embodiments of the present invention the coupling device will be a fixed value device, for example a 100 Pico farad capacitor. However it should be noted that in some other preferred embodiments the coupling device can be a variable device that can either be preset at the time of manufacture or in some cases can be adjusted during the final installation or servicing of the device to which the circuit is connected.

It should also be appreciated that in some preferred embodiments of the present invention the auxiliary winding will be manufactured with a fixed number of turns, wherein the number of turns is calculated to ensure that in combination with the coupling device the correct amplitude and phase of the required cancelling signal is achieved.

However, this should not be seen to be a limitation on the present invention in any way as in some other preferred embodiments the auxiliary winding can be constructed so that its number of turns can be varied in order that when in circuit with the coupling device the number of turns can be adjusted in order for optimum performance.

It should also be noted that more than one common mode noise reduction circuit can be used in a single application where more than one common mode noise reduction signal is required.

It is envisaged that in most applications of the present invention it will be used to minimise or cancel common mode noise that is produced by noisy circuitry such as switches etc.

However, it should be appreciated (particularly in the fields of digital electronics and communications) that in some applications there can be a repetitive noise signal (e.g. a system clock) that can be coupled by stray compacitance to ground and which therefore is a problem.

In cases such as this one or more logic gates can be used to generate the anti-phase signal.

It should be appreciated that within the specific applications the noise may be from fairly low frequencies (i.e. a few kilohertz) up to very high frequencies that can be in excess of 1 gigahertz.

In theory, cancelling the effects of a noise source by adding an equal but anti-phase (or opposite) signal applies over the entire frequency spectrum for both conducted (i.e. propagated through power and control cables connected to a product) and radiated emissions.

In order to achieve this the cancelling network / circuit must reproduce the amplitude, waveshape and phase of the noise-cancelling signal accurately over the entire frequency range.

In practice it is more difficult to create an accurate anti-phase signal as the frequency increases above a few tens of MHz due to the effects of parasitic and / or non-linear circuit elements.

There are three effects caused by this: altered amplitude, imperfect waveshape and inaccurate phasing (i.e. a phase shift from the desired 180°) of the noise cancelling signal.

As these non-ideal circuit elements form a filter the amplitude and phase of the cancelling signal will vary with frequency preventing complete cancelling over the entire spectrum.

By using the present invention a reduction in common mode emissions of 30 dB or more are achievable and the range of frequencies over which a satisfactory level of reduction of emissions can be achieved can be extended by the addition of a resistor in series with the coupling device.

It should be appreciated that in most preferred embodiments of the present invention the cancelling circuit is constructed as a single series path to ground. However, it is envisaged that in some applications, particularly where higher order solutions to problems are required, the cancelling circuit can be composed of a plurality of parallel paths where each path can if necessary operate in a different frequency band in order that the overall cancelling circuit covers a greater bandwidth.

It should be understood that in some embodiments of the present invention specialised combinations of inductive and capacitive coupling can be used, for example strip-line and directional couplers.

It should be further understood that in some embodiments of the present invention the coupling device will be in the form of a mutual inductance rather than as a capacitor so that in application a winding carrying the noise cancelling signal would be wound around a magnetic core that is coupled around the phase and neutral conductors of the equipment (or circuit) that is generating the common mode noise.

This type of circuit can be described as a common mode choke with a noise cancelling winding.

As previously disclosed, in most embodiments of the present invention where a high frequency roll-off circuit is used, then the component of choice will be a resistor.

However, it should be appreciated that in some embodiments of the present invention the component (or components) used could be in the form or conductors or higher order networks that could be used to tune the circuit to have optimum performance at a particular frequency or band of frequencies. It should be noted however that this tuning of the circuit to a particular frequency or band of frequencies will impair the performance of the circuit at frequencies other than those to which it is tuned.

Whilst the present invention is aimed at the reduction or cancellation of common mode noise it should be appreciated that the principles behind the present invention could be used for cancelling differential mode noise if required.

According to another aspect of the present invention there is provided a method of passively reducing common mode noise in an electrical circuit that contains a noise signal, including

a coupling device, and

an auxiliary winding, that are connected in series

characterised by the steps of

a) measuring the approximate level of common mode noise present in a particular circuit, and

b) calculating the values of the coupling device and auxiliary winding that will produce a similar signal, and

c) connecting the coupling device and the auxiliary winding in series between the noise generating element and the system ground in order to produce a signal that is substantially in anti-phase with, and of a comparative magnitude to, the noise signal that is to be reduced.

In some embodiments of the present invention where the coupling device is an adjustable device there is an additional step in which the coupling device is adjusted in order to fine tune the circuit.

In some embodiments of the present invention where the auxiliary winding is an adjustable device there is an additional step in which the auxiliary winding is adjusted in order to fine tune the circuit.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a diagrammatical representation of a typical common mode noise source in a switching power supply, and

Figure 2 is a diagrammatical representation of the circuit of Figure 1 with the addition of one embodiment of the present invention, and

Figure 3 is the circuit of Figure 2 in accordance with one embodiment of the present invention that is configured for a high frequency roll-off, and

Fi ure 4 is a diagrammatical representation of one embodiment of the present invention for use in a buck converter, and

Figure 5 is a diagrammatical representation of one embodiment of the present invention for use in a boost converter, and

Figure 6 is a diagrammatical representation of a flyback converter fitted with a plurality of circuits in accordance with the present invention, and

Fi ure 7 is a diagrammatical representation of a three phase motor drive circuit that is fitted with a plurality of circuits in accordance with the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to the figures there is illustrated a common mode noise reduction circuit generally indicated by arrow 1.

In Figure 2 the present invention (1) is attached to a simple switching power supply in which the switch (2) generates a common mode noise signal that flows to ground through the stray compacitance (3) and flows through the stray compacitance (4) and across the transformer (5).

The magnitude of the current injected to cancel the unwanted noise is determined by the turns ratio of the primary winding of the transformer (5) and of the auxiliary winding (6) and the value of the coupling capacitor (7),

The auxiliary winding (6) is connected into the power supply circuit in a manner that ensures the current generated across it is in anti-phase to the current within the noise signal.

Figure 3 shows in similar circuit to that of figure 2 with the addition of a coupling resistor (8) connected in series between the auxiliary winding (6) and the coupling device (7).

The inclusion of the coupling resistor (8) achieves a high frequency attenuation roll-off which limits the frequency range over which the present invention can function adequately.

Figure 4 shows a typical application of the present invention in a buck converter wherein the auxiliary winding (6) is connected as an additional winding on the system coil (9). Figure 5 shows another typical example of an application for the present invention, this time in a boost converter. It can be seen that once again the auxiliary winding (6) is wound as an additional winding onto the system coil (9).

Figure 6 shows a more complex application for the present invention wherein it is incorporated into the circuit for a flyback converter.

In this application it can be seen that there is a plurality of common mode noise reduction circuits wherein the auxiliary windings (6) are wound as additional windings on the system transformer (5).

Figure 7 shows one possible arrangement of another typical application of the present invention, in a three phase motor drive circuit.

It can be seen within this circuit that there is a common mode noise reduction circuit for each set of phase switches (2) in order to reduce or cancel the common mode noise generated by the switches (2).

It should be appreciated that there is a vast number of arrangements and applications in which the present invention can be used however the examples given here show a good indication of most of the common types of applications in which the benefits of using the present invention are required.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the appended claims.

Claims

WHAT WE CLAIM IS:

1. A passive common mode noise reduction circuit for an electrical circuit that contains a noise signal, including

a coupling device, and

an auxiliary winding, wherein the auxiliary winding and the coupling device are connected in series

characterised in that

the passive common mode noise reduction circuit components form a second signal generator which is configured to produce a signal substantially in anti-phase with, and of a comparative magnitude to, the noise signal to be reduced.

2. A passive common mode noise reduction circuit as claimed in claim 1 wherein the coupling device is a capacitor.

3. A passive common mode noise reduction circuit as claimed in claim 1 wherein the coupling device includes a resistor.

4. A passive common mode noise reduction circuit as claimed in any previous claim wherein the auxiliary winding is a winding that was already present in the equipment's circuitry, to which the common mode noise reduction circuit is applied.

5. A passive common mode noise reduction circuit as claimed in any one of claim 1 wherein the auxiliary winding is a dedicated winding to the common mode noise reduction circuit.

6. A passive common mode noise reduction circuit as claimed in any previous claim wherein the coupling device is of a fixed value.

7. A passive common mode noise reduction circuit as claimed in any one of claim 1 to claim 5 wherein the coupling device has an adjustable value.

8. A passive common mode noise reduction circuit as claimed in any previous claim wherein the auxiliary winding has a fixed number of turns.

9. A passive common mode noise reduction circuit as claimed in any one of claim 1 to claim 7 wherein the auxiliary winding is constructed with an adjustable number of turns.

10. A passive common mode noise reduction circuit as claimed in any previous claim wherein the range of operational frequencies extends from a few kilohertz up to frequencies in excess of 1 gigahertz.

11. A passive common mode noise reduction circuit as claimed in any previous claim wherein the reduction in common mode emissions exceeds 30 db.

12. A passive common mode noise reduction circuit as claimed in any previous claim wherein the operation range of the circuit is decreased by the addition of a resistor in series with the coupling device.

13. A passive common mode noise reduction circuit as claimed in any previous claim wherein the operational range of the circuit is increased by the addition of other coupling device plus auxiliary winding combinations connected in parallel, with each combination tuned to a different frequency range.

14. A passive common mode noise reduction circuit as claimed in claim 1 wherein the coupling device is in the form of a mutual inductance.

15. A method of passively reducing common mode noise within a circuit that contains a noise signal, including

a coupling device, and

characterised by the steps of

b) calculating the values of the coupling device and auxiliary winding that will produce a similar signal, and select the coupling device and auxiliary winding to give these values.

c) connecting the coupling device and the auxiliary winding in series between the noise generating element and the system ground, in a manner wherein they are configured to produce a signal that is substantially in anti-phase with, and of a comparative magnitude to, the noise signal that is to be reduced.

16. A method of passively reducing common mode noise as claimed in claim 15 wherein the coupling device has an adjustable value, and the method further includes the additional step of adjusting the value of the coupling device as required.

17. A method of passively reducing common mode noise as claimed in claim 15 or claim 16 wherein the auxiliary winding has an adjustable number of turns and the method further includes the additional step of adjusting the number of turns of the auxiliary winding as required.

18. A passive common mode noise reduction circuit substantially as herein described with reference to and as illustrated by the accompanying drawings.

19. A method of passively reducing common mode noise substantially as herein described with reference to and as illustrated by the accompany drawings.