AU2012238246B2 - Light emitting diode load protection circuit - Google Patents

Abstract

Disclosed is a protection circuit and method for protecting a load comprising at least one non incandescent load upon reconnection to a constant current load driver after disconnection. The protection 5 circuit comprises a reconnection sensor for sensing reconnection of the load to the load driver; a switch controllable to limit a rate of rise of current through the load upon the reconnection sensor sensing reconnection of the load to the load driver; and a control circuit for controlling the switch. 130 100 60 22 Figure 5 Figure 6A 350mA Figure 6B

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Schneider Electric South East Asia (HQ) Pte Ltd Actual Inventor: Donald Murray Terrace Address for Service: C/- MADDERNS, GPO Box 2752, Adelaide, South Australia, Australia Invention title: LIGHT EMITTING DIODE LOAD PROTECTION CIRCUIT The following statement is a full description of this invention, including the best method of performing it known to us.

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 5 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. 2011904150 entitled "Light Emitting Diode Load Protection Circuit", filed on 7 October 2011. 0 The entire content of this provisional application is hereby incorporated by reference. INCORPORATION BY REFERENCE The following documents are referred to in the present application: 5 PCT/AU03/00365 entitled "Improved Dimmer Circuit Arrangement"; PCT/AU03/00366 entitled "Dimmer Circuit with Improved Inductive Load"; PCT/AU03/00364 entitled "Dimmer Circuit with Improved Ripple Control"; PCT/AU2006/001883 entitled "Current Zero Crossing Detector in A Dimmer Circuit"; PCT/AU2006/001882 entitled "Load Detector For A Dimmer"; 0 PCT/AU2006/001881 entitled "A Universal Dimmer"; PCT/AU2008/001398 entitled "Improved Start-Up Detection in a Dimmer Circuit"; PCT/AU2008/001399 entitled "Dimmer Circuit With Overcurrent Detection"; and PCT/AU2008/001400 entitled "Overcurrent Protection in a Dimmer Circuit". 25 The entire content of each of these documents is hereby incorporated by reference. BACKGROUND Non-incandescent loads are becoming very popular devices 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 30 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 current 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 35 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. Current flow from the capacitor 1, flows via a connection point 2 to the series-connected LED load 3, 5 then returns through the connection point 2 to a current sensing resistor 4 and back to the reservoir capacitor I terminal. The current sensing 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 resistance so that negligible power loss will be experienced when the circuit is operating, and 0 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 1 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 5 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 maintain 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 incorporate some voltage limiting protection to prevent 0 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 capacitor I without damage, but if the LED load 3 is reconnected to the circuit, the low resistance of current sense resistor 4 and the low dynamic resistance of the LEDs 3 -5 themselves will not be sufficient to limit the current enough to protect the LEDs from damage. Figures 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 driver, the voltage at the output 30 point 2 is about 3V (Figure 2A) and the current through LED load 3 is about 350mA (Figure 2B). At time I Oms, 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, driving the voltage at the output to a maximum of nearly 40V in this example (Figure 2A). 35 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 instant high current spike of nearly 18A through the LED load since the current is limited only by the low (for example I 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. 5 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 10 will be disconnected while the system is powered, but these conditions are not always possible in many commercial installations and still allow for the potential of damage to the LED load. SUMMARY According to a first aspect, there is provided a protection circuit for protecting a load comprising at least 15 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 20 to closed circuit to limit a rate of rise of current through the load upon the reconnection sensor sensing reconnection of the load to the load driver. In one embodiment, the non-incandescent load is a Light Emitting Diode (LED) and the load driver is an LED driver. 25 In one form, the circuit further comprises a current limiting impedance in parallel with the current switch. In one form, the circuit further comprises a disconnection sensor for sensing when the load is disconnected from the LED driver. 30 In one embodiment, 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. In one embodiment, the reconnection sensor is a comparator. 35 In another embodiment, the reconnection sensor is an amplifier with a finite gain. 4 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. 5 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. [0 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 reconnection to a terminal of a constant current load driver after [5 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 reconnection sensor for sensing reconnection of the load to the load driver; a capacitor switch; and a current switch; and 20 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 25 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 driver. 30 In one embodiment, the control circuit controls the transition 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 Emitting Diode (LED) driver comprising a protection circuit according to any of the first and second aspects. 35 According to a fourth aspect, there is provided a dimmer circuit comprising an LED driver according to the third aspect. 5 According to a fifth aspect, there is provided 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 5 controlling the transition 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. According to a sixth aspect, there is provided a method of protecting a load comprising at least one non ,0 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 capacitor 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 constant current load driver so that power delivered to the 15 reservoir capacitor can be substantially reduced to allow the reservoir 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. 20 According to a seventh aspect, there is provided 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 reservoir capacitor for providing a voltage to the terminal of the constant current load driver, the protection circuit comprising; 25 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 constant 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 reservoir capacitor to discharge, and the 30 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 protecting a non-incandescent load such as a Light Emitting Diode (LED) load from damage upon reconnection to a load driver. A 35 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. 6 In another general aspect, there is provided 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 5 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: [0 Figure 1 - shows a prior art example of a typical constant current LED load arrangement; Figure 2A - shows a waveform of the LED driver terminal voltage as it varies over disconnection and reconnection 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 1; 15 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 waveform of the LED driver terminal voltage as it varies over disconnection 20 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 of the protection circuit; Figure 8 - shows a circuit diagram of one embodiment of the protection circuit; 25 Figure 9 - shows one embodiment of an LED lighting arrangement with another embodiment of a protection circuit; Figure 10 - shows a circuit diagram of another embodiment of the arrangement of Figure 7 with disconnection and reconnection of the LED load represented by switches; Figure 1 IA - shows the LED terminal voltages of the arrangement of Figure 10; 30 Figure 1 B - shows the switch gate voltage in the arrangement of Figure 10; Figure 11 C - shows the fault detection and regulator signals in the arrangement of Figure 10; Figure I1D - 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; 35 Figure 13 - shows a circuit diagram of an arrangement that disables the regulator to allow discharging of the capacitor 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; 7 Figure 14C - shows the fault detection and regulator 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; 5 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 circuit of Figure 15; Figure 16D - shows the load current in the circuit of Figure 15; Figure 17 - shows a flow chart of one method of protecting a load; 10 Figure 18 - shows a flowchart of another method of protecting a load; and Figure 19 - shows a flowchart of another method of protecting the load. 7a DESCRIPTION While the various aspects described herein are described with reference to a Light Emitting Diode (LED) as the non-incandescent load, however, it will be appreciated that the various aspects are applicable to many other types of non-incandescent loads including but not limited to, Compact Fluorescent Lamps 5 (CFLs), plasma lamps and Organic Light Emitting Diodes (OLEDs). According to one embodiment disclosed herein, there is provided a protection circuit that reduces the likelihood of damage to an LED load upon reconnection to a constant current LED driver. 0 Figure 3 shows a block diagram of an LED arrangement 10 similar to that shown in Figure 1, but with a protection circuit included. Shown in Figure 3 is LED arrangement 10 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 sensing of the current flowing through the load 50 as 5 previously described. 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 0 incorporated within LED driver 10. 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 reconnected to the terminals 21, 22 of the LED driver 10. Protection circuit 100 also has current switch 110 which is controlled by control .5 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 through reconnected load 50 so that a large current spike as previously described will not occur. 30 The output of reconnection sensor 130 is applied to the control circuit 120 to, in this embodiment, 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 110 is located in series with current sense resistor 60. 35 Reconnection sensor 130 can be any arrangement that provides 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 8 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 130 is a mechanical sensor that actuates upon mechanical deflection caused by reconnection of the load 50 to the terminal 21 and/or 22. 5 In one embodiment, upon detection of the connection, reconnection sensor 130 provides a connection signal to control circuit 120 to slowly close current switch 110 to re-establish current through load 50 at a controlled rate. 0 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 Vr 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. Similarly, Figure 6B shows the current IL through the load 50 at a time when the load is connected to driver 10, and then when the load 50 is disconnected 5 (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 2B), 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 110 under control of control circuit 120. 0 In another embodiment as shown in Figure 7, there is a current limiting element such as a current limiting impedance, and in one embodiment, a current limiting resistor 140 in parallel with current switch 110 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 current to a value that will not damage the LEDs of the load 50 at the time of reconnection. This allows another embodiment of -5 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. 30 In this embodiment, upon reconnection of load 50 to the driver 10, the current immediately flows through sense resistor 60 and current 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. 35 Upon sensing a current flowing through current sense resistor 60, reconnection sensor 130 initiates control circuit 120 to start the controlled turn-on of current switch 1 10. As the current switch 110 becomes more conductive, more and more current is diverted from current limiting resistor 140 through current switch 110, until current switch 110 is fully on and providing minimal resistance. At this time, the

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majority of the current has bypassed the larger current limiting resistor 140 and is passing only through the much smaller 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 current limiting resistor is not required and the circuit looks electrically substantially like that shown in Figure 5. Accordingly, the 5 effect of larger current limiting resistor 140 on power drain is minimal 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. 0 Load 50 comprises four LEDs. Protection circuit 100 comprises current switch 110 with current 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 (inverting) input terminal of comparator 131 5 is connected to a reference voltage 132, which in this embodiment provides a "trip threshold" indicating connection 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 .0 finite gain, wherein the 'trip threshold' is determined by the operation of elements at the output of the amplifier. For example, a MOSFET will have a gate threshold voltage which can become the defining threshold of operation of the reconnection sensor. In one embodiment, when the load 50 is not connected to the driver 10, the current through current sense .5 resistor 60 is zero and below the reference voltage. When the load 50 is reconnected, the current through current sense resistor 60 begins to increase and when the voltage across current sense resistor exceeds the reference voltage, comparator 131 will output a signal to control circuit 120 indicating a reconnection. In response, control circuit 120 will act to slowly turn on current switch 110 via application of a control voltage to the gate of current switch 1 10 as previously described. 30 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 35 open and thereby turn off. In one embodiment, the reference voltage is substantially equal to the second reference voltage. 10 Figure 9 shows yet another embodiment of the LED lighting arrangement 10 with load 50 connected to constant current driver 10 via terminals 21, 22. In this example, LED load 50 comprises three LEDs 50a, 50b and 50c. 5 Protection circuit 100 is provided around current 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 provided is current switch 1 10 as well as parallel impedance or current limiting resistor 140 in the main current path. While current switch 110 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 0 previously discussed, current limiting 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 1. Current switch 110 is controlled by control circuit 120 which exhibits the characteristics of a slow 5 transition from open-circuit to closed, and a short 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 adjusting values of resistor 121 and capacitor 122, but it will be appreciated that there are many other possible ways to control the turn-on 0 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 voltage across sense resistor -5 60, and comparing the measured voltage with a low level reference, in this case shown as being derived 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 110 to open circuit. At any time subsequent to the break in connection, if the LED load 30 50 is re-connected 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. 35 Figure 10 shows a circuit diagram of another embodiment 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 U5A will quickly turn off current switch 110 via diode connection 1 1 to the gate of the switch. Once off, the voltage limiting current regulator will behave as for the circuit with no protection, 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 selected to be within the capacity of the LEDs will flow through current 5 limiting resistor 140 and current sense resistor 60, and the disconnect sensing comparator U5A will allow the gate capacitor Cl to charge slowly through the high series resistance R9. As the gate voltage rises, the channel resistance of the current switch 1 10 will eventually become lower than that of current limiting resistor 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 0 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 circuit shown in figure 10 includes a second amplifier U4 connected to represent the action of voltage 5 limiting as is normally implemented in constant current drivers of this type. Figures 1 A, I IB, 1 IC and 1 D show various waveforms at points in the circuit of Figure 10. Figure 1 IA shows the LED terminal voltages of the arrangement of Figure 10. Figure 1 B shows the gate voltage of current switch 110 as it transitions from an on state to off state and back to a slow turn on. Figure 11 C 0 shows the detection of reconnection and regulator signals through the load disconnection and reconnection described above and Figure IID 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 herein. This arrangement provides an .5 alternative means of protecting the load 50 upon reconnection, in that it temporarily prevents the regulator Il l from charging 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 of the load 50, a signal is sent to regulator 30 111 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 110. 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 voltage across the 35 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. 12 In one embodiment of this aspect, once signalled to turn on, current switch 110 can be controlled as previously described to control the rate of rise of the current flowing through load 50 to provide a slow turn on function. 5 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 providing 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 feedback signal means by which power delivered to 0 the main capacitor 23 can be substantially reduced to allow the capacitor to discharge; and a current switch 110 controllable to control current flowing through the load 50 after the capacitor 23 has started to discharge. In one embodiment, the feedback signal means is provided by a signal output from (or indirectly from) 5 the sensor 130, to the regulator 111. In one embodiment, current switch 110 has current limiting impedance or resistor 140 in parallel. In this arrangement, limited current will flow through load 50 upon reconnection via current limiting resistor 140, but current switch 110 will not turn on until the reservoir capacitor 23 has discharged at least 0 partially. In another embodiment, there need not be a current limiting resistor in parallel with current switch 110 and so no current will flow through load 50 upon reconnection until current switch 110 is turned on. In this embodiment, reconnection sensor 130 detects the reconnection of the load 50 via means other than a .5 current flow through a sense resistor as previously described. Upon this detection, switchmode regulator 11 is disabled as previously described, and reservoir capacitor 23 is allowed to begin discharging. At this time, current switch I 10 remains open so that no current flows through the load 50 until the voltage across the reservoir capacitor 23 (and in turn the terminals 21, 22 of the driver) , has reduced from the maximum disconnected value. When the voltage of across reservoir capacitor 23 has reduced, for 30 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. 35 These different embodiments are discussed in more detail with reference to the following figures. Figure 13 shows an embodiment of this enhancement to the slow-turn-on protection to reduce recovery 13 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 RI 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 5 connected. In addition, power dissipation in the current switch 110 is reduced. Figures 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 110 in the circuit of Figure 13; 0 Figure 14C shows the fault detection and regulator signals 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 dropping, until the current switch 1 10 allows the current 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 compared to 5 that of Figure I lD. Figure 15 shows a further circuit embodiment of a controlled protection circuit in which the discharge of the reservoir capacitor 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 110 back on. This embodiment .0 provides a fast turn on of current switch 110 while still providing load reconnection protection. Figures 16A, 16B, 16C and 16D show various waveforms at corresponding points in the circuit of Figure 15. Figure 16A shows the terminal voltages which show a continuously reducing voltage due to the continuous discharge of reservoir capacitor 23. Figure 16B shows the gate voltage of current switch I10 .5 and shows how the turn on starts later than in previous circuit 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 110. The various aspects described herein also provide for various methods of protecting a load upon 30 reconnection to a load driver. In one aspect, as shown in Figure 17, there is provided method of protecting 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, controlling the rate of rise of current through the reconnected load. 35 In another embodiment, as shown in figure 18, the method further comprises, in step 203, upon detecting a disconnection of the load from the load driver, causing 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. 14 In another aspect described herein, there is provided another method of protecting a load upon reconnection of the load to a load driver having a reservoir capacitor. In this method, an embodiment of which is shown in Figure 19, the method comprises the steps of, in step 300, detecting reconnection of the 5 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 partially discharged. It will also be appreciated that the various protection circuits and methods can be applied to load drivers 0 such as LED drivers such that an LED driver will incorporate a protection circuit as shown in any one or more or of the embodiments described herein. Furthermore, it will also be appreciated that the various protection circuits 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 5 previously-referenced patent applications whose contents are incorporated herein in their entirety. It will also be appreciated that the above has been described with reference to particular illustrative embodiments only, and that many variations and modifications may be made to the circuits, devices and methods described. 0 It will be understood that the term "comprise" and any of its derivatives (e.g. comprises, comprising) as used in this specification 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 implied. .5 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)

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 current through the load upon the reconnection sensor sensing reconnection 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 claimed in claim 2 further comprising a current limiting impedance in parallel with the current switch.

4. A protection circuit as claimed 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 wherein the reconnection sensor is a comparator.

7. A protection circuit as claimed in claim 5 wherein the reconnection 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 representative 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 voltage across the current sense resistor reduces to below the second reference voltage.

10. A protection circuit as claimed in claim 5 wherein 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. 16

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 disconnection 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 transition 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 claimed in claim 12 further comprising the steps of: upon detecting reconnection of the load to the load driver, causing a voltage at terminals of the load driver to reduce prior to turning on the current switch.

14. A protection circuit as claimed in any one of claims I to 11 substantially as herein described with reference to the accompanying Figures 3 to 19. 17