The present invention generally relates to ballasts and more particularly, to an inductively coupled ballast for non-contact power transfer to a secondary circuit or load.
Ballasts are commonly used to supply power to a wide variety of electrically powered components. Often ballasts are connected directly to the component (or load), for example, by xe2x80x9cpermanentxe2x80x9d connections, such as wires or soldered leads on a circuit board, or by xe2x80x9cremovablexe2x80x9d connections, such as plugs or other connectors. Direct electrical connections present a number of problems. First, direct electrical connections make it difficult to install and remove the load from the ballast. With permanent connections, the electrical leads must be soldered or otherwise secured directly between the ballast and the load. If the ballast or the load is damaged, replacement is complicated by the permanent connections. Removable connections make separation of the ballast and the load easier, but still require some manual manipulation. Removable connectors are also subject to corrosion and may be inadvertently or unintentionally disconnected, for example, by vibrations. Second, in many environments, direct electrical connections must be insulated from the environment to prevent damage to the circuit. For example, in wet environments, exposed electrical connections are subject to a short circuit. Third, direct electrical connections provide a direct and essentially unimpeded path for electricity to flow between the ballast and the load. As a result, power surges and other potentially damaging abnormalities in one element can be directly transfer to the other, thereby permitting problems in one component to damage or even destroy the other.
To address these and other significant problems, there is an increasing trend to replace conventional direct electrical connections with inductive connections. Inductively coupled systems provide a number of significant advantages over direct connections. First, inductive couplings do not include permanent or removable physical connectors. Instead, the secondary coil of the load (or secondary circuit) simply needs to be placed in the close proximity to the primary coil of the ballast. This greatly simplifies installation and removal of the load. Second, the inductive coupling provide a significant level of isolation between the ballast and the load. This isolation can protect one component from power surges and other potentially damaging abnormalities in the other component.
Unfortunately, conventional inductively coupled ballasts suffer from a number of problems associated primarily with efficiency. To provide maximum efficiency, it is desirable for the circuit to operate at resonance. Conventional ballasts are designed to operate at resonance by carefully selecting the components of the ballast in view of the precise characteristics of the load. Any variation in the load can move the circuit dramatically out of resonance. Accordingly, conventional ballasts require very precise selection of the components of the ballast circuit and secondary circuit. In some applications, the impedance of the secondary circuit will vary over time, thereby changing the resonant frequency of the circuit. For example, in many conventional lighting applications, the impedance of the lamp will vary as the lamp is heated and will also vary over the life of the lamp. As a result of these changes, the efficiency of conventional, fixed-frequency ballasts will vary over time.
Conventional ballast control circuits employ bipolar transistors and saturating transformers to provide power. The ballast control circuits oscillate at frequencies related to the magnetic properties of the materials and winding arrangements of these transformers. Circuits with saturating transformer oscillators produce an output in the category of a square wave, require the transistors of the half bridge to hard-switch under load and require a separate inductor to limit the current through the load. Conventional circuits chop the available power supply voltage, developing voltage spikes at the corners of the square wave as a consequence of the current limiting inductor. Inductive couplings rely on electromagnetic induction to transfer power from a primary coil to a secondary coil. The amount of current induced in the secondary coil is a function of the changes in the magnetic field generated by the primary coil. Accordingly, the amount of current transferred through an inductive coupling is dependent, in part, on the waveform of the current driving the primary. A square waveform has relatively small regions of change and therefore provides relatively inefficient transfer of power.
These and other deficiencies in prior ballasts are addressed by the present invention.
The present invention discloses an inductively powered ballast circuit having a current sensing circuit that automatically adjusts the frequency of the ballast to maintain operation of the ballast at or near unity power factor.
In one embodiment, the inductively coupled ballast circuit is a self-oscillating half-bridge switching design that operates at high frequencies. In addition, the inductively coupled ballast circuit self-oscillates partly as a function of the current sensing circuit to maintain resonance, uses MOSFET transistors as switching elements, and is designed to accommodate an air-core transformer coupling arrangement.
One embodiment of the inductively coupled ballast circuit includes a control circuit, an oscillator, a driver, a half-bridge switching circuit, and a series resonant tank circuit. The secondary circuit preferably includes a secondary coil and a load. During operation, the control circuit provides electrical signals to the oscillator, which, in turn, provides electrical signals to direct the driver. The driver then causes the half-bridge switching circuit to become energized. The half-bridge switching circuit energizes the series resonant tank circuit, which includes a primary coil. Once the series resonant tank circuit, and consequently the primary coil, is energized, the secondary coil becomes inductively energized, thereby providing power to the load.
In one embodiment, the resonant frequency for the inductively coupled ballast circuit is about 100 kHz. In addition, the secondary circuit preferably resonates at about 100 kHz as well. The resonant frequency of operation can be adjusted up or down by the control unit to accommodate for convenient component selection. In addition, selection of the resonant frequency is a function of the component selection in the series resonant tank and the characteristics of the secondary circuit.
An interesting feature of the inductively coupled ballast circuit is the inductive coupling. The series resonant tank circuit includes an inductive coupler. In one embodiment, the inductive coupler is positioned adjacent the secondary coil with an air gap therebetween to form an air core transformer. When voltage is applied to the inductive coupler, magnetic flux in the air gap induces voltage in the secondary coil thereby energizing the secondary load.
Another interesting feature of the inductively coupled ballast circuit involves the air gap of one embodiment. The air gap is the distance between the inductive coupler and the secondary coil. The air gap may be selected to provide a current limiting function. In addition, the air gap provides a magnetic flux path for inducing sufficient voltage in the secondary coil to establish and maintain an operating point for the secondary load.
Yet another interesting feature involves the frequency of operation of the inductively coupled ballast circuit. Both the series resonant tank and the secondary load may be tuned by proper selection of components to operate at a similar resonant frequency. In addition, impedance matching between the series resonant tank and the secondary load may occur at the resonant frequency. Accordingly, power transfer from the inductive coupler to the secondary coil may be optimized at a resonant frequency to maximize power efficiency.
Still another interesting feature involves self-oscillation of the inductively coupled ballast circuit with the oscillator. The oscillator may include feedback control for monitoring the series resonance tank. The feedback control may allow the oscillator to adjust the frequency to minimize reflected impedance from the secondary circuit. Adjusting the frequency to maintain resonance minimizes the reflected impedance and maintains optimum power transfer as the impedance of the secondary circuit varies.
In another aspect, the present invention preferably includes a current limit circuit that monitors the ballast circuit and disables the ballast circuit if the current to the primary exceeds a desired threshold. The current limit circuit protects both the load and the ballast circuit from excessive current. The current limit circuit is preferably latched to keep the ballast circuit disabled until reset, for example, by a manual reset switch.
In an alternative embodiment, the current limit circuit may be configured to disengage the ballast circuit if the current falls outside of a desired operating range. This embodiment is particularly useful in application where the load may be damaged or function improperly when operating under low current.
These and other features and advantages of the invention will become apparent upon consideration of the following detailed description of the presently preferred embodiments of the invention, viewed in conjunction with the appended drawings.
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