APPARATUS AND METHOD FOR DETECTING A SHORT CIRCUIT IN A LIGHTING SYSTEM

Abstract: An apparatus and method for detecting a short circuit in a load which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the load is connected to the secondary winding. The apparatus comprises a waveform conversion circuit for converting voltage and current signals that represent the voltage and current, respectively, of the primary winding into generally squarewave voltage and current signals, respectively. The apparatus further comprises a phase detector circuit for outputting a control signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference. The apparatus further comprises a filter for filtering the signal outputted by the phase detector circuit. In one embodiment, the apparatus further comprises a circuit for deactivating the load when the control signal has the second state. In one embodiment, the apparatus and method of the present invention are used to detect a short in a lighting system that receives power from a transformer that has a primary winding and a secondary winding wherein the load is connected to the secondary winding.

Description: Apparatus and method for detecting a short circuit in a lighting system The present invention generally relates to an apparatus for detecting a short circuit in a load that is connected to a secondary winding of a transformer. Resonant transformers are typically used in lighting systems. Resonant transformers usually have a relatively high turn ratio. As a result of this high turn ratio, the impedance of the leakage inductor between the primary winding and the secondary winding is relatively high. When a switching frequency is applied to the transformer that is relatively higher than the resonant frequency (inductive mode), the impedance of the leakage inductor is generally the same as the impedance of the transformer's output load. Due to the high impedance of the leakage inductor, the primary current of the transformer will not increase much more even if the output load is shorted. The conventional technique of detecting the amplitude of the primary current cannot determine if the output load is shorted. It is therefore an object of the present invention to provide a unique apparatus and method for detecting a short circuit in a load connected to the secondary winding of a transformer. Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention. In one aspect, the present invention is directed to an apparatus for detecting a short in a load which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the load is connected to the secondary winding. The apparatus comprises a waveform conversion circuit for converting voltage and current signals representing the transformer primary winding voltage and current signals, respectively, into generally squarewave voltage and current signals, respectively. The apparatus further comprises a phase detector circuit for outputting a control signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference. The apparatus further comprises a filter for filtering the signal outputted by the phase detector circuit. In one embodiment, the apparatus further comprises a circuit for deactivating the load when the control signal has the second state. In one embodiment, the apparatus included means for indicating that a short exists in the load when the control signal has the first state. In a related aspect, the present invention is directed to an apparatus for detecting a short in a lighting system which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the lighting system is connected to the secondary winding. The apparatus comprises means for converting voltage and current signals representing the transformer primary voltage and current signals, respectively, into generally squarewave voltage and current signals, respectively, means for generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference, and means for filtering the signal outputted by the phase detector circuit. In yet a further aspect, the present invention is directed to a method for detecting a short in a load that receives power from a transformer that has a primary winding and secondary winding wherein the load is connected to the secondary winding. The method comprises the steps of providing voltage and current signals that represent the transformer primary voltage and current signals, respectively, converting the signals that represent the transformer primary voltage and current signals into generally squarewave voltage and current signals, respectively, generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference, and filtering the signal outputted by the phase detector circuit. The features of the invention are believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The invention itself, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: FIG. 1 is a schematic diagram of a transformer. FIG. 2 is a phase diagram of a transformer's voltage and current for normal operation. FIG. 3 is a phase diagram of a transformer's voltage and current when a short circuit exists. FIG. 4 is a block diagram of the apparatus of the present invention. FIG. 5 is a circuit diagram of one embodiment of the apparatus of FIG. 4. FIG. 6 illustrates the signal waveforms outputted by a waveform converter circuit and phase detector circuit shown in FIG. 4 during normal operation. FIG. 7 illustrates the signal waveforms outputted by the waveform converter circuit and a filter circuit shown in FIG. 4 during normal operation. FIG. 8 illustrates the signal waveforms outputted by the waveform converter circuit and the phase detector circuit shown in FIG. 4 during short circuit conditions. FIG. 9 illustrates the signal waveforms outputted by the waveform converter circuit and the filter circuit shown in FIG. 4 during short circuit conditions. In order to facilitate understanding of the present invention, the ensuing description pertains to the basic operational theory of resonant transformers. Referring to FIG. 1, there is shown an equivalent circuit of a resonant transformer. When a relatively high working frequency is applied to a resonant circuit, the transformer primary winding current Ip and primary winding voltage Np can be detected without difficulty. The transformer primary voltage Np increases with the transformer secondary voltage Ns. Similarly, the transformer primary current Ip increases with the transformer secondary current Is. The transformer primary voltage Np and current Ip include the amplitude and phase information. The phase of voltage Np is substantially the same as the phase of voltage Ns. Similarly, the phase of current Ip is generally the same as the phase of current Is. The load voltage, current and resistance are represented by Vo, Io . and Ro, respectively. The transformer magnetizing current is represented by I M and is typically very low. Referring to FIG. 2, there is shown a phase diagram of the transformer's voltage and current for normal operation. In view of the foregoing discussion, the voltage Ns represents the voltage Np and the current Is represents current Ip. Under normal operation, the phase angle Alpha between the voltage N s and the current I s is less than fifty degrees. Referring to FIG. 3, there is shown the phase diagram of the transformer's voltage Vs and current I s for short circuit operation, i.e. the transformer output is shorted. It has been found that, as a result of the short circuit, the phase angle Alpha between the voltage Vs and the current Is is greater than seventy degrees. Therefore, in accordance with the present invention, the phase angle between the voltage Ns and the current Is is measured to determine if the output of the transformer is shorted. This is accomplished by measuring the phase angle between the voltage Np and the current Ip since, as described in the foregoing discussion, the phase of voltage Np and current Ip is substantially the same as Ns and current Is, respectively. Referring to FIG. 4, there is shown a block diagram of apparatus 10 of the present invention. Although the ensuing description is in terms of apparatus 10 being used with lighting systems, it is to be understood that apparatus 10 can be used with any type of load that receives power from a resonant transformer. Apparatus 10 generally comprises waveform conversion circuit 12, phase detection circuit 14 and output filter 16. The transformer primary voltage Vp and current Ip waveforms, respectively, are inputted into waveform conversion circuit 12. Waveform circuit 12 shapes these waveforms into squarewave signals. These shaped waveforms are inputted into phase detector circuit 14. Phase detector circuit 14 detects the phase difference between the voltage and current squarewave signals outputted by circuit 12 and outputs a signal having a first state (e.g. TTL logic level "LOW") if the voltage and current squarewave signals are substantially in phase and a second state (e.g. TTL logic level "HIGH") if the difference in phase between the voltage and current squarewave signals exceeds a predetermined phase difference. A short in the load exists when the difference in phase between the voltage and current squarewave signals exceeds the predetermined phase difference. In one embodiment, the predetermined phase difference is 50 degrees. In another embodiment, the predetermined phase difference is 70 degrees. If the difference in phase between the voltage and current squarewave signals does not exceed the predetermined phase difference, then no short exists in the load. Referring to FIG. 4, the output of phase detector circuit 14 is inputted into filter circuit 16 which removes any A.C. (Alternating Current) ripple, voltage spikes or other transient signals. The output of filter circuit 16 is then inputted into a control circuit (not shown). If the voltage outputted by filter circuit 16 is above a predetermined level, the control circuit (not shown) deactivates the load to which the transformer secondary winding is connected. For example, if the signal outputted by filter circuit 16 is a logic level ("1"), then the control circuit (not shown) deactivates the load. Referring to FIG. 5, there is shown one embodiment of apparatus 10 of the present invention. In this particular embodiment, apparatus 10 is configured to detect short circuits for two transformers in series wherein each transformer is associated with a corresponding lamp. However, it is to be understood that the circuit of FIG. 5 can be used with one lamp or more than two lamps. Waveform conversion circuit 12 generally comprises input signal circuitry for receiving the transformer primary voltage waveforms associated with the pair of lamps and for receiving the transformer primary current waveform. Circuit 12 includes resistors Rl and R2 and capacitor Cl which are configured as a "T" network. One end of resistor Rl is connected to a transformer primary auxiliary winding portion TPAI to receive voltage VPAI which is in phase with voltage Vp. The other end of resistor Rl is connected to one end of resistor R2 and capacitor C2. The primary auxiliary winding portion TPAI corresponds to the transformer secondary winding voltage which is applied to a first lighting system or lamp. The other end of capacitor C2 is connected to ground potential. In one embodiment, resistor Rl has a resistance of 10KΩ and resistor R2 has a resistance of 100KΩ. In one embodiment, capacitor Cl has a capacitance of lOOpf (picofarads). Circuit 12 further includes resistor R3, capacitor C2 and Zener diode Dl. One of resistor R3 is connected to one end of capacitor C2 and the anode of Zener diode Dl . The other end of capacitor C2 is connected to ground potential. In one embodiment, resistor R3 has a resistance of 100KΩ. Capacitor C2 functions as a bypass capacitor and prevents A.C. signals from being applied to Zener diode Dl and in one embodiment, has a capacitance of 0.01 uf (microfarads). In one embodiment, the Zener voltage of Zener diode Dl is about 3.3 volts. Resistor R3 and Zener diode Dl provides a D.C. offset voltage for the voltage signal V P AI. Referring to FIG. 5, waveform conversion circuit 12 further includes resistors R4 and R5 and capacitor C3 which are configured in a "T" network. One end of resistor R4 is connected to transformer primary auxiliary winding portion T P 2 to receive voltage VPA2 which is in phase with voltage Vp. The other end of resistor R4 is connected to one end of resistor R5 and capacitor C3. Voltage V P A 2 is applied to a second lighting system or lamp. The other end of capacitor C3 is connected to ground potential. In one embodiment, resistor R4 has a resistance of 10KΩ and resistor R5 has a resistance of 100KΩ. In one embodiment, capacitor C3 has a capacitance of lOOpf (picofarads). Circuit 12 further includes resistor R6. Resistor R6 is connected between resistor R5 and Zener diode Dl . In one embodiment, resistor R6 has a resistance of 100KΩ. Resistor R6 and Zener diode Dl cooperate to provide a D.C. offset voltage for voltage signal VPA_. Referring to FIG. 5, circuit 12 further includes resistors R7 and R8 and capacitor C4 which are arranged in a "T" network. In one embodiment, resistors R7 and R8 have resistances of 10KΩ and 100KΩ, respectively. In one embodiment, capacitor C4 has a capacitance of about lOOpf. The current flowing through the transformer secondary winding is inputted into one end of resistor R7. Resistor R10 and Zener diode Dl cooperate to provide a D.C. offset for current IP A which flows through transformer auxiliary winding portions TP A I andTp 2- The current IPA is inputted into comparator 22. Referring to FIG. 5, waveform converter circuit 12 further comprises resistors R9, RIO, Rl 1, R12, R14 and R15, and comparators 18, 20 and 22. Resistor R9 is connected between supply voltage +Vcc and the anode of Zener diode Dl . In one embodiment, supply voltage +Vcc is about 5.0 volts D.C. Resistor RIO is connected between resistor R8 and the anode of Zener diode Dl. Resistor RIO is also connected to the non-inverting input of comparator 22. Resistors Rl 1 and R12 are configured in a voltage divider circuit. Resistor Rl 1 is connected between the anode of Zener diode Dl and one end resistor R12. The other end of resistor Rl 2 is connected to ground potential. The junction of resistors Rl 1 and Rl 2 is connected and provides a bias voltage to the inverting inputs of comparators 18, 20 and 22. The junction of resistors R2 and R3 is connected to the non-inverting input of comparator 18. The junction of resistors R5 and R6 is connected to the non-inverting input of comparator 20. Resistors R13, R14 and R15 function as pull-up resistors and are connected between +Vcc and the outputs of comparators 18, 20 and 22, respectively. In one embodiment, resistor Rl 0 has a resistance of 100KΩ. In one embodiment, each resistor R9, Rl 1, R12, R13, R14 and R15 has a resistance of 10KΩ. Referring to FIG. 5, phase detector 14 generally comprises XOR (Exclusive OR) gates 24 and 26. One input of gate 24 is connected to the output of comparator 18. The other input of gate 24 is connected to the output of comparator 22. One input of gate 26 is connected to the output of comparator 20. The other input of gate 26 is connected to the output of comparator 22. The outputs of gates 24 and 26 are inputted into output filter circuit 16. Referring to FIG. 5, filter circuit 16 generally comprises two R-C type filter circuits. Each R-C filter circuit receives an output from a respective XOR gate. One filter circuit comprises resistor R16 and capacitor C5. The output of XOR gate 24 is connected to resistor R16. The other filter circuit comprises resistor R18 and capacitor C6. The output of XOR gate 26 is connected to resistor R18. In one embodiment, each resistor R16 and R18 has a resistance of 100KΩ. In one embodiment, each capacitor C5 and C6 has a capacitance of 0. luf. The R-C filter circuits smooth out the output of the XOR gates 24 and 26 and filter out any AC ripple, ringing or transient spikes. Operation Referring to FIG. 5, voltage VPAI inputted into waveform converter circuit 12. Specifically, voltage V PA ι is inputted into the input circuit comprising resistors Rl and R2 and capacitor Cl. As described in the foregoing description, resistors Rl and R3 and Zener diode Dl provides a D.C. offset voltage for the voltage VP A I- Voltage V P AI and the offset voltage are then inputted into the non-inverting input of comparator 18. Similarly, voltage Vp 2 is inputted into waveform converter circuit 12. Specifically, voltage VP 2 is inputted into the input circuit comprising resistors R4 and R5 and capacitor C3. Referring to FIG. 5, the current IPA is inputted into waveform converter circuit 12 at resistor R7. The voltages V P AI and V P A 2 and current IP A are in the form of sinusoidal waveforms. As will be explained below, comparators 18, 20 and 22 convert the sinusoidal waveforms into generally squarewave signal waveforms. Voltage VPAI, which is associated with the first lamp, is inputted into the non-inverting input of comparator 18. The output of comparator 18 is inputted into phase detector circuit 14. Specifically, the output of comparator 18 is inputted into one input of XOR gate 24. The current waveform IPA is outputted by comparator 22 and is inputted into the second input of XOR gate 24. Referring to FIGS. 6, waveforms 30 and 32 are the IPA and V P AI waveforms, respectively, that are outputted from comparators 18 and 22, respectively, during normal conditions (i.e. no short). Waveforms 30 and 32 have an offset voltage Vos- Waveforms 30 and 32 are inputted into XOR gate 24. Waveform 34 is the output of XOR gate 24. Since the IPA and V P AI waveforms are in phase, waveform 34 is generally at TTL logic level "LOW" indicated by voltage Ni. The output of XOR gate 24 is inputted into the R-C filter circuit comprising resistor R16 and capacitor C5. Referring to FIG. 7, waveform 36 is outputted by this R-C filter circuit and is at a TTL logic level "LOW" level indicated by voltage N 2 which, in one embodiment, is between about 0.1 and 0.2 volts. Referring to FIG. 8, waveforms 40 and 44 are current signal Is and voltage N PA i when the first lamp is shorted. Waveforms 40 and 44 are outputted from comparators 18 and 22, respectively, and are inputted into XOR gate 24. Since there is a significant phase difference between waveforms 40 and 44, XOR gate 24 outputs waveform 46 which has a TTL "HIGH" level that is indicated by voltage V 3 . Waveform 46 is then inputted into the R- C filter circuit comprising resistor R16 and capacitor C5. This R-C filter circuit outputs waveform 48 shown in FIG. 9. Waveform 48 is generally a D.C. voltage N 4 that is at a TTL "HIGH" level of about 3.3 volts. Referring to FIG. 5, it is to be understood that due to the symmetry of the circuit design used to realize apparatus 10, the phase difference between current I PA and voltage signal VP A2 is detected in the same manner via comparators 20 and 22, XOR gate 26 and the R-C filter comprising resistor R18 and capacitor C6. Thus, the aforementioned R-C filter circuit outputs a waveform similar to waveform 36 when the second lamp is not shorted, and a waveform similar to waveform 48 when the second lamp is shorted. The waveforms outputted by the R-C filter comprising resistor R16 and capacitor C5 and the R-C filter comprising resistor R18 and capacitor C6 can be inputted into control circuitry (not shown) that deactivates either or both lamps if a short develops in either or both lamps. Since the voltage swing of the output of the R-C filter circuits is relatively large, each R-C filter may be configured to have a relatively low time constant even if the output ripple of the filter is high. The relatively low time constant also allows apparatus 10 to detect short circuits in a deep-dimming configuration which typically uses relatively low, duty-cycle PWM (pulse- width-modulation). An important advantage of apparatus of the present invention is that the per- unit cost of system 10 is relatively low because it is not necessary for apparatus 10 to detect the amplitude of the transformer's voltage and current, or circuit parameters such as leakage inductance Ls and coupling coefficients. Although FIG. 5 shows a circuit configured for use with two lamps, it is to be understood that the present invention can be used for one lamp or more than two lamps. Furthermore, the component values (i.e. resistances and capacitances) for the resistors and capacitors described above are just examples of suitable component values. It is to be understood that other suitable component values can also be used. Furthermore, although the comparators shown in FIG. 5 are configured to operate with a single power source +Vcc, it is to be understood that the comparators can be configured to operate with two power sources. The principals, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations in changes may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not limited to the scope and spirit of the invention as set forth in the attached claims.

Claims: CLAIMS: 1. An apparatus (10) for detecting a short in a load which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the load is connected to the secondary winding, the apparatus comprising: a waveform conversion circuit (12) for converting a voltage signal and a current signal that represent a voltage signal and current signal, respectively, of the primary winding into generally squarewave voltage and current signals, respectively; a phase detector circuit (14) for outputting a control signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference; and a filter (16) for filtering the signal outputted by the phase detector circuit. 2. The apparatus according to claim 1 wherein the phase detector circuit (14) comprises an XOR gate having a first input for receiving the generally squarewave voltage waveform and a generally squarewave current waveform. 3. The apparatus according to claim 1 wherein the filter (16) comprises an R-C filter. 4. The apparatus according to claim 1 further including means for indicating that a short exists in the load when the control signal has the first state. 5. The apparatus according to claim 1 wherein the phase detector (14) outputs the control signal with the second state when the phase difference between the generally squarewave voltage and current signals is greater than about 70 degrees. 6. The apparatus according to claim 1 further including a circuit responsive to the control signal for deactivating the load when the control signal has the second state. 7. A ballast for supplying power to a lamp comprising a transformer having a primary and a secondary winding and comprising an apparatus (10) for detecting a short in a lighting system that is connected to and receives power from the secondary of the transformer, the apparatus comprising: means (12) for converting voltage and current signals that represent the transformer primary winding voltage and current signals, respectively, into generally squarewave voltage and current signals, respectively; means (14) for generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference; and means (16) for filtering the signal outputted by the phase detector circuit. 8. A method for detecting a short in a load which receives power from a transformer that has a primary winding and secondary winding wherein the load is connected to the secondary winding, the apparatus comprising: providing voltage and current signals that represent the voltage and current signals, respectively, of the transformer primary winding; converting the provided voltage and current signals into generally squarewave voltage and current signals, respectively; generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference; and filtering the signal outputted by the phase detector circuit. 9. The method according claim 8 further including the step of indicating a short exists in the load when the control signal has the second state. 10. The method according to claim 8 further including the step of deactivating the load when the control signal has the second state.

Inventors: CHANG CHIN, WENG DA F, CHANG, CHIN, WENG, DA, F.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H02H7
IPC codes: H02H3, H02H7, H05B41