Circuit for detecting near or below resonance operation of a...

Details Circuit for detecting near or below resonance operation of a... Circuit for detecting near or below resonance operation of a...

1998-06-10

2001-12-18

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Periodic switch in the supply circuit

Periodic switch cut-out

C315S119000, C315S307000

Reexamination Certificate

active

06331755

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fluorescent lamp with a resonant type circuit driven by a half-bridge circuit and, more particularly, to a circuit for detecting near or below resonance operation of such a lamp.
2. Description of the Related Art
FIG. 1
shows a typical lamp resonant output stage being driven by a half-bridge circuit including MOSFETS M
1
and M
2
providing current I
L1
through inductor L
1
to the lamp connected in parallel with capacitor C
1
. A sense resistor R
CS
is connected between the source of lower MOSFET M
2
and ground. As shown in the timing diagram of
FIG. 2
, under normal operating conditions, the phase of the inductor current I
L1
with respect to the half-bridge voltage V
S
lies somewhere between 0 and −90 degrees. Should the phase approach 0 degrees, however, the frequency is approaching resonance. At or near resonance, non-zero voltage switching can occur at the half-bridge, resulting in a large current spike at turn-on in either of the two half-bridge switches, as shown in the timing diagram of FIG.
3
.
Referring to the transfer function for running and before ignition conditions depicted in
FIG. 4
, it is also possible for the resonant lamp output stage to be operating above the resonance frequency of the low-Q circuit (during running), but below the resonance frequency of the high-Q (before ignition) circuit. If the lamp is then removed, the transfer function jumps from the low-Q to the high-Q curve while the frequency remains unchanged and below the resonance frequency of the high-Q circuit. This results in almost immediate destruction of the half-bridge.
Another condition which can cause below resonance operation is if the filaments of the lamp are intact, but the gas within the lamp escapes (e.g., the glass cracks). In this condition, the load operating condition would instantaneously change from the damped (above resonance) to the undamped (below resonance).
Accordingly, it would be desirable to be able to detect of operation of the lamp near or below the resonance frequency, and to shutdown operation of the lamp under such conditions to prevent catastrophic failure of the switching devices (usually MOSFETs) of the half-bridge driver circuit.
SUMMARY OF THE INVENTION
The present invention is a circuit which detects near or below resonance operation of a fluorescent lamp being driven with a half-bridge circuit and which deactivates the half-bridge circuit before any damage to the circuit can result.
More specifically, in the circuit of the present invention, the voltage across a sense resistor disposed either the lower transistor switch and ground, or between the lower lamp filament and ground, is compared against a predetermined reference voltage to generate an output comparison signal. The output comparison signal is gated to the turn-off edge of the lower MOSFET (in the case of the sense resistor disposed between the lower MOSFET and ground) or to the turn-off edge of the upper MOSFET (in the case of the sense resistor between the lower lamp filament and ground) to generate a signal for shutting down the half-bridge circuit in the event of near or below resonance operation of the lamp resonant circuit.
The circuit of the present invention is advantageously applicable to any resonant circuit, such as a resonant-mode power supply.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.


REFERENCES:
patent: 5049790 (1991-09-01), Herfurth et al.
patent: 5420779 (1995-05-01), Payne
patent: 5550433 (1996-08-01), Tobler
patent: 5699238 (1997-12-01), Lee et al.
patent: 5705894 (1998-01-01), Krummel
patent: 5808455 (1998-09-01), Schwartz et al.
patent: 5883473 (1999-03-01), Li et al.
patent: 5969483 (1999-10-01), Li et al.
patent: 0399613 (1990-11-01), None
patent: WO9325952 (1993-12-01), None
patent: WO9743878 (1997-11-01), None
patent: WO9846053 (1998-10-01), None

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