Resonant converter circuit with suppression of transients...

Details Resonant converter circuit with suppression of transients... Resonant converter circuit with suppression of transients...

1998-11-18

2003-04-01

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S2090SC, C315S307000

Reexamination Certificate

active

06541925

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to resonant converters such as commonly used as an electronic ballast for fluorescent and other gas discharge lamps, and more particularly to a resonant converter which can provide a change in operating condition, such as lamp turn-on or turn-off, while suppressing generation of high voltage and current transients.
2. Description of the Related Art
A well-known half-bridge resonant converter circuit which is commonly used as an electronic ballast for fluorescent and other gas discharge lamps is shown in FIG.
1
A. Therein a power source
1
supplies a DC input voltage V
in
across a pair of series connected electronic switches Q
1
, Q
2
which may be power MOSFET's, the gates of which are actuated by substantially square wave gating signals V
GS1
. and V
GS2
which are complementary; i.e., V
GS2
has a duty ratio D and V
GS1
has a duty ratio (1−D), where D is the ratio of ON time to period of the signal. The power source
1
may comprise a full wave rectifier followed by a preconditioner circuit, as shown in U.S. pat. No. 5,742,134, issued Apr. 21, 1998, assigned to Philips Electronics N.A. Such gating signals are supplied by a conventional square wave generator
3
. The frequency of the gating signals may be of the order of 45 kHz. Connected across switch Q
2
, in series, are a blocking capacitor C
b
, inductor L
r
and the primary winding of a step-up isolation transformer T having a primary magnetizing inductance L
m
which is shunted by a capacitor C
r
. The inductor L
r
and capacitor C
r
have a resonant frequency which is somewhat below the gating signal frequency. An output voltage V
0
is produced across the secondary winding of the transformer T, and is applied to a fluorescent lamp connected thereto represented by a resistance R
1
in series with a current limiting capacitance C
1
.
The circuit will be described starting from an initial state in which the lamp is off. It is assumed that the lamp is of the instant-start type which requires a starting voltage in the vicinity of 550 volts rms. During the “on” portion or “1” state of each; a cycle of gating signal V
GS1
it turns switch Q
1
ON. The input voltage V
in
, typically about 250 volts DC, then produces current in the path including C
b
, L
r
and L
m
and also charges capacitance C
r
. Capacitance C
b
is much larger than C
r
and serves only a blocking function to prevent DC from reaching transformer T. When gating signal V
GS1
returns to the “
0
” state it turns switch Q
1
OFF, and simultaneously gating signal V
GS2
turns switch Q
2
ON. The energy stored in inductance L
r
and capacitance C
r
then results in a very large reactive voltage across the transformer primary winding inductance L
m
, since the voltage gain in the vicinity of the resonant frequency of L
r
and C
r
can be 10 to 15 or even greater. That is still further amplified by the step-up turns ratio of transformer T, and so it is readily achieved that adequate starting voltage is applied to the lamp to cause it to start-up.
After the lamp has ignited the lamp resistance R
1
loads the resonant circuit, reducing the effective gain. Cyclic operation then continues at a frequency somewhat above the resonant frequency, producing a sufficiently high voltage to maintain the lamp in the ON condition. When it is desired to turn the lamp off the gating signal V
GS1
applied to switch Q
1
is turned off. It is thereby switched to the “
0
” state (duty ratio=0), remaining open, so that voltage is no longer supplied to the resonant circuit. At the same time, the common practice is to also turn gating signal V
GS2
to the ON state (duty cycle=1), so that switch Q
2
remains closed. The reactive energy existing in the resonant circuit at the instant turn-off is commenced will again result in a very large transient voltage across the transformer winding and also across inductance L
r
which will then decay over an interval determined by the time constant of the reactive circuit loop.
The large reactive voltages and current produced in the converter circuit during lamp turn-on and turn-off requires the circuit designer to use circuit elements rated for voltages and currents many times the levels encountered during steady state circuit operation after the lamp has been turned on. That significantly increases the cost of the converter circuit. In addition, during turn-off the cyclic reactive voltage produced across the transformer winding may be sufficient to cause low frequency re-ignition of the lamp during several of the reactive cycles, causing repeated flickering which can be quite disturbing to an observer.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a resonant converter which, while still providing an adequate instant-start voltage for a fluorescent or other gas discharge lamp, suppresses generation of the excessive transient reactive voltages and currents conventionally produced during lamp turn-on and turn-off. A further object is to provide definite turn-off of the lamp, without cyclic production of reactive voltages sufficient to cause low frequency repetitive re-ignition of the lamp.
Applicants have found that these objectives can be achieved by appropriate control of the duty ratios of the cyclic gating signals V
GS1
and V
GS2
which are supplied to the switches in the converter circuit. In particular, instead of immediate transition of each gating signal to or from the completely on or completely off state, the duty ratios of the gating signals are swept so as to incrementally reach the required altered duty ratio over several cycles of the gating signals. For lamp turn-on, the duty cycle of gating signal V
GS1
is swept over several cycles from 0% (constant OFF) to 50% (ON time=OFF time), while simultaneously the duty ratio of gating signal V
GS2
is swept over several cycles from 100% (constant ON) to 50% (ON time=OFF time). For lamp turn-off, the aforesaid sweeps of the duty cycles of the gating signals are reversed. Applicants have also found that control of the duty ratios of the gating signals may be employed to efficiently control the intensity of the light produced by the lamp, as contrasted with conventional control of light intensity by alteration of the cyclic frequency of the gating signals.
Such gradual alteration of the duty ratios of the cyclic gating signals has been found to substantially suppress generation of high voltage and current transients during lamp turn-on or turn-off, as well as low frequency repeated re-ignition of the lamp during turn-off.


REFERENCES:
patent: 4685040 (1987-08-01), Steigerwald et al.
patent: 5424614 (1995-06-01), Maheshwari
patent: 5583402 (1996-12-01), Moisin et al.

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