WO1991009459A1

WO1991009459A1 - Display board provided with a regulated power supply

Info

Publication number: WO1991009459A1
Application number: PCT/BE1990/000070
Authority: WO
Inventors: Ludo De Ceuster; Eddy Giron
Original assignee: Ludo De Ceuster
Priority date: 1989-12-18
Filing date: 1990-12-17
Publication date: 1991-06-27
Also published as: BE1003655A3; AU6953991A

Abstract

Display board, in particular an illuminated advertisement member, provided with at least two light bulbs branched in parallel, which are connected to a low power output formed by a secondary winding of a transformer (52), which is part of a transformer circuit provided with a rectifier. The transformer circuit comprises a pulse width modulation control circuit (29) provided with a frequency generator (30, 31), and wherein between a first and a second output of the rectifier there is provided a first transistor circuit (48, 49) the basis of which being connected with an output of said pulse width modulation control circuit (29), which transistor circuit comprises a connection point connected with the primary winding of the transformer.

Description

"Display board provided with a regulated power supply"

The invention relates to a display board provided with at least two light bulbs branched in parallel, which are connected to a low power output formed by a secondary winding of a transformer, which is part of a transformer circuit provided with a rectifier. Such a display board is commercially available and is used as an illumated advertisement member. By the known illumina¬ ted advertisement members two or more light bulbs, which are provided to operate an a power tension of 2k volts, are connected in parallel to each other. The 2 volt power is supplied by a usual transformer circuit which transforms the mains into a 2k volt tension.

A drawback of the known display boards is that the transformer is heavy, especially when the display board comprises five or more light bulbs which consume the necessary energy. The use of heavy transformers makes it rather impossible to fit the transfor- mer into the display board itself s, which implies that it is necessary to put the transformer on a suitable support, most of the time inside a housing.

It is an object of the invention to provide a solution for the above mentioned drawback. Thereto a display board according to the invention is characterized in that said transformer circuit comprises a pulse width modulation control circuit provided with a frequency generator, and wherein between a first and a second output of the rectifier there is provided a first transistor circuit, the basis of which being connected with an output of said pulse width modulation control circuit to control said transistor circuit, which comprises a connection point connected with the primary winding of the transformer for controlling the current streaming to the primary winding. By using a pulse width modulation control circuit, it is possible to use much lighter transformer circuits which thereupon is - very compact. The pulse width modulation and the transformer easily fits into the housing of the display board without giving thereto a substantial weight, the whole member can then easily be fixed to a wall. The transistor circuit further provides a dedicated power supply for the light bulbs.

The use of a pulse width modulation circuit for transform ing a power supply, such as for example the mains, is known per se and is for example applied by computers. Computers are apparatuses which however have to satisfy other requirements than those imposed on display boards or illuminated advertisement members, and therefore it can be stated that a completely different technical field is dealt with. The invention is mainly based on the insight that, when using a pulse width modulator, a solution is provided for the weight and dimension problems occuring when use is made of the well known heavy transformers.

A first preferred embodiment of a display board according to the invention is characterized in that said light bulbs are connected to a power supply line which is connected via a further rectifier with the secundary winding of said transformer. By using a further rectifier the high frequency alternative current formed by the pulse width modulator is transformed into a direct current through which the parallel branching of the light bulbs will not act as an antenna, which would be the case when an alternative current would be present thereon.

A second preferred embodiment of a display board according to the invention is characterized in that the transistor circuit formed by a MOSFET, the drain of which being connected with said primary winding of said transformer.

A third preferred embodiment of a display board according to the invention is characterized in that said transistor circuit comprises two serially connected MOSFET's, the source of a first MOSFET being connected via a connection line with the drain of a second MOSFET, the primary winding of the transformer being connected with said connection line. The use of a MOSFET enables the operation with higher frequency and sharper pulse edges.

It is advantageously the the basis of the transistor circuit is galvanically separated from the output of the pulse width modulation control circuit.

A fourth preferred embodiment of a display board according to the invention is characterized in that the light bulbs are connected to a power supply line which is connected via a further rectifier to the secundary winding of the first transformer, which power supply line is further connected with a first control input of said pulse width modulation control circuit for controlling in function of the tension present on said power supply line the pulse width of a pulse signal output by said pulse width modulation control circuit. This enables a tension dependent control. A fifth preferred embodiment of a display board according to the invention is characterized in that said power supply line is connected via a light sensitive cell with said first control input. A control in function of the intensity of the day light is herewith possible. The invention will now be described in more details by means of the drawings. It will be clear that the invention is not limited by the embodiment represented in the drawings and that within the scope of the invention different alternatives are possible. In the drawings : Figure 1 shows a schematic view of an example of the inside of a display board according to the invention ;

Figure 2 shows a first embodiment of a transformer circuit for supplying power to the lamps of a display board according to the invention ; Figure 3 shows a second embodiment of a transformer circuit according to the invention.

A same reference number has been assigned to the same or analogous elements in the drawings.

Figure 1 shows a schematic representation of an example of the inner side of a display board according to the invention. - k -

The display board is for example formed by an illumated advertisement member or an indication board provided with an illumination as is for example used in public places (railway stations, airports, etc.). In this example the display board comprises a housing 100 wherein a transformer circuit 2 is provided. At an input of the transformer ciricuit a first power supply 3 is applied, which for example originates from the mains and has a 220 volt alternative tension. An ouput of the transformer circuit 2 is connected with a power supply line 101 to which light bulbs 102, 103, 10k, 105 and 106 are connected in parallel. It will be clear the the number of five light bulbs is only given by way of example and that this number can have any value.

That number is at least two in such a manner that the rupture of one light bulb does not render the complete member unusable. By connecting the light bulbs in parallel there is avoided that a defect of one lamp causes all lamps to go out which is the case by a serial connection of all the light bulbs. The light bulbs are preferably automo¬ bile light bulbs of the elongated type, such as for example a light bulb of the type of N.V. Philips, and suitable for a power supply of approximately 20 volts. Low tension light bulbs have the advantage to be safer for example for use in illuminated advertisement members.

An embodiment of a transformer circuit 2 is illustrated in Figure 2. A power 3, for example the mains, is supplied via a mains filter, for example formed by the solenoids LI en L2 en de capacitor C, to a rectifier 20, for example formed by a diode bridge. In order to make the transformer circuit suitable for feeding different powers, for example 220 V and 110 V a switch 21 is provided which is switchable in two or more positions. Over an output of the rectifier 20 there is connected a parallel connec¬ tion of two serially connected capacitors 22, 23 and two serially connec- ted resistances 2k, 25. The capacitors 22, 23 have for example each a value of 470 μF and the resistances 2k, 25 have a value of 220 kJl . thus enabling on the connection point 26 between the capacitor 22, the resistance 2k on the one hand, and the capacitor 23, the resistance 25, on the other hand, a tension of 0 V. A primary winding of a first transformer 27 is connec- ted on the one hand with .he point 26 and on the other hand with a + output of the rectifier 20. The secondary winding of the first transformer 27 has a first pole which is connected via a diode and a polarised capacitor 28 with a VCC input 12 of a pulse width modula- tion control circuit (PWM) 29. The first transformer 27 is provided for i.a. supplying a power supply of for example Ik Volt to the PWM 29. By the use and the branching of the first transformer 27 there is obtained that the power supply to the PWM 29 is galvanically comple¬ tely separated, on the one hand, from the power supply 3 and, on the other hand, from the load formed by the light bulbs. Due to this a perturbation on the load has no influence on the power supply of the PWM 29.

The pulse width modulation control circuit (PWM)

29 is for example formed by an IC of the type Fairchild μA k9k. The basis oscillator for the PWM 29 is formed by a first connection point of the capacitor 30, of for example 1 nF resp. a resistance 31 of for example k.7 KJt wich are connected with the input 5 resp.

6 of the PWM. A second connection point of the capacitor 30 resp. the resistance 31 is connected to earth. The RC loop formed by the capacitor 30 and the resistance 31 thus forms a basis oscillator which is suitable to produce an oscillator frequency of for example 100 KHz.

A random positive potential, for example supplied by the parallel connection of a resistance and a capacitor, is applied at the input k and 13 of the PWM. At the input 1* of the PWM 29 a reference voltage of for example 5 Volt is applied which is supplied by a voltage divider 32, which is for example formed by two resistances of 10

KΛ each.

The inputs 1 and 2 of PWM 29 are connected with a comparison element (not shown in the figure) which is part of the PWM. Input 2 is connected with a branching point of the voltage divider 32. Input 1 is connected with a branching point of a further voltage divider 33, which is formed by the resistance 3k and 35 and the variable resistance 35. Resistance 36 is connected with a power supply line 101 on which the light bulbs of the illuminated advertisement member are connected. In such a manner a feedback is realized between the PWM 29 and the load formed by the light bulbs of the advertisement member. At the input 2 of the PWM 29 there is supplied via a voltage divider 32 a fixed voltage value of for example 2.5 Volt. The power supplied at the input 1 of the PWM will now vary in function of the power consumed by said load. In order not to supply at the input 1 of the PWM the full voltage picked up by the load, use is made of a voltage divider 33. The variable resistance 35 has the purpose to delimit the treshold values within which the voltage picked up by the load may vary. The reaction of the PWM on the voltage variations at the input 1 will be described hereafter.

The PWM includes a further comparison element (also not shown in figure 2) the inputs of which are connected with the respective inputs 15 and 16 of the PWM. Input 15 is connected with a branching point of a voltage divider, while input 16 is connected via a rectifying capacitor 37 and a further variable resistance 38 with a secondary winding of a second transformer 39. The connections 8 and 11 form the outputs of the PWM 29, both outputs being each time connected with a collector of a transistor belonging to the PWM. At the outputs 8 and 11 there is supplied a square wave the pulse width of which being modulated in function of the preset RC-time and the variations such as sensed at the control inputs 1 and 2 as well as 15 and 16. At both outputs there are supplied signals which have opposite fase. The connections 7, 9 and 10 of the PWM 29 are earthed. Output 11 resp. 8 is connected with the basis of a first npn kO and a second pnp 41 respectively a third npn 42 and a fourth pnp 43 transistor. The collector of the first 40 and the third 42 transistor are connected with the secondary winding of the first transformer 27 and thus receive, when active, a positive voltage. The collector of the second 41 and the fourth 43 transistor is connected to earth. The emitters of the first 40 and the second 41, respectively the third 42 and the fourth 43 tansistor are connected to each other via a connection line 44 resp. 45. Those respective connection lines are each connected via the capacitor with a first connection point of a primary winding of a third 46 resp. a fourth 47 transformer. A second connection point of the primary winding of the third and the fourth transformer is earthed. Over the secondary winding of the third 46 resp. the fourth 47 transformer there is applied a resistance of for example 1.2 Kj_,. A first connection point of the secondary winding of the third respectively the fourth transformer is connected via a resistance of for example kl.fl with a first connection point (gate) of a fifth 48 respectively a sixth 49 transistor. The fifth and the sixth transistors are preferably MOSFET's because they are suitable for a higher branching frequency and thereupon dissipate less energy. A second connection point of the secondary winding of the third transformator 46 is connected with a second connection point (source) of the fifth transistor 48. That type of transistor circuit is generally designated as a half bridge amplifier.

The source of the sixth transistor 49 is connected with a first connection point of the primary winding of the second transformer 39, and a second connection point of the primary winding is connected via a connection line 50 with the secondary winding of the fourth transforer 47. That connection line is further connected with the - output of the rectifier 20. A third connection point (drain) of the sixth transistor 49 is connected via connection line 51 with the source of the fifth transistor 48. Connection line 51 is connected with a first connection point of a primary winding of a fifth transformer

52. A second connection point of the primary winding of that fifth transformer is connected via a capacitor with the connection point 26. A secondary winding of a fifth tranformer is connected over two diodes, for double side rectifying, and over an LC circuit for smoothening the supplied tension, with the power supply line 101 on which the light bulbs are connected.

When now at the inputs 1 and 2 of the PWM 29 a substantially equal tension is supplied, the PWM will supply a square wave at the outputs 8 and 11. When the tension of the square wave has a positive level at the output I I, then the first transistor 40, which is an npn transistor will conduct while the second pnp transistor 41 will block when a positive tension is applied at its basis. Because transistor 40 is conducting, the current at its collector will stream towards its emitter and thus load the capacitor 53.

When now the square wave supplied by the PWM 29 reaches a negative level, the transistor 40 will block and transistor 41 will conduct, according to which a loop formed by the emitter-collec- ter path of the second transistor 41, the primary winding of the third transformer 46 and the capacitor 53 will get closed. The closing of that electrical loop will have as consequence that the capacitor will discharge over the primary winding of the transformer 46 and thus will induce a voltage pulse into the secondary winding. An analogous process occurs by the third 42 and the fourth 43 transistor and at the fourth transformer 47 where also the discharging of the capacitor 54 over the primary winding of the transformer 47 will induce a voltage pulse into the secondary winding. Because however the outputs 8 and 11 are of opposite fase, the capacitor 53 will charge while capacitor 54 will be discharged and vice versa thus causing the alternative induction in the secondary winding of the transformer 46 resp. 47 of a voltage pulse. The emitter-emitter connection of the transistor pairs 40, 41, resp. 42, 43 will provide that the voltage pulses induced in the secondary winding will show sharp edges, because transistors are generally excellent switches. The sharp edges are in fact particularly advantageously by the used high switching frequency of for example 100 kHz. Further it is necessa¬ ry when using MOSFET's to present pulse with sharp edges at the basis of the MOSFET's because they are thereby optimally brought into conduction.

The voltage pulse induced in the secondary winding of the thirth 46 resp. the fourth 47 transformer^' is supplied to the gate of the fifth 48 resp. 49 transistor. The serial branching of two resistances prevents a floating of the MOSFET gate. When now a tension pulse is supplied to the gate of the fifth transistor 48 the latter will be conductive causing a tension originating from the + connection of the rectifier to be applied to a primary winding of the fifth transformer 52. A tension is thus induced into the secondary winding of the transformer 52. When, on the other hand, a tension pulse is supplied at the gate of the sixth transistor 49, the latter will be conductive whereas transistor 48 will block at the same moment. Because transistor 49 is conductive, the circuit formed by the - pole of the rectifier 20, the sixth transistor 49 and the primary winding of transformer 52 and connection point 26 is closed, according to which current is again streaming through the primary winding of the transformer 52 and thus again a tension is induced into the secondary winding of the fifth transformer 52. The fifth and the sixth transistors are thus switched as a push-pull thus enabling to supply continuously voltage pulses in the primary winding of the fifth transformer 52.

The voltage pulses supplied at the primary winding of the fifth transformer 52 will now induce an alternative voltage in the secondary winding. That alternatvie voltage is converted into a direct voltage by the diodes 53 and 54. The use of two diodes allows a double side rectifying. The thus formed direct tension is supplied via an LC circuit formed by a solenoid 55 and a capacitor 56 to the power supply line 101, to which the light bulbs of the illuminated advertisement member are connected. In this way, the lighting bulbs are fed by a direct tension. In fact, would the alternative current not be transformed into a direct current, the power supply line would operate as an antenna, especially in view of the high switching frequency of for example 100 KHz which is used. The fact that the power supply line would act as an antenna would involve that the display board would operate as a perturbation emitter which of course is inacceptable. The ratio of the windings of the fifth transformer

52 is preferably chosen in such a manner that on the power supply line a tension ranging between 15 and 30 Volt is presented.

By using a PWM, it is now possible to use lighter and more compact transformers, thus offering a construction which can be included without any problem in a display board. Ths use of a pulse width modulator offers the possibility to realize a regulated power supply which is light and compact.

A feedback line 57 between the power supply line 101 and the further voltage divider 33 enables to hold tension variations on the power supply line between definite thresholds. A variation of the tension on the power supply line 101 is supplied at the further voltage divider 33, which supplies a fraction thereof at the input

1 of the PWM 29. The amount of that fraction is determined by the position of the variable resistance 35. It will be clear that the use of a variable resistance is a preferred embodiment and that a resistance

35 and eventually also a resistance 36 can be changed by a resistance with a fixed value.

When now the tension at the input 1 of the PWM varies due to variations on the power supply line 101, and in such a manner that the comparison unit connected with the inputs 1 and

2 detects a too big difference between the fixed tension at the input 2 and the variable tension at the input I, the comparison unit will stop the oscillation of the PWM, in such a manner that at the outputs 8 and 11 no more pulses are supplied. Thereby the voltage on the power supply line 101 will drop thus enabling that a lower tension is now applied at the input 1. Thereby the comparison unit will then again detect that a substantially equal value is presented at the inputs 1 and 2 and the oscillation will be started again according to which pulses will again be output at the outputs 8 and 11. Thereby the tension on the power supply line 101 is kept between defined thres¬ holds, for example between 15 and 30 Volt and the bulbs are not over¬ charged. Due to this the bulbs have a longer life time since they sense a nearly continuous voltage at a nearly equal level.

A further advantage of the use of a pulse width modulator is that it is now possible to realize in an easy manner a control in function of the light intensity. Indeed, for example by illuminated advertisement members, it is necessary to vary the intensity in function of the intensity of the daylight. The feedback line 57 between the power supply line 101 and the further voltage divider 33 enables such a control. By this embodiment, the variable resistance

35 is replaced by a light sensitive cell which measures the intensity of the daylight. Variations in the intensity of the daylight are now converted by that light sensitive cell in variations of the resistance value, through which the voltage at the input 1 of the PWM changes and in such a manner the pulse width thus resulting in another tension on the power supply line 101.

A further protection is formed by the fourth transfor¬ mer 39, a secondary winding of which being connected via a variable resistance 38 with the input 16. When the tension over the secondary winding of the transformer 39 becomes too high, a fraction thereof, determined by the position of the variable resistance 38, will be supplied to the input 16. Inputs 15 and 16 are connected with a further compari¬ son unit of the PWM. A too large difference between the fixed voltage at the input 15 and the variable voltage at the input 16 will also lead to the fact that the oscillation will be interrupted by the further comparison unit. This, at its turn, leads to a drop of the tension over the secondary winding of the transformer 39 which, then again, will bring the further comparison unit into equilibrium and will again start the oscillation. In such a manner the whole circuit is protected against short-cuttings. It will be clear that the variable resistance 38 can be replaced by a resistance with a fixed value. The use of the second transformer 39 for this purpose has the advantage that the voltage supplied at the PWM 29 will thus be separated from the voltage at the output of the rectifier 20. Figure 3 shows a second embodiment of a transformer circuit according to the invention. By this embodiment no use is made of a half bridge amplifier. The low tension supplied to the secondary winding of the first transformer 27 is supplied via a resistance resp. via a further resistance to the power supply inputs 63 resp 64 of the PWM 29. The PWM is here for example formed by an IC of the type ....

A capacitor 30 resp. a resistance 31 are connected with the input 5 resp. 6 of the PWM. The thus formed RC circuit forms a basis oscillator for the PWM. The inputs 66, 67, 68 of the PWM are earthed via a capacitor, and a power tension is supplied at input 65, which is supplied by the secondary winding of the first transformer 27. Input 69 is earthed.

The feedback line 57 comprises a variable resistance 35 which is part of a voltage devider, a branching of which is connected with input 62 of the PWM. As described by the circuit illustrated in figure 2 that feedback line enables the PWM to be controlled in function of the variations such as observed on the power supply line 101.

A further feedback line 70 connects the secondary winding of the transformer 39 with an input 16 of the PWM 29. With this further feedback line there is connected a capacitor 71 and a diode 72, which prevents the tension supplied by the capacitor 71 reaches the secondary winding of the transformer 39. The circuit formed by the diode 72 capacitor 71 serves to supply a continuous direct tension at the PWM 29.

At an output 11 of the PWM 29 a square wave is output the pulse width of which is modulated. The output 11 is connec¬ ted with the basis of transistor 40 resp. 41. The circuit formed by the transistors kO and 41, capacitor 53 and transformer 46 operates in an analogue manner as the one described in figure 2. The discharging of capacitor 53 induces a voltage pulse in the secondary winding of the transformer 46, which voltage pulse is then supplied at the gate of the MOSFET 49.

The supply of a voltage pulse at the MOSFET 49 has now for consequence that the MOSFET 49 becomes conducting according to which the loop formed by the primary winding of transfor¬ mer 52, the MOSFET 49 and the primary winding of transformer 39 is closed and that a current will now stream between the + and the - output of the rectifier 20. Through this a voltage pulse is induced in the secondary winding of transformer 52 thus supplying a voltage at the power supply line 101. Also capacitor 73 is charged, which capacitor 73 is branched in parallel with the primary winding of the transformer 52. In order to limit the current through the capaci¬ tor 73 a resistance 47 is connected in parallel with capacitor 73. During the half period the capacitor 53 is charging, no voltage pulse is supplied at the gate of the MOSFET 49 thus causing the circuit wherein the primary winding of the transformer 52 is present to be interrupted. Through this, the charge present in capacitor 73 will discharge in the circuit formed by capacitor 73, primary winding of transformer 52 and diode 75. In such a manner a continuous tension is supplied at the power supply line 101.

In the figures 2 and 3 there is respesented a circuit with discrete components, but it will be clear that the circuit can mainly be integrated in a IC, according to which the weight and the dimensions of the circuit can be further reduced.

Claims

1. Display board, in particular an illuminated adverti¬ sement member, provided with at least two light bulbs branched in parallel, which are connected to a low power output formed by a secundary winding of a transformer (52), which is part of a transformer circuit provided with a rectifier, characterized in that said transformer circuit comprises a pulse width modulation control circuit (29) provided with a frequency generator (30,31), and wherein between a first (+) and a second (-) .output of the rectifier there is provided a first transis- tor circuit (48, 49) the basis of which being connected with an output of said pulse width modulation control circuit (29) for controlling said transistor circuit, which transistor circuit comprises a connection point connected with the primary winding of the transformer for controlling the current streaming through the primary winding.

2. Display board as claimed in claim 1, characterized in that said light bulbs are connected to a power supply line (101) which is connected via a further rectifier with the secundary winding of said transformer (52).

3. Display board as claimed in claim 1 or 2, characte- rized in that the basis of the transistor circuit is gaivanically separated from the output of the pulse width modulation control circuit (29).

4. Display board as claimed in claim 3, characterized in that said galvanic separation is realized by a further transformer (46, 47), the primary winding of which being connected via a further transistor circuit with said output of said pulse width modulation control circuit, and the secondary winding of which being connected with the basis of said transistor circuit (48, 49).

5. Display board as claimed in anyone of the claims I to 4, characterized in that the transistor circuit is formed by a MOSFET, the drain of which being connected with said primary winding of said transformer (52).

6. Display board as claimed in claim 5, characterized in that a capacitor is branched in parallel with said primary winding.

7. Display board as claimed in anyone of the claims l-k, characterized in that said transistor circuit comprises two serially connected MOSFET's, the source of a first MOSFET (48) being connected via a connection line (51) with the drain of a second MOSFET (49), the primary winding of the transformer (52) being connected with said connection line (51).

8. Display board as claimed in claim 7, characterized in that said first (48) resp. said second (49) MOSFET is connected with a first ( 11) resp. a second (8) output of said pulse width modulation control circuit (29).

9. Display board as claimed in anyone of the claims 1-8, characterized in that the light bulbs are connected to a power supply line ( 101) which is connected via a further rectifier with the secundary winding of the transformer (52), which power supply line is further connected with a first control input of said pulsewidth modula¬ tion control circuit (29) for controlling in function of the tension present on said power supply line (101) the pulse width of a pulse signal output by said pulse width modulation control circuit.

10. Display board as claimed in claim 9, characterized in that said power supply line (101) is connected via a light sensitive cell with said first control input.

11. Display board as claimed in claim 7, characterized in that a second control input (16) of the pulse width modulation control circuit (29) is connected with an output of said transistor circuit.

12. Transformator circuit for use into a display board as claimed in anyoreof the claims 1- 11.