Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-08-25
2001-10-09
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S414000
Reexamination Certificate
active
06300777
ABSTRACT:
BACKGROUND OF THE INVENTION
An important feature of a dimmable electronic ballast is the ability to ignite the lamp at any dim position. It is difficult to determine when the lamp strikes, since lamp ignition is a function of filament pre-heating, temperature, frequency, distance to the nearest earth plane (usually the fixture) and age of the tube. If the regulation loop is not closed immediately after ignition (hundreds of micro-seconds), the result is an unwanted “flash” over the tube before going to the user dim setting (or lamp brightness setting).
In a conventional lamp resonant output stage, the frequency of operation at which the tube strikes is usually close to the maximum brightness operating frequency during running. Therefore, all present day fluorescent lamps are ignited close to 100% brightness. The travel time to a low brightness setting after ignition results in a “flash” seen by a user.
Furthermore, if a conventional lamp circuit falsely detects an ignition when the lamp has not yet ignited, the circuit will close the loop and regulate back out of ignition to somewhere between the ignition and preheat frequencies. This results in dangerously high voltages and currents across and through the tube and in the ballast output stage for an indefinite amount of time.
SUMMARY OF THE INVENTION
The present invention addresses the above-described problems by providing a circuit for detecting ignition of the lamp at any dim position.
The ignition detection circuit of the present invention detects the ignition of a fluorescent lamp by comparing the phase of the total lamp resonant circuit current to a predetermined reference phase value. When the phase of the total lamp resonant circuit current is coincident with the predetermined reference phase, lamp ignition has occurred and the circuit automatically “closes the loop” so that the lamp power can be regulated (via phase control) to a user designated setting.
The phase of the total lamp resonant circuit current is measured in the present invention by detecting the zero-crossing of the inductor current as measured across a resistor disposed in the lamp resonant circuit. The only way for the phase of the inductor current to reach the reference phase is if the lamp ignites (provided the frequency is above the resonance frequency and is ramping smoothly from a high frequency down to the ignition frequency).
The circuit of the present invention is advantageously designed to wait approximately 10 cycles to avoid closing the loop before the lamp has struck.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.
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International Rectifier Corporation
Kerveros James C
Metjahic Safet
Ostrolenk Faber Gerb & Soffen, LLP
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