WO2021219785A1

WO2021219785A1 - Surge current detection

Info

Publication number: WO2021219785A1
Application number: PCT/EP2021/061261
Authority: WO
Inventors: Johannes Adrianus Cornelis Misdom
Original assignee: Signify Holding B.V.
Priority date: 2020-05-01
Filing date: 2021-04-29
Publication date: 2021-11-04

Abstract

A circuit for surge current detection makes use of a sensing coil physically located alongside a first supply conductors, or between first and second supply conductors, but not electrically connected to the supply conductor(s). The sensing coil generates a sense current which depends on the magnetic field generated by the supply conductor(s). This sense current may be used to detect current surge events.

Description

FIELD OF THE INVENTION

The invention relates to a circuit for surge current detection.

BACKGROUND OF THE INVENTION

Surge robustness of LED drivers is a topic of increasing interest. The higher the surge voltage the driver can withstand, the more robust the driver will be. It is also of interest to monitor surge events. For example, if the end of life of the driver due to current surges can be predicted, preventive maintenance can be carried out. Especially for road lighting applications, predictable maintenance is of importance.

One of the key parameters for end of life prediction is a measurement of the surge current and a count of surge current events. However, surge current measuring for example using a current sense resistor is a high cost function. The monitoring microprocessor needs to be isolated from the point where the surge current is sensed, so an isolator is required.

The typical surge current values to be measured range from 100A up to 5kA.

If a sense current of e.g. lkA is to be measured with a sense resistor, a 1 hiW resistor will already result in a IV output signal. A lmQ resistor is not a simple component. The sensing function then also needs an instrumentation amplifier as well. Additionally, a further building block is required to send the signal to the controller as there is no common ground.

A passive solution based upon a current transformer has therefore been proposed as it is lower cost than an optical isolation system, and more reliable for coping with high surge currents (and corresponding voltages).

The advantage of the use of current transformer is the desired isolation is provided by the transformer itself. However, to avoid transformer core saturation, a large core is needed. This results in a bulky and expensive part that will not for example fit inside an LED driver.

Thus, there are difficulties in providing a practical implementation with a sense resistor or with a current transformer. SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a circuit for surge current detection, comprising: an input for receiving power; a printed circuit board; a first supply conductor extending along the printed circuit board between the input and a downstream unit; and a sensing coil mounted on the printed circuit board and located laterally to the side of the first supply conductor for generating a sense current which depends on the magnetic field generated by the first supply conductor.

This circuit uses a sensing coil to detect the magnetic field produced by the current flowing along at least one supply conductor. In this way, isolation is provided by the sensing coil itself, and the sense output can be provided to a monitoring circuit. In this way, surge events can be detected and counted. The sensing coil is not electrically connected to the supply conductor, i.e. there is no galvanic connection between the sensing coil and the conductor. There is only a coupling by means of the electromagnetic field generated by the first supply conductor. The input is preferably for connection to an ac power supply.

Under laterally to the side, it is understood that the sensing coil is placed in its entirety on one side of the first supply conductor. This means that the sensing coil is not placed coaxially around the first supply conductor, which would also make the placement of the sensing coil more complex.

The sensing coil may be located at the input of a driver circuit, immediately downstream of the ac input to the driver circuit. However, the input of the circuit for current surge detection does not have to be the input of the overall circuit in which the surge current detection circuit is used.

The sensing coil for example comprises a discrete component mounted on the printed circuit board. This provides a compact and low cost arrangement, with a small discrete sensing coil mounted on the PCB and with the current flowing along a PCB track or wire being sensed, without the need to introduce any other additional components to the circuit or indeed to make any additional electrical connections along the PCB track or wire.

The sensing coil may comprise a dumbbell magnetic core. A dumbbell coil is a standard off the shelf component which may be used for this application. This gives a low cost and practical implementation. The circuit may further comprise a second supply conductor between the input and the downstream unit, and spaced from the first supply conductor, wherein the sensing coil is located between the first and second supply conductors for generating a sense current which depends on the magnetic field generated by the first and second supply conductors.

This increases the field strength so allows a smaller sensing coil to be used and/or simpler sensing circuitry.

The first and second supply conductors are preferably in parallel at the location of the sensing coil. Thus, they may provide an equal contribution to the magnetic field when the same current flows in opposite directions through the supply conductors (i.e. a supply current and a return current).

The sensing coil is for example spaced from the first and second supply conductors by an air space. This provides the desired isolation of the sensing coil from the conductors with no electrical (resistive/galvanic) connection.

The air space for example has a shortest length of between 2mm and 20mm. This range is suitable for preventing saturation of the coil based on the typical expected surge current.

The first and second supply conductors preferably comprise printed circuit board tracks. This provides a compact and low cost implementation of the surge detection with no modification needed to the existing circuit.

The sensing coil is for example located between a first portion of the first supply conductor and a first portion of the second supply conductor, and the circuit further comprises a second sensing coil between a second portion of the first conductor and a second portion of the second conductor.

The use of two sensing coils enables common mode surge events as well as differential mode surge events to be detected. For this purpose, when the first portions have the same current directions relative to each other, the second portions may have opposite current directions relative to each other, and vice versa. Thus, for one coil, equal current directions to/from the input provide a superposed magnetic field whereas for the other coil, equal current directions to/from the input provide a cancelled/suppressed magnetic field

The circuit preferably further comprises a monitoring circuit for monitoring the sense current and determining a surge event from the monitored sense current. A level shifting circuit may be provided between the sensing coil and the monitoring circuit.

The invention also provides a LED lighting circuit comprising: a LED unit; and the circuit for surge current detection defined above for surge current sensing and counting.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Fig. 1 shows an LED driver circuit in accordance with the invention;

Fig. 2 shows how the current sensing functions.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

The invention provides a circuit for surge current detection, which makes use of a sensing coil physically located alongside a first supply conductors, or between first and second supply conductors, but not electrically connected to the supply conductor(s). The sensing coil generates a sense current which depends on the magnetic field generated by the supply conductor(s). This sense current may be used to detect current surge events.

Figure 1 shows a LED driver circuit in accordance with the invention.

The circuit has an input 1 which is connected to an AC source of power. The input comprises a first supply conductor 2 between the input and a downstream unit and a second supply conductor 4 between the input and the downstream unit. The first and second supply conductors are spaced from each other. The circuit typically comprises a printed circuit board, and the first and second supply conductors 2, 4 comprise printed circuit board tracks. However, wire connections may equally be used for the current sensing as described further below.

The AC power is provided to an EMI filter 10 before AC/DC conversion in AC/DC converter 12. Thus, in this example, the EMI filter 10 is the downstream unit to which the first and second supply conductors 2, 4 connect. The EMI filter includes surge protection devices such as metal oxide varistors.

At the output of the AC/DC converter, power factor correction (PFC) may be implemented in PFC circuit 14 and the resulting signal is provided to an LED driver 16 which drives an LED load 17.

The LED driver may be an isolated driver, which provides an isolation barrier between the input and the LED load 17. However, isolation may be provided at other locations in the circuit. The invention may also be applied to non-isolated configurations.

The circuit may be a single stage driver (e.g. with power factor correction as part of the LED driver) or a multi-stage driver with separate power factor correction, as shown.

The driver may be based on a switch mode power converter or it may comprise a linear driver.

Thus, any known overall driver architecture may be employed.

A current sensing unit 18 provides detection of the current delivered from the input 1 to the rest of the circuit. This current may be interpreted as corresponding to a surge event by a master control unit 22. The current sensing signal will typically have a voltage in a different range to the input voltage range suitable for the control unit 22 so a level shifting circuit 20 may be used.

The current sensing unit 18 comprises a sensing coil 19 located between the first and second supply conductors 2, 4 for generating a sense current which depends on the magnetic field generated by the first and second supply conductors.

The circuit thus uses a sensing coil to detect the magnetic field produced by the current flowing along the first and second supply conductors. In this way, isolation is implemented by the sensing coil itself so that the sense signal does not need to cross a separate isolation barrier before connecting to the master control unit 22. Surge events can be detected and counted at the control unit 22.

The sensing coil 19 is not electrically connected to the supply conductor, i.e. there is no galvanic connection between the sensing coil and the conductors. There is only a coupling by means of the electromagnetic field generated by the first and second conductors. This coupling is across an air space between the sensing coil 19 and the supply conductors. Preferably, the sensing coil is equidistant between the supply conductors.

As shown in Figure 1, the first and second supply conductors 2, 4 are preferably in parallel at the location of the sensing coil 19. Thus, they may provide an equal contribution to the magnetic field when the same (alternating) current flows through the supply conductors (i.e. a supply current and a return current).

Figure 2 shows how the current sensing function is implemented.

The sensing coil 19 is a discrete component mounted on the printed circuit board. The example shows a dumbbell coil, i.e. a coil wrapped around the narrow shaft of a dumbbell shaped magnetic core. A dumbbell coil is a standard off the shelf component which may be used for this application. This gives a low cost and practical implementation.

The coil axis (and hence the shaft of the dumbbell) is oriented vertically, i.e. perpendicular to the plane of the PCB and hence perpendicular to the plane in which the supply conductors extend. As seen in Figure 2, this means a significant component of the magnetic field generated by the supply conductors extends along the coil axis thereby generating an induced current around the coil.

Because the sensing coil is between the supply conductors, currents in opposite directions generate a contributory field components at the location of the sensing coil, as shown.

The first supply conductor 2 is represented as delivering a current into the page, and the second supply conductor 4 is represented as delivering a current out of the page. In normal operation, these currents are equal, in that there is no leakage current; all delivered current from one supply conductor is returned along the other.

A conductor carrying an electric current will generate a magnetic field according to the formula:

B is the magnetic field, I is the current, d is the air distance and mo is the permittivity of free space. If there is a surge current of lkA and the distance between the conductor and the sensing coil is 10mm, this will result in a magnetic field of 20mT.

When the core of the sensing coil is placed between the two supply conductors (i.e. Live and Neutral) it will pick up the field generated by both supply conductors. The local field will then be 40mT/kA in this example.

As the ferrite core of the sensing coil has a high relative permittivity p_r, there is a densification of the field lines of about a factor of three. The actual field strength will thus be approximately 120mT/kA.

As the saturation level of the commonly used ferrites is around 400mT, such a configuration is capable of measuring surge currents up to 3kA.

For the current generated by the sensing coil:

L * I = n * B * A

L is the coil inductance, I is the current generated, n is the number of turns, A is the area and B is the field to which the coil is exposed.

Thus, the output current of the coils is given by: lout

By way of example, if n = 120 turns, L = 470mH, A = 20mm²:

. n*B*A 120*120 10 — 3*20 10 — 6 lout = - L = - 470 10-6 = 0.6A

If the system is terminated with 15W this will result in an output voltage of approximately 10V (for a lkA surge current).

By way of example, the sensing coil inductance is for example in the range IOmH to lOmH, for example IOOmH to 2mH.

The level shifting circuit 20 converts the current to a voltage and connects the voltage signal to the MCU 22. The level shifting circuit for example comprises diodes for peak-peak rectification, a peak hold capacitor and a resistive divider to match the voltage to the required MCU input. The MCU for example includes an analog to digital converter. The digital signal may be used for determining both the surge amplitude and duration.

A common mode surge current, where the supply conductors carry a current in the same direction, will result in no magnetic field generated in the core of the sensing coil due to local cancellation of the magnetic field. Thus, the basic arrangement shown in Figure 1 will measure differential mode surges only.

A second sensing coil and a modified PCB track pattern may be used to monitor common mode surge events as well. For this purpose, the (first) sensing coil is for example located between a first portion of the first supply conductor and a first portion of the second supply conductor.

The second sensing coil is then between a second portion of the first conductor and a second portion of the second conductor.

When the first portions have the same current directions relative to each other, the second portions may have opposite current directions relative to each other, and vice versa. Thus, for the first coil, equal current directions relative to the input provide a cancelled/suppressed magnetic field, whereas for the second coil equal current directions relative to the input to the output provide a superposed magnetic field. Similarly, for the first coil, opposite current directions relative to the input provide a superposed magnetic field, whereas for the second coil opposite current directions relative to input provide a cancelled/suppressed magnetic field.

This may require crossovers between the first and second supply conductors (or vias to the opposite side of a double sided PCB) to have one pair of portions with one pair of orientations and another pair of portions with an opposite pair of orientations.

The example above makes use of a sensing coil between a pair of supply conductors. This provides a large magnetic field for sensing. The invention may however be applied for sensing the current flowing along a single supply conductor.

The LED and driver are not described in detail since any known arrangement may be used.

The invention is of particular interest for LED drivers, but it may also be used in a wide range of applications where a surge counter is desired.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A circuit for surge current detection, comprising: an input (1) for receiving power; a printed circuit board; a first supply conductor (2) extending along the printed circuit board between the input and a downstream unit; and a sensing coil (19) mounted on the printed circuit board and located laterally to the side of the first supply conductor (2) for generating a sense current which depends on the magnetic field generated by the first supply conductor.

2. The circuit of claim 1, wherein the sensing coil (19) comprises a discrete component mounted on the printed circuit board.

3. The circuit of claim 2, wherein the sensing coil comprises a dumbbell magnetic core.

4. The circuit of any one of claims 1 to 3, further comprising a second supply conductor (4) between the input and the downstream unit, and spaced from the first supply conductor, wherein the sensing coil (19) is located between the first and second supply conductors (2, 4) for generating a sense current which depends on the magnetic field generated by the first and second supply conductors.

5. The circuit of claim 4, wherein the first and second supply conductors are in parallel at the location of the sensing coil (19).

6. The circuit of claim 4 or 5, wherein the sensing coil (19) is spaced from the first and second supply conductors by an air space.

7. The circuit of claim 6, wherein the air space has a shortest length of between 2mm and 20mm.

8. The circuit of any one of claims 4 to 7, wherein the first and second supply conductors (2,4) comprise printed circuit board tracks.

9. The circuit of any one of claims 4 to 8, wherein the sensing coil (19) is located between a first portion of the first supply conductor and a first portion of the second supply conductor, and the circuit further comprises a second sensing coil between a second portion of the first conductor and a second portion of the second conductor.

10. The circuit of claim 9, wherein when the first portions have the same current directions relative to each other, the second portions have opposite current directions relative to each other, and vice versa.

11. The circuit of any one of claims 1 to 10, further comprising a monitoring circuit (22) for monitoring the sense current and determining a surge event from the monitored sense current.

12. The circuit of claim 11, further comprising a level shifting circuit (20) between the sensing coil (18) and the monitoring circuit (22).

13. A LED lighting circuit comprising: a LED unit (17); and the circuit for surge current detection of any one of claims 1 to 12 for surge current sensing and counting.