Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2001-06-15
2003-07-15
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S2090CD, C315S243000, C315S360000
Reexamination Certificate
active
06593703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit arrangement and controller that generates a high frequency resonant ignition voltage to ignite a high intensity discharge (HID) lamp, to maintain a low frequency square wave lamp current while spikes are superimposed on the low frequency square wave lamp current within an envelope of the low frequency square wave lamp current, to reduce stresses on switching devices during a polarity transition of the lamp voltage or lamp current when the HID is being extinguished, and to improve a lamp glow-to-arc transition by applying an imbalanced high frequency current after a lamp breakdown.
2. Discussion of Background and Relevant Information
Two distinctly different methods exist to ignite an electronic high intensity discharge (HID) lamp. In a first method, the lamp is ignited using a pulsed method. In a second method, the lamp is ignited using a resonant method. Because of safety concerns associated with a peak magnitude of an ignition voltage, the resonant method, which requires a voltage having a smaller peak magnitude (in comparison with the pulsed method), remains the preferred technique for use in electronic high intensity discharge (HID) lamp ballasts.
Two distinctively different methods of operating the lamp after ignition are employed with electronic high intensity discharge lamp ballasts. In a first method, the lamp is operated at a high frequency range of, for example, several tens of kilohertz. In a second method, the lamp is operated at a low frequency range of, for example, several hundreds of hertz. Because of acoustic resonance problems associated with operating the lamp in the high frequency range, it is preferable to operate the lamp at the low frequency range.
FIGS. 8 and 9
illustrate two approaches for generating a high frequency voltage with sufficient energy to ignite the lamp and to run a lamp in a low frequency operation. In
FIG. 8
, a discharge lamp driving circuit includes a combined chopper and high frequency inverter. Depending upon the control scheme implemented with switches Q
1
to Q
4
, this configuration can serve many design purposes.
U.S. Pat. No. 4,912,374 describes one such implementation. It is well known that HID lamps may exhibit an acoustic resonance when it is operated at a high frequency. U.S. Pat. No. 4,912,374 discloses a method that reduces the acoustic resonance problem by interrupting the high frequency current with a smoothed DC current. Buck inductor L
1
and buck filter capacitor C
1
form a buck resonant network. Transformer T and ignition capacitor C
2
form an inverting resonant network. If semiconductor switches, such as, for example, transistor switching pair Q
1
and Q
4
and transistor switching pair Q
2
and Q
3
are complementarily switched at a high frequency rate, two high frequency AC currents will flow through the lamp. A first high frequency AC current will flow through the capacitor C
1
and the inductor L
1
. A second high frequency AC current will flow through the ignition capacitor C
2
and the transformer T.
As a result, a loop current is formed between the buck filter capacitor C
1
, the transformer T, and the lamp. In a chopper (or buck) configuration, switch Q
1
is ON and switches Q
2
and Q
3
are completely OFF while switch Q
4
is switched at a high frequency rate. As a result, a DC current flows through the lamp from left to right (of the circuit) during this period. When switches Q
1
and Q
2
change state (e.g., the operation state of switch Q
1
changes from ON to OFF and the operation state of switch Q
2
changes from OFF to ON), the voltage at the junction of switches Q
1
and Q
2
changes from HIGH to LOW within a very brief period of time, such as, for example, a couple of hundred nanoseconds. This rapid (sudden) voltage change causes a spike current to flow out of the ignition capacitor C
2
and the lamp, from the left to the right, and for current to flow out of the buck filter capacitor from right to left, back to the junction of switches Q
1
and Q
2
.
Note that the direction of the spike current is the same as the direction of a DC lamp current. When switch Q
3
is switched at a high frequency rate, switch Q
2
is ON and switches Q
1
and Q
4
are completely OFF. Thus, a DC current flows from right to left through the lamp during this period. When switch Q
2
changes from an ON state to an OFF state and switch Q
1
changes from an OFF state to an ON state, the voltage at the junction of switches Q
1
and Q
2
changes from LOW to HIGH within a couple of hundred nanoseconds. This rapid (sudden) change on voltage causes a spike current to flow into the ignition capacitor C
2
and the lamp from right to left, and for current to flow in the buck filter capacitor C
1
, from left to right. The spike current has the same direction as the DC lamp current. Unfortunately, in both cases, the spike current re-enforces the DC lamp current, causing the instantaneous lamp current at the spike to be higher than the average DC current. In the case where only a DC current (e.g., low frequency square wave current) is applied to the lamp during a normal operation, the spike current is higher than an envelope of the low frequency square wave current. This is not desirable.
A more detailed explanation on how the circuit behaves during the polarity transition of the lamp voltage or lamp current will now be provided. Generally, during the transition, the four switches Q
1
to Q
4
operate as shown in FIG.
10
. During a first half cycle, switch pair Q
1
and Q
4
is active. At the end of the first half cycle, all switches are turned OFF at time t equals t
1
to avoid cross conduction. Then, a so-called “dead time” begins, during which time there is no electrical conduction. After the dead time (e.g., when time t equal t
2
), switching pair Q
2
and Q
3
become active. The load current reverses its polarity and flows in the opposite direction, as compared with the active half cycle of switch pair Q
1
and Q
4
. Because of the switching at time t equals t
1
, the voltage at the junction of switching pair Q
1
and Q
2
suddenly goes from being substantially equal to bus voltage V(
1
), to either float or become substantially equal to a negative rail voltage, to continue “free-wheeling”. Unfortunately, this instantaneous change in voltage causes a spike current to flow through the ignition capacitor C
2
and the lamp in the same direction as the low frequency square wave current.
FIG. 9
illustrates a modification of U.S. Pat. No. 4,912,374. According to this circuit, switch Q
5
and diode D
5
are added, so that the lamp current exhibits a clean square wave. Switch (e.g., MOSFET) Q
5
is turned (switched) OFF after the lamp is ignited, or whenever the high frequency current is not needed for the operation of the lamp. When switch Q
5
is switched OFF, the ignition capacitor C
2
, is electrically disconnected from the circuit. Thus, no current flows through the ignition capacitor C
2
and the lamp, due to the switching of switch pair Q
1
and Q
2
. Diode D
5
functions to prevent any voltage overshoot during the switching of switch (MOSFET) Q
5
.
Unfortunately, modifying the circuit of U.S. Pat. No. 4,912,374 to include the high voltage MOSFET Q
5
, the high voltage diode D
5
, and a driving circuitry required to drive switch Q
5
increases the complexity of the lamp driving circuit. Further, the inclusion of these additional components increases manufacturing costs.
The lamp voltage (or lamp current) may be sensed to detect a light dropout during a normal operation. If the lamp voltage exceeds a predetermined maximum voltage for the lamp to operate normally, the lamp is determined to have dropped out (e.g., light from the lamp is extinguished). The controller is then quickly switched from a (normal) operating mode to a starting (ignition) mode to re-ignite the lamp. The transition from the operation mode to the starting mode usually requires at least one low frequency cycle. Any time duration less than approximately one low f
Greenblum & Bernstein P.L.C.
Matsushita Electric & Works Ltd.
Tran Thuy Vinh
Wong Don
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