A circuit used for suppressing current spike

Abstract

A circuit used for suppressing current spike, comprising an input (L, N), a damping resistor (R5) connected in series with said input (L, N) and adapted limit a current from the input (L, N), an instant conduction component (U2) connected in parallel with said damping resistor (R5) and adapted to bypass said damping resistor (R5) in certain condition, and a downstream circuit (C2) connected across said input and after said damping resistor (R5), further comprising an inductor (L5) connected in parallel with said resistor (R5) and adapted to gradually bypass the current through the damping resistor (R5) and reduce a voltage thereon, wherein said instant conduction component (U2) is adapted to bypass said damping resistor (R5) when the voltage thereon has been reduced below a threshold. The inductor helps to gradually decrease the voltage across the damping resistor thus the voltage change on the downstream circuit is smoothed without undesired effect such as EMI or oscillation.

Description

[0001] A CIRCUIT USED FOR SUPPRESSING CURRENT SPIKE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of electronics, and in particular to a circuit used for suppressing current spike.

[0004] BACKGROUND OF THE INVENTION

[0005] LED lighting become popular, and driver performance for LED lighting is also become higher. On the other hand, the legacy Triac-based phase cut dimmer is already widely used / installed in Europe and north America market, so the new LED driver also needs to special design to comply with the legacy Triac-based phase cut dimmer to achieve dimming.

[0006] For Triac-based phase cut dimmer, basically it has two states: a first state of allowing a very small current to pass the charge a timing circuit in the dimmer, to decide an off time and a second state of allowing full current pass to the load to decide an on time. The more ratio of the off time to the one time, the lower brightness the load would emit. The Triac is also a semi-conductor having a special requirement on the current passing though it: after the Triac is turned on, it current needs to above a holding current otherwise the Triac would turn off by itself. In old incandescent lamp, the lamp is a tungsten filament which effectively a pure linear resistance / impedance thus it works well with the triac phase cut dimmer. But the LED driver is usually a switched mode power supply which is a non-linear impedance. Thus there are several issues need to deal with by LED driver.

[0007] 1. Hold current: LED driver need to keep enough current when Triac conducts.

[0008] 2. Inrush current: Since it’s hard to control the turn on speed of Triac, the voltage sharply applied to the LED driver and often causes huge current spike to charge the capacitor inside LED driver when Triac turns on.

[0009] 3. Since Phase cut dimmer operate in series with LED driver, the driver needs to provide current float channel when Triac is off to keep dimmer internal control circuit operate normally.

[0010] Figure 1 shows a traditional circuit inside an LED driver to comply with a Triac- based phase cut dimmer. The RC bleeder block normally connects at the input of LED driver before bridge diode. It provides short time charging current at input to prevent triac current cross zero because of oscillation when it turns on. The current damper block limits current spike and damp the oscillation. At the beginning dimmer turns on, the resistor connects in series to limit current surging, then MOSEFT kicked in to bypass the resistor to minimize the power loss after the current has become stable. The delay time determines the power loss on resistor. And low voltage bleeder used to provide current flow channel to allow the above- mentioned vert small current during triac off, it’s key to keep the control circuit work for dimmers.

[0011] RC bleeder (resistor with several Watts power + 200~400nF film cap) and current damper (600V MOSFET + 2w resistor) has high power loss and occupy lots of PCB space. They are hard to meet the requirement of driver miniaturization design. Thus there is a need to provide a simpler circuit for the current damper to suppress current spike.

[0012] Another disadvantage of using MOSFET is high cost, especially when the MOSFET has a high rating in current / voltage.

[0013] US20150342004A1 discloses lighting apparatus with inducutor current limiting, to be used with a phase cut dimmer. It includes a TVS component in parallel with the inductor.

[0014] US20140300288A1 discloses lighting emitting device power supply circuit and damping circuit. It has an impedance circuit 381 and a SCR 383.

[0015] US20130057167A1 discloses a damping circuit for switched dimming. It also contains a resistor 164 and a thyristor 162.

[0016] US20140239832A1 discloses a power conversion apparatus and system for solid state lighting. A resistive impedance 440 serves to damp oscillation.

[0017] SUMMARY OF THE INVENTION

[0018] A basic idea of the invention is using an instant conduction component to replace the above-mentioned MOSFET for controlling a damping resistor. The instant conduction component is for example a Thyristor, more specially could be a Triac or SCR, is low cost especially compared with high rating MOSFET.

[0019] However, the inventor also finds that the instant conduction component’s conduction is very hard to control. For example, a Triac is one typical instant conduction component and has different switching characteristics from regular MOSFET: due to the positive feedback working mechanism of the two transistors inside the Triac, it always turns on at the fastest speed. The high switching speed causes several problems: before Triac turns on, the current is flowing through the damping resistor, and a relatively high volage builds on the damping resistor. So there’s a voltage gap between input voltage and a downstream circuit. When the Triac turns on to bypass the damping resistor, the voltage gap would be suddenly applied to the downstream circuit. The downstream circuit may respond abnormally upon such a sudden voltage change. For example, the downstream circuit is often a buffer capacitor such an X-cap connected across the AC input. A capacitor draws a high inrush current from mains when its voltage suddenly changes. The higher turn-on speed of the Triac results the higher inrush current. The inrush current may again cause oscillations which may results in turning off of the phase cut dimmer and make the EMI worse. The oscillations may also cause visible light flicker.

[0020] Figure 2 shows waveform, the small curve on the right is an enlarged portion of input current in the left curve. At the beginning of the cycle of AC signal, the AC signal is phase cut. The Triac is turned off and the damping resistor is in series with the input. The moment the phase cut dimmer turns on, the input current starts a first oscillation which is damped by the damping resistor. And after the current become stable, the Triac in parallel with the damping resistor (note: not the Triac in the dimmer) becomes conductive, and the input current starts a second oscillation. This second oscillation not only makes the input current oscillate to a low level and possibly turn off the phase cut dimmer due to the low level, but also makes the EMI worse.

[0021] At the beginning of next cycle of the phase cut signal, the Triac should be turned off. The above procedure happens again.

[0022] Even further, a lamp capable of being used with phase cut dimmer should also be capable of being used with the AC mains directly without the phase cut dimmer. Figure 3 shows a waveform when the circuit is used with the AC mains directly. It can be seen that, compared with fig. 2, the current oscillation caused by the turn on of the phase cut dimmer is gone due to non-existence of the phase cut dimmer, but the Triac in parallel with the damping resistor still causes current oscillation when the Triac turns on to bypass the damping resistor, for a similar reason as explained above referring to Fig. 2. This oscillation makes the EMI worse..

[0023] The present application aims for suppressing undesired effect, such as the current oscillation / spike, generated by the downstream circuit due to the sudden voltage change caused by the instant conduction component in parallel with the damping resistor.

[0024] Thus the basic idea of the invention then does not only comprise the above mentioned instant conduction component in parallel with the damping resistor, but also further comprise an inductor in parallel with the damping resistor and the instant conduction component, wherein the inductor is adapted to gradually take over the current through the damping resistor and reduce the voltage thereacross. Therefore, the voltage is gradually changed on the downstream circuit, without causing serious problem. At last, when the voltage across the parallel resistor and inductor is already small enough or substantially zero, the instant conduction component becomes conductive and bypasses them to reduce power loss, and would not cause undesired effect at the downstream circuit.

[0025] A first aspect of the invention proposes a circuit used for suppressing current spike, comprising an AC input, a damping resistor connected in series with said AC input and adapted limit a current from the AC input, an instant conduction component connected in parallel with said damping resistor and adapted to bypass said damping resistor in certain condition, and a downstream circuit connected across said input and after said damping resistor. The circuit for suppressing current spike further comprising an inductor connected in parallel with said resistor and adapted to gradually bypass the current through the damping resistor and reduce a voltage thereon, wherein said instant conduction component is adapted to bypass said damping resistor when the voltage thereon has been reduced below a threshold.

[0026] In this first aspect, the inductor gradually reduces the voltage across the damping resistor and gradually increases the voltage on the downstream circuit, without causing undesired effect on the downstream circuit. And the instant conduction component bypasses the damping resistor and the inductor when the voltage thereacross is so small that would not cause undesired effect on the downstream circuit. Thus the current spike is suppressed without causing additional problems.

[0027] In one embodiment, said instant conduction component is adapted to bypass said damping resistor and said inductor. So the power loss on the inductor is also reduced.

[0028] The instant conduction component comprises a Thyristor which is low cost. Those skilled in the art would understand that: any lost cost conduction component whose conduction is instant / very hard to control can be used as the instant conduction component of the invention. The Thyristor could further be a SCR (Silicon controlled rectifier) or a Triac (Triode for alternating current).

[0029] In alternative embodiment, the instant conduction component could be relay.

[0030] In one further embodiment, the certain condition is the current spike has gone. Thus there is no need to keep the damping resistor in series with the input anymore, and the instant conduction component bypass the damping resistor to save power loss.

[0031] In one embodiment, the inductor is adapted to become saturate such that the voltage thereon is below the threshold. Using inductor’s saturation to reduce the voltage thereon is easily controlled because of the inductance of the inductor without needing a complex and active control mechanism, and is with less power loss because of the inductor is a reactive component.

[0032] In one preferred embodiment, the downstream circuit comprises a capacitive component adapted to draw a current spike less than a critical value when the voltage below the threshold is suddenly applied thereon when the instant conduction component bypasses said damping resistor. The capacitive component is for example a buffer capacitor.

[0033] This embodiment prevents a sudden and large voltage change on a capacitive component and prevent large current spike and oscillation thereon. It can reduce the EMI, or even prevent overcurrent or undercurrent (due to oscillation) in the input current.

[0034] Alternatively, the downstream circuit is a battery which can neither accept a sudden voltage change. Those skilled in the art would be inspired to use the present invention to solve problems in any other applications and the concerned circuit in those applications should also fall into the scope of the downstream circuit.

[0035] In one preferred embodiment, the circuit used for suppressing current spike is used with a phase cut dimmer, optionally used for suppressing an oscillation of the current from the phase cut dimmer when the phase cut dimmer is turn on.

[0036] In this embodiment, the circuit, especially the damping resistor provides compatibility with the phase cut dimmer by suppressing the current spike when the phase cut dimmer turns on. Even more, it also prevents the large current spike and oscillation when the damping resistor is bypassed, and prevents the phase cut dimmer from turning off due to the current oscillates to low level after the phase cut dimmer turns on.

[0037] In one embodiment, the circuit for suppressing current spike comprises a driving circuit for driving the instant conduction component, said driving circuit is adapted to turn off the instant conduction component when the phase cut dimmer is turn on so as to make the damping resistor in series with the AC input and the phase cut dimmer.

[0038] This embodiment provides a control circuit to control the instant conduction component according to whether the damping resistor is needed for damping.

[0039] In one further embodiment, said driving circuit comprising a delay circuit adapted to turn on the instant conduction component by a delay after when the phase cut dimmer is turn on. Optionally, said delay circuit is adapted with the delay sufficient to allow the inductor to bypass the current through the damping resistor and become saturate with a voltage below the threshold. Using a delay circuit can ensure that the voltage has been reduced before bypassing the damping resistor.

[0040] In a more specific embodiment, said delay circuit comprises a timing, optionally RC timing, circuit adapted to provide a low voltage to the instant conduction component to turn it off when the phase cut dimmer is off, and adapted to be charged, by the phase cut dimmer after the phase cut dimmer is on for the delay, to a voltage level sufficient to turn on the instant conduction component.

[0041] The RC timing circuit is relative low cost in providing a fixed delay. As long as the parameter of the RC timing circuit is properly selected according to the practical application, it turns on the Thyristor at proper turn on time instant to ensure the voltage across the damping resistor and the inductor has just become low, achieving both low current spike and low power loss.

[0042] Alternatively, the delay circuit can also be implemented by a voltage detection circuit to detect the voltage across the damping resistor and turn on the instant conduction component when the voltage across the damping resistor has become low enough. This implementation is more accurate but is also more complex than the above timing circuit.

[0043] In one embodiment, a series connection of the said damping resistor and the capacitive component is used for conducting a current to allow the phase cut dimmer to operate.

[0044] This embodiment reuses the damping resistor and the capacitive component as the RC bleeder to provide short time charging current at input to prevent the triac current in the phase cur dimmer from crossing zero because of oscillation.

[0045] In one preferred embodiment, the circuit used for suppressing current spike is placed before a rectifier bridge. Accordingly, in one embodiment, the instant conduction component comprises a Triac.

[0046] An advantage of using instant conduction component is that the damping resistor as well as the instant conduction component can be placed at the AC input side, thus it is possible to integrate the above mentioned AC RC bleeder and the current damper. Since the instant conduction component has to conduct current in alternative directions, it had better to be a Triac. Alternatively, the instant conduction component could also be anti-parallel connected SCR.

[0047] In an alternative embodiment, the circuit used for suppressing current spike is placed after a rectifier bridge. Accordingly, in one embodiment, the instant conduction component comprises a SCR which is a unidirectional conduction component for that it only needs to handle current in a single direction. In a second aspect of the invention, it provided a LED lighting circuit, comprising the circuit used for suppressing current spike of the first aspect, and a rectifier bridge connected after the circuit used for suppressing current spike.

[0048] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment s) described hereinafter.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] 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:

[0051] Fig. 1 illustrates a known circuit to be used with a phase cut dimmer;

[0052] Fig. 2 illustrates the measured current waveform in case an instant conduction component is used for bypassing a damping resistor, when used with a phase cut dimmer;

[0053] Fig. 3 illustrates the measured current waveform in case an instant conduction component is used for bypassing a damping resistor, when not used with a phase cut dimmer;

[0054] Fig. 4 illustrates the circuit schematic according to an embodiment of the invention used with a phase cut dimmer;

[0055] Fig. 5 illustrates the simulated current waveform of the circuit schematic in figure 3 when used with a phase cut dimmer; and

[0056] Fig. 6 illustrates the circuit schematic according to another embodiment of the invention.

[0057] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0058] The invention will be described with reference to the Figures.

[0059] 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. Figure 4 shows the basic circuit of a Triac based AC current damper for phasecut dimmer compatible application. Here the triac is one implmentation of the instant condution component.

[0060] The inputs are L and N.

[0061] A damping resistor R5, a Triac U2, and a small inductor L5 are connected parallel, the input current flow through these three components with a particular order to as to reduce the voltage change stress on downstream circuits.

[0062] C6, R7, R16 and C9 form a driving circuit for the Triac U2. These components determine the delay time of triac U2 turn-on.

[0063] C2 is a regular X-cap connect at the input of driver as part of EMI filter which further comprises the inductor LI and L2. The X-cap C2 is one typical downstream circuit. Alternatively, the downstream circuit could be a buffer capacitor before or after a PFC stage. It should be noted that other circuits which is susceptible to the sudden volage change caused by bypassing the damping resistor can also be taken as the downstream circuits to be protected by the invention.

[0064] A diode bridge comprising diodes DI to D4 is connected after the EMI filter. The output of the diode bridge is to be connected to the LED driving circuit and is omitted for simplicity.

[0065] An advantage is a series connection of the damping resistor R5 and the X-cap C2 shows similar performance of a dedicated RC bleeding circuit as that in figure 1 ; meanwhile R5 can also suppress the current spike when the phase cut dimmer turns on. So, the cost and space of a dedicated RC bleeding or a current damper in figure 1 could be saved.

[0066] The damping resistor is placed at the AC side before the rectifier bridge thus needs a bidirectional conduction component to bypass it. For example, the Triac U2 is used.

[0067] But an instant conduction component has different switching characteristics with regular MOSFET or BJT, its turn-on speed is hard to control. Due to the positive feedback working mechanism of the two transistors inside the Triac, it always turns on at the fastest speed.

[0068] If without using the inductor L5, the high switching speed causes several problems. Before Triac U2 turns on, current flows into the damping resistor R5, a relatively high volage on the damping resistor R5 because of high resistance. So there’s a voltage gap between the X-cap C2 and input voltage. When the triac U2 turns on to bypass the damping resistor R5, a sudden voltage change is applied on the X-cap and a high inrush current flow through Triac to charge the X-cap from mains. The higher turn-on speed results the higher inrush current. As explained in summary of the invention, this inrush current brings additional oscillations on the circuit, it worsens the dimmer compatibility especially at light load. It also bad for the EMI and the light smoothness.

[0069] So, the invention proposes to add an inductor L2 in parallel with triac U2 and the damping resistor R5, and it makes the input current goes through the damping resistor R5 first, and to inductor L2 gradually for all or a big part. And at last the triac U2 turns on to bypass both the damping resistor R5 and the inductor L2 to reduce power loss thereon.

[0070] The inductor L2 should be designed to have enough inductance such as >10mH to prevent quick current changing in the inductor L2. And the inductor L2’s saturate current should be small to keep the inductor small enough and low saturate current helps inductor take over the current from resistor quickly. Practically, a drum coil with 6mm diameter with lOmH inductance and -lOohrn conduct resistor is enough for a 50w LED driver. The saturate current may be selected as 0.1 A to 0.3 A, depending on the power of the appliance.

[0071] Figure 5 shows the simulation results of the current. When the phase cut dimmer turns on, the voltage shows on driver input with a high dV / dT, which is already be damped by the damping resistor R5. The current also quickly drops to charge the driver of the lamp. At this moment, the triac U2 is still off and the inductor L2’s inductance suppresses the current going through the inductor. Thus the current mainly flow through the damping resistor R5 at first. The amplitude of inrush current is determined by the resistance of the damping resistor R5.

[0072] Then the current inside the inductor L2 gradually increases with time and the current inside the damping resistor R5 decreases. After the current inside the inductor L2 reaches its saturation current, the inductor L2 loses its inductance and take most of the current from the damping resistor quickly. In this duration, the voltage on the inductor L2 / damping resistor R5 also gradually decreases. Since inductor resistance <10ohm is much smaller than resistance on damping resistor -lOOohrn, the final voltage drop on resistor / inductor are also smaller (<5V, if 0.5A) than the condition without inductor(~50V).

[0073] After several hundred micro-seconds which is timed by the delay circuit C6, R16, C9 and R7, the delay circuit turns on the triac U2. The Triac U2 takes over the current inside inductor L2 and the damping resistor R5 to keep system power loss small enough. Since the voltage on the damping resistor / inductor is already reduced to small when the triac U2 conducts, the voltage change on the X-cap C2 is small. The current transition on the X-cap X2 is relatively smooth without substantial spike. The spike is shown in figure 5 and is much smaller than that in the figure 2. When used with the AC mains voltage directly without a phase cut dimmer, the circuit behaves in a similar manner without cause the current spike as in figure 3.

[0074] The present invention is not limited to be used with phase cut dimmer for limiting the current spike during the turning on of the phase cut dimmer. The present application can also be used as an inrush limiter for limiting inrush from the AC mains into the appliance especially when the appliance has a large filter capacitor.

[0075] For appliance with a large filter capacitor, when power up, the AC mains has to charge the filter capacitor first and the current at power up is often large and may even trigger circuit breakers. Thus such appliance often has a circuit used for suppressing current spike which essentially comprises a damping resistor as that in the previous description, and an instant conduction component, such a circuit used for suppressing current spike can be put either at the AC side, similar as that in figure 3, or at the DC side after the rectifier, as figure 6 shows. Wherein when put at the DC side, the instant conduction component could use a SCR U2’ instead of a Triac. The conduction behavior of SCR is similar as that of Triac, and is very fast and can hardly be controlled. Similarly, the circuit for suppressing current spike in figure 6 also comprises a damping resistor R5 and an inductor L2. The filter capacitor C is after a power factor corrector, and the power factor corrector is connected after the circuit for suppressing current spike. The auxiliary secondary winding is the driving circuit for the SCR which can be magnetic coupled with the inductor L2 or to an inductor in the power factor corrector. Starting from startup, the SCR is open. The resistor R5 limits the inrush current into the whole appliance. The inductor L2 gradually takes over the current. The auxiliary secondary winding also obtains power via the magnetic coupling. The capacitor and resistor connected to the auxiliary secondary winding provides a certain delay to wait for the voltage across the damping resistor R5 drop, then the SCR is turned on by the capacitor and resistor connected to the auxiliary secondary winding.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A circuit used for suppressing current spike, comprising an input (L, N), a damping resistor (R5) connected in series with said input (L, N) and adapted limit a current from the input (L, N), an instant conduction component (U2) connected in parallel with said damping resistor (R5) and adapted to bypass said damping resistor (R5) in certain condition, and a downstream circuit (C2) connected across said input and after said damping resistor (R5), further comprising an inductor (L5) connected in parallel with said resistor (R5) and adapted to gradually bypass the current through the damping resistor (R5) and reduce a voltage thereon, wherein said instant conduction component (U2) is adapted to bypass said damping resistor (R5) when the voltage thereon has been reduced below a threshold.

2. The circuit used for suppressing current spike of claim 1, wherein said instant conduction component (U2) is adapted to bypass said damping resistor (R5) and said inductor (L5), said instant conduction component (U2) comprises a Thyristor, said certain condition comprises the current spike has gone, the inductor (L5) is adapted to become saturate such that with the voltage thereon is below the threshold and the downstream circuit (C2) comprises a capacitive component (C2) adapted to draw a current spike less than a critical value when the voltage below the threshold is suddenly applied thereon when the instant conduction component (U2) bypasses said damping resistor (R5).

3. The circuit used for suppressing current spike of claim 2, being used with a phase cut dimmer.

4. The circuit used for suppressing current spike of claim 3, being used for suppressing an oscillation of the current from the phase cut dimmer when the phase cut dimmer is turn on.

5. The circuit used for suppressing current spike of claim 3, further comprising a driving circuit for driving the instant conduction component (U2), said driving circuit is adapted to turn off the instant conduction component (U2) when the phase cut dimmer is turn on so as to make the damping resistor (R5) in series with the input and the phase cut dimmer.

6. The circuit used for suppressing current spike of claim 5, wherein said driving circuit comprising a delay circuit adapted to turn on the instant conduction component (U2) by a delay after when the phase cut dimmer is turn on.

7. The circuit used for suppressing current spike of claim 6, wherein said delay circuit is adapted with the delay sufficient to allow the inductor to bypass the current through the damping resistor and become saturate with the voltage thereon below the threshold.

8. the circuit used for suppressing current spike of claim 6, wherein said delay circuit comprises a RC circuit (C6, R16, C9, R7) adapted to provide a low voltage to the instant conduction component to turn it off when the phase cut dimmer is off, and adapted to be charged, by the phase cut dimmer after the phase cut dimmer is on for the delay, to a voltage level sufficient to turn on the instant conduction component.

9. The circuit used for suppressing current spike of claim 3, wherein a series connection of the said damping resistor (R5) and the capacitive component (C2) is used for conducting a current to allow the phase cut dimmer to operate.

10. The circuit used for suppressing current spike of claim 3, being placed before a rectifier bridge and the Thyristor comprises a Triac.

11. A LED lighting circuit, comprising the circuit used for suppressing current spike of claim 1, anda rectifier bridge connected after the circuit used for suppressing current spike.

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