WO2015028922A1

WO2015028922A1 - Power supply circuit for led lighting apparatus and apparatus with it

Info

Publication number: WO2015028922A1
Application number: PCT/IB2014/064021
Authority: WO
Inventors: Angelo FORBICI
Original assignee: 2 A Srl
Priority date: 2013-08-29
Filing date: 2014-08-22
Publication date: 2015-03-05
Also published as: ITMI20131422A1

Abstract

A circuit for electrically powering LEDs from an AC mains voltage in a lighting apparatus, comprises a rectifier bridge (11) with an input (12) connected to the electricity mains and with an output (14) which supplies with a pulsating rectified voltage a constant-current stage (15), at the output (16) of which an LED array (17) and a smoothing capacitor (18) are arranged in parallel. The circuit may also comprise a phase-regulating dimmer (26) arranged at the input of the rectifier bridge (11). An apparatus according to the invention may comprise a printed circuit board with at least one mains power supply circuit and LED thereon.

Description

Power supply circuit for LED lighting apparatus and apparatus with it DESCRIPTION

The present invention relates to a circuit for powering LED lighting apparatus from the AC electricity mains, which also allows adjustment of the luminosity of the LEDs by means of phase-control dimmers. The invention also relates to lighting apparatus equipped therewith.

In the prior art the problems associated with the power supplying of power LEDs used as a lighting source are known.

These LEDs generally require low DC voltages (around 3 V) and currents in the region of 30mA. In order to obtain a luminous intensity suitable for replacing that of a conventional incandescent or fluorescent lamp, usually a large number of LEDs (even a hundred or so) are used, these being connected in series or parallel until DC power supply voltages for the LED group or array in the region of 12V or 24V and a current intensity in the region of 0.5-1.0 A are obtained. If this LED group is to be powered with mains AC voltage (for example 240 V) a special power supplier is required.

Initially linear power supply systems which rectified or decreased the mains voltage down to the low- voltage value suitable for the LED array were proposed. However, a linear system, which reduces the voltage from the mains value to the value for example of 12-24V with a current of 0.5 A usually has a high heat dissipation. The efficiency of these systems is therefore very low and there are problems of heat disposal.

Moreover, the known systems which were proposed offer a limited possibility of regulating the luminous intensity emitted by the LEDs without involving complex circuits and further heat dissipation.

In order to be able to obtain a greater efficiency in the powering of LED groups, therefore, switching power suppliers have been proposed and are now universally employed, said power suppliers converting the AC mains voltage into the appropriate DC voltage by means of a switching system.

These switching power suppliers, while having a high efficiency (in the region of 85%), always give rise to a not insignificant power loss which makes the replacement of conventional lighting apparatus with LED apparatus less advantageous from an energy point of view. Moreover, switching power suppliers are relatively complex, bulky and costly.

The complexity increases, moreover, the probability of malfunctions. Because of its size, it is difficult to position the power supply unit inside, for example, lighting apparatus with dimensions generally suitable for conventional apparatus. In addition to these drawbacks, the cost also poses a problem for replacement of conventional systems with LED systems.

The size of the power supplier also poses a problem in the case of lighting apparatus which in their conventional form have a lot of empty space internally. For example, in the case of ceiling lights with fluorescent or discharge lamps, most of the internal space is empty and generally there is no difficulty in arranging the reactor which this type of conventional lamp requires. The power supply units of the prior art for LED lighting have dimensions which are comparable with those of a reactor for fluorescent lamps, but in reality, in order to obtain the same luminosity, the LEDs must be distributed in a large number uniformly over a large area substantially corresponding to the entire transparent or semi-transparent light-emitting area of the ceiling light.

The problem therefore arises of where to position the switching power supplier. This problem is accentuated by the fact that any lack of uniformity in the distribution of the LEDs is immediately noticeable as a less bright patch in the light-emitting surface of the ceiling light, in particular owing to the substantially point-like characteristic light- emitting fonn of the LEDs. Often it is therefore not possible to move the LEDs in order to create space for the power supplier.

A further problem of switching power suppliers is that it is not possible to perform adjustment of the luminosity of the LED light apparatus using normal regulators or phase-regulating dimmers which are universally used for incandescent lamps. This complicates once again rapid replacement of the conventional lighting systems with LED systems and this means that it is extremely costly to provide a good system for adjusting the luminosity of an LED apparatus,

A further problem of the known power supply systems is that the power factor is relatively low and the electrical disturbances generated are relatively high, something which results in the need to use filters and power correction circuits (PFC) of the active or passive type, which increase even further the cost, size and complexity of the power supplier.

The general object of the present invention is to provide a power supply circuit which allows suitably efficient powering of LED lighting apparatus, easy miniaturization and phase-control regulation using relatively low-cost systems. A further object is to obtain LED lighting surfaces with fewer problems for positioning of the power supplier and to provide satisfactory LED lighting apparatus.

In view of these objects the idea which has occurred is to provide, according to the invention, a circuit for electrically powering LEDs from an AC mains voltage in a lighting apparatus, comprising a rectifier bridge with an input intended to be connected to the electricity mains and with an output which supplies with a pulsating rectified voltage a constant-current stage, at the output of which an LED array and a smoothing capacitor are arranged in parallel.

Still according to the invention the idea which has occurred is to provide an LED lighting apparatus of the type using AC mains voltage, comprising an insulating printed circuit board provided with at least one circuit of the aforementioned type and LEDs distributed over the surface of the board.

In order to illustrate more clearly the innovative principles of the present invention and its advantages compared to the prior art, an example of embodiment applying these principles will be described below with the aid of the accompanying drawings. In the drawings:

- Figure 1 shows an electrical diagram of a power supply circuit according to the invention;

- Figures 2 and 3 show input voltage and current waveforms for a circuit according to the invention and for a known circuit, respectively;

- Figure 4 shows a diagrammatic view of lighting boards provided according to the invention;

- Figures 5 shows a partial view of a lighting apparatus provided according to the invention.

With reference to the figures, Figure 1 shows a circuit 10 for electrically powering

LEDs from a mains AC voltage, intended for a lighting apparatus.

The circuit comprises a rectifier bridge 11 with an input 12 which is connected to the power supply network 13 (for example 240 V) and with an output 14 which supplies with a pulsating rectified voltage a constant-current stage 15. An array of LEDs 17 and a smoothing capacitor 18 are arranged in parallel at the output 16 of the constant-current stage.

Advantageously, the LED array must be formed by a maximum number of LEDs such as to ensure that, with the mains voltage supplied, there is always a sufficient voltage on the array to ensure use by the array of its nominal operating current. On the other hand, the number of LEDs, must be sufficiently high to ensure a voltage drop on the constant- current generator which is sufficiently low to achieve a desired low (preferably minimum) dissipation on the generator itself.

Advantageously, the smoothing capacitor 18 has a value equal to or greater than 5 microfarad and, preferably, in the region of 6.8 microfarad. The capacitor may be for example of the electrolytic type and have a nominal voltage suitable for the maximum voltage on the LED array. The greater the value of the capacitor the smaller will be the ripple on the LED array and the better will be the response of the system.

It has been found that a particularly advantageous definition of the number of LEDs in the array is provided by the formula:

n_LED=INT[(l .41 V_min^V_RIPPLE@I_LED)/V_f]

where V_mm is the minimum effective value envisaged for the sinusoidal supply voltage, rippie is the voltage ripple on the capacitor 18 at the desired nominal current I_LED for the array and Vf is the nominal voltage on each LED. I_LED and Vf are generally characteristic of the LED chosen.

For example, let us consider an LED array formed by LEDs with IuED⁼20mA and V_f= 3V. Moreover, let us consider a sinusoidal mains power supply voltage V_rete of 230V-/+ 10%. Let us consider moreover a V_rippie of 20V on the capacitor (for example, such a ripple may be detected with

20mA using a capacitor 18 of l OmicroF). The minimum sinusoidal power supply voltage V_mi_n in terms of effective value will be V_rettr10% = 207Vac (most unfavourable condition). The optimum number of LEDs will therefore be:

n_LED=lNT[(1.41 V_mra-V_NPPLE@I_LED)/V_f]= INT[(1.41 207-20)/3]=INT[90.62]=90

With the values given, the maximum number of LEDs is therefore 90.

The preferred number of LEDs all arranged in series with each other will therefore be in the region of the number (advantageously rounded down) defined by (1.41 V_mi„- rippie@lLED)/ f and, advantageously, is therefore equivalent to 90 LEDs. A different (greater or smaller - preferably greater) number of LEDs may also be envisaged in order to optimize for example the distribution of the LEDs on the printed circuit board.

The circuit formed by rectifier bridge 1 1 , constant-current stage 15 and smoothing capacitor 18 forms a power supply stage indicated generically by 19 in Figure 1. It should be noted that the constant-current stage is supplied at its input with a pulsating rectified voltage.

Advantageously, the constant-current stage 15 may comprise a mosfet 20 with drain connected to the rectifier bridge 1 1 , source connected via a first electric resistance 21 to a common point or terminal 22 of the parallel arrangement of LED array 17 and smoothing capacitor 18, and gate connected to the drain via a second electric resistance 23. A transistor 24 is connected with its base to the source of the mosfet 20, emitter connected to the said common terminal 22, and collector connected to the gate of the mosfet 20.

This constant-current circuit controls the current supplied by the mosfet 20 on the load (namely the LED array) via the npn transistor which drives the gate.

The power supply voltage is divided between the voltage on the LED array and the voltage V_DS of the mosfet. When the mains voltage is applied, the mosfet is in the disabled state since the voltage on the gate is insufficient to cause it to conduct. The LEDs, which have a behaviour similar to that of Zener diodes, start to be crossed by current once a certain threshold voltage (dependent on the number of LEDs in series) has been exceeded. The result is that on the mosfet there is a very low voltage until the LED threshold is exceeded. Once the threshold has been exceeded, the voltage on the mosfet rises rapidly and, seeing that the power consumption of the gate is practically zero, all the voltage of the source is situated on the gate via the resistance 23. Once the threshold voltage Vth is reached, the mosfet starts to conduct, but at this point the current Ids also passes through the resistance 21 and a voltage Ids*R is generated at its terminals. When, at the terminals of the resistance 21 , there is a voltage higher than the Vbe of the transistor 24, the latter starts to conduct, cutting the gate voltage of the mosfet.

In this way the current output by the constant -current stage is kept at the value of Vbe/resistance 21. Generally, the voltage Vbe is approximately equal to 0.6 V.

The first resistance 21 is therefore designed with dimensions depending on the desired output current of the circuit.

For example, if the LED array 17 is formed by a series of LEDs with a nominal power supply current of 30 mA, the resistance 21 will be equal to 0.6/0.03=20 ohm which may be rounded up to the closest higher standard value which is 22 ohm.

The resistance 23 is instead designed with dimensions to provide a suitable collector current to the transistor 23. For example, this resistance may be in the region of 330

Kohm.

Advantageously, a third resistance 25 of value suitable for rapidly discharging the capacitor 18 when the mains voltage is disconnected may be arranged in parallel with the output 14 of the rectifier bridge, so as to prevent the risk of electric shocks due to the residual charge of the capacitor. The value of the resistance is chosen advantageously as a compromise between the discharge speed of the capacitor and the dissipation of electric power during operation of the circuit. This resistance 25 may be for example in the region of 680 Kohm.

The resistance 25 situated at the output of the bridge 11 will discharge the capacitor 18 via the diode of the mosfet. Alternatively, the resistance 25 may be arranged directly in parallel with the capacitor, with the disadvantage, however, of drawing current from the constant-current circuit during normal operation.

It has been found that, by providing a circuit according to the principles of the invention (with a rectifier bridge, a constant-current stage and a smoothing capacitor in parallel with the LED array), it is possible to obtain a high power factor (of even more than 0.8) without the need for particular filters or PFC circuits.

By way of example, Figure 2 shows a graph which compares the mains power supply circuit entering the circuit with the current drawn from the mains. It can be seen how the current is a substantially rectangular wave in the centre of the half-waves of the sinusoidal mains voltage. By way of comparison, Figure 3 shows a similar graph in the case of a conventional linear power supply circuit. It can be seen how the current used is formed substantially by a sequence of current pulses, not acceptable owing to the disturbances and the low power factor. Since the voltage drop on the mosfet is small (most of the voltage drop occurs on the LED array), the power dissipation on the mosfet is low, favouring for example the efficiency of the system (as high as 98%) and possible miniaturization, a large-size heat dissipater not being required.

As a result of the circuit according to the invention it has been found that the current which passes through the LEDs is much less affected by the incoming voltage fluctuations, unlike the known circuits, and this ensures that the lighting apparatus has suitably constant luminosity.

As diagrammatically shown in Figure 1 , according to the principles of the present invention, a phase-regulating dimmer 26 for controlling the light emitted by the LED array 17 may also be advantageously inserted between the mains power supply 13 and the input 12 of the rectifier bridge 1 1. The dimmer 26 may be of any type substantially known per se with, for example, a manual control input 27, and therefore will not be described or shown in detail here. The dimmer may be for example of the well-known type using a TRIAC and a DIAC connected to the mains by means of a suitable adjustable phase-shaft network.

Using the dimmer 26 the rectifier bridge receives the mains half-waves regulated, as diagrammatically shown again in Figure 1.

It has been found that, with the circuit according to the invention, the luminosity control functions adequately also using a simple phase-regulating dimmer and this even when the LED current is modulated by regulating the phase of the mains voltage during the first 90° of adjustment, differently from that which occurs with the prior art systems (where regulation results in a serious instability in the light emission of the LEDs, in particular at the low luminosity levels).

As shown in Figure 1, the lighting circuit may comprise several circuits 19 which are powered by a single dimmer 26.

It is thus possible to have several lighting apparatus according to the invention powered by the same dimmer and/or have a lighting apparatus composed of several circuits 19. Owing to this latter feature, it is possible to divide up into several circuits the number of LEDs required to achieve the desired luminous intensity. In this way there is both a smaller heat dissipation for each circuit, and a greater resistance of the lighting apparatus to faults. For example, in the case of breakage of one LED in an array of LEDs in series, only one section of the apparatus is switched off and not the entire apparatus. Moreover, with this system a fault affecting one of the power suppliers does not result in switching-off of the entire apparatus, as instead occurs with the apparatus of the known art powered by means of a switching power supplier, it not being possible to easily provide two or three such amplifiers inside the same lighting apparatus because of their cost and size.

Figure 4 shows in diagrammatic form an example of a lighting apparatus 40 provided in accordance with the invention.

The apparatus 40 comprises at least one insulating printed circuit board 41 on which there is at least one circuit 19 with an area 42 in which the LEDs of the array, powered by the circuit 19, are arranged. Advantageously, inside the area 42, the LEDs are arranged in an orderly manner in a matrix formed by rows and columns of LEDs. The LEDs in the area are advantageously connected in series so as to reach the given number of LEDs needed to obtain the optimum voltage drop (for example, 90-92 LEDs).

The circuit components may be soldered directly onto the printed circuit board, as may now be easily imagined by the person skilled in the art.

As can be seen in Figure 4, the board 41 may comprise advantageously several circuits 19 arranged alongside each other so as to obtain substantially geometric continuity of the LED areas 42 (understood as meaning, for example, a uniform distribution of the LEDs). In this way a substantially continuous lighting surface is obtained (with the LEDs equally spaced from each other). Advantageously, the circuits 1 formed by the rectifier bridge, the constant-current stage and the smoothing capacitor may be arranged aligned along one edge of the board. The modules formed by a circuit 19 and by the corresponding array of LEDs may be of any number. For example, the figure shows a board 41 which has three modules arranged alongside each other. Several boards may also be advantageously arranged alongside each other so as to increase the lighting surface. Figure 4 shows for example two adjacent boards, each with three modules. A different number of modules and boards may obviously be provided. For quick connection of the boards a mains power supply bus may be provided, with an input 43 on one edge of the board and a mains power supply output 44 which is arranged symmetrically on an opposite edge of the board so that it may be easily connected via short jumpers (for example wire lugs or engaging pins) to the input 43 of an adjacent board.

Figure 5 shows by way of example an embodiment of the apparatus according to the invention with two boards each with three modules and with the power supply circuit shown in Figure 1, As can be seen, all the components may be advantageously arranged along the edge of the apparatus, and the LEDs may be arranged equally spaced in an ordered manner in both the orthogonal directions. The apparatus which is shown in Figures 4 or 5 with, removed, the front part through which the light is emitted may be for example a square ceiling light with dimensions of a conventional ceiling light designed for a circular fluorescent lamp.

The same printed circuit board may provide or incorporate a radiating surface for the heat produced by the circuit, owing to the fact that this heat is limited.

At this point it is clear how the predefined objects have been achieved. With a circuit according to the invention an energy efficiency much greater than that of the known switching systems is obtained. Moreover, the dimensions are very small and it is possible to provide LED lighting boards with integrated the electric mains power supply system. The resultant lighting apparatus is thus very compact and, in order to function, requires solely connection to the electricity mains. The use of an apparatus according to the invention in place of conventional apparatus with incandescent or fluorescent lamps is thus for example greatly facilitated. Furthermore, control of the luminosity may be achieved using known means and simple phase-regulating systems.

Based on the principles of the invention, boards with an integrated power supply system may be easily provided and this allows the most varied geometric forms to be catered for, as required, and dispenses with the need for a mechanical design for housing the power supply section, with consequent significant economic advantages (lower-cost components, no mechanical machining in order to house various power suppliers, and a major reduction in wiring both between electricity mains and power suppliers, and between power suppliers and LEDs). With the system according to the invention, the wiring may be practically non-existent, as may be easily deduced from the description and from Figures 4 and 5, with a consequent saving in labour, ties, adhesive supports for fixing cables, etc.

Moreover, with the solution according to the invention it is possible to cover areas to be illuminated without limitation in the continuity of the light-emitting area, such as to be able to obtain a greater uniformity of the luminous flux and have a distributed power supply system, with the advantage that, in the event of a fault, only a part of the apparatus becomes inoperative.

Each power supply circuit may power an array of LEDs all in series with a constant current and each series is independent of the other ones.

Obviously the description above of an embodiment applying the innovative principles of the present invention is provided by way of example of these innovative principles and must therefore not be regarded as limiting the scope of the rights claimed herein. Although the diagram described is preferable, the constant-current stage may be also provided using a dual circuit with PNP transistor and/or P channel mosfet. Owing to the small dimensions and the limited number of components in the circuit according to the invention, these components may also be arranged on the board interspersed with the LEDs, as may be now easily imagined by the person skilled in the art.

Claims

1. LED electrical power supply circuit using AC mains voltage in an LED lighting apparatus, said circuit comprising a rectifier bridge (1 1) with an input (12) intended to be connected to the electricity mains and with an output (14) which supplies with a pulsating rectified voltage a constant-current stage (15), at the output (16) of which an LED array (17) and a smoothing capacitor (18) are arranged in parallel.

2. Circuit according to Claim 1, characterized in that the LED array (17) is formed by a number of LEDs equal to about the number defined by the formula:

(1.41 V_min-V_rippie@I_LED) V_f

where V_mi„ is the minimum effective value envisaged for the sinusoidal supply voltage of the citcuit, V_nppie is the voltage ripple on the smoothing capacitor at the desired nominal current Iy^D for the LED array and Vf is the nominal voltage on each LED.

3. Circuit according to Claim 1, characterized in that the smoothing capacitor (18) has a value equal to or greater than 5 microfarad.

4. Circuit according to Claim 1, characterized in that the constant -current stage (15) comprises a mosfet (20) with drain connected to the rectifier bridge (11 ), source connected, via a first electrical resistance (21 ), to a common terminal of the parallel arrangement of LED array ( 17) and smoothing capacitor ( 18), and gate connected to the drain via a second electrical resistance (23), a transistor (24) being connected with its base to the source of the mosfet, with its emitter to the said common terminal and with its collector to the gate of the mosfet.

5. Circuit according to Claim 4, characterized in that a third electrical resistance (25) is connected in parallel to the output of the rectifier bridge (1 1).

6. Circuit according to Claim 4, characterized in that a phase-regulating dimmer for controlling the light emitted by the LED array is present between mains and input of the rectifier bridge (11).

7. LED lighting apparatus using AC mains voltage, said apparatus comprising an insulating printed circuit board (41) on which at least one circuit according to any one of Claims 1 to 6 is arranged and the LEDs of the array distributed on the surface of the board.

8. Apparatus according to Claim 7, characterized in that the board (41) comprises several circuits arranged alongside each other so as to obtain substantially continuity in the distribution of the LED arrays of the circuits.

9. Apparatus according to Claim 7, characterized in that it comprises several boards arranged alongside each other.

10. Apparatus according to Claim 7, characterized in that at least the rectifier bridge (1 1) and the constant-current stage (15) are arranged along one edge of the board (41) and the LEDs are arranged as a matrix of rows and columns.