Electric lamp and discharge devices: systems – Pulsating or a.c. supply – With power factor control device
Reexamination Certificate
2001-10-19
2003-09-09
Nguyen, Hoang (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
With power factor control device
C315S291000, C315S307000, C315S324000, C315SDIG007
Reexamination Certificate
active
06617805
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for controlling a fluorescent lamp, and more particularly, to an integrated circuit that includes ballast control circuitry, power factor correction circuitry and a half bridge driver in a single monolithic chip.
2. Brief Description of the Related Art
Electronic ballasts for controlling fluorescent or high-intensity discharge (HID) lamps usually require electronics necessary for preheating the lamp filaments, striking the lamp, driving the lamp to a given power, detecting lamp fault conditions, and safely deactivating the circuit.
Electronic ballasts for gas discharge circuits have come into widespread use because of the availability of power MOSFET switching devices and insulated gate bipolar transistors (IGBTs) that can replace previously used pwer bipolar switching devices. A number of integrated circuits (ICs) have been devised for driving gates of power MOSFETs or IGBTs in electronic ballasts. Examples include the IR2155, IR2157, and IR21571 products sold by International Rectifier Corporation and described in U.S. Pat. Nos. 5,545,955 and 6,211,623, the disclosures of which are incorporated herein by reference in their entireties.
The IR2155 gate driver IC offers significant advantages over prior circuits: The driver is packaged in a conventional DIP or SOIC package. The package contains internal level shifting circuitry, under voltage lockout circuitry, deadtime delay circuitry, and additional logic circuitry and inputs so that the driver can self-oscillate at a frequency determined by external resistors and capacitors.
The IR2157 and IR21571 products provide fully integrated ballast control ICs with several features not available in the IR2155. The IR2157 and IR21571 products function in five basic modes of operation and can make transitions between modes based on IC inputs. The modes include undervoltage lockout (UVLO) mode, preheat mode, ignition ramp, run mode, and fault mode. Other features of these ICs include: (i) a start-up procedure that insures a flash-free start without an initial high voltage pulse across the lamp; (ii) non-zero voltage switching protection circuitry; (iii) over-temperature shutdown circuitry; (iv) DC bus and AC on/off control circuitry; and (v) near or below resonance detection circuitry.
Previously available ballast ICs require external components for power factor correction (PFC) control. An example of a PFC control circuit is described in U.S. Pat. No. 6,259,614, the disclosure of which is incorporated herein by reference in its entirety.
In addition, previously available ballast ICs are subject to several operation problems:
One ballast operation problem, the “initial flash problem,” can arise when driver circuitry for a half bridge circuit first begins to switch. If the half bridge is powering a load such as a lamp, inductive (L) and capacitive (C) components in the load circuit initially contain no stored energy. For a few initial switching cycles, a very high voltage appears across the C components and also across the lamp. This initial high voltage can be substantially higher than steady state voltage during preheating and can cause the lamp to momentarily ignite or strike, producing an initial flash of the lamp. Because the cathodes of the lamp have not been properly heated at this time, they may be weakened, undesirably shortening the lamp's life.
Another ballast operation problem, the “DC bus droop problem,” can arise when the lamp ignites or “strikes.” Prior to lamp ignition, current through the load circuit is relatively low compared to load currents when the lamp ignites and runs. Meanwhile, a PFC control circuit conventionally drives a boost switch transistor and inductor to maintain DC bus voltage during this light load, pre-ignition period. When the ballast control circuit begins ignition ramping, the frequency of half bridge switch devices decreases or ramps downward to an ignition frequency. During this downward ramp, current through the L and C components of the load circuit increases as the resonant frequency is approached. The voltage developed across the C component also increases, and when the magnitude of this voltage reaches the ignition voltage of the lamp, the lamp ignites. At lamp ignition, the load current seen by the DC bus increases very abruptly, causing a momentary droop in DC bus voltage. If large enough, this droop could undesirably cause lamp ignition to fail, the lamp to extinguish, and the ballast to sense a fault and shut down.
In the DC bus droop problem, DC bus load can cause a change in DC bus voltage because the PFC control circuit has a finite loop response time. A step change in load current causes a momentary change in the DC bus voltage until the control loop can catch up. Several techniques can be used to reduce this change in DC bus voltage, such as increasing the value of the DC bus storage capacitor and/or increasing control loop speed. Neither of these solutions is ideal in practice, however, because increasing the value of the capacitor also increases its cost and physical size, while increasing control loop speed can lead to instability.
Another ballast operation problem, the “false shutdown problem,” can arise during ignition ramping when the lamp ignites. In some cases, current in the lamp circuit, which includes L and C components and the lamp, can momentarily go to zero. If this occurs, no energy is stored and therefore the half-bridge switch devices, such as FETs, will hardswitch, possibly over several cycles. The ballast control circuit may detect this hardswitching as an over-current condition and may therefore shut down. This is undesirable because such false shutdowns make it uncertain whether the lamp will reliably strike.
Previous designs used filter/delay components external to the ballast control IC to alleviate the false shutdown problem. Such components may, however, prevent proper sensing of real faults.
Another ballast operation problem, the “end-of-life detection problem,” arises when a lamp approaches the end of its life. The IR2157 and 21571 products described above each have a shutdown (SD) pin, used to shutdown the oscillator, pull gate driver outputs low, and put the IC in an interim micropower state. Input voltage on the SD pin above a threshold indicates lamp fault, lamp exchange, or lamp removal. It would be advantageous to provide circuitry for easier detection of the end of life of a lamp.
Another ballast operation problem, the “delayed restart problem,” arises when a lamp is turned off such as by a momentary power outage or brown-out. Restarting of the lamp is conventionally delayed for an undesirably long time while preheating is performed.
SUMMARY OF THE INVENTION
The present invention provides a circuitry for controlling a load circuit including a fluorescent lamp which reduces the number of external components required and alleviates the ballast operation problems described above.
The circuitry of the present invention reduces the number of external components by providing an IC that includes both ballast control circuitry, power factor correction (PFC) circuitry, and a half bridge driver in a single monolithic chip. The ballast control and driver circuitry provides drive signals to a power supply circuit, which delivers power to a load circuit including a fluorescent lamp. The ballast control and driver circuitry also receives sense signals indicating operating conditions of the power supply circuit and/or the load circuit, and responds to the sense signals by modifying the drive signals. The PFC circuitry regulates the voltage at which power is provided to the load circuit by the power supply circuit. Therefore, PFC circuitry and the ballast control and drive circuitry together can control the power supply circuit and the load circuit.
By including PFC circuitry with the ballast control and driver circuitry, the invention eliminates the need for external PFC circuitry. This is advantageous because a full function ballast control IC can be implemented,
Ribarich Thomas J.
Wilhelm Dana S.
International Rectifier Corporation
Nguyen Hoang
Ostrolenk, Farber, Gerb & Soffen, LLP
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