WO2009095890A2

WO2009095890A2 - Switched-mode power supply

Info

Publication number: WO2009095890A2
Application number: PCT/IB2009/050387
Authority: WO
Inventors: Paul Joseph Jungwirth; William Peter Coetzee
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-02-01
Filing date: 2009-01-30
Publication date: 2009-08-06
Also published as: TW201004119A; WO2009095890A3

Abstract

Disclosed herein are a switched-mode power supply and a method therefor. The switched-mode power supply has an input and an output and includes an input voltage conditioner, a transformer, a switching control circuit, an output voltage conditioner and a feedback voltage conditioner, which are operatively coupled. In addition, the switched mode power supply includes a start-up circuit operatively coupled to the input voltage conditioner and the switching control circuit, wherein the start-up circuit is configured to substantially provide electrical power to the switching control circuit during start-up and until the feedback voltage conditioner provides a predetermined voltage to the switching control circuit.

Description

SWITCHED-MODE POWER SUPPLY

Technical Field

[0001] The present invention is directed generally to power supplies. More particularly, various inventive methods and apparatus disclosed herein relate to switched mode power supplies.

Background

[0002] Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Patent Nos. 6,016,038 and 6,211,626.

[0003] Switched-mode power supplies (SMPS) are commonly used in electronic systems to provide adequate voltage by converting one form of electricity into another. They are generally used because of their relatively low cost. SMPS employ certain types of transformers that can operate efficiently at high frequencies and therefore be made smaller and lighter than those conventionally used to transform electricity at typical line frequencies. Inductors and capacitors are additionally used to condition the output voltage of the transformer.

[0004] A SMPS is based on the principle of repeatedly interrupting the provision of electricity to the primary side of a transformer in accordance with a fed back output of the SMPS. For this purpose, an SMPS includes a pulse generator for generating a switching signal comprising a train of pulses during operation that is used to control the opening and closing of a switching element that is connected in series to the primary side of the transformer. The pulses in the pulse train may be determined according to a number of different schemes but are typically pulse width modulated (PWM). The switching signal determines the duration of applications and interruptions of power to the transformer which determines the average power that is provided to the primary side of the transformer. The average power in turn determines the output voltage at the secondary side of the transformer which is further conditioned and smoothed so the SMPS provides a desired output voltage during operating conditions. Certain conventional SMPS are able to generate high quality DC output voltages with low harmonic content, for example, from low quality forms of AC input voltages. The self-regulation mechanism provided by the feedback control enables the SMPS to maintain a desired output voltage under fluctuating load conditions and within a wide range of input voltages and input frequencies.

[0005] In general, SMPSs include an alternating-current (AC) to direct-current (DC) converter such as a rectifier, for example, that provides DC voltage to a transistor switch that is connected in series to a primary winding of a transformer. SMPSs further include a pulse generator for generating a train of pulses of varying pulse width as a function of the present value of the output voltage. The transistor switch is also connected to the pulse generator and controlled by the pulses generated by the pulse generator such that the switch closes for the duration of each pulse. The pulse generator monitors the output DC voltage generated by the power supply, compares the output voltage at a secondary winding of the transformer to a reference voltage and either expands the pulse width to raise the output voltage or narrows the pulse width to lower the output voltage in order to maintain the output DC voltage within a predetermined range of the reference voltage. The reference voltage can be a certain desired, for example, predetermined voltage. A rectifier and filter circuit, connected to the secondary winding, provides the output DC voltage. Feedback of the output of the SMPS is provided to compensate for certain otherwise occurring variations in the output voltage.

[0006] Generally, energy is stored in the transformer when the transistor switch is ON and then extracted from the transformer by the load primarily when the switch is OFF. The transformer converts the electrical energy into electromagnetic energy which is stored in a magnetic field. The magnetic field drives electrical current through the secondary side of the transformer primarily when the transistor switch is open. The electrical current at the secondary side of the transformer charges an output capacitor as well as feeds power through the output of the SMPS to a connected load. The PWM pulse generator compares the output voltage with the reference voltage to generate pulses of proper pulse width based on the deviation between these voltages. The two standard types of switching power supplies are boost (step-up) and buck (step-down) power supplies. The conventional boost switching power supply is also called a flyback power supply or flyback converter.

[0007] Conventional buck power supplies include forward converter power supplies which operate in a similar manner to flyback power supplies except that an additional inductor on the secondary side of the transformer is used to store energy rather than the transformer alone. For these types of SMPS, when the switching transistor is on and current flows through the primary winding, current is also caused to flow from the secondary winding through a diode rectifier into an inductor and out to the output load. When the transistor switch is OFF, the additional inductor continues to provide electrical current to the load.

[0008] Without proper configuration of the SMPS, the pulse generator may not gain sufficient power to be able to operate the transistor switch during start-up. SMPSs therefore generally require a start-up circuit that can operate the power switch during start-up until the pulse generator can, or provide power to the pulse generator sufficiently rapidly to enable the corresponding feedback loop to establish proper control of the transistor switch and to transition the SMPS into stable operating conditions. Many start-up circuits can also additionally limit otherwise occurring current spikes in the SMPS during start-up.

[0009] A number of start-up circuits are known in the art, but all of them have disadvantages. Start-up circuits for SMPS that maintain the pulse generator electrically insulated from the primary side of the transformer replicate certain circuit components already provided by the pulse generator and are typically more complex. As many applications require the SMPS to isolate the load from the input of the SMPS, transformers with an auxiliary winding on the secondary side are often employed in order to provide feedback of the output to the pulse generator without having to establish an electrical connection between devices that are electrically connected to the secondary winding and devices that are connected to the primary side of the transformer. Other known SMPS configurations employ optoelectronic devices to isolate the secondary side from the primary side of the transformer. In light of their functionality, these start-up circuits are complex.

[0010] A key challenge in SMPS is the adequate operation of the pulse generator during startup. In particular, enabling this adequate operation in a relatively simple, cost-effective and/or energy-efficient manner is desired. Thus, there is a need for a new switched mode power supply start-up circuit that addresses at least some of the deficiencies of existing systems.

Summary

[0011] The present disclosure is directed to inventive methods and apparatus for switched mode power supplies. For example, a switched mode power supply, according to various embodiments of the present invention, enables the operation of a pulse generator during startup of the switched mode power supply in a relatively simple, cost -effective and/or energy - efficient manner.

[0012] Generally, in one aspect of the present invention there is provided a switched mode power supply having an input and an output, the switched mode power supply including: an input voltage conditioner operatively coupled to the input; a transformer having a primary side and a secondary side, the primary side operatively coupled to a switching control circuit , wherein the primary side and the switching control circuit are operatively coupled to the input voltage conditioner; an output voltage conditioner operatively coupled to the output and the secondary side; a feedback voltage conditioner operatively coupled to the secondary side and the switching control circuit; and a start-up circuit operatively coupled to the input voltage conditioner and the switching control circuit, the start-up circuit configured to substantially provide electrical power to the switching control circuit during start-up and until the feedback voltage conditioner provides a predetermined voltage to the switching control circuit.

[0013] In some embodiments, the start-up circuit includes a field effect transistor configured to shut off at the predetermined voltage. In other embodiments, the start-up circuit includes a RC element which is configured to control the gate voltage of the field effect transistor such that it substantially follows variation in the input.

[0014] In one particular embodiment, the start-up circuit includes a RC element configured to define the time constant indicative of the amount of time required to reach the predetermined voltage. The start-up circuit can be operatively coupled to the feedback voltage conditioner.

[0015] In another aspect of the present invention, there is provided a method for start-up of a switched mode power supply having an input and an output and including an input voltage conditioner operatively coupled to the input, a transformer having a primary side and a secondary side, the primary side operatively coupled to a switching control circuit, wherein the primary side and the switching control circuit are operatively coupled to the input voltage conditioner; an output voltage conditioner operatively coupled to the output and the secondary side; and a feedback voltage conditioner operatively coupled to the secondary side and the switching control circuit, the method including the steps of: receiving a first voltage from the input voltage conditioner; receiving a second voltage from the switching control circuit; providing, in response to the first voltage and the second voltage, electrical power to the start-up circuit until the feedback voltage conditioner provides a predetermined voltage to the switching circuit.

[0016] In one embodiment, the method further includes receiving a third voltage from the feedback voltage conditioner and wherein providing electrical power is performed in further response to the third voltage.

[0017] As used herein for purposes of the present disclosure, the term "LED" should be understood to include any electroluminescent diode or other type of carrier injection/junction- based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization.

[0018] For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum "pumps" the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum.

[0019] It should also be understood that the term LED does not limit the physical and/or electrical package type of an LED. For example, as discussed above, an LED may refer to a single light emitting device having multiple dies that are configured to respectively emit different spectra of radiation (e.g., that may or may not be individually controllable). Also, an LED may be associated with a phosphor that is considered as an integral part of the LED (e.g., some types of white LEDs). In general, the term LED may refer to packaged LEDs, non-packaged LEDs, surface mount LEDs, chip-on-board LEDs, T-package mount LEDs, radial package LEDs, power package LEDs, LEDs including some type of encasement and/or optical element (e.g., a diffusing lens), etc.

[0020] The term "light source" should be understood to refer to any one or more of a variety of radiation sources, including, but not limited to, LED-based sources (including one or more LEDs as defined above), incandescent sources (e.g., filament lamps, halogen lamps), fluorescent sources, phosphorescent sources, high-intensity discharge sources (e.g., sodium vapor, mercury vapor, and metal halide lamps), lasers, other types of electroluminescent sources, pyro-luminescent sources (e.g., flames), candle-luminescent sources (e.g., gas mantles, carbon arc radiation sources), photo-luminescent sources (e.g., gaseous discharge sources), cathode luminescent sources using electronic satiation, galvano-luminescent sources, crystallo- luminescent sources, kine-luminescent sources, thermo-luminescent sources, triboluminescent sources, sonoluminescent sources, radioluminescent sources, and luminescent polymers.

[0021] A given light source may be configured to generate electromagnetic radiation within the visible spectrum, outside the visible spectrum, or a combination of both. Hence, the terms "light" and "radiation" are used interchangeably herein. Additionally, a light source may include as an integral component one or more filters (e.g., color filters), lenses, or other optical components. Also, it should be understood that light sources may be configured for a variety of applications, including, but not limited to, indication, display, and/or illumination. An "illumination source" is a light source that is particularly configured to generate radiation having a sufficient intensity to effectively illuminate an interior or exterior space. In this context, "sufficient intensity" refers to sufficient radiant power in the visible spectrum generated in the space or environment (the unit "lumens" often is employed to represent the total light output from a light source in all directions, in terms of radiant power or "luminous flux") to provide ambient illumination (i.e., light that may be perceived indirectly and that may be, for example, reflected off of one or more of a variety of intervening surfaces before being perceived in whole or in part).

[0022] The term "spectrum" should be understood to refer to any one or more frequencies (or wavelengths) of radiation produced by one or more light sources. Accordingly, the term "spectrum" refers to frequencies (or wavelengths) not only in the visible range, but also frequencies (or wavelengths) in the infrared, ultraviolet, and other areas of the overall electromagnetic spectrum. Also, a given spectrum may have a relatively narrow bandwidth (e.g., a FWHM having essentially few frequency or wavelength components) or a relatively wide bandwidth (several frequency or wavelength components having various relative strengths). It should also be appreciated that a given spectrum may be the result of a mixing of two or more other spectra (e.g., mixing radiation respectively emitted from multiple light sources).

[0023] The term "lighting fixture" is used herein to refer to an implementation or arrangement of one or more lighting units in a particular form factor, assembly, or package. The term "lighting unit" is used herein to refer to an apparatus including one or more light sources of same or different types. A given lighting unit may have any one of a variety of mounting arrangements for the light source(s), enclosure/housing arrangements and shapes, and/or electrical and mechanical connection configurations. Additionally, a given lighting unit optionally may be associated with (e.g., include, be coupled to and/or packaged together with) various other components (e.g., control circuitry) relating to the operation of the light source(s). An "LED-based lighting unit" refers to a lighting unit that includes one or more LED- based light sources as discussed above, alone or in combination with other non LED-based light sources. A "multi-channel" lighting unit refers to an LED-based or non LED-based lighting unit that includes at least two light sources configured to respectively generate different spectrums of radiation, wherein each different source spectrum may be referred to as a "channel" of the multi-channel lighting unit.

[0024] The term "controller" is used herein generally to describe various apparatus relating to the operation of one or more light sources. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A "processor" is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs). [0025] In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as "memory," e.g., volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM, floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at least some of the functions discussed herein. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein. The terms "program" or "computer program" are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program one or more processors or controllers.

[0026] In one network implementation, one or more devices coupled to a network may serve as a controller for one or more other devices coupled to the network (e.g., in a master/slave relationship). In another implementation, a networked environment may include one or more dedicated controllers that are configured to control one or more of the devices coupled to the network. Generally, multiple devices coupled to the network each may have access to data that is present on the communications medium or media; however, a given device may be "addressable" in that it is configured to selectively exchange data with (i.e., receive data from and/or transmit data to) the network, based, for example, on one or more particular identifiers (e.g., "addresses") assigned to it.

[0027] It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. Brief Description of the Drawings

[0028] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[0029] FIG. 1 illustrates a block diagram of a SMPS with a start-up circuit according to an embodiment of the present invention.

[0030] FIG. IA illustrates an example input-voltage conditioner for a SMPS according to an embodiment of the present invention.

[0031] FIG. IB illustrates an example feedback-voltage conditioner for a SMPS according to an embodiment of the present invention.

[0032] FIG. 2 illustrates a start-up circuit for an SMPS according to an embodiment of the present invention.

[0033] FIG. 3 illustrates a start-up circuit for an SMPS according to another embodiment of the present invention.

[0034] FIG. 4 illustrates a start-up circuit for an SMPS according to another embodiment of the present invention.

[0035] FIG. 5 illustrates a start-up circuit for an SMPS according to another embodiment of the present invention.

Detailed Description

[0036] A number of start-up circuits are known in the art wherein they typically replicate certain circuit components already provided by the pulse generator of a SMPS and are therefore typically complex. A key challenge in SMPS is the adequate operation of the pulse generator during start-up. In particular, enabling this adequate operation in a relatively simple, cost-effective and/or energy-efficient manner is desired.

[0037] In view of the foregoing, various embodiments and implementations of the present invention are directed to a start-up circuit for a switched-mode power supply (SMPS). The SMPS has an input and an output. The SMPS includes an input voltage conditioner (IVC) operatively coupled to the input, a switching-control circuit (SCC) and a transformer having a primary and a secondary side. The primary side of the transformer is operatively coupled to the SCC. The primary side and the SCC are operatively coupled to the IVC. The SMPS further includes an output voltage conditioner (OVC) operatively coupled to the output and the secondary side, a start-up circuit (SUC) operatively coupled to the IVC and the SCC, and a feedback voltage conditioner (FVC) operatively coupled to the secondary side. The FVC is further operatively coupled to the SCC and optionally to the SUC. The SUC is configured to substantially provide electrical power to the SCC during start-up of the SMPS until the FVC provides a predetermined voltage to the SCC.

[0038] FIG. 1 illustrates a block diagram of a SMPS with a start-up circuit according to one embodiment of the present invention. The SMPS 100 has an input and an output and includes an IVC 110 operatively coupled to the input and a SCC 140. The SMPS further includes a transformer 130 having a primary and a secondary side. The primary side of the transformer is operatively coupled to the SCC 140 via an electrical series connection. Moreover, the primary side and the SCC are operatively coupled to the IVC. The purpose of the IVC includes the preconditioning of the electricity provided at the input into electrical power suitable for supplying to the transformer.

[0039] The SMPS 100 further includes an OVC 160 operatively coupled to the output and the secondary side of the transformer and a SUC 120 operatively coupled to the IVC 110 and the SCC 140. The SMPS further includes a FVC 150 that is operatively coupled to the secondary side of the transformer, the SCC and the SUC. The SUC is configured to provide electrical power to the SCC during start-up of the SMPS until the feedback voltage V_fb provided by FVC 150 on the feedback line 190 to the SCC reaches a predetermined voltage.

[0040] As is illustrated in FIG. 1, the IVC 110 is connected in series to the primary winding of the transformer 130, which in turn is connected in series with the SCC 140. The IVC, the SCC and the transformer can alternatively be connected in a different sequence (not illustrated). For example, the IVC may be connected in series to the SCC and the SCC may be series connected to the primary winding of the transformer. Depending on the configuration of the sequence, the SMPS may include different circuitries (not illustrated) for suppressing high induced voltages and undesired harmonic content that may be caused, for example, by the switching of the primary winding of the transformer.

[0041] According to an embodiment of the present invention, the SMPS 100 includes an optional electronic valve 153 connected between the feedback line 190 and the SCC 140. Further optionally, the FVC 150 may be operatively connected to the OVC 160 via connection 193. The SUC 120 may, also optionally, be operatively connected to the FVC 150 via connection 191 and the feedback voltage line.

[0042] The SMPS 100 may additionally include optional power-factor correction circuitry (not illustrated) in order to mitigate the amount of reactive power drawn by the SMPS. The SMPS may include further circuitry to enhance electromagnetic compatibility, for example, active or passive filters or shielding in order to suppress undesired harmonics from being superimposed back onto the power supply line or being otherwise released into the environment.

Input Voltage Conditioner

[0043] The input voltage conditioner (IVC) 110 preconditions the input voltage and may include a rectifier and filtering components for suppressing harmonic content that, for example, originates from the rectification itself or that may otherwise be superimposed on the voltage V_bus- The IVC may include a number of different components for accomplishing the rectification and filtering function, including, for example, a bridge or line rectifier and one or more capacitors, inductances or resistors or other devices or components as would be readily understood by a worker skilled in this art.

[0044] FIG. Ia illustrates an example IVC including a bridge rectifier 111 with four diodes Dl, D2, D3 and D4 113, and a filter element with an inductance Ll 115 and a capacitor Cl 117 which can form a part of a SMPS according to an embodiment of the present invention.

Transformer

[0045] The transformer for use with a SMPS according an embodiment of the present invention may include, for example, a ferrite core for enabling efficient operation at switching frequencies of between about 20 kilohertz to several megahertz. Other readily known types of transformers, including core-less transformers, may be used. Adequately designed transformers can provide low power loss, small physical size, low weight and little audible vibration. Operating frequencies beyond the audible range can provide quiet operation.

[0046] The transformer has a primary and a secondary side and includes at least a primary and a secondary winding. In one embodiment, a transformer with an auxiliary winding at the secondary side may be used to preserve galvanic isolation between the primary and the secondary winding and to avoid employing other electrically insulating devices, for example, electro-optical devices, in the feedback voltage conditioner of the SMPS. Different embodiments of the present invention, however, may utilize different types of transformers, including transformers with a primary and a secondary winding only.

Output Voltage Conditioner

[0047] The output voltage conditioner (OVC) smoothes, filters and stabilizes the voltage provided by the transformer at the secondary winding and provides a suitable quality output voltage under operating conditions to a load connected to the output of the SMPS. The OVC for use with the SMPS according to one embodiment of the present invention may include a number of different devices including, diodes, capacitors, inductors and optional resistors, or other devices or components as would be readily understood by a worker skilled in this art, for example, for smoothing, filtering and stabilizing the output voltage. Other electronic devices such as integrated regulators, operational amplifiers and/or other parts as would be readily understood by a worker skilled in the art, may be employed in different output voltage conditioners.

Feedback Voltage Conditioner

[0048] According to an embodiment of the present invention and as illustrated in FIG. 1, the feedback voltage conditioner (FVC) is coupled to an auxiliary winding of the transformer. The purpose of the FVC is to provide a feedback voltage V_fb indicative of the output voltage of the SMPS. [0049] Referring to FIG. IB, an exemplary FVC 150 suitable for use with a SMPS according to many embodiments of the present invention, includes a series connection of forward biased diode D3 152, resistor R4 154, polarized capacitor C3 156 and ground. FIG. IB also illustrates an example electronic valve D2 153 and a schematic of transformer 130. As illustrated in FIG. IB, the RC-element including resistor R4 and capacitor C3, is connected to the auxiliary winding of the transformer via diode D3 and filters and smoothes the voltage provided by the auxiliary winding to provide feedback voltage V_fb at the anode of capacitor C3.

[0050] Optionally, in one embodiment, an adequately configured FVC (not shown) may operatively be connected to the output voltage conditioner via connection 193 in order to provide a feedback voltage that more accurately correlates with the output voltage of the SMPS. The connection may be used to provide a signal that directly indicates the instant output voltage, for example, via an optoelectronic device. Such a connection may provide a feedback signal that more accurately follows the output voltage of the SMPS under varying load conditions. The feedback voltage is used by the SCC 140 to determine the train of pulses for switching the transformer and it can be used to power at least parts of the switching-control circuit (SCC) under operating conditions.

[0051] Alternatively, in another embodiment, a transformer with only a primary and a secondary winding may be employed (not illustrated) and an adequately configured FVC (not illustrated) may be coupled to the secondary winding. For galvanic isolation such an FVC may include, for example, further optoelectronic devices. The FVC can be configured so that the feedback voltage provided by it adequately correlates to the output voltage in a linear, proportional or other suitable predetermined way so that the SCC can be calibrated to adequately respond to variations in the feedback voltage in order to maintain a desired output voltage of the SMPS.

Switching-Control Circuit

[0052] The switching-control circuit (SCC) for use with an SMPS according to the present invention includes a switching element 145 and a pulse generator 143 as illustrated in FIG. 1. The pulse generator is used to operate the opening and closing of the switching element. The switching element is connected in series with the primary winding of the transformer in order to be able to interrupt the flow of electrical current and thereby the provision of electrical energy to the primary side of the transformer. The switching element can be a make-or-break contact electronic switch, for example, a power field-effect transistor or other suitable controllable power electronic switch as would be readily understood by a worker skilled in the art.

[0053] The pulse generator 143 provides a switching signal to the switching element 145. The pulse generator can include, for example, a pulse width modulator for generating pulses at fixed intervals with variable pulse width or a pulse code modulator or pulse position modulator for generating fixed length pulses at variable intervals or repetitions or other configurations as would be readily understood by a skilled worker. The switching control circuit is configured to repeatedly open and close the switching element in correspondence with voltage V_cc at the input of the switching control circuit. Depending on the embodiment of the present invention, V_cc may correspond to the feedback voltage V_fb or it may deviate from V_fb by a predetermined amount, for example, in embodiments that employ an optional electronic valve 153.

[0054] The SCC determines if the feedback voltage deviates from a predetermined value, which may be fixed or adjustable within a predetermined range, and determines an error signal. The SCC determines the pulse train in correspondence with the error signal to either increase or decrease the amount of electrical energy provided on average to the primary winding of the transformer 130. For example, in PWM-controlled SMPS, the SCC extends or contracts the duration of the ON period of switching pulses in correspondence with the error signal. Generally, a PWM-controlled SCC is configured to extend the duration of flow of electrical current and thereby increase the time-averaged amount of electrical energy provided to the transformer, whenever the error signal indicates that the output voltage is lower than the desired output voltage. Vice versa, the SCC reduces the amount of electrical energy provided when the output voltage is too high. The SCC is configured to determine the magnitude of the extension and contraction based on the magnitude of the error signal.

[0055] The switching element 145 can include a power transistor such as a bipolar transistor, insulated-gate bipolar transistor, a suitable FET such as a power MOSFET or a suitable thyristor such as a gate turn-off thyristor depending on the amount of power transformed by the SMPS, or other power transistor as would be readily understood.

Start-Up Circuit

[0056] The start-up circuit for use with a SMPS according to an embodiment of the present invention is configured to provide power for the operation of the switching-control circuit, for example, after switching on the SMPS during the transitional period until the feedback voltage reaches a predetermined value. Such a transitional period typically begins with the initiation of switch-ON of the SMPS and lasts until sufficient electrical power is provided by the feedback voltage line 190 to operate and maintain the operation of the SCC. FIGs. 2, 3, 4 and 5 illustrate examples of start-up circuits for different embodiments of SMPS according to the present invention. Each of the example start-up circuits can be used to power the SCC during transitional periods until the feedback voltage V_fb is sufficiently high and the SCC can be operated substantially via the secondary side of the transformer. A transitional period may occur, for example, immediately after switching ON the SMPS.

[0057] The invention will now be described with reference to particular examples. It will be understood that the following examples are intended to describe embodiments of the invention and are not intended to limit the invention in any way.

[0058] It is noted that for the purposes of this description, including all figures, like references in electronic circuitries to resistors, capacitors, inductors, diodes or transistors, which begin with respective letters R, C, L, D or Q and are followed by a number, do not necessarily indicate like properties of the referenced devices. For example, capacitor C2 of FIG. 2 does not need to be of the same type or have the same characteristics as capacitor C2 of FIG. 3 or another figure or circuit.

Examples Example 1:

[0059] The start-up circuit illustrated in FIG. 2 includes a number of devices including a RC- element having series resistors Rl 201, R2 202 and forward polarized capacitor C2 203 for providing a gate voltage to field effect transistor (FET) Ql 204. The source of FET Ql is connected to V_bUS and the drain of FET Ql is connected to a series connection of forward biased diode D5 205, resistor R3 206 and reverse biased Zener diode D6 207. FET Ql can be a p- channel enhancement mode DMOS FET, for example. Polarized capacitor C3 209 is connected in parallel to the Zener diode D6, as illustrated. Polarized capacitors C2 and C3 can be of electrolytic type. The anode of the capacitor C3 is connected to V_cc. The capacitor C3 is additionally connected to Vb_US via resistor R4 208.

[0060] Capacitor C3 209 can charge up through resistor R4 208 and the series connection of FET Ql 204, diode D5 205 and resistor R3 206. Zener diode D6 207 limits V_cc while capacitor C3 209 helps stabilizing V_cc. The charge and voltage at capacitor C2 203 varies in correspondence with V_bus and the rate of change of the charge and voltage is determined by the time constant of the RC-element including resistors Rl 201, R2 202 and capacitor C2 203. The RC-element ensures that the gate voltage of FET Ql follows variations of V_bUS during transient periods. The gate voltage follows V_bUS in a delayed fashion determined by the time constant of the RC- element Rl, R2, and C2. As the capacitor C2 charges up, V_cc and the gate voltage at the FET Ql rise, and the source-drain resistance of FET Ql increases, and the charge current for C3 209 provided through FET Ql and the power loss through the resistor R3 decrease until FET Ql shuts OFF. Once the capacitor C3 is substantially fully charged, V_cc stabilizes at about the reverse breakdown voltage of the Zener diode D6.

[0061] Resistor R4 208 provides residual charge current past the end of a transient period and also provides extra charge current to capacitor C3 209 during transient periods which can help improve V_cc rise time. Alternatively, the resistor R4 may be infinite or absent from the start-up circuit (not illustrated). Moreover, the cathode of Zener diode D6 207 may be connected to the anode of the capacitor C3 via an optional resistor (not illustrated) rather than the illustrated direct negligible-resistance connection. Example 2:

[0062] FIG. 3 illustrates another example of a start-up circuit for an SMPS according to embodiments of the present invention. The objective is to bring V_cc to a predetermined magnitude within a predetermined time period so the start-up circuit can provide sufficient power to be able to operate the SCC and transition the SMPS into stable operating conditions until sufficient power for the operation of the SMPS can be provided through the secondary side of the transformer.

[0063] The example start-up circuit includes a series connection of the V_bus terminal connected to resistor Rl 220, FET Ql 221 and resistors R2 222 and R3 223. The gate of FET Ql 221 is connected to the series connection of R2 and R3, as illustrated. The FET Ql can be a depletion mode FET such as a depletion mode MOSFET, for example. The source of the FET Ql is connected in series to diode Dl 226 which in turn is connected in series to capacitors Cl 225 and C2 224. Capacitors Cl and C2 are connected in parallel. The voltage terminal of the start-up circuit providing V_cc for operating the switching-control circuit is connected to the cathode of the capacitor Cl.

[0064] For the example start-up circuit illustrated in FIG. 3, the time that it takes for V_cc to reach a predetermined magnitude depends on the time constant formed by the RC-element including resistor Rl 220 and capacitors Cl 225 and C2 224.

[0065] During increases of Vb_US or decreases of V_cc, FET Ql 221 switches ON and at least partially conducts current through its source-drain channel thereby effectively connecting resistor Rl 220 in series with parallel capacitors Cl 225 and C2 224. The voltage V_cc rises as capacitors Cl and C2 charge up through the resistor Rl. The start-up circuit can be configured so that, as V_cc rises, the source voltage of the FET Ql rises more rapidly than the gate voltage of the FET Ql, as determined by the values of the resistors R2 222 and R3 223. Once the gate- source voltage across the FET Ql becomes negative, the FET Ql switches OFF its source-drain channel. The resistors Rl, R2, and R3 are configured so that the FET Ql switches OFF when V_cc reaches a predetermined magnitude that is sufficient to operate the SMPS until the feedback voltage provided via the second side of the transformer reaches a magnitude that can transition the SMPS into stable operating conditions.

[0066] In the example embodiment of the start-up circuit illustrated in FIG. 3, V_cc is determined by the ratio of the magnitudes of resistors R2 222 and R3 223, which is amplified by FET Ql 221. A variation in the ratio of R2 and R3 is, therefore, magnified into a variation in V_cc. Hence, accurate calibration of resistors R2 and R3 in combination with the FET Ql may be required in order to obtain adequately accurate and stable magnitudes of V_cc during operation.

Example 3:

[0067] FIG. 4 illustrates another example start-up circuit similar to the one illustrated in FIG. 3. The start-up circuit of FIG. 4 differs from the one of FIG. 3 in that resistor R3 223 is replaced by Zener diode D4 230 as illustrated, which aids to mitigate the effects of deviations of device properties from their nominal values and stabilize the operating conditions of the startup circuit. As is readily known, deviations of device properties from their nominal values may be due to manufacturing tolerances. For example, under operating conditions the drain-source voltage spreads of typical MOSFET devices may be substantial. The start-up circuit of FIG. 4 may be effectively reproduced and manufactured without requiring stringent selection or accurate binning of certain types of devices employed in the start-up circuit.

[0068] As illustrated in FIG. 4 the gate of FET Ql 221 is connected to the cathode of Zener diode D4 230. The gate voltage of the FET Ql is hence substantially limited to the breakdown voltage of the Zener diode D4 whenever the source-drain channel of the FET Ql is sufficiently conducting. This results in a relatively narrow V_cc operating voltage range.

Example 4:

[0069] FIG. 5 illustrates a further example start-up circuit for an SMPS according to the present invention. The illustrated example SUC provides for improved reproducibility and stable operating conditions and reduced vulnerability to variations in device characteristics. This example SUC ensures that FET Ql 221 completely and rapidly switches OFF when necessary thereby providing low power losses in resistor Rl 220. [0070] In comparison to the example SUC of FIG. 4 and as illustrated in FIG. 5, the gate voltage generation for FET Ql 221 is assisted by a combination of transistor Q2 240, Zener diode D5 242 and resistor R5 241. The collector of the transistor Q2 is connected to the gate of FET Ql 221 and the cathode of Zener diode D4 230. The emitter of the transistor Q2 is connected to ground. Resistor R5 241 connects the base of the transistor Q2 to ground. Zener diode D5 242 connects the base of transistor Q2 240 also to V_fb provided by the feedback voltage conditioner 150. Zener diode D5 242 is biased as illustrated in FIG. 5.

[0071] The additional components including transistor Q2 240, Zener diode D5 242 and resistor R5 241 can be configured to provide a start-up circuit with very short response times. For example, when V^ rises, the voltage at the base of the transistor Q2 rises and switches ON the transistor Q2, which effectively shorts the Zener diode D4. The gate-source voltage on the FET Ql 221 can so be rapidly dropped to a value that ensures that the FET Ql turns OFF the start-up circuit within a short predetermined period of time. Furthermore, if V_cc drops, for example, when the output of the SMPS is shorted or if certain types of faults occur in the SMPS, the collector emitter path of the transistor Q2 becomes highly resistive quickly and switches ON the FET Ql, which, in turn, raises V_cc and restarts the SMPS.

[0072] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0073] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0074] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0075] The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0076] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0077] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0078] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0079] In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," "composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of" shall be closed or semi-closed transitional phrases, respectively.

Claims

1. A switched mode power supply having an input and an output, the switched mode power supply comprising: an input voltage conditioner operatively coupled to the input; a transformer having a primary side and a secondary side, the primary side operatively coupled to a switching control circuit , wherein the primary side and the switching control circuit are operatively coupled to the input voltage conditioner; an output voltage conditioner operatively coupled to the output and the secondary side; a feedback voltage conditioner operatively coupled to the secondary side and the switching control circuit; and a start-up circuit operatively coupled to the input voltage conditioner and the switching control circuit, the start-up circuit configured to substantially provide electrical power to the switching control circuit during start-up and until the feedback voltage conditioner provides a predetermined voltage to the switching control circuit.

2. The switched mode power supply according to claim 1, wherein the start-up circuit comprises a field effect transistor configured to shut off at the predetermined voltage.

3. The switched mode power supply according to claim 2, wherein the start-up circuit further comprises a RC element configured such that a gate voltage of the field effect transistor substantially follows variations in the input.

4. The switched mode power supply according to claim 2, wherein the start-up circuit further comprises a RC element configured with a time constant indicative of a required period of time to reach the predetermined voltage.

5. The switched mode power supply according to claim 1, wherein the start-up circuit further comprises a Zener diode configured to mitigate operational deviations based on one or more differences in actual circuit element properties from defined circuit element properties.

6. The switched mode power supply according to claim 1, wherein the start-up circuit is operatively coupled to the feedback voltage conditioner.

7. The switched mode power supply according to claim 1, further comprising suppression circuitry configured to suppress high induced voltages and/or harmonic content in the output.

8. The switched mode power supply according to claim 1, further comprising power factor correction circuitry configured to control an amount of reactive power drawn by the switched mode power supply.

9. The switched mode power supply according to claim 1, further comprising suppression means for suppressing undesired harmonics in the input.

10. The switched mode power supply according to claim 9, wherein the suppression means is an active filter and/or a passive filter and/or shielding.

11. The switched mode power supply according to claim 1, wherein the secondary side of the transformer further comprises an auxiliary winding, wherein the auxiliary winding is configured to provide galvanic isolation between a primary winding on the primary side and a secondary winding on the secondary side.

12. A method for start-up of a switched mode power supply having an input and an output and including an input voltage conditioner operatively coupled to the input, a transformer having a primary side and a secondary side, the primary side operatively coupled to a switching control circuit, wherein the primary side and the switching control circuit are operatively coupled to the input voltage conditioner; an output voltage conditioner operatively coupled to the output and the secondary side; and a feedback voltage conditioner operatively coupled to the secondary side and the switching control circuit, the method comprising the steps of: receiving a first voltage from the input voltage conditioner; receiving a second voltage from the switching control circuit; and providing, in response to the first voltage and the second voltage, electrical power to the start-up circuit until the feedback voltage conditioner provides a predetermined voltage to the switching circuit.

13. The method according to claim 12, wherein the method further comprises receiving a third voltage from the feedback voltage conditioner and wherein providing electrical power is performed in further response to the third voltage.

14. The method according to claim 12, wherein the method further comprises configuring a RC element with a time constant indicative of a required period of time to reach the predetermined voltage.