LED driving circuit and LED lighting device comprising the same

Abstract

A LED driving circuit comprising a power input adapted to connect to an input power (Vin), an LED output (LED+, LED-) adapted to connect to an LED load (LED), a power inductor, a high frequency power switch (Q1), adapted to alternate in conductive and non- conductive state so as to alternate power charging into the power inductor from the power input and discharging from the power inductor to the LED output, wherein said power inductor comprising a first (L2), a second (L3) and an equivalent leakage (Lleak) inductor segments magnetically coupled together, wherein a series connection of the first and second inductor segments (L2, L3) is coupled to the LED output so as to discharge to the LED output, and a series connection of the first and the leakage inductor segments (L2, Lleak) is coupled across the high frequency power switch (Q1) without the second inductor segment (L3), characterized in that, further comprising a voltage connecting circuit coupled from the series connection of the first and the leakage inductor segments (L2, Lleak) to a voltage smoothing terminal which is adapted to connect to a voltage smoothing component (Csn, LED) thereby outputting an induced voltage / overshoot on the first and the leakage inductor segments to the voltage smoothing component (Csn, LED) via the voltage smoothing terminal.

Description

[0001] LED driving circuit and LED lighting device comprising the same

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of electrical circuits, and in particular, to LED driving circuit.

[0004] BACKGROUND OF THE INVENTION

[0005] SEPIC (Single-Ended Primary Inductance Converter) and Buck-boost topologies are widely used in LED driver. In those topologies, the power inductor is connected in parallel with the LED load and is also connected across the power switch. When the power switch is turned off, the output LED voltage is induced on the power inductor thus is applied across the power switch, optionally superimposed with another voltage such as input voltage or a resonant voltage. In some applications, the LED voltage is very high (such as >220V), and the voltage it induces on the power switch is therefore very high and it will force to choose higher rating power switch (800V or 950V Mosfet). Such high rating power switch is very expensive. If using cost effective power switch such as 700V&600V, we would like to use taped inductor. Such as figure 1 and figure 2 shows wherein figure 1 shows a buck-boost converter using tapped inductor and figure 2 shows a SEPIC converter using tapped inductor.

[0006] In figure 1, Vin is AC mains voltage, D1-D4 is Rectified bridge, L2 and L3 are coupled inductor, turn ratio U=NL2:NL3. Ideally if not consider leakage inductance of L2 and L3, power switch Qi’s voltage stress Vds_stress= Vrect+ ^ * VLED -

[0007] Similarly, figure 2 shows a Taped Sepic Topology, Vin is AC mains voltage, D1-D4 is Rectified bridge, LI is boost PFC inductor, L2 and L3 are coupled inductor. Turn ratio n= NL2:NL3. Ideally if not consider leakage inductance of L2 and L3, power switch Qi’s voltage stress

[0008] It can be seen that this voltage stress is significantly reduced from that in the SEPIC or normal buck-boost converter.

[0009] However, the real tapped inductor has leakage inductance of L2 and L3 which will lead to very high voltage on QI Vds_stress Vds stress= Vrect+ voltage even might be higher than Vrect+VLED. More specifically, figure 3 shows the SEPIC converter with the equivalent leakage inductance, wherein there is a leakage inductor segment Lleak in series with the inductor L2 and this leakage inductor induces a voltage and superimpose on the power switch QI. Figure 4 shows the amplitude of the voltages. Vovershoot is almost 2 times of n / (n+l)*VLED. And if the leakage inductance is higher, Vovershoot might be higher till to break down the power switch. As shown, the peak amplitude of the voltage stress is around 850V. Additionally, there is also a leakage inductor segment Lleakl in series with the inductor L3 but it would not harm the power switch QI so it would not be discussed by this application.

[0010] Moreover, besides the leakage inductance, the tapped inductor further has parasitic capacitances between the terminals of the inductor segments. Depending on the topology, some parasitic capacitance eventually being connected in parallel with some existing capacitor in the circuit and would not influence the circuit, but some parasitic capacitance not. Figure 5 shows a parasitic capacitance that causes power loss in the SEPIC converter. When the MOSFET QI turn off, the parasitic capacitance Cp will be charged via the inductor segment L3 as the dash line 50. And when MOSFET QI turns on, the parasitic capacitance Cp will discharge throughout the inductor segment L3, the resonance capacitor Cb, and power switch QI as shown by the line 52. So Cp energy will be wasted on QI and Ploss=l / 2Cp*Vcp2*fs, here fs is switch frequency.

[0011] Similar problem exists for the buck-boost converter with tapped inductor. Figure 6 shows the buck-boost converter with the equivalent leakage inductance. Similarly, there is a leakage inductor segment Lleak in series with the inductor L2 and this leakage inductor induces a voltage and superimpose on the power switch QI (there is also a leakage inductor segment Lleakl in series with the inductor L3 but it would not harm the power switch QI so it would not be discussed by this application). Figure 6 also shows a parasitic capacitance in the buck-boost converter. The parasitic capacitance Cp will be charged during MOS QI off. The charging loop is L3, L2, and the parasitic capacitance Cp shown by the dash line 60. The parasitic capacitance Cp discharges during MOS QI turn on, and the loop is the parasitic capacitance Cp, power switch QI, input, the inductor segment L3, and the parasitic capacitance Cp as shown by the line 62. Thus there is High loss on QI.

[0012] SUMMARY OF THE INVENTION

[0013] A first goal of the embodiments of the invention is solving the voltage stress caused by the leakage inductance of the tapped inductor. More specifically, a further goal is solving the voltage stress across the power switch. A basic idea of the invention for achieving this goal is using a voltage connecting circuit to connect the leakage inductance to a voltage smoothing component thereby the induced voltage on the leakage inductance can be buffered by the voltage smoothing component. Thus the overshoot / induced voltage on the leakage inductance can be smoothed. As a result of the voltage on the leakage inductance being smoothed without overshoot, the voltage across the power switch is also clamped and the voltage stress on the power switch is reduced. This allows using a lower voltage rating power switch and reduces the cost of the circuit.

[0014] An embodiment is using a capacitor as the voltage smoothing component.

[0015] An alternative or additional embodiment is using the LED load as the voltage smoothing component.

[0016] A second goal of the embodiment of the invention is solving the power loss of a parasitic capacitance of the tapped inductor which discharges through the power switch. An idea of the invention for achieving this goal is using a unidirectional component to block a path of discharging of the parasitic capacitance via the power switch. Thus the power loss can be reduced and efficiency can be increased. In an example, the efficiency can be increased by 1%.

[0017] To achieve the first goal, it is provided a LED driving circuit comprising a power input adapted to connect to an input power, an LED output adapted to connect to an LED load, a power inductor, and a high frequency power switch, adapted to alternate in conductive and non-conductive state so as to alternate power charging into the power inductor from the power input and discharging from the power inductor to the LED output, wherein said power inductor comprising a first, a second and an equivalent leakage inductor segments magnetically coupled together, wherein a series connection of the first and second inductor segments is coupled to the LED output so as to discharge to the LED output, and a series connection of the first and the leakage inductor segments is coupled across the high frequency power switch without the second inductor segment, characterized in that, further comprising a voltage connecting circuit coupled from the series connection of the first and the leakage inductor segments to a voltage smoothing terminal which is adapted to connect to a voltage smooth component thereby outputting an induced vol tage / over shoot on the first and the leakage inductor segments to the voltage smoothing component via the voltage smoothing terminal.

[0018] This voltage smoothing circuit is capable of smoothing the voltage across the first and the leakage inductor segments, thus can reduce the induced voltage / overshoot on the first and the leakage inductor segments. In an embodiment, the LED driving circuit comprises a capacitor as the voltage smoothing component and connected in parallel with the series connection of the first and the leakage inductor segments.

[0019] The advantage of using a capacitor to smooth the overshoot is good performance because the capacitor can absorb the excessive energy well.

[0020] In a further embodiment, the voltage connecting circuit comprises a connecting diode to apply the induced vol tage / over shoot on the first and the leakage inductor segments to capacitor.

[0021] Since the capacitor had better to be discharged so as to be able to absorb a next voltage overshoot, the driving circuit further comprises a discharging diode forward connected between the interconnection point of the connecting diode and the capacitor and the second inductor segment and is adapted to discharge the energy in the capacitor, preferably via the second inductor segment. Besides allowing continuous overshoot absorption, this embodiment also have good power efficiency since the discharged energy goes to the second inductor segment and eventually goes to the LED load, without being dissipated as power loss.

[0022] In an embodiment, a series connection of the discharging diode and the connecting diode is between the first, the second and the equivalent leakage inductor segments. In this embodiment, the discharging diode and the connecting diode would cause some power loss since the power output to the LED load also goes through them. In order to reduce this power loss, a further embodiment of the invention further comprises a bypassing diode connected in parallel with a series connection of the discharging diode and the connecting diode. The total power loss of this further embodiment is substantially lower than the power loss of the embodiment without the bypass diode.

[0023] In an alternative or additional embodiment, the voltage smoothing terminal comprises the LED output and the voltage connecting circuit comprises a voltage clamping circuit to output the induced voltage / overshoot on the first and the leakage inductor segments to the LED load as the voltage smoothing component. This embodiment re-uses the LED load to smooth the voltage overshoot thus it has double functions of the LED load.

[0024] In a further embodiment, said voltage clamping circuit is adapted to clamp the voltage across the first and the leakage inductor segments to a voltage across the LED output. The LED loas has an inherent forward voltage and the present embodiment re-uses the LED load to clamp the voltage on the leakage inductance and the first inductance segments to the forward voltage. As a result of smoothing the voltage the leakage inductance and the first inductance segments, the voltage smoothing circuit is adapted to limit the voltage across the high frequency power switch not exceeding a certain threshold. Thus it can prevent the high frequency power switch from being break down. A lower voltage rated power switch can be used and the cost of the circuit is reduced.

[0025] Specifically, said LED driving circuit comprises a SEPIC converter or a buckboost converter. These converters often use tapped inductors thus may suffer from the induced overshoot voltage. This invention thus can beneficially used with these converters and prevent the tapped inductor from overstressing the power switch.

[0026] In order to achieve the second goal, said power inductor further comprises a parasitic capacitance, and said LED driving circuit further comprising a unidirectional component connected between the series connection of the first and the leakage inductor segments and the second inductor segments and is reversely biased from an anode of the parasitic capacitance via the high frequency power switch to a cathode of the parasitic capacitance for preventing a discharging of the parasitic capacitance through the high frequency power switch in conductive state. In this embodiment, the parasitic capacitance is prevented from discharging through the power switch thus its power loss is reduced.

[0027] In one embodiment, said unidirectional component is a diode. In an alternative embodiment, the unidirectional component can also be an actively controlled switch which is turned off when the parasitic capacitance intends to discharge through the power switch. Using an actively controlled switch has a benefit of low power loss because such an actively controlled switch usually does not have a voltage drop when it is conductive. This actively controlled switch can be implemented by a MOSFET or a BJT.

[0028] The application also proposes a LED lighting device comprising a LED driving circuit as mentioned above, and a LED load connected to the LED output of the LED driving circuit.

[0029] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0031] Figure 1 illustrates an existing buck-boost converter with a tapped inductor;

[0032] Figure 2 illustrates an existing SEPIC converter with a tapped inductor;

[0033] Figure 3 illustrates the existing SEPIC converter of figure 2 with a leakage inductance;

[0034] Figure 4 illustrates the voltage stress over the power switch in the existing SEPIC converter in figure 2 and 3;

[0035] Figure 5 illustrates the power loss caused by a parasitic capacitance of the tapped inductor in the existing SEPIC converter in figure 2 and 3;

[0036] Figure 6 illustrates the existing buck-boost converter of figure 2 with a leakage inductance and parasitic capacitance;

[0037] Figure 7 illustrates a new SEPIC converter proposed by a first embodiment of the invention;

[0038] Figure 8 illustrates the voltage stress over the power switch in the new SEPIC converter of figure 7;

[0039] Figure 9 illustrates a new buck-boost converter proposed by the first embodiment of the invention;

[0040] Figure 10 illustrates a new SEPIC converter proposed by a second embodiment of the invention;

[0041] Figure 11 illustrates the voltage stress over the power switch in the new SEPIC converter of figure 10;

[0042] Figure 12 illustrates an improved embodiment on top of the new SEPIC converter of figure 10; and

[0043] Figure 13 illustrates a new buck-boost converter proposed by the second embodiment of the invention.

[0044] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The invention will be described with reference to the Figures.

[0046] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0047] Basically the embodiments of the invention propose a LED driving circuit comprising a power input adapted to connect to an input power (Vin), an LED output (LED+, LED-) adapted to connect to an LED load (LED), a power inductor, and a high frequency power switch (QI), adapted to alternate in conductive and non- conductive state so as to alternate power charging into the power inductor from the power input and discharging from the power inductor to the LED output, wherein said power inductor comprising a first (L2), a second (L3) and an equivalent leakage (Lleak) inductor segments magnetically coupled together, wherein a series connection of the first and second inductor segments (L2, L3) is coupled to the LED output so as to discharge to the LED output, and a series connection of the first and the leakage inductor segments (L2, Lleak) is coupled across the high frequency power switch (QI) without the second inductor segment (L3).

[0048] Most distinctive from the known technologies, the embodiments of the invention propose a voltage clamping circuit (Da) coupled from the series connection of the first and the leakage inductor segments (L2, Lleak) to the LED output (LED+, LED-).

[0049] Figure 7 illustrates a new SEPIC converter proposed by a first embodiment of the invention. The basic operation of a SEPIC converter is already known for those skilled in the art and would not be described in full details here.

[0050] When the power switch QI is switched off, the induced voltage on the leakage inductance segment Lleak and on the inductor segment L2 is clamped by the diode Da to the LED output, namely to the LED forward voltage. Thus the power switch Qi’s voltage stress is only Vb+VLED and maintain only several tens ns which will not damage QI. Figure 8 shows waveforms including the voltage stress. The peak amplitude of the voltage stress is only 630V significantly lower than 850V in figure 4. Thus the voltage stress problem is addressed.

[0051] Even further, in order to address the power loss on the parasitic capacitance, the embodiment of the invention proposes a unidirectional component Do. It reversely biases from an anode of the parasitic capacitance Cp via the high frequency power switch QI to a cathode of the parasitic capacitance Cp for preventing a discharging of the parasitic capacitance Cp through the high frequency power switch QI in conductive state.

[0052] As shown in figure 7, even if the parasitic capacitance Cp can be charged via Do by the input when the power switch QI is turned off, the parasitic capacitance Cp would not discharge via the power switch QI when it is turned on since the diode Do blocks the discharging. The parasitic capacitance Cp would discharge / charge with the output capacitor Cout in a power loss-less manner. More specifically, the parasitic capacitance Cp may be charged by the output capacitor Cout as shown by the dash line 70; and the parasitic capacitance Cp may discharge to the output capacitor Cout as shown by the dash line 72.

[0053] The invention is also applicable to buck-boost converter, figure 9 illustrates a new buck-boost converter proposed by the first embodiment of the invention. The basic operation of a buck-boost converter is already known for those skilled in the art and would not be described in full details here.

[0054] When the power switch QI is switched off, the induced voltage on the leakage inductance segment Lleak and on the inductor segment L2 is clamped by the diode Da to the LED output, namely to the LED forward voltage. Thus the power switch Qi’s voltage stress is only Vrect+VLED and maintain only several tens ns which will not damage QI.

[0055] Even further, in order to address the power loss on the parasitic capacitance, the embodiment of the invention proposes a unidirectional component Do. It reversely biases from an anode of the parasitic capacitance Cp via the high frequency power switch QI to a cathode of the parasitic capacitance Cp for preventing a discharging of the parasitic capacitance Cp through the high frequency power switch QI in conductive state.

[0056] As shown in figure 9, the parasitic capacitance Cp would discharge / charge with the output capacitor Cout in a power loss-less manner. More specifically, the parasitic capacitance Cp may be charged by the output capacitor Cout as shown by the dash line 90; and the parasitic capacitance Cp may discharge to the output capacitor Cout as shown by the dash line 92. What is to be noted is that, this parasitic capacitance Cp’s location is different from the parasitic capacitance in figure 6. The reason is that there are parasitic capacitances between the two inductor segments L2 and L3 between each end, and some parasitic capacitances become parallel connection with existing capacitors in the circuit and does not influence the operation. For example, in the circuit of figure 9, there is also a parasitic capacitance between the right end of the inductor segment L3 and the lower end of the inductor segment L2, similar as that in figure 6, but it is in parallel with the output capacitor Cout thus do not cause power loss anymore. In explaining the above first embodiment, the voltage smoothing is described as being implemented mainly by the voltage clamping circuit and the LED load. As shown in figures 7 and 9, the applicant submits that it also discloses connecting a capacitor Cout across the series connection of the first and leakage inductor segments and the capacitor Cout is also used as the voltage smoothing component. Thus the application further provides a second embodiment wherein the voltage smoothing component is mainly implemented by such a capacitor. Figure 10 illustrates a new SEPIC converter proposed by a second embodiment of the invention. The basic structure and operation of the SEPIC converter is similar as the SEPIC converter disclosed in the above embodiment and will not be elucidated again. Here the application will focus on the voltage smoothing which is implemented by a capacitor Csn.

[0057] As shown in figure 10, the capacitor Csn is connected in parallel with the series connection of the first and the leakage inductor segments. The voltage connecting circuit comprises a connecting diode Dsn to apply the induced vol tage / over shoot on the first and the leakage inductor segments to the capacitor Csn.

[0058] In order to discharge the capacitor Csn so that the capacitor Csn can smooth a new voltage overshoot, the LED driving circuit further comprises a discharging diode Do forward connected between the interconnection point of the connecting diode Dsn and the capacitor Csn and the second inductor segments L3 and is adapted to discharge the energy in the capacitor Csn. A series connection of the discharging diode Do and the connecting diode (Dsn) is between the first (L2), the second (L3) and the equivalent leakage (Lleak) inductor segments

[0059] Figure 11 shows the waveform voltage stress over the power switch in the new SEPIC converter of figure 10. Although there is a leakage inductance (L leak) of the first and the second inductor segments L2 and L3, the capacitor Csn and the diode Dsn are put in this circuit to absorb the MOSFET Qi’s Vds spike. Comparing figure 11 and figure 4, it can be seen that the dark and large oscillation on the Vds is substantially smoothed and the power

[0060] Even more, the discharging diode Do’s reverse voltage stress is VDo_stress= 1 1

[0061] - * Vcb+ * VLED, which is easily for diode selection. And the diode Dsn’s reverse voltage stress is VDsn stress= Vcb * VLED, which is also easily for diode selection.

[0062] System efficiency is relatively low due to average output current ILED will always flow through the diodes Do and Dsn. Because the total loss in these 2 diodes are Pioss=Isn*Vf Dsn+Ioo*Vf DO=ILED*( Vf_Dsn+ Vf DO). When designed output current ILED=0.84A as example. Pioss=Ln*Vf Dsn+Ioo*Vf DO=0.84* 1.4+0.84*0.85=1.89W.

[0063] In order to improve the efficiency, as shown in figure 12, the LED driving circuit further comprises a bypassing diode Da connected in parallel with a series connection of the discharging diode Do and the connecting diode Dsn. Assuming the current on the bypassing diode is lDa=0.65A, and the current on the connecting diode and the discharging are IDsn=lDO=0.19A. Ploss=Isn*V f_Dsn+IDO*V fDO+ ILEDa* Vf Da =0.19* 1+0.19*0.73+0.65* 1.4 = 1.24W.

[0064] Similar as the first embodiment, the second embodiment is not limited to SEPIC converter but can also apply to other type of converters. Figure 13 shows a new buck-boost converter according to the second embodiment. The bypassing diode Do in figure 13 can be removed.

[0065] What is to be noted is that, the leakage inductor segment and the parasitic capacitance are not a standalone component in the real circuit, but stands for inherent electrical characteristics in the tapped inductor. As long as a tapped inductor has inherent leakage inductance or parasitic capacitance, it falls into the scope of the invention.

[0066] 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. For example, besides using LED load or capacitor as the voltage smoothing component, other suitable component can also be used, for example, Zener diodes. Besides the embodiments of SEPIC converter and buck-boost converter, the principle of the application can also be applied to other type of LED converter where a tapped inductor induces a high voltage stress and optionally causes a high voltage stress over the power switch. The application can be used in such converter to clamp the leakage inductance to the LED output so as to reduce the voltage stress on the leakage inductance and optionally the power switch.

[0067] 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.

[0068] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0069] If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". If the term "arrangement" is used in the claims or description, it is noted the term "arrangement" is intended to be equivalent to the term "system", and vice versa. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A LED driving circuit comprising a power input adapted to connect to an input power (Vin), an LED output (LED+, LED-) adapted to connect to an LED load (LED), a power inductor, and a high frequency power switch (QI), adapted to alternate in conductive and non- conductive state so as to alternate power charging into the power inductor from the power input and discharging from the power inductor to the LED output, wherein said power inductor comprising a first (L2), a second (L3) and an equivalent leakage (Lleak) inductor segments magnetically coupled together, wherein a series connection of the first and second inductor segments (L2, L3) is coupled to the LED output so as to discharge to the LED output, and a series connection of the first and the leakage inductor segments (L2, Lleak) is coupled across the high frequency power switch (QI) without the second inductor segment (L3), characterized in that, further comprising a voltage connecting circuit coupled from the series connection of the first and the leakage inductor segments (L2, Lleak) to a voltage smoothing terminal which is adapted to connect to a voltage smoothing component.

2. The LED driving circuit of claim 1, further comprises a capacitor (Csn, Cout) as the voltage smoothing component and connected in parallel with the series connection of the first and the leakage inductor segments, and the voltage connecting circuit coupled from the series connection of the first and the leakage inductor segments (L2, Lleak) to the voltage smoothing terminal without the second inductor segment (L3).

3. The LED driving circuit of claim 2, wherein the voltage connecting circuit comprises a connecting diode (Dsn) to apply the induced voltage / overshoot on the first and the leakage inductor segments to the capacitor (Csn).

4. The LED driving circuit of claim 3, wherein the LED driving circuit further comprises a discharging diode (Do) forward connected between the interconnection point of the connecting diode (Dsn) and the capacitor (Csn) and the second inductor segments (L3) and is adapted to discharge the energy in the capacitor (Csn).

5. The LED driving circuit of claim 4, wherein a series connection of the discharging diode (Do) and the connecting diode (Dsn) is between the first (L2), the second (L3) and the equivalent leakage (Lleak) inductor segments, and the LED driving circuit further comprises a bypassing diode (Da) connected in parallel with a series connection of the discharging diode (Do) and the connecting diode (Dsn).

6. The LED driving circuit of claim 1 or 2, wherein said voltage smoothing terminal comprises the LED output (LED+, LED-) and the voltage connecting circuit comprises a voltage clamping circuit (Da) to output the induced vol tage / over shoot on the first and the leakage inductor segments to the LED load (LED) as the voltage smoothing component, wherein the voltage connecting circuit coupled from the series connection of the first and the leakage inductor segments (L2, Lleak) to the voltage smoothing terminal without the second inductor segment (L3).

7. The LED driving circuit of claim 6, wherein said voltage clamping circuit (Da) is adapted to clamp the voltage across the first and the leakage inductor segments (L2, Lleak) to a voltage across the LED output (LED+, LED-).

8. The LED driving circuit of claim 1, 2 or 6, wherein said voltage connecting circuit is adapted to limit the voltage across the high frequency power switch (QI) not exceeding a certain threshold thereby preventing the high frequency power switch (QI) from being break down.

9. The LED driving circuit of any one of claims 1 to 8, wherein said LED driving circuit comprises a SEPIC converter.

10. The LED driving circuit of any one of claims 1 to 8, wherein said LED driving circuit comprises a buck-boost converter.

11. The LED driving circuit of any one of claims 6 to 8, said power inductor further comprises a parasitic capacitance (Cp), and said LED driving circuit further comprising a unidirectional component (Do) connected between the series connection of the first and the leakage inductor segments (L2, Lleak) and the second inductor segments (L3) and is reversely biased from an anode of the parasitic capacitance (Cp) via the high frequency power switch (QI) to a cathode of the parasitic capacitance (Cp) for preventing a discharging of the parasitic capacitance (Cp) through the high frequency power switch (QI) in conductive state.

12. The LED driving circuit of claim 7 or 11, wherein said voltage clamping circuit is a diode, and / or said unidirectional component (Do) is a diode or is an actively controlled switch.

13. A LED lighting device comprising a LED driving circuit of any one of claims 1 to 12, and a LED load (LED) connected to the LED output of the LED driving circuit.

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