Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-09-24
2001-02-20
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C315S219000, C363S019000
Reexamination Certificate
active
06191956
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a power supply apparatus for providing efficient high frequency power to a transformer. More specifically, the present invention relates to a power supply designed to minimize dissipation while providing power to a high frequency transformer by providing that the power supplied to a resonant tank circuit driving the transformer is timed to occur when the drive pulse to the resonant tank will result in providing maximum energy to the load and minimum dissipation in the circuit.
BACKGROUND OF THE PRIOR ART
An oil burner typically consists of a fan that blows air past a nozzle spraying oil under pressure. The oil-air mixture is ignited by placing arcing electrodes slightly upstream of the oil spray and using the high velocity air from the fan to blow the hot gas from the arc into the oil spray. The heat from the gas causes combustion of the oil-air mixture. In these oil burners, the voltage needed to provide the appropriate arc is typically between five to ten thousand volts or more. In previous oil burners, such high voltages were normally produced with a low frequency, step-up transformer connected to a standard 60 Hz power line. However, due to the core requirements of power transformers designed to operate at such low frequencies, these transformers were large, heavy, and expensive.
Additionally, gas discharge devices such as those for neon signs, as well as discharge ignition gas burner systems such as those used in furnaces, also require a high voltage for operation. These devices have also used expensive, heavy, low frequency step-up transformers to provide the high voltage from a 60 Hz power line.
Similarly, natural gas and liquefied propane (LP), hereinafter both referred to as “gas,” are commonly ignited in gas appliances either by a standing pilot flame, an electric spark, or a hot-surface ignitor. Each of these ignition methods may not be preferable since standing pilot flames waste gas, hot-surface ignitors are expensive and fragile, and spark ignition is typically noisy. In previous spark ignitors, the snapping noise made during operation may be objectionable to some users. There are also several disadvantages in operating transformers for oil burners at low frequencies, such as 60 Hz. In the formation of the arc, the moving air surrounding the electrodes carries away the ions from between the electrodes. This has the same effect as lengthening the air gap between the electrodes. As a result of this constructive increase in length, there is a gradual increase in the voltage needed to hold the arc. At some point, the ionized path becomes so extended that the holding voltage exceeds the voltage at which an ionized path is created; this is known as the breakdown voltage. When this happens, a new arc is formed and the fan out process begins again. Below about 400 Hz, the electrodes retain the same polarity from the start of an arc until the next arc begins. Above 400 Hz, the voltage changes polarity and the arc current goes to zero while the holding voltage is less than the breakdown voltage. With no arc current, the ionized path begins to dissipate. The electrode voltage must now form the ionized path again before the current can flow. If only a short period passes after the current stops flowing, the voltage required to re-establish the arc through the old path, the restrike voltage, is only slightly greater than the holding voltage.
Consequently, much smaller, lighter, and less expensive transformers may be used to realize the power requirements if powered by a higher operating frequency. Thus, solid state power supplies have been developed to provide this higher operating frequency. As frequencies less than around 10 kilohertz are generally audible and annoying to their owners, transformers which operate at frequencies above 25 kilohertz are generally preferred.
Solid state power supplies used to provide the high frequency voltage to the transformers have been used in a variety of applications. Such known designs, however, have not been of satisfactory cost or efficiency. Since the lifetime of electronic equipment is shortened at elevated operational temperature, the risk of failure increases both as a function of time and as a function of temperature. Similarly, the lifetime of nearby equipment may be shortened if they surround components operating at an elevated temperature. The need then exists for a low cost, cool running, solid state power supply for an oil, gas, and neon sign ignitor.
For comparison, U.S. Pat. No. 4,698,741 (Pacholok) depicts a solid state power supply with a free-running oscillator to generate a high frequency power signal. The oscillator uses an arrangement of transistors such that a high-speed switching transistor rapidly deactivates a high voltage transistor. Although this provides a high frequency signal, the circuit results in dissipation of a significant amount of power in the switching transistor, which necessitates a heatsink. Because of the high temperatures at which this switching transistor runs, the lives of the transistor and nearby electrical components are potentially shortened. The switching transistor in this circuit is also significantly overdriven under “no load” conditions, resulting in peak voltages in the tank circuit becoming excessive. Consequently, the transistors used must be rated for 600 to 800 volts, even for operation on U.S. domestic 120 volt power, and the insulation of the transformer must be designed to withstand these extremes.
SUMMARY OF THE INVENTION
To overcome the problems of excessive voltages and significant power dissipation in the switching transistor, the present invention utilizes a novel arrangement whereby a parallel-resonant circuit operates with a clamping apparatus to control a single switching transistor, which also operates to minimize the power dissipation in the transistor.
The improved solid state power supply includes a current sensing circuit, designed to operate in conjunction with a resonant tank circuit for detecting the appropriate time to supply power to the switching transistor. An amplifier then amplifies the current waveform existing in the resonant tank circuit and passes this amplified waveform to a clamping circuit. The clamping circuit operates to provide to or remove power from the switching transistor. The timing of the power supplied is controlled to occur at times when the operation of the transistor will result in providing maximum energy to the load and minimum power dissipation in the transistor. This event occurs around the time when the voltage at the juncture between the resonant tank circuit and the switching transistor is at a minimum.
The duration of the power supplied is also chosen to provide just enough drive to operate the circuit under conditions of low supply voltage and full load. Thus, the resonant tank is supplied only as much energy per cycle as it needs to sustain oscillation and supply its load.
Accordingly, the present invention is a quiet ignition source that readily ignites gas or oil. It is has the advantages of being low in cost and operates at a cool temperature, while still maintaining high efficiency.
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.
REFERENCES:
patent: 4232252 (1980-11-01), Peil
patent: 5747942 (1998-05-01), Ranganath
Honeywell International , Inc.
Leonard Robert B.
Riley Shawn
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