The invention is m the field of emergency lighting, and ballasts for driving an emergency light in a maintained mode and in particular concerns a driver including PFC circuit in boost topology and implementing an isolation barrier.

An emergency lighting system uses battery-backed lighting devices that switch to a battery energized mode automatically when a power outage is detected.

An emergency light is required to provide illumination when the power provided by regular power supply such as mains supply goes out. Every emergency light requires some sort of energy storage device, for example a battery, preferably a rechargeable battery that provides electrical energy to the light, during a mains failure.

Modern emergency lighting is often installed in commercial buildings and high occupancy residential buildings. The lights often include one or more clusters of high-intensity LEDs as lighting devices.

Modern emergency lighting systems operate with relatively low voltages, for example in voltage ranges from 6-12 volts. These low voltages both reduce the size of the required batteries and reduce the load on a driving circuit, which drives the emergency light. A small transformer of an emergency lighting device is arranged to step-down the voltage from mains supply to the low voltage required by the lights in order to supply the LEDs with electric energy in a maintained mode. In such maintained mode, the LEDs are turned on even when there is no power failure. Then, the LEDs are driven from power supply by means of a converter. An emergency lighting installation may be either a central standby source such as a bank of batteries, control units and battery chargers supplying slave fittings throughout the building, or may be constructed using self-contained emergency fittings which incorporate the light emitting device, for example an LED, a battery, a battery charger and a control unit such as application specific circuits or a microcontroller.

Self-contained emergency light devices may operate in a maintained mode, in which the light devices are illuminated all the time or controlled by a switch, or in a non-maintained mode, in which the lighting devices are illuminated only when the mains supply fails.

Further popular battery backup ballasts for emergency lighting applications are adapted to be installed within or adjacent to standard lighting fixtures. Upon sensing power loss, the ballasts switch into emergency mode turning the existing lighting devices into emergency lighting in order to meet both the requirements for standard lighting and the emergency lighting without the need of wiring separate electric circuits or external wall mounts.

A conventional emergency lighting driver device connected to a mains supply often includes an EMI filter circuit followed by a power factor correction circuit (PFC) , which supplies a converter circuit , for example a flyback converter, which powers LEDs used as lighting devices in a maintained mode. The flyback converter also fulfils the role of separating mains supply on one hand and a low voltage side on the other hand by an isolation barrier.

The isolation barrier serves to guaranty safety extra-low voltage (SELV - also separated extra-low voltage) by separating circuitry with high voltages, such as a mains supply voltage, from circuitry with low voltages. A SELV circuit includes electrical-protective isolation (double insulation) from all circuits other than SELV, particularly all circuits that may carry higher voltages and simple separation from other SELV circuits

A known emergency lighting device as shown in fig. 2 of the accompanying drawings includes an additional controller controlling a further flyback converter. The further flyback converter feeds on a secondary side of its transformer a battery charger for charging an internally or externally arranged energy storage device.

The power factor correction circuit is regularly implemented as a boost converter. A major drawback of the known emergency lighting device is that the boost PFC circuit, the standard fly back converter for driving LEDs in maintained mode and the flyback converter for supplying the battery charger each require control by a circuit such as an ASIC or microcontroller. The different controllers each involve costs and increase space requirements . Furthermore the number of converter circuits itself drives the complexity and cost of the emergency lighting converter device according to prior art.

The invention addresses the drawbacks of the known emergency lighting converter device by addressing the technical problem of reducing the complexity, the space requirements as well as the costs.

The technical problem is solved by the converter device according to claim 1. Further advantageous embodiments are claimed in the dependent claims .

The converter device according to claim 1 solves the technical problem by including a power factor correction circuit including an inductor and a charging circuit configured to provide a charging current to an energy storage device. The converter device according to the invention comprises a secondary winding magnetically coupled to the inductor and connected to the charging circuit for supplying the charging circuit with electric energy. The inventive approach enables to omit a second flyback converter used in prior art for providing a current to the charging circuit. Replacing the flyback converter also enables to reduce the control circuitry necessary because no microcontroller for controlling the second flyback converter is needed. Accordingly, the number of separate design blocks for the converter device is reduced, resulting to reduced unit cost of the converter device and advantageously decreased space requirements over the entire converter device. These advantages are achieved without having to accept drawbacks with respect to SELV protection of the converter device. While galvanic isolation is provided using the transformers of the flyback units in known converter devices, the inventive converter device provides galvanic isolation between a primary mains side on one hand and secondary side energy storage and load side on the other hand due to the flyback converter and the galvanic isolation between the winding of the inductor (boost choke) of the power factor correction circuit and the new secondary winding of the inductor.

The converter device of a preferred embodiment comprises a converter circuit arranged at a secondary side of an isolation barrier configured to drive the load from energy drawn from the energy storage device.

The converter device according to a further advantageous embodiment has the charging circuit comprising a linear charger circuit or a switched mode power supply circuit (SMPS) connected to the secondary winding for charging the energy storage device.

A particularly advantageous converter device connects the secondary winding via a rectifier circuit to the charging circuit .

The rectifier circuit converts the current provided by the secondary winding to a current suitable to directly drive the charging circuit. The combination of the secondary winding of the inductor in the power factor correction circuit and the subsequent rectifier circuit enables to eliminate the second flyback converter and to simultaneously provide a stable voltage and corresponding current as input to the battery charger circuit. The converter device according to an embodiment has the rectifier circuit including s a first diode and a second diode, wherein an anode of the first diode is connected to a first output of the secondary winding and a cathode of the first diode is connected to an output of the rectifier circuit. A cathode of the second diode is connected to the fist output of the secondary winding and an anode of the second diode is connected to ground potential .

Using the first and second diodes in the claimed circuit topology enables to benefit from the advantages of full wave rectification. Both a positive and a negative half of the period of the input waveform to the rectifier provided by the secondary winding of the power factor correction circuit can be used to acquire a single polarity voltage at its output.

An advantageous converter device according to an embodiment includes the rectifier circuit showing a first capacitor and a second capacitor. The first capacitor connects an output of the rectifier circuit to a second output of the secondary winding and the second capacitor connects the second output of the secondary winding to the ground potential . A conventional full-wave-rectifier circuit requires four diodes arranged in a diode bridge and a transformer without centre tapped secondary winding, or alternatively a centre tapped transformer and two diodes arranged in a back-to-back, e.g. cathode-to-cathode or anode-to-anode arrangement. Contrary, the claimed rectifier topology eliminates the need for either four diodes or a more complex inductor with a centre tap and generates a sufficiently smooth waveform benefiting from both positive and a negative portions of the input waveform for driving the battery charger circuit. The rectifier circuit of a further embodiment includes a third capacitor arranged between the output of the rectifier circuit and ground potential.

The voltage over the third capacitor corresponds to the voltage provided to the charger circuit and provides a smooth and only moderately pulsating waveform for feeding to the charger circuit .

The converter device may be an emergency LED converter.

The description of an embodiment of the invention refers to the enclosed drawings in which

Fig. 1 provides a general overview of major elements of a converter device according to an embodiment,

Fig. 2 is a general overview of major elements of a converter device according to prior art, Fig. 3 provides an overview of major elements of a boost power factor correction circuit and a rectifier circuit according to an embodiment,

Fig. 4 provides a time diagram of the input current in a case, in which no secondary winding L _se c is present in the power factor correction circuit according to prior art,

Fig. 5 provides a time diagram of the input current in a case, in which a secondary winding L _se c is present in the power factor correction circuit according to an embodiment using the circuit design of fig. 3, and

Fig. 6 provides a time diagram of voltages in the rectifier circuit according to an embodiment using the circuit design of fig. 3.

In the figures, same numerals denote same or corresponding elements. For sake of conciseness, the description of the figures omits repeating the description of same reference signs in different figures.

Fig. 1 provides a general overview of major elements of a converter device according to an embodiment. For sake of clarity and conciseness, those structures of depicted preferred embodiment of the converter device 1 corresponding to respective structural elements in a known emergency converter 20 are discussed with reference to fig.2 first.

In fig. 2 a general overview of major elements of a converter device according to prior art is depicted. The converter device 20 is fed via a mains interface 21 with an AC voltage. The AC voltage is provided to an EMI circuit 22. The EMI circuit 22 is adapted to address requirements of electromagnetic interference (EMI, sometimes also termed radio frequency interference RFI), for example suppressing any disturbances that affect an electric circuit due to either electromagnetic conduction or electromagnetic radiation emitted from external sources. The EMI circuit 22 is in particular adapted to ensure electromagnetic compatibility (EMC) . The EMI circuit 22 ensures that the converter device 20 works as intended and as required by the applicable rules in its environment. The EMI circuit 22 suppresses external interference to the converter device, in particular via the mains lines over the mains interface 21. Alternatively or additionally the EMI circuit 22 ensures that the converter device 20 does not provide any interference with other electronic equipment nearby and/or operating in the vicinity. The EMI circuit 22 may use one or more of components such as RFI filters, for example feedthrough elements, IEC inlet filters , power entry modules single phase filters , 3- phase and neutral line filters chokes, pulse transformers, shielding products. Typically, a rectifier not shown in figs 1 and 2 is arranged before the subsequent power factor correction circuit 23 in order to convert the AC voltage from mains to DC voltage for further processing. The converter device 20 arranges the power factor correction circuit 23 in a mains-load-direction after the EMI circuit 22. The power factor correction circuit 23 is adapted to shape an input current supplied to the converter device 20 from the mains supply to be in synchronization with the mains voltage in order to maximize the real power drawn from mains supply. Ideally the input current follows the input AC mains voltage acting as a resistor, for example without any input current harmonics. The power factor correction circuit 23 is regularly an active power factor correction circuit, which is often, but not exclusively, implemented using a boost converter topology.

The boost power factor correction circuit topology is generally known and uses an inductor, often termed a boost inductor or boost choke and a boost switch besides a rectifier diode and possibly a bulk capacitor at the output of the power factor correction circuit 23.

The power factor correction circuit 23 may operate in different modes, for example a continuous conduction mode (CCM) controlled via the switch, for example a transistor such as a MOSFET, driven by a controller integrated circuit 24 (IC) such as an ASIC or microcontroller.

The inductor of the power factor correction circuit 23 is typically selected for a specified maximum inductor current ripple besides other requirements such as core losses, stability of inductance value, etc.

A first converter circuit, for example a first flyback converter 25 is fed by the output current of the power factor correction circuit 23. The first flyback converter circuit 25 generates a load current for driving the load 2, in particular one or more lighting devices as light emitting elements such as LEDs or fluorescent lamps.

A controller IC, for example the controller IC 24, may contro 1 the load current provided by the first flyback converter 25 The controller IC 24 can be an application specific integrated circuit (ASIC) or a microcontroller.

The converter device 20 can be adapted to provide a predetermined load current value to the load 2. The load current value may be selectable. A current selector 28, for example an interface for accommodating one out of plural resistors with different values, or a dedicated programming interface may be used to select one out of plural possible load current values.

The first flyback converter circuit 25 additionally provides an isolation barrier by galvanic isolation between the inputs of the first flyback converter circuit 25 from the outputs of the first flyback converter circuit 25, for example by a using a transformer of the first flyback converter circuit 25. Fig. 2 depicts the isolation barrier 27 being ensured via a separate first SELV element 26, however the first SELV element 26 may also be functionally incorporated into the first flyback converter circuit 25 and its transformer.

The converter device 20 includes a switch 29 which switches either an output of the first converter device 25 in a standard mode of operation or an output of an emergency converter circuit 4 in an emergency mode of operation to an output of the switch for driving the LED as a load 2 with a LED current.

The depicted emergency converter device 20 powers the LED 2 in a maintained mode, which means that the LED 2 are powered in a normal mode of operation by the first flyback converter circuit 25 from mains supply and in an emergency mode of operation by a second converter circuit 4 (emergency converter circuit 4) from electrical energy drawn from an energy storage device 5.

The emergency converter circuit 4 is controlled by a second controller integrated circuit (IC) 33 arranged on a secondary side of the isolation barrier 27. A low voltage power supply (LVPS) 32 may provide a supply voltage to the second controller IC 33. In normal mode of operation the mains supply is present at the input of the mains supply interface 21. A battery charger 34 generates a battery charging voltage, a battery charging current and provides the battery charging current via an interface 35 to the energy storage device 5. The energy storage device 5 is preferably a rechargeable battery.

In an emergency mode of operation the battery charging circuit 34 draws electrical energy from the energy storage device 5 and provides the emergency converter circuit 4 with a current for driving the LED 2 with an emergency load current. This means that the battery charging circuit 34 acts in a battery discharging operation during an emergency mode of operation. The emergency load current value may be selectable, for example via a select interface not shown in fig. 2, in order to ensure a predetermined light level for a predetermined time duration, for example a regulatory minimum time.

A third converter circuit 36, for an example flyback converter, simultaneously performing the function of a second SELV element 37 for maintaining the galvanic isolation between a mains supply side and an energy storage side of the converter device 20 provides the battery charger circuit 34 with an input current. The second SELV element 37 can also be implemented using a transformer of the third converter circuit 36 as indicated by the dotted line in fig. 2. The third converter circuit 36 may include a switch which is controlled by a third controller integrated circuit (IC) 38 arranged on the mains supply side of the isolation barrier 27. The third controller IC 38 is provided with a supply voltage by the low voltage supply circuit 39. It is to be noted, that the first controller IC 24 and the third controller IC 38 and/or their respective low voltage supplies may be functionally integrated into one circuit element such as an ASIC so that a single integrated circuit performs their respective functions. In fig. 1, a general overview of major elements of a converter device 1 according to an embodiment is provided. The converter device 1 may be an emergency lighting converter for driving one or more LED 2 as a load. The emergency converter device 1 powers the LED 2 in a maintained mode, which means that the LEDs 2 are powered in a normal mode of operation by a first converter circuit 3 from mains supply and in an emergency mode of operation by a second converter circuit 4 from electrical energy drawn from an energy storage device 5. The converter device 1 includes a power factor correction circuit 6, in particular in boost converter topology with an inductor 7. The inductor 7 acts as a boost choke in the boost converter. The inductor 7 comprises a secondary winding L _se c The secondary winding L _se c is connected with a first connecting line to a first input of a rectifier circuit 8 and with a second connecting line to second input of the rectifier circuit 8. The second connecting line is connected to ground potential 9 at the secondary side of the converter device 1.

A primary winding of the inductor 7 and the secondary winding Lsec of the inductor 7 are magnetically coupled and transfer electric energy from the power factor correction circuit 6 to the rectifier circuit 8 and further to the battery charging circuit 34. Simultaneously the primary winding of the inductor 7 and the secondary winding L _se c of the inductor 7 are magnetically coupled, however electrically isolated from each other and therefore act as a galvanically isolated barrier. The inductor 7 of the power factor correction circuit 6 therefore forms a SELV barrier (isolation barrier) between the primary side of the converter device 1 and a secondary side of the converter device 1.

When comparing the diagrams of fig. 1 and fig. 2, it becomes immediately apparent that the inventive power factor correction circuit 6 enables to simplify the emergency converter device 1 when compared to the known emergency converter device 20. In particular the circuit blocks of the third converter device 37 with the second SELV element 37 and the third controller IC 38 with its LVPS 39 are not required. Complexity and involved costs are significantly improved for the emergency converter device 1. The rectifier circuit 8 will be discussed with more detail with respect to fig. 3, which provides an overview of major elements of a boost power factor correction circuit 6 and a rectifier circuit 8 according to a preferred embodiment.

The lower portion of fig. 3 shows a power factor correction circuit 6 in boost converter topology. The boost power factor correction circuit 6 is a DC-to-DC power converter that steps up voltage from its input to its output load. The power factor correction circuit 6 includes at least two semiconductors, a diode 43 and a transistor acting as switch 41, and at least one inductor 7 as an energy storing element. To reduce voltage ripple at the output of the power factor correction circuit 6, a filter represented by a capacitor 44, possibly in combination with further capacitors and/or inductors may be added to the power factor correction circuit's output as a load-side filter. Further filter circuits may be added to the input of the power factor correction circuit 6 as supply-side filter.

When the switch 41 is closed by applying a control signal via control terminal 42 of the power factor correction circuit 6, a current flows through the inductor 7 in a clockwise direction and the inductor 7 stores energy by generating a magnetic field. Polarity of the left side of the inductor is positive.

When the control IC 24 issues a control signal to open the switch 41, current through the inductor will decrease as the impedance is higher. The magnetic field previously created decreases to maintain the current towards a load 30 of the power factor correction circuit 6. Thus, the polarity will be reversed, in particular a left side of inductor 7 will be on a negative electric potential. As a result, two sources will be in series causing a higher voltage to charge the capacitor 44 through the diode 43.

If the switch 41 is controlled with a cycle frequency high enough, the inductor 7 will not fully discharge between charging stages, and the load will always see a voltage greater than that of the input voltage to the power factor correction circuit 6, provided by the EMI circuit 22 alone when the switch 41 is opened. Also, while the switch 41 is opened, the capacitor 44 in parallel with the load is charged to this combined voltage. When the switch 41 is then closed and the right hand side is shorted out from the left hand side, the capacitor 44 is therefore able to provide a voltage and to supply energy by a load current to a load represented by a load resistor 30 in fig. 3. During this time, the diode 43 prevents the capacitor 44 from discharging through the switch 41. The switch 41 is opened again fast enough to prevent the capacitor 44 from discharging entirely.

Thus, a basic principle of a boost converter consists of two distinct states. In an ON-state, the switch 41 is closed, resulting in an increase in the inductor current through inductor 7. In an OFF-state, the switch 41 is open and the only path offered to the inductor current is through the diode 43, the capacitor 44 and the load 30. This results in transferring magnetic energy accumulated during the ON-state into the capacitor 44, in particular into its electric field. When a boost converter operates in continuous conduction mode, the current through the inductor 7 never decreases to zero.

The power factor correction circuit 6 according to the embodiment comprises a secondary winding L _se c of the inductor 7 magnetically coupled with a winding of the inductor 7 as primary winding.

The primary winding and the secondary winding L _se c of the inductor 7 act as a transformer for electrical isolation, for example double insulation, reinforced insulation or protective screening of all circuits other than SELV and PELV (protected extra low voltage) , from all circuits that might carry higher voltages of the converter device 1 and from earth ground potential 40 of the converter device 1. The circuits carrying higher voltages, in particular mains voltages such as 230 VAC, of the converter device 1 are in particular the EMI circuit 22, the mains supply interface 21 or a mains rectifier circuit not explicitly shown in figs. 1 and 2. The secondary winding L _se c of the inductor 7 is connected to the rectifier circuit 8. The inductor 7 therefore contributes to forming the isolation barrier 27. The ground potential 9 of the rectifier circuit 8 is not electrically conductive connected with earth ground potential 40. The secondary winding L _se c is connected via the rectifier circuit 8 to the battery charging circuit 34. In the following description a specific embodiment of the rectifier circuit 8 is provided, however other circuit topologies are also possible without departing from the invention. The rectifier circuit 8 shown in fig. 3 comprises a first diode 46 and a second diode 47. The first diode 46 and the second diode 47 are connected in an antiparallel arrangement, wherein an anode of the first diode 46 is connected to a fist output of the secondary winding Lsec and a cathode of the first diode 46 is connected to an output of the rectifier circuit 8. A cathode of the second diode 47 is connected to the first output of the secondary winding L _se c and an anode of the second diode 47 is connected to ground potential 9.

The rectifier circuit 8 further includes a first capacitor 49 and a second capacitor 48 in a serial arrangement. The first capacitor 49 connects the output 53 of the rectifier circuit 8 to a second output of the secondary winding L _se c and the second capacitor 48 connects the second output of the secondary winding L _se c to the ground potential 9. The rectifier circuit 8 depicted in fig. 3 further includes a third capacitor 50 arranged between the output 53 of the rectifier circuit 8 and ground potential 9. Thus the third capacitor 50 is in parallel to the serially arranged first capacitor 49 and second capacitor 48. A voltage over the third capacitor 50 corresponds to a sum of voltages over the first capacitor 49 and the second capacitor 48.

In fig. 3, a load resistor 52 represents a load 51 to the rectifier circuit 8. The load 51 may correspond to the charging circuit 34 of the converter device 1.

The circuit diagram of fig. 3 including a power factor correction circuit 6 with a secondary winding L _se c according to the invention and followed by a rectifier circuit 8 is simulated to show the feasibility of the inventive concept. The rectifier circuit 8 with first and second diodes 46, 47 and parallel thereto the first and second capacitors 48, 49 adds the voltages from the forward conduction stage and the fly-back stage of the boost converter acting as power factor correction circuit 6 coupled via the secondary winding L _se c to the rectifier circuit 8. Accordingly, an almost constant voltage is generated over the third capacitor 50 corresponding to the output voltage of the rectifier circuit 8 between output terminal 53 and ground potential 9. The output voltage of the rectifier circuit 8 reflects the controlled output voltage of the boost power factor correction circuit 6 over the load 52.

In case the LED 2 fails, for example when a short circuit occurs at the output of the boost power factor correction circuit 6, in particular only a remaining resistance value of about 1 to 2 Ώ will apply in case of resistor 30 in fig. 3. Given this case, the capacitor 44 will not store energy in its electrical field, meaning that loading the capacitor 44 does not occur. In this case, the boost power factor correction circuit 6 will perform similar to a flyback converter and transfer energy via the first winding and the secondary winding Lsec of the inductor 7 to the rectifier circuit 8. Capacitor 44 and resistor 30 operate now as a snubber circuit, in particular suppressing a phenomenon such as voltage transients.

Fig. 4 provides a time diagram of the input current in a case, in which no secondary winding L _se c is present on the inductor 7. The input current corresponds to the input current of a known boost converter used as a power factor correction circuit, for example .

Contrary thereto, Fig. 5 provides a time diagram of the input current according to the embodiment using the circuit design of fig. 3 and when exclusively the load 51 via the secondary winding L _se c is connected. The inductor 7 includes the secondary winding L _se c and the additional winding L _se c effects some distortions in the forward conduction stage when loading the first capacitor 49. These additional distortions become evident when comparing figs. 4 and 5 for the times ranging from about 11 to 14 ms and 21 to 24 ms .

Fig. 6 provides a time diagram of voltages over capacitors in the rectifier circuit 8 according to an embodiment using the circuit design of fig. 3. The voltage curve 55 over the second capacitor 48 varies considerably over time.

The voltage curve 56 over the third capacitor 50 corresponds to the output voltage of the rectifier circuit 8 provided at the output of the rectifier circuit 8 for driving a current to the load 51. The voltage curve shows only small variations (voltage ripple) over time. The voltage curve 56 provides a suitable input to the battery charging circuit 34.

A voltage curve over the first capacitor 49 is not shown in fig. 6, however, the voltage curve over first capacitor also varies considerably over time. The voltage curve 56 over the third capacitor 50 corresponds to a sum of the voltage curve 55 over the second capacitor 48 and the omitted voltage curve over the first capacitor 49. The converter device 1 according to the embodiment shows the advantages of the inventive approach which are achieved at acceptable drawbacks. In particular, the converter device 1 provides energy to the battery charging circuit 34 without incurring additional cost of the separate third flyback converter 38 and its associated cicuitry. The converter device 1 according to the embodiment provides this energy over an isolated rail additionally maintaining advantageous SELV characteristics. The drawback of a slightly more complex power factor correction stage 6 also implementing an isolation barrier proved acceptable and the voltages and currents at the input of the power factor correction circuit 6 as well as at the input of the battery charging circuit were simulated to have acceptable characteristics.