In Europe there are new noise reduction requirements for all electrical and electronic equipment that will be sold in the near future, and two of these requirements are known as the "harmonic" requirement IEC 61000-3-2, and the "flicker" requirement IEC 61000-3-3. Laser printers contain a high wattage heating element, such as a 750 W tungsten-filament lamp, which are used to provide heat to the fuser. When alternating current electrical power is first provided to such high-wattage lamps, there is typically a large inrush current that primarily produces harmonic noise and an instantaneous voltage drop that can affect other electrical equipment connected on the same or a nearby electrical branch circuit.
For example, previous laser printers manufactured by Lexmark International, Inc. used a strictly ON-OFF control system to control the fuser temperature by turning the high-wattage lamp either full ON or full OFF. A tungsten halogen lamp has typically been used to act as this heating element, which acts as a nonlinear load, and which will observe a quite high inrush current when the lamp is first turned on under the prior control circuits. For example, if the lamp undergoes a "cold start," the resistance characteristic of a standard 750 W tungsten halogen lamp filament is around 5.2 ohms at 25.degree. C. However, when the lamp is burning at a full ON steady state, at about 2000.degree. C., its resistance is about 64.5 ohms while providing a 750 W output.
The low filament resistance when started from a "cold start" results in a light flicker for electrical light bulbs that are previously energized on the same or a nearby branch circuit.
To satisfy the European flicker requirement, one alternative is to use a phase-angle control method to provide power to the tungsten-filament lamp so as to gradually increase the amount of current that flows through the lamp filament when it is cold and is initially being energized. The advantage of the phase-angle control is that the power supplied to the load can be initially reduced by delaying the firing pulse of the output stage triac relative to the starting of each half cycle of AC voltage. However, phase control also introduces significant distortion of the sine wave that normally represents the AC current waveform. A distorted current waveform can cause many undesirable effects on the AC power supply, thus leading to a failure of the equipment to comply with the European harmonic requirement.
To meet this European harmonic requirement while using a phase-angle controller, a large AC harmonic attenuation inductor has been placed in series with the tungsten halogen lamp in conventional designs. Unfortunately, this relatively large inductor dramatically increases the cost of the product, and additionally causes an uncomfortable humming noise when the lamp is turned on. In the past, no practical solution has been found to completely eliminate the inductor humming noise.