EP0710052B1 - Electronic starter for a fluorescent lamp - Google Patents

Description

The present invention relates to an electronic starter for a fluorescent lamp.

The electrical behavior of fluorescent lamps which contain pressurized gases (neon, argon) is similar to that of a zener diode in an avalanche, with a resistance in the gas which becomes very weak and negative after breakdown.

A choke is necessary to obtain the breakdown of the gas in the lamp: it is a question of causing an overvoltage at the terminals of the lamp to ionize the gas. We are thus used to using an inductor and a device for short-circuiting the lamp and passing current. When the device is opened, the energy stored in the inductor turns into an overvoltage which causes the gas to snap.

It is also preferable to preheat the filaments of the lamp in order to bring them to a temperature at which they easily emit electrons.

In practice, the duration of the short circuit is normalized according to the category of lamp considered. For example, it is around 1.5 seconds for low-pressure fluorescent lamps. This standardization makes it possible to use a lamp from a given manufacturer with a choke from another manufacturer.

A commonly used type of choke is a bimetallic strip (with a suppressor capacitor in parallel). This inexpensive electromechanical device makes it possible to maintain a short circuit for the necessary time long enough to warm up (1.5 seconds), then open the short circuit to snap the gas.

However, the lamp does not always light up. As long as it is not lit (that is to say that the breakdown did not take place), the bimetallic strip will work: there is then a permanent flashing of the particularly troublesome lamp. In addition, the bimetallic strip may be damaged. Finally, the bimetallic strip opens for any current. It can therefore open at a time when the current is almost zero: the energy is then too small to be effective, or at a time when the current is very strong: the lamp itself can be damaged.

To overcome these drawbacks, attempts have been made to use electronic circuits generally using a triac or a thyristor as a power device and a counter to limit the number of lamp ignition tests.

These circuits consume a lot of current, requiring a very high capacity for the capacitor to maintain the logic voltage necessary for controlling the electronic power device (triac) and for counting, for the entire duration of the short circuit.

In addition, these circuits must measure the duration of the short circuit, to then control the opening of the electronic power device. However, this preheating time is relatively long. We saw in an example, that it had a normalized value of 1.5 seconds (low pressure fluorescent lamp). It is not desirable to use a slow RC circuit to measure such a long duration, in particular by the need in this case to use a high resistance (1 Megaohm for example), which reduces noise immunity (high input impedance).

We prefer to use a fairly fast RC circuit, followed by a large capacity counter to measure the desired duration.

These electronic circuits are however large consumers of current, and in practice require a capacitor for maintaining the logic voltage of high capacity, of the order of 100 or 1000 microfarads for example, of the electrochemical type, which moreover weakens these circuits. due to the limited lifetime of these capacitors.

An object of the invention is to overcome these various drawbacks.

• sur détection d'une tension d'entrée correspondant à la première référence de tension, délivrer un signal de détection du début du préchauffage, pour commander la fermeture du commutateur de puissance et commuter la deuxième référence de tension sur le comparateur,
• sur détection d'une tension d'entrée correspondant à la deuxième référence, délivrer un signal de détection de fin du préchauffage, pour commander l'ouverture du commutateur de puissance.

As characterized, the invention relates to an electronic starter of a fluorescent lamp, comprising a power switch in parallel on the lamp and supplied with high voltage, a gate control circuit of said switch comprising a measurement circuit d a determined preheating time and an auxiliary supply circuit in parallel on said switch and comprising a capacitor, for supplying a logic supply voltage to the gate control circuit on a terminal of said capacitor.
According to the invention, the preheating time measurement circuit comprises a comparator with two voltage references, a first voltage reference greater than a second voltage reference, the comparator having an input connected to the terminal of the capacitor, and an output connected to a logic circuit to switch the first voltage reference on the comparator at high voltage, and for:

• on detection of an input voltage corresponding to the first voltage reference, delivering a detection signal of the start of preheating, for controlling the closing of the power switch and switching the second voltage reference on the comparator,
• on detection of an input voltage corresponding to the second reference, deliver a detection signal of end of preheating, to control the opening of the power switch.

Advantageously, the gate control circuit further comprises a preheating current measurement circuit, for delivering a command to open the switch after reception of the end of preheating detection signal, upon detection of an optimal current of preheating in the lamp.

According to another characteristic of the invention, to reduce the consumption current during the measurement of the preheating time, the gate control circuit comprises a gate circuit placed between the logic supply voltage and the current measurement circuit and controlled by the logic circuit to de-energize the current measurement circuit upon detection of the first voltage reference and energize it upon detection of the second voltage reference.

• la figure 1 est une première variante d'un schéma d'un starter électronique appliqué à une lampe fluorescente selon l'invention,
• la figure 2 est une deuxième variante d'un schéma d'un starter électronique selon l'invention,
• la figure 3 est un schéma d'un circuit de commande de grille avec un circuit de génération de courant selon l'invention et
• la figure 4, est un schéma d'un circuit de mesure de courant utilisé dans l'invention.

Other characteristics and advantages of the invention will appear on reading the description which follows, given by way of indication and in no way limiting and with reference to the accompanying drawings, in which:

• FIG. 1 is a first variant of a diagram of an electronic starter applied to a fluorescent lamp according to the invention,
• FIG. 2 is a second variant of a diagram of an electronic starter according to the invention,
• FIG. 3 is a diagram of a gate control circuit with a current generation circuit according to the invention and
• FIG. 4 is a diagram of a current measurement circuit used in the invention.

FIG. 1 represents a first variant of an electronic starter for a fluorescent lamp 1 according to the invention. The choke is connected between a terminal e1 of a first filament f1 of the lamp and a terminal e2 of a second filament f2 of the lamp. An inductor 2 has a first end 11 connected to the other terminal el 'of the first filament f1. The lamp and inductance assembly is supplied at high alternating voltage, in the example 220 volts - 50 hertz, applied between the other end l2 of the inductor and the other terminal e2 'of the second filament f2 of the lamp.

The electronic choke comprises a power device 3 with gate control G, a gate control circuit 5 and an auxiliary power supply circuit AUX.

The gate control power device is connected between the terminals e1, e2 of the lamp, through a rectifier stage 4 with diodes.

The gate control power device may for example be a MOS transistor with field as shown in FIG. 1 (N-type MOSFET), or an insulated gate bipolar transistor, or a controlled gate thyristor, etc. In the following, it is designated by the general term of power switch. It is closed, similar to a short circuit if it passes current, or open, to allow the breakdown of gas in the lamp.

The gate G of the power switch is voltage-controlled by the gate control circuit 5, to make it conducting, equivalent to a short circuit for a determined period of preheating of the lamp, then open, to allow ignition.

An auxiliary power supply circuit AUX is connected in parallel on the power switch, to supply the logic voltage Vdd necessary for the gate control circuit 5 and maintain this voltage while the power switch is closed, equivalent to a short circuit . It comprises a capacitor C on a terminal from which it supplies the logic supply voltage Vdd necessary for the control circuit. More specifically, the auxiliary supply circuit comprises a resistor R connected between the high voltage and the terminal c1 of the capacitor which supplies the voltage Vdd, the other terminal c2 of the capacitor being connected to ground. A diode D1, having the anode connected to the high voltage and the cathode connected to the resistor, makes it possible to prevent the discharge of the capacitor C by the power switch, when the latter is closed (on).

According to the invention, the gate control circuit 5 comprises a circuit for measuring a time of determined preheating, during which the power switch 3 must be closed (on), equivalent to a short circuit.

The preheating time measurement circuit comprises a comparator 8 with two voltage references V1, V2 and a logic circuit 9. This comparator receives as input the logic supply voltage Vdd, supplied by the auxiliary supply circuit, on the terminal c1 of the capacitor. The comparator compares this input voltage Vdd with the voltage reference V1 or V2 applied by a switch 10 controlled by the logic circuit 9. The voltage V2 is lower than the voltage V1. They both take their value between the maximum logic voltage supplied by the auxiliary supply circuit and a minimum voltage for which the electronics of the control circuit can still operate, for example between 15 and 3.5 volts.

The logic circuit 9 delivers a signal sl for detecting the start of preheating, upon detection by the comparator of an input voltage Vdd greater than or equal to the first voltage reference V1. It delivers a signal s2 for detecting the end of preheating, upon detection by the comparator of a logic voltage Vdd which has become less than or equal to the second voltage reference V2. Finally, it commands (s3) the switching of the voltage reference V1 or V2 to a voltage reference node v of the comparator 8.

The signals s1 and s2 are applied to a voltage control circuit (COM) of the gate G of the power switch. Depending on the signals s1 and s2, it applies an appropriate voltage VG to the gate G, to close the switch on the detection of the start of preheating (s1) or open it at the end of preheating detection (s2). The signal s3 is applied to a switch 10 to switch the voltage V1 or the voltage V2 on a reference node of the comparator 8.

In the example shown, where the power switch is an N-type MOSFET transistor, a positive logic voltage is required on the gate G to close the switch (transistor on) and a zero logic voltage to open the switch (transistor blocked) . In this example, the voltage control circuit then advantageously comprises a diode 6 for switching the positive logic voltage on the gate G. The signal s1 for detecting the start of preheating is then applied to the anode of diode 6, the cathode of the diode being connected to the grid G of the power switch. The control circuit also includes a transistor 7 to force the gate to ground. The end of preheating detection signal s2 is applied to the gate of transistor 7, one electrode of which is connected to gate G of the power switch, the other being connected to ground. In the example, transistor 7 is a bipolar NPN transistor, passing for a positive gate voltage and off for a zero gate voltage.

In an improvement shown in FIG. 2, a circuit 11 for measuring the lamp preheating current is advantageously provided for controlling (s2 ') the opening of the power switch, for an optimal value of current, after the detection of the end of preheating.

Measuring the current determines the appropriate time to open the short circuit after the necessary preheating time has elapsed. It is a question of having an optimum current to light the lamp: neither too small, to have a sufficient overvoltage, nor too large, not to damage the lamp.

Preferably, a gate circuit 12 is placed between the logic supply voltage Vdd and the current measurement circuit 11. It is controlled by the end of preheating detection signal s2 to de-energize the current measurement circuit 11, during the preheating period, and to energize it, at the end of the preheating time, to control the opening of the switch for an optimal value of the preheating current flowing in the power switch (signal s2 ').

A current bypass circuit 13 is then provided between the power switch 3 and ground to derive a small current Ip to the current measurement circuit 11.

The current measurement circuit 11 comprises, as shown in FIG. 4, a current amplifier 14 followed by a comparator 15 to a current or power reference (ref) corresponding to an optimal preheating current Ip for lighting the lamp . However, there is a problem in measuring the preheating current, since the power is applied to any value of the preheating current. If the current measurement circuit is re-energized at a time when the current is increasing and greater than the reference: the comparator 15 will switch for a too high preheating current. To overcome this drawback, the comparator 15 is of the window type, such that it does not switches only for passing through the reference value ref with a negative slope. It could also be a comparator with a first comparison to zero: thus, we are sure not to switch for a too high current value.

The gate control circuit 5 preferably comprises a counter 16 of the number of lamp ignition tests (Figures 1 and 2). An incrementation (or decrementation) command is provided by the logic circuit 9. In the example, it is the detection signal for the start of preheating sl which is used, but one could as well use the detection signal for end of preheating s2.

This counter delivers an inhibition signal inh to stop the choke, if the lamp is still not lit after a number n of authorized tests. This signal inh is applied in the example to logic circuit 9.

The operating principle of the choke according to the invention will now be explained, in relation to FIG. 2.

At high voltage, the logic circuit 9 controls the switch 10 to apply the first voltage reference V1 to the comparator, for example equal to 15 volts. The comparator sv output is at zero.

The detection by the comparator of a logic voltage Vdd which becomes greater than or equal to V1, provides the starting point for the lamp preheating phase: the output sv of the comparator goes to 1.

The logic circuit 9 then generates the preheating start detection signal s1 to control the closing of the power switch and switches the second voltage reference V2 to the comparator. The counter 16 is incremented (or decremented by one).

The signal s1 being applied to the anode of the diode 6, the latter then switches the corresponding logic level (15 volts at this time) on the gate G of the power switch 3. A preheating current Ip then flows through the switch 3. The voltage across the power switch drops to practically zero.

The logic voltage Vdd is maintained by the auxiliary supply circuit, but the capacitance C is gradually discharged through the consumption current of the gate control circuit 5. The comparator 8 will then detect the transition to the lower voltage V2.

As the consumption current of the gate control circuit is perfectly known, the time after which the voltage Vdd will drop from the voltage V1 to the voltage V2 is perfectly known: it depends on the capacitance of the capacitor, the consumption current and of the voltage excursion (V2-V1).

Thus, according to the invention, the gate control circuit 5 uses its own consumption to precisely measure the duration of preheating. And the capacitor C of the auxiliary supply circuit is used both to maintain the logic voltage and to measure the necessary duration of preheating.

When the voltage comparator 8 detects a logic voltage Vdd corresponding to the second voltage reference V2, the logic circuit 9 activates the end of preheating detection signal s2, to re-energize the circuit 11 for measuring the preheating current. The latter can then command (s2 ') the blocking of the power switch for an optimum value of the lamp preheating current IP.

The logic circuit can then switch the first reference voltage V1 again, for a new preheating phase.

As one preferably seeks to have a small capacitor, one seeks to have the lowest possible current consumption. In the invention, provision is made to cut the logic supply to the circuits which are not useful during the measurement of the preheating time.

In the example shown in FIG. 2, the supply of the current measurement circuit 11, which consumes a lot of current (amplifier), is thus cut off and which is only useful after the preheating time has elapsed. The power supply is cut by the door circuit 12 controlled by the end of preheating detection signal s2, delivered by the logic circuit 9.

On the other hand, the counter 16 remains supplied, so as not to lose its information.

In practice, it is then sufficient, in manufacturing, to measure precisely the current consumed in the gate control circuit, as used during the measurement period, to determine the value of the capacitance of the capacitor C and the value of the references of voltage V1 and V2, to be able to measure the preheating time. When using the current measurement circuit, the consumption of the latter, which is energized, must also be taken into account. after detection of the end of preheating, knowing that the detection of the optimum current requires at most 1 to 2 alternations of current.

The use of diode 6 to switch the positive voltage on the grid of the power switch, to turn it on is particularly advantageous, because this grid voltage is then maintained, regardless of the level of the logic supply voltage Vdd. It is thus possible to use a large voltage excursion of the logic supply voltage. We are only limited by the classical logic of the control circuit (and not by the minimum gate voltage necessary to keep the switch closed).

The choke according to the invention thus makes it possible to obtain a small consumption current, of the order of a micro-ampere, and a large voltage excursion (10 volts for example) authorizing the use of a small capacitor, with a capacity of the order of microfarad.

In an improvement represented in FIG. 3, the gate control circuit 5 comprises a current generator, with current mirrors. In this way, a current is imposed in each current branch of the gate control circuit. There is no longer any need to measure the current at the end of manufacture to adjust the reference values (reference voltages, capacity). In dynamic operation, the current is imposed and the preheating time measured very reliably.

The current generator comprises a reference branch 17 with a reference load element 18 (resistance) and a transistor 19 mounted as a diode with its drain connected to its gate, and transistors of the same type (20 to 23), all having their gate controlled by the gate of the transistor of the reference branch: the respective current in each transistor is identical to that imposed in the reference branch, except for the geometry ratio of the transistors.

In the example, a reference current is imposed in this way on the circuit for measuring the preheating current, which can also be very useful for a particularly reliable measurement of the preheating current.

Finally, to prevent the starter from continuing to operate once the lamp is on, provision is made (FIG. 2) preferably for a zener diode D2 between the high voltage and the auxiliary power supply circuit AUX, with a high zener threshold, so as to bring a zero voltage across the auxiliary power circuit when the lamp is on.

In one example, for a lamp supplied with 220 volts, and which has 100 volts at its terminals when it is switched on, it suffices to take a zener threshold of 120 volts.

Other switching devices are possible. It is thus possible to provide a circuit for reading the voltage at the terminals of the power switch, in order to inhibit logic circuit 9.

Finally, the counter itself makes it possible to stop, as we have seen, the choke.

The switch 10 of the voltages V1, V2 on the voltage reference node v of the comparator 8 comprises in one example (FIG. 2) an N-type MOS transistor 24, connected between the voltage reference V2 and the reference node v, and controlled in the state passing on its grid by the signal s3 delivered by the logic circuit 9, upon detection by the comparator 8 of an input voltage greater than or equal to V1. The voltage reference V1 is directly connected to the reference node v. Thus, when the transistor 24 is blocked, it is the voltage V1 which is applied. When the transistor 24 is on, the voltage V2 lower than V1 is imposed on the node v.

The logic of circuit 9 is simple, dependent on the technologies selected for the various elements that it controls. In the example more particularly described in relation to FIG. 2, the circuit 9 comprises a power switch 3, constituted by a MOSFET transistor of type N. It also includes a gate circuit 12 with MOS transistor of type P and a switch 10 of voltage type MOS transistor 24 of type N, the signals s1, s2 and s3 are a copy of the signal sv of the comparator output 8. The logic circuit 9 preferably comprises an inhibition transistor 25 for transmitting the signal sv to the outputs s1, s2 and s3 of the logic circuit. This inhibition transistor is controlled on its gate by the inhibition signal inh of the counter 16 or of an ignition detection circuit not shown.

When the comparator 8 detects an input voltage Vdd greater than or equal to V1, its output sv goes to 1. If the inhibition transistor 25 is on (ignition test authorized), the signals s1, s2 and s3 follow this transition: the corresponding logic voltage level is switched by the diode 6 on the gate G of the power switch 3; the door circuit 12 is blocked, thereby putting the current measurement circuit 11 off; the transistor 24 turns on, which imposes the reference voltage V2 on the node v of the comparator 8. When the comparator detects an input voltage less than or equal to V2, its output sv goes to 0: the circuit gate 12 is returned passing, restoring the circuit 11 for measuring the current under voltage and the transistor 24 becomes blocked, which again imposes the reference voltage V1 on the voltage reference node of the comparator 8.

The entire gate control circuit 5 can be easily produced in the form of an integrated electronic circuit, which is a definite advantage.

In an improvement, it will be provided in the case where the control circuit comprises a generator with current mirrors as shown in FIG. 3, that the reference resistor 18 is transferred outside the integrated circuit. This optionally makes it possible to easily adjust this value, depending on the desired preheating time and above all to use high precision resistors, which the integrated technology does not allow.

The electronic starter according to the invention, using its consumption current during the preheating period, to perform a precise time measurement based on its own consumption characteristics (in particular discharge of the capacitor), allows particularly simple and reliable control of the power switch.

In addition, it allows reduced and controlled consumption, and a much smaller capacity than those required in the circuits of the prior art.

In one example, the capacity of the holding capacitor is of the order of microfarad. We no longer have to use electrochemical capacitors, which makes it possible to extend the life of these starters.

Claims (14)

1. Electronic starter for a fluorescent lamp (1), comprising a power switch (3) in parallel with the lamp (1) and supplied with high voltage, a gate control circuit (5) for the said switch (3) having a circuit for measuring a given preheating time and an auxiliary supply circuit (AUX) in parallel with the said switch (3) and comprising a capacitor (C) for supplying a logic supply voltage (Vdd) to the gate control circuit (5) at a terminal (c1) of the said capacitor (c),
   characterised in that the circuit for measuring the preheating time comprises a comparator (8) for comparing the voltage at the terminal (c1) of the said capacitor (c) with a first reference voltage in order to deliver a signal for the detection of the start of preheating controlling the closure of the power switch (3) and for comparing the voltage at the terminal (c1) of the said capacitor with a second reference voltage in order to deliver an end of preheating signal controlling the opening of the power switch.
2. Electronic starter according to Claim 1, characterised in that the gate control circuit (5) also comprises a circuit (11) for measuring the preheating current in order to deliver a signal controlling the opening of the switch (s2') after reception of the signal detecting the end of preheating (s2), on detection of an optimum preheating current in the lamp.
3. Electronic starter according to Claim 2, characterised in that the gate control circuit (5) comprises a gate circuit (12) positioned between the logic supply voltage (Vdd) and the current measuring circuit (11) and controlled by the logic circuit (9) in order to de-energize the current measuring circuit when the first voltage reference (V1) is detected and energize it when the second voltage reference (V2) is detected.
4. Electronic starter according to Claim 1 or 2, characterised in that the gate control circuit (5) comprises a diode (6) whose cathode is connected to the gate (G) of the power switch and whose anode receives the signal for detection of the start of preheating (s1), so as to switch a positive logic voltage onto the gate (G) of the power switch in order to control its closure.
5. Electronic starter according to Claim 4, characterised in that the gate control circuit (5) comprises a first transistor (7) connected between the gate (G) of the power switch and earth, and receiving at its gate the end of preheating detection signal (s2) delivered by the logic circuit (9) or the opening control signal (s2') delivered by the current measuring circuit (11), to apply a nil voltage to the gate (G) of the power switch, in order to control its opening.
6. Electronic starter according to Claim 2, characterised in that the circuit (11) measuring the preheating current (Ip) of the lamp comprises an amplifier (14) of a derived current (Ip) and a comparator (15) with a reference current value (ref).
7. Electronic starter according to Claim 2, characterised in that the circuit (11) measuring the preheating current (Ip) comprises an amplifier (14) of a derived current and a comparator (15) with a reference power value (ref).
8. Electronic starter according to Claim 6 or 7, characterised in that the comparator with a reference value (15) is a window comparator so that it switches when the reference value changes to a negative gradient.
9. Electronic starter according to any one of the preceding claims, characterised in that the gate control circuit (5) comprises a circuit for generating a given current, with current mirrors, with a current reference branch comprising a reference resistor (18).
10. Electronic starter according to any one of the preceding claims, characterised in that the logic circuit (9) again switches the first voltage reference (V1) to the comparator (8) on detection of an input voltage corresponding to the second reference, for a new preheating step.
11. Electronic starter according to Claim 10, characterised in that the gate control circuit (5) also comprises a counter (16) which receives a counting/downcounting command for each new preheating step, in order to deactivate the gate control circuit (5) after a given number of preheating commands.
12. Electronic starter according to any one of the preceding claims, characterised in that a Zener diode (D2) is placed between the high voltage and the auxiliary supply circuit (AUX) in order to de-energize the starter when the lamp is lit.
13. Electronic starter according to any one of the preceding claims, characterised in that the gate control circuit (5) is an integrated circuit.
14. Electronic starter according to Claim 9 or 11, characterised in that the gate control circuit (5) is produced in one and the same integrated circuit, except for the reference resistor (18), which is placed outside.

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