Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch in the primary circuit of the supply...
Reexamination Certificate
2001-03-30
2002-09-10
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch in the primary circuit of the supply...
C315S243000, C315S244000, C315S245000
Reexamination Certificate
active
06448720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to an apparatus and method for driving a high intensity discharge (HID) lamp. Specifically, the present invention is directed to generating a high frequency resonant ignition voltage to ignite (start) the HID lamp, and to maintain a stable circuit operation with minimal interference from a high frequency resonant ignition circuit to a peak current sense signal that is used for output power and current control during a normal state of operation. A variation of the magnitude of the resonant ignition voltage with respect to a parasitic capacitance related to a length of the lamp leads is minimized by the inclusion of a damping resistor connected in series with a resonant ignition capacitor.
2. Discussion of Background and Relevant Information
Electronic high intensity discharge lamps generally employ one of two techniques for igniting (starting) the lamp. In a first technique, the lamp is ignited using a pulsed ignition method. In a second technique, the lamp is ignited using a resonant ignition method. The peak magnitude of the ignition voltage associated with the resonant ignition method is lower than the peak magnitude of the ignition voltage associated with the pulse ignition method. Accordingly, from the standpoint of safety, the resonant ignition method is generally preferred over the pulsed ignition method.
Further, two distinctively different methods exist to continue operating the lamp after it has been ignited. In a first method, the lamp is operated with a high frequency signal that is typically in the kilo Hertz (kHz) range. In a second method, the lamp is operated with a low frequency signal that is typically measured in the hundreds of Hertz range. Due to acoustic resonance problems associated with high frequency operations, it is generally preferred to employ the low frequency operation method to maintain the operation (e.g., illumination) of the lamp.
In order to generate a high frequency voltage having sufficient energy to ignite the lamp or to run the lamp (after ignition) with a high frequency signal, three fundamental approaches are generally taken, as shown in FIGS.
3
(
a
) to
3
(
c
).
FIG.
3
(
a
) illustrates a discharge lamp driving circuit having a chopper and a high frequency inverter. Depending upon different control schemes applied to switches Q
1
to Q
4
, this configuration can serve many design purposes.
It is known that HID lamps exhibit an acoustic resonance when operated at a high frequency. U.S. Pat. No. 4,912,374 discloses a method to interrupt the high frequency current with a smoothed DC current. Inductor L
1
and capacitor C
1
form a buck resonant network. Transformer T and capacitor C
2
form an inverting resonant circuit. When transistor pair Q
1
and Q
4
and transistor pair Q
2
and Q
3
are alternately switched at a high frequency, two high frequency AC currents flow through the lamp. The first high frequency AC current is produced by the buck resonant network. The second high frequency AC current is produced by the inverting resonant network. As a result, a loop current is formed between the capacitor C
1
, the transformer T, and the lamp. When transistor Q
4
is switched at a high frequency, transistor Q
1
is ON, and transistors Q
2
and Q
3
are completely OFF (due to the chopper, or buck, configuration.), so that a DC current flows from left to right through the lamp. When transistor Q
3
is switched at a high frequency, transistor Q
2
is ON, and transistors Q
1
and Q
4
are completely OFF, so that a DC current flows from right to left through the lamp.
To control the DC current, some sort of buck current sensing is required. Such a system is not disclosed in detail in U.S. Pat. No. 4,912,374. The simplest method to sense the buck current is to add a sense resistor in series with input bus voltage V
1
. However, unless special care is taken to separate the inverting resonant network current from the buck network current, a coupling may occur between the inverting resonant network and the buck resonant network. U.S. Pat. No 4,912,374 does not disclose the separation of the inverting high frequency operation and the buck DC or low frequency operation, but the inverting high frequency operation is utilized just for starting (igniting) the lamp and the DC (or low frequency) operation is utilized for the normal (continuous) operation of the lamp after it has been started.
FIG.
3
(
b
) illustrates a modification of U.S. Pat. No. 4,912,374, in which MOSFET Q
5
and diode D
5
are added. The inclusion of these components results in the lamp current comprising a clean square wave, while the sensed buck current comprises a clean triangular wave. It is noted that MOSFET Q
5
can be switched OFF any time after the lamp is ignited (started), or whenever the high frequency current is not needed for the lamp operation. When MOSFET Q
5
is switched OFF, the buck network, formed by inductor L
1
and capacitor C
1
, and the ignition network, formed by transformer T and ignition capacitor C
2
, are completely decoupled. That is, ignition capacitor C
2
is electrically disconnected from the circuit. There is no charge (or discharge) current flowing through the ignition capacitor C
2
or the current sensing resistor Rs, due to the switching of transistors Q
1
and Q
2
. Further, diode D
5
prevents any voltage overshoot during the switching of MOSFET Q
5
.
The disadvantage of this modification is that a high voltage MOSFET Q
5
and a high voltage diode D
5
is required, along with any associated driving circuitry required to drive MOSFET Q
5
. This increases the circuit complexity and increases the manufacturing costs. It is noted that if the composite waveform of the high frequency current and the DC current is required to prevent an acoustic resonance, MOSFET Q
5
has to be turned ON during the high frequency period and turned OFF during the low frequency period.
A dual stage output filter of U.S. Pat. No. 6,020,691 is illustrated in FIG.
3
(
c
), in which a chopper (or buck) power regulator with a high frequency resonant ignition, a discontinuous first resonant stage inductor current, and a continuous second resonant stage inductor current are related to each other.
In U.S. Pat. No. 6,020,691, a first stage resonant frequency fr
1
, formed by inductor L
1
and capacitor C
1
, is lower than a second stage resonant frequency fr
2
, formed by inductor L
2
and capacitor C
2
. In addition, a distance between the first stage resonant frequency fr
1
and the second stage resonant frequency fr
2
is somewhat confined not to be less than a selected minimum value, in order to avoid an excessive resonant current circulating in the circuit. The ignition voltage is generated by sweeping the frequency over the second stage resonant frequency, fr
2
. For example, if the second stage resonant frequency fr
2
is selected to be, for example, approximately 40 kHz and a minimum sweeping frequency is selected to be, for example, approximately 30 kHz, the first stage resonant frequency fr
1
may be selected to be, for example, approximately 22 kHz. This kind of circuit arrangement suffers from frequency inaccuracies and component tolerance problems, because the circulating current of the first stage resonant network is highly related to the frequency fr
1
and the minimum sweeping frequency. A further disadvantage of this circuit arrangement is that the magnitude of the ignition pulse, which is mainly generated by the second stage network, is a function of both resonant frequencies, since two stages are cascaded together. The input voltage signal, with its frequency near the second stage resonant frequency, is damped by the first stage network and amplified by the second stage network. Thus, the Q factor of the second stage network has to be significantly high so that enough ignition voltage can be generated.
SUMMARY OF THE INVENTION
The present invention overcomes the inability of the prior art to electrically separate (isolate) the first resonant network design and the second
Greenblum & Bernstein P.L.C.
Matsushita Electric Works R&D Laboratory, Inc.
Vu David
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