EP4287779A1

EP4287779A1 - Method for operating an led converter and led converter

Info

Publication number: EP4287779A1
Application number: EP22176366.7A
Authority: EP
Inventors: Hans Auer; Clemens KUCERA; Fabio Romano; Lukas Saccavini; Stefan Stark
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2022-05-31
Filing date: 2022-05-31
Publication date: 2023-12-06
Also published as: WO2023232576A1

Abstract

A method for operating an LED converter (1) which is connected to a power supply via a mains input (10) and to an LED light source (50) via a load connection (30) is provided, wherein the method comprises: determining, by the LED converter (1), whether a fault condition occurs during operation of the LED converter (1); in case a fault condition is determined, operating the LED converter (1) in a safe operating mode.

Description

The present invention relates generally to the operation of light-emitting diodes (LEDs), whereby light-emitting diodes are understood to mean inorganic light-emitting diodes, but also organic light-emitting diodes (OLEDs). The term LED will be used hereinafter as representative of both types of light-emitting devices. In particular, the present invention relates to the operation of an LED converter in situations where a malfunction or abnormal behaviour has been detected.
It is known that the light emission or brightness of an LED correlates with the current flow through the LED. For brightness control (so-called dimming), the LEDs are therefore preferably operated in a mode in which the current flow through the LED is controlled. This is done using an LED converter that converts the input supply voltage into an output voltage suitable for operating the LEDs, whereby in the case of dimming operation, the current flowing through the LEDs is regulated to a certain current that depends on a corresponding input dimming signal. This input dimming signal can be, for example, a control command according to the DALI standard.
During operation of the converter, conditions may arise that require the LED converter to the switched off to avoid a severe damage of components of the converter and/or of the LED load. These conditions can relate to a fault of the supply voltage on the input side of the converter, of the LED load on the output side of the converter and/or of components of the converter itself. Usually, a control unit of the converter detects such an abnormal condition and switches to converter off. In case the error remains even after a restart of the converter, the converter is completely turned off by the control unit or it switches into a standby mode in which no further light is emitted.
The solution explained above and known from the prior art can prevent damage to the components of a lighting system when abnormal conditions occur. However, turning off the converter or switching to a standby mode will cause the light output to fail completely. Not only does the customer have to completely do without lighting, but he also has very limited possibilities to determine the fault causing the shutdown (for example, a lamp defect). One possibility would be to use a special analyser marketed by the applicant under the name "deviceAnalyser".
The present invention overcomes the problem that in the event of a fault, the LED converter shuts down completely and no more light is emitted from the luminaire. The solution according to the invention to overcome this problem is based on the realization that under certain circumstances it is possible to operate an LED converter in a so-called safe operating mode or emergency mode, whereby operation in safe operating mode or emergency mode continues to provide light to the customer. In certain situations, it is also possible to notify the customer that there is a fault by reducing the light output. In this way, customer service is supported by fault narrowing and faster processing is made possible.
Accordingly, in accordance with the present invention, a method for operating an LED converter which is connected to a power supply via a mains input and to an LED light source via a load connection is provided, wherein the method comprises:

a) determining, by the LED converter, whether a fault condition occurs during operation of the LED converter;
b) in case a fault condition is determined, operating the LED converter in a safe operating mode.

Also in accordance with the present invention, an LED converter for operating an LED light source is provided, wherein the converter comprises a mains input connectable to a power supply and a load connection for connection to an LED light source, and wherein the LED converter comprises control means being adapted to:

a) determine whether a fault condition occurs during operation of the LED converter;
b) in the event that a fault condition is determined, operate the LED converter in a safe operating mode.

Accordingly, contrary to the solution to the prior art explained above, the converter not simply tries to restart operation once an abnormal condition has been recognized and completely switches off in case the restart is not successful. Instead, the LED converter changes operation to a safe operating mode which allows to continue operating the light source although the abnormal condition still exists. Although the flexibility of the operation might be reduced in the safe operating mode, nevertheless operating of the LED load is still possible and therefore at least a reduced amount of a light can be generated.
Preferred embodiments of the inventive concept are subject matter of the dependent claims. These further developments in particular relate to the type of fault condition which can occur in different components of the lighting system.
In a first scenario, the converter comprises a power factor correction (PFC) stage followed by a DC/DC stage, the fault condition being due to a defect of the power factor correction stage. Whereas in the prior art solution, a fault in the power factor correction stage ultimately results in a shutdown of the converter, the concept according to the invention now proposes that operation of the converter is maintained in the safe operating mode, in which safe operating mode the DC/DC stage is operated with a deactivated power factor correction stage. As will be shown in more detail later, it is indeed possible to continue operation of the converter even if a fault has occurred at the PFC stage.
In particular, according to a first alternative, in the safe operating mode the DC/DC stage - which is e.g. realized in the form of a resonant converter - is regulated to a set point output current which was set before the fault condition has been determined wherein preferably regulation takes place above a predetermined minimum frequency. Alternatively, it would also be possible that in the safe operating mode the DC/DC stage is regulated to a predetermined smallest output current. A third option would be that in the safe operating mode the DC/DC stage is regulated at a predetermined frequency. While in the first alternative the converter attempts to maintain the output current at a value that was set before the abnormal condition occurred, which corresponds to a constant light output, the last two options have the additional effect that the amount of light generated by the LED load changes, in particular changes to a lower amount. This can be used as a signal showing to the costumer that the converter operates under abnormal conditions. Preferably, in the embodiments explained above, the DC/DC stage comprises a self-resonant topology, in particular an LLC or an LCC circuit.
However, it has to be emphasized that the inventive concept explained above is also usable for other converter topologies. In particular, the DC/DC stage could also be formed by a non-resonant converter, e.g. a switched converter like a Buck converter, a Flyback converter... For such other DC/DC stages like the Buck converter, instead of the frequency other parameters like the peak current through the switch are controlled in order to operate in the safe operating mode. Here, instead of considering different frequencies, different peak current levels must be considered.
However, in addition to a damage of the power factor correction stage, a fault or abnormal condition also can occur in case an input voltage supplied to the converter drops below a nominal voltage. In this scenario, the converter preferably switches to a safe operating mode wherein the converter is operated at this safe operating mode at a predetermined reduced load, in particular at a predetermined minimum dimming level.
This operation allows to continue operating the LEDs although the input voltage does no longer satisfy the predetermined requirements. Again, in this case of an inventive safe operating mode the light output of the LED load changes to a reduced light output which again can be used to signal to the costumer that an abnormal condition has occurred.
Another situation where an error condition can occur is when an error is detected in the LED load. This does not necessarily mean that the LEDs themselves are damaged. However, although the LEDs are still capable of producing light, a fault in the LED load can lead to a situation where the output voltage becomes higher than a specified safety threshold. In this case, the LED converter can again switch to the safe operating mode according to the invention, in which the converter is now regulated to a predetermined lowest output current that ensures that the output voltage is well below a safety threshold.
Similar to the solution explained above and known from the prior art, the converter can - in a first step - try to restart once an abnormal condition has been detected. The switch to the inventive safe operating mode then only occurs in case also the restart was not successful, i.e. the abnormal condition remains.
Accordingly, with the aid of the present invention, it is possible to continue to emit light despite an abnormal condition. Furthermore, if the safe operating mode according to the invention results in reduced light output, the customer is indirectly informed of a malfunction so that he can take steps to correct the problems. Nevertheless, the lamp is not completely switched off, but at least a reduced illuminance remains.
In the following, the inventive solution is discussed in more detail with reference to the accompanying drawings.

Figure 1: schematically shows the structure of an LED converter.
Figure 2: shows the behaviour of an converter according to the prior art in case a critical error occurs.
Figure 3: shows the behaviour of an inventive converter is case a critical error occurs.
Figure 4: shows a simplified boost PFC circuit in an preferred embodiment of an inventive converter.
Figure 5: schematically shows the LLC gain transfer curve for different bus voltages.
Figure 6: shows the initial PFC performance in case of an "brown out" voltage input.
Figure 7: shows an improved PFC performance in accordance with the present invention with a "brown out" voltage input.
Figure 8: shows the initial converter behaviour in case of a load error.
Figure 9: shows the behaviour of the LED converter in accordance with a preferred embodiment of the present invention.

Figure 1 shows a block diagram illustration of a lighting system 100 comprising an LED converter 1 operating an LED light source 50. The converter 1 can be connected to a physical communication line or to a wireless communication system via a communication interface in order to receive dimming commands and/or to output or exchange status information. In the example shown in figure 1, communication takes place using the known DALI-standard.
The shown LED converter 1 is connected to a power supply - not shown - via a mains input 10 and to the LED light source 50 via a load connection 30. Also not shown in the figures, converter 1 usually has a rectifier following the mains input 10 for rectifying a supply voltage, for example, the power supply system voltage. The rectified supply voltage is then forwarded to a power factor correction (PFC) stage 15 which provides an output voltage for components of the converter 1 that are connected downstream. The output voltage provided by the power factor correction stage 15 is usually designated as a bus voltage V_bus. In addition, the converter 1 typically includes EMI filter components and other filtering/protection circuitry against bursts/surges, which are not shown in figure 1 for reasons of clarity.
Further voltage conversion and/or dimming functions are implemented by means of a DC/DC stage 18, which may be implemented as an LLC resonant converter (or LCC resonant converter). As will be explained later, also other converter topologies could be used for the DC/DC stage 18. In operation of the converter 1, the DC/DC stage 18 controls the power provided at the load terminal 30 such that the LEDs of the light source 50 are operated at an intended forward voltage, the current provided to the LED light source 50 corresponding to a desired light output. The operation of the PFC stage 15 and the DC/DC stage 18 is controlled by one or more corresponding control units, which in the present case are implemented by a combination of an ASIC 21 and a microcontroller 22. In the case of the dimming functionality, the DALI commands received via the corresponding interface 25 are forwarded via optocoupler 26 to the microcontroller 22, which then controls the operation of the converter 1 so that the current supplied to the LED light source 50 corresponds to the desired dimming value.
Basically, there are three possibilities to interact with the converter 1 shown in figure 1 from the outside, i.e. the mains connection 10, the load connection 30 and the communication interface 25. With all these connections, conditions may arise that require the LED converter 1 to be switched off as these situations prevent a continued normal operation.
As already discussed above, the usual behaviour of an LED converter in case that an error condition occurs is shown in figure 2. As shown in this figure, in case the control unit of the converter detects an error condition (ERROR), the normal behaviour is to switch off the converter and to restart the system (2nd start). Many problems indeed can be solved by simply switching off the connector and starting it again.
However, if the error occurs again even after the restart, a converter switches to a standby mode (STANDBY) according to the state of the art, i.e. the operation of the LED load is completely switched off. No further light is emitted and the customer only recognizes that operation has been stopped for unknown reasons. However, it is not possible for the customer to determine the reason for the shutdown without a detailed analysis of the entire system, e.g. with a device analyser.
The present invention overcomes this problem by amending the behaviour of the converter as shown in figure 3.
Again, also the converter according to the present invention in a first step switches off in case an abnormal condition has been detected. The converter then tries to restart in order to begin again normal operation.
However, in contrast to the solution shown in Figure 2, the converter no longer switches to a standby mode if the restart was not successful. Now, in certain situations, a safe operating mode (SOM) is started, which allows the LED light source to continue to operate and generate light. In contrast to the solution in the prior art, the light emission is not completely stopped, but a basic illumination is still realized. Additionally, if desired, the light emission can be changed to a specific pattern to inform the customer that a fault condition has occurred. This solution not only allows the LED load to maintain operation, but also provides a signal to the customer. The customer then knows that specific action is required to correct the remaining problems.
It should be mentioned that the restart shown in Figure 3 is not absolutely necessary. In particular, it would also be possible to switch directly to safe operating mode. However, as mentioned above, many problems can be solved by simply restarting the converter, and therefore it is preferable that at least one restart is attempted before initiating the safe operating mode.
Accordingly, the present invention solves the problem that in the event of a fault of the lighting system, the LED converter switches off and there is no more light emitted by a luminaire. Based on the inventive solution, it is possible to still operate the LED converter in the safe operating mode and to still provide a light to a costumer. Operation of the converter is thus significantly improved compared to solutions known in the prior art.
In the following, specific error situations are discussed in more detail and it is shown in which way operation of the converter is changed to the inventive safe operating mode. The following examples distinguish between faults, which occur at different section of the lighting system 1. The following examples in particular discuss a partial destruction of the PFC stage and errors in the mains supply and the output load.

a) Broken PFC

At first, abnormal conditions resulting in a partial destruction of the PFC stage shall be discussed. Such problems can result for example from a mains fault, which leads to a partial destruction of the PFC stage 15. For example, very high inrushing currents or other power system abnormalities such as burst/surge pulses can cause components of a PFC stage to break. Figure 4 schematically shows a simplified structure of a boost PFC circuit wherein in particular the resistors R or the switch M can break in response to the problems mentioned above.
In case some of these components break, current can still run via inductive element L and diode D and therefore the rectified supply voltage is more or less directly submitted to the DC/DC stage 18. The idea according to the invention is now to operate the DC/DC stage 18 without operating the boost PFC circuit 15. Self-resonant topologies such as LLC or LCC circuits, which are used in the present example to implement a DC/DC stage, can still handle the reduced input voltage to a certain extent.
Figure 5 for example shows how the transfer ratio of a LLC topology circuit changes when the input voltage V_bus decreases from 405V (corresponding to an active PFC stage) to 320V (corresponding to a double rectified line voltage with deactivated PFC stage). The figure shows that at a constant switching frequency (fo) the LLC gain and therefore also the output current I_LED decreases. Nevertheless, figure 5 also shows that even with deactivated PFC stage, the DC/DC stage 18 still can be operated in order to supply a suitable supply voltage and a suitable current to the LED load in order to maintain operation of the light source.
Based on this finding, several ways are suggested to maintain operation of the power source DC/DC stage 18 in a safe operating mode in case the PFC stage 15 has been deactivated or is damaged.

The first possibility is to continue regulating the output current provided to the LED load 50 to the last set point current which was set before the abnormal condition was detected. As can be obtained from figure 5, this means in particular that the driving frequency f_sw of the DC/DC stage 18 should be adapted accordingly, wherein nevertheless control of the output current is possible. Besides a corresponding adaptation of the frequency f_sw, normal operation of the light source can be maintained. However, it should be ensured that the frequencies are not too low for large currents as this increases the risk of capacity of switching and of noise. Therefore, operation of the DC/DC stage 18 is preferable restricted to a frequency range above a predetermined minimum frequency.
A second possibility would be to regulate the DC/DC stage 18 to output a predetermined smallest current. This is possible because the reduced input voltage (V_bus) provides an extended frequency range for driving the DC/DC stage.
Finally, a third possibility would be to operate the DC/DC stage 18 which is formed by a resonant converter by means of a fixed frequency in a range where the oscillating circuit safely works. Also this unregulated operation allows to continue with operating the LED light source to emit light.

All three possibilities have the advantage that the customer continues to be supplied with light despite a fault occurring. The last two possibilities have the additional advantage that a kind of signal effect of the reduced light emission is achieved, which signals to the customer that problems in the operation of the light source have been detected.
As already mentioned above, the inventive concept of maintaining operation of the power source DC/DC stage 18 in a safe operating mode in case the PFC stage 15 has been deactivated or is damaged is also usable for other converter topologies. In particular, the DC/DC stage could also be formed by a non-resonant converter, e.g. a switched converter like a Buck converter, a Flyback converter, other synchronous converter topologies or hybrid solutions as also these converter types are able to handle a reduced input voltage at least to a certain extent. With such other DC/DC stages, other parameters such as e.g. peak current through the switch may then be controlled instead of frequency in order to operate in the safe operating mode.

b) "Brown out voltage"

Another potential problem is the case of so-called "brown out voltage". In this specific case of a mains fault, the input voltage at the entry of the converter 1 drops to a value far below the nominal voltage. Figure 6 shows the voltage obtained after the bridge rectifier as "rectified brownout voltage". In case the LED converter 1 is now operated with a large load, the bus voltage output by the PFC stage 15 drops as soon as the input voltage drops, since the PFC stage 15 no longer has the energy required to boost. If this voltage drop is large too large, the PFC stage 15 detects under voltage and switches off the complete LED converter 1.
In accordance with the present invention, a safe operation mode is suggested which allows to continue the operation of the converter 1 even in case of such an brown out voltage.
In particular, if a brown out voltage is detected by the ASIC 21 or the microcontroller 22, the load can be set to a minimum by operating the LED light source 50 at a minimum dimming level. This change in operation has the consequence that the DC/DC stage 18 no longer needs so much energy from the PFC stage 15 resulting in the effect that the voltage ripple of the bus voltage is significantly reduced. This is shown in figure 7. In this situation, an under voltage can no longer occur and thus a shutdown of the LED converter 1 in view of a detected under voltage by the PFC stage 15 is prevented.
Accordingly, current is still supplied to the light source 50 and operation of the converter 1 is maintained. Again, the switch to the safe operation mode also results in a change of the light emission providing a signal to the costumer that an error has been detected.

c) Load errors

A third situation for an abnormal condition is an error in the load. Typically, SELV LED converters must prevent output voltages higher than 60V under all circumstances. For this reason, the maximum output voltage of an SELV LED converter is often limited to 54V.
Figure 8 shows the classical behaviour of an LED converter wherein at the beginning the light source is operated at 50% of the maximum current (I_LED = 50% I_LED,max) and wherein the output voltage amounts to 54V. If the LED current is changed (to in figure 8), then the forward voltage of the LED load increases and an over voltage shutdown occurs. The LED converter 1 usually tries to operate the light source again (second cycle as mentioned above in connection with figure 2). In case over voltage is again detected, the converter 1 switches to a standby mode wherein no longer light is emitted.
In accordance with the present invention, in case the second start attempt failed, the LED converter now operates in a safe operating mode which is shown in figure 9. This means in this situation that the converter 1 is operated with the smallest current (I_LED = I_LED,min). This results in a reduction of the forward voltage of the LED light source and therefore the risk of an over voltage shutdown is avoided.
It has to be noted that in addition to a current changed by the costumer, also other circumstances may cause an increased LED voltage risking a shutdown of the converter. For example, in case a costumer replaces the installed LED module with an LED module that has a higher forward voltage, again the problems mentioned before can occur. There is also the possibility that due to a temperature change of the LED module the forward voltage increases. Also in these situations switching to the safe operating mode allows to continue with operation of the LED light source and to guarantee at least a basic light emission.
Accordingly, the present invention allows the LED converter to operate in a certain safe mode that ensures continued light emission in a luminaire despite a fault at the power input, at the load, or even a faulty boost PFC. Thus, the customer is provided with light even if abnormal operation occurs, which normally results in a complete shutdown of the luminaire. By reducing the amount of light, the customer is also indirectly informed of a malfunction. This information can be used by the customer to more quickly isolate possible faults together with a customer support team.

Claims

Method for operating an LED converter (1) which is connected to a power supply via a mains input (10) and to an LED light source (50) via a load connection (30), wherein the method comprises:
a) determining, by the LED converter (1), whether a fault condition occurs during operation of the LED converter (1);

b) in case a fault condition is determined, operating the LED converter (1) in a safe operating mode.
Method for operating an LED converter according to claim 1,
wherein the converter (1) comprises a power factor correction stage (15) followed by a DC/DC stage (18),

wherein the fault condition relates to a defect of the power factor correction stage (15), and

wherein the safe operating mode is an operation of the converter (1) in which the DC/DC stage (18) is operated with deactivated power factor correction stage (15).
Method for operating an LED converter according to claim 2,
wherein in the safe operating mode the DC/DC stage (18) is regulated to a set point output current which was set before the fault condition has been determined, wherein preferably regulation takes place above a predetermined minimum frequency.
Method for operating an LED converter according to claim 2,
wherein in the safe operating mode the DC/DC stage (18) is regulated to a predetermined smallest output current.
Method for operating an LED converter according to claim 2,
wherein in the safe operating mode the DC/DC stage (18) is regulated at a predetermined frequency.
Method for operating an LED converter according to one of claims 2 to 5,
wherein the DC/DC stage (18) includes
• a resonant converter which preferably comprises a self-resonant topology, in particular an LLC or an LCC circuit;
or

• a non-resonant converter.
Method for operating an LED converter according to one of the preceding claims, wherein the fault condition relates to a drop of an input voltage supplied to the converter (1) below a nominal voltage.
Method for operating an LED converter according to claim 7,
wherein in the safe operating mode the converter (1) is operated at a predetermined reduced load, in particular at a predetermined minimum dimming level.
Method for operating an LED converter according to one of the preceding claims, wherein the fault condition relates to an error in the LED load.
Method for operating an LED converter according to claim 9,
wherein in the safe operating mode the converter (1) is regulated to a predetermined smallest output current.
Method for operating an LED converter according to one of the preceding claims, wherein the method further comprises:
prior to operating the LED converter (1) in the safe operating mode, restarting the LED converter (1) and switching to the safe operating mode if the fault condition persists.
LED converter (1) for operating an LED light source (50), wherein the converter (1) comprises a mains input (10) connectable to a power supply and a load connection (30) for connection to an LED light source (50),
and wherein the LED converter (1) comprises control means (21, 22) being adapted to:
c) determine whether a fault condition occurs during operation of the LED converter (1);

d) in the event that a fault condition is determined, operate the LED converter (1) in a safe operating mode.
LED converter according to claim 12, wherein the converter (1) comprises a power factor correction stage (15) followed by a DC/DC stage (18),
wherein the fault condition relates to a defect of the power factor correction stage (15), and

wherein the safe operating mode is an operation of the converter (1) in which the DC/DC stage (18) is operated with deactivated power factor correction stage (15).
LED converter according to claim 12 or claim 13,
wherein the fault condition relates to a drop of an input voltage supplied to the converter (1) below a nominal voltage.
LED converter according to one of the preceding claims 12 to 14, wherein the fault condition relates to an error in the LED load.