WO2014170291A1

WO2014170291A1 - Self-oscillating power supply circuit and led lamp having the same

Info

Publication number: WO2014170291A1
Application number: PCT/EP2014/057560
Authority: WO
Inventors: Guy Louis Paul De Bondt; Christian Hattrup; Georg Sauerlaender
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-04-18
Filing date: 2014-04-15
Publication date: 2014-10-23

Abstract

A self-oscillating power supply circuit (10, 10a) is configured to provide an operating current to at least one load (5). The power supply circuit (10, 10a) comprises an input (2) for receiving power from a power source, an output (3) to provide said operating current to said at least one load, a switching converter (4, 4a) comprising an inductive element (8) and a power switching circuit (9, 9a), and a control circuit (27, 27a, 27b). The power switching circuit (9, 9a) is adapted to control said operating current, provided to the load (5). To provide an accurate and stable operating current to the at least one load (5), the control circuit (27, 27a, 27b) comprises a start-up switching device (15), connected with said power switching circuit (9, 9a) to set the power switching circuit (9, 9a) to a conducting state and a current limiting circuit (17, 17a), connected with the power switching circuit and having a controllable voltage reference (18) configured to set the power switching circuit (9, 9a) to a non-conducting state in dependence of a feedback signal, corresponding to the momentary operating current.

Description

Self-oscillating power supply circuit and LED lamp having the same

FIELD OF THE INVENTION

The present invention relates to the field of power supply for loads and in particular switched-mode power supply circuits, such as buck, boost, buck-boost or SEPIC converters.

BACKGROUND OF THE INVENTION

In many applications, a need exists to transform electrical energy according to the operating specifications of a respective load. For example, in the emerging market of light emitting diode (LED) lighting, it is typically necessary to provide the one or more LEDs with a constant current due to the steep voltage/current behavior of such diodes. Several alternative circuit designs exist to provide a constant current from a voltage source, such as a mains connection. A main design goal of these circuits typically is to keep the current stable and within specified limits to provide flicker- free light output.

One example of a suitable circuit design is a switched-mode power supply (SMPS). Here, a switching converter is typically used to convert the input voltage and current to the specifications of the respective one or more LEDs used. Switched-mode power supplies allow for relatively low losses, which is in particular beneficial to the present energy conservation efforts in the field of general office and residential lighting.

An example of a switched-mode power supply circuit is disclosed in WO 99/13559. The document discloses a DC/DC converter having a switching transistor for switching the unregulated DC voltage received, a coil in series with said switching transistor, and a control circuit for modulating the switching transistor to provide a regulated direct current at an output. The switching operation is controlled on the basis of the sensed current over a resistor.

The operation of the disclosed circuit corresponds to the operation of a typical SMPS: upon connection with power, current flows and builds up linearly in the coil and the connected load until an upper threshold is reached. Then, the switching transistor is set to a non-conducting state to disconnect the unregulated input DC voltage from the output. Current flow through the load is maintained by the coil over a freewheeling diode until the current reaches a lower threshold. The switching transistor is then set to the conductive state again and the above cycle is repeated.

While the above SMPS circuit provides, as mentioned in the preceding, relatively low losses, the circuit design disclosed suffers from sensitivity to changes of the input voltage, the load and the temperature, as they appear even during the normal operation of such circuits, e.g. due to components warm up. For example, the power transferred to the load is dependent from the input voltage, so that an increase in the input voltage may lead to a higher power level provided to the load. This may not be acceptable for some applications, in particular for high-quality LED lighting applications where it is important to keep the power provided to the load constant.

Therefore, an object exists to provide a cost-efficient power supply that provides stable output characteristics even under varying temperature conditions as well as input voltage and load variations.

SUMMARY OF THE INVENTION

The object is solved by a self-oscillating power supply circuit and a LED lamp with a self-oscillating power supply circuit according to the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims and in the following description.

According to the invention, a self-oscillating power supply circuit is arranged to provide an operating current to at least one load. The power supply circuit is "self- oscillating", i.e. does not require external control such as by an additional clocking source. In self-oscillating circuits, the switching frequency typically results from the design of the components of the power supply circuit and the load. The power supply circuit comprises an input for receiving electrical power from a power source and an output to provide the operating current to said at least one load.

A switching converter is further arranged comprising an inductive element and a power switching circuit, said power switching circuit being adapted to control said operating current, e.g. between a high and a low current threshold. To control said power switching circuit, a control circuit is provided, comprising a start-up switching device and a current limiting circuit.

The start-up switching device is connected to said power switching circuit to set said power switching circuit to a conducting state. The current limiting circuit is connected to said power switching circuit to set said power switching circuit to a non- conducting state. The current limiting circuit comprises a controllable voltage reference to set the power switching circuit to the non-conducting state in dependence of a feedback signal, corresponding to the momentary operating current.

The basic idea of the invention is to provide the control circuit of the power supply with a current limiting circuit having a controllable voltage reference, which advantageously stabilizes the operation, i.e. reduces the temperature dependence and the influence of input voltage and load fluctuations from the control of the switching circuit.

In the present context, the term "voltage reference" is understood to comprise a device that generates a constant voltage substantially independent of the load of the device, power source variations and temperature variations. For example, the controllable voltage reference may be a controllable series or shunt voltage reference, as will be explained further below.

As discussed in the preceding, the self-oscillating power supply comprises an input for receiving power from a power source, such as a mains or battery connection. An output is arranged to provide the operating current to the at least one load. The input and/or output may be of any suitable type for connection to the power source and the at least one load, respectively, and may have corresponding input and output terminals. Both, the input and/or the output may comprise additional components. For example, the input may comprise a smoothing capacitor and/or a rectifier e.g. in case of an AC power source. The output may comprise suitable terminals for a connection with the load, e.g. such as a plug and socket connection.

In the context of the present invention, the term "connected" means an electrically conductive connection, which may be direct or indirect, i.e. over one or more intermediate components. Each connection may be permanent or temporary. Certainly, with respect to the input / output, the power source and the at least one load the connection should be electrically conductive at least during an operational state of the power supply circuit.

The power supply circuit according to the present aspect of the invention further comprises a switching converter as discussed in the preceding. The switching converter comprises at least an inductive element and a power switching circuit, where the power switching circuit being adapted to control the operating current, provided to the at least one load.

The switching converter may be of any suitable setup, such as a boost, buck- boost, buck, or SEPIC setup to provide the operating current to the at least one load.

Topologies with isolated outputs (through transformers) like flyback or forward converters are also conceivable. Preferably, the switching converter is a buck converter, i.e. a step-down converter. Most preferably and in particular in the latter case, the inductive element, the switching circuit and the load are connected in series with each other to the input terminals. The inductive element may preferably be an inductor, such as a coil. Most preferably, the coil has no auxiliary winding, since such may be omitted due to the special self-oscillating operation of the inventive circuit.

The power switching circuit according to the present aspect of the invention may be of any suitable type to control the operating current, provided to the load, and may comprise a switch, relay, transistor, thyristor or a suitable type of semiconductor switching device. In one embodiment and according to the setup of a switching mode power supply, the power switching circuit allows to switch the connection between the load and the input, i.e. one of the input terminals, from a connected or conducting state to a disconnected or nonconducting state and vice versa. Certainly, a minor current may be present during operation even in the disconnected or non-conducting state of the power switching circuit. The power switching circuit may preferably comprise one or more NPN-type bipolar transistors, which are advantageously available at a reduced cost.

The switching converter according to the present invention may comprise further components in addition to the inductive element and the power switching circuit. For example, the switching converter may comprise an output capacitor, connected in parallel to the output terminals and the at least one load, respectively. Alternatively or additionally, an auxiliary capacitor may be coupled in parallel to the power switching circuit to delay the current flow through the power switching circuit upon a change of the switching state referred to as "valley switching", so that power losses upon the change of switching state are further reduced. In another embodiment, the switching converter comprises a freewheeling diode so that a closed circuit is formed between the inductive element and the at least one load even in the disconnected or non-conducting state of the power switching device.

According to the present aspect of the invention, the power supply circuit further comprises the control circuit with at least the start-up switching device and the current limiting circuit.

The control circuit is arranged to control the power switching device and thus the operating current through the at least one load, e.g. over a corresponding control connection of the power switching circuit. Both, the start-up switching device and the current limiting circuit are connected to the power switching device. The start-up switching device is configured to set the power switching device from the non-conducting state to the conducting state. The current limiting circuit is configured to set the power switching device from the conducting to the non-conducting state. Accordingly, the start-up device and the current limiting circuit allow a two-point control of the operating current. Preferably, the control circuit is configured for critical conduction mode operation, sometime also referred to as "boundary conduction mode", i.e. where the start-up switching device is configured to set the power switching device to the conducting state when the current in the inductive element is close to OA.

Both, the start-up switching device and the current limiting circuit may be of any suitable type. For example, one or more of the aforesaid components may comprise a switch, relay, transistor, thyristor or a suitable type of semiconductor switching device.

Certainly, more than one of the aforementioned components or further components may be present in the start-up switching device and the current limiting circuit.

Preferably, the start-up switching device and/or the current limiting circuit comprise at least a PNP-type bipolar transistor.

The current limiting circuit according to the invention is arranged to set the power switching circuit to the non-conducting state in dependence of the feedback signal. The current limiting circuit thus is also referred to as "feedback circuit". The feedback signal corresponds to the momentary operating current, i.e. the current through the inductive element. Thus the current limiting circuit may preferably be coupled to the input and/or output, e.g. over a current sensing resistor. Certainly, the current limiting circuit may comprise further components in addition to the aforementioned voltage reference.

The current limiting circuit according to the present aspect of the invention comprises at least the voltage reference. In one embodiment, the voltage reference serves to "stabilize" the feedback signal so that temperature, voltage and/or load fluctuations do not influence the switching behavior of the current limiting circuit or are at least substantially attenuated. Preferably and with regard to the mains voltage, a variation in the mains voltage of +-10% should provide not more than +-3% of variation in the output voltage.

Due to the thus "stabilized" feedback signal and the accordingly stabilized switching behavior of the current limiting circuit, the self-oscillating power supply circuit advantageously allows providing a stable and constant power level to the load.

According to the present invention, the voltage reference is a controllable voltage reference, i.e. a device that provides the stabilized voltage on the basis of a control setting. Such control setting may e.g. be a voltage divider, applied to a control input of the device. The control input could however, also be used as is, in order to not change the predefined voltage reference threshold of e.g. 1.25V or 2.5V. To provide the functionality discussed above, the controllable voltage reference may comprise a switching device, such as a transistor, and/or an internal e.g. band gap reference. The switching device may in particular be used to set/program voltages that differ from the internal reference.

The controllable voltage reference may be of any suitable type to provide a voltage that is substantially independent from the load of the device, power source and temperature variations, as discussed in the preceding.

According to a preferred embodiment, the controllable voltage reference is a controllable shunt reference. For example, the controllable shunt reference may be of TL431, TLV431 or TS431 type.

The voltage reference may preferably be adapted for filtering and/or stabilizing, so that the threshold signal corresponds to the filtered and/or stabilized feedback signal to remove the line, load and/or temperature dependency.

According to another preferred embodiment of the invention, the current limiting circuit further comprises a hysteresis control switch, i.e. that upon the power switching circuit being set to the non-conducting state, this state is safely maintained until the power switching circuit is reactivated by the start-up switching device. The hysteresis control switch may be of any suitable type, described in the preceding with reference to the start-up switch.

According to a preferred embodiment, the controllable voltage reference is configured to receive the feedback signal and to set the power switching circuit to the nonconducting state in case the feedback signal corresponds to a switch-off threshold. According to the present embodiment, the controllable voltage reference is connected as an intermediate component, e.g. between input and/or output and a control terminal of the power switching circuit. The switch-off threshold certainly depends on the respective application and should be set accordingly.

In a further preferred embodiment, the controllable voltage reference is configured to set the power switching circuit to the non-conducting state by connecting the power switching circuit to a reference potential, such as ground potential. In this

embodiment, a control terminal of the power switching circuit may, e.g. over an intermediate resistor, be connected to the reference potential to set the power switching circuit to the nonconducting state.

For example, the controllable shunt reference may preferably be connected with its cathode and anode between the control terminal of the power switching circuit and the defined reference potential, such as a ground connection. The control input of the shunt reference may further be connected to receive the feedback signal to provide the behavior as described in the preceding. The present embodiment advantageously provides a combined functionality and thus a very compact setup, since the shunt reference serves, as mentioned before, to stabilize the switching behavior of the power supply circuit, and furthermore also acts as a comparator to trigger the turn off of power switching circuit.

In a further preferred embodiment, the power switching circuit comprises at least a first power switching device with a control terminal, an input terminal and an output terminal, where the input and output terminals are connected in series with the input and output of the power supply circuit to control the operating current, provided to the load.

The present embodiment provides a compact and cost efficient setup to control the operating current to the load. The control terminal of the first power switching device may be connected to the start-up switching device and/or the current limiting circuit either directly or over one or more intermediate components.

The first power switching device may be of any suitable type, such as a switch, relay, transistor, thyristor or a suitable type of semiconductor switching device.

Preferably, the power switching device is a bipolar transistor and most preferably a NPN-type bipolar transistor.

To further enhance the switching behavior of the power supply circuit, the power switching circuit may in a further preferred embodiment comprise a capacitive discharging circuit, connected to the control terminal of the first switching device, e.g. in series between the start-up switching device/current limiting circuit and the control terminal of the power switching device. The capacitive discharging circuit comprises at least a capacitive element, such as a suitable capacitor, e.g. together with further components and in particular resistive elements.

When the power switching circuit and more precisely the first power switching device is set to the non-conducting state by the current limiting circuit, the capacitive discharging circuit allows to discharge charge carriers from the input terminal of the first power switching device, so that the switching speed to the non-conductive state is increased, allowing higher switching frequencies.

Alternatively or additionally to the above, the switching behavior of the power supply circuit may be further improved by providing an auxiliary capacitive element, connected between the input and output terminals of the first power switching device. As discussed in the preceding, the first power switching device may be of any suitable type and preferably is a bipolar transistor. According to a further preferred embodiment, the first power switching device is a high-gain transistor, i.e. having a current gain of 20 or more, to further increase the efficiency of the present power supply circuit. In this context it is noted that the higher the transistor gain, the better the electrical efficiency of the overall circuit.

According to a further preferred embodiment, the power switching circuit comprises a second power switching device having a further control terminal, an input terminal and an output terminal, wherein the output terminal of the second power switching device is connected to the control terminal of the first power switching device.

The present embodiment provides a cascaded configuration, which enhances the switching behavior of the inventive circuit by providing a further increased gain.

According to the configuration of the present embodiment, the start-up switching device and the current limiting circuit are connected to the input terminal of the second power switching device, so that the second power switching device amplifies the control signal and provides the amplified signal to the input of the first power switching device.

The second power switching device may be of any suitable type, such as a switch, relay, transistor, thyristor or a suitable type of semiconductor switching device.

Preferably, the second power switching device is a bipolar transistor and most preferably a NPN-type bipolar transistor. In the configuration of the present embodiment, the first and second power switching devices are preferably low cost transistors, typically having a current gain of less than 10 to further improve the cost efficiency of the present power supply circuit. Most preferably, the capacitive discharging circuit is connected between the first and second power switching devices, i.e. between the output terminal of the second power switching device and the control terminal of the first power switching device to further increase the switching speed of the arrangement.

According to a further preferred embodiment, the at least one load is an LED unit, comprising at least one light emitting diode (LED). In the present context, an LED may be any type of solid state light source, such as an inorganic LED, organic LED or a solid state laser, e.g. a laser diode. The LED unit may certainly comprise more than one of the before mentioned components connected in series and/or in parallel. The LED unit may preferably be of low-power type, i.e. having a power consumption of below 25 W (sum of LED power plus driver losses), preferably below 15 W and most preferably below 10 W. According to a further aspect of the present invention, an LED lamp is provided having at least one LED unit and a self-oscillating power supply circuit according to one or more of the above embodiments.

These and other aspects of the invention will become apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

Fig. 1 shows a circuit diagram of a first embodiment of an LED lamp with a self-oscillating power supply circuit,

Fig. 2 shows a circuit diagram of a second embodiment of an LED lamp with a self-oscillating power supply circuit and

Fig. 3 shows a circuit diagram of a third embodiment of an LED lamp with a self-oscillating power supply circuit.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a circuit diagram of a first embodiment of an LED lamp 1 with a self-oscillating power supply circuit 10 and an LED unit 5. The LED unit 5 according to the present embodiment comprises a high-intensity white LED for illumination purposes. The LED unit 5 certainly could comprise more than one LED, e.g. in a serial connection. For reasons of clarity, the housing of the LED lamp 1 and any further mechanical part such as plug/socket connectors are not shown in Fig. 1. In a particular preferred embodiment for connection to AC mains, the LED unit 5 shows a forward voltage in the range of 20V - 150V.

The self-oscillating power supply circuit 10 comprises an input 2 having two input terminals 6 and a smoothing capacitor 7. The input 2 provides power to the circuit 10 from a connected power source (not shown), such as a battery or a rectified mains connection. The input 2 is connected to switching converter 4, which comprises an inductor 8, a freewheeling diode 25, a power switching circuit 9 and an output capacitor 11. As will be apparent to one skilled in the art, the setup of switching converter 4 corresponds to the setup of a step-down buck converter. Outputs 3 connect the switching converter 4 to the LED unit The power switching circuit 9 comprises a high-gain NPN bipolar power transistor 12, i.e. having a current gain of 20 or more, a capacitive discharging circuit 13 and an auxiliary capacitor 14.

The circuit 10 further comprises a start-up switching device 15 and a current limiting circuit 17, which comprises a controllable shunt voltage reference 18, a current sensing resistor 19, an auxiliary resistor 20 and hysteresis control switch 16. Both, the startup switching device 15 and the hysteresis control switch 16 according to the present embodiment are PNP bipolar transistors.

The shunt voltage reference 18 according to the present embodiment is a TL431, i.e. a controllable shunt voltage reference comprising a switch (not shown).

The start-up switching device 15 and the current limiting circuit 17 control the ON/OFF state of the power switching circuit 9, i.e. the power transistor 12, and thus the current flow through the buck converter 4 and the LED unit 5. While, as will be explained in the following in more detail, the start-up transistor 15 sets the power transistor 12 to the ON- state when the current through the inductor 8 is substantially OA, the current limiting circuit 17 controls an upper current threshold and sets the power transistor 12 to the OFF-state when the momentary operating current through the inductor 8 corresponds to the predefined switch-off threshold in a closed-loop operation. Accordingly, a feedback voltage signal is derived over current sensing resistor 19, which corresponds to the momentary operating current. The feedback signal is provided over auxiliary resistor 20 to a control input of the shunt voltage reference 18. The shunt voltage reference 18 provides a corresponding threshold voltage signal to the integrated transistor. Once the voltage across current sensing resistor 19 reaches the defined switch-off threshold, set by voltage reference 18, the shunt voltage reference 18 is set conductive and thus connects the resistor 23 to ground potential, i.e. over the shown additional resistor. The connection causes discharging the base of power transistor 12 to set the power transistor 12 to the OFF state. Simultaneously, hysteresis control switch 16 is set conductive and provides that the voltage reference 18 remains in the conductive state, so that the power transistor 12 safely remains in the OFF state until reactivated by the start-up switching device 15. Diode 26 prevents negative currents from flowing through the shunt voltage reference 18.

During operation of the circuit 10, i.e. upon connection with a power source, an input voltage is applied to capacitor 7. The start-up transistor 15 is set conductive by resistor 21, which is driving its base. Resistor 22 provides a current to the base of power transistor 12 through the conductive start-up transistor 15 and resistor 23, resulting in the power transistor 12 being switched to the ON state. Accordingly, current flows through the collector of power transistor 12 and output capacitor 11 is charged via inductor 8, power transistor 12 and sense resistor 19. Due to the series inductance of inductor 8, the current in this "main current path" of capacitor 11, LED unit 5, inductor 8, power transistor 12 and sense resistor 19 rises linearly.

Once the current in the main current path reaches the previously discussed switch-off threshold, set by shunt voltage reference 18, shunt voltage reference 18 is set conductive and switches off power transistor 12. To increase the speed for switching off the power transistor 12, a capacitive discharging circuit 13 with capacitor 24 and resistor 23 is provided. At the moment the shunt voltage reference 18 is set conductive, the charge of capacitor 24 helps switching off power transistor 12 faster by providing a negative base current. The capacitor 24 is charged by resistor 23 during the ON-state of power transistor 12. The resistor 23 limits the voltage over capacitor 24.

When the power transistor 12 is set to the OFF-state, freewheeling diode 25 conducts and provides a negative base/emitter voltage at the start-up transistor 15, causing the start-up transistor 15 to be set non-conductive. Accordingly, no current is provided further to the base of power transistor 12 so that the transistor 12 remains in the OFF-state.

The current flow through the inductor 8 and the LED unit 5 is kept through the freewheeling diode 25. The current in inductor 8 however decreases and reaches after some time OA. At this moment, the inductor 8 and in particular the parasitic capacitance of inductor 8 and the auxiliary capacitor 14 are ringing. Then, start-up transistor 15 starts to conduct and sets the power transistor 12 to the ON-state. The process, also referred to as "critical conduction mode", is then repeated.

Fig. 2 shows a circuit diagram of a second embodiment of an LED lamp la with a self-oscillating power supply circuit 10a. The setup of circuit 10a corresponds to the setup of the circuit 10 according to Fig. 1 with the exception of the setup of control circuit 27a, switching converter 4a and more particularly of power switching circuit 9a.

As will become apparent from the figure, the high-gain power transistor 12 of the embodiment of Fig. 1 has been replaced by an arrangement of two low-gain transistors 30, 31 in a cascaded configuration. The capacitive discharging circuit 13 is now arranged "between" the transistors 30, 31. Diode 26 is now provided in series with shunt voltage reference 18. Also here, diodes 26 and 40 prevents negative currents through the shunt voltage reference 18. Since the transistors 30, 31 have a lower gain, it is of advantage to amplify the control signal provided by the start-up transistor 15 by the (auxiliary) transistor 31, thus compensating for the reduced gain by adding an additional low cost device. The main current path however is also here provided through transistor 30. To switch this transistor OFF rapidly, the charge carriers have to be removed fast, which is provided by the discharging circuit 13.

Fig. 3 shows a circuit diagram of a third embodiment of an LED lamp lb with a self-oscillating power supply circuit 10b. The setup of circuit 10b corresponds to the setup of the circuit 10 according to Fig. 1 with the exception of the setup of input 2a, the control circuit 27b and in particular current limiting circuit 17a. As will be apparent from the drawing, input 2a now is adapted for AC operation and comprises a bridge rectifier 32, connected with a mains power source 33 over an input resistor. Control circuit 27b according to the present embodiment comprises shunt voltage reference 18, as explained in the preceding. Here, shunt voltage reference 18 is of TS431-type, a low voltage version of the TL431 of Fig. 1, so that lower sense voltages are possible. Instead of the TS431-type controllable shunt voltage reference it may be possible to alternatively use a zener diode or bandgap/brokaw diode as shunt voltage reference 18.

According to the present embodiment, shunt voltage reference 18 is connected to a comparator arrangement, formed by transistors 34 and 35. Shunt voltage reference 18 is supplied via resistor 36. Transistor 34 serves to compensate the Vbe of transistor 35, both are of the same type, having almost the same Vbe. The Vb of transistor 34 is Vb34=REF=l .25 V, which provides that point ce is one Vbe step lower as Vref (Vce=Vref-Vbe). The emitter of transistor 35 is connected to a point having this voltage. The comparator arrangement switches, once the basis of transistor 35 reaches the reference voltage Vref, i.e. in this example 1.25V. Then, transistor 35 starts to conduct, setting power transistor 12 to the OFF- state. The functionality of hysteresis control switch 16 corresponds to the functionality explained with reference to Fig. 1.

Diode 37 limits the voltage drop over resistor 20 to limit the losses produced in this resistor 20. However, diode 37 may be omitted if resistor 20 is small enough. The advantage of the control circuit 27b and the current limiting circuit 17a is that here, the shunt voltage reference 18 is used as a reference only and the comparator functionality is provided by the comparator arrangement. Accordingly, the present embodiment allows higher switching frequencies and lower sense voltages, thus further reducing sensing losses and increasing the overall efficiency of circuit 27b. Since both transistors 34 and 35 have the same temperature coefficient, there is no temperature influence on the circuit 27b.

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.

For example, it is possible to operate the invention in a further embodiment having one or more of the following changes

the circuit 10, 10a is adapted for AC input, i.e. the input 2 comprises a rectifier,

- the input 2, 2a and/or output 3 comprises plug/socket connections for a disconnectable connection to power and/or LED unit 5,

the LED unit 5 comprises more than one LED and/or

the shunt reference 18 is a TLV431 , TS431 or a further suitable type of shunt reference such as a zener, bandgap or brokaw diode.

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 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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Self-oscillating power supply circuit to provide an operating current to at least one load, comprising

an input (2, 2a) for receiving power from a power source,

an output (3) to provide said operating current to said at least one load, - a switching converter (4, 4a) comprising an inductive element (8) and a power switching circuit (9, 9a), said power switching circuit (9, 9a) being adapted to control said operating current, and

a control circuit (27, 27a, 27b), comprising

a start-up switching device (15), connected with said power switching circuit to set said power switching circuit to a conducting state and

a current limiting circuit (17, 17a), connected with the power switching circuit and having a controllable voltage reference (18), the controllable voltage reference (18) being configured to set the power switching circuit (9, 9a) to a non-conducting state in dependence of a feedback signal, corresponding to the momentary operating current.

2. Self-oscillating power supply circuit according to claim 1, wherein the controllable voltage reference (18) is configured to receive the feedback signal and to set the power switching circuit (9, 9a) to the non-conducting state in case the feedback signal corresponds to a switch-off threshold.

3. Self-oscillating power supply circuit according to one of claims 1 or 2, wherein the controllable voltage reference (18) is configured to set the power switching circuit to the non-conducting state by connecting the power switching circuit to a reference potential.

4. Self-oscillating power supply circuit according to one of the preceding claims, wherein said controllable voltage reference (18) is a controllable shunt reference.

5. Self-oscillating power supply circuit according to one of the preceding claims, wherein said power switching circuit (9, 9a) comprises at least a first power switching device (12, 30) with a control terminal, an input and an output terminal and wherein said input and output terminals are connected in series with said input (2) and output (3) to control said operating current, provided to the load.

6. Self-oscillating power supply circuit according to claim 5, wherein said power switching circuit (9, 9a) comprises a capacitive discharging circuit (13), connected to said control terminal of said first power switching device (12, 30).

7. Self-oscillating power supply circuit according to one of claims 5-6, wherein said first power switching device (12, 30) is a high-gain transistor.

8. Self-oscillating power supply circuit according to one of claims 5-7, wherein said power switching circuit (9, 9a) further comprises a second power switching device (31) having a control terminal, an input and an output terminal, wherein the output terminal of the second power switching device (31) is connected to the control terminal of said first power switching device (12, 30).

9. Self-oscillating power supply circuit according to claim 8, wherein said first and second switching devices (30, 31) are low-gain transistors.

10. Self-oscillating power supply circuit according to one of the preceding claims, wherein said at least one load is at least one LED unit (5).

11. LED lamp with at least one LED unit (5) and a self-oscillating power supply circuit (10, 10a, 10b) according to one of the claims 1-10.