Lamp current balancing topologies

A method according to on embodiment may include generating AC voltage and current and a striking voltage. The method of this embodiment may also include generating striking voltage and steady-state voltage for at least two lamp loads. The method of this embodiment may also include coupling at least two lamp loads in parallel. The method of this embodiment may also include coupling current balancing circuitry to the at least two lamp loads and providing, by the current balancing circuitry simultaneous striking voltage to the at least two lamps loads. The method of this embodiment may also include balancing, by the current balancing circuitry, AC current through the at least two lamp loads. Of course, many alternatives, variations, and modifications are possible without departing from this embodiment.

FIELD

The present disclosure relates to lamp current balancing topologies.

BACKGROUND

In conventional circuits driving multiple lamps, there are two primary configurations. Both configurations may include an inverter controller capable of receiving feedback and fault protection signals, switches, ballast and resonant capacitors to amplify the signal, multiple lamps to be driven by the amplified signal and fault protection circuitry to generate the fault protection signal. The first configuration drives the lamps with a single transformer. In the conventional arrangement, when the lamps are driven by a single transformer, the current flowing through each individual lamp is not balanced, creating difficulties in acquiring a reliable feedback signal. An unreliable feedback signal may allow some of the lamps to stay off during the ignition process and may present safety issues. The second configuration drives each lamp with a dedicated transformer. Driving each lamp individually solves the feedback issue, but introduces a new component which increases cost of production and requires physical space in the device. Therefore, an inexpensive way to drive multiple lamps while maintaining a reliable feedback signal is needed.

SUMMARY

One system embodiment described herein may provide a transformer capable of generating, at least in part, AC voltage and current and a striking voltage. The system may also include at least two lamp loads coupled in parallel to the transformer. The system may also include current balancing circuitry coupled to the plurality of lamp loads, the current balancing circuitry is capable of balancing AC current supplied by the transformer through the at least two lamp loads, the current balancing circuitry is also capable providing simultaneous striking voltage, supplied by the transformer, to the at least two lamps loads.

A method according to on embodiment may include generating AC voltage and current and a striking voltage. The method of this embodiment may also include generating striking voltage and steady-state voltage for at least two lamp loads. The method of this embodiment may also include coupling at least two lamp loads in parallel. The method of this embodiment may also include coupling current balancing circuitry to the at least two lamp loads and providing, by the current balancing circuitry simultaneous striking voltage to the at least two lamps loads. The method of this embodiment may also include balancing, by the current balancing circuitry, AC current through the at least two lamp loads.

One apparatus embodiment may include current balancing circuitry coupled to a plurality of lamp loads and a transformer capable of generating, at least in part, AC voltage and current and a striking voltage. The current balancing circuitry may be capable of balancing AC current supplied by the transformer through the at least two lamp loads. The current balancing circuitry may also be capable of providing simultaneous striking voltage, supplied by the transformer, to the at least two lamps loads.

Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly, and be defined only as set forth in the accompanying claims.

DETAILED DESCRIPTION

FIG. 1illustrates one exemplary system embodiment100of the claimed subject matter. System100, in this embodiment, may include inverter controller circuitry101, DC/AC inverter circuitry102, current balancing circuitry116and one or more loads112,114. In this embodiment, loads112and114comprise lamp loads, for example cold cathode fluorescent lamps, as may be used in a liquid crystal display (LCD) panel. DC/AC inverter circuitry102may comprise, for example, a plurality of switches (not shown) arranged in a full-bridge, half-bridge, push-pull, active clamp, and/or Class D topology and/or other conventional and/or custom inverter topology. Inverter controller circuitry101may control the switches of the inverter circuitry102to generate a rectangular AC signal from a DC source. System100may also comprise a transformer104. Transformer104may receive the rectangular AC signal and, in conjunction with one or more resonant capacitors108, generate a smooth sinusoidal (or quasi-sinusoidal) AC signal to supply power to the lamp loads112and114. In this embodiment, current balancing circuitry116may be capable of balancing current supplied to each lamp load, in a manner described below.

As used in any embodiment herein, “circuitry” may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry. For ease of understanding, the drawings herein depict inverter controller circuitry101, DC/AC inverter circuitry102and current balancing circuitry116as separate components. However, it should be understood that inverter controller circuitry101, DC/AC inverter circuitry102and current balancing circuitry116may, in whole or in part, or collectively or individually, be comprised in one or more integrated circuits. As used in any embodiment herein, an “integrated circuit” means a semiconductor device and/or microelectronic device, such as, for example, a semiconductor integrated circuit chip.

The inverter controller circuitry101may be capable of controlling DC/AC inverter circuitry102to generate an AC voltage and current from a DC signal. The inverter controller circuitry101may also be capable of controlling the DC/AC inverter circuitry102to generate striking voltage and steady-state voltage for at least two lamp loads112and114. Striking voltage may include, for example, sufficient voltage to ignite the lamp. If the lamp is a CCFL, for example, a striking voltage on the order of 1500 Volts may be used to ignite the lamp. Once struck, the lamp may require less voltage during steady-state operation. If the lamp is a CCFL, for example, a steady state voltage on the order of a few hundred volts may be used.

Current balancing circuitry116may include respective ballast capacitors106aand106bfor each respective lamp load112,114. The lamp loads may be coupled in parallel with each other and to the transformer104. Current balancing circuitry116may also include a balancing inductor110disposed between each lamp112and114, for example, between the high voltage sides118and120, respectively, of the lamps, and each respective ballast capacitors106aand106b. Ballast capacitors106aand106band inductor110may operate to balance current through each lamp load IL1and IL2. “Balance”, as used in any embodiment herein with reference to the current in each lamp load, may be defined as approximately equal, or, alternatively, as a condition where the current in one lamp is a multiple of the current in another lamp load. Current balancing circuitry116may be capable of supplying lamp striking voltage to each lamp in the system100simultaneously. “Simultaneously”, as used in any embodiment herein with reference to striking voltage, may mean that each lamp is provided striking voltage at the same time. Lamp striking voltage may last for several seconds, and the aforementioned definition of “simultaneously” means that at least a portion of the time intervals for the striking voltage for each lamp overlap (which does not necessarily mean that the time intervals overlap completely).

For example, the current in each lamp load may be balanced when the respective currents IL1and IL2are approximately equal. “Approximately equal”, as used herein with reference to current, may be defined as within a selected and/or pre-defined tolerance and/or within a tolerance that may be defined by component values. In this embodiment, ballast capacitors106aand106bmay be given a common value Cb, then according to Kirchhoff's voltage law, a proper current balance may be achieved at a certain operating frequency fo if the balancing inductor Lb110and the ballast capacitors Cb106aand106bare given values according to:
w2LbCb=2; wherew=2πfo

Since the inductor110is located on the high voltage sides (118and120) of the lamps112and114, the safety and voltage ratings of the inductor110may be considered.

The low voltage terminals of the n lamps210,212,214may be connected by n−1 differential mode inductors216,218. Thus, for example, if system200includes 3 lamps, than system200may also include two differential mode inductors216and218. Each differential mode inductor may include a primary winding220and228and a secondary winding222and230. Each lamp load210,212, . . . ,214may be coupled to a respective primary winding220or228or a respective secondary winding222or230. In this embodiment, each differential mode inductor216, . . . ,218may operate as 1-1 current transformers, meaning the current through the primary windings is approximately equal to the current through the secondary windings.

The high voltage side of each lamp may be coupled in series to respective ballast capacitors204,206, . . . ,208. Each lamp and ballast capacitor may be coupled in parallel to each other and to transformer104.

The inductance values for the differential mode inductors216,218may be calculated using Kirchoff's laws. The differential mode inductors216,218may be arranged so that the low voltage terminal of lamp L1210is connected to the primary winding of differential mode inductor DM1216, the low voltage terminal of lamp L2212is connected to the secondary winding of differential mode inductor DM1216, the low voltage terminal of lamp Ln214is connected to the secondary winding of differential mode inductor DMn218. The differential mode inductors216,218may be further arranged so the secondary winding of differential mode inductor DM1216is in series with the primary winding of differential mode inductor DMn218.

As with the previous embodiment, current balancing circuitry224coupled to the lamp loads may operate to balance current through each lamp load. Also, current balancing circuitry224may be capable of supplying lamp striking voltage to each lamp in the system200simultaneously.

FIG. 3illustrates another exemplary embodiment300of the claimed subject matter. This particular embodiment may include inverter controller circuitry101′, DC/AC inverter circuitry102, current balancing circuitry350, and one or more loads112,114. In this embodiment, loads112and114comprise lamp loads, for example cold cathode fluorescent lamps (CCFLs), as may be used in a liquid crystal display (LCD) panel. DC/AC inverter circuitry102may comprise, for example, a plurality of switches (not shown) arranged in a full-bridge, half-bridge, push-pull, active clamp, and/or Class D topology and/or other conventional and/or custom inverter topology. Inverter controller circuitry101′ may control the switches of the inverter circuitry102to generate a rectangular AC signal from a DC source.

System300may also comprise transformer104. Transformer104may receive the rectangular AC signal and, in conjunction with one or more resonant capacitors302, generate a smooth sinusoidal (or quasi-sinusoidal) AC signal to supply power to the lamp loads112and114. In this embodiment, current balancing circuitry350may be capable of balancing current supplied to each lamp load, in a manner described below.

The inverter controller circuitry101′ may have all of the capabilities of the inverter controller circuitry101′ described above with reference toFIGS. 1 and 2, and, in this embodiment may also comprise soft start circuitry322, fault protection circuitry326and a feedback comparator320. During initial power on, after reset, and/or at other time periods, soft start circuitry322may be capable of controlling the switches102of the DC/AC inverter circuitry to generate a nominal or minimal current value applied to transformer T1104. Soft start circuitry322may also be capable of ramping up power delivered to the CCFL loads, based on, for example, a user-defined and/or programmable interval.

The soft start circuitry322may increase the signal VS324which may cause the inverter controller circuitry101′ to control the DC/AC inverter circuitry102. If the signal VS324reaches a threshold voltage VT328, the soft start circuitry322may enable the fault protection circuitry326. The fault protection circuitry326may also be enabled by a fault protection signal FP314. Fault protection circuitry326may control switches102to reduce, minimize, and/or shut-off power delivered to one or more lamp loads (for example, as may be desirable if an open-lamp condition is detected). The inverter controller circuitry101′ may control the DC/AC inverter circuitry102based on, at least in part, lamp current feedback information.

To that end, inverter controller circuitry101′ may also include a feedback comparator320. The feedback comparator320may compare lamp current from one or more loads112,114to a reference signal ADJ318, for example, a signal proportional to the brightness setting in a liquid crystal display (LCD) panel. Inverter controller circuitry101′ may also receive voltage feedback indicative of the voltage across one or more lamp loads, and may also control switches102based on, at least in part, voltage feedback information. Lamp current information for lamp112may be generated by voltage divider circuitry, for example a voltage divider comprising resistor354and356. Voltage divider resistors354and356may generate a current feedback signal indicative of, or proportional to, the current in lamp112. Similarly, lamp current information for lamp114may be generated by voltage divider circuitry, for example a voltage divider comprising resistor358and360. Voltage divider resistors358and360may generate a current feedback signal indicative of, or proportional to, the current in lamp114.

Current balancing circuitry350may include respective ballast capacitors304aand304bfor each respective lamp load112,114. Current balancing circuitry may also include differential mode inductor circuitry306electrically coupled to the high voltage side of the lamp load. The ballast capacitors304aand304band the differential mode inductor306may operate to balance the current, as defined above, through each lamp load IL1and IL2. Additionally, current balancing circuitry may be capable of supplying lamp striking voltage to each lamp in the system300simultaneously, as may defined above.

In this particular embodiment, ballast capacitors304aand304bmay be given a common value C. The inductance of the primary and secondary windings of the differential mode inductor306may be given a common value L and contain a mutual inductance M. The lamp loads may be replaced with loads RL1and RL2for the purposes of calculations and the inductance of the differential mode inductor may be found using Kirchoff's Laws:
RL12−RL22=(4 L/C)*(1−K), where K=M/L.

For the purpose of calculation, the values of RL1and RL2may be assumed to be a worst case scenario, for example 20% apart with values of 120 KΩ and 100 KΩ respectively, and the inductance L may be calculated from those assumed values. When the RL1/RL2differential is near the assumed differential, the current balancing circuitry350will correct the current imbalance. When the RL1/RL2differential is near zero, the current will be naturally balanced because there will be no lamp impedance differential. The range may be picked realistically because the current balance may suffer when the RL1/RL2differential is between the assumed value and zero.

When the above formula is followed, the differential mode inductor306may act as a 1-1 current transformer and balance the current, IL1and IL2, flowing to each lamp112and114, respectively. However, it is equally contemplated herein that inductor306may be configured to operate in other modes, for example, a m to n current transformer, where m does not equal n. The ballast capacitors304aand304band the inductance of the primary and secondary windings of the differential mode conductor do not have to be equal for the differential mode inductor306to balance the lamp currents IL1and IL2, and may have unequal capacitance and inductance values respectively. If the values are unequal, the differential mode inductor306may be chosen according to Kirchoff's laws. Since the differential inductor306is located on the high voltage side of the lamp loads112and114, the safety and voltage ratings of the differential mode inductor306may be considered.

The fault protection signal generating circuitry310may generate a fault protection signal FP314that may be used by the inverter controller circuitry101′. The fault protection generating circuitry310may comprise switches330and332that may process the output voltages of each individual lamp112,114. In this embodiment, there may be a transistor332,330for each individual lamp112,114that may be gated by the output voltage of respective lamp112,114output voltage. The transistors332,330may be connected in series in such a manner that when all transistors332,330are turned on the fault protection signal FP314is pulled to ground. If one of the lamps112,114stops producing voltage, the associated transistor332,330may be turned off causing the fault protection signal314to be pulled to approximately VDD312. This may operate to trigger the fault protection circuitry326in the inverter controller101′ to reduce and/or shut off power delivered to the lamp loads (via switches102).

FIG. 4illustrates another exemplary embodiment400for the claimed subject matter. This particular embodiment may include inverter controller circuitry101′, DC/AC inverter circuitry102, current balancing circuitry422, and one or more loads112and114. The inverter controller circuitry101′, DC/AC inverter circuitry102, and current balancing circuitry422may have similar capabilities and configurations to their respective counterparts in system300except that the current balancing circuitry422in system400may comprise a differential mode inductor408located on the low voltage side of the lamp loads. Shifting the differential mode inductor408to the low voltage side of the lamp may reduce the safety hazards created by providing the differential mode inductor408with too much voltage, while still maintaining a current balance between the respective lamps.

The fault protection signal generating circuitry424may generate a fault protection signal FP314that may be used by the inverter controller circuitry101′. The fault protection signal generating circuitry424may comprise transistor logic412and414that processes the output voltages of each individual lamp112and114. The output voltages of each lamp may be sensed and OR-ed together, by diodes D1and D2420, to create a signal VB410. The signal VB410may be used to gate switch Q1412, controlling the signal VD416. The signal VD416controls switch Q2414, thereby controlling the fault protection signal FP314. When the lamps112and114are functioning in a normal manner, the signal VB410may be small and unable to trigger the switch Q1412. If the switch Q1412is not turned on, the signal VD416may be pulled to approximately VDD418. If VD416is large, the switch Q2414may be turned on, pulling the fault protection signal314to ground. However, if the lamps112and114are removed or malfunctioning, VB410may be large and may enable the switch Q1412. If the switch Q1412is enabled, the signal VD416may be pulled to ground, the switch Q2414may be turned off. If the switch Q2414is turned off, then the fault protection signal316may be approximately VDD418. If the fault protection signal output314is high, then the fault protection circuitry326may be enabled.