NL8900703A - HIGH-FREQUENT BALLAST. - Google Patents

Description

NEDAP N.V.

Groenlo

High frequency ballast

The invention relates to a high-frequency ballast for medium and high-pressure metal vapor gas discharge lamps. High-frequency ballasts can be very compact and are easily adjustable. High-frequency ballasts for the said type of lamps are known, inter alia, from publications DE 3445817, DE 3623306, US 4170747 and EUR 0240049.

The ballasts described in publication DE 3445817 and EUR 0240049 contain an adjustable DC-DC converter, followed by an inverter. This solution prevents acoustic resonances in the gas discharge arc, but the ballast becomes relatively expensive and bulky, because two series-connected power converters must be used. Although the ballasts described in publication DE 3623306 and US 4170747 consist of a single bridge inverter, separate control circuits are used to drive one side of the bridge at a high frequency and the other side at a low frequency. This entails additional costs compared to a bridge inverter in which one drive frequency is applied. Furthermore, in all the publications mentioned, a separate circuit is used to ignite the lamps, which makes the circuit more expensive and more extensive.

It is known, inter alia from publication DE 3511661, to ignite gas discharge lamps by using the oscillation of the voltage in a series resonant circuit. However, the known circuits cannot simply be used for large powers. None of the above-mentioned ballasts have taken special measures to stabilize lamp power, light intensity or lamp temperature, as is desirable in many industrial applications of medium and high pressure medium discharge gas discharge lamps of the order of 500 W to 3000 ff.

Furthermore, none of the aforementioned publications addresses the problem of the power factor. This problem arises when the DC voltage of the bridge inverter is obtained by rectifying and smoothing a mains AC voltage.

This results in higher harmonic currents in the supply network, which can lead to additional losses in the alternating voltage distribution network, and which can trigger fuses earlier and lead to distortion of the voltage form in the supplying alternating voltage network.

Circuits are known to deal with this problem. However, an additional self-inductance and semiconductor switching element with associated control circuits is used, which does not lead to additional costs and makes the device more extensive.

Such a method is described, for example, in article Simplified Control Algorithm for Active Power Factor Correction by Neil J. Earabas, Proceedings of the tenth international PCI-85 Conference, pp 1-9. Simpler methods for power factor correction, especially in high-frequency ballasts, are also described in Power Factor Corrected, MOSFET Multiple Output,

Flyback Switching supply1 by J.J. Spangler, Proceedings of the tenth international PCI-85 Conference, pp 19-32. However, this method also requires additional power components and therefore entails additional costs.

It is known from publications NL 8600812 and DE 3505182 to keep and control the lamp power of a high-pressure gas discharge lamp. However, use is made of a low lamp frequency.

It is generally known to use so-called full bridge circuits for converting a DC voltage into a high-frequency AC voltage. Here, control transformers are often used to drive the switching elements of the bridge circuit, followed if necessary by a buffer circuit in order to drive the control connection of the switching element, low-ohmic. However, these buffer circuits then require supply voltage, for which separate windings from a separate transformer are used, which entails quite a lot of extra costs and extra space.

The object of the invention is to enable the construction of a high-frequency ballast using a single bridge inverter, the frequency of which can be varied over a wide range in order to control the lamp power, which also serves to control the lamps without special additional starting circuits. to ignite, and also avoiding said acoustic resonances.

Another object is to enable simple adjustment and stabilization of the lamp power or the light intensity.

Yet another goal is to allow controlled forced cooling of the lamp, which allows for still more convenient lamps with a large

power density can be constructed, while at the same time I

an optimal lamp temperature can be maintained over a wide power range.

Yet another object is to use a control circuit for the power switching elements of the bridge inverter, which does not require a separate power transformer for the buffer stages connected behind the control transformer, and which therefore entails little additional costs and can be well miniaturized.

Yet another object is to obtain a good power factor in the order of 0.8 to 0.9 without using additional components when supplying power from an AC voltage network.

These goals are achieved in the following manner.

Firstly, use is made of a bridge inverter consisting of power MOSFETS or other power switching elements, on the output side of which the lamp is connected in series with one or more current-limiting self-inductors, while one or more capacitors are arranged in parallel with the lamp, for ignition of the lamp, the drive frequency of the bridge above the resonant frequency of the series circuit, formed by said current-limiting self-inductors and capacitors, is selected, and this frequency is then gradually lowered, whereby at a drive frequency close to the resonant frequency, a strong oscillation of the voltage across the lamp arises, which can ignite it, while in normal operation frequency modulation of the output voltage can be applied, so that the acoustic vibration circuits in the lamp cannot be strongly excited.

In particular, the frequency modulation can be synchronized with the double mains frequency. A high frequency gives a low power to the lamp due to an increase in the impedance of the current-limiting coils. By now choosing the frequency modulation so that most of the power is delivered to the lamps around the positive and negative peaks of the mains voltage, the smoothing capacitor, which follows the mains rectifier, can be kept small and essentially only serves to supply the zero crossings of the mains voltage with a residual DC voltage in order to keep the lamps conductive via the inverter, but with a low instantaneous power. The said frequency modulation can be provided by a microprocessor included in the control of the inverter and therefore does not cost any additional components.

In addition, the operating status of the lamps is checked by measuring the lamp voltage, input current and input voltage of the inverter and, if necessary, the light intensity of the lamps. Data stored in the microprocessor program then allows settings of the drive frequency for the bridge inverter and fan speed to be set, so that the lamps provide the desired power or the desired light intensity and also maintain the desired lamp temperature.

Furthermore, the buffer circuit for the power switching elements of the bridge inverter which allows a wide frequency range to drive the bridge inverter is fed by a capacitor, which is between the buffer circuit of the power switching element which follows the output AC voltage and one of the input supply poles o £ the associated buffer circuit is connected. The operation of the bridge converter according to the invention will be described in more detail below with reference to the figures and some exemplary embodiments.

The following are shown successively in the figures. Fig. 1 shows the block diagram of a possible embodiment of the ballast, FIG. 2 and FIG. 3 show alternative embodiments of the ballast, FIG. 4 shows the supply circuit for the buffer stages driving the power switching elements. Fig. 5 depicts a possible embodiment of the buffer stages, FIG. 6 shows a possible block diagram for the control and regulation of the ballast, and FIG. 7 shows some important waveforms of currents and voltages in the ballast. Finally, fig. 8 shows some important parameters as a function of time, during the ignition and heating phase of the lamp.

In the circuit of FIG. 1, the input voltage, which is applied between terminals 1 and 2, is obtained, for example, by rectifying and smoothing the 220 V input voltage, by switching elements 2 and 5, and 3 and 4 alternately in conduction converted to a high-frequency block voltage which is applied to the series circuit, formed by inductors 6 and 7, and capacitor 8.

For igniting the lamp, the drive frequency of the bridge inverter is above the resonant frequency of the series circuit, formed by inductors 6 and 7, and capacitor 8. The series circuit is now inductive and the waveforms of voltage and current are as shown in Fig. 7. A negative value of the current means that the freewheeling diodes 12, 13, 14 or 15 are conducting.

It is important in high-frequency operation of the bridge that the diodes first conduct, and then the controllable power switching elements, because the diodes can then recover freely, and are not brutally recovered by the opposite switching element, which leads to very large peak currents and even failure of the inverter can result. The drive frequency is now gradually lowered and as it approaches the resonant frequency of the above series circuit, the voltage across capacitor 8 becomes very high, igniting the lamp. The lamp now loads the circuit and dampens it supercritically, so that elements 6, 7 and 8 now form a low-pass filter. The drive frequency can now be lowered even further and the load on the bridge remains inductive. The resistance of the ignited medium or high pressure gas discharge lamp is initially very low, so that a relatively high frequency is required, such that the impedance of self-inductance 6 and 7 is sufficiently high to draw the current on one for the switching elements 2, 3, 4, 5 and lamp 9 to limit an acceptable value. As the lamp heats up, the resistance of the lamp increases, further driving the drive frequency down to keep the lamp current at the same limit.

The final operating condition of the lamp is determined by the

6, which will be described later. The control unit determines which control frequencies should be selected on the basis of lamp voltage, input current of the bridge, supply voltage of the bridge, and external information, for example desired lamp power, or difference between desired and actual light intensity. A lower frequency hereby leads to a lower impedance of the self-inductors 6 and 7 and therefore to higher currents with the same lamp resistance. The peak value of lamp voltage is always smaller than the value of the supply voltage that is applied to terminals 1 and 2.

An alternative embodiment is shown in Fig. 2. Here additional capacitors 16, 17, 18 and 19 are connected to one of the supply terminals. The capacitors 16, 18 and resistors 20 and 21 are in this case connected as a capacitive voltage divider, which is frequency-independent and which reduces the peak voltage that can arise on one side of the lamp from, for example, 2000 V to, for example, 10 V. Capacitors 17 , 19 and resistors 22, 23 are switched in the same manner. Peak voltage detector 10 can now give a control signal 24 to the control unit when a certain value of the circuit voltage is exceeded, after which the control of the bridge is cut off and all power switching elements are cut off. The voltage threshold at which the driving is stopped is below the value that can occur with loop resonance, which is usually determined by nuclear saturation phenomena of inductances 6 and 7. The limit value is chosen so that the currents through the switching elements remain limited to acceptable values, while on the other hand the lamps still ignite reliably. By proceeding in the above manner, damage to the ballast if the lamp is not connected or is still very hot and then cannot ignite due to the high gas pressure.

Yet another embodiment is shown in Fig. 3. Here, a half bridge consisting of switching elements 2 and 3 and freewheeling diodes 12 and 13 is used. An additional coupling capacitor 11 is also used here to block the DC voltage component in the drive voltage.

In this case, the lamp voltage has half the value of that in FIGS. 1 and 2 in proportion to the source voltage across terminals 1 and 2 at otherwise equal operating conditions.

Fig. 4 shows how the driving circuits 39 and 40 for the power switching elements 2, 3 can be supplied. A capacitor 30 is connected between the nodes of diode pair 28, 29 on the one hand and diode pair 31, 32 on the other. Before the inverter starts to operate, the control circuits are supplied with voltage via resistors 35 and 36; This supply voltage is stabilized by parallel zener diodes 34 and 38 and high-frequency decoupled via capacitors 33 and 37. It is important here that the control circuits 39 and 40 in the switched-off state have a low current consumption in order to avoid unnecessary dissipation in resistors 35 and 36. As soon as the converter is put into operation, by activating control signals and $ 2 of control circuit 25 and actuating the buffer circuits via torque transformers 26 and 27, the block voltage shown in FIG. 7 arises at point A. This voltage causes alternating charging and discharging of capacitor 30, which alternately conducts diodes 28 and 32, and diodes 29 and 31. The capacitance value of capacitor 30 is chosen such that this charge is amply sufficient to accommodate the charge required to power the gate-source and gate-drain capacities of the MOSFETs, IGBTs, or Darlington combination of MOSFET and BJT power switching elements. to deliver. Capacitor 30 also serves as a dV / dt limiter, and reduces switching losses in the MOSFETs. In principle, the buffer circuit 40 could also be supplied from the same supply voltage as, for example, the control and one connection of capacitor 30 can be connected directly to pole 2 instead of to the junction of diode 31 and 32. However, as a result of the very fast switching (toff 30 nsec), large voltage peaks occur, which makes it advantageous to still isolate the power supply for circuit 40, and to supply and disconnect it directly at the switching element 3.

An exemplary embodiment of the control circuits 39 and 40 is shown in FIG. 5. The output voltage of transformer 26 is dipped through the network consisting of diodes 42, 43 and zener diodes 41, 44 and has the form shown in FIG. 7 at 26. MOSFET 2 is turned on by a positive voltage from transformer 26, driving transistor 47 open. It is important here that the control voltage on the base of transistor 47 is lower than the voltage on the collector of this transistor, so that it does not saturate and can be dissipated very quickly. To turn off MOSFET 2, the output voltage of transformer 26 becomes negative and P channel FET 51 is turned on. Diode 46 is now blocked and due to the action of diode 48 and resistor 52, a negative bias voltage of approx.

0.7 V applied to the base-emitter junction of transistor 47. As can be seen from Fig. 7, the MOSFET is switched on in normal operation when the parallel-connected freewheeling diode is already conducting. As a result, the control circuit does not have to load the Miller-drain-gate capacitance and a relatively high ohmic value can be selected for resistor 49. This choice then prevents an excessive peak current from flowing during the first switching on of the M0SFET switch as a result of the discharge of capacitor 30. However, when the MOSFET 2 is switched off, the Miller drain-source capacity is active. Now, to minimize the switching losses at high operating frequencies, a low switch-off resistance is required, which in this case is formed by the channel resistance of MOSFET transistor 51.

A sizing example follows below.

MOSFET switches 2, 3, 4, 5: 500 V - types with an on-resistance of 0.3 ohm and a drain current of 13 A continuous and 50 A peak. Supply voltage: 250-380 VDC.

Resistance value of 49:47 Ohm.

Channel resistance of 51:10 Ohm.

Output voltage of transformer 26: + 12V and -12V,

Supply voltage of the control circuit 39 and 40: 15V.

Dead time td: 500 nsec.

Lamp power: 1200 W, Fire voltage 150 V effective.

A possible embodiment of the control of the ballast is shown in Fig. 6. A control unit 54, which may be equipped with a microcomputer, receives information from the outside world 56 via an optical coupling member 55. This information may be: lamp on / off, desired lamp power, desired light intensity, measured light intensity. Furthermore, this control unit measures the following internal quantities: input voltage between terminals 1 and 2, average input current of the bridge inverter by measuring the voltage drop across resistor 63, which is filtered low-pass through connections 61 and 62 if necessary, and lamp voltage measured by circuit 10 and transmitted via signal 64. Furthermore, the microcomputer can, if necessary via battery backup, hold data from the past, how long have the lamps burned, when were they extinguished, and so on. Furthermore, via signal 64, a control circuit 65 can control a fan 66 for lamp cooling. The output variable of the control unit consists of a signal for voltage controlled oscillator 57, which in turn generates signals ^ 1 and φΐ (59 and 60), with voltage forms as shown in Fig. 7, as U26 and U27. The control unit ensures that, before ignition, the driving frequency is above the series resonance frequency of the aforementioned series circuit, that this frequency is gradually reduced until the lamps ignite, or the maximum peak voltage value for the series circuit, measured by circuit 10, is then determined based on measured lamp voltage, input current of the converter and input voltage of the converter, the desired output frequency. This will generally be chosen as low as possible in order to bring the lamp to temperature as quickly as possible, without exceeding the aforementioned operating parameters. If forced cooling is now used, from the moment the lamp voltage has reached the value associated with the measured power, which corresponds to the optimum working temperature of the lamp, this voltage will be kept constant by energizing the fan. Should this not happen, the gas pressure in the lamp would increase further due to an increase in temperature, which would result in a higher lamp voltage. The lamp voltages and temperatures are measured in advance on one or a few copies of the lamps to be fed, after which these data are processed in the control program of the microcomputer. The control signal to the voltage controlled oscillator can be amplitude modulated with a frequency from approx. 100 Hz to approx. 1 kHz, to achieve frequency modulation of the control signal for the bridge inverter, so that acoustic resonances in the lamps are prevented. It is possible to modulate the duty cycle of control or both frequency and duty cycle with comparable frequencies. In that case, a low-frequency off-set current will also flow through the lamp, which may possibly prevent resonances in the gas.

In particular, the frequency modulation can be synchronous with the double mains frequency, whereby a large power is supplied to the lamps around the negative and positive peaks of the mains voltage by choosing a low driving frequency, and around the zero crossings a small power, so that the smoothing condensation -tor, which is switched after the mains rectifier only needs to have a small value. The reference signal required to synchronize this frequency modulation may be the DC ripple of the smoothed mains voltage, which is also the supply voltage for the bridge. In this way a power factor of 0.8 to 0.9 is achievable.

A typical variation of the parameters which occur during ignition, starting, burning and cooling of the lamps is shown in fig. 8. The variation of the peak value of the lamp voltage u-lamp, the peak value of the lamp current ^ lamp are shown successively. , the drive frequency of the inverter 'freq.', the air displacement of the fan 'airflow', whereby very low speeds can be realized by on / off control of the fan, and the light intensity, weighted according to a certain spectral distribution 'light output' .

The time scale in interval A is 10 msec / div, in interval B 1 min / div. At time tO the inverter is started up under the influence of an external control command 'on'. The desired light intensity has level 1. The inverter is started at the highest frequency, after which the frequency is continuously decreased. At time t1, the voltage across the series circuit is so high that the lamp ignites. The frequency reduction continues until the maximum permissible value of the lamp peak current is reached at time t2. This value is not measured directly, but is determined based on the input power of the converter (product input current and input voltage) and the lamp voltage. Then the lamp heats up, causing the lamp voltage to increase. The i-lamp value is now kept approximately constant by the indirect measurement described above.

The basis for the calculation can be that the lamp power P-lamp is given by:

Cl is a constant which depends on the waveform of lamp voltage and lamp current, it is also assumed that the lamp can be regarded as a purely resistive, linear element. If the waveform remains constant during warm-up, and also the efficiency

of the inverter, and we assume that the inverter is supplied with a pure DC voltage ü-i and that it draws an average current I-i from this source, then:

or

or

At known

and so Cl can

indeed, the processor present in the control unit can be calculated from U-i, I-i and

which are measured. If the aforementioned frequency modulation is used synchronously with the double mains frequency, the above-mentioned relationships will remain valid for the time being, but will vary periodically within half a network period. The microprocessor can now, by means of the calculated instantaneous powers, regulate the frequency in such a way that on the one hand the average lamp power is kept constant, the maximum permissible currents through the switching elements are not exceeded, and the largest input current is also around the maximum and minimum of the mains voltage. The microprocessor program may therefore contain a number of preprogrammed modulation curves or an algorithm to generate these modulation curves, in particular the ripple of the DC voltage U-i being used as a reference signal for the frequency modulation, so that no additional components are required. At time t3 the desired value of the light intensity has been reached. However, the lamp temperature is not yet optimal. The optimum working temperature of the lamp is reached at time t4. The lamp voltage now increases a little further and the lamp current drops a little further, the lamp current / lamp voltage ratio being used as the control variable for the air displacement of the fan. A thermally stable situation has arisen at time t5. At time t6, a new desired light intensity 2 is prescribed by an external signal. This is achieved almost immediately by lamp voltage and lamp current by means of adjust frequency control. The lamps are already at the right temperature. A slight change in the lamp voltage / lamp current ratio again serves as a control variable for the air displacement of the fan, so that a new thermal equilibrium is finally reached at time t7.

As mentioned earlier, an optimal control program will have to be compiled on the basis of temperature measurements on lamps. These temperature measurements are one-off in advance. If desired, it can also be corrected with aging, while a certain lamp power / light output ratio can also indicate that the lamps have reached the end of their service life and need to be replaced. This can then be made known to the outside world 56 via a signal via coupling member 55.

Claims (16)

1. Ballast for medium and high-pressure gas discharge lamps, consisting of a full- or half-bridge converter with two or four power switching elements, which power switching elements consist of a main current path with a positive and a negative connection, with the positive connection in reverse mode a higher voltage then has the negative terminal, and giving a control signal with which the switching elements are conductively switched relative to the negative terminal of the main current path and which inverter is powered by a DC voltage source and which power switching elements are controlled by buffer circuits, which in turn control transformer or other coupling element capable of transmitting a control signal is controlled from a control unit, while on the output side of the bridge converter a series circuit consisting of one or more inductors and one or more capacitors is arranged, wherein the lamp to be supplied is connected in parallel to the capacitors, characterized in that at least the supply voltage for each of said buffer circuits, which follow the output AC voltage signal, measured with respect to one of the DC input terminals, is supplied by a capacitor, the first connection of which is connected to the anode of a first diode, the cathode of which is connected to the positive supply terminal of said buffer circuit, and also to the cathode of a second diode, the anode of which is connected to the negative supply terminal of the buffer circuit, while the The second terminal of this capacitor is connected to a point in the circuit which is approximately at the potential of one of the DC input terminals. Ballast according to claim 1, characterized in that the control signal for the power switching elements is not given with respect to the negative main current path connection, but with respect to the positive main current path connection. Ballast according to claim 1 or 2, characterized in that the second terminal of said capacitor is connected to the anode of a third diode, the cathode of which is connected to the positive terminal of the buffer circuit, which is connected to one of the terminals. of the DC input voltage, and also to the cathode of a fourth diode, the anode of which is connected to the negative supply voltage terminal of said buffer circuit. Ballast according to claim 1, 2 or 3, characterized in that before igniting the gas discharge lamp, the bridge circuit is actuated such that the frequency of the output voltage is above the resonant frequency of the above-mentioned circuit, and the frequency is then gradually is lowered. Ballast according to claim 4, characterized in that if the voltage across the series resonant circuit exceeds a predetermined value, the drive of the bridge inverter is blocked, and all power switching elements are brought into the non-conducting state. Ballast according to any one of the preceding claims, characterized in that, during the heating of the lamps, the control unit of the inverter controls the drive frequency on the basis of the measured value of input DC voltage, input DC current and output lamp voltage, so that the maximum permissible lamp current does not is exceeded. Ballast according to claim 6, characterized in that the frequency of the bridge converter is regulated such that the lamp current has a predetermined nominal value during heating. Ballast according to any one of the preceding claims, characterized in that if the measured light intensity of the lamp deviates from the desired light intensity of the lamp, the average driving frequency is adjusted such that the desired light intensity is achieved. Ballast according to any one of the preceding claims, characterized in that if the measured inverter power deviates from the desired power, the inverter is adjusted such that the desired power is achieved. Ballast according to any one of the preceding claims, characterized in that if the ratio of the measured lamp voltage and the lamp current calculated by the control unit exceeds a predetermined value, partly due to the measured power, a fan is activated, such that said ratio between lamp voltage and lamp current remains approximately constant. Ballast according to any one of the preceding claims, characterized in that the control frequency is modulated. Ballast according to one of the preceding claims, characterized in that the duty cycle of the control signal is modulated. Ballast according to any one of the preceding claims, characterized in that both duty cycle and frequency of the control signal are modulated. Ballast according to claim 12 or 13, wherein the DC input voltage for the bridge converter is obtained from an AC voltage source by rectifying and smoothing the AC voltage in two phases, characterized in that the power modulation due to duty cycle and / or frequency modulation of the drive signal of the bridge converter is synchronous with the dual frequency of the AC voltage source, the phase of this modulation being selected such that the inverter draws the largest current around the positive and negative peaks of the AC input voltage, and around the zero crossings of the input AC voltage the smallest current. Ballast according to claim 14, characterized in that the AC voltage component of the DC input voltage of the bridge inverter is used for determining the correct phase of said modulation. Ballast according to any one of the preceding claims, characterized in that if the ratio between the inverter power and light intensity calculated by the control unit exceeds a certain value depending on said power or light intensity, a signal is given indicating that the lamps have been replaced should be.