Abstract:
Disclosed is an overcurrent protection circuit for use in a dimmer circuit having a switching device for controlling power delivered to a load. The overcurrent protection circuit comprises means for sensing a load current passing through the load and means for comparing the sensed load current with a threshold, wherein the threshold is a dynamic current threshold. Also disclosed is a dimmer circuit comprising the overcurrent protection circuit.

Description:
TECHNICAL FIELD 
     The present invention relates to dimmer circuits and in particular, to detecting overcurrent conditions. 
     This Application is a U.S. National Stage filing under 35 U.S.C. §371 of International Application No. PCT/AU2008/001400, filed 19 Sep. 2008, which claims priority of Australian Provisional Patent Application No. 2007905110, filed on 19 Sep. 2007, Australian Provisional Patent Application No. 2007905108, filed on 19 Sep. 2007 and Australian Provisional Patent Application No. 2007905109, filed on 19 Sep. 2007, The contents of the aforementioned applications are incorporated by reference as if set forth fully herein. Priority to the aforementioned application is hereby expressly claimed in accordance with 35 U.S.C. §§119, 120, 365 and 371 and any other applicable statutes. 
     The entire content of each of these applications is hereby incorporated by reference. 
     INCORPORATION BY REFERENCE 
     The following documents are referred to in the following description: 
     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”; and 
     PCT/AU2006/001881 entitled “A Universal Dimmer” 
     Co-pending Australian Provisional Patent Application entitled “Dimmer Circuit With Overcurrent Detection”. 
     The entire content of each of these applications is hereby incorporated by reference. 
     BACKGROUND 
     Dimmer circuits are used to control the power provided to a load such as a light or electric motor from a power source such as mains power. Such circuits often use a technique referred to as phase controlled dimming. This allows power provided to the load to be controlled by varying the amount of time that a switch connecting the load to the power source is conducting during a given cycle. 
     For example, if voltage provided by the power source can be represented by a sine wave, then maximum power is provided to the load if the switch connecting the load to the power source is on at all times. In this way the, the total energy of the power source is transferred to the load. If the switch is turned off for a portion of each cycle (both positive and negative), then a proportional amount of the sine wave is effectively isolated from the load, thus reducing the average energy provided to the load. For example, if the switch is turned on and off half way through each cycle, then only half of the power will be transferred to the load. The overall effect will be, for example in the case of a light, a smooth dimming action resulting in the control of the luminosity of the light. 
     Modern dimming circuits generally operate in one of two ways—leading edge or trailing edge. In leading edge technology, the dimmer circuit “chops out” or blocks conduction of electricity by the load in the front part of each half cycle (hence the term “leading edge”). In trailing edge technology, the dimmer circuit “chops out “or blocks conduction of electricity by the load in the back part of each half cycle. 
     Since the load is connected to a high voltage or current source such as mains power, a defect in the circuit such as a short circuit, can lead to a sudden surge of high current, which can damage the load and any circuitry connected to the load. It is useful for the dimmer circuit to be able to detect the presence of such high, or overcurrent conditions, and act so as to remove the load and/or connected circuitry from the high current source. 
     The decision to act so as to remove the load and/or connected circuitry may be based upon the sensed current exceeding a preset threshold. A number of methods exist which provide a means and method of comparing the sensed current with a preset threshold. 
     In one method, the criteria for cutout is determined by the instantaneous current flowing through the dimmer exceeding a pre-determined threshold level, particularly for the condition when a power device is commencing conduction using controlled transition time while the instantaneous line voltage is high, in that the instantaneous power dissipation imposed upon the power device while a short-circuit load condition exists, is high. 
     In another existing method, the criteria for cutout is determined by the product of the instantaneous voltage appearing across the dimmer and the instantaneous current flowing through the dimmer exceeding a pre-determined threshold level i.e. instantaneous power level in the power semiconductor, however, such circuit designs are complex and expensive to design and manufacture. 
     SUMMARY 
     An overcurrent protection circuit for use in a dimmer circuit comprising a switching device for controlling power delivered to a load, the overcurrent protection circuit comprising:
         means for sensing a load current passing through the load; and   means for comparing the sensed load current with a threshold; wherein the threshold is a dynamic current threshold.       

     An overcurrent protection circuit wherein the switching device comprises a first switch and a second switch. 
     In one form, the dynamic current threshold is inversely proportional to the voltage appearing across the switching device. 
     In one form, the overcurrent protection circuit generates a cut out signal when the sum of the instantaneous voltage appearing across the dimmer and the instantaneous current flowing through the dimmer exceeds the threshold. 
     In one form, the overcurrent protection circuit further comprises a trip signal generator to generate a trip signal for disconnecting gate drive signals from the first and second switches. 
     In one form, the trip signal is applied to a latch circuit for disconnecting the gate drive signals from the controlling first and/or second switch. 
     In one form, the means for sensing the load current passing through the load comprises a current sense resistor RS 1  connected between the current path between the first and second switches. 
     In one form the first and second switches are MOSFETs and the current sense resistor is connected between a source of the first switch and the source of the second switch. 
     In another form, the first and second switches are IGBTs and the current sense resistor is connected between a collector of the first switch and the collector of the second switch. 
     In one form, the dynamic current threshold I T  is determined by:
 
 I   T   =[V   ref   −R 2· V   LL /( R 1+ R 2)]/ RS 1
 
Where:
 
     V LL =Line voltage−Load voltage 
     R 1 =Voltage sense resistor 
     R 2 =voltage converter resistor 
     RS 1 =Current sense resistance 
     V ref =reference voltage 
     According to another aspect of the present invention, there is provided a method for providing overcurrent protection in a dimmer circuit comprising a switching device for controlling power delivered to a load, the method comprising:
         sensing a load current passing through the load; and   comparing the sensed load current with a threshold; wherein the threshold is a dynamic current threshold.       

     In one form, the method further comprises generating a trip signal when the sensed load current exceeds the threshold, to isolate the load from the power. 
     In one form, the method further comprises calculating the threshold I T  according to the following relation:
 
 I   T   =[V   ref   −R 2· V   LL /( R 1+ R 2)]/ RS 1
 
Where:
 
     V LL =Line voltage−Load voltage 
     R 1 =Voltage sense resistor 
     R 2 =voltage converter resistor 
     RS 1 =Current sense resistance 
     V ref =reference voltage 
     According to another aspect of the present invention, there is provided a dimmer circuit comprising the overcurrent protection circuit of the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of the present invention will now be described in detail with reference to the following figures in which: 
       FIG.  1 —shows a circuit arrangement according to one aspect of the present invention, using dynamic current sensing; 
       FIGS.  2 A to  21 —show waveforms at various points in the circuit arrangement of  FIG. 1  during normal and short-circuit/overcurrent conditions; 
       FIG.  3 —shows the variation of trip current I T  with Line-Load voltage V LL  resulting from the present invention; and 
       FIG.  4 —shows a graph showing the short-circuit instantaneous power vs line voltage comparing the present invention with the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     A short-circuit protective cutout mechanism for the power semiconductors within a phase-control dimmer, whereby the criteria for cutout is determined by the sum of the instantaneous voltage appearing across the dimmer and the instantaneous current flowing through the dimmer exceeding a pre-determined threshold level. 
       FIG. 1  shows a dimmer circuit  10  controlling power delivered to the load as shown in  FIG. 1 . Dimmer  10  has a switching device, in this example provided by first and second switches MOSFETs Q 1  and Q 2  (for example SPA20N60C3). The switches turn on and off in response to dimmer gate drive signals provided by block  11  as will be understood by the person skilled in the art. The switch elements Q 1  and Q 2  operate/control the load alternately, each operating at different polarities during subsequent half-cycles of the power applied by the line. Each switch element has an associated respective anti-parallel diode D 1  and D 2 . 
     It will be understood that the various aspects of the present invention may be applied to any form of dimmer circuit, such as those described in 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”; and 
     PCT/AU2006/001881 entitled “A Universal Dimmer”, the entire content of each of which is hereby incorporated by reference. 
     The present example illustrates the operation of the circuit as switch Q 1  turns on.  FIG. 2A  shows the substantially-sinusoidal portion of the line current I L , with the turn-on of switch Q 1  and Q 2  (second half-cycle), whether by alternate or simultaneous gate activation. The corresponding line voltage V L  is shown in  FIG. 2B , with a peak value of 350V. At the scale shown in  FIG. 2A , the turn-on appears to be a step function, however, as will be appreciated by the person skilled in the art, there is a transition from non-conduction to full conduction, as shown in  FIG. 2C . In this example, the transition time from 0V to 350V is about 50 μS.  FIG. 2C-1  shows the transition of load current IL and  FIG. 2C-2  shows the corresponding transition of dimmer voltage VLL. 
     Referring back to  FIG. 1 , the instantaneous voltage appearing across the load controlling power device (Q 1 ) is represented as a signal current flowing through the shunt voltage sense resistor R 1 . This “Voltage” signal current is converted to a corresponding “Voltage” signal voltage by resistor R 2 —in series with the shunt voltage sense resistor R 1 . R 2  has small value compared to R 1 , and hence does not significantly influence the signal current. 
     The instantaneous current flowing through switch Q 1  is represented as a signal voltage across the series current sense resistor RS 1 . The series resistor circuit arrangement of R 1 , R 2  &amp; RS 1  results in addition of the “Voltage” signal voltage and the “Current” signal voltage to form a composite signal voltage at the junction of R 1  and R 2  relative to 0V reference potential. 
     The magnitude of the composite signal voltage is compared to a reference voltage Vref and when greater, will activate the fault current cutout latch. 
     It will be understood that various other means of sensing the current flowing through the load may be used, including the method as described in a co-pending PCT Patent Application entitled “Dimmer Circuit With Overcurrent Detection”, claiming priority from Australian Provisional Patent Application No. 2007905108 also entitled “Dimmer Circuit With Overcurrent Detection”, the entire content of which is hereby incorporated by reference. 
     Referring again to  FIG. 2 ,  FIGS. 2D to 2I  show various waveforms at different points in the circuit of  FIG. 1 , during the transition time of about 50 μS as shown in  FIG. 2C  described above. 
     In  FIG. 2D , it can be seen that VRS 1  increases as a constant ramp up to IRS 1 ×RS 1 , where IRS 1  is the corresponding instantaneous load current under normal load conditions. 
     In  FIG. 2E , it can be seen that as V LL  drops from 350V, the voltage at point A in  FIG. 1  (V A ) decreases as a continuous ramp function, from a value determined by VLL×R 2 /(R 1 +R 2 ) to a small offset determined by IRS 1 ×RS 1 . 
     Under short circuit conditions, in  FIG. 2F , it is seen that V RS1  across current sense resistor RS 1  also increases as a constant ramp function towards a substantially greater level than under normal conditions ( FIG. 2D ). 
       FIG. 2G  shows the value of VLL×R 2 /(R 1 +R 2 ), which under short circuit conditions, remains a constant. 
       FIG. 2H  shows the value of VA under short circuit conditions. The actual value of VA=[(VLL−2VRS 1 )(R 2 /(R 1 +R 2 )]+VRS 1  but under short circuit conditions, the value of VRS 1  is very small compared to the value of VLL and so it can reasonably be approximated that VA=VLL×(R 2 /R 1 +R 2 )+VRS 1 . Thus  FIG. 2H  shows the value of VA as the sum of  FIGS. 2G and 2F . 
       FIG. 2H  also shows the value of Vref, which crosses the function for VA. The constant reference voltage V ref  set in this example at a constant 0.5 volts. 
     It can be seen that at some point, VA crosses the value of Vref.  FIG. 2I  shows that at this crossover point, the voltage VC at point C in  FIG. 1 , jumps to the level of Vref, providing the trigger signal to latch circuit  12  ( FIG. 1 ), to disconnect the switches Q 1  and Q 2  from Dimmer Gate Drive Signal block  11 . 
     The Trip Current or dynamic current threshold, I T  can be calculates as:
 
 I   T   =[V   ref   −R 2· V   LL /( R 1+ R 2)]/ RS 1
 
Where:
 
     V LL =Line voltage−Load voltage 
     R 1 =Voltage sense resistor 
     R 2 =voltage converter resistor 
     RS 1 =Current sense resistance 
     V ref =reference voltage 
       FIG. 3  shows a plot of I T  as it varies with V LL  ranging from 0V to 350V, with the values of the components as shown in  FIG. 1 , and V ref  equal to about 0.5V.  FIG. 3  shows that the higher the line-voltage to load voltage, the lower the trip current is. This reduces the excessive power dissipation problems associated with prior methods where the trip current is static. 
       FIG. 4  shows a plot of Power (W) vs Line Voltage-Load Voltage V LL  for prior art methods using static current sensing and static power sensing as well as for the “dynamic current” sensing of the present invention. It can be seen that the power dissipated by the switching device before cutting out at a high line voltage is greatly reduced as compared to the static current method, and equal to that of the static power method. The complexity of the circuit design of the present invention is also far less than that required for the static power method. 
     It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications in its scope. 
     For example, in one particular application, it is possible to remove gate drive from one only of the first and/or second switches (whichever is controlling at the time), and allow the other switch to continue conducting every half cycle 
     It is also possible to apply the protection circuit of the present invention to a DC application, in which the switching device comprises only one switch. 
     The invention is equally applicable to other types of switching elements, including bi-polar transistors. Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. 
     The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.