Electric lamp and discharge devices: systems – Condenser in the supply circuit – Condenser in shunt to the load device and the supply
Reexamination Certificate
2001-12-20
2004-01-06
Nguyen, Hoang V. (Department: 2821)
Electric lamp and discharge devices: systems
Condenser in the supply circuit
Condenser in shunt to the load device and the supply
C315S24100S, C315S20000A
Reexamination Certificate
active
06674247
ABSTRACT:
BACKGROUND
1. Field of the Invention
Embodiments relate to the field of photographic flashes and in particular to efficient flash termination and charging.
2. Related Art
Photographic flashes use a high percentage of the battery power available to modem cameras. Despite the level of commercial interest in photography, electronic flashes remain highly inefficient. In a typical flash, only 30 percent of the energy drained from the battery reaches the flash capacitor.
FIG. 1
is a schematic diagram of a typical flash system. Capacitor
114
is charged from battery
108
by charge circuit
116
. To make a flash, controller
121
closes switch
120
and sends trigger signal
119
to cause trigger circuit
118
to send a pulse through electrode
112
of flash tube
110
. Trigger signal
119
partially ionizes the gas in flash tube
110
; capacitor
114
then discharges through the gas, causing a flash of light energy to be radiated. The flash stops when the voltage on capacitor
114
falls below a threshold or switch
120
is opened.
Prior-art photo flashes use minority-carrier semiconductor switching devices, also known as conductivity-modulated devices or bipolar devices, as switch
120
. Use of such devices incurs problems with timing uncertainty and parasitic power losses, due to a turn-off delay, of typically many microseconds, that depends on minority carrier storage and recombination times. Some of these flash systems emit multiple flashes of light for one picture; however, timing uncertainty lowers performance or renders the circuits complex.
FIG. 2
is a schematic diagram of flyback converter charge circuit
200
, typical in photographic flashes.
FIG. 3
is a timing diagram. Current flows through primary winding
242
of coupled inductor
241
when drive circuit
244
turns on transistor
246
, completing a circuit through primary
242
from battery
108
. Transistor gate voltage and primary voltage are shown by traces
301
and
302
, respectively, in
FIG. 3
; trace
303
shows the drain voltage of transistor
246
. When drive circuit
244
turns off transistor
246
, mutual inductance generates current in secondary winding
243
. Voltage across secondary
243
is shown by trace
304
in FIG.
3
. Diode
248
allows current to flow from secondary
243
into capacitor
114
, and not back out. Thus, the circuit charges capacitor
114
over many cycles.
Typical flyback converters have inefficient coupled inductors that waste power, and that can create overshoot voltages at transistor
246
, potentially damaging it. Also, the current drained from battery
108
may have steep spikes and dips, lowering battery life.
FIGS. 4A and 4B
are cross-section illustrations of the winding of a typical coupled inductor. Primary winding
242
is wound around plastic bobbin
460
; then, insulation
468
is placed over winding
242
; finally, layers of secondary winding
243
are wound over insulation
468
. Ferrite core
250
with axis
464
is made in two halves,
455
and
456
. Plastic bobbin
460
supports windings
242
and
243
, shown with an “X.”
Typical coupled inductors suffer from primary-winding leakage inductance and skin effect. Leakage inductance is caused by poor magnetic-field coupling between primary winding
242
and secondary winding
243
. Primary leakage inductance causes overshoot voltages that can damage switching transistor
246
. Skin effect causes energy losses by increasing the impedance of the windings at high frequencies. Skin effect dominates the resistive losses in primary windings that are made from thick wire.
Many coupled inductors are wound on iron cores, rather than on core materials that do not easily saturate. Such an inductor reaches saturation while the current in the primary winding is still increasing, and wastes energy that cannot be stored in the core's magnetic field.
FIG. 5
is taken from FIG. 5 of U.S. Pat. No. 5,430,405, a schematic diagram of a coupled-inductor charging circuit and driver. A typical problem with such circuits is that, as capacitor
114
approaches higher charge voltages, the cyclical action of circuit
500
speeds up to drive higher voltage into capacitor
114
, causing the current drained from battery
108
to increase beyond a limit where the battery may be damaged, and thus shortening battery life.
Thus, it would be desirable to have a flash system that saves battery energy, extends battery life, and enhances flash performance by controlling flash timing accurately, with little energy loss, and by including a charge circuit with an efficient coupled inductor that also limits overshoot voltages and battery-current spikes, and that has a switching rate controlled by a drive circuit that limits the amount of current drained from the battery and uses energy-efficient components.
A more detailed background of related flash and charge circuits is included in Appendix A.
SUMMARY OF THE INVENTION
In accordance with the present invention, energy efficiency of a photographic flash is improved by provision of several unique circuits that significantly increase the efficiency of the flash. Efficiency, measured by energy stored on the flash capacitor divided by energy drained from the battery, is conserved by precisely timed flash termination, a low-loss flyback converter, a high-efficiency coupled inductor, and a battery-saving charge circuit, including a new drive. When the several improvements are combined, total energy efficiency is improved from a nominal 30-percent efficiency to close to 90-percent efficiency.
In some embodiments, a majority-carrier switching-device circuit controls flash termination, starting and stopping the flow of current from the flash capacitor through the flash tube. This circuit eliminates the problems of timing uncertainty and transient energy dissipation, which are associated with previous designs, thereby making possible more precisely timed flashes, including multiple flashes. Thus, energy is not wasted by being dumped from the flash capacitor or in transient energy dissipation. The disclosed flash-control method may also be used in conjunction with a through-the-lens (TTL) exposure control that determines how much flash energy is needed for capture of a given image, and that commands the flash control to deliver only that much flash energy, thereby further saving energy.
Some embodiments use a high-efficiency coupled inductor to save energy during charging of the flash. This coupled inductor makes use of both an overlapping winding configuration and multiple primary winding strands. Multiple primary strands lower energy losses caused by skin effects. The winding configuration enables the primary and secondary windings to share the magnetic field of the core more efficiently, thus lowering primary leakage inductance, which is another source of energy loss. Lower primary leakage inductance also results in smaller voltage spikes during turn-off of the primary winding.
A charge circuit that uses the high-efficiency inductor does not require an active snubber to damp voltage spikes. Omitting the snubber circuitry saves energy. Several embodiments of such an energy-saving charge circuit are disclosed; each has simple and efficient damping circuits that control effectively the reduced overshoot voltages and that smooth battery current drain. Because overshoot voltages are controlled, the field-effect transistor (FET), which is used to drive the charge circuit, can also be small and energy efficient. The circuit extends battery life by smoothing out peaks in the battery-current drain.
Some embodiments of the present invention include a new drive circuit that keeps battery-current drain below a threshold value, thus further extending battery life. Some embodiments of the drive circuit save additional energy by using discrete transistor circuits rather than operational amplifiers.
By combining several novel circuits and devices, the various embodiments of the resent invention improve overall energy efficiency.
REFERENCES:
patent: 3821635 (1974-06-01), Kimmel et al.
patent: 3947720 (1976-03-01), Breit
Keller Glenn J.
Mead Carver A.
Foveon, Inc.
Nguyen Hoang V.
Sierra Patent Group Ltd.
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