EP2732679A2

EP2732679A2 - Electronic ballast for a lamp

Info

Publication number: EP2732679A2
Application number: EP12743411.6A
Authority: EP
Inventors: Jamie Kelly; Deepak MAKWANA; Kevin Mcdermott; Paul Dalby; Wayne Bell
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2011-07-11
Filing date: 2012-07-10
Publication date: 2014-05-21
Anticipated expiration: 2032-07-10
Also published as: EP2732679B1; GB201111860D0; WO2013007727A3; WO2013007727A2; GB2492776A; GB2492776B

Abstract

Electronic ballast for a lamp, such as a gas discharge lamp, preferably for a fluorescent lamp (LA), having an inverter, connected with a d.c. voltage source (Vbus), and a load circuit, which has the lamp (LA) and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches (Q1, Q2) arranged in a half-bridge arrangement, which switches are so alternatingly controllable by a control circuit (1) during an ignition phase that their switch-on time (Ton) is increased to reduce the control frequency, characterised in that the control circuit (1) in the ignition phase monitors an output parameter of the half-bridge to determine if ignition has occurred by evaluation of an output parameter or a half bridge parameter after a given time. The control circuit (1) may monitor a current or voltage, or a frequency.

Description

ELECTRONIC BALLAST FOR A LAMP

TECHNICAL FIELD

The present invention relates to an electronic ballast for a lamp, such as a gas discharge lamp, preferably for a fluorescent lamp, and to a corresponding method.

BACKGROUND TO THE INVENTION

The use of electronic ballasts for operating gas discharge lamps is preferred over the use of conventional ballasts due to lesser losses and an improved lamp efficiency, leading to significant energy savings. The input of a typical electronic ballast is formed by means of a high frequency filter connected to the voltage supply mains, which high frequency filter is connected with a rectifier circuit. The rectified supply voltage from the rectifier circuit is fed to a smoothing circuit for the generation of an intermediate circuit voltage, and finally an inverter fed with the intermediate circuit voltage generates a high frequency a.c. voltage, which is applied to the load circuit with the gas discharge lamp connected thereto. The operation with high frequency a.c. voltage has as a consequence a reduction of electrode losses and an increase of the light yield. Further, there arises the possibility of igniting the lamp in a controlled and power conserving manner.

For igniting the gas discharge lamp normally its electrodes are initially preheated with an increased frequency of the inverter. At the end of this pre-eating period the frequency generated by the inverter is then reduced, so that it approaches the resonance frequency of the load circuit, which is primarily determined by a series resonance circuit arranged in the load circuit, and as a consequence thereof the voltage applied to the lamp increases. At a certain time point during the reduction of the frequency there is finally effected the ignition of the gas discharge lamp. The lamp then enters a run mode. SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an electronic ballast for a lamp, such as a gas discharge lamp, preferably for a fluorescent lamp, having an inverter, connected with a d.c. voltage source, and a load circuit, which has the lamp and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches arranged in a half-bridge arrangement, which switches are so altematingly controllable by a control means during an ignition phase of the lamp that their switch-on time is increased to reduce a control frequency, characterised in that the control means is operable in the ignition phase to monitor an output parameter to determine if ignition has occurred.

The control means may monitor a current or voltage.

The control means, for monitoring the current, may detect a voltage drop across a resistance arranged at the foot at the half-bridge (between the lower switch and ground). In the first embodiment, the control means compares the voltage drop across the resistance arranged at the foot at the half-bridge with a reference voltage (Vref Detection) and detects that ignition has occurred if the voltage drop across the resistance arranged at the foot at the half-bridge is below the reference value ( Vref Detection). The control means detects a critical condition when the current or voltage exceeds a threshold value ( V_{ref Rej},_ula(lon) and opens a closed one of the switches in response thereto.

The control means adjusts the frequency or the switch-on-time during the ignition phase.

In the second embodiment, the control means detects a critical condition when the current or voltage exceeds a threshold value (V_{rof egu}m, _n) and stops adjusting the frequency or the switch-on-time in response thereto. The control means monitors the frequency or the switch-on-time of the half bridge and determines whether ignition has occurred in dependence upon the frequency after a predetermined time period.

According to a second aspect of the present invention, there is provided a corresponding method, as defined in the claims. The present invention also provides an electronic driver for a lightsource comprising an electronic ballast according to this invention.

The present invention also provides an electronic ballast for a lamp, such as a gas discharge lamp, preferably for a fluorescent lamp, having an inverter. connected with a d.c. voltage source, and a load circuit, which has the lamp and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches arranged in a half-bridge arrangement, which switches are so alternatingly controllable by a control means during an ignition phase of the lamp that their switch-on time is increased to reduce a control frequency, characterised in that the control means is operable in the ignition phase to monitor the control frequency and when the control frequency reaches a predetermined minimum value the control means automatically ends the ignition phase and starts the run phase of operation. A corresponding method is also provided.

In the embodiments, for operation of the half bridge, a control circuit detects, for monitoring the load circuit current, preferably the voltage dropping over a resistance arranged at the foot of the half-bridge of the inverter, and compares this with reference voltages. The monitoring of the current is then effected during the switch-on phase of the lower switch of the half-bridge. Beyond this, after the switching off of one of the two switches and the following switching on of the other switch there is provided a predetermined delay time, in order to exclude a short-circuiting of the inverter. The switches are preferably MOS field effect transistors, the gates of which are controlled by the control circuit by means of pulse-width modulated signals.

The embodiments provide an arrangement for detecting when ignition of a lamp occurs and which prevents damage to the ballast when the lamp is faulty or absent. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention embodiments will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a switching arrangement for a ballast;

Figure 2 shows the detection of lamp ignition by obtaining an indication of the half-bridge current according to a first embodiment of the invention; Figure 3 shows a flow chart of the procedure for lamp ignition by obtaining an indication of the half-bridge current according to a third embodiment of the invention;

Figures 4A and 4B show the detection of lamp ignition by the frequency according to a second embodiment of the invention; and

Figure 5 shows the state chart of lamp ignition by frequency detection according to a second embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The main components of a converter for instance a half bridge are illustrated in the circuit diagram in Figure 1 . The further elements of the ballast arranged upstream of the inverter - for example the rectifier and smoothing circuit - are already well known and thus are not explained in more detail below. The inverter is formed by means of a half-bridge of two electronic switches Ql and Q2 connected in series. These switches Q l and Q2 may be formed for example by means of two MOS field effect transistors. The switches have respective body diodes D l and D2 associated therewith. The foot of the half- bridge is connected to ground via a shunt resistance R l . whilst at the input of the half-bridge the d.c. voltage Vbus is applied, which can for example be generated by shaping the mains voltage by means of a combination of radio frequency suppressor and rectifier. Alternatively to this there can. however, also be applied to the half-bridge any other d.c. voltage source. At the common node point of the t o sw itches Ql and Q2. there is preferably connected the load circuit containing the gas discharge lamp LA. which is preferably a fluorescent lamp. This load circuit is comprised first of a series resonance circuit, which is made up of a choke LI and a resonance capacitance CI. At the connection point between the choke LI and the resonance capacitance CI there is further connected a series circuit of a coupling capacitance C2 and the gas discharge lamp LA such that it lies parallel to the resonance capacitance CI.

The control of the two switches Ql. Q2 of the inverter is effected by means of a control circuit 1. which passes control signals to the gates of the two field effect transistors Q l and Q2.

A switching period begins with a switching on or closing of the upper switch Q l of the half-bridge for a certain sw itching-on time ton. At the end of this switch-on time ton the switch Q l is again opened, and altematingly the switch Q2 closed. Betw een the opening of the switch Ql and the following closing of the switch Q2 a predetermined dead time td is waited out. in order in any event to avoid a simultaneously closing of the two switches Ql. Q2 and therewith a short-circuiting of the inverter. Also the second switch Q2 is closed for the switch-on time ton and thereafter again opened. After a further waiting out of the dead time td the upper switch Ql is again closed, with which a complete switching period is ended.

The overall time Tp of a switching period is thus: Tp = 2.( ton + td)

The frequency of the inverter is correspondingly calculated as: f = 1 /Tp = 1 / [2. (ton + td)]

During a start phase of the ballast, initially the electrodes of the lamp LA are pre-heated. which is effected in that there is applied to the load circuit an a.c. voltage having a frequency which lies significantly above the resonance frequency of the load circuit. The voltage yielded thereby is then too low to be able to bring about an ignition of the lamp LA.

At the end of the pre-heating time, the ignition of the lamp LA is initiated ( ignition phase), which is effected in that the switch on time ton for the two switches Q l . Q2 of the inverter is stepwise increased and coiTespondingly the operating frequency of the inverter is reduced. The frequency then approaches ever closer the resonance frequency of the load circuit, until the voltage yielded thereby is so great that it brings about an ignition of the gas discharge lamp LA. After igniting the fluorescent lamp LA electrically behaves essentially as an ohmic resistance, so that the lamp voltage after ignition falls, which is maintained in the run mode by a normal operating frequency of the switches Q l and Q2 (run mode).

According to a first embodiment of the invention, during the ignition phase, the control circuit 1 detects the voltage V_d drop across the shunt resistance l . The control circuit 1 detects if the voltage exceeds a first lower threshold V, _t Detect i n- The control circuit 1 also detects if the voltage exceeds a second higher threshold V_ie, Regulat ion - When the voltage exceeds the first lower threshold V_iet- Detection may be determined by comparator 2A. When the voltage exceeds the second higher threshold V_{ret Reg Ll}i_{al l0l1} may be determined by comparator 2. Preferably, the comparators 2 and 2A are implemented within the control circuit 1 (rather than separately, as shown), the control circuit being an ASIC.

The ignition phase lasts a maximum time period Time_out.

As shown in Figure 2. if Vj exceeds both the first lower threshold V_rct- De tion and the second higher threshold V_lct Regu lat ion - ^at the end of the ignition phase (time period Time_out). then it is determined that the lamp LA is broken or absent, and the system is shut down immediately to prevent damage (whichever of the switches Q l and Q2 that is closed is opened).

As also shown in Figure 2. if V_d is below the first lower threshold V_ret- _Deteclion at the end of the ignition phase (time period shorter than Time_out) then the circuit enters the run mode and applies a normal operating frequency to the switches Q l and Q2.

As not shown in Figure 2, if V_u exceeds both the first lower threshold V_ret Detect ion and the second higher threshold V_ref Regulat ion- during the ignition phase then the active switch will be opened immediately to prevent damage of the half bridge ( whichever of the switches Q l and Q2 that is closed is opened). In this case ignition will continue with its current frequency and switch-on time (Ton).

The lower threshold V_{ref D}, _{c lion} is used for the ignition detection but without direct impact on the half bridge operation. After the ignition time period Time_out ( when the ignition window ends) the control circuit 1 checks whether the lower threshold V_{iet Delcctlon} is exceeded. If the lower threshold V_ret Detecti n is exceeded, this is a signal that lamp has not struck ( as there is still a high current in the resonant circuit). This can mean that the upper threshold V,-ef Ref lation is not exceeded and thus the halfbridge will not be damaged, but as the lower threshold V_ief Detection is exceeded there is a clear indication that the resonant circuit is not damped, which only occurs when the lamp has not struck (as the arc resistance of the struck lamp dampens the resonant circuit).

A third embodiment is summarised in the flow chart of figure 3.

At step a the pre-heat phase is performed.

At step b the ignition phase is entered. At step c the control circuit 1 sets an ignition phase timer to zero.

At step d it is determined by the control circuit 1 if the higher threshold V_ref Regulati n ^ls exceeded. If the higher threshold V_ret R_{e aU}iati n is exceeded, then at step e the half bridge is shut down.

If the higher threshold V_iet Reflation is not exceeded, then it is determined by the control circuit 1 at step f whether the lower threshold V,_{et Detectjon} is exceeded.

If the lower threshold V_ret Detection ⁿ°t exceeded, then at step g the run mode is entered. In the run mode the control circuit operates the half bridge at a run frequency Frun of 58kHz.

If the lower threshold V_iet- Dete tion is exceeded, then at step h the ignition phase counter is incremented.

At step i the control circuit 1 determines if the ignition phase counter has reached a value corresponding to the ignition phase having lasted the time period Time_out.

If at step i the control circuit 1 determines that the ignition phase counter has reached a value corresponding to the ignition phase having lasted the time period Time_out, then at step e the half bridge is shut down. If at step i the control circuit 1 determines that the ignition phase counter has not reached a value con'esponding to the ignition phase having lasted the time period Time_out. the procedure returns to step d and the thresholds continue to be monitored. According to a second embodiment of the invention there is no direct detection through a monitored output parameter such as the V_{ret De(ectio} mentioned above. Instead of that the frequency itself is used to determine whether the lamp has ignited successfully. In general we can differentiate two cases for the ignition. Firstly, there is no lamp or a broken lamp connected to the ballast which does not allow the ballast to ignite and run the lamp. Secondly, the lamp is present and operating correctly, so that the ballast can ignite and run the lamp. As explained before, in the ignition phase the operating frequency of the inverter is stepwise reduced. The frequency then approaches the resonance frequency of the load circuit. Thus the resistance of the circuit is reducing as well and the current in the output circuit increasing and the voltage across the lamp increasing. The control circuit 1 usually provides means to protect the half bridge from destruction. Such destruction could happen due to high current through the switches Ql and Q2 or to high voltage across some of the parts in the circuit. To realise such protection, the control circuit 1 has means to monitor one or more output parameters to prevent the circuit from overload. The output parameters which can be monitored by the control circuit 1 can be a current or a voltage at the output circuit, for instance the half bridge. For example the control circuit 1 detects the voltage V_d drop across the shunt resistance R l . as in the first embodiment. In this example, the control circuit 1 detects if the voltage V_d exceeds the threshold V,._el- egulati n- When the voltage exceeds the threshold V_ier Regulati n ^maY be determined by comparator 2 in Figure 1. Preferably, the comparator 2 is implemented within the control circuit 1 (rather than separately, as shown), the control circuit being an ASIC. In the second embodiment, the second comparator 2A is not required. In the second embodiment, as the control circuit 1 reduces the frequency during the ignition phase, the monitored parameter (in this example, the current in the half bridge given by the voltage V_d) increases. The control circuit 1 reduces the frequency until one of the following events occurs: 1 . The frequency reaches a predetermined minimum value, F_{endstop ii;n}; or

2. Voltage V_d reaches the threshold V_{ret Re2U}|_ati011.

If the Voltage V_d reaches the threshold V,._et Regulation (as detected by the comparator 2) the control circuit 1 has reached its maximum switch-on time. Ton. (minimum frequency of the half bridge) to prevent the circuit from overload. That means that the control circuit 1 cannot go further to increase switch-on time. Ton, (which would result in a decrease of the operating frequency of the half bridge). So the control circuit 1 stays at this operation point also called regulation level as shown in Fig. 4. In this example, the regulation level leads to a half bridge frequency of 60kHz.

After a given time period (e.g. Time_out) the control circuit 1 evaluates a half bridge parameter, preferably the frequency or the switch-on time. Ton. According to this second embodiment, the circuit 1 can determine whether the lamp has ignited successfully or not in dependence upon the frequency or the switch-on time, Ton.

In the case that there is no lamp or a broken lamp the operating frequency of the half bridge will stay on the regulation level (60kHz in this example) until the ignition time period Time_out ends, and the predefined value of F_endsto __ign cannot be reached. This is shown in Figure 4A. F_cndstop __ign is 50kHz in this example.

In the case that a lamp is connected to the ballast, and is operating correctly, the increasing voltage through decreasing the operating frequency of the half bridge will result in an ignition of the lamp. This is shown in Figure 4B. In Figure 4B the lamp ignites at 70Hz. At ignition the resonant circuit itself changes because the lamp as load is added and the current in the half bridge given by the voltage V_d reduces. The Voltage V_d consequently does not reach the higher threshold V_ref egulati n- Therefore the control circuit 1 is able to continue to reduce the operating frequency further until the frequency reaches the predetermined minimum value F_endsto _i_t:n without an intervention of the protection circuit (the V_{ref Regu}|_{ati n} level will not be reached). When F_endstop__igI1 is reached the control circuit automatically ends the ignition phase and starts the run phase of operation. In the run phase of operation, as shown in Figure 4B. the control circuit operates the half bridge at a run frequency Frun of 58kHz.

This second embodiment is based on a principle with no active ignition detection criteria. Thus in such case there is no need for a dedicated comparator for the ignition detection by monitoring of a voltage level (that is. the second comparator 2A of the first embodiment can be omitted).

The principle of operation of the second embodiment will now be described with reference to Figure 5. which shows a state chart.

After initialisation the preheat state A is entered for a predetermined preheat period. When the predetermined preheat period ends, the ignition state B is entered.

If the lamp is present and operating correctly, at the time where the halfbridge frequency reaches the predetermined minimum frequency F_{en(lst pj2n} the ignition state will be left and the "Lamp Operation' state C will be initiated similar to the first and third embodiment.

On the other hand, if the lamp is defective or absent (no load), the frequency is swept down until the V_{ret RegU}|_ation (of the over current comparator 2) is hit. The operating frequency of the half bridge will stay on the regulation level (operation frequency will stay at 60kHz in this example) until the ignition time period Time_out ends, after which the lamp circuit will go into the Standby state D. Instead of the voltage V_{r t}- _Re„_LI|_ation the control circuit may also monitor another output parameter, e.g. the voltage of the resonant choke L I . Advantageously the predetermined minimum frequency F_{cndstop i}„_n is not close to the regulation level. Additional shutdown may be in place as well. A shutdown may also occur in case where the predetermined minimum frequency F_ends!op__lgn has not been reached after a given time. Another example would be if there is a very high current in the half bridge detected (e.g. twice V_ref- egulation )

With a working lamp the lamp will strike at some point as the frequency is being swept down, jumping to a damped curve. The control means 1 then continues to sweep the frequency down until the predetermined minimum frequency F_ena_stop__li;n is reached. When the frequency F_{cndst Jgn} is reached the control circuit 1 then exits the ignition state B after F_cndstop __ign is confirmed and goes to "Lamp Operation^' state C. This is based on the idea that if the control circuit 1 can reach the frequency F_{eildstop isn} then the lamp must be struck. For this to work correctly the predetermined minimum frequency F_cndstop __ign should be set a few kHz less than the real resonant frequency.

The frequency to which we sweep in ignition should be predetermined minimum frequency F_{endst p}__i!;n. It is advantageously to set this beyond resonance, e.g. 5kHz. so that the only time it is reached is if the lamp is struck.

In a defective or deactivated lamp condition this parameter is not relevant as the regulation level is reached first. The regulation level is greater than the resonant frequenct of the load circuit. The frequency values given in the embodiments are given by way of example only. The values appropriate for a particular application will be determined according to the circumstances. However, the regulation level frequency should be greater than the resonant frequency, and the predetermined minimum frequency F_ena_stop__ign should be less than the resonant frequency of the load circuit.

Claims

Electronic ballast for a lamp (LA), such as a gas discharge lamp, preferably for a fluorescent lamp, having an inverter, connected with a d.c. voltage source ( Vbus). and a load circuit, which has the lamp (LA) and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches (Ql. Q2) arranged in a half-bridge arrangement, which switches are so altematingly controllable by a control means ( 1 ) during an ignition phase of the lamp (LA) that their switch-on time (Ton) is increased to reduce a control frequency, characterised in that the control means ( 1 ) is operable in the ignition phase to monitor the control frequency and when the control frequency reaches a predetermined minimum value. (Fendstop_ign) the control means (T ) automatically ends the ignition phase and starts the run phase of operation.

The ballast of claim 1. wherein the control means ( 1 ) monitors an output parameter, preferably a current or voltage, during the ignition phase and wherein the control means is operable to stop further increase to switch-on time (Ton) in case the output parameter reaches a threshold (Vref Regulation).

The ballast of claim 2. wherein the control means ( 1 ) is operable to enter a Standby state (D) in case where the operating frequency of the half bridge is still above the predetermined minimum frequency (Fendstop_ign) at the end of the ignition time period (Time_out). The ballast of claim 2 or 3, wherein the control means ( 1 ). for monitoring the current, detects a voltage (VR 1 ) drop ( Vd) across a resistance <R1) arranged at the foot at the half-bridge.

The ballast of claim 4, wherein the control means ( 1 ) is operable to compare the voltage (VR 1 ) drop ( Vd) across the resistance (R l ) arranged at the foot at the half-bridge with a reference voltage (V,._ef D te tion) and to stop the increase of the switch-on time (Ton) if the voltage (VR 1 ) drop ( Vd) across the resistance (R l ) arranged at the foot at the half-bridge exceeds the reference value ( V_ref Regulation) -

The ballast of claim 2, 3, 4 or 5, wherein the control means ( 1 ) is operable to detect a critical condition when the current or voltage exceeds a threshold value (V,._{ct Regu l(um}) and to open a closed one of the switches ( Ql. Q2) in response thereto.

The ballast of anyone of claims 1 to 6. wherein the control means ( 1) is operable to monitor the frequency or the switch-on-time (Ton) of the half bridge.

The ballast of anyone of claims 1 to 7, wherein the control means (T) is operable to adjust the frequency or the switch-on-time (Ton) during the ignition phase.

The ballast of claim 8. wherein the control means ( 1 ) is operable to detect a critical condition when the current or voltage exceeds a threshold value (V_{ret Re}„_ui_{at ion}) and to stop adjusting the frequency or the switch-on-time (Ton) in response thereto.

The ballast of claim 7, 8 or 9, wherein the control means ( 1 ) is operable to monitor the frequency or the switch-on-time (Ton) of the half bridge and to determine whether ignition has occurred in dependence upon the frequency after a predetermined time period.

1 1. Electronic ballast for a lamp (LA), such as a gas discharge lamp. preferably for a fluorescent lamp, having an inverter, connected with a d.c. voltage source (Vbus), and a load circuit, which has the lamp (LA) and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches (Ql. Q2) arranged in a half-bridge arrangement, which switches are so altematingly controllable by a control means ( 1 ) during an ignition phase of the lamp (LA) that their switch-on time (Ton) is increased to reduce a control frequency, characterised in that the control means ( 1 ) is operable in the ignition phase to monitor an output parameter to determine if ignition has occurred.

The ballast of claim 1 1. wherein the control means ( 1 ) monitors current or voltage.

13. A method of operating an electronic ballast for a lamp (LA), such as a gas discharge lamp, preferably for a fluorescent lamp, the ballast having an inverter, connected with a d.c. voltage source ( Vbus). and a load circuit, which has the lamp (LA) and a series resonance circuit, connected to the inverter, wherein the inverter is formed by means of two switches (Ql, Q2) arranged in a half-bridge arrangement, the method including controlling the switches (Ql. Q2) by a control means

( 1 ) during an ignition phase of the lamp (LA) such that their switch-on time (Ton) is increased to reduce a control frequency, characterised in that the control means ( 1 ) in the ignition phase monitors an output parameter to determine if ignition has occurred.

14. The method of claim 13. wherein the control means ( 1 ), for monitoring the current, detects a voltage (VR l ) drop (Vd) across a resistance ( R l) arranged at the foot at the half- bridge. 15. The method of claim 14. wherein the control means ( 1 ) compares the voltage (VR l ) drop ( Vd) across the resistance (Rl ) arranged at the foot at the half-bridge with a reference voltage ( V_ref Detection ) ^and detects that ignition has occurred if the voltage (VR l ) drop (Vd) across the resistance ( R l ) arranged at the foot at the half-bridge is below the reference value ( V_{iet Deteclion}).

The method of claim 14 or 15. wherein the control means ( 1) detects a critical condition when the current or voltage exceeds a threshold value (V_{vef Rc!;il}|_ati_0n) and opens a closed one of the switches (Ql, Q2) in response thereto.

17. The method of claim 13 or 14, wherein the control means ( 1 ) monitors the frequency or the switch-on-time (Ton) of the half bridge. 18. The method of claim 17, wherein the control means ( 1 ) adjusts the frequency or the switch-on-time (Ton) during the ignition phase.

The method of claim 18, wherein the control means ( 1 ) detects a fault when the current or voltage exceeds a threshold value ( V_iet Regulati n) and stops adjusting the frequency or the switch-on-time (Ton) in response thereto.

The method of claim 19. wherein the control means ( 1 ) monitors the frequency or the switch-on-time (Ton) of the half bridge and determines whether ignition has occurred in dependence upon the frequency after a predetermined time period.