WO1995010168A1

WO1995010168A1 - Electronic ballast for gas discharge lamps

Info

Publication number: WO1995010168A1
Application number: PCT/NL1994/000237
Authority: WO
Inventors: Paul Adriaan Coster; Marcus Wilhelmus Van Look; Eward Don Stoffer
Original assignee: C.M. Personnel Participation B.V.
Priority date: 1993-10-01
Filing date: 1994-09-30
Publication date: 1995-04-13
Also published as: AU1077695A; NL9301694A

Abstract

This invention provides in a number of measures in the form of sections for the electronic ballast by which the efficiency of the combination of the discharge lamp and the electronic ballast is increased. This is done by two measures. Measure 1 concerns a warm start circuit for the discharge lamp, by which a constant pre-heating current is available for a period of time which depends on the temperature characteristics of the discharge lamp's electrodes and by which a starting voltage is immediately offered to the discharge lamp after the pre-heating phase. Measure 2 concerns an electrode current limiter which reduces the current through the electrodes of the discharge lamp after the starting phase to a minimal value. The invention provides in an integrated use of both measures 1 and 2 in a modular way.

Description

Electronic Ballast for Gas Discharge Lamps:

According to the head in conclusion 1, this invention relates to an electronic ballast for gas discharge lamps. A ballast for a discharge lamp is common knowledge. It limits the current through the discharge lamp (f.i. fluorescent lamp) by using a choke which is connected in series with the lamp. The ignition of a lamp is usually done with the help of a starter connected in series with the filaments of the fluorescent lamp.

An electronic ballast is a generally known circuit with the same function as a ballast like the one mentioned before. This function will be achieved by placing two in serial connected switches across a DC-power supply. The switches are controlled by a predefined frequency and so that both switches are never closed at the same time.

Disadvantages of these known electronic ballasts are:

1. High inrush current as a result of charging a large capacity, which will cause in general a high peak-load at the main supply. As a result only a unnecessary limited number of electronic ballasts can be placed at one and the same distribution group.

2. No adequate, non-destructive detection of, and protection against exposure to high mains voltage. 3. No controllable and flexible pre-heat time, which is tmed to the temperature characteristics of the filaments of the discharge lamp, therefore not guaranteeing an, under all circumstances, optimal warm start of the discharge lamp according to the IEC 929 standards. 4. A capacity placed in series with the filaments of the fluorescent lamp causes a current rise through them after the lamp is ignited. The extra current is not necessary for the burning of the lamp, but it leads to a shorter lamp life and an extra energy loss in the lamp, as in the ballast.

In the applicant's Dutch patent application number NL9301397, a protection circuit for electronic devices in general and an electronic ballast in particular is given, which offers a solution for the previous mentioned disadvantages number one and two, hereafter refered to, which contains an integrated inrush current limitor and an overvoltage protection.

The invention aims to give a solution for disadvantage number 3 by the characteristic measurements described in conclusion 1. Other beneficial design forms are characterised by the underconclusions 2-10, which are refering to conclusion 1.

Also the invention aims to give an integrated design, preferably with a warm start according to conclusion 1, by which the mentioned disadvantage 4, closely connected with disadvan¬ tage 3, will not occur. The invention is characterised by the characteristic measure of conclusion 11, which application forms are characterised by the to conclusion 11 refering subconclusions 12 to 16.

The advantages of the measures taken in the conclusions 1 to 10 will be:

1. Under any circumstances a warm start of a fluorescent lamp will be performed in accordance to the IEC 929 standards, even under extra-ordinary conditions, such as a short interruption of the main supply. 2. During short main supply interruptions caused by a switch¬ over to emergency supply, a short warm start procedure will take place. 3. Because a warm start will take place under any cir¬ cumstances, the discharge lamp's filaments will wear out less, resulting in the increase of the lamp life. 4. During the fluorescent lamp's pre-heating phase, the open lamp voltage will be actively kept under the level of the starting voltage, herewith preventing a premature ignition of the lamp. 5. After the pre-heating phase of the discharge lamp, the starting voltage will become abrupt available instead of gradual, and therefore the pre-heat time is presented in a more definable manner.

The advantages of the measures taken in the conclusions 11 to 16 will be:

1. A controllable minimization of the current through the lamp's electrodes after the ignition of the discharge lamp.

2. As a result of the limitation of the electrodes' current, energy loss will be reduced.

3. Because of the electrodes' current limitation, the electrodes will wear out less and this results again in an increase of the lamp life.

4. Because a reduction of current provided to the fluorescent lamp by the electronic ballast, the ballast will produce less heat, and therefore the ballast will have a longer lifetime.

The invention will be further explained by means of a block diagram of a total electronic ballast (figure 1) and by detail schematics of the various sections (figure 2 and 3) .

Figure 1 shows a block diagram of an electronic ballast according to the invention.

Figure 2 shows the schematics of the warm start circuit according to the invention. Figure 4 shows the schematics of the electrodes current limitor according to the invention. General description of electronic ballast's sections according the invention by means of figure l.

Section A, an EMI-filter, serves as an noise suppressor to the mains as well as a protection circuit against high voltage spikes at the mains to the electronic ballast.

Section B, the diode bridge, has the purpose to rectify the mains voltage.

Section C, see patent application number NL9301397 of the applicant, is a protection circuit, which eliminates the disad- vantages 1 and 2.

Section D corrects the current consumption of the electro¬ nic ballast so that the current waveform is a sine-shape, which is in compliance with the IEC 555-2 requirements regarding the prevention of higher harmonic current components. Section E serves as an energy buffer for section F as smoothing component for the powersupply.

Section F consists of a variable free oscillating half H- bridge with the purpose to generate the high-frequency power supply for the discharge lamp. Section H generates the control signal for section D by which it controls the voltage of section E.

Section I consists of a charge pump driven by a high amplitude and high-frequency square voltage's waveform, which takes care of the 30V low voltage supply for section F. Section J, according to the invention, controls section F and takes care of pre-heating, ignition and burning of the fluorescent lamp.

Section K triggers section F to start the oscillation and is supplied from Section E. Section L detects defective lamps, and brings section F out of oscillation when a defective lamp is detected. It also suppresses the trigger pulses from section K, preventing section F from starting the oscillation.

Section M, according to the invention, reduces the fluorescent lamp electrodes' current in an active and definable way after the starting phase. Description of Division Function J according to invention:

A more detailed description of section J, the warm start schematic, can be found in Figure 2. This section can be described as a starting circuit providing a defined warm start at all times for a fluorescent lamp.

The starting circuit refers to the provision of warm start conditions for the discharge lamp under normal as well abnormal circumstances (This especially during short mains supply interruptions when the mains supply is taken over by emergency power supply systems) . A(n) (electronic) ballast may not have an adverse influence on a discharge lamp's behavior or function, certainly not during the starting phase. Traditionally there are two methods for starting a discharge lamp: the so-called warm start, by which the electrodes are pre-heated before the actual start has taken place, and a start by which the electrodes are not pre-heated before the actual starting voltage is presented, the so-called cold start. The lamp life depends to a large extent on the duration of the lamps' electrodes. The duration of the electrodes, however, depends largely on the starting con- ditions. The emitter coating of the electrodes will be damaged during cold start procedures, leading to larger signs of wear.

A warm start is generally recommended, because here the lamp life is positively influenced compared to the cold started lamp. The requirements for the warm start are precisely described in the IEC-standards.

There are several methods possible to create correct warm start conditions for discharge lamps. These methods are either based on starting circuits by which warm start conditions are defined in such a way that the pre-heat time is not flexible, with the disadvantage that during abnormal conditions starting conditions are not optimal anymore, or starting circuits, in particular those for which thermistors (especially Positive Temperature Coefficient (P.T.C.)) are used, wherein the warm start conditions are to a large extent not defined and control- lable presented to the discharge lamp. This results in the disadvantage that no optimal starting conditions are present. But both kinds of methods, concerning the starting circuit for discharge lamps, are not meeting the requirements for a warm start in circumstances of a quick re-start. One method pre-heats unnecessary too long and therefore does not meet the require- ments for use with an emergency power supply, while the other method will perform a cold start.

Section J intends to avoid the disadvantages mentioned before by taking the lamp's characteristics as a fundamental base for the starting circuit's functional behavior. A pre-heat time will depend on the temperature of the discharge lamp's electrodes. This provides a warm start under normal and abnormal starting conditions.

The warm start circuit is based on a variable oscillation frequency of the LC-circuit. Due to an arrangement of diverse components, the electric potential of the capacitor placed between both lamp electrodes will determine the starting voltage, and the current through this capacitor determines the pre-heat current.

A variable oscillation frequency will be created by making the half H-bridge controllable by placing in series with the half H-brigde's lower switch (TI) a variable resistor consisting of Rl and T2 parallel as Tl's emitter resistor.

The amount of saturation of the transistor TI can now be controlled. This alters the on-time. By enlarging Tl's emitter resistance, the on-time will decrease, which means an increase of the oscillation frequency.

We can distinguish the pre-heating phase; the starting phase; and the burning phase of the discharge lamp. During the pre-heat phase, Tl's emitter resistor has a "high' resistance, the oscillation frequency will become much higher than the resonance frequency of the LC-circuit. In this state, the open lamp voltage will be kept under the starting voltage, but the current will be high enough to pre-heat the electrodes. After the pre-heating phase, the discharge lamp must be started with a starting voltage which is high enough to ignite the lamp. This will happen by altering the frequency of the LC-circuit, so it will approach the resonance frequency of the oscillationcircuit, resulting in a high voltage across the discharge lamp. The emitter resistance of TI will be reduced.

The control of the emitter resistance of TI is done as followed: The control voltage of the variable emitter resistor (Rl and T2 parallel) of TI is 0 Volt when the ballast will be switched on. T3 will start conducting after a certain period of time, depending on the charge time of C4 by R5, untill Dl reaches its zener voltage, through which T4 is brought into conduction. T4 will make sure that T3 stays in conductance. Cl and C2 will suppress noise at the bases of T3 and T4, with the result that T2 never will be false triggered.

The oscillation frequency of the half H-bridge, after the discharge lamp starts burning, is determined by the voltage divider constisting R2, R3, R5 in series supplied by the voltage of section I. The voltage's level of R2 is the control signal for the controllable resistor, consisting of Rl and T2.

The duration of the pre-heating current is determined by R5, R4, D2 and C4. At the moment the Half H-bridge starts to oscillate, C4 will be charged via R5 to the trigger level to which T2 is triggered by Dl. The time necessary to charge C4 to the trigger level is equal to the pre-heating time of the discharge lamp's electrodes. If the half H-bridge stops oscillating, section I's voltage will become zero volt, herewith enabling C4 to discharge through the parallel circuit of R4 and R5. This happens via a definable time curve, which is tuned to the temperature characteristics of the discharge lamp's electrodes. If a re-start takes place, a pre-heat current will be started, which duration is determined by the cooling down of the discharge lamp's electrodes. In other words: the residual charge of C4 is the measure for the residual warmth of the discharge lamp's electrodes at the moment of switching on. The duration and the magnitude of the pre-heating current are therefore as such defined that under any circumstances the electrodes will be brought at the required emission temperature. Description of Section M according to the Invention:

Figure 3 represents section M, the electrode current limitor. This section reduces the discharge lamp's electrodes' current after completing the starting phase. Characteristic of this section is the reduction of the electrodes' current in a definable and active way and the fact that this section can be used additionally.

The principle of section M is that the impedance, which is placed parallel to the discharge lamp and in series with the electrodes, will be increased after the starting phase by means of the decrement of the total capacity's impedance placed parallel with the lamp (lamp capacitor) .

In known electronic ballasts, a similar increase of the impedance parallel to the discharge lamps happens by placing a parallel circuit consisting of a P.T.C. and a capacitor in series with the lamp capacitor. The primary goal of the circuit with a P.T.C. is, however, to create a warm start. The disadvantages of this solution are: The P.T.C. continues to consume energy which is in discrepancy with the objective to come to a maximal energy reduction; the capacitor's value parallel to the P.T.C. can not be minimised in respect to the stated safety requirements of an electronic ballast; the P.T.C. provides a non-definable pre-heat time for a warm start. The increasement of the impedance parallel to the discharge lamp and in series at the electrodes is realized by switch TI, a MOSFET, and diode Dl. When TI is in conduction together with Dl it will form a conduction for an alternating current. If TI is brought out of conduction, the current can only go in one direction, namely through Dl. Lamp capacitor Cl, in serial connected with TI and Dl, will charge itself then untill its voltage equals the peak-peak voltage of the lamp. At that moment, the impedance will be infinite high so that no current can flow through this branch. To ensure a stable oscillation of the total circuit C2 is included, with the characteristic that C2's capacity is much smaller than the capacity of the lamp capacitor. This has as a result that the current through this branch and therefore the current through the electrodes will be reduced.

Other characteristics of switch Tl are that this switch will be hold actively out of conduction; that Tl will conduct in a defined and active way before the beginning of the fluorescent lamp's pre-heating phase; that Tl will stop to conduct in a defined and active way shortly after the starting phase; that Tl starts conducting in an active and defined way directly after the discharge lamp is extinguished.

The control voltage of switch Tl is realized by the compo¬ nents Rl, R2, and D2 parallel with C5, which forms a voltage divider. The voltage divider is supplied by the DC supply voltage of the oscillation circuit and will directly, after the DC supply voltage is offered, charge C5 to the zener diode's, D2, voltage level, which will bring Tl into conduction before the pre-heat current starts running.

Switch T2, a transistor, will be brought into conduction after the discharge lamp is ignited, whereby C5 will be decharged through T2. Tl will stop conducting whereby the impedance parallel to the discharge lamp will be increased.

T2 stops conducting immediately after the oscillation circuit stops to oscillate, thus at the moment the discharge lamp extinguishes. C5 will recharge to the level of D2's zener voltage, so Tl will conduct and the conditions of a correct warm start are present again.

Claims

10 Conclusions:

1. Warm start circuit for an electronic ballast with the characteristic that a constant pre-heating current will be delivered during a pre-defined period of time (pre-heating phase) , of which starting time synchronize the pre-heat current start, and that the starting voltage which will be offered as a step function directly after the pre-heating phase.

2. Warm start circuit for an electronic ballast according to conclusion 1, with the characteristic that the pre-heating time of the discharge lamp's electrodes is determined by a network consisting of a resistor and a capacity with a Re¬ curve which depends on the electrodes' temperature characteristics of the discharge lamp used, and which is supplied by a DC-voltage which depends directly on the ballast's oscillation circuit.

3. Warm start circuit for an electronic ballast according to conclusions 1 and 2 with the characteristic that the charging time of the capacity differs from the decharging time of this capacity.

4. Warm start circuit for an electronic ballast according to conclusions 1, 2 and 3 with the characteristic that the voltage level at the capacity is a measure for the residual warmth of the discharge lamp's electrodes.

5. Warm start circuit for an electronic ballast according to conclusion 1 with the further characteristic that the curve, which the capacitor's charge time follows, is adjusted to the size of the predefined pre-heat current.

6. Warm start circuit for an electronic ballast according to conclusion 1 with the characteristic that a controllable emitter resistor is placed by one of the oscillation circuit's transistors, which alters the oscillation frequency.

7. Warm start circuit for an electronic ballast according to conclusions 1 and 6 with the characteristic that the 11 controllable emitter resistor is formed by a parallel circuit of a resistor and a MOSFET.

8. Warm start circuit for an electronic ballast according to conclusions 1, 6 and 7 with the characteristic that during the pre-heating phase the oscillation frequency is higher than during the starting phase.

9. Warm start circuit for an electronic ballast according to conclusions 1, 6 and 7 with the characteristic that the impedance of the oscillation circle is significant higher during the pre-heating phase than during the starting phase.

10. Warm start circuit for an electronic ballast according to conclusions 1, 6 and further with the characteristic that the MOSFET will be brought into total conduction with a step-function after the pre-heating.

11. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristic that the electrodes current will be reduced to a minimal value after the starting phase.

12. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristic that the reduction is accomplished by means of reducing the capacity which stands parallel with the discharge lamp and switched in series with its electrodes.

13. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristic that the reducing of the capacity is accomplished by a parallel circuit of a MOSFET, a diode and a capacity, which is in its totality in a serial connection with the lamp capacitor. This total circuit is placed parallel with the discharge lamp and in series with its electrodes.

14. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristics that the capacity parallel to the MOSFET is smaller than the capacity which is herewith in series.

15. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristic that the MOSFET is in conduction during the pre-heating phase and the starting phase of the discharge lamp.

16. An electrodes current limiter, preferably suitable for an electronic ballast according to one of the preceding conclusions, with the characteristic that the MOSFET is not conducting during the burning phase of the discharge lamp.

17. An electronic ballast with the characteristic that the device is built up as a combination of a warm start circuit according to conclusions 1 through 10 and an electrodes current limiter according to conclusions 11 through 16.

18. An electronic ballast to one of the preceding conclusions with the characteristic that this device can be build up modular at the basis of diverse sections.