Patent Grant
9936551
The present invention relates to LED based lighting applications, in particular to lighting applications that are powered from an electronic transformer and/or TRIAC dimmer. Such an arrangement is often encountered in a retrofit situation whereby a conventional halogen light is replaced by an LED unit while a power converter such as an electronic transformer is maintained.
In general, LED based lighting applications (also referred to as LED units, comprising one or more light emitting diodes (LEDs)) are powered by an LED driver (e.g. comprising a switched mode power supply such as a Buck or Boost converter) which is supplied from a DC voltage source. In such an arrangement, dimming of the light (in response to a user interface action) is typically realised by adjusting the duty cycle of the LED or LEDs of the application. As such, conventional LED drivers are not suited for being powered by a voltage source which differs from a DC voltage source, such as e.g. provided by an electronic transformer or a standard TRIAC dimmer. When an LED driver is supplied from a varying voltage source, the instantaneous voltage available as input to the LED driver may be momentarily insufficient to power the LED or LEDs of the lighting application. This could result in flicker of the lighting application which could result into a range of effects in an observer, from awkwardness via irritation to nausea.
When such a varying voltage source such as an electronic transformer or a TRIAC dimmer is used to power a normal halogen light, the power received will be averaged out and will not result in flicker, although even with halogen lights, the low output levels are cumbersome and flicker can be seen in many cases.
In case a lighting application is powered from an electronic transformer, it is further required to, in order for the electronic transformer to provide an output voltage for supplying the LED unit, maintain a current as provided by the transformer above a certain level. As such, when the supply current (i.e. the current supplied to the LED driver) is insufficient, the electronic transformer will cease to provide an output voltage. Subsequently, the electronic transformer will, after a certain amount of time, attempt to resume its proper operation. Meanwhile however, the light output of the LED unit could be interrupted, whereas a continuous light output would be desired. In order to ensure proper operation of the electronic transformer, it has been proposed in literature to provide a load in parallel to the LED driver in order to ensure that a minimum supply current is being supplied by the electronic transformer. Maintaining such a current may result in an Important dissipation, adversely affecting the efficiency of the lighting application.
Ensuring that a sufficiently high supply current is provided by an electronic transformer powering one or more LED drivers, is rendered even more difficult because of the high power to light conversion of LED based lighting applications, compared to conventional halogen lights. As will be understood, when a 20 W halogen light bulb is replaced by a 5 W LED unit, the power to be supplied by the electronic transformer can be reduced significantly and may result in a supply current insufficient for proper operation of the electronic transformer.
In view of the above, it is an object of the present invention to facilitate the powering of LED based lighting applications by an electronic transformer (optionally preceded by a TRIAC based dimmer), thereby facilitating conventional applications such as halogen lights to retrofit with LED based lighting application.
According to an aspect of the invention, there is provided an LED driver comprising
The LED driver according to the Invention comprises a power converter, such as a Buck, Boost or hysteretic converter and a control unit (e.g. a controller or microprocessor) for controlling the power converter. In accordance with the invention, the control unit controls the power converter by providing a control signal to the power converter, the control signal being based on a supply signal that is received at an input of the control unit.
The control signal may further, in an embodiment, be based on a set-point (e.g. representing a required intensity or colour setting), e.g. received via RF or any other communication means.
In accordance with the invention, the supply signal represents the supply voltage that is supplied to the power converter. As such, the supply signal can e.g. be a signal that is proportional to the supply voltage (e.g. obtained via an A/D conversion of the supply voltage). The supply signal can be derived or retrieve at various positions, e.g. at the output terminal of the electronic transformer, after a rectification of the transformer's output voltage, at the input terminals of the LED driver, . . . . In accordance with the invention, the control unit is arranged to control the power converter in such manner that an LED unit, in use powered by the LED driver, is not provided with a substantially constant current, rather, the LED unit is, in use, provided with a current with varying amplitude, the amplitude variation (or modulation) being in synchronism or in phase with the supply voltage, in use, a rectified AC voltage. By modulating the current as supplied to the LED driver as stated, it has been observed that, when the rectified AC supply voltage originates from an electronic transformer, the electronic transformer can more easily sustain the supply voltage. It has thus been found that, in order to ensure that an electronic transforms keeps providing an output voltage, it is not required to maintain the current supplied to the LED unit at a constant comparatively high value in order to sustain the electronic transformer. Rather it has been found sufficient to provide a comparatively high current to the LED unit when the supply voltage is comparatively high. As such, an amplitude modulation of the current provided to the LED unit in synchronism with the rectified AC supply voltage facilitates the power source (e.g. an electronic transformer and/or a TRIAC dimmer) in providing an output voltage used for generating the supply voltage of the LED driver. In an embodiment, the current as provided to the LED unit is arranged to vary in phase with the rectified AC voltage.
Assuming a rectified AC voltage having a main frequency component of 100 Hz, (e.g. obtained by transforming and rectifying an AC mains supply of 230 V, 50 Hz), it has been found sufficient, for most electronic transformers, to ensure that the current as supplied by the transformer comprises a 100 Hz component (or a multiple thereof) substantially in phase with the main frequency component of the rectified AC voltage. As the power supplied to an LED unit can be approximated to be proportional to the current through the LED unit, the average power supplied to the LED unit can be substantially smaller than the peak power which is supplied when the current supplied is at its peak value.
In an embodiment, the LED driver further comprises a rectifier arranged to receive an AC supply voltage and provide the rectified AC voltage to the input terminal. The AC supply voltage can e.g. be provided by an electronic transformer or a TRIAC dimmer modulating an AC supply voltage, or a combination thereof.
In an embodiment, the LED driver further comprises an energy storage element connectable to the input terminal and a switch for connecting and disconnecting the energy storage element to the input terminal, the switch being controlled by the control unit, based on the input signal. As an example, such an energy storage element can comprise a capacitor or an assembly of capacitors which can be charged (by the supply voltage) and discharged (towards the LED driver) when the switch is operated at appropriate instances.
The application of such an energy storage element can improve the performance in different ways. As the energy storage element is charged from the rectified AC voltage, e.g. originating from an electronic transformer, the charging current increases the instantaneous current demand of the LED driver which can thus facilitate sustaining an electronic transformer. The energy storage element can further improve the LED driver's performance by applying it as a power source when the supply voltage is comparatively low.
As will acknowledged by the skilled person, in order to provide a current to an LED unit, a minimal voltage (known as the forward voltage Vf) is required in order to provide a current to the LED unit. Depending on the type of power converter applied, supplying such a minimal voltage at the output terminal of the power converter may equally require a minimal voltage at the input terminal of the power converter. When this voltage is not available, the power converter cannot supply the required current to the LED unit. However, in case e.g. a charged capacitor (e.g. charged to a voltage level corresponding to the peak value of the rectified AC voltage) would be available, this capacitor could be applied, temporarily, as a supply source, thereby improving the current supply towards the LED unit. In the absence of an energy storage element that can be applied as a temporary power source, the current supplied to the LED unit could reduce to zero during part of the period of the rectified AC supply voltage. Depending on the main frequency of the supply voltage, this could be observed by a user or could even result in the user experiencing nausea. The application of an energy storage element as described also enables to adjust the frequency content of the current supplied to the LED unit, thereby mitigating any adverse effects such as flicker. In case a comparatively large amplitude modulation would be required to sustain an electronic transformer to provide the supply voltage, this could e.g. result in the current provided to the LED unit comprising a comparatively large 100 Hz component. Such an 100 Hz component could be undesired for certain observers. By applying an energy storage element for providing a current to the LED unit when the supply voltage is comparatively low, the frequency content of the current to the LED unit can be altered. By introducing current peaks (e.g. by discharging a charged capacitor) when the supply voltage is comparatively low, the main frequency component of the current supplied to the LED unit can become a 200 Hz current instead of a 100 Hz current. In general, due to the application of the switchable capacitor (in general, the energy storage element), which can be applied as a voltage source when the rectified AC voltage is comparatively low, a current component at twice the main frequency of the rectified AC voltage (e.g. a 200 Hz current in case the rectified AC voltage originates from a 50 Hz mains supply) can be introduced in the current as supplied to the LED unit. By doing so, adverse effects such as an observable flicker or nausea can be reduced significantly.
In addition to (or as an alternative to) the application of an energy storage device, the LED driver according to the invention can be provided with a power factor correction device. Various embodiments of such a power factor correction device are discussed in more detail below. In an embodiment, the power factor correction device can be applied as an energy storage device, e.g. comprising one or more capacitances.
In an embodiment, the power factor correction device can be connected and disconnected from the input terminal via a switch that is controlled by the control unit of the LED driver, e.g. in accordance with the supply signal. Connecting and disconnecting the power factor correction device can thus be synchronised with the rectified AC supply voltage.
The control unit of the LED driver according to the invention can, in an embodiment, be arranged to determine a minimum value for the amplitude modulation in order to sustain the supply voltage. This can e.g. be done by starting with a comparatively large amplitude modulation, gradually reducing the amplitude modulation applied to the current, monitor if the supply voltage is sustained, and, if the supply voltage is no longer sustained, gradually increase the amplitude modulation until the supply voltage is sustained again.
By doing so, the current variation, and any possible adverse effects of it, can be reduced to a minimum while sustaining the electronic transformer.
In this respect, it is worth noting that the required amplitude modulation (required to sustain an electronic transformer supplying the LED driver) can depend on the total load to be powered by the transformer. In case a single transformer is used to provide a supply voltage to a plurality of LED drivers, the required amplitude modulation can be comparatively small or even zero, compared to the case whereby the transformer only supplies a single LED driver.
With respect to maintaining an electronic transformer to supply an output voltage, it is worth noting that the amplitude modulation required to sustain the transformer, may depend on the maximum amplitude of the supply voltage as provided. As will be understood by the skilled person, this maximum amplitude may vary in time, e.g. due to load changes in the electric grid supplying the electronic transformer. As such, it may be required to increase the amplitude modulation or the amplitude of the current profile as provided to the LED driver, when the supply voltage maximum amplitude increases, in order to sustain the electronic transformer. As in general, there is a limited number of current levels available that can be selected (e.g. 16 current levels ranging from zero to 120% of the nominal current). If the level of the current supplied to the LED unit would be raised by one level, such a change would become visible to an observer. Instead of applying such a sudden current increase, it is proposed in the present invention to gradually raise the average current level of the current supplied to the LED unit. This can be realised by raising the current supplied to the LED unit to the next available current level for only a comparatively small portion of a period of the rectified AC voltage. This small portion can e.g. correspond to T1=1/F whereby F represents the frequency at which a new current set-point can be provided to the LED driver. In case a new current set-point can e.g. be provided every 52 μsec, (T1=52 μsec), the average current over a period of the supply voltage could be incremented in very small steps by increasing the current during each period of the supply voltage only over a period equal to T1, rather than adjusting (raising or decreasing) the current profile entirely to a next current level. Phrased differently, the current as provided to the LED unit can e.g. have a staircase profile, ascending when the supply voltage increases and descending when the supply voltage decreases. The levels of the staircase would thus correspond to the available current levels. Instead of incrementing each level of the staircase profile with one level, the average level is gradually increased by increasing only one level (or part of one level, e.g. only during a period T1) of the staircase profile with one current level. By doing so, the resolution at which the average current can be varied is increased significantly, compared to a resolution solely based on the available number of current levels.
The LED driver according to the present invention thus enables the powering of a comparatively low number of LED units by an electronic transformer even if the average power to the LED units is lower than a minimum power requirement of the transformer. This facilitates the application of the LED driver according to the invention in retro-fit situation. It is further worth noting that the LED driver according to the invention may also be applied when the supply voltage is provided from a conventional magnetic transformer, which e.g. merely transforms a 230V, 50 Hz mains voltage to a suitable lower voltage by an inductive coupling. In case the supply voltage originates from a magnetic transformer, there is no need to perform the amplitude modulation and conventional current control can be applied by the LED driver. In such case, it may however still be advantageous to apply an energy storage element to avoid a visible flicker of the LED unit's light output. Therefore, in an embodiment, the LED driver according to the invention is arranged to detect what type of transformer (either a conventional transformer or an electronic transformer) is providing the supply voltage. This can e.g. be realised by applying a rapid current fluctuation (i.e. a comparatively large increase or decrease of the current provided to the LED unit) and monitoring the effect of such current fluctuation on the supply voltage. It has been devised by the inventors that the application of a rapid current fluctuation during either the ascending or descending slope of the supply voltage, can result in an electronic transformer ceasing to provide an output voltage. Because a conventional transformer is not or hardly affected by such a current fluctuation, distinction can be made between a conventional transformer and an electronic transformer providing the supply voltage.
The following figures provide further details of embodiments of the present invention whereby corresponding reference numbers indicate corresponding features.
At present, lighting applications such as halogen lights are often supplied from an electronic transformer or a conventional magnetic transformer. In the latter case, a mains input voltage (e.g. 230V, 50 Hz) is converted to a comparatively low voltage, e.g. 12V, 50 Hz. In case an electronic transformer is applied, a mains AC voltage (or a TRIAC dimmer output voltage) is modulated (e.g. at 50 or 60 kHz) to a voltage form as schematically shown in
In order to remain providing a supply voltage (as e.g. shown in
The output voltage can subsequently be rectified to obtain a rectified AC supply voltage which can be provided at an input terminal of an LED driver according to the invention. The rectifier providing the AC supply voltage may also be implemented as part of the LED driver.
Conventionally, the output voltage of the electronic transformer can be applied to supply a halogen light. In such a situation, in order to properly operate, the power rating of the electronic transformer should match the power requirements of the halogen light that is powered because, as is known to the skilled person, an electronic transformer requires a minimum load in order to keep providing an output voltage. As will be illustrated in the following figures, when a supply current of an electronic transformer drops below a certain value, the output voltage cannot be maintained and gradually reduces to zero. Subsequently, a start-up circuit of the electronic transformer (e.g. the circuit associated with the transistor Q3 as shown in
In case of an LED based lighting application, the output voltage of the electronic transformer can be applied to supply a power converter of an LED driver. Examples of such power converters are Buck or Boost converters. In order for such an LED driver to power an LED unit, similar constraints with respect to load requirements have to be met, in order for the electronic transformer to keep providing a supply voltage to the power converter.
In accordance with the present invention, a strategy has been devised which enables an LED based lighting application having a power rating below the power rating of an electronic transformer, to be powered by such a transformer, substantially without any noticeable intensity variations. In order to achieve this, an embodiment of the LED driver according to the invention is controlled in such manner that a current can be supplied to a LED unit, essentially in an uninterrupted manner.
In case an LED driver or a power converter of an LED driver is supplied from a varying supply voltage such as a rectified AC-voltage or a voltage originating from an electronic transformer (optionally preceded by a TRIAC dimmer), it may be advantageous or even required to provide the LED driver with an energy storage element. Such an energy storage element can e.g. comprise one or more capacitors connectable or connected to the input terminals of the power converter of the LED driver. In such case, the one or more capacitors can be charged by the supply voltage when this voltage is comparatively high and discharged, thereby supplying the power converter of the LED driver, when the supply voltage is comparatively low.
In order to ensure the operation of an electronic transformer providing a supply voltage, at least during part of the period of the supply voltage, the LED driver according to the present invention is arranged to draw a current from the supply voltage, the current being amplitude modulated in synchronism or in phase with the rectified AC supply voltage. This will be explained in more detail below. By applying an energy storage element such as the capacitor 60, the variation of the current as supplied to the LED unit (which could be noticed by some observers), can be mitigated. The use of such an energy storage element as a temporary supply source also enables the frequency content of the current as supplied to the LED unit to be raised. As an example, by appropriate switching of the switched capacitor (as is explained in more detail below) and thus applying the charged capacitor as a voltage source when the supply voltage is comparatively low, the current as supplied to the LED unit may have a main frequency component at twice the main frequency of the rectified AC voltage (i.e. a 200 Hz current component in case the rectified AC supply voltage originates from a 50 Hz mains voltage and thus has a main frequency component of 100 Hz).
In case a switchable storage element is used, the control unit of the LED driver can be arranged to control both the current as provided to the LED unit and the switching of the switchable storage element such that the total current as provided by the power supply (e.g. the sum of the load current of the storage element and the current provide to the LED unit) is synchronised or in phase with the rectified AC supply voltage.
As an alternative or in addition to the application of a switchable storage element, the load of the LED driver according to an embodiment of the invention as perceived by the electronic transformer providing the supply voltage, can gradually be raised thereby facilitating the proper operation of the electronic transformer. Gradually increasing the load as perceived by the electronic transformer e.g. be done by connecting one or more comparatively small capacitors as an additional load to the LED driver. In an embodiment, such an arrangement of e.g. n capacitors provided with a switch to selectively connect the capacitors can be integrated in the LED driver. In practice, a single electronic transformer is often applied to power a plurality of LED units (or LED drivers). It has been observed by the inventors that care should be taken not to add too much load to the LED driver or drivers, as this may cause damage to the electronic transformer. When the power drawn by the LED units (or the LED drivers powering the LED units) is too small, the output voltage of the electronic transformer can drop to zero. In order to sustain the output voltage of the transformer, the control unit of the LED drivers according to an embodiment of the invention can be arranged to increase the power consumption of the LED drivers, by adding an extra load. However, as it may be a-priori unknown how many LED drivers (say in a case: N) are powered from a single electronic transformer, the additional load may rise to N times the load which is minimally necessary for a particular type of electronic transformers to remain outputting power. This follows from the observation that such an additional load can only be determined at design-time of the LED driver. With certain types of load (f.e. capacitive loads), adding a comparatively high load may damage the electronic transformer, thus limiting N to only 1 or 2 nodes. By making the LED driver adapting to the situation, that is to the number of LED drivers N, a situation can be reached whereby only the bare minimum of additional load is added over the entire system (i.e. over all N LED drivers) to sustain the electronic transformer.
Assuming the additional load needed to keep the electronic transformer operating to be a capacitor of X nF. In case more than one LED unit is powered from the transformer, it may be sufficient to add to each lighting application a load which is only a fraction of X nF, namely substantially 1/N times X nF. As N is a priori unknown, it is proposed according to the invention, to gradually increase the additional load whereby an assessment is made whether the added load is sufficient, each time a load, e.g. X/Y nF is added. Each time a load is added, the electronic transformer will, as indicated above, attempt to output power again. In case the load of the transformer is insufficient, the transformer will cease to output power indicating that further additions of the load are required. As such, it may typically take a few periods before the total added load by the N LED drivers equals or exceeds the minimal extra load. As an example, assuming a minimal load requirement to be 15 nF whereby the load represented by each LED driver can be increased in steps of 2 nF during each period. In such a situation, it would take three periods to obtain or exceed the minimal load when 6 LED drivers are powered by the transformer. In case 10 LED drivers are powered, it would only take one period to obtain or exceed the minimal load. As soon as the minimal load required is added, the lighting applications can stop adding load. Using this approach, one can avoid that the total load to be powered by the electronic transformer increases to a level that would cause damage to the electronic transformer. As indicated above, the application of an additional load can be combined with the application of an LED current being amplitude modulated in phase with the rectified AC supply voltage. Due to the additional load, a smaller amplitude modulation may be applied to sustain the transformer during part of the supply voltage period, compared to the situation where no additional load is applied.
An example of an LED unit being powered by an LED driver is schematically shown in
In an embodiment, the energy storage element 155.1 may also function as a power factor correction device. In the embodiment as shown, this would thus result in a switchable power factor correction device. As an alternative, a static power factor correction device can be applied in combination with the switchable storage element as shown. Embodiments of a (switchable) power factor correction device are explained in more detail below.
Conventionally, the current I as supplied by the power converter to the LED unit or the LEDs of the LED unit is kept at a nominal value. In order to change an intensity of the light emitted, the duty cycle at which the current is provided, is changed, e.g. by operation of a switch in parallel with the LED unit or an LED of the LED unit (such as switches T1, T2 and T3) as shown in
With respect to the application of an electronic transformer as a supply source for an LED driver (or a power converter of the LED driver), it has been devised, according to the invention, that such application can be facilitated by applying a varying current I to the LED unit. More specifically, it has been devised by the inventors that by applying an amplitude modulation to the current supplied, the amplitude modulation being in phase with the supply voltage (i.e. the rectified AC voltage), the proper operation of the electronic transformer can be more easily sustained. Within the meaning of the present invention, an amplitude modulation in phase with the supply voltage can be described as, but is not limited to:
In accordance with the present invention, the current as provided to the LED unit need not necessarily be in phase with the rectified AC supply voltage, as it may be sufficient to synchronise the current as provide to the LED unit with the rectified AC supply voltage. By synchronising the current to the LED unit, the supply voltage (e.g. provided by an electronic transformer or a TRIAC dimmer) used as input for providing the rectified AC supply voltage can at least be sustained for a considerable part of a period of the supply voltage. As such, during a comparatively small part of the period of the supply voltage, the supply voltage may reduce to zero, the current to the LED unit thus being reduced to zero as well. For some application, having the LED current reduce to zero during a comparatively small part of the period of the supply voltage may be acceptable. In case this is not acceptable, an embodiment of the LED driver according to the invention is provided with an energy storage element (such as a capacitance) which can be used, as explained in more detail below, as a power supply for generating an LED current when the supply voltage is absent.
With respect to the required current profile as depicted in graph b of
In order to overcome this, an energy storage element such as a capacitor can be applied in an embodiment of the LED driver according to the present invention, to supply the LED driver when the supply voltage is low. The application of an energy storage element is illustrated below using one or more capacitors. The same principles as explained below can however also be implemented when one or more inductances are applied as energy storage elements.
In order to charge the capacitor, different approaches can be applied, as illustrated in
In an embodiment, the capacitor is charged in a continuous mode (see graph b) thereby connecting the capacitor to the input terminals (i.e. to the rectified AC supply voltage) until the capacitor is substantially charged. In such mode, the capacitor can e.g. remain connected to the supply voltage until the voltage reaches its maximum. At this point, the capacitor will be charged entirely and can be disconnected. By doing so, the capacitor remains charged to this maximum voltage. In graph b the capacitor is connected to the supply voltage from instance 510 and remains connected, e.g. until the supply voltage reaches its maximum value, instance 501. Graph b shows the supply current Is as drawn from the supply voltage. Is comprises a component Iload, the current as supplied to the LED unit and a component due to the charging of the capacitor. As from instance 502, the capacitor is reconnected and the available voltage (at a level 505) can be used to supply the load current to the LED unit. Consequently, the supply current Is can reduce to zero. At instance 503, the supply voltage becomes sufficiently high, compared to the voltage available at the capacitor, for the electronic transformer to restart and provide the supply voltage. As from that instance, the capacitor can be charged again.
In an embodiment, illustrated in graph c of
The operation of an embodiment of the LED driver according to the invention is further illustrated in
In
When the capacitor is disconnected from the supply voltage, the LED driver is operated in the run-mode. During this mode, the LED unit is powered by the electronic transformer. In order to keep the electronic transformer providing the supply voltage, the current as supplied to the LED unit should thus be larger or equal to the minimum current. In order to realise this, the LED unit can be controlled to operate above its nominal current during this mode, i.e. during the run-mode. In general, the run-mode starts when the capacitor is disconnected from the supply voltage (this disconnection preferable occurs when the supply voltage is at its maximum value) and ends when the capacitor is reconnected.
When the capacitor is reconnected to the LED driver, the charged capacitor can be applied as a voltage source for powering the LED driver. This mode of operation is referred to as the boost-mode.
In an embodiment, the boost-mode is started when the supply voltage as provided by the electronic transformer to the LED driver is too low to power the LED unit. As will be understood by the skilled person, in order to supply a current to an LED unit, a minimum voltage equal to the required forward voltage of the LED unit needs to be available. Based on the topology of the LED unit and the specifications of the LED or LEDs applied, a control unit of the LED driver can determine the required minimal voltage that should be available at the LED driver input terminals in order to supply a current to the LED unit. When the available voltage would become insufficient to power the LED driver, the control unit of the LED driver can thus control the switch associated with the capacitor thereby connecting the charged capacitor to the input terminals of the LED driver. When operating in the boost-mode, e.g. starting from instance 502, the LED driver is supplied from the charged capacitor (in general, the energy storage element). Supplying the LED driver from the charged capacitor enables powering the LED driver and thus providing the LED unit with a current. Note that, in the absence of the charged capacitor, no power could be delivered to the LED unit when the supply voltage is below the required forward voltage (indicated by dotted line 505) of the LED unit. As such, no current would be drawn by the LED driver and the electronic transformer would cease providing the supply voltage. When the charged capacitor is applied to power the LED driver during the boost-mode, the electronic transformer will also cease to provide the supply voltage. However, such an interruption of the supply by the electronic transformer can remain unnoticed due to the power supply by the charged capacitor. As such, by applying an energy storage element as a supply source during part of the period of the rectified AC voltage (e.g. when the voltage is below a certain level), the electronic transformer need not be sustained during that part and can cease to provide an output voltage. When the electronic transformer has stopped providing an output voltage, the transformer will attempt, e.g. every 0.5 ms, depending on the type of transformer, to restore the output voltage again. Such attempt will fail however as long as the electronic transformer cannot supply a current to the LED driver. As long as the output voltage is smaller than the available voltage over the capacitor, an attempt to restart the transformer will thus fail. When the output voltage exceeds the voltage available at the capacitor, the electronic transformer can resume supplying a current to the load (i.e. the LED driver powering the LED unit) and to the capacitor, thereby charging the capacitor. In graph d of
In practice, as indicated in graph e, a larger margin between the required current 540 and the actual current 542 supplied to the LED unit can be applied. When a sufficiently high supply voltage is available, the current supplied to the LED unit 542 can e.g. be above the nominal value (e.g. at 120%) and decrease below the nominal value at other instances. Similar to curve 531 of graph d, the current 542 comprises a component substantially in phase with the main frequency component of the rectified AC supply voltage (i.e. a 100 Hz component in case the supply voltage originates from a 50 Hz mains supply). In addition, current 542 comprises current peaks 544 occurring at times when the supply voltage is comparatively low, i.e. when the LED driver is supplied from the energy storage element. By doing so, a current component at twice the frequency of the main component of the rectified AC voltage (e.g. a 200 Hz component in case the supply voltage originates from a 50 Hz mains supply) is introduced. By doing so, adverse effects of the intensity variation of the LED unit can be mitigated.
Graph f finally describes the current Is as provided by the electronic transformer 550, together with the minimum current requirement 510. As can be seen, when the current 550 drops below the minimum current 510 (because the charged capacitor has taken over supplying the load current), the current rapidly drops to zero, due to the electronic transformer stopping. At times when the electronic transformer is not providing an output voltage, the LED driver can rely on the energy storage element (e.g. a charged capacitor or capacitors) to provide the required input power to supply a current to the LED unit.
In order to synchronise the operation of a switch connecting or disconnecting a capacitor to the supply voltage, a reference instance can be determined relative to the period of the rectified AC voltage. The timing of the operation of the switchable energy storage element can then be controlled by the control unit, relative to the reference instance. Given the reference instance, the peak value of the supply voltage and frequency, the control unit can determine at each instance the available supply voltage and thus determine whether or not to operate the switchable energy storage element. As a reference instance, the control unit can e.g. determine (during a number of periods of the supply voltage), when the voltage is reduced by e.g. 3 or 5% compared to the peak value. This is schematically illustrated in
When a phase angle modulated supply voltage (as shown in
In an embodiment, the LED driver according to the invention is arranged to gradually increase the average current to the LED unit when an increase in the amplitude of the supply voltage is noticed. Such an Increase can be due to load changes in the electric grid supplying an electronic transformer supplying the LED driver. Such change in the supply voltage amplitude is in general, a phenomenon that occurs on a comparatively large time scale (˜minutes). A change in the amplitude of the available voltage may however affect the required current supplied to the LED unit in order to sustain the transformer. As such, the current supplied to the LED unit may need to be changed (e.g. increased) when the supply voltage changes (increases). In accordance with the invention, such an increase is done gradually, in order for the change in brightness (due to the change in current) to remain unnoticed to the observer. Assuming that a limited number of current levels is available that can be selected (e.g. 16 current levels ranging from zero to 120% of the nominal current). If the level of the current supplied to the LED unit would be raised by one level, such a change would become visible to an observer. In accordance with the invention, a gradual increase of the current is realised by raising the current supplied to the LED unit to the next available current level for only a comparatively small portion of a period of the rectified AC voltage. This small portion can e.g. correspond to T1=1/F whereby F represents the frequency at which a new current set-point can be provided to the LED driver, or a larger portion. In case a new current set-point can e.g. be provided every 52 μsec, (T1=52 μsec), the average current over a period of the supply voltage could be incremented in very small steps by increasing the current during each period of the supply voltage only over a period equal to T1, rather than adjusting (raising or decreasing) the current profile entirely to a next current level. This is illustrated in
With respect to the current profile as schematically depicted in
Referring to
In
It is worth noting that the power factor correction device as shown may also be applied without the electronic switch 910, as schematically shown in
In
With respect to the use of a power factor correction device as described above, it can be mentioned that the application of such a device can result in the LED driver operating at an improved power factor. The application of such a device may however also be considered as it can allow the profile of the current to the LED unit to be altered. In the absence of a power factor correction device, particular requirements can be posed upon the current profile in order to obtain a power factor that is sufficiently high; as an example, it may be required to have a sufficiently large current component in phase with the supply voltage. By using a power factor correction device, the requirements for the current profile can become less strict which can result in an improved illumination quality; e.g. less flicker.
It should further be mentioned that the embodiments of the LED drivers as described are mere illustrations of the various aspects of the invention, the invention only being limited by the scope of the claims as set forth.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5414340 | Gannon | May 1995 | A |
| 5786670 | Nguyen | Jul 1998 | A |
| 5930130 | Katyl | Jul 1999 | A |
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