Electronic HID ballast with current source/sink to power recessed can insulation detector

Electronic ballasts, driver apparatus, and methods are provided in which a heating current is generated by the driver to power a resistive heating element of an insulation detector associated with a recessed can fixture using a regulated current source/sink circuit.

BACKGROUND OF THE DISCLOSURE

Ballasts are used in the artificial illumination arts for starting and controlling power applied to lamps, such as fluorescent lamps and high intensity discharge (HID) lamps. These types of lamps are often installed as recessed down-facing lights (also known as recessed can lights) in so-called recessed can fixtures that are recessed into ceilings or walls to provide unobtrusive directed lighting source. Ballasts installed in such recessed can fixtures are not typically covered by insulation. If the insulation is improperly installed, then heat may build up, increasing the heat retained by the lighting system and a rise in temperature. This poses a fire danger and safety hazard. To prevent this hazard, a detection system may be employed to detect a rise in temperature and shut off the electrical current before the increase in temperature and the heat buildup poses a safety concern. A conventional insulation detector includes a resistive heating element which is thermally coupled with a bi-metallic switch, with the heating element connected across the voltage supply mains and the bi-metallic switch in series with the input to the ballast. In the event the insulation is packed too close in the fixture, the internal temperature of the insulation detector will rise and cause the bi-metallic to open, thereby discontinuing the power to the ballast circuitry. However, the connection of the resistive heating element across the input supply mains renders the ballast and associated insulation detector appropriate for only a fixed input voltage rating. Prior attempts to regulate the voltage across the insulation detector heating element have degraded the total harmonic distortion (THD) generated by the ballast. Consequently, there remains a need for improved ballasts and techniques for powering heating elements of insulation detectors without exacerbating THD levels for recessed can lighting installations.

SUMMARY OF THE DISCLOSURE

An electronic ballast is provided for operating a lamp in a recessed can fixture. The ballast includes an input coupleable to an AC power source and an output coupleable to at least one lamp, along with a ballast circuit operative to receive AC electrical input power from the input and to provide AC output power to the output to drive the at least one lamp and a regulated current source/sink circuit that provides a regulated amount of heating current to power the resistive heating element independent of the input voltage. Certain embodiments of the current source/sink circuit provide RMS heating current to power the resistive heating element via a linear mode MOSFET or other semiconductor-based switching device including bipolar, IGBT, etc., coupled in series with the resistive heating element and a first regulator circuit controlling an impedance of the switching device according to a first reference voltage. The current source/sink circuit may include a rectifier circuit operative to rectify the AC electrical input power from the input to provide a rectified bus, and wherein the switching device is coupled in series with the resistive heating element across the rectified bus, with a second regulator circuit to regulate the first reference voltage independent of the voltage of the input power.

A method is provided for powering a resistive heating element of an insulation detector associated with a recessed can fixture using an electronic ballast. The method includes generating a heating current in the electronic ballast, such as by rectifying AC input power provided to the ballast and generating the heating current from the rectified bus, as well as providing the heating current to the resistive heating element, and regulating the amount heating current provided to the heating element independent of an input voltage provided to the electronic ballast.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, where like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale,FIGS. 1 and 2illustrate an exemplary electronic ballast102that provides a regulated RMS heating current to a resistive heating element RH of an insulation detector210associated with a recessed can fixture200for different input voltage levels of an AC power source104without introducing low or high frequency chopping of the AC mains, so that the input current total harmonic distortion (THD) will not be significantly impacted. The disclosed ballast and method embodiments find particular utility in applications involving recessed can fixtures where an insulation detector circuit is used to prevent fires, although the disclosed ballasts may be used and the disclosed methods may be employed in other applications. In the illustrated ballast, the resistive heating element RH may be of any suitable size and rating, for example, a 7.2 kOHM device with a rated supplied power level of about 2 W at normal operating conditions where the ballast102supplies a RMS heating current IH (e.g., approximately 17 mA in one example).

As best shown inFIG. 1, the heating element RH is thermally coupled with a normally closed bi-metallic switch SW1to form an insulation detector210. The switch SW1is connected in series with the AC power source104and the thermal coupling of the switch SW1and the resistive heating element RH can be any appropriate mechanical construction or relative location thereof by which the switch SW1opens in the event that insulation is packed too close the fixture200or otherwise when thermal conditions of a given installation of the fixture create a potential for overheating of the surrounding installation materials, the electrical power wiring, or of the ballast102itself. Thus, in potentially hazardous situations, the internal temperature of the insulation detector210rises to a point where the bi-metallic switch SW1will open, thereby removing power to the ballast102. In conventional ballast/insulation detector products, a different resistive element RH would be needed for use in applications of different source voltages, since the heating element RH was powered from the AC mains voltage itself. The illustrated ballast102instead includes an internal regulated current source/sink circuit130with a rectifier134and regulated current source circuitry132to provide regulated heating current IH to the heating element RH independent of the voltage level supplied by the source104. This novel concept allows the use of one heating element resistance RH independent of the application input voltage, by which the fixture manufacturer can use the same thermal protector210/ballast102for all fixtures200regardless of input voltage.

The electronic ballast102operates one or more lamps108of any given type coupled to an output106using input power obtained at an input105coupled to the AC power source104. The ballast102in one embodiment includes a pair of input terminals105and a pair of resistive heating element terminals107for connection with the power source104and the insulation detector210, although other wiring configurations are possible. A main ballast circuit120is coupled to the input105via an optional EMI filter circuit and operates to receive AC electrical input power from the input105and to provide AC output power to the output106to drive the lamp(s)108. Any suitable ballast circuitry120can be used which provides an output suitable to drive one or more lamps, including without limitation instant start ballast circuitry, program start ballast circuitry, dimming ballast circuitry and the like. The filter circuit110in the illustrated embodiment couples power via connections112from the input105to the ballast circuit120and also couples the input power to the regulated current source/sink circuit130, although other embodiments are possible in which the filter circuit110is omitted.

The regulated current source/sink circuit130is coupled with the terminals107and operates to receive AC electrical power from the input105(via the optional filter110) and provides a regulated amount of heating current IH to power the resistive heating element RH, where the regulated amount is substantially independent of a voltage of the input power. In this regard, no heating current IH is provided when the AC source voltage goes to zero and only a certain maximum amount of current can be supplied by the circuit130, but in normal operation the current IH is a generally constant RMS value whether the ballast102and detector210are powered by a 120 volt, 240 volt, or 277 volt AC supply104. This operation of the circuit130is illustrated and described further below in connection withFIG. 3. While the exemplary current source/sink circuit130provides a regulated amount of RMS heating current to power the resistive heating element RH, other embodiments are possible in which the circuit130provides a time varying current (e.g., AC current or other forms of time varying current) with a value that is regulated independently of the input AC voltage from the source104.

Referring in particular toFIG. 2, the regulated current source/sink circuit130includes a rectifier134as well as first and second regulator circuits132aand132b, respectively, along with a switching device Q2, such as a MOSFET (other switching device types could be used, such as bipolar, IGBT, etc.) coupled to the terminal107in series with the resistive heating element RH and operated in linear mode operation to provide a series impedance that varies with gate-source voltage Vgs applied to a gate terminal thereof by the regulator circuit132a. The first regulator circuit132a, in turn, provides the gate control signal to control the impedance of the switching device Q2according to a first reference voltage CREF. The rectifier circuit134includes a full diode bridge D1-D4to rectify the AC electrical input power from the input105(received in one embodiment via the EMI filter110) and provides a rectified bus. The upper bus is connected to a first terminal107aand a circuit path is formed across the rectified bus including series connection of the resistive heating element RH of the detector210, the linear mode switching device Q2and a resistor R5to the lower rectified bus line (ground). Thus configured, the heating current IH flows through this series circuit path across the rectified bus provided by the rectifier circuit134. The first regulator circuit132areceives as an input the voltage across R5and provides as an output the gate control signal for the switching device Q2so as to regulate the voltage across R5(and hence the current IH is regulated) according to the reference CREF. In the illustrated embodiment, the first regulator132aincludes an op-amp (error amp) U4with a non-inverting input coupled to the reference CREF and an inverting input coupled to the connecting node of Q2and R5via a resistor R6. The output of U4is coupled to the inverting input by a feedback circuit formed by the parallel connection of two circuit branches, one including the series combination of a resistance R7and a capacitance C1and the other including a capacitance C2. The output of U4is coupled to drive the gate control signal of the switching device Q2via a resistance R8. The circuit132athus controls the impedance of Q2based on the reference voltage CREF such that the amount of heating current IH applied to the resistive heating element RH is regulated independent of the voltage of the AC source104.

As also shown inFIG. 2, the reference voltage CREF is generated at the center node of a resistive voltage divider circuit that includes an upper resistance R1and a lower resistance R2, where R2is coupled in parallel with the series combination of a resistance R3and a JFET Q1of the second regulator circuit132b. The second regulator circuit132boperates to regulate the first reference voltage CREF by controlling the JFET Q1as a variable resistance to adjust the CREF node voltage in closed loop fashion independent of the voltage of the input power source104. A gate control signal is provided to the JFET Q1via a resistance R4from an error amp U3. A first buffer amplifier U1in the circuit132bbuffers the reference voltage value CREF and the buffered output signal of U1is provided to a resistive divider circuit formed by resistors R9and R10with a filter capacitor C3in parallel across R10, and the joining node of R9and R10is connected to the non-inverting input of a second buffer amplifier U2. U2provides an output through a resistance R11to the inverting input of U3, with the non-inverting input of U3being coupled to a second reference VREF (VREF is the equivalent of the DC value of CREF when Q1is off). The output of U3drives the gate of JFET Q1via resistor R4with the output coupled with the inverting input through a feedback network including C5in parallel with the series combination of a resistance R12and a capacitance C4. Thus, the exemplary second regulator circuit132bcontrols the impedance of Q1to adjust the CREF node voltage by closed loop operation independent of the voltage of the input power source104.

Referring also toFIG. 3, graphs300and310respectively illustrate the regulated operation of the circuit130showing the AC input voltage302of the source104and the heating current IH312over time. As the AC input voltage curve302changes from an initial value of 120 volts to 240 volts and then to 277 volts, the current IH provided to the resistive heating element RH remains essentially constant. It is noted that the ballast102is thus ideally suited for universal application without having to provide a different RH value for different input source voltage levels. Moreover, unlike other approaches, the first and second regulator circuits132aand132bdo not involve AC switching operations and hence to not contribute to THD (total harmonic distortion) at the source104.

Referring also toFIG. 4, a method400is illustrated for powering a resistive heating element RH of an insulation detector210associated with a recessed can fixture200using an electronic ballast102. The method includes generating a heating current IH at410in the electronic ballast102, providing the heating current IH to the resistive heating element RH at420, and regulating the amount heating current IH provided to the resistive heating element RH at430independently of an input voltage provided to the electronic ballast102. The heating current generation at410in the illustrated method400includes rectifying AC electrical input power provided to the electronic ballast102at412to provide a rectified bus, and generating the heating current IH at414from the rectified bus. This can be done in one embodiment by coupling a linear mode MOSFET or other suitable switching device (e.g., Q2inFIG. 2above) in series with the resistive heating element RH across the rectified bus. The regulation of the heating current amount IH at430in this embodiment includes controlling the impedance of the switching device Q2according to a reference voltage CREF, which may be generated using electrical power from the rectified bus.

FIG. 5illustrates an exemplary driver apparatus130in a different embodiment in which the regulated current source/sink circuit130is separate from the ballast102. The driver apparatus130derives power from the input source104via suitable terminals and includes an output107with suitable terminals for connection to the resistive heating element RH of the insulation detector210. The driver apparatus130operates as described above in connection with the circuit130ofFIGS. 1 and 2to provide heating current IH to power the resistive heating element RH using the above described regulated current source/sink circuitry.