Two-stage converter circuit for low-cost isolated driver circuits

Abstract

The invention relates to a two-stage converter circuit comprising a first stage that is configured to provide power factor correction and a second stage that is configured to provide electric isolation and fixed voltage-to-voltage conversion. The converter circuit is configured to fulfil input requirements (e.g., total harmonic distortion (THD), power factor (PF) etc.) and apply isolation by means of a self-oscillating converter. Thereby efficiency can be improved and costs reduced for LED drivers or other driver circuits that require an isolated output.

Description

[0001] TWO-STAGE CONVERTER CIRCUIT FOR LOW-COST ISOLATED DRIVER CIRCUITS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of power supplies for lighting systems, such as - but not limited to - solid-state lighting systems or other loads, for use in various different applications for home, office, retail, hospitality and industry.

[0004] BACKGROUND OF THE INVENTION

[0005] A luminaire can be any type of lighting unit or lighting fixture which comprises one or more light sources (e.g., visible or non-visible (infrared (IR) or ultraviolet (UV)) light sources) for illumination and / or communication purposes and optionally other internal and / or external parts necessary for proper operation of the lighting, e.g., to distribute the light, to position and protect the light sources and ballast (where applicable), and to connect the luminaires to a power supply.

[0006] Solid-state lighting (SSL) is a type of lighting that uses semiconductor lightemitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas. Solid state electroluminescence is used in SSL, as opposed to incandescent bulbs (which use thermal radiation) or fluorescent tubes. Compared to incandescent lighting, SSL creates visible light with reduced heat generation and less energy dissipation.

[0007] A driver circuit is required to deliver a highly stable constant load current to the luminaire(s) irrespective of variations in the luminaire characteristics or the supply voltage while complying with increasingly stringent regulations covering input power requirements such as power factor and total harmonic distortion.

[0008] Switch mode power supplies (SMPS) with isolated output requirements such as luminaires, mobile phone charger, laptop chargers etc. have been dominated mostly by flyback topology for cost reasons. The fly-back topology uses a fly-back transformer that is configured to transfer power from the input side to the output side only when a primary-side switch is open and its current flow is zero or close to it. When the switch is closed, the primary side of the transformer is directly connected to an input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so that a secondary-side diode is reverse-biased (i.e., blocked). When the switch is opened, the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode and allowing current to flow on the secondary side. The energy from the transformer core recharges a secondaryside capacitor and supplies the load.

[0009] Evolutionary improvements have increased the efficiency of the fly-back topology by the addition of synchronous rectifiers at the output and / or hybrid fly-back converters.

[0010] However, fly-back converters in lighting applications have more stringent requirements compared to other technologies and industrial applications. Such requirements include the parameters total harmonic distortion (THD) of the input current and power factor (PF). The parameter THD is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency (distortion factor, a closely related term, is sometimes used as a synonym). The parameter PF is an expression of energy efficiency and may be expressed as the ratio of working power (measured in kilowatts (kW)) to apparent power (measured in kilovolt amperes (kVA)). It is usually expressed between 0 and 1 indicating the ratio between working power and apparent power. The lower PF the lower the ratio between working power and apparent power. A lower PF may be unfavorable for users, as it results in high losses in power lines, causes distortion in the mains voltage, etc. Therefore, standards and regulations have set limits for applications, wherein e.g., lower-power lighting products may have a lower PF compared to higher-power lighting products. Alternatively, IT products may have lower PF compared to lighting.

[0011] In addition, the output power of the light source also needs to meet requirements for temporary lighting artifacts (TLD) and stroboscopic visibility measure (SVM). Moreover, although the fly-back topology is appreciated for its versatility, it is less efficient compared to buck or boost converter designs, as energy related to the output power needs to be stored in the fly-back transformer. For isolated fly-back designs, additional losses are introduced by leakage inductance of the fly-back transformer.

[0012] SUMMARY OF THE INVENTION It is an object of the present invention to provide a converter circuit with improved efficiency and compact design compared to converters with fly-back topology.

[0013] This object is achieved by a converter circuit as claimed in claim 1, by a driver circuit as claimed in claim 11, by a luminaire as claimed in claim 12, and by a lighting system as claimed in claim 13.

[0014] According to a first aspect, a converter circuit is provided for generating an output current of a driver, the converter circuit comprising:

[0015] a first converter stage configured to provide power factor correction and to provide a regulated voltage; and

[0016] a second converter stage configured to receive the regulated voltage and configured to provide electric or galvanic isolation towards a driver output channel that supplies the output current, wherein the second converter stage does not provide any control function for controlling the output current, wherein the second converter stage (103) is configured to be operated at a fixed frequency and fixed duty cycle for voltage conversion by means of a transformer turns ratio.

[0017] Furthermore, according to a second aspect, a driver circuit comprising a converter circuit of the first aspect is provided.

[0018] Moreover, according to a third aspect, a luminaire comprising a driver circuit of the second aspect is provided.

[0019] Additionally, according to a fourth aspect, a lighting system comprising one or more driver circuits of the second aspect for driving respective luminaires is provided.

[0020] Accordingly, isolation of converter circuits can be realized more effectively through non-fly-back topologies such as self-oscillating or forced-frequency half-bridge or full-bridge converters. The proposed combination of a non-isolating first converter stage with power factor correction, followed by an electrically or galvanically isolating second converter stage realized e.g. by a self-oscillation bridge converter topology provides higher efficiency, compacter designs and lower cost compared to boost converters with fly-back topology.

[0021] More specifically, the proposed topology achieves lower losses and compacter designs compared to fly-back converters, and lower costs compared to dual-stage converters with isolated PFC boost converter and fly-back converter or PFC boost converter and LLC converter.

[0022] According to a first option of any of the first to fourth aspects, the first converter stage may comprise a buck converter, a boost converter, or a buck-boost converter. According to a second option that may be combined with the first option or any one of the first to fourth aspects, the second converter stage may comprise a bridge converter, in particular self-oscillating half-bridge or full-bridge converter.

[0023] The proposed (power) converter may thus comprise a PFC buck, boost or buck-boost converter followed by a fixed-control, non-resonant tank (such as resonant converters that apply variable frequency, of which the power is partially reactive in order to change the input to output voltage ratio by changing the oscillation frequency) half-bridge isolating circuit that performs only a fixed voltage-to-voltage conversion through a high frequency transformer independently from its operating frequency. Thus, the second converter stage may apply a voltage-to-voltage conversion with a fixed ratio for a very wide frequency range.

[0024] Additionally, a slow output current control loop may be provided e.g. by a headroom control function, and a fast current control loop may be provided by a direct output current control function.

[0025] According to a third option which may be combined with the first or second option or any one of the first to fourth aspects, the second converter stage may be configured to be operated at a fixed frequency and fixed duty cycle for voltage conversion e.g. by means of a transformer turns ratio.

[0026] According to a fourth option which may be combined with any one of the first to third options or any one of the first to fourth aspects, the converter circuit may be configured to control the output current by at least one of a current limiter at the output of the second converter stage, a headroom voltage control circuit at the driver output channel, and a control function at the output of the first converter circuit.

[0027] In particular, an optional ripple cancelling circuit may be provided at the output or at an intermediate bus voltage of the proposed two-stage converter circuit.

[0028] According to a fifth option which may be combined with any one of the first to fourth options or any one of the first to fourth aspects, a controller of the first converter stage may be configured to receive a feedback control input from the driver output channel or the second conversion stage and to use the feedback control input to control an application or a generation of a switching / modulation control signal for controlling the output current of the driver output channel.

[0029] According to a sixth option which may be combined with any one of the first to fifth options or any one of the first to fourth aspects, a controller may be configured to measure a current headroom voltage at a linear current source of the output channel via a measurement signal and to control a minimum head room voltage of the linear current source within a predetermined time period by outputting a feedback control signal to the first converter stage through a galvanically isolating coupling device in order to control the output voltage of the first converter stage to gradually adapt the measured headroom voltage to the minimum headroom voltage.

[0030] According to a seventh option which may be combined with any one of the first to fifth options or any one of the first to fourth aspects, a controller may be configured to measure a voltage at a return path of the second converter stage via a measurement signal and to control a minimum voltage at the return path within a predetermined time period by outputting a feedback control signal based on the measurement signal through a coupling device in order to control the output voltage of the first converter stage to gradually adapt the measured return path voltage to the minimum voltage.

[0031] According to an eighth option which may be combined with any one of the first to seventh options or any one of the first to fourth aspects, the second converter stage may comprise a pulse / power transformer or other isolating coupling element for achieving the electric or galvanic isolation.

[0032] According to a ninth option which may be combined with any one of the first to third options or any one of the first to fourth aspects, the converter circuit may be configured to control the output current by a third converter stage.

[0033] It is noted that the above converter circuit may be implemented based on discrete hardware circuitries with discrete hardware components, integrated circuits, or arrangements of integrated modules, or based on signal processing devices or integrated circuits controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet.

[0034] It shall be understood that the converter circuit of claim 1, the driver circuit of claim 11, the luminaire of claim 12, and the lighting system of claim 13 may have similar and / or identical preferred embodiments, in particular, as defined in the dependent claims.

[0035] It shall further be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

[0036] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS In the following drawings:

[0038] Fig. 1 shows schematically a block diagram of a two-stage converter according to various embodiments;

[0039] Fig. 2 shows schematically exemplary waveforms of an input voltage and respective input currents of a buck converter;

[0040] Fig. 3 shows schematically exemplary waveforms of a fluctuating bus voltage and a resulting fluctuating LED output current;

[0041] Fig. 4 shows schematically a circuit diagram of a two-stage converter according to a first embodiment with headroom control at the output of the second stage; and Fig. 5 shows schematically a circuit diagram of a two-stage converter according to a second embodiment with ripple cancellation at the output of the first stage.

[0042] DETAILED DESCRIPTION OF EMBODIMENTS

[0043] Various embodiments of the present invention are now described, which are applicable to luminaires of a solid-state lighting system, such as semiconductor LEDs, semiconductor lasers, vertical -cavity surface emitting lasers (VCSELs), organic lightemitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination or light sources in visible or non-visible light spectra.

[0044] More specifically, the following embodiments are directed to LED luminaires. They can be implemented in connection with any type of LED module or board and are applicable to various kinds of drivers or converters of luminaires or other types of loads (e.g., machines, motors, heating elements or the like) that may suffer from switching distortions.

[0045] Fig. 1 shows schematically a circuit diagram of a two-stage converter for an LED driver according to various embodiments.

[0046] It is noted that - throughout the present disclosure - the structure and / or function of blocks or circuit components with identical reference numbers that have been described before are not described again, unless an additional specific functionality is involved. Moreover, only those structural elements and functions are shown, which are useful to understand the embodiments. Other structural elements and functions are omitted for brevity reasons.

[0047] The converter circuit of Fig. 1 comprises a first converter stage (SI) 102 for providing at least a power factor correction (PFC) and a second converter stage (S2) 103 for providing galvanic / electric isolation only towards an output channel 201 that supplies a controlled load current to an output load (e.g., LED string) 104. Without the consideration of energy storage components, the first converter stage 102 is configured to meet PF requirements and outputs a sinusoidal output power with double the mains frequency e.g., P

[0048]

[0049] outtf)=^Vmains^Zl mains sin2(27r / t) . Furthermore, the first converter stage 102 provides a regulated voltage to its output 106. This regulated voltage is provided to the second converter stage 103.

[0050] In an embodiment, the first converter stage 102 may comprise a buck converter or step-down converter which is configured to decrease voltage (while increasing average current) from its input at supply terminals 1001 and 1002 to its output 106. It is a type of switched-mode power supply (SMPS) and its name derives from the use of an inductor that “bucks” or opposes the supply voltage. Switching converters (such as buck converters) provide much greater power efficiency (e.g., up to more than 90%) than linear regulators for higher voltage differences between input and output voltage, which are simpler circuits that dissipate power as heat, but do not step up the output current.

[0051] Buck converters typically contain at least two semiconductors (a diode and a transistor, although modem buck converters frequently replace the diode with a second transistor used for synchronous rectification) and at least one energy storage element (a capacitor, inductor, or the two in combination). To reduce voltage ripple, filters made of capacitors (sometimes in combination with inductors) may be added to such a converter's output (load-side filter) and input (supply-side filter).

[0052] In an embodiment, the second converter stage 103 may only be used for galvanic isolation and voltage conversion and may be operated at a fixed frequency and fixed duty cycle without any output regulation or control function.

[0053] As explained in more detail in the below embodiments, the load current may be controlled in various ways (not shown in Fig. 1), e.g., at the output of the second converter stage 103 (the load current may for example be controlled by a current limiter, and / or a headroom voltage control may for example be introduced by a bottom control circuit) and / or at the output of the first converter stage 102 (for example by a ripple cancellation or the like). To achieve this, a corresponding feedback control input 1004 (e.g., parallel or serial digital or analog input) may be provided to a controller (not shown) of the first converter stage 103.

[0054] The controller of the first converter stage 102 may be configured to use the feedback control input to control application or generation of a switching / modulation control signal (not shown in Fig. 1) for controlling the load current of the output channel 201.

[0055] As an example, the controller of the first converter stage 102 may supply control signals (e.g., switching parameters) to an optional drive circuit (not shown in Fig. 1) that generates and supplies a control signal for a controllable switch or modulator to thereby control the load current of the output channel 201. To this end, the optional drive circuit may comprise control terminals for allowing the controller of the first converter stage 102 to apply the control signals for controlling switching / modulation functions of the load current.

[0056] The first converter stage 102 may be supplied with power via the supply terminals 1001 and 1002. More specifically, a power supply AC voltage (e.g., a power grid voltage of 110 or 220V at a mains frequency of 50 or 60Hz) may be supplied to an electromagnetic interference (EMI) filter (not shown) which is an electronic device that attenuates electromagnetic interference caused by any of the electronics circuits of the first and second converter stages 102 and 103 to enter the mains terminals and result in EMI to other devices. Such EMI can pass through conduction e.g., mains terminals or through radio frequencies.

[0057] The first converter stage 102 thus serves to convert the input AC voltage into a DC voltage while meeting power intake requirements such as power factor (PF) and total harmonic distortion (THD).

[0058] A constant voltage with reduced ripple at the output 106 of the first converter stage 102 may then be converted e.g. by or at the second converter stage 103 into a ripple-free (mains-ripple-free) constant load current that is supplied via the output channel to the output load 104.

[0059] In an example, the drive train of the proposed two-stage converter circuit may comprise a high-PF (HPF) and low-THD buck converter in the first converter stage 102 and a half-bridge converter in the second converter stage 103. The principle of operation is similar to a HPF buck converter, i.e., a wide input range of mains volage and a wide output range of current can be provided, wherein the wide voltage range is covered by the buck converter of the first converter stage 102. A characteristic of the buck converter in the first converter stage 102 is that the parameters THD and PF are affected by the input current wave form, as shown in Fig. 2 below.

[0060] The second converter stage 102 may comprise a self-oscillating converter (e.g., half-bridge converter) that is configured to utilize positive feedback to create an oscillating mode of operation. An advantage of self-oscillating converters is their low cost. A fly-back converter can be modified into a self-oscillating converter by using a half-bridge or full-bridge circuit. The self-oscillating half-bridge converter is a type of DC-DC converter that uses a self-excitation mechanism to generate its output voltage. The self-oscillating structure ensures automatic turn-on and turn-off of transistors without any exterior control. Alternatively, the self-oscillating converter may also be supplied by a fixed frequency and fixed duty cycle.

[0061] Thus, according to various embodiments, the first converter stage 102 is configured to fulfil predetermined input requirements (e.g., THD and PF) and the second converter stage 103 is configured to apply isolation, e.g., by means of a self-oscillating (halfbridge) converter. Thereby efficiency can be improved, and cost reduced for LED drivers or other drivers that require isolated output.

[0062] Fig. 2 shows schematically exemplary waveforms of an input voltage v(t) and respective first and second input currents

[0063]

[0064] ii(t) and of a buck converter during a half period (Tmams / 2') of the mains voltage.

[0065] Generally speaking for buck converters, PF decreases with increasing ratio of output voltage to input voltage (Vout / Vin) of the buck converter.

[0066] Furthermore, higher harmonics with respect to the fundamental input voltage waveform of the input current deteriorate the THD of the LED driver. For comparison, the first input current ii(t) of Fig. 2 is more distorted (e.g., due to limitation effects at times h and L where the input voltage exceeds a bus voltage Vbus of the buck converter) and would therefore have a higher THD than the second input current i2(t).

[0067] A power converter that meets PF and THD requirements may however result in a fluctuating supply power at the output (e.g., bus voltage) of the first converter stage 102 with the buck converter and may thus also generate undesirable fluctuation in the light output (commonly known as stroboscopic light effects).

[0068] Fig. 3 shows schematically exemplary waveforms of a fluctuating bus voltage Vbus(t) and a resulting fluctuating LED output current iiFp(t) over one period of the means

[0069]

[0070] voltage.

[0071] In the example of Fig. 3, the fluctuating bus volage Vbus(t) will cause a fluctuating load current through the LED load 104 and thereby a fluctuating light output. To overcome this problem, large energy storage devices (e.g., buffer capacitors) may be added to the driver circuit in combination with a second power conversion stage.

[0072] Compared to conventional two-stage LED drivers, where the first converter stage is designed to meet the input power requirements and the second converter stage is designed to output a constant power, the proposed two-stage converter circuit comprises the first converter stage 102 (e.g., buck converter) with high PF and the second converter stage 103 (e.g., with self-oscillating half-bridge converter) with a limitation in that it is only designed for electric / galvanic isolation and has no means to cancel out signal variations (e.g., ripple). The second converter stage 103 is only intended to apply electric / galvanic isolation. Ripple cancellation can be realized by other effective measures such as mini converter or linear current source.

[0073] Fig. 4 shows schematically a circuit diagram of a two-stage converter according to a first embodiment with headroom control at the output of the second converter stage 103.

[0074] The first converter stage 102 comprises a high-power-factor (HPF) buck converter and the second converter stage 103 comprises a self-oscillating half-bridge converter with output headroom control feedback.

[0075] Power from a mains grid is supplied to the buck converter of the first converter stage 102 through the input terminals 1001 (Line) and 1002 (Neutral) via a bridge rectifier 101 and a parallel smoothing capacitor. The output of the buck converter of the first converter stage 102 supplies the half-bridge converter of the second converter stage 103 with a DC voltage, wherein a primary and secondary winding ratio of a transformer of the halfbridge converter can be optimized for PF, THD and efficiency.

[0076] As mentioned above, a voltage ripple at the output of the buck converter of the first converter stage 102 may be present (e.g., in the same or a similar proportion) at the output of the half-bridge converter of the second converter stage 103. This voltage ripple can be smoothened by a relatively large output capacitor of the half-bridge converter and / or (further) smoothened / cancelled by a linear current source 105 connected in series with the LED load 104.

[0077] The buck converter of the first converter stage 102 comprises of a controlled switch (e.g., MOSFET in Figs. 4 and 5), diodes, capacitors (including the parallel smoothing capacitor at the input side), inductors or transformer windings and a controlled driving circuitry with a controller. The switch controls the flow of input power towards the output by turning on and off periodically. The on-time of the switch is known as duty cycle. Optionally, a transformer may be provided for stepping-up or stepping-down the AC voltage generated by the controlled switch and finally a rectifier for converting back into the required DC output voltage.

[0078] The working operation of the buck converter can be explained in two modes: a first mode where the switch is turned by the controller and a second mode where the switch is turned off by the controller. By turning on the switch, the diode at the switch will become reverse biased to the applied input. Therefore, all the input current will flow through inductor at the switch. Hence the DC input current flowing in the buck converter is equal to the inductor current and the inductor will charge during turn on-time of the switch. After turning off the switch, the first mode changes to the second mode where the polarity of the inductor reverses and it starts acting as a source. The change of the polarity of the inductor leads to a forward bias of the diode. The anode voltage of the diode become more positive than cathode during this period and hence starts conducting. In the second mode, the current flows due to the stored energy in the inductor. Therefore, the current flows in the circuit until the inductor discharges.

[0079] The half-bridge converter of the second converter stage 103 comprises a gate driver circuit (e.g., an integrated PWM generator) which may operate at logic levels depending on the gate drive characteristics of two switching elements (e.g., MOSFETs in Figs. 4 and 5) which are configured as a half bridge. These two switching elements may have an integrated current sense pin so that the driver circuit may be adjusted in case of any output fluctuations.

[0080] Oscillation of the half-bridge converter may be achieved by an oscillator function provided in the driver circuit, wherein the conversion frequency may be set by suitably selecting externally connected elements (e.g., a resistor and a capacitor) of the driver circuits (forced-frequency converter).

[0081] Additionally, a pulse / power transformer or other isolating coupling element (e.g., optocoupler) is provided for achieving the desired isolation function of the second converter stage 103. The transformer may be configured with a tapped secondary winding structure to provide a symmetrical full-wave rectifying structure via rectifying elements (e.g., diodes in Figs. 4 and 5) on the output side to force the output current of both half waves to flow in the same direction.

[0082] Furthermore, the second converter stage 103 may be operated at a fixed frequency and fixed duty cycle for voltage conversion by suitably selecting a turns ratio of the pulse / power transformer.

[0083] Alternatively, a full-bridge converter structure with four switching elements (e.g., MOSFETs) arranged as a full bridge with diagonal output may be provided on the primary side of the transformer.

[0084] However, note that in both half-bridge and full-bridge topologies, the switching elements may be arranged on the secondary output side of the second converter stage 103.

[0085] Depending on the application, a pulse width modulation (PWM) or amplitude modulation (AM) or other modulation control signal 1005 may be applied e.g. by the controller of the buck converter of the first converter stage 102 to the load current 1006 via an input terminal of a control circuit of the linear current source 105 and used to generate a drive signal for a transistor (e.g., metal oxide semiconductor field effect transistor (MOSFET) that controls the load current 1006. The load current 1006 is measured via a voltage across a measurement resistor and supplied to the control circuit of the linear current source 105.

[0086] To maximize efficiency, a controller 106 may be provided, that is configured to measure a current headroom voltage at the linear current source 105 via a measurement signal 1003 and to control a minimum head room voltage of the linear current source 105 within a predetermined time period (e.g., a full mains period). The controller 106 may then output the feedback control signal 1004 to a controller of the buck converter of the first converter stage 102 through an optocoupler 107 (or other galvanically separating coupling device) in order to control the output voltage of the buck converter to gradually adapt the measured headroom voltage (indicated by the measurement signal 1003) to the targeted minimum headroom voltage in order to minimize power losses.

[0087] For the specific two-stage drive train (i.e., buck converter with half-bridge converter), it may be beneficial from an efficiency point of view to apply ripple control / cancellation at the output of the buck converter (first converter stage 102), as this output voltage is higher compared to the output volage of the half-bridge converter of the second stage 103, such that sensing and controlling can be performed more efficiently.

[0088] Fig. 5 shows schematically a circuit diagram of a two-stage converter according to a second embodiment with ripple cancellation at the output of the first converter stage 102.

[0089] In the second embodiment, the load current through the LED load 104 is controlled at the first controller stage 102 on the primary side of the two-stage converter circuit by sensing the load current 1006 at a return path of the self-oscillating half-bridge converter of the second converter stage 103 via a measurement resistor at the output of the first converter stage 102. Alternatively, the load current may be directly measured at the secondary side of the two-stage controller circuit and then error is fed back to the controller at the primary side via an additional optocoupler (not shown).

[0090] To maximize efficiency, a controller 106 of the second embodiment may be configured to measure a voltage at the return path of the half-bridge converter circuit of the second converter stage 103 via a measurement signal 1003 and to control a minimum voltage at the return path within a predetermined time period (e.g., a full mains period). The controller 106 may then generate and output the feedback control signal 1004 based on the measurement signal 1003, the pulse width modulation (PWM) or amplitude modulation (AM) or other modulation control signal 1005 (received e.g. from the controller of the buck converter of the first converter stage 102) and the measured load current 1006, through an optocoupler 107 (or other level-shifting or functional isolation separating coupling device) in order to control the output voltage of the buck converter to gradually adapt the measured return path voltage (indicated by the measurement signal 1003) to the targeted minimum voltage in order to minimize power losses.

[0091] Alternatively, in the above first and second embodiments, the converter topology of the first converter stage 102 may be changed from a buck converter topology to a boost converter topology or a buck-boost converter topology e.g. by suitably rearranging the locations and connections of the respective inductor, diode and capacitor at the output of the switching MOSFET.

[0092] Furthermore, in the above embodiments, a third converter stage may be provided, that is configured to control the output current of the second converter stage 103. The third converter stage may be configured to control the output current through the load 104 to maintain a constant output current regardless of changes to the input voltage and output resistance. As a result, a change in the output resistance causes the output voltage to adjust as the load resistance varies. The higher the output resistance, the greater the output voltage. In LED drivers or batteries / supercapacitor chargers, the third converter stage may be configured to control the output current for a range of LED strings or battery cells. Should the resulting load voltage increase beyond a certain level, the voltage may be controlled or “clamped.”

[0093] In an example, a slow output current control loop may be provided e.g. by a headroom control function and / or a fast current control loop may be provided by a direct output current control function.

[0094] To summarize, a two-stage converter circuit has been described, that comprises a first stage configured to provide power factor correction and a second stage configured to provide electric isolation and fixed factor voltage division of multiplication with respect to input voltage. The converter circuit is configured to fulfil input requirements (e.g., total harmonic distortion (THD), power factor (PF) etc.) and apply isolation by means of a self-oscillating (half-bridge) converter. Thereby efficiency can be improved and costs reduced for LED drivers or other driver circuits that require an isolated output. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments concerning solid-state luminaires (e.g., LED luminaires). The proposed embodiments can be applied in connection with any type of loads, any types of DC / DC converters of the first converter stage and any type of isolating self-oscillating converters of the second converter stage.

[0095] More specifically, elements and components mentioned in the above embodiments may be embedded in one or more integrated circuits (ICs), e.g., application specific ICs (ASICs) or programmable logic arrays (PLAs) or the like.

[0096] The above embodiments may be implemented in (e.g., integrated or combined with) various high-efficiency products such as office luminaires, outdoor lighting, LED strips, color-tuneable spots or the like.

[0097] In the provided examples, the transformer preferably has no or very little energy storage, and therefore a very low leakage inductance, compared to a transformer commonly used in e.g. a flyback converter. The coupling between the windings of the transformer can be close to unity. This allows a compact and more efficient power conversion with galvanic isolation.

[0098] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in the text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated. A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0099] The described procedures of the controller circuit 200 can be implemented as program code means of a computer program and / or as dedicated hardware of the receiver devices or transceiver devices, respectively. The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:

1. A converter circuit for generating an output current of a driver, the converter circuit comprising:a first converter stage (102) configured to provide power factor correction and to provide a regulated voltage; anda second converter stage (103), configured to receive the regulated voltage and configured to provide electric or galvanic isolation towards a driver output channel (201) that supplies the output current, wherein the second converter stage (103) does not provide any control function for controlling the output current, wherein the second converter stage (103) is configured to be operated at a fixed frequency and fixed duty cycle for voltage conversion by means of a transformer turns ratio.

2. The converter circuit of claim 1, wherein the first converter stage 102 comprises a buck converter, a boost converter or a buck-boost converter.

3. The converter circuit of claim 1 or 2, wherein the second converter stage (103) comprises a bridge converter, in particular self-oscillating half-bridge or full-bridge converter.

4. The converter circuit of any one of the preceding claims, wherein the converter circuit is configured to control the output current by at least one of a current limiter at the output of the second converter stage (103), a headroom voltage control circuit at the driver output channel (201), and a control function at the output of the first converter circuit (102).

5. The converter circuit of any one of the preceding claims, further comprising a controller of the first converter stage (103), wherein the controller is configured to receive a feedback control input (1004) from the driver output channel or the second conversion stage (103) and to use the feedback control input (1004) to control an application or a generation ofa switching / modulation control signal for controlling the output current of the driver output channel (201).

6. The converter circuit of any one of the preceding claims, further comprising a controller (106) that is configured to measure a current headroom voltage at a linear current source (105) of the output channel (201) via a measurement signal (1003) and to control a minimum headroom voltage of the linear current source (105) within a predetermined time period by outputting a feedback control signal (1004) to the first converter stage (102) through a galvanically isolating coupling device in order to control the output voltage of the first converter stage (102) to gradually adapt the measured headroom voltage to the minimum headroom voltage.

7. The converter circuit of any one of claims 1 to 5, further comprising a controller (106) that is configured to measure a voltage at a return path of the second converter stage (103) via a measurement signal (1003) and to control a minimum voltage at the return path within a predetermined time period by outputting a feedback control signal (1004) based on the measurement signal (1003) through a coupling device (107) in order to control the output voltage of the first converter stage (102) to gradually adapt the measured return path voltage to the minimum voltage.

8. The converter circuit of any one of the preceding claims, wherein the second converter stage (103) comprises a pulse / power transformer or other isolating coupling element for achieving the electric or galvanic isolation.

9. The converter circuit of any one of the preceding claims , wherein the converter circuit is configured to control the output current by a third converter stage.

10. A driver circuit comprising a converter circuit according to any one of the preceding claims.

11. A luminaire comprising a driver circuit of claim 10.

12. A lighting system comprising one or more driver circuits of claim 10 for driving respective luminaires.

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