NZ602890B2 - Light emitting diode load protection circuit - Google Patents

Abstract

Disclosed is a protection circuit (100) for protecting a load (50) comprising at least one non-incandescent load upon reconnection to a constant current load driver (not shown) after disconnection. The protection circuit (100) includes a reconnection sensor (130) for sensing reconnection of the load (50) to the load driver. The protection circuit (100) also includes a switch (110) connected in series between the constant current driver and the load (50). A control circuit (120) is also provided to control the switch (110) to limit a rate of rise of current through the load (50) upon the reconnection sensor (130) sensing reconnection of the load (50) to the load driver. The protection circuit may be particularly useful for protecting LEDs. d (50) to the load driver. The protection circuit (100) also includes a switch (110) connected in series between the constant current driver and the load (50). A control circuit (120) is also provided to control the switch (110) to limit a rate of rise of current through the load (50) upon the reconnection sensor (130) sensing reconnection of the load (50) to the load driver. The protection circuit may be particularly useful for protecting LEDs.

Description

PATENTS FORM NO. 5 Appln Fee: $250 Our Ref: 39028NZ MPzAP PATENTS ACT 1953 COMPLETE SPECIFICATION LIGHT EMITTING DIODE LOAD TION CIRCUIT We, Schneider Electric South East Asia (HQ) Pte Ltd of 10 Ang Mo Kio Street 65, #02-01/06 Techpoint,569059,Singapore, do hereby declare this invention to be described in the following statement:— LIGHT EMITTING DIODE LOAD PROTECTION CIRCUIT TECHNICAL FIELD The present application relates to the protection of a non-incandescent load such as a Light Emitting Diode (LED) load driven by a constant current load driver such as a constant current LED Driver.

PRIORITY The present application claims priority from Australian Provisional Patent Application No. 201 l904l 50 entitled “Light Emitting Diode Load Protection Circuit”, filed on 7 October 2011.

The entire content of this provisional ation is hereby incorporated by reference.

INCORPORATION BY REFERENCE The following nts are ed to in the present application: ‘03/00365 entitled “Improved Dimmer Circuit Arrangement”; PCT/AU03/00366 entitled “Dimmer Circuit with ed Inductive Load”; PCT/AL'03/00364 entitled “Dimmer Circuit with Improved Ripple Control”; entitled “Current Zero ng Detector in A Dimmer Circuit”; PCT/AL'2006/001882 entitled “Load Detector For A Dimmer”; PCT/AL'2006/001 881 entitled “A Universal ”; entitled “Improved Start—Up Detection in a Dimmer Circuit”; PCT/AL 2008/00] 399 ed “Dimmer Circuit With Overcurrent Detection”; and ed “Overcurrent Protection in a Dimmer Circuit”.

The entire content of each of these documents is hereby incorporated by reference.

BACKGROUND Non-incandescent loads are ng very popular s for use as light sources. One example of a non—incandescent load used as a light source is a Light Emitting Diode or LED. Special circuits known as LED Drivers are used to drive a load made up of one or more LEDs. One aspect of LEDs is that they can be damaged by excess t and so it is desirable to limit the current that can be applied to an LED load.

One form of LED Driver can provide a constant current source for the LEDs, which allows the maximum current to be controlled, thereby reducing the risk of damaging the LED load.

Figure 1 shows one example of a typical configuration of a constant current LED load arrangement 6. In this example, the arrangement 6 comprises LED load 3 comprising one or more LEDs (3), a switchmode ac-dc or dc-dc converter providing a switching power supply 5, which delivers power to one or more reservoir capacitors 1, t flow from the capacitor 1, flows via a tion point 2 to the series-connected LED load 3, then returns through the connection point 2 to a current sensing resistor 4 and back to the reservoir tor 1 terminal. The current g resistor 4 is included so that the voltage developed across it can be measured (regulation circuitry not shown for clarity) and the switching power supply 5 can be regulated to maintain a constant current through the LED load 3. The current sensing resistor 4 is selected to be a low ance so that negligible power loss will be experienced when the circuit is operating, and is included in the current path so that control circuitry can feed back current information if required to the switchmode converter circuit 5.

The circuit shown in Figure l is a basic representation of LED drivers commonly available today. Such LED drivers suffer from a drawback. That is, if the LED load 3 is disconnected from the connection point 2 while the driver is powered, the nature of the current regulation circuit will always increase the voltage across the reservoir capacitor 1 in an effort to in voltage drop across resistor 4 representing a target current level. The voltage across capacitor 1 can rise to a point at which the capacitor itself will be damaged if there is no protection circuitry included. This overvoltage problem is sufficiently common that most available LED drivers of this type orate some voltage limiting protection to prevent damage to the capacitors and other components of the LED driver from damage.

As will be appreciated, if the LED load 3 is left unconnected, the system can be left in a state with higher- than—normal voltage across tor 1 without damage, but if the LED load 3 is reconnected to the circuit, the low resistance of current sense or 4 and the low dynamic resistance of the LEDs 3 themselves will not be sufficient to limit the current enough to protect the LEDs from damage. s 2A and 2B show the output at connection points 2 of Figure 1. Figure 2A shows the voltage at the LED load terminal output and Figure 2B shows the current through the LED load. At time Oms, when an LED exhibiting approximately 3V drop is connected to the operating , the e at the output point 2 is about 3V (Figure 2A) and the current through LED load 3 is about 350mA (Figure 2B). At time lOOms, LED load 3 is disconnected from the driver and the current through the sense resistor 4 goes to zero (Figure 2B). The regulation circuitry immediately increases the voltage in an attempt to maintain the current through current sense resistor, g the voltage at the output to a maximum of nearly 40V in this example (Figure 2A). 500ms later, at time 600ms, the LED load 3 is reconnected to the terminals of the driver, which has an output voltage of nearly 40V. This creates an t high current spike of nearly 18A through the LED load since the t is limited only by the low (for example 1 ohm) current sense resistor, and the LED load dynamic impedance, before the regulation circuitry acts to reduce the terminal voltage back to about 3V to bring the LED load current back to its target level of about 350mA.

This massive spike can destroy or damage the LEDs in the load upon reconnection.

For this reason, most LED drivers of this design are marked with warning notices that the power must be disconnected from the driver for a minimum length of time before the LED load 3 can be re—connected.

A warning notice is useful where the LED driver is visible and where it is not likely that the LED load will be disconnected while the system is powered, but these ions are not always possible in many commercial installations and still allow for the ial of damage to the LED load.

SUMMARY According to a first , there is provided a protection circuit for protecting a load comprising at least one non-incandescent load upon reconnection to a constant current load driver after disconnection, the protection circuit comprising; a reconnection sensor for sensing reconnection of the load to the load driver; a current switch arranged in series between the nt current load driver and the load; and a control circuit for controlling the current switch by controlling the transition from open circuit to closed circuit to limit a rate of rise of t through the load upon the reconnection sensor sensing reconnection of the load to the load driver.

In one embodiment, the candescent load is a Light Emitting Diode (LED) and the load driver is an LED driver. In one form, the circuit further comprises a t limiting impedance in parallel with the current . In one form, the circuit further comprises a disconnection sensor for sensing when the load is disconnected from the LED .

In one embodiment, the reconnection sensor has a first input for receiving a reference e and a second input for receiving a voltage input representative of a e across a current sense resistor. In one embodiment, the reconnection sensor is a comparator. In another embodiment, the reconnection sensor is an amplifier with a finite gain.

In one embodiment, the disconnection sensor comprises a comparator having a first input for receiving a second reference voltage and a second input for receiving a voltage input representative of a voltage across a current sense resistor. In one embodiment, disconnection of the load is sensed when the voltage across the current sense resistor reduces to below the second reference voltage.

In another embodiment, reconnection of the load to the LED driver is sensed when the voltage across the current sense resistor reaches the value of the reference voltage. In one embodiment, the reference voltage is substantially equal to the second reference voltage.

According to a second aspect, there is provided a protection circuit for protecting a load comprising at least one non-incandescent load upon ection to a terminal of a constant current load driver after disconnection, the constant current load driver comprising a reservoir capacitor for providing a voltage to the terminal of the constant current load driver, the protection circuit comprising; a ection sensor for g reconnection of the load to the load driver; a tor ; and a current switch; and a control circuit for controlling the capacitor switch to discharge the reservoir capacitor upon the reconnection sensor sensing reconnection of the load to the load driver, and for controlling the current switch to delay turn on of the current switch until after the reservoir capacitor has started to discharge.

In one embodiment, the current switch turns on when the voltage across the reservoir capacitor is substantially equal to the load voltage. In one embodiment, the non—incandescent load is a Light Emitting Diode (LED) and the load driver is an LED . In one ment, the control circuit controls the tion from open circuit to closed circuit in the current switch to limit a rate of rise of current through the load.

According to a third aspect, there is provided a Light ng Diode (LED) driver comprising a protection circuit according to any of the first and second aspects. According to a fourth aspect, there is provided a dimmer circuit comprising an LED driver according to the third .

According to a fifth aspect, there is provided a method of ting a load upon reconnection to a constant current load driver, the load sing at least one non-incandescent load, the method comprising the steps of: detecting reconnection of the load to the load driver; and 3O controlling the transition from open circuit to closed circuit of a current switch d n and in series with the load and load driver so as to limit a rate of rise of current through the reconnected load upon detecting the reconnection of the load to the load driver.

According to a sixth aspect, there is provided a method of protecting a load comprising at least one non— incandescent load upon reconnection of the load to a constant current load driver having a reservoir capacitor, the constant current load driver comprising a main tor for providing a voltage to the terminal of the constant current load driver, the method comprising the steps of: detecting reconnection of the load to the load driver; providing a feedback signal to the nt current load driver so that power delivered to the reservoir tor can be ntially reduced to allow the oir capacitor to begin discharging through the load at a reduced level; and delaying a turn on of a current switch arranged in series between the constant current load driver and the load to control the flow of current through the reconnected load until the reservoir capacitor has at least partially discharged. ing to a seventh aspect, there is provided protection circuit for protecting a load comprising at least one non-incandescent load upon reconnection to a al of a constant current load driver after disconnection, the constant current load driver comprising a reservoir capacitor for providing a e to the terminal of the nt current load driver, the protection circuit comprising; a reconnection sensor for sensing reconnection of the load to the load driver; a current switch arranged in series between the constant current load driver and the load; and a control circuit for providing a feedback signal to the nt current load driver so that upon the reconnection sensor sensing reconnection of the load to the load driver, power delivered to the reservoir capacitor can be substantially reduced to allow the oir capacitor to discharge, and the current switch is controlled to delay turn on of the current switch until after the main capacitor has started to discharge.

In a general aspect, there is provided a protection circuit and method for ting a non-incandescent load such as a Light Emitting Diode (LED) load from damage upon reconnection to a load driver. A reconnection sensor detects when the load is reconnected to the driver and a switch controls a rate of rise of current flowing through the load to limit the potential for damage to the load.

In another general aspect, there is ed a protection circuit and method for protecting a load from damage upon reconnection to a load driver. The circuit comprises a reconnection sensor for detecting reconnection of the load to the driver and a means for allowing a reservoir or charging capacitor connected to the driver terminals to begin discharging through the load at a safe level of current. When the capacitor has discharged to a certain level, current is allowed to either begin flowing through the load, or its rate of rise is controlled.

DRAWINGS Various aspects will be described with reference to the following drawings in which: Figure l — shows a prior art example of a typical constant current LED load arrangement; Figure 2A -— shows a rm of the LED driver terminal voltage as it varies over disconnection and ection of the LED load in the prior art arrangement of Figure 1; Figure 2B — shows a waveform of LED load current as it varies over disconnection and reconnection of the LED load in the prior art arrangement of Figure l; Figure 3 — shows a block diagram of one example of one embodiment of an arrangement with load protection; Figure 4 — shows a block diagram of one embodiment of a protection circuit; Figure 5 — shows the protection circuit of Figure 4 implemented in the arrangement of Figure 3; Figure 6A — shows a rm of the LED driver terminal voltage as it varies over disconnection and reconnection of the LED load in the arrangement shown in Figure 5; Figure 6B ~ shows a waveform of LED load current as it varies over disconnection and reconnection of the LED load in the arrangement shown in Figure 5; Figure 7 — shows another embodiment ofthe protection circuit; Figure 8 - shows a circuit diagram of one embodiment ofthe protection circuit; Figure 9 — shows one embodiment of an LED lighting arrangement with r embodiment ofa protection circuit; Figure 10 ~ shows a t diagram of another embodiment of the arrangement of Figure 7 with disconnection and reconnection of the LED load represented by switches; Figure 11A — shows the LED terminal voltages of the arrangement of Figure 10; Figure 11B — shows the switch gate voltage in the arrangement of Figure 10; Figure 11C - shows the fault ion and regulator signals in the arrangement of Figure 10; Figure l 1D — shows the LED current in the arrangement of Figure 10; Figure 12 — shows a block diagram of another embodiment providing for discharge of a main capacitor prior to controlling current through the load; Figure 13 — shows a circuit diagram of an arrangement that disables the regulator to allow discharging of the tor through the load; Figure 14A — shows the terminal voltages in the circuit of Figure 13; Figure 14B — shows the gate voltage of the current switch in the circuit of Figure 13; Figure 14C —— shows the fault detection and tor signals in the circuit of Figure 13; Figure 14D — shows the load current in the circuit of Figure 13; Figure 15 — shows a circuit diagram of another embodiment allowing disabling of the regulation circuitry; Figure 16A — shows the terminal voltages in the circuit of Figure 15; Figure 16B — shows the current switch gate voltage in the circuit of Figure 15; Figure 16C — shows the fault detection and regulator signals in the t of Figure 15; Figure 16D — shows the load current in the t of Figure 15; Figure 17 — shows a flow chart of one method of protecting a load; Figure 18 — shows a flowchart of another method of protecting a load; and Figure 19 — shows a flowchart of another method of ting the load.

DESCRIPTION While the various aspects described herein are described with reference to a Light Emitting Diode (LED) as the non-incandescent load, r, it will be appreciated that the s aspects are applicable to many other types of non-incandescent loads ing but not limited to, Compact Fluorescent Lamps (CFLs), plasma lamps and Organic Light Emitting Diodes (OLEDs).

According to one embodiment disclosed herein, there is provided a protection circuit that s the likelihood of damage to an LED load upon reconnection to a constant current LED driver.

Figure 3 shows a block diagram of an LED arrangement 10 similar to that shown in Figure l, but with a protection circuit ed. Shown in Figure 3 is LED arrangement l0 comprising an LED load 50 comprising one or more LEDs, connected to the terminals 21, 22 of a constant current LED driver 20. A reservoir capacitor 23 is used as previously described to be charged by the power supply (not shown).

Current sense resistor 60 is also provided to enable g of the t flowing h the load 50 as previously bed.

In the embodiment shown in Figure 3, protection circuit 100 is provided to protect the load 50 from damage in the event that the load 50 is reconnected to a powered up driver 10 as will be described in more detail below. Protection circuit 100 may be provided as a separate component to LED driver 10 or may be orated within LED driver l0.

Figure 4 shows the main elements of protection circuit 100. In this embodiment, protection circuit 100 comprises a reconnection sensor 130 for detecting when the load 50 is ected to the terminals 2i, 22 of the LED driver 10. Protection circuit 100 also has current switch l 10 which is controlled by control circuit 120 to turn on when required, such as in this embodiment, to slowly turn on by changing effective resistance from a high value to a lower value, upon detection of reconnection to thereby limit a rate of rise of the current flowing h reconnected load 50 so that a large current spike as previously described will not occur.

The output of reconnection sensor 130 is applied to the control circuit 120 to, in this ment, initiate the slow turn on of the current switch 110 in response to the detection of reconnection.

Figure 5 shows the arrangement of Figure 3 with the protection circuit of Figure 4. In this embodiment, current switch I 10 is located in series with current sense resistor 60.

Reconnection sensor 130 can be any arrangement that es an indication that the load 50 has been reconnected to the terminals of the LED driver 10. In one embodiment, reconnection sensor 130 is an optical sensor that detects a drop in light level in terminal 21 and/or 22 when the load 50 terminal is inserted. In another embodiment, reconnection sensor 130 is a short—circuit sensor that detects an electrical connection directly at the terminal 21 and/or 22. In another embodiment, reconnection sensor I30 is a mechanical sensor that actuates upon mechanical deflection caused by reconnection of the load 50 to the terminal 21 and/or 22.

In one embodiment, upon detection of the tion, reconnection sensor 130 provides a connection signal to control t 120 to slowly close current switch 110 to re—establish current through load 50 at a controlled rate.

Figures 6A and 6B show illustrative and idealised waveforms of the effect of the protection circuit 100 in the arrangement of Figure 5. Figure 6A shows the voltage VT across the terminals of the driver at a time when the load 50 is connected, then the sudden increase in voltage as the load 50 is disconnected, and then the smooth ramp down as the load is reconnected. rly, Figure 6B shows the current IL h the load 50 at a time when the load is connected to driver 10, and then when the load 50 is disconnected (essentially dropping to zero) and then when the load 50 is reconnected. As can be seen in this case (in contrast to the waveform in Figure ZB), there is no current spike upon reconnection, but rather a slow and gradual increase over time up to the desired target of, for example, about 350mA. This gradual rise is due to the slow turn on of current switch l 10 under control of control circuit 120.

In another embodiment as shown in Figure 7, there is a current limiting element such as a current limiting 2O impedance, and in one ment, a current limiting resistor 140 in el with current switch 1 l0 and in series with current sense resistor 60. In one embodiment, the value of current limiting resistor 140 is selected so as to provide sufficient resistance to limit the maximum load t to a value that will not damage the LEDs of the load 50 at the time of ection. This allows another embodiment of reconnection sensor 130 which uses the sensed current through current sense resistor 60 to determine when the load 50 has been reconnected.

It will be appreciated that current limiting impedance can be any suitable impedance, including an active device.

In this embodiment, upon reconnection of load 50 to the driver 10, the current ately flows through sense resistor 60 and t limiting resistor 140, but at a limited level so as to not damage the LEDs in load 50 as will be understood by the person skilled in the art.

Upon sensing a current flowing through current sense resistor 60, reconnection sensor 130 initiates l circuit 120 to start the controlled n of current switch 110. As the current switch 110 becomes more conductive, more and more current is diverted from t limiting resistor 140 through current switch 110, until current switch 110 is fully on and providing minimal ance. At this time, the majority of the current has bypassed the larger current ng resistor 140 and is passing only through the much r current sense resistor 60. By this time, the driver has had sufficient time to regulate the load current to its target level of for example about 350mA so that the effect of t limiting resistor is not required and the circuit looks electrically substantially like that shown in Figure 5. Accordingly, the effect of larger current limiting or 140 on power drain is l since it is only electrically present for a short period of time after reconnection.

Figure 8 shows a circuit diagram of one embodiment of the arrangement 10 including protection circuit 120. Figure 8 shows driver 10 with reservoir capacitor 23 and power source 5, with terminals 21, 22.

Load 50 comprises four LEDs. Protection circuit 100 comprises current switch 1 10 with t limiting resistor 140 in parallel with it, and in series with current sense resistor 60. In this embodiment, reconnection sensor 130 is provided by comparator 131 having one input terminal connected between current sense resistor 60 and current limiting resistor 140 to provide a voltage input representative of the current flowing through current sense resistor 60. The other ting) input terminal of comparator 131 is connected to a reference voltage 132, which in this ment provides a “trip threshold” indicating tion and disconnection of the load 50. The value of the reference voltage may be set by various means and may be varied as will be described in more detail below.

It will be appreciated that in some embodiments, comparator 131 can be replaced by an amplifier with finite gain, wherein the ‘trip threshold’ is ined by the operation of elements at the output of the amplifier. For example, a MOSFET will have a gate threshold e’which can become the g threshold of operation of the reconnection sensor.

In one embodiment, when the load 50 is not ted to the driver 10, the current through current sense resistor 60 is zero and below the reference voltage. When the load 50 is reconnected, the current through current sense resistor 60 begins to se and when the voltage across t sense resistor exceeds the reference voltage, comparator 131 will output a signal to control circuit 120 indicating a reconnection. In control response, control circuit 120 will act to slowly turn on current switch 110 via application of a voltage to the gate of current switch 1 10 as previously described. 3O In this embodiment, reconnection sensor 130 can also act as a disconnection sensor. In this case, when load 50 is disconnected from driver 10, the current through current sense resistor 60 drops rapidly until the voltage across current sense resistor 60 falls below a second reference voltage. In this event, comparator 131 generates a signal to control circuit 120 that can then act to cause current switch 110 to open and thereby turn off. In one embodiment, the reference voltage is substantially equal to the second reference voltage.

Figure 9 shows yet another embodiment of the LED lighting arrangement 10 with load 50 connected to constant current driver 10 via als 21, 22. In this example, LED load 50 comprises three LEDs 50a, 50b and 500.

Protection circuit 100 is ed around t sensor 60 of the original driver circuit. In another embodiment, current sense resistor 60 may be provided in addition to the original current sense resistor.

Also ed is current switch 1 10 as well as parallel impedance or current ng resistor 140 in the main current path. While current switch l 10 is shown as a MOSFET in this embodiment, it will be appreciated that any suitable switching device or devices can be used in other embodiments. As previously sed, current ng resistor 140 is chosen to allow a small current to flow when the LED load 50 is reconnected, such that the LEDs will not be damaged even if the highest voltage is available on the reservoir capacitors l.

Current switch 1 10 is lled by l circuit 120 which exhibits the characteristics of a slow transition from ircuit to closed, and a sh01t transition from closed to open. The time taken to transit to the closed condition can be chosen so that the reservoir capacitors 1 will be discharged to allow current regulation to occur without any risk of an over—current surge in the LED load 50. In this embodiment, tailoring of the turn-on time in the circuit shown can be achieved by ing values of resistor 121 and capacitor 122, but it will be appreciated that there are many other possible ways to control the turn—on time including making use of digital circuit techniques such as A/D conversion. Diode 123 provides a simple means to control current switch 110 to the open state rapidly.

In operation, if the LED load 50 is ever disconnected from the circuit while power is applied, comparator 131 is used to establish the resulting loss of current flow by monitoring the e across sense resistor 60, and comparing the ed voltage with a low level reference, in this case shown as being d from a zener—diode reference provided by zener diode 124 and reference resistor 125 and voltage divider network provided by divider resistors 126 and 127. If the load current drops below a predetermined minimum level, comparator 131 will discharge capacitor 122 via diode 123, and quickly change the state of current switch 1 10 to open circuit. At any time subsequent to the break in connection, if the LED load 50 is nected to the terminals 21 and/or 22, the current flow through impedance 140 will be sufficient to bring the voltage across resistor 60 to the point where the output of comparator 131 will swing positive, reverse-biasing diode 123 so that current switch 110 can then slowly change state to fully conducting.

Figure 10 shows a circuit diagram of another ment of the general arrangement of Figure 7, and an alternative to the circuit shown in Figure 9. Figure 10 shows a circuit implementing the slow turn-on method of protection described previously. When the LED load 50 (in this figure shown as diodes D10 and D15) is disconnected, comparator USA will quickly turn off current switch 1 10 via diode connection to the gate of the switch. Once off, the e ng current regulator will behave as for the circuit with no tion, and raise the LED + terminal voltage to the maximum level. At this point, the LED - terminal is pulled low via current limiting resistor 140 and current sense resistor 60. When the LEDs are re—connected, a limited current ed to be within the capacity of the LEDS will flow h current limiting resistor 140 and current sense resistor 60, and the disconnect sensing comparator USA will allow the gate capacitor C1 to charge slowly through the high series resistance R9. As the gate voltage rises, the channel resistance of the t switch 110 will eventually become lower than that of current limiting or 140 and the LED current will begin to rise toward the regulation level. To keep a fast response, a small amount of non-damaging overshoot is allowed in the LED current. If the LED target current was set to a much lower value than the maximum, this circuit will always result in a significant overshoot, and consequently a flash of light from the LED. This does not however, detract from the function of the circuit in protecting the LED load from damage.

The t shown in figure 10 includes a second amplifier U4 connected to represent the action of voltage limiting as is normally implemented in constant current drivers of this type.

Figures 1 1A, 11B, 11C and 11D show various rms at points in the circuit of Figure 10. Figure 11A shows the LED terminal voltages of the arrangement of Figure 10. Figure 11B shows the gate voltage of current switch l 10 as it transitions from an on state to off state and back to a slow turn on. Figure 1 1C shows the detection of reconnection and regulator signals through the load nection and reconnection described above and Figure 1 1D shows the LED current in the load 50 as the load is disconnected and subsequently reconnected.

Figure 12 shows a block diagram of another aspect as described . This arrangement provides an alternative means of ting the load 50 upon ection, in that it temporarily prevents the regulator 111 from ng the reservoir capacitor 23 and allows the reservoir capacitor 23 to discharge at least to some extent, before current switch 110 turns on to allow current to flow through the load 50.

As shown in Figure 12, when sensor 130 detects a reconnection ofthe load 50, a signal is sent to regulator 1 11 to shut it down while the signal is present. This will allow capacitor 23 to discharge through load 50, sense resistor 60 and in some embodiments (not shown in Figure 12), an impedance across current switch 1 10. In this way, at the point when the circuit is closed by the turning on of current switch 110, the current that flows through the load is reduced because the terminal voltage has reduced compared to prior to reconnection. In one embodiment, the current switch 110 turns on when the e across the reservoir capacitor 23 is substantially equal to the load voltage. This value can be set by appropriate - selection of circuit components, or by the use of a microprocessor.

In one embodiment of this aspect, once signalled to turn on, current switch I 10 can be controlled as previously described to control the rate of rise of the current flowing h load 50 to provide a slow turn on function.

Generally then, this aspect provides a protection circuit for protecting a load comprising at least one non- incandescent load upon reconnection to a terminal of a constant current load driver after disconnection, the constant current load driver comprising a main capacitor for ing a voltage to the terminal of the constant current load driver. In this aspect, the protection circuit comprises a reconnection sensor 130 for sensing reconnection of the load to the load driver, a ck signal means by which power delivered to the main capacitor 23 can be ntially reduced to allow the capacitor to discharge; and a current switch I 10 llable to control t flowing through the load 50 after the capacitor 23 has d to discharge.

In one embodiment, the feedback signal means is provided by a signal output from (or indirectly from) IS the sensor 130, to the regulator 111.

In one embodiment, current switch 110 has t limiting impedance or resistor 140 in parallel. In this arrangement, d current will flow through load 50 upon reconnection via current limiting resistor I40, but current switch I 10 will not turn on until the reservoir capacitor 23 has discharged at least partially.

In another embodiment, there need not be a current limiting resistor in parallel with current switch I l0 and so no t will flow through load 50 upon reconnection until current switch 1 10 is turned on. In this embodiment, reconnection sensor 130 detects the reconnection of the load 50 via means other than a current flow through a sense resistor as previously described. Upon this detection, switchmode regulator l l l is disabled as previously described, and reservoir capacitor 23 is allowed to begin discharging. At this time, current switch 110 remains open so that no current flows h the load 50 until the voltage across the reservoir capacitor 23 (and in turn the als 21, 22 of the driver) has reduced from the m disconnected value. When the voltage of across reservoir capacitor 23 has reduced, for example to a predefined value, current switch 110 will begin to turn on to allow current to flow through load 50. In one embodiment, current switch 110 will turn on slowly to control the rate of rise of the current through the load until it has reached a steady value. In another embodiment, the current switch 110 will turn on quickly to allow the current through the load to reach the steady or final value almost instantaneously.

These different embodiments are sed in more detail with reference to the following figures.

Figure 13 shows an embodiment of this enhancement to the urn~on protection to reduce recovery time and current overshoot. This circuit provides a signal to the regulation circuitry so that while the reservoir capacitor 23 is discharging through the load 50 and resistors R1 and R2, the regulation circuit is not simultaneously charging the capacitor 23. This removes the need to precisely adjust the slope of the current switch 110 gate voltage, and can result in faster circuit recovery after the LED load 50 is re- connected. In addition, power dissipation in the current switch 1 10 is reduced. s 14A, 14B, 14C and 14D show various waveforms at points in the circuit of Figure 13. Figure 14A shows the terminal voltages in the circuit of Figure 13; Figure 14B shows the gate voltage of the current switch 1 10 in the circuit of Figure 13; Figure 14C shows the fault detection and regulator s in the circuit of Figure 13 and Figure 14D shows the load current in the circuit of Figure 13. It can be seen in Figure 14D that the current drops upon reconnection of the load 50 to the driver because the voltage at the terminals is ng, until the current switch 110 allows the t to increase to its stable state with slow turn on as described above. It can also be seen that the current overshoot as shown in Figure 14D is reduced or eliminated as ed to that of Figure 11D.

Figure 15 shows a further circuit embodiment of a controlled protection circuit in which the rge of the reservoir tor 23 is maintained until the current through current limiting resistor 140 drops to a predefined level, at which point it is safe to simply turn the current switch 1 10 back on. This ment provides a fast turn on of current switch 1 10 while still providing load reconnection protection.

Figures 16A, 168, 16C and 16D show various waveforms at corresponding points in the circuit of Figure . Figure 16A shows the terminal voltages which show a continuously reducing voltage due to the continuous rge of reservoir capacitor 23. Figure 16B shows the gate voltage of current switch 1 10 and shows how the turn on starts later than in previous t embodiments, but turns fully on much more quickly. Figure 16C shows the reconnection and regulator voltages and Figure 16D shows the load current which reaches its stable and safe state very quickly due to the fast turn on of current switch 1 10.

The various aspects described herein also provide for various methods of protecting a load upon 3O reconnection to a load driver. In one aspect, as shown in Figure 17, there is provided method of ting a load upon reconnection to a load driver. In one embodiment, the method comprises the steps of detecting reconnection of the load to the load driver (step 200) and then in step 201 the rate of , controlling rise of t through the reconnected load.

In another embodiment, as shown in figure 18, the method further ses, in step 203, upon detecting a disconnection of the load from the load driver, g a switch to open to protect the load, then upon detecting a subsequent connection, to allow the voltage at terminals of the load driver to reduce before reclosing the switch.

In another aspect described herein, there is provided another method of protecting a load upon reconnection ofthe load to a load driver having a reservoir capacitor. In this , an embodiment of which is shown in Figure 19, the method comprises the steps of, in step 300, ing reconnection of the load to the load driver, in step 301, causing or otherwise allowing the reservoir capacitor to discharge, and in step 302 delaying the turn on of a current switch to control flow of current through the reconnected load until the capacitor has at least lly discharged.

It will also be appreciated that the various protection circuits and methods can be applied to load drivers such as LED drivers such that an LED driver will orate a protection circuit as shown in any one or more or of the ments described herein. Furthermore, it will also be appreciated that the various protection ts and methods can be applied to dimmer circuits such that a dimmer circuit will incorporate a protection circuit as shown in any one or more or of the embodiments described herein.

Suitable dimmer circuits can be any conventional dimmer circuit including any of those described in the previously—referenced patent applications whose contents are incorporated herein in their entirety.

It will also be appreciated that the above has been bed with reference to particular illustrative embodiments only, and that many variations and modifications may be made to the circuits, devices and methods described.

It will be understood that the term “comprise” and any of its tives (eg. comprises, comprising) as used in this cation is to be taken to be inclusive of features to which it refers, and is not meant to exclude the presence of any additional features unless otherwise stated or d.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge of the technical field

Claims (14)

CLAIMS :

1. A protection circuit for protecting a load comprising at least one non—incandescent load upon reconnection to a constant current load driver after disconnection, the protection circuit comprising: a reconnection sensor for sensing reconnection of the load to the load driver; a current switch arranged in series between the constant current load driver and the load; and a control circuit for controlling the current switch by controlling the transition from open circuit to closed circuit to limit a rate of rise of t through the load upon the reconnection sensor sensing ection of the load to the load driver.

2. A protection circuit as claimed in claim 1 wherein the non-incandescent load is a Light Emitting Diode (LED) and the load driver is an LED driver.

3. A protection circuit as d in claim 2 r comprising a current limiting impedance in parallel with the current switch.

4. A tion circuit as d in claim 3 further comprising a disconnection sensor for sensing when the load is disconnected from the LED driver.

5. A protection circuit as claimed in claim 2 wherein the reconnection sensor has a first input for receiving a reference voltage and a second input for receiving a voltage input representative of a voltage across a current sense resistor.

6. A protection circuit as claimed in claim 5 n the reconnection sensor is a comparator.

7. A protection circuit as claimed in claim 5 wherein the ection sensor is an amplifier with a finite gain.

8. A protection circuit as claimed in claim 4 wherein the disconnection sensor comprises a comparator having a first input for receiving a second reference voltage and a second input for receiving a voltage input entative of a voltage across a current sense resistor.

9. A protection circuit as claimed in claim 8 wherein disconnection of the load is sensed when the e across the current sense or reduces to below the second reference voltage.

10. A protection circuit as claimed in claim 5 n reconnection of the load to the LED driver is sensed when the voltage across the current sense resistor reaches the value of the reference voltage.

11. A protection circuit as claimed in claim 10 wherein the reconnection sensor is a comparator, and the reconnection sensor also acts as a disconnection sensor, and nection of the load is sensed when the voltage across the current sense resistor reduces below the reference voltage.

12. A method of protecting a load upon reconnection to a constant current load driver, the load comprising at least one non-incandescent load, the method comprising the steps of: detecting reconnection of the load to the load driver; and controlling the tion from open circuit to closed circuit of a current switch located between and in series with the load and load driver so as to limit a rate of rise of current through the reconnected load upon detecting the reconnection of the load to the load driver.

13. A method as d in claim 12 further sing the steps of: upon detecting reconnection ofthe load to the load driver, causing a voltage at terminals of the load driver to reduce prior to g on the current switch.

14. A protection circuit as claimed in any one of claims 1 to 11 substantially as herein described with reference to the accompanying