GB2253077A - Power control system for gas discharge tubes - Google Patents

Abstract

The control system uses pulsed width modulation techniques to drive a constant power generator, the output of which is rectified giving a constant power D.C. which is then used to drive an inverter, supplying a fluorescent tube. This combination of circuitry enables the striking and running requirements of gas discharge tubes to be met. This approach effectively "pumps" prescribed amounts of power to the tube, and is fully controllable so that not only can the tube be set to its correct level of operation but it may also be dimmed smoothly. By using pulse width modulation techniques the control unit can be made very efficient. Furthermore these techniques allow the control unit to operate from a wide variety of power sources. The control system described can replace existing control chokes and starters on conventional lighting and also has application to emergency lighting. <IMAGE>

Description

CONTROL SYSTEM FOR FlCtFESCENT AND GAS DISCHARGE TUBES This invention relates to an electronic control system which enables flourescent and gas discharge tubes to be controlled.

The e conventional control for driving flourescent tubes from Alternating Current supplies generally comprises a ballast choke and a starter system.

To drive flourescent tubes from a Direct Current source there exist a range of electronic units which generally provide a limited and fixed amount of drive power.

There are limitations with these systems. Generally the lamp output varies with supply Voltage.

The e general market place does not have available a control unit which can precisely control and maintain the output of a flourescent lamp automatically at it's correct output level regardless of supply Voltage variations.

The initial design work behind this invention was to provide emergency /standby lighting from a low Voltage DC source, giving a much higher illumination level than is currently available. With a sufficient illumination level to enable normal working to continue in offices or shops and also to provide an emergency lighting system with several times the illumination of units currently commercially available.

The resultant design which ensued not only met these requirements, but also provided additional features which have wide applications to the whole field of lighting, in addition to it's application for emergency lighting.

According to the present invention there is a control unit. The control unit comprises two main sections A constant power generator and an output generator.

The purpose of the constant power generator is to ensure that the correct amount of power is delivered in the form of DC to the output generator.

mis DC power is constant; i.e. (The Load Voltage) x (The Load Current) remains constant regardless of the load.

See Figure 1 1.......

The purpose of the output generator is to convert the constant power DC to a constant power AC. When connected to a flourescent or other gas discharge tube this conibination allows a high Voltage at low current to be available for starting the lamp which automatically drops in Voltage with a resultant increase in current as the lamp strikes and settles to the correct running level. mis control characteristic directly accomnodates the negative resistance normally associated with gas discharge tubes.

A more detailed technical description follows......

CONSTANT POWER GENERATOR The circuit configuration used to generate constant power is similar to the conventional and well known flyback technique in which current is allowed to flow in an Inductor and at a predetermined current level, the current flow is interrupted and the back E.M.F. so generated is added to the supply Voltage.

In this invention only the back E.M.F. generated by the Inductor is used.

The equation Q = + L 12 denotes the stored energy in the Inductor.

This equation indicates that the energy stored in the Inductor is dependent only on the Inductance value and the current flowing at the time of interruption. Energy thus stored and discharged on a repetitive basis to a load will provide constant power which is independent of the Voltage source and load.

See Figures 2 and 3 The frequency of operation is chosen so that the Inductor can reach it's predetermined current level within the first part of a cycle and is permitted to discharge to the load during the second part of the same cycle.

This technique is commonly used in switch mode supply design.

Provided that the supply Voltage is large enough to allow the current rise in the Inductor to reach the predetermined value within the first part of the cycle, then a constant energy will be stored in the Inductor.

Also, provided that the output from the Inductor can be transferred to the load in the remaining part of the duty cycle, constant power output will ensue.

To ensure a constant flow of power to the output a bi-phase "pump" circuit with steering diodes has been used to allow the power stored in one Inductor to be transmitted to the output circuit whilst the other Inductor is storing energy. The inclusion of diodes gives a DC output.

See Figure 4 4......

A single-phase "pump" could also be used operating at a higher frequency so that energy is fed to the output generator at not less than one half cycle period of the output generator.

Equally a multi-phase "pump" can be used to provide a continuous flow of power to the output.

The output storage Capacitor value is chosen so that sufficient energy can be stored to supply one half cycle of output generation.

A shutdown facility is provided which puts a Voltage override control on the constant power "pump" designed to inhibit the maximum Voltage that can be supplied to the output circuit and the connected tube. It also inhibits the operation of the circuit when a tube will no longer strike or is removed. The circuit going into an idling condition.

To ground reference the load a secondary winding is added to the Inductors as shown .. See Figure 5 OUTPUT DRIVER The circuit configuration used to convert the DC power to a constant AC power consists of a push pull driven generator. This ensures that AC Voltage is correctly applied to the tube with a bi-directional current flow. The frequency of operation of the output generator is locked to the frequency of the constant power generator. This ensures that equal amounts of energy are taken from the constant power generator by the output generator during each half cycle.

Locking the frequencies in this manner ensures a symmetrical output wave form to the tube.

For convenience, the output generator is coupled to the tube by a transformer which permits utilisation of low Voltage components in the driver circuit and provides a facility for obtaining suitable heater Voltages for a flourescent tube and additionally isolation of same.

An alternative output circuit using an H - Bridge configuration can also be used.

See Figure 6 6.....

GENERAL The power delivered by the control unit is constant, and as a consequence, the power taken from the incoming supply is also costant. This means that a variation in input supply Voltage is automatically compensated for by a corresponding change in supply current.

E.g. 50 Watts from the supply could be 2.5 Amps at 20 Volts or 4 Amps at 12.5 Volts.

The range of input Voltage that can be accommodated being a function of the permitted duty cycle amd the asmponents used.

The control unit's characteristics are additionally utilised when starting a flourescent tube by providing a heater Voltage which reduces to a low level when the tube has struck. By offsetting part of this Voltage by a diode, resistive or other Voltage dropping device in the heater circuit the remaining heater Voltage is reduced to virtually zero.

See Figure 7 ... General schematic circuit diagram An outstanding feature of the control unit is it's ability to not only preset the power output for a given tube but also to provide a smooth adjustment of the tube output over a wide range i.e. a dinning facility.

Claims (10)

1 A control system for flourescent and gas discharge tubes which will drive a wide range of flourescent tubes at constant power. The power level of which may also be adjustable, Power being taken from an incoming supply converted and pumped to the tube in precisely measured quantity.

2 A control system as claimed in Claim 1 which provides for efficient power conversion. Typical efficiency of between 80-85%.

3 A control system as claimed in Claim 1 or Claim 2 which can be operated from a wide range of power sources both AC and DC.

4 A control system as claimed in Claim 3 which has no power factor when used on an AC supply and requires no power factor correction.

5 A control system as claimed in Claims 1-3 which automatically corrects for changes in input supply whilst maintaining constant output.

6 A control system as claimed in Claim 1 which can provide smooth "Dimming" of a flourescent or other tube.

7 A control system as claimed in Claim 1 which provides for virtually instant start up of a flourescent or other tube.

8 A control system as claimed in Claims 1 and 7 which provides a rapid "warm up time" attaining full brightness within seconds.

9 A control system as claimed in Claims 1 and 2 which consumes less energy than conventional systems.

10 A control system substantially as described herein with reference tb Figures 1- 7 of the accompanying drawings.