DE102009047714A1 - Circuit arrangement and method for operating at least one discharge lamp - Google Patents

Abstract

The present invention relates to a circuit arrangement for operating at least one discharge lamp (La) having an input with a first (E1) and a second input terminal (E2) for coupling to a _DC 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 (La); a bridge circuit having at least a first (S1) and a second electronic switch (S2), wherein a series connection of the first (S1) and the second electronic switch (S2) to form a first bridge center (HBM) between the first (E1) and the second input terminal (E2) is coupled; a second order LRC resonant load circuit having a lamp inductor (L1) coupled between the first bridge center (HBM) and the first output terminal (A1) and at least one trapezoidal capacitor coupled in parallel with one of the electronic switches (S1, S2) ( C _T ); and a control device (10) for driving at least the first (S1) and the second electronic switch (S2) with a drive signal (AH, AL), wherein the drive signal (AH, AL) an operating frequency (f) for operating the LRC resonant load circuit in the frequency range with inductive phase position; the circuit arrangement further comprising: a determining device (L2) designed to derive the time derivative. the current (I _S2 ) by at least one of the electronic switches (S1, S2) to determine; and a comparator (R2, D3, R3, S3, C1, R4) coupled to the determining device (L2) and adapted to derive the value of the time derivative of the current (I _S2 ) by at least one of the electronic switches. (S1, S2) to compare against a predetermined threshold value, wherein the comparison device (R2, D3, R3, S3, C1, R4) is further designed to control the control device (10) upon detection of exceeding the predetermined threshold value such that this Operating frequency (f) of the drive signal (AH, AL) increased. It also relates to a corresponding method for operating at least one discharge lamp (La) on such a circuit arrangement.

Description

Technical area

The present invention relates to a circuit arrangement 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, a bridge circuit having at least one first and a second electronic switch, wherein a series circuit of the first and second electronic switches is coupled to form a first bridge center between the first and second input terminals, a second order LRC resonant load circuit having a lamp inductor connected between the first bridge center and the first bridge center Output terminal is coupled, and at least one, parallel to one of the electronic switch coupled trapezoidal capacitor, and a control device for controlling at least the first and the second electr onischen switch with a drive signal, wherein the drive signal has an operating frequency for operating the LRC resonant load circuit in the frequency range with inductive phase position. It also relates to a corresponding method for operating at least one discharge lamp.

State of the art

Typical load circuits for discharge lamps, in particular low-pressure discharge lamps, are LRC (L = inductance, R = resistance, C = capacitance) resonant circuits which are operated at frequencies above their resonance frequency. Half-bridge or bridge circuits are commonly used as drivers of such load circuits.

In order to ensure soft switching, so-called soft switching, the switch of the bridge circuit, that is a lossless switching, it is necessary to provide a certain amount of reactive power. With an appropriate design of the circuit overload conditions, component tolerances, fluctuations in the DC supply voltage and the lamp temperature and, moreover, the modification of various parameters due to the lamp aging must be considered. This means that in normal operation, ie in operation far away from the worst case, usually large amounts of reactive power are available. The transmission of these large amounts of reactive power causes unwanted losses.

If insufficient reactive power is present in the circuit arrangement, then the switches of the bridge circuit turn hard, so-called hard switching, which causes undesirably high current peaks, losses and electromagnetic interference. Operation near the resonant frequency of the LRC load circuit is also desirable because then maximum energy can be transferred to the LRC load circuit.

Presentation of the invention

The object of the present invention is therefore to improve a generic circuit arrangement or a generic method such that a soft switching with as little reactive power to be transmitted and largely independent of various boundary conditions, such as lamp temperature, lamp aging, component tolerances, is possible.

This object is achieved by a circuit arrangement having the features of patent claim 1 and by a method having the features of patent claim 10.

The present invention is based on the finding that during commutation of the current from a first to a second electronic switch of the bridge circuit, first a current flows through the freewheeling diode of the switch to be turned on, before this switch itself is turned on. The period during which both switches are switched off is called dead time. The second switch is turned on in the period during which current flows through its freewheeling diode to ensure smooth switching. This period is greater the more reactive power is present in the load circuit. Part of the reactive power is used to reload the trapezoidal capacitor. The further excess of reactive power is fed back into the supply via the freewheeling diodes.

If there is no longer sufficient reactive power, a recharging of the trapezoidal capacitor fails, as a result of which the switch to be switched conductive is hard-wired. The present invention is based, in particular, on the finding that a hard switching through a peak in the current can be recognized by the switched-on switch. This indicates that the operating frequency of the drive signal for the switches of the half-bridge is too close to the resonance frequency, that is, tends to sink from the inductive range into the capacitive range.

The present invention therefore comprises a detection device which is designed to determine the time derivative of the current through at least one of the electronic switches. she further comprises a comparison device which is coupled to the determination device and is adapted to compare the value of the time derivative of the current through at least one of the electronic switches against a predefinable threshold value. If an exceeding of this predefinable threshold value is determined, the comparison device controls the control device in such a way that it increases the operating frequency of the drive signal. As a result, the load circuit is operated again in the inductive range, whereby the mentioned peaks and thus a hard switching of the switches is avoided. By doing so, operation of the circuitry close to the resonant frequency but on the inductive side thereof is possible. This results in that the circuit arrangement can be operated with minimum reactive power. As a result, the power components, such as coils, capacitors, switches and so on, can be made smaller. Moreover, simpler circuit concepts can be realized, for example the integration of preheating into the lamp inductor.

In a preferred embodiment, the control device is designed to increase the operating frequency. This can be done by fine dimensioning predefinable steps in digital implementation or by a continuous control in an analog implementation. Thus, an operation of the circuit arrangement is made possible close to the optimum, that is, in view of the present problem with minimal reactive power, while still a soft switching can be ensured.

In a further preferred embodiment, the control device is designed to reduce the operating frequency on the basis of a predefinable initial value, in particular in predefinable steps or steplessly, as long as no exceeding of the predefinable threshold value is determined by the value of the time derivative of the current through at least one of the electronic switches , This measure serves to gradually lower the operating frequency from a value at which sufficient reactive power is present in the load circuit to critical regions in order to achieve the optimum. Here, too, a fine dimensioning of the predefinable steps enables a reliable finding of the optimum, independently of the already mentioned varying operating parameters.

Preferably, the detection device comprises a shunt resistor coupled serially to the first or second electronic switch. In order to determine the time derivative of the current through at least one of the electronic switches, the voltage drop across the shunt resistor must therefore be differentiated. Therefore, the determining device preferably further comprises a differentiating device which is designed to determine the time derivative of the current through the shunt resistor.

However, it is particularly preferred if the determining device comprises an inductance which is coupled in series to the first or second electronic switch. This variant takes advantage of the circumstance; the voltage drop across the inductance is proportional to the time derivative of the current through the inductance, that is, U = L * di / dt. An additional differentiating device, as in the evaluation of the current through a shunt resistor, can therefore be dispensed with in this embodiment.

In a preferred embodiment, the comparison device comprises a voltage divider, which is connected in parallel at least the inductance, wherein the tapping point of the voltage divider is coupled to the control electrode of a controllable resistor. By means of this measure, it is possible in a simple manner to determine the predefinable threshold value for the time derivative of the current through at least one of the electronic switches. Preferably, the arrangement is designed so that the controllable resistor when the above-mentioned predefinable threshold value is exceeded activates the control device in such a way that it increases the operating frequency.

The voltage divider may further comprise a diode, wherein the diode between the coupled to the inductance terminal of the voltage divider and the tapping point of the voltage divider is coupled. By this measure, it can be considered that for different control tasks of the control device, the determination of the current through one of the electronic switches is required, that is not its time derivative. If, for this purpose, a shunt resistor is coupled serially to the inductance, in particular in such a way that the inductance is coupled between the second electronic switch and the shunt resistor, the voltage drop at the shunt resistor at maximum current can be taken into account and disturbed by the diode Therefore, the evaluation of the time derivative of this stream by a comparison device designed as mentioned not.

In a preferred embodiment, a device for averaging is coupled between the working electrode of the controllable resistor and the control device. Thus, pulses at the input of the controllable resistor are not transmitted to its output, which is coupled to the control device.

Further advantageous embodiments will become apparent from the dependent claims.

The preferred embodiments presented with reference to the circuit arrangement 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)

In the following, an embodiment of the present invention will now be described in detail with reference to the accompanying drawings. These show:

1 a schematic representation of an embodiment of a circuit arrangement according to the invention;

2 a more detailed view of a range of in 1 illustrated embodiment; and

3a to 3c the time course of the current through the second electronic switch of in 1 illustrated embodiment at different operating frequencies.

Preferred embodiment of the invention

1 shows a schematic representation of an embodiment of a circuit arrangement according to the invention. The circuit arrangement comprises an input with a first E1 and a second input terminal E2, between which a DC supply voltage is applied, in particular the so-called intermediate circuit voltage U _Zw . Between the input terminals E1, E2 a bridge circuit is coupled, which mainly comprises a first switch S1 and a second switch S2. The actual switch is in each case a freewheeling diode D1 or D2 connected in anti-parallel, which in the present case is expressed by the dashed border expressed that in a MOSFET, the respective freewheeling diode is realized by the respective body diode. In contrast, when using bipolar transistors for the switches S1, S2, a separate freewheeling diode would be provided.

Between the switches S1, S2, a half-bridge center HBM is formed, which is coupled via a lamp inductor L1 to a first output terminal A1. Between the first output terminal A1 and the second output terminal A2, a discharge lamp La is coupled. Parallel to the discharge lamp La, a resonance capacitor C _{R is} coupled. Between the second output terminal A2 and the second input terminal E2, a coupling capacitor C _{K is} coupled. Parallel to the second switch S2, a trapezoidal capacitor C _{T is} coupled.

As will be apparent to those skilled in the art, other embodiments of LRC resonant circuits are possible. By way of example only, the number and position of the trapezoidal capacitor (s), coupling capacitor (s), and resonant capacitor (s) may be varied without affecting the present invention.

For measuring the current through the second switch S2, a shunt resistor R1 is provided, which is coupled in series with the switch S2. Between the switch S2 and the shunt resistor R1, an inductance L2 is coupled. The drop across the inductance L2 voltage U _L2 is proportional to the time derivative of the current Is through the inductance L2. The voltage U _L2 , as well as the voltage U _R1 , which drops across the shunt resistor R1, a processing device 12 supplied, which is designed to provide at its output AL a drive signal for the switch S2 and at its output AH, a drive signal for the switch S1. The drive signals have an operating frequency f, which can be varied. In the processing device 12 a comparison device is provided to which the voltage U _{L2 is} supplied. It is designed to compare the voltage U _L2 against a predefinable threshold value, and upon detection of an exceeding of the predefinable threshold value, the processing device 12 causes the operating frequency f of the drive signals for the switches S1, S2 to increase.

2 shows a more detailed representation of a section of in 1 illustrated embodiment of a circuit arrangement according to the invention. As can be seen, the voltage drop across the inductor L2 voltage U _L2 fed to a voltage divider comprising the resistors R2 and R3 and a diode D3. Its tapping point is coupled to the control electrode of a transistor S3, which is operated here as a controllable resistor. Parallel to the path working electrode reference electrode of the transistor S3, a capacitor C1 is coupled, which together with an ohmic resistor R4 represents a device for averaging. The processing device 12 further comprises a control device 10 , The ohmic resistor R4 is connected to an input of the control device 10 coupled.

How it works: If the switch S1 is turned on, the current first flows in the circle S1, L1, La. If the switch S1 is now switched to blocking, the lamp inductor L1 continues to drive the current. As a result, first the trapezoidal capacitor C _{T is} reloaded. When it is recharged, the diode D2 becomes conductive, causing the voltage across the switch S2 to become almost zero. The switch S2 must be turned on during the phase during which the diode D2 conducts to ensure smooth switching.

If the switch S2 is turned on, the current first flows through the switch S2, the lamp inductor L1 and the discharge lamp La. At this time, the capacitor CR is charged. The potential at the half-bridge center HBM drops to almost 0 V. Since the potential at point N is greater than 0 V as a result of the charged capacitor C _R , then a current flows from the discharge lamp La via the point N, the lamp inductor L1 through the switch S2.

In order to transmit as much power as possible to the discharge lamp, the operating frequency f of the drive signals for the switches S1, S2 is lowered as far as possible in the direction of the resonance frequency of the LRC load circuit. As a result, there is little excess reactive energy in the load circuit, resulting in only small losses.

However, this leads to the fact that after the blocking of the switch S1, the current through the lamp inductor L1 is so small that it is no longer sufficient to completely recharge the trapezoidal capacitor C _T. As a result, at the time when the switch S2 is turned on, the voltage at the half-bridge center HBM is not equal to zero. As a consequence, the switch S2 is switched hard.

In this context, reference is made to the 3 , 3a shows first the time course of the current I _S2 through the switch S2 and the current I _D2 through the diode D2 at a dimensioning of the operating frequency f greater than the resonant frequency f _{o of} the load circuit. It can be seen that a current I _{D2 flows} through the diode D 2 over a period of time t ₁ , that is to say the long time period t _{1 is available} for conducting the switching of the switch S2.

If now the operating frequency f is lowered to a range in the vicinity of f ₀ , but still on the inductive side of the resonance frequency, that is f> ≈ f _o , see 3b , so the period during which the switch S2 can be switched soft, has shrunk to the period t ₂ .

An even further lowering of the operating frequency f leads to the representation in 3c , where the operating frequency is f ≈ f _o . Here there is no longer a period in which a current would flow through the diode D2. The circuit arrangement is operated at the resonant frequency f _o . In this case, there is so little reactive energy in the load circuit that the peak of the current I _{S2 flows} only in the circuit of the switch S2 and the trapezoidal capacitor C _T. Since at the time of the peak, the voltage across the switch S2 is not equal to zero, all energy stored in the trapezoidal capacitor C _T is converted into heat. The switch S2 is switched hard.

An accurate setting of the operating frequency f to a result as in 3b To obtain shown is not possible in practice, since both the _{DC link} voltage U _Zw varies, the temperature of the discharge lamp La and as a result of the aging of the discharge lamp La whose burning voltage. According to the invention it is now provided to determine the operating frequency at which the in 3c shown peak of the current I _S2 arises. For this purpose, the voltage U _L2 , which drops across the inductance L2, evaluated. For these: U _L2 = L2 · di _S2 / dt; it thus directly reflects the time derivative of the current _is again. If it is determined that the time derivative of the current Ist exceeds a predefinable threshold, the operating frequency of the switches S1, S2 is changed in such a way that it continues to be in the inductive range, ie is increased. As a result, more inductive reactive power is provided in the load circuit, so that a reliable swing and thus a soft switching can be ensured.

Will be like in 2 shown, the sum of U _L2 and U _R1 supplied to the comparison device, this is harmless since at the time of the peak of I _S2 , see 3c , the current through the resistor R1 is very small and thus irrelevant.

In a preferred implementation was for the drive device 12 used a block which is designed so that it increases the operating frequency of the drive signals at its outputs AH and AL, the more current flows from the terminal to which the ohmic resistor R4 is connected via the resistor R4 through the transistor S3.

Claims (10)

Circuit arrangement for operating at least one discharge lamp (La) with - an input having a first (E1) and a second input terminal (E2) for coupling to a _DC 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 (La); A bridge circuit having at least a first (S1) and a second electronic switch (S2), wherein a series connection of the first (S1) and the second electronic switch (S2) to form a first bridge center (HBM) is coupled between the first (E1) and the second input terminal (E2); A second order LRC resonant load circuit having a lamp inductor (L1) coupled between the first bridge center (HBM) and the first output terminal (A1) and at least one trapezoidal capacitor coupled in parallel with one of the electronic switches (S1, S2) (C _T ); and a control device ( 10 ) for driving at least the first (S1) and the second electronic switch (S2) with a drive signal (AH, AL), wherein the drive signal (AH, AL) an operating frequency (f) for operating the LRC resonant load circuit in the frequency range with inductive phase position having; characterized in that the circuit arrangement further comprises: - a detection device (L2) which is designed to determine the time derivative of the current (I _S2 ) by at least one of the electronic switches (S1, S2); and - a comparison device (R2, D3, R3, S3, C1, R4), which is coupled to the determining device (L2) and designed to reduce the value of the time derivative of the current (I _S2 ) by at least one of the electronic switches (S1, S2) against a predefinable threshold, wherein the comparison device (R2, D3, R3, S3, C1, R4) is further designed, upon detection of exceeding the predefinable threshold, the control device ( 10 ) in such a way that it increases the operating frequency (f) of the drive signal (AH, AL). Circuit arrangement according to Claim 1, characterized in that the control device ( 10 ) is designed to increase the operating frequency (f). Circuit arrangement according to one of Claims 1 or 2, characterized in that the control device ( 10 ) is designed to reduce the operating frequency (f) starting from a predefinable initial value as long as no exceeding of the predefinable value for the value of the time derivative of the current (I _S2 ) by at least one of the electronic switches (S1, S2) is detected. Circuit arrangement according to one of the preceding claims, characterized in that the determining device (L2) comprises a serially to the first (S1) or second electronic switch (S2) coupled shunt resistor (R1). Circuit arrangement according to claim 4, characterized in that the determining device (L2) further comprises a differentiating device which is designed to determine the time derivative of the current (I _S2 ) through the shunt resistor (R1). Circuit arrangement according to one of claims 1 to 4, characterized in that the determining device comprises an inductance (L2), which is coupled in series to the first (S1) or second electronic switch (S2). Circuit arrangement according to Claim 6, characterized in that the comparison device (R2, D3, R3, S3, C1, R4) comprises a voltage divider (R2, D3, R3), which is connected in parallel with at least the inductance (L2), the tapping point of the Voltage divider (R2, D3, R3) is coupled to the control electrode of a controllable resistor (S3). Circuit arrangement according to Claim 7, characterized in that the voltage divider (R2, D3, R3) comprises a diode (D3), wherein the diode (D3) is connected between the terminal of the voltage divider (R2, D3, R3) coupled to the inductance (L2) ) and the tapping point of the voltage divider (R2, D3, R3) is coupled. Circuit arrangement according to claim 8, characterized in that between the working electrode of the controllable resistor (S3) and the control device ( 10 ) a device (R4, C1) is coupled for averaging. Method for operating at least one discharge lamp (La) on a circuit arrangement having an input with a first (E1) and a second input terminal (E2) for coupling to a _DC 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 (La); a bridge circuit having at least a first (S1) and a second electronic switch (S2), wherein a series connection of the first (S1) and the second electronic switch (S2) to form a first bridge center (HBM) between the first (E1) and the second input terminal (E2) is coupled; a LRC resonant load circuit of at least second order with a lamp inductor (L1) which is coupled between the first bridge center (HBM) and the first output terminal (A1) and at least one, in parallel to one of the electronic switches (S1, S2) coupled trapezoidal capacitor (C _T ); and a control device ( 10 ) for driving at least the first (S1) and the second electronic switch (S2) with a drive signal (AH, AL), wherein the drive signal (AH, AL) an operating frequency (f) for operating the LRC resonant load circuit in the frequency range with inductive phase position having; characterized by the following steps: a) determining the time derivative of the current (I _S2 ) by at least one of the electronic switches (S1, S2); b) comparing the value of the time derivative of the current (I _S2 ) by at least one of the electronic switches (S1, S2) against a predefinable threshold value; and c) increasing the operating frequency (f) of the drive signal (AH, AL) upon detection of exceeding the predefinable threshold in step b).

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