Transformer Ballast

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More Great Information on Transformer Ballast:

Variable impedance Electronic Ballast for a gas discharge device by ballast123

This invention relates to an electronic ballast for driving a set of gas discharge lamps which is capable of driving the lamps independently of lamp loading, i.e., with less than all of the lamps in the set connected or operating, and in a transition period during which a lamp already connected stops or starts operating, or a during which a lamp is changed. Electronic Ballasts may include a resonant inverter which is current-fed or voltage-fed. The invention herein relates to an electronic ballast with a voltage-fed resonant inverter.

Electronic ballasts that drive a set of gas discharge lamps should regulate the output to the lamps to drive the lamps with all, or less than all, of the lamps in the set operating or connected. Thus, if a lamp either degrades, stops lighting altogether or is disconnected, the ballast should continue to provide a satisfactory output to the remaining operating lamps. The same applies in a transition period during which a lamp already connected stops or starts operating and when a lamp is changed. It is highly desirable for a ballast to continue operating the remaining lamps connected thereto during a lamp change and to ignite a newly connected lamp so that it is not necessary to turn the power off to the ballast during lamp changing or to restart all of the lamps after a lamp change. Thus, electronic ballasts for multiple lamp operation advantageously should operate all connected lamps independently of lamp loading as indicated above. Such ballasts advantageously should also permit instant and/or Rapid Start operation.

An electronic ballast with a current-fed inverter requires a large, heavy inductor in series with the input of the inverter. Such ballasts are less affected by lamp loading due largely to the presence of the large input inductor, and are capable of multiple lamp operation independently of lamp loading, However, the large input inductor required by current-fed inverters increases the cost and the size and weight of the ballast.

U.S. Pat. No. 5,438,243 discloses an electronic ballast for multiple independent lamp operation of instant start gas discharge lamps, which includes a quasi-voltage-fed half-bridge parallel resonant inverter, as shown in FIG. 2 of the patent. Intrinsically, the electronic ballast operates in current-fed parallel resonant mode where the current source is derived from an ideal voltage source in series with a first resonant tank circuit. In order to maintain a high voltage output for lamp starting, which requires a high Q, a second resonant circuit is provided. The electronic ballast circuit disclosed in this patent thus contains two cascaded resonant tanks. As a result, circuit losses and cost are increased. At the output, two ballast inductors are provided in series with two lamps and two ballast capacitors are provided in series with two other lamps, which reduces the reactive power requirement on the secondary side of the output transformer. However, this electronic ballast may impose a high voltage hazard across the output terminals when the lamps are removed while the circuit is hot. Other types of electronic ballast circuits with current-fed push-pull or half-bridge inverters for multiple florescent lamps are described at pages 583 and 584 of Power Semiconductor Applications Handbook 1995, Philips Semiconductor, 1994. In those configurations, each lamp is parallel connected to the output of a resonant inverter via a series ballast capacitor. The current source is usually obtained via a choke inductor in series with a voltage source. Good independent lamp operation is achieved via a constant relation between the output high frequency voltage and the input DC voltage. The disadvantages of this type of circuit include higher switch stresses and the added bulky choke inductor.

On the other hand, an electronic ballast with a voltage-fed resonant inverter, including well known half-bridge LC or LLC resonant inverters, do not require a large input inductor but are affected more by lamp loading, and thus require additional circuitry or resonant components in order to achieve multiple lamp independent operation.

U.S. Pat. Nos. 5,438,243, 5,394,064, 5,075,599 and 4,535,399 disclose electronic ballasts for multiple lamp independent operation which include voltage fed inverters. The electronic ballast described in U.S. Pat. No. 5,394,064, which permits dimming operation, includes a voltage-fed half-bridge resonant inverter which generates a high frequency square-wave voltage. Control is provided by varying the input voltage to the inverter or the inverter frequency, or both using voltage feedback from the input to the inverter and feedback from the lamp circuit to the oscillator driving the inverter switches. As shown in FIG. 1 of the patent, a transformer couples the lamps to the inverter, with each lamp being driven through an individual resonant tank circuit consisting of a series inductor and a parallel capacitor. The circuit for connecting the lamps may be considered as a parallel connection of multiple resonant tanks each loaded with a single lamp. Although multiple lamp operation is achieved, a large number of magnetic components and capacitors is required, which results in higher cost.

The electronic ballast described in U.S. Pat. No. 4,535,399 includes a voltage-fed resonant inverter and a separate resonant tank circuit for each lamp (FIG. 6). Each of the separate resonant tank circuits includes a small inductor and a capacitor, which adds to the cost of the ballast. This ballast also includes a current feedback loop which controls switching of a pulse width modulator ("PWM") that supplies the timing for driving the inverter. The current feedback loop includes a phase locked loop ("PLL") which provides a DC output to the PWM proportional to the change in phase of the current sensed at the output of the inverter. The PLL forces the inverter to operate at a frequency where the modulating pulses are initiated at the load current zero crossing. The PLL includes a low pass filter (FIG. 2) and has a suitable low frequency response which allows the PLL to maintain tracking during steady state operation. However, the PLL has a poor high frequency response which prevents the PLL from tracking fast transients that are produced, for example, when a lamp is disconnected or connected. As a result, the PLL may cease tracking, i.e., become unlocked during a lamp change, which can cause loss of zero voltage switching and destruction of the inverter.

U.S. Pat. Nos. 5,075,599 and 4,277,728 both disclose electronic ballasts for driving a single gas discharge lamp with a phase related feedback loop for control. In the '599 patent, correction towards a target phase angle difference (e.g., 0) is made when a minimum reference phase angle difference is greater than the measured phase angle difference (FIG. 5). If the measured phase angle difference is greater than reference phase angle difference, then no correction is made. In the '728 patent, a phase detector (FIGS. 4a and 4b) senses the phase difference between the input to the inverter (push-pull in FIG. 4a and half-bridge in FIG. 4b) and the voltage across the load. For proper operation, the phase difference should be 90 degrees, indicating that the inverter is operating at the resonant frequency. If the phase difference is not 90 degrees, the phase detector provides an error signal causing the inverter to change frequency towards the resonant frequency. The electronic ballast disclosed in the '728 patent also includes an inductor in the resonant circuit which limits current to the load.

Ningbo Zhengjia Electric Appliance Co., Ltd is a professional R & D manufacturer and exporter of electronic ballast for fluorescent lamps(FL) and Compact Fluorescent

Lamps(PL).

Article Source: http://www.earticlesonline.com/Article/Variable-impedance-electronic-ballast-for-a-gas-discharge-device/1034678

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