WO2014044909A2

WO2014044909A2 - Control system for light matrices, particularly led matrices

Info

Publication number: WO2014044909A2
Application number: PCT/FI2013/050900
Authority: WO
Inventors: Heikki SEPPÄ
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2012-09-21
Filing date: 2013-09-18
Publication date: 2014-03-27
Anticipated expiration: 2015-03-21
Also published as: FI124042B; FI20125981A7; WO2014044909A3

Description

Control System for Light Matrices, particularly LED Matrices

The invention relates to a method, according to the preamble to Claim 1 , for feeding power to light matrices.

The invention also relates to a control system for light matrices.

The use of LEDs in lighting is increasing greatly, because good efficiency can be obtained with them.

A drawback with the prior art is that several LED rows must be piit into a' lamp with a- large ) output, because only 40 V/ 3 V LEDs can be put into a single row. The voltage restriction is because 40 V is regarded as a safe voltage. Rows cannot be connected in parallel, because the specific curve of a LED is very non-linear and voltage control leads to an uncertain output in each LED row.

The known solution is thus that a chopper operating with a normal electric coil develops a voltage of >40 V and utilizing this voltage each LED row (different colours or a single colour) is controlled with separate choppers.

Thus in practical known solutions, it is necessary to measure the current of each row and to control a chopper made for each row. This leads to a situation in which a lamp with N rows requires N+l choppers and a colour regulated LED system 3N+1 choppers.

The invention is intended to solve the problems of the prior art described above and for this purpose to create an entirely new type of method and system for power supply. The method is based on using a normal chopper, using either mechanical or electrical energy storage, to generate several different frequencies by pulse-width modulation. Each frequency is directed through a filter (either mechanical or electrical) to the desired LED row.

The amplitude and phase of the current running through the LED rows (or the total current of all the sets of rows) are measured and the phase information is used to lock each frequency to the frequency of the relevant resonance circuit. The amplitude is used to control the light output of the LED row. In connection with the use of mechanical resonances a piezo crystal is be used, to which an additional oscillator is connected to regulate the impedance level and the frequency. Another alternative is to use a mechanical oscillator based on a permanent- magnet, which is already suitable as a point of departure in terms of level of impedance. In particular, a magnetically controlled oscillator can also be used to cool the LEDs. In such a case, a larger output automatically leads to more powerful ventilation.

More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The system according to the invention is, in tum, characterized by what is stated in the characterzing portion of Claim 10.

Considerable advantages are gained with the aid of the invention.

The method according to the invention permits the control of LED lights, and also other lights, with good efficiency, using small and cheap electronics.

The invention permits a) the size of the main chopper to be reduced (if a mechanical 'coil' is used) and b) the choppers controlling the LED rows to be replaced with a resonance circuit (mechanical or electrical). The method also permits the control of several separate lamps or lamp rows, in such a way that only the current of the main chopper is measured. In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

Figure 1 shows schematically one light according to the invention, together with its power- supply equipment.

Figure 2 shows graphically the waveform of the power supply used in the invention.

Figure 3 shows graphically the waveform according to the invention, which is the filtered output of the chopper.

Figure 4a shows the equivalent circuit of a piezo crystal.

Figure 4b shows the equivalent circuit of a magnetically controlled oscillator.

Lighting is generally controlled by means of separate switches and in LED lights for each LED there is a chopper, which measures the current and regulates the voltage as required. The present invention discloses a method, in which every lamp or row of lamps is controlled by means of a single chopper, utilizing frequency multiplexing.

Figure 1 shows on a general level the method intended for controlling a light. A chopper consisting of a switch 5 generates a pulse for width modulation, based on a signal according to Figure 2. The coil (generally also an additional capacitance against ground) connected to the chopper filters the signal in such a way that the final curtain curve is essentially according to Figure 3.

Typically, the chopper's 5 output is direct voltage, but in the present invention 'direct voltage' varies in that it contains . V different frequency signals. After filtering, the signal is according to Figure 3. In Figure 3, the three standardized frequencies are: 1, 1.4, 1.7. The large intervals between the frequencies have been chosen only so that the shape of the signal can be seen from a few cycles. In practice, the differences in frequency are smaller. Because the energy store 4 remaining in the chopper 5 is also loaded capacitively, this should be taken into account in dimensioning.

The signal according to Figure 3 is connected through the resonance circuits 2 of Figure 1 to the LEDs 1 forming the light 20, which are formed of several LED rows 21 connected in series. Each row-specific resonance circuit 2 'selects' from the signal only the frequency corresponding to its own resonance frequency. The amplitudes of the different sines determine how much power is fed to each set of LED rows 21. Because the specific curve of a LED is very non-linear (the voltage over it is nearly constant), the regulation of power must take place with the aid of current. Because the current of each set of rows has a different frequency, it is enough to measure only the total current and, by separating the various frequency components, for example with the aid of a stage-locked loop, to obtain the currents for each set of LED rows separately. The measurement can be performed directly by v induction from the current of the coil connected to the chopper 5, by placing a magnetometer 3 (MEMS or some other) on the lead, or of course also with the aid of a separate resistance. A resistance is indeed an unfavourable alternative, because it consumes power. The - ' magnetometer 3 is advantageous, because it measures both the direct-current component and all the alternating-current components. Of course, it is possible to measure the current of each row separately, as is done at present, but this unnecessarily increases the price of the device. This alternative is also shown in Figure 1. The current is made as desired by adjusting the amplitudes of the different voltages separately, by controlling the phase-modulated signal (see Figure 2).

Because the frequency of the resonance circuit 2 can deviate nominally significantly, the frequencies produced by the chopper 5 must be adjusted adaptivety, so that they correspond to the frequencies of the resonance circuits 2. This can be done easily by measuring the phases of the currents relative to the phases of the voltages and adjusting the frequencies of the voltages correspondingly. All in all, the measurement takes place simply by using a phase-sensitive detector to measure both the amplitudes and the phases of the currents, and by adjusting the amplitudes of the voltages to regulate the power and to fit the phases of the frequencies to the resonance circuits.

Resonance circuits

A resonance circuit 2 can be realized by the series connection of a coil and capacitor in the normal manner.

By means of the connection, a situation is reasonably obtained, in which only less than 2 % of the power is lost. The problem is rather one of size, because a small -loss transformer at a reasonably low frequency requires a large coil.

Taken generally, a mechanical resonance is able to store more energy than an electric coil of corresponding size. This means that the resonance circuit is best realized by a mechanical resonance circuit. We can use either an oscillator operating with a piezo transformer or a magnetic resonance circuit controlled by current. The advantage of a mechanical resonance circuit is not only small size but also a high quality factor. In addition, it is possible to integrate a mechanical oscillator close to the LEDs, when we can also use the movement for ventilating the LEDs. Of course we are then using power for ventilation, so that the quality factor of the oscillator will drop. However, this is more advantageous than placing a fan or a large metal cooler grille in the light.

Figure 4a shows the equivalent circuit of a piezo crystal. Thanks to the parallel capacitance 12 of the piezo, the structure creates both a series resonance determining the mechanical resonance and a parallel resonance, which depends on both the mechanical resonance and the stray capacitance. However, the oscillator is preferably used in a series resonance, so that the stray capacitance is of no significance. However, it appears as a capacitive load, so that it should be taken into account when designing the whole system. Typically, with a crystal the effective series resistance 13 is reasonably large, so that the efficiency remains poor. Of course, by making a thin and wide crystal the series resistance 13 is made smaller. However, it is advantageous to connect a mechanical resonance on top of the crystal, in such a way as to make the effective series resistance smaller. In this situation, the resonance frequency is determined by the excess resonance circuit and the piezo actuator acts only as a device that converts the voltage into movement. In Figure 4a, the reference number 10 depicts the coil in the equivalent circuit and the reference number 11 the series capacitance.

If we use a magnetically controlled oscillator in the resonance circuit, we can depict it with the equivalent circuit according to Figure 4b. The magnetic resonance circuit is based on the force of a current running through a permanent magnet 15 and a coil 14. It this solution, it is possible to select whether the coil 14 moves (like a loudspeaker) or the magnet 15. If the coupling factor is large, the circuit represents a parallel resonance circuit against the ground. Power connects well to the LEDs at a single frequency, but tends to 'short-circuit' the currents intended for the other LEDs. It is preferable to place a capacitor in series after the coil, when the mechanical resonance circuit operates as an effective coil and creates a series resonance circuit with the capacitor. In this case, the other frequencies see this resonance circuit as open.

A mechanical resonance circuit can also be used to reduce losses in the chopper. In this connection, it is preferable also to make a transformer in the circuit. In the case of a piezo, this means that the piezo piece forms a single structure in terms of the mechanical oscillation, but an intermediate electrode is connected to it to pick up output power. In a magnetic oscillator, two coils are placed in the system and the number of windings is selected so as to achieve the desired transformation ratio. Because all the diodes are not completely the same, and one of them can break, the set of LED row can rectify the signal. Rectification as such is not detrimental, because it only biases the diodes to a different operating point. In this solution the failure of one diode does not put the light out completely. On account of self-biasing and alternating electricity the remaining diodes will produce light. Summary

The present invention discloses a method, in which lamps and especially LED lamps can be controlled by a single chopper, by exploiting frequency multiplexing. The procedure also means that the currents (outputs) of all the lamps can be measured by a single ammeter. In addition, this application discloses the utilization of a mechanical resonance in order to make frequency filters. The method according to the invention is clearly cheaper and of a smaller size than present solutions. The efficiency is at least of the same order as with present solutions, but because several choppers are not required it may be possible to also improve the device's efficiency, particularly if a mechanical resonance circuit is also used as the chopper's energy store.

In practice, in the invention each LED row is supplied with chopped DC power through band-pass filters, in such a way that each light row receives power from the whole spectrum of its frequency spectrum.

The invention is suitable for lighting in general. Because LED is a new growing lighting technology, it is worth introducing it there first. One application is also LED-backlit displays. In the present application, the term light 20 refers to both a traditional light, in which the elements are in the vicinity of each other, and also a larger lighting totality, in which the various parts of the light can be located, for example, in the different rooms of a building.

Claims

Claims:

1. Method for feeding power to a light, in which the light (20) is formed of several rows (21) consisting of light elements ( 1 ), in which method power is fed to the light (20) row-specifically (21 ), characterized in that each light row ( 21 ) is fed with power at a predefined frequency band.

2. Method according to Claim 1, characterized in that power is directed to each row

through a band-pass filter (2).

3. Method according to either of the above Claims, characterized in that an electric filter is used as the filter (2).

4; Method according to any of the above Claims, characterized in that a piezo-electric filter is used as the filter (2).

5. Method according to any of the above Claims, characterized in that a mechanical filter is used as the filter (2).

6. Method according to any of the above Claims, characterized in that the power being fed is produced by pulse-width modulation.

7. Method according to any of the above Claims, characterized in that the light elements (1) are LEDs:

8. Method according to any of the above Claims, characterized in that the total current fed to the light is measured (3).

9. System for feeding power to a light, in which the light (20) is formed of several rows (21) consisting of light elements (1), which system comprises means for feeding power to the light (20) row-specifically (21 ), characterized in that the system comprises means for feeding power at a predefined frequency band to each light row ( 21).

10. System according to Claim 9, characterized in that power is directed to each row

through a band-pass filter (2).

11. System according to any of the above Claims, characterized in that an electric filter is used as the filter (2).

12. System according to any of the above Claims, characterized in that a piezo-electric filter is used as the filter (2).

13. System according to any of the above Claims, characterized in that a mechanical filter is used as the filter (2).

14. System according to any of the above Claims, characterized in that the power being fed is produced by pulse-width modulation.

15. System according to any of the above Claims, characterized in that the light elements (l) are LEDs.

16. System according to any of the above Claims, characterized in that the total current fed to the light is measured (3).