Abstract:
A risk of shock (ROS) protection circuit is disclosed, which comprises a capacitor that charges whenever there is a voltage between a ground on a lamp ballast and earth ground. If the capacitor exceeds a predetermined threshold voltage, the capacitor causes a gate to shunt current away from a tertiary winding in a control circuit, which in turn reduces impedance reflected by the tertiary winding back on to primary and secondary windings in the ballast circuit. The reduced reflected impedance causes the operating frequency of the ballast to increase, reducing the voltage between the ballast ground and earth ground until it is safe for human contact. In this manner, a human replacing a lamp connected to the ballast can be protected from shock despite a failure to disconnect the power to the lamp ballast prior to lamp replacement.

Description:
BACKGROUND OF THE INVENTION 
     This application claims the benefit of provisional patent application Ser. No. 60/968,219, filed Aug. 27, 2007, which is incorporated by reference in its entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present application is directed to electronic ballasts. It finds particular application in conjunction with the resonant inverter circuits that operate one or more fluorescent lamps and will be described with the particular reference thereto. However, it is to be appreciated that the following is also amenable to high intensity discharge (HID) lamps and the like. 
     A ballast is an electrical device which is used to provide power to a load, such as an electrical lamp, and to regulate the current provided to the load. The ballast provides high voltage to start a lamp by ionizing sufficient plasma (vapor) for the arc to be sustained and to grow. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp. 
     Typically, after the alternating current (AC) voltage from the power source is rectified and appropriately conditioned, the inverter converts the DC voltage to AC. The inverter typically includes a pair of serially connected switches, such as MOSFETs which are controlled by the drive gate control circuitry to be “ON” or “OFF.” 
     Linear fluorescent lamp ballasts are required to meet a UL safety standard which calls for the quantification of the Risk of Shock (ROS). To meet such standards, the current that may flow through a human body model (HBM) when one end of a linear fluorescent lamp (LFL) is removed from its socket is measured, and is required to be less than the limit prescribed by UL. Inverters of the type described above typically do not have transformer isolation and are capable of producing ROS currents that may exceed the UL safety requirement. When such lamps need replacing, power to the lamps should be removed, in order to make changing the lamps safe for a human carrying out the replacement procedure. However, in practice, the step of cutting the power is often omitted. Even more dangerous is that workers often use their fingers to line up the pins on the lamp with the sockets in the lamp housing. If any other part of the worker is in contact with earth ground, then the workers body completes a circuit and the worker suffers a potentially lethal shock when high-frequency (e.g., 70 kHz-150 kHz or so) current pulses through the worker. 
     The following contemplates new methods and apparatuses that overcome the above referenced problems and others. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to an aspect, a risk-of-shock (ROS) protection system for a lamp comprises a ROS sensor circuit with a capacitor that charges when there is a voltage between earth ground and a ballast ground, a ballast circuit that is connected to one or more lamps and to the ballast ground, and a control circuit coupled to the ballast circuit and to the ROS circuit, wherein when the capacitor charges to a voltage that exceeds a predetermined threshold voltage level, the voltage between earth ground and the ballast ground is reduced. 
     According to another aspect, a ballast circuit for reducing the risk of shock to a human comprises an inverter circuit with primary and secondary inductor windings around a ferrite core, a resonant circuit, coupled to the inverter circuit and to at least one lamp, a control circuit that is hardwired to the inverter circuit and the resonant circuit and comprises a tertiary winding around the ferrite core to inductively couple the control circuit to the inverter circuit, and a ROS sensor circuit, hardwired to the control circuit, with a capacitor that charges when there is a voltage between a ground on the resonant circuit of the ballast and earth ground. 
     According to yet another aspect, a risk of shock protection circuit comprises a first diode with a cathode connected to a control circuit for a lamp ballast, and an anode connected to a first resistor and a first capacitor, a second diode with a cathode connected to the anode of the first diode, the first capacitor, and the first resistor, and an anode connected to a second capacitor, and a third diode with a cathode connected to the anode of the second diode and the second capacitor, and an anode connected to the first capacitor, the first resistor, and ground. The circuit further includes a second resistor that is coupled to the second capacitor and to earth ground, wherein the first resistor, the first capacitor, and the third diode are connected in parallel relative to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic illustration of a ballast circuit that includes a plurality of components for detecting whether a risk of shock (ROS) is present, and if so, folding back an inverter voltage to a safe level to prevent harm to a human; 
         FIG. 2  is an illustration of the ballast circuit and a corresponding control circuit coupled thereto, as well as a ROS protection circuit that detects wither a current level exceeds an acceptable ROS threshold level and folds back voltage supplied to an inverter to mitigate an ROS condition, if present; 
         FIG. 3  is an illustration of a more detailed diagram of the control circuit; 
         FIG. 4  is an illustration of the ROS circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a ballast circuit  6  includes a plurality of components that facilitate determining whether a risk of shock (ROS) is present, and if so, folding back an inverter voltage to a safe level to prevent harm to a human. The ballast is coupled to one or more lamps  24 ,  26 , . . . ,  28 , such as linear fluorescent lamps or the like. The ballast circuit  6  can be employed with an ROS circuit ( FIG. 4 ) to ensure that an ROS condition is mitigated when a user is replacing a lamp and fails to cut power to the lamp during the replacement procedure. 
     The ballast circuit  6  includes an inverter circuit  8 , a resonant circuit or network  10 , and a clamping circuit  12 . A DC voltage is supplied to the inverter  8  via a voltage conductor  14  running from a positive voltage terminal  16  and a common conductor  18  connected to a ground or common terminal  20 . A high frequency bus  22  is generated by the resonant circuit  10  as described in more detail below. Additionally, the high-frequency bus  22  is connected to a node labeled “+B,” which in turn is connected to a controller circuit  108 , described in greater detail below. First, second, . . . , nth lamps  24 ,  26 , . . . ,  28  are coupled to the high frequency bus via first, second, . . . , nth ballasting capacitors  30 ,  32 , . . . ,  34 . Thus if one lamp is removed, the others continue to operate. It is contemplated that any number of lamps can be connected to the high frequency bus  22 . E.g., each lamp  24 ,  26 , . . . ,  28  is coupled to the high frequency bus  22  via an associated ballasting capacitor  30 ,  32 , . . . ,  34 . Power to each lamp  24 ,  26 , . . . ,  28  is supplied via respective lamp connectors  36 ,  38 . 
     The inverter  8  includes analogous upper and lower or first and second switches  40  and  42 , for example, two n-channel MOSFET devices (as shown), serially connected between conductors  14  and  18 , to excite the resonant circuit  10 . Two P-channel MOSFETs may also be configured. The high frequency bus  22  is generated by the inverter  8  and the resonant circuit  10  and includes a resonant inductor  44  and an equivalent resonant capacitance which includes the equivalence of first, second and third capacitors  46 ,  48 ,  50 , and ballasting capacitors  30 ,  32 , . . . ,  34  which also prevent DC current flowing through the lamps  24 ,  26 , . . . ,  28 . The ballasting capacitors  30 ,  32 , . . . ,  34  are primarily used as ballasting capacitors. 
     The switches  40  and  42  cooperate to provide a square wave at a common or first node  52  to excite the resonant circuit  10 . Gate or control lines  54  and  56 , running from the switches  40  and  42  are connected at a control or second node  58 . Each control line  54 ,  56  includes a respective resistance  60 ,  62 . 
     With continuing reference to  FIG. 1 , first and second gate drive circuitry or circuit, generally designated  64 ,  66 , is connected between the nodes  52 ,  58  and includes first and second driving inductors  68 ,  70  which are secondary windings mutually coupled to the resonant inductor  44  to induce in the driving inductors  68 ,  70  voltage proportional to the instantaneous rate of change of current in the resonant circuit  10 . First and second secondary inductors  72 ,  74  are serially connected to the respective first and second driving inductors  68 ,  70  and the gate control lines  54  and  56 . 
     The gate drive circuitry  64 ,  66  is used to control the operation of the respective upper and lower switches  40  and  42 . More particularly, the gate drive circuitry  64 ,  66  maintains the upper switch  40  “ON” for a first half of a cycle and the lower switch  42  “ON” for a second half of the cycle. The square wave is generated at the node  52  and is used to excite the resonant circuit  10 . First and second bi-directional voltage clamps  76 ,  78  are connected in parallel to the secondary inductors  72 ,  74  respectively each including a pair of back-to-back Zener diodes. The bi-directional voltage clamps  76 ,  78  act to clamp positive and negative excursions of gate-to-source voltage to respective limits determined by the voltage ratings of the back-to-back Zener diodes. Each bi-directional voltage clamp  76 ,  78  cooperates with the respective first or second secondary inductor  72 ,  74  so that the phase angle between the fundamental frequency component of voltage across the resonant circuit  10  and the AC current in the resonant inductor  44  approaches zero during ignition of the lamps. 
     Serially connected resistors  80 ,  82  cooperate with a resistor  84 , connected between the common node  52  and the common conductor  18 , for starting regenerative operation of the gate drive circuits  64 ,  66 . Upper and lower capacitors  90 ,  92  are connected in series with the respective first and second secondary inductors  72 ,  74 . In the starting process, the capacitor  90  is charged from the voltage terminal  16  via the resistors  80 ,  82 ,  84 . A resistor  94  shunts the capacitor  92  to prevent the capacitor  92  from charging. This prevents the switches  40  and  42  from turning ON, initially, at the same time. The voltage across the capacitor  90  is initially zero, and, during the starting process, the serially-connected inductors  68  and  72  act essentially as a short circuit, due to a relatively long time constant for charging of the capacitor  90 . When the capacitor  90  is charged to the threshold voltage of the gate-to-source voltage of the switch  40 , (e.g., 2-3 volts), the switch  40  turns ON, which results in a small bias current flowing through the switch  40 . The resulting current biases the switch  40  in a common drain, Class A amplifier configuration. This produces an amplifier of sufficient gain such that the combination of the resonant circuit  10  and the gate control circuit  64  produces a regenerative action which starts the inverter into oscillation, near the resonant frequency of the network including the capacitor  90  and inductor  72 . The generated frequency is above the resonant frequency of the resonant circuit  10 , which allows the inverter  8  to operative above the resonant frequency of the resonant network  10 . This produces a resonant current which lags the fundamental of the voltage produced at the common node  52 , allowing the inverter  8  to operate in the soft-switching mode prior to igniting the lamps. Thus, the inverter  8  starts operating in the linear mode and transitions into the switching Class D mode. Then, as the current builds up through the resonant circuit  10 , the voltage of the high frequency bus  22  increases to ignite the lamps, while maintaining the soft-switching mode, through ignition and into the conducting, arc mode of the lamps. 
     During steady state operation of the ballast circuit  6 , the voltage at the common node  52 , being a square wave, is approximately one-half of the voltage of the positive terminal  16 . The bias voltage that once existed on the capacitor  90  diminishes. The frequency of operation is such that a first network  96  including the capacitor  90  and inductor  72  and a second network  98  including the capacitor  92  and inductor  74  are equivalently inductive. That is, the frequency of operation is above the resonant frequency of the identical first and second networks  96 ,  98 . This results in the proper phase shift of the gate circuit to allow the current flowing through the inductor  44  to lag the fundamental frequency of the voltage produced at the common node  52 . Thus, soft-switching of the inverter  8  is maintained during the steady-state operation. 
     With continuing reference to  FIG. 1 , the output voltage of the inverter  8  is clamped by serially connected clamping diodes  100 ,  102  of the clamping circuit  12  to limit high voltage generated to start the lamps  24 ,  26  . . . ,  28 . The clamping circuit  12  further includes the second and third capacitors  48 ,  50 , which are essentially connected in parallel to each other. Each clamping diode  100 ,  102  is connected across an associated second or third capacitor  48 ,  50 . Prior to the lamps starting, the lamps&#39; circuits are open, since impedance of each lamp  24 ,  26 , . . . ,  28  is seen as very high impedance. The resonant circuit  10  is composed of the capacitors  30 ,  32 , . . . ,  34 ,  46 ,  48 ,  50  and the resonant inductor  44  and is driven near resonance. As the output voltage at the common node  52  increases, the clamping diodes  100 ,  102  start to clamp, preventing the voltage across the second and third capacitors  48 ,  50  from changing sign and limiting the output voltage to the value that does not cause overheating of the inverter  8  components. When the clamping diodes  100 ,  102  are clamping the second and third capacitors  48 ,  50 , the resonant circuit  10  becomes composed of the capacitors  30 ,  32 , . . . ,  34 ,  46  and the resonant inductor  44 . E.g., the resonance is achieved when the clamping diodes  100 ,  102  are not conducting. When the lamps ignite, the impedance decreases quickly. The voltage at the common node  52  decreases accordingly. The clamping diodes  100 ,  102  discontinue clamping the second and third capacitors  48 ,  50  and the ballast  6  enters steady state operation. The resonance is dictated again by the capacitors  30 ,  32 , . . . ,  34 ,  46 ,  48 ,  50  and the resonant inductor  44 . 
     In the manner described above, the inverter  8  provides a high frequency bus at the common node  52  while maintaining the soft switching condition for switches  40 ,  42 . The inverter  8  is able start a single lamp when the rest of the lamps are lit because there is sufficient voltage at the high frequency bus to allow for ignition 
     With reference to  FIGS. 2 and 3 , a tertiary circuit  108  is coupled to the inverter circuit  8 . More specifically, a tertiary winding or inductor  110  is mutually coupled to the first and second secondary inductors  72 ,  74 , and the circuit  108  is hardwired to the ballast circuit  6  via node +B. The resonant circuit  10  also includes a node −B, which may be considered a ground. In this embodiment, the first and second bi-directional voltage clamps  76 ,  78  are optionally omitted. An auxiliary or third voltage clamp  112 , which includes first and second Zener diodes  114 ,  116 , is connected in parallel to the tertiary inductor  110 . Because the tertiary inductor  110  is mutually coupled to the first and second secondary inductors  72 ,  74 , the auxiliary voltage clamp  112  simultaneously clamps the first and second gate circuits  64 ,  66 . 
     Different values of the Zener diodes  114 ,  116  of the voltage clamp  112  are useful in allowing the ballast  6  to change the current and subsequently the power provided to the lamps  24 ,  26 , . . . ,  28 . As is known, in an instant-start ballast, the initial mode of the lamp operation is glow. In the glow mode, the voltage across the lamp electrodes is high, for example, 300V. The current that flows in the lamp is typically lower than the running current, for example, 40 or 50 mA instead of 180 mA. The electrodes heat up and become thermionic. Once the electrodes become thermionic, the electrodes emit electrons into the plasma and the lamp ignites. Once the lamp ignites, the different amount of power is to be delivered to the each of the ballasts since each ballast runs at a nominal current different level of a nominal current. 
     For example, during ignition of the lamps  24 ,  26 , . . . ,  28 , the clamping voltage of the tertiary winding  110  is increased to allow more glow power. After the lamps have started, the voltage can be folded back to allow the correct steady-state current to flow. This function can be implemented via a controller  120 . 
     More specifically, prior to ignition, a capacitor  122  is discharged, causing a switch  124 , such as a MOSFET, to be in the “OFF” state. When the inverter  8  starts to oscillate, the capacitor  122  charges via lines  126  and  128 . The tertiary winding  110  is clamped by parallel-connected first and second Zener diodes  114 ,  116  that are coupled to the drain and source of the MOSFET  124 . When a high-power start mode is employed in the controller  120 , a high-frequency of the input signal causes the capacitor  122  to charge, which causes Zener diode  116  to turn on, which in turn causes MOSFET  124  to turn ON and the control circuit to start regulating. That is, once the capacitor  122  charges to a predefined voltage, such as the threshold voltage of the MOSFET  124 , the MOSFET  124  turns ON and current is shunted away from the second Zener diode  116  that is connected to the source terminal of the MOSFET  124 . The capacitor  122  is connected in series with a resistor  140 , and a capacitor  132  is connected to the gate and drain of the MOSFET  124 . A diode  150  is connected in parallel to the resistor  140  and capacitor  122 . Thus, the higher voltage clamping of the tertiary winding  110  allows more glow power to be achieved until the lamps  24 ,  26 , . . . ,  28  start. A resistor  148  is coupled to the gate of the MOSFET  124  and to the anode of the Zener diode  116 . The circuit  108  further includes a diode  152 , a resistor  154 , a capacitor  156 , and a resistor  158 , which is connected to node +B (e.g., the tie-in point to high-frequency bus  22  of the ballast circuit  6 ). 
     After a period of time, such as for example from about 0.5 to about 1.0 seconds, the MOSFET  124  turns ON, causing the tertiary winding  110  to be clamped at a lower voltage. This allows the lower steady-state lamp power to be achieved. Thus, the switching of the clamping voltage, such as the switching of the voltage clamping of the tertiary winding  110  via the Zener diodes  114 ,  116 , causes an increase in the power applied to the lamps  24 ,  26 , . . . ,  28  during the glow stage but folds back this power to allow the lamps  24 ,  26 , . . . ,  28  to operate under normal predetermined power levels of the lamps  24 ,  26 , . . . ,  28 . The circuit  108  additionally is coupled to a node “A,” which is in turn coupled to the ROS circuit  200 , described below with regard to  FIG. 4 . 
     In addition to the normal instant start function and the setting of various predetermined steady-state power limits, by controlling the tertiary winding  110 , the ballast  6  can be used as a program start, rapid start ballast or instant start ballast in a variety of applications for different ballast factors. 
       FIG. 4  is an illustration of the ROS protection circuit  200 , which is coupled to control circuit  108  via node A. The ROS circuit  200  comprises a diode  202  that is connected to node A. Diode  202  is coupled in parallel to a diode  204 , and to a resistor  208  and capacitor  210 , which in turn are connected in parallel to each other. A diode  206  is also connected in parallel with capacitor  210  and resistor  208 . Diode  204  and diode  206  are connected to a capacitor  212 , which is in turn connected serially to a resistor  214 . The resistor  214  is then serially connected to earth ground  216 . 
     According to an example, the ROS protection circuit senses a potential between the ballast ground (e.g., node −B) and earth ground  216  in the ROS circuit  200 . If a voltage is present, capacitor  210  charges. If the capacitor  210  exceeds a predetermined threshold voltage (e.g., approximately 8V, according to an example), then the potential voltage across node −B and earth ground is unacceptably high and poses a serious risk of injury. If such is the case, then the voltage a node A rises, and the MOSFET  124  increasingly shunts current away from winding  110 , which lowers the impedance of winding  110 , and consequently the impedance reflected back to windings  72  and  74  in the inverter circuit  8 . This in turn increases the operating frequency of the inverter to rise, which causes the voltage across node −B and earth ground to decrease to a safe level. Thus, when capacitor  210  charges up, potential across the nodes −B and earth ground folds back. 
     It is to be appreciated that the foregoing example(s) is/are provided for illustrative purposes and that the subject innovation is not limited to the specific values or ranges of values presented therein. Rather, the subject innovation may employ or otherwise comprise any suitable values or ranges of values, as will be appreciated by those of skill in the art. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.