EP2484183B1 - Electronic ballast and method for operating at least one discharge lamp - Google Patents

Abstract

In various embodiments, ballast for a discharge lamp includes input and output connections; inverter with bridge circuit with electronic switches and control device for controlling electronic switches, wherein switches are connected in series between input connections, wherein one electronic switch is coupled to first input connection and second electronic switch to second input connection, wherein a bridge midpoint is between electronic switches; including a current measuring device for measuring second electronic switch current; lamp choke series-connected between bridge midpoint and first output connection; capacitor parallel-connected with one of electronic switches; and coupling capacitor; wherein control device is coupled to current measuring device and renders an electronic switch conducting, if negative threshold value is exceeded when electronic switch is rendered nonconducting; or if negative threshold value of current through electronic switch is not exceeded after another electronic switch is rendered nonconducting, wherein control device increases first frequency in second case.

Description

Technical area

The present invention relates to an electronic ballast for operating at least one discharge lamp having an input with a first and a second input terminal for coupling with a DC supply voltage, an output having a first and a second output terminal for coupling to the at least one discharge lamp, an inverter having a bridge circuit with at least a first and a second electronic switch and a control device for controlling at least the first and the second electronic switch such that the first and the second electronic switch are alternately turned on at a first frequency, the first and the second switch connected in series between the first and second input terminals are coupled, wherein the first electronic switch is coupled to the first input terminal and the second electronic switch is coupled to the second input terminal, wherein between the first and the second electronic switch, a first bridge center is formed, a current measuring device for measuring the current at least by the second electronic switch, a lamp inductor, which is serially coupled between the first bridge center and the first output terminal, at least one trapezoidal capacitor, which is parallel to a the two electronic switches is coupled and at least one coupling capacitor for coupling the load, wherein the control device is coupled to the current measuring device and is designed to switch the second electronic switch conductive, either if a predetermined negative threshold value of the current is exceeded by the second electronic switch after the Sperrend switching of the first electronic switch or after a predetermined period of time, if the predetermined negative threshold value of the current is not exceeded by the second electronic switch after the Sperrend switching of the first electronic switch. It also relates to a corresponding method for operating a discharge lamp.

State of the art

For some time, the term multi-lamp electronic ballasts electronic ballasts on the market, which are designed to operate different lamps, especially lamps of different power. A problem in this context is to ensure a switch-relieved operation of the bridge circuit of the inverter at different loads.

In the following, it is assumed that the inverter is equipped with a half-bridge. As will be readily apparent to those skilled in the art, the following remarks are applicable to inverters having full bridge switches.

In an Infineon discharge lamp controller known from the prior art, switching during the conducting phase of the free-wheeling diode via the second electronic switch is ensured as follows: Using a half-bridge shunt resistor, the current in the lower bridge branch is measured. Falling below a negative threshold of this current is equated with the time at which the freewheeling diode of the lower switching element becomes conductive. This event triggers the switching on of the lower half-bridge switch and thus determines the dead time of the drive signals for the switches of the half-bridge.

This control is problematic in operation of the bridge circuit with a frequency immediately above the phase jump, that is above the transition from inductive operation to capacitive operation at high loads. In this operating mode, the available current for the recharging of the trapezoidal capacitor can be very low. As a result, there is the danger that the negative threshold of the current through the half-bridge shunt resistor will not be reached. The deadtime control known from the prior art in this case sets the maximum dead time, i. a maximum predefinable period of time. Thereby, the switching operation of the lower half-bridge switch is carried out after the current flow through the freewheeling diode has already ended. Since at this time, the voltage across the lower half-bridge switch is not equal to zero, the lower switch of the half-bridge is no longer switching-relieved. This leads to undesirable switching losses and overstressing of the transistors involved. The latter results inter alia in a shortening of the life of such electronic ballasts.

In order nevertheless to ensure a reliable switching discharge of the half-bridge, the usually existing Resonant circuit can be designed with large resonance capacities. However, this measure leads to increased reactive currents and thus to undesirably large losses in the inverter.

WO 2009/037613 A1 discloses an electronic ballast wherein the operating frequency is controlled.

Presentation of the invention

The present invention is therefore based on the object of developing a generic electronic ballast or a generic method such that even with an operation of the electronic ballast in the vicinity of the phase jump at different connected loads a switch-unloaded operation can be provided with minimum losses.

This object is achieved by an electronic ballast with the features of claim 1 and by a method having the features of claim 11.

The present invention is based on the finding that the above problem can be met if the frequency at which the switches of the half-bridge are operated is increased when a switching operation is detected after reaching the maximum dead time. By increasing this frequency, the operating frequency is shifted from inductive operation to a transient frequency between capacitive and inductive operation. This results in an increase of the negative current amplitude when taking over the current through the freewheeling diode of the lower switch. If the operating frequency of the two switches is increased so far that the specifiable negative threshold value of the current through the lower switch is exceeded again, so the known dead time control works again; a switch-unloaded operation of the switches of the inverter can be ensured.

This solution works without an increase in the capacitance of the resonant capacitor and therefore involves virtually no additional losses.

Each of the two electronic switches comprises a control electrode, a working electrode and a reference electrode. It can now be provided that the path working electrode reference electrode is connected in parallel with a discrete diode as a freewheeling diode or that the freewheeling diode is a body diode of the electronic switch. The latter is the case, for example, when mosfet transistors are used as switches.

Preferably, the control device of an electronic ballast according to the invention comprises a memory in which the predefinable period of time is stored. This opens up the possibility, in particular, of modifying these for specific lamps.

Furthermore, it is preferred if the control device comprises a time-measuring device which is designed to determine the time duration after the blocking-end switching of the first electronic switch until the second electronic switch is turned on.

Preferably, the control device is designed to carry out the following step: c1) If the measured time duration is equal to the predefinable time duration: Increase the first frequency by a predefinable step. In this context, the control device is preferably designed to carry out the following step: c2) Repeat step c1) at any rate until the measured time duration is less than the predefinable time duration. This results in the sum that the operating frequency of the switches of the half-bridge is increased in predetermined stages until the dead time no longer corresponds to the maximum dead time. Since too much increase in the operating frequency of the switches of the half-bridge would reduce the power transferable to the lamp, this approach represents an optimal compromise between a switch-unloaded operation of the switches of the half-bridge and a maximum transmitted to the connected lamp power.

Further preferably, the control device is designed to carry out the following step: d1) If the measured time duration is less than the predefinable time duration: decrease the first frequency by a predefinable step. In this context, the control device is preferably designed to carry out the following step: d2) Repeat step d1) until a predefinable value for the first frequency has been reached. These measures take into account in particular the situation when first a discharge lamp with higher power or higher operating voltage is connected to the output of the electronic ballast, the operating voltage decreases again during operation due to temperature effects. Would maintain the operating frequency for the switches of the half-bridge, which are set during operation of the lamp with higher power has, would transmit less power to the lamp with lower burning voltage than would actually be possible. By gradually reducing the operating frequency of the switches of the half-bridge can be ensured that on the one hand, the switches are operated switch relieved and that on the other hand, a maximum power is transmitted to the output of the electronic ballast connected discharge lamp. In this context, algorithms for selecting the step size can be implemented, which only very seldom provoke non-switch-relieved switching of the switches of the half-bridge, for example every 100th or 1000th switching operation. Such rare non-switching relieved switching leads only to unimportant losses, but allows an optimized in terms of power transmission operation of the electronic ballast.

Further advantageous embodiments will become apparent from the dependent claims.

The preferred embodiments presented with reference to the electronic ballast according to the invention and their advantages apply correspondingly, as far as applicable, to the method according to the invention.

Short description of the drawing (s)

Fig. 1

in schematischer Darstellung ein Ausführungsbeispiel eines erfindungsgemäßen elektronischen Vorschaltgeräts;

Fig. 2

in schematischer Darstellung die Abhängigkeit der Ausgangsspannung von der Betriebsfrequenz der Schalter des Wechselrichters für zwei unterschiedliche Lasten;

Fig. 3

den zeitlichen Verlauf verschiedener elektrischer Größen für das Ausführungsbeispiel von Fig. 1; und

Fig. 4

in schematischer Darstellung einen Signalflussgrafen eines Ausführungsbeispiels einer erfindungsgemäßen Totzeitregelung.

In the following, an embodiment of an electronic ballast according to the invention will now be described in more detail with reference to the accompanying drawings. Show it:

Fig. 1

a schematic representation of an embodiment of an electronic ballast according to the invention;

Fig. 2

a schematic representation of the dependence of the output voltage of the operating frequency of the switches of the inverter for two different loads;

Fig. 3

the time course of various electrical variables for the embodiment of Fig. 1 ; and

Fig. 4

a schematic representation of a signal flow graph of an embodiment of a dead time control according to the invention.

Preferred embodiment of the invention

Fig. 1 shows a schematic representation of an embodiment of an electronic ballast according to the invention. Although the invention is presented below using the example of an inverter with a half-bridge circuit, it will be obvious to a person skilled in the art that the principles according to the invention can also be applied to an inverter with a full-bridge circuit.

This in Fig. 1 illustrated electronic ballast has an input with a first E1 and a second input terminal E2 for coupling with a DC supply voltage. In the present case, this is the so-called intermediate circuit voltage U _Zw , which is usually obtained from an AC line voltage. This intermediate _circuit voltage U _Zw is applied to an inverter 10, comprising a first S1 and a second electronic switch S2 in a half-bridge arrangement. For controlling the switches S1, S2, a control device 12 is provided. The control device 12 controls the switches S1, S2 in particular such that the first and the second switches S1, S2 are alternately turned on with a first frequency. For this purpose, the control device 12 is coupled to a current measuring device, which in the present case comprises a shunt resistor R _S , which is arranged in series with the first switch S1. The current flowing through the shunt resistor R _S is denoted by I _S. The switches S1, S2 are designed as Mosfet, wherein for simplification of the following explanations, the respective body diode D1, D2, which acts here in each case as a freewheeling diode, is located.

Between the switches S1, S2, a first half-bridge center HBM is formed, wherein the voltage dropping at the half-bridge center is designated U _HBM . Parallel to the lower half-bridge branch a trapezoidal capacitor C _{t is} coupled. A lamp inductor ELF is coupled between the first half-bridge center HBM and a first output terminal A1 of the electronic ballast. Between the first output terminal A1 and a second output terminal A2, which here represents a second half-bridge center, an output voltage U _{R is provided} to a load R _L , which in the present case comprises at least one discharge lamp. Between the second output terminal A2 and the reference potential, represented by the terminal E2, a coupling capacitor C _{C is} coupled. Parallel to the series connection of the load R _L and the coupling capacitor C _C is a resonant capacitor C _R coupled.

Fig. 2 shows a schematic representation of the dependence of the provided between the output terminals A1, A2 voltage U _R of the operating frequency f _R , with which the control device 12 controls the switches S1, S2, for two different loads R _L. Curve 1) represents a low-impedance load 1) (low burning voltage, low output power) with a resonant frequency f _R1 , curve 2) a higher-impedance load 2) with a resonant frequency f _R2 . As can be clearly seen, the frequency f _{R2 is} greater than the frequency f _R1 . In operation with the frequency f _{o of} the resonant circuit with the first-mentioned load (curve 1)) would be operated inductively, with the second-mentioned load (curve 2)) capacitive.

Fig. 3 shows the time courses of different sizes of the embodiment of Fig. 1 , It shows in particular the time course of the on and off state of the switch S2 (curve a)), the voltage U _HBM (curve b)) and the on and off state of the switch S1 (curve c)). In addition, the course of the current I _{S is} shown, first for an inductive operation (f _R = f _R2 ) at load 1) (curve d)), for a capacitive operation in the phase jump at load 2) (f _R = f _R2 ) (Curve e)), and for the same load as curve e), but now when operating at a frequency f _R greater f _R2 (curve f)).

The respective temporal courses are divided into four different phases. In phase 1, the switch S2 is on, that is conductive. This is the reason Potential at the half-bridge center on the potential of the intermediate _{circuit voltage} U _Zw . The switch S1 is off during this time. The current through the shunt resistor R _{S is} also zero. In phase 1, therefore, the current flows through the switch S2, the inductor L _R to the load R _L.

The transition to phase 2 is characterized in that the switch S2 goes into the off state, while the switch S1 is not yet turned on. The current driven by the inductor L _R thus flows from the trapezoidal capacitor C _t through the inductor L _R to the load R _L. The potential at the half-bridge center is linearly reduced to zero. The beginning of phase 2 corresponds to the beginning of the dead time t _dead .

The transition from phase 2 to phase 3 is characterized in that the trapezoidal capacitor is discharged. The freewheeling diode D1 becomes conductive and clamps the voltage at the half-bridge center to approximately -0.7 V. The current now flows through the freewheeling diode D1, the inductor L _R to the load R _L. With reference to curve d), therefore, a negative current I _{S flows} from the time when the freewheeling diode D1 has become conductive. If this reaches a Threshold I _Thres , then this is used according to the prior art to trigger the switch-on of the switch S1. The switch-on operation of the switch S1 represents the beginning of the phase 4. The period between the beginning of the phase 2 and the end of the phase 3 represents the dead time t _dead . The phase 3 designates the time interval within which the switch S1 can be switched switched-off , The voltage U _HBM dropping across the switch S1 is equal to zero within this period.

In phase 4, the current now begins to flow through the switch S1, as a result of which the current flow in phase 4, see curve d), runs approximately sinusoidally until the switch S1 is switched off.

The marked with respective superscript curve curves result in an increase in the load R _L , ie with reference to Fig. 2 at load 2). Accordingly, after a switch-off operation of the switch S2, the potential at the half-bridge center falls much slower, see U ' _HBM in curve b). However, at the instant when the voltage U ' _{HBM reaches} the ground potential, the negative current peak of the current I's, see curve e), is not negative enough to reach the threshold I _Thres . As a result, a switching operation of the switch S1, see the course S1 'in curve c), only after reaching the maximum predetermined time t _timeout triggered.

When switching on the switch S1, see the course S1 ', now occurs a needle-shaped current I' _S , which results from the discharge of the trapezoidal capacitor C _t . Since U ' _{HBM is} no longer equal to zero at this point in time, the switch S1 is not switched switched-off.

While curves d) and e), as mentioned, were recorded at a first operating frequency f _R equal to f _R2 for the switches of the half bridge, a second operating frequency f _R greater than _R2 is now selected for curve f). By increasing the frequency f _R , the negative current pulse increases at the time when the potential at the half-bridge center goes to zero, compare curve f) with curve e). The threshold I _Thres will be back achieved and a switch-relieved switching on the switch S1 allows.

Fig. 4 shows a schematic representation of a signal flow graph for controlling the dead time t _dead . The method starts in step 100. In step 120, it is checked whether the dead time t _dead measured by the time measuring device is equal to the predefinable time period t _timeout .

If this is the case, then in step 140, the frequency f _R , with which the switches of the half-bridge are operated, increased. Subsequently, step 120 is repeated. By the action of step 140, see Fig. 2 , the resonant frequency again shifted into the inductive range. This results in a larger negative current amplitude when taken over by the freewheeling diode, whereby the dead time control works again.

However, if it is determined in step 120 that the dead time t _{dead is} less than the predetermined time period t _timeout , it is checked in step 160 whether the current operating frequency f _{R is} greater than a nominal operating frequency f _nom . The nominal operating frequency f _nom represents a minimum operating frequency of the electronic ballast. If it is determined that the current operating frequency f _{R is} above the nominal operating frequency f _nom , the operating frequency f _{R is} reduced in step 180 and then branched back to the start.

However, if it is determined in step 160 that the nominal operating frequency f _{nom has been} reached, then without a change in the current operating frequency f _{R, it} is branched back to the start.

The execution of steps 160, 180 is of particular importance when initially a lamp with a higher operating voltage has been operated on the electronic ballast and its burning voltage has subsequently dropped, for example due to thermal effects. Without regulation to the nominal operating frequency f _nom , the lamp would in this case be operated permanently at increased frequency and thus at reduced power. Thus, the in Fig. 4 illustrated regulatory relationship on the one hand, a functioning of the dead time control, on the other hand, an operation of each connected to the electronic ballast lamp with the optimum operating frequency.

The respective achievement of the predefinable time duration t _timeout can be _detected particularly simply digitally. Depending on the implementation, the increase of the half-bridge frequency can be digital, for example by digital PWM registers for the switch-on times of the switching elements, or analogously by an offset at the input of a VCO or CCO.

The in Fig. 4 However, the sequence should only be activated in the burning mode and not during the preheating or the ignition of the discharge lamp in order to avoid unwanted interactions with other protection and control mechanisms.

As is apparent to those skilled in the art, the trapezoidal capacitor C _t and the coupling capacitor C _{C may} also be located elsewhere. Moreover, it is also possible to provide a plurality of trapezoidal capacitors and coupling capacitors, as is obvious to a person skilled in the art.

Claims (11)

1. Electronic ballast for operating at least one discharge lamp (R_L) comprising

   - an input having a first (E1) and a second input terminal (E2) for coupling to a direct supply voltage (U_Zw);

   - one output having a first (A1) and a second output terminal (A2) for coupling to the at least one discharge lamp (R_L);

   - an inverter (10) having a bridge circuit with at least one first (S1) and one second electronic switch (S2) and a control device (12) for driving at least the first (S1) and the second electronic switch (S2) in such a manner that the first (S1) and the second electronic switch (S2) are alternately switched to conduct with a first frequency (f_R), wherein the first (S1) and the second switch (S2) are coupled serially between the first (E1) and the second input terminal (E2), wherein the second electronic switch (S2) is coupled to the first input terminal (E1) and the first electronic switch (S1) is coupled to the second input terminal (E2), wherein a first bridge centre (HBM) is formed between the first (E2) and the second electronic switch (E1);

   - a current measuring device (R_S) for measuring the current (Is) at least through the first electronic switch (S1);

   - a lamp choke (L_R) which is coupled serially between the first bridge centre (HBM) and the first output terminal (A1);

   - at least one trapezoidal capacitor (C_t) which is coupled in parallel with one of the two electronic switches (S1; S2); and

   - at least one coupling capacitor (C_C) for connecting the
   load;
   wherein the control device (12) is coupled to the current measuring device (R_S) and is designed for switching the first electronic switch (S1) to conduct,

   a) if a predeterminable negative threshold value (I_Thres) of the current (I_S) through the first electronic switch (S1) is exceeded after the second electronic switch (S2) has been switched to block; or

   b) if the predeterminable negative threshold value (I_Thres) of the current (I_S) is not exceeded by the first electronic switch (S1) after the second electronic switch (S2) has been switched to block: after a predeterminable period (t_timeout);

   characterized in that the control device (12) is designed for increasing the first frequency (f_R) in case b).
2. Electronic ballast according to Claim 1, characterized in that each of the two electronic switches (S1, S2) comprises a control electrode, an operating electrode and a reference electrode, wherein a free running diode (D2, D1) is connected in parallel with the operating electrode - reference electrode path.
3. Electronic ballast according to Claim 2, characterized in that the free running diode (D2, D1) represents a body diode of the electronic switch (S2, S1).
4. Electronic ballast according to Claim 2, characterized in that the free running diode (D2, D1) represents a discrete diode.
5. Electronic ballast according to one of the preceding claims, characterized in that the control device (12) comprises a memory in which the predeterminable period (t_timeout) is deposited.
6. Electronic ballast according to one of the preceding claims, characterized in that the control device (12) comprises a time measuring device which is designed to determine the period after the second electronic switch (S2) has been switched to block up to when the first electronic switch (S1) has been switched to conduct.
7. Electronic ballast according to Claim 6, characterized in that the control device (12) is designed for carrying out the following step:

   c1) if the measured period (t_dead) is equal to the
   predeterminable period (t_timeout) (step 120) :

   increase the first frequency (f_R) by a predeterminable step (step 140).
8. Electronic ballast according to Claim 7, characterized in that the control device (12) is also designed for carrying out the following step:

   c2) repeat step c1), in any case until the measured
   period (t_dead) is shorter than the predeterminable period (t_timeout).
9. Electronic ballast according to one of Claims 7 and 8, characterized in that the control device (12) is also designed for carrying out the following step:

   d1) if the measured period (t_dead) is shorter than the
   predeterminable period (t_timeout) (step 160) :

   reduce the first frequency (f_R) by a predeterminable step (step 180).
10. Electronic ballast according to Claim 9, characterized in that the control device (12) is also designed for carrying out the following step:

    d2) repeat step d1) until a predeterminable value for the
    first frequency (f_R) is reached.
11. Method for operating a discharge lamp (R_L) at an electronic ballast having an input with a first (E1) and a second input terminal (E2) for coupling to a direct supply voltage (U_Zw); an output having a first (A1) and a second output terminal (A2) for coupling to the at least one discharge lamp (R_L); an inverter (10) having a bridge circuit with at least one first (S1) and a second electronic switch (S2) and a control device (12) for driving at least the first (S1) and the second electronic switch (S2) in such a manner that the first (S1) and the second electronic switch (S2) are alternately switched to conduct at a first frequency (f_R), wherein the first (S1) and the second switch (S2) are coupled serially between the first (E1) and the second input terminal (E2), wherein the second electronic switch (S2) is coupled to the first input terminal (E1) and the first electronic switch (S1) is coupled to the second input terminal (E2), wherein a first bridge centre (HBM) is formed between the first (S1) and the second electronic switch (S2); a current measuring device (R_S) for measuring the current (Is) at least through the first electronic switch (S1); a lamp choke (L_R) which is coupled serially between the first bridge centre (HBM) and the first output terminal (A1); at least one trapezoidal capacitor (C_t) which is coupled in parallel with one of the two electronic switches (S1; S2); and at least one coupling capacitor (C_C) for connecting the load; wherein the control device (12) is coupled to the current measuring device (R_S) and is designed for switching the first electronic switch (S1) to conduct

    a) if a predeterminable negative threshold value (I_Thres) of the current (Is) is exceeded by the first electronic switch (S1) after the second electronic switch (S2) has been switched to block; or

    b) if the predeterminable negative threshold value (I_Thres) of the current (I_S) is not exceeded by the first electronic switch (S1) after the second electronic switch (S2) has been switched to block: after a predeterminable period (t_timeout);

    characterized by the following step:

    increase the first frequency (f_R) in case b) (step 140).

Images