High-Power Factor Circuit for Energizing Gas Discharge Lamps

Gas discharge lamps can be operated most efficiently by AC (alternating current) power at a relatively high frequency (on the 35 KHz [kilohertz]). However, line AC power is supplied by utility companies at low frequencies (around 50 Hz [hertz] or 60 Hz). To obtain high efficiency operation of the lamps, the AC power at the first low frequency is converted to AC power at a second high frequency.

The conversion of the AC power from one frequency to another is accomplished by a ballast circuit. The AC power at the first low frequency is rectified into DC (direct current) power, and then stored as energy in a relatively large electrolytic capacitor. The energy stored in the electrolytic capacitor is then "chopped" by an inverter into AC power at a second high frequency.

In this kind of circuit, whenever the voltage of the line AC power is greater than the voltage stored in the electrolytic capacitor, a relatively large surge of current passes into the electrolytic capacitor, causing the line current drawn to be "peaky" and having a poor power factor.

One solution is to place a floating voltage supply in series with the incoming line to the capacitor. Such a supply presents several problems. First, the voltage of the supply must be controlled so as to match the voltage on the electrolytic capacitor, otherwise the waveform of the power drawn from the line will be distorted. Second, the impedance level of the supply must be adjustable so as to control the amount of power drawn from the power line. If not, the inverter will either produce too much power or there will be little correction of the power factor. Finally, the source of the power for the floating voltage supply must be stable and have a low impedance.

FIG. 1 is a diagram of a circuit for energizing gas discharge lamps.

FIG. 1 shows a circuit suitable for energizing gas discharge lamps.

Terminals 100, 102 of line rectifier 104 are coupled to a source of AC power at a relatively low frequency, such as 60 cycles per second. Rectifier 104 may be a bridge rectifier. Line rectifier 104 converts the AC voltage to a DC voltage. The positive output terminal of line rectifier 104 is coupled to a high frequency rectifier 106. (High frequency rectifier 106 could be connected to the negative output terminal of line rectifier 104 with appropriate orientation of the remaining circuit elements.) High frequency rectifier 106 is suitable for operation at relatively high frequencies such as 35 KHz.

The positive output terminal of line rectifier 104 is coupled to the negative DC output terminal of high frequency rectifier 106. The positive DC output terminal of high frequency rectifier 106 is coupled to the negative output terminal of line rectifier 104 by storage capacitor 108.

Storage capacitor 108 provides a stable reservoir of charge at a relatively constant voltage for running driving circuit 110. Storage capacitor 108 is charged by the output from series connection of line rectifiers and high frequency rectifier 106.

Driving circuit 110 consists of switches and inductors connected to induce alternating currents around the parallel resonant tank circuit 112. Suitable driving circuits would be the high efficiency, self-oscillating LC (inductor-capacitor) multivibrator circuit shown in Konopka, U.S. Pat. No. 5,150,013, the circuit for driving a gas discharge lamp load shown in Moisin, U.S. Pat. No. 5,148.087, as well as other well known inverter circuits, such as those containing two transistors and such a circuit has an electrically symmetric output waveform.

Driving circuit 110 has a pair of input terminals and two or more output terminals. The input terminals of driving circuit 110 are connected across storage capacitor 108. The output terminals are coupled to parallel resonant tank circuit 112 in a manner suitable to excite parallel resonant tank circuit 112 into oscillation.

Parallel resonant tank circuit 112 consists of tank inductor 114 connected in parallel with tank capacitor 116. Preferably, tank inductor 114 is part of a combined inductor and transformer assembly, so that additional output windings are present on core 118.

A feedback circuit from the output to the input of driving circuit 110 is formed by power factor correction winding 120, high frequency rectifier 106 and capacitor 122.

Power factor correction winding 120 is present on core 118. Power factor correction winding 120 is connected through capacitor 122 to the AC inputs of high frequency rectifier 106. Thus, high frequency rectifier 106 is energized by power factor correction winding 120, producing a DC voltage across the output terminals of high frequency rectifier 106. The number of turns on the power factor correction winding determines the amplitude of the voltage of the AC power coupled to the inputs of high frequency rectifier 106. By adjusting the number of turns in power factor correction winding 120, there is a means for controlling the level of the DC voltage across the output terminals of high frequency rectifier 106. Matching the level of the DC voltage across the output terminals of high frequency rectifier 106 to the voltage level of the DC energy stored in storage capacitor 108 significantly improves the power factor of the circuit.

The DC output voltage of high frequency rectifier 106 is approximately equal to the voltage across capacitor 108. Therefore, the current from line rectifier 104 is limited only by the output impedance at 60 Hz of high frequency rectifier 106. The output impedance of high frequency rectifier 106 is controlled by capacitor 122.

The operating power level of the inverter is controlled by the capacitance of capacitor 122. If capacitor 122 has a large capacitance, the output impedance of high frequency rectifier 106 is low, resulting in a high level of power transfer into the inverter. Conversely, if capacitor 122 has a small capacitance, the level of power transfer into the inverter is low. Thus, capacitor 122 acts as a means to control the impedance of high frequency rectifier 106.

At low frequencies, the output impedance of high frequency bridge rectifier 106 is almost purely resistive, and is inversely proportional to the capacitance of capacitor 122. The incoming line current therefore is proportional to the incoming line voltage, and the waveform of the line current is the same shape as the waveform for the incoming line voltage and in phase. For a sinusoidal input voltage, a sinusoidal input current results, thus achieving a high power factor.

Thus, it is possible with this circuit to control both the DC voltage level at the output of high frequency rectifier 106 and the impedance of high frequency rectifier 106. By carefully selecting the voltage level and the impedance, extremely high power factors can be obtained.

Load winding 124 is wound on core 118. Load winding 124 is connected to loads 126, 128 through current limiting capacitors 130, 132. When energized, load winding 124 powers loads 126, 128.

Control winding 134 is wound on core 118. Control winding 134 is coupled to control 136. Control 136 may govern the opening and closing of a switch, such as a transistor, a silicon controlled rectifier, or any other electric or electronic device used to open and close electrical connections. In normal operation, the switch would be open.

Control 136 senses when loads 126, 128 are removed from the circuit. A power surge occurs in control winding 134, and is sensed by control 136. Control 136 then disables high speed rectifier 106. Thus, driver 110 is energized only by the peak of the line voltage which appears across storage capacitor 108. If high speed rectifier 106 was not disabled, the power feedback via winding 120 would continually increase the voltage on storage capacitor 108, eventually damaging the inverter.

If high speed rectifier 106 is a bridge rectifier, control 136 comprises in part a switch between the DC outputs of the high speed rectifier. If loads 126, 128 are removed, the switch closes, connecting the DC outputs of the bridge rectifier.