Controller and control device for a low-pressure fluorescent lamp

A control device for a fluorescent lamp comprises two independent circuits based on a power transistor and a switching control circuit, series-connected between a high voltage and the ground. The power transistor has a diode that is reverse mounted between its two electrodes. The switching control circuit comprises a circuit for the detection of a voltage at the terminals of the diode greater than a voltage reference value and a circuit for the detection of the integral of the current flowing in the transistor that is greater than a current reference value corresponding power of the lamp.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to control devices for a low-pressure fluorescent 
lamp. 
2. Discussion of the Related Art 
Fluorescent lamps contain gases (neon, argon) at low pressure. The 
electrical behavior of a fluorescent lamp is similar to that of a zener 
(avalanche) diode with a resistance in the gas that may become very low 
and negative after breakdown. Ions moving at high speeds lead the atoms of 
the gas to assume excited states in which they give out luminous lines. 
A control device for the lamp is typically needed, comprising a current 
source. However, to avoid a migration of ions, the current discharges 
applied between two electrodes of the lamp should pass in one direction 
and then in the other, alternately. The practice has been to use an 
inductor as a discharge control device but the development of electronics 
has led to the use of control devices typically comprising two electronic 
switches based on power transistors supplied with DC high voltage and a 
current transformer to control these transistor-based control devices. A 
resonant circuit comprising an inductor and a capacitor applies an AC 
current to the fluorescent lamp. According to the prior art, the 
transformer is a saturation transformer that limits the current in the 
lamp by saturation of its magnetic core and leads to the switching of the 
switches of the control devices. The electronic switches generally use 
bipolar technology power transistors for the switching and parallel and 
reverse-connected diodes to let through the current during the 
alternations, and various protection elements such as diodes and 
capacitors. 
These transformer devices are very bulky and costly because they require 
many components and allow only a very low degree of integration. 
Furthermore, the storage time of the bipolar transistors is a highly 
variable characteristic, for example ranging from 2 to 7 microseconds. 
This variation is not negligible as compared with the time at the end of 
which the transformer gets saturated for a current half-wave: it is about 
three microseconds for an alternation time of about ten microseconds. 
Hence, the time at the end of which the bipolar transistor goes off after 
saturation of the transformer in an alternation varies from 5 to 10 
microseconds. This is very troublesome. In practice, the storage time of 
each transistor is measured at the end of its manufacture in order it may 
be classified in a group corresponding to a narrow range of values for use 
in a control device matched by means of resistors with this range of 
values. All this entails heavy penalties and is very costly. 
SUMMARY OF THE INVENTION 
One illustrative embodiment of the present invention relates to a switching 
control circuit for a low-pressure fluorescent lamp. The switching control 
circuit includes a power transistor, a diode reverse-mounted between two 
electrodes of the transistor, a circuit for the detection of the voltage 
at the terminals of the diode and of the transistor to activate the ON 
state of the transistor when the voltage is below a voltage reference 
value, and a circuit to measure the current flowing into the transistor to 
activate the OFF state of the transistor when the integral of the current 
is greater than a current reference value. 
In another illustrative embodiment of the present invention, a low-pressure 
fluorescent lamp control device is provided that comprises two switching 
control circuits and a power transistor series-connected between a high 
voltage and the ground. An inductor, the low-pressure fluorescent lamp and 
a capacitor are series-connected between the midpoint of the two switching 
circuits and the ground. A starting capacitor (Cp) is designed in parallel 
on this lamp.

MORE DETAILED DESCRIPTION 
FIG. 1 shows a control device according to the invention. 
It comprises mainly two circuits Com.sub.a and Com.sub.b series-connected 
between a high voltage and the ground. In the example, the high voltage is 
given by a supply stage E with rectifier and filtering capacitor C0 which 
maintain a DC supply high voltage of the order of 300 V. 
The circuits Com.sub.a and Com.sub.b shall be described in detail 
hereinafter with reference to FIGS. 1 and 2. They comprise three external 
terminals referenced B1.sub.a, B2.sub.a, B3.sub.a for the circuit 
Com.sub.a and B1.sub.b, B2.sub.b, B3.sub.b for the circuit Com.sub.b. The 
terminals B1.sub.a and B2.sub.a (and B1.sub.b and B2.sub.b respectively) 
are the connection terminals of the circuit. The terminal B3.sub.a (and 
B3.sub.b respectively) is a decoupling terminal for the logic supply of 
the circuit. 
The terminals B3.sub.a and B3.sub.b are each connected to a decoupling 
capacitor C.sub.a and C.sub.b connected to the reference point of the 
circuit, namely B2.sub.a and B2.sub.b. The role of these capacitors is to 
keep the level of the internal logic voltage of the circuits Com.sub.a and 
Com.sub.b when they have a null voltage between their connection terminals 
B1.sub.a and B2.sub.a, B1.sub.b and B2.sub.b. 
Each of the circuits Com.sub.a and Com.sub.b comprises chiefly a power 
transistor T.sub.a, respectively T.sub.b and its switching control circuit 
(CC.sub.a respectively CC.sub.b) between the two connection terminals 
B1.sub.a and B2.sub.a, respectively B1.sub.b and B2.sub.b. The switching 
control circuit controls the gate ga, respectively gb of the associated 
power transistor. A diode D.sub.a, respectively D.sub.b is placed in 
parallel and in reverse on the transistors T.sub.a, respectively T.sub.b. 
The midpoint M between the two circuits Com.sub.a and Com.sub.b is 
connected to a terminal of an inductor L connected at the other terminal 
to a first electrode e1 of a low-pressure fluorescent lamp F. The other 
electrode e2 of the lamp is connected to a capacitor Cs connected to the 
ground. The inductor L, the lamp F and the capacitor Cs are therefore 
series-connected between the midpoint M and the ground, and form an 
oscillator circuit. 
Finally, a starting capacitor Cp is parallel-connected to the lamp. 
The general principle of operation of the device shall now be explained. 
The current flows into the lamp F in one direction and then in the other. 
This current flows, for example, in a first period through the power 
transistor T.sub.a which is then on, from the circuit Com.sub.a which is 
in the closed state: it has a null voltage between its terminals B1.sub.a 
and B2.sub.a, all the high voltage being at the terminals of the other 
circuit Com.sub.b. When the switching control circuit detects that there 
is sufficient current flowing in the transistor, it activates the open 
state of the circuit Com.sub.a, in turning its power transistor T.sub.a 
off: the current in the lamp then goes into the other circuit Com.sub.b 
through the parallel and reverse-mounted diode Db. The voltage at the 
terminals of this diode becomes negative with reference to the reference 
point B2.sub.b of the circuit. The control circuit CC.sub.b detects this 
voltage drop at the terminals of the diode and activates the ON state of 
the associated power transistor T.sub.b : the external resonant circuit 
will now cause a change in the direction of the current which could now go 
through the switch in the ON state, and so on and so forth. 
The two circuits Com.sub.a and Com.sub.b then work independently, each one 
detecting a voltage drop at its terminals to go into the closed (or ON) 
state and ascertaining that there is sufficient current flowing between 
its terminals to go into the open (or OFF) state. 
The criterion of current corresponds to the nominal power of the lamp to be 
controlled. Thus, a reference value of current is made to correspond with 
the nominal power of the lamp, and the current detection operation 
consists in measuring and comparing an integral of the current flowing 
into the transistor with a current reference value. 
For the turning on of the lamp, it will be recalled that the element 
conventionally used is the starting capacitor Cp which short-circuits the 
lamp. The over voltage on the starting capacitor prompts the breakdown of 
the gas in the lamp and all the current then flows through the lamp: it is 
the starting capacitor that is then short-circuited and the resonance 
circuit then comprises only the capacitor in series with the lamp and the 
inductor. 
FIG. 2 is a detailed diagram of the circuit Com.sub.a comprising the 
switching control circuit CC.sub.a and the power transistor T.sub.a 
according to the invention. It has three external connection terminals: 
two connection terminals B1.sub.a and B2.sub.a and a third decoupling 
terminal B3.sub.a of the logic supply V1. 
The power transistor T.sub.a is connected between the terminals B1.sub.a 
and B2.sub.a with a parallel and reverse-connected diode D.sub.a. Thus, 
the drain of the transistor and the cathode of the diode are connected to 
the terminal B1.sub.a and the source of the transistor and the anode of 
the diode are connected to the terminal B2.sub.a. 
The switching control circuit comprises a circuit for the detection of a 
voltage at the terminals of the diode and of the transistor that is 
greater than or lower than a voltage reference value Vref and a circuit to 
measure the current flowing into the transistor to determine a 
current-related surface area greater or lower than a current reference 
value Iref. 
The voltage detection circuit comprises a resistive divider 3 comprising, 
in the example, two resistive elements referenced R1 and R2 
series-connected between the first terminal B1.sub.a and the second 
terminal B2.sub.a. The resistive elements may be diffusions or a 
transistor in a state of saturation for example. This resistive divider is 
notably designed to reduce the voltage excursion between the two terminals 
for it may be 500 volts in the example, to reduce it to a logic level, for 
example in the range of 15 volts, that is acceptable by a logic circuit. 
The midpoint P between the two resistive elements is connected to an input 
of a voltage comparator 4 which, at another input, receives a reference 
voltage Vref. In the example, it is sought to detect a practically null 
voltage: the voltage reference is a voltage close to zero volts. 
The voltage comparator delivers, at output, a detection signal pertaining 
to a voltage greater than or lower than the voltage reference s1, which is 
used to control the gate of the power transistor T.sub.a. 
The current measurement circuit comprises a current bypass circuit 
comprising, in the example, two resistive charges. A first charge 5a is 
placed between the power transistor and the input of an integrator 6. 
Another resistive charge 5b is placed between the power transistor and the 
reference point B2.sub.a. 
Other bypass circuits may be envisaged. For example, since the power 
transistor is formed by millions of MOSFET cells, it is also possible to 
provide for a fourth terminal on this transistor to reroute the current by 
a few cells only: the ratio of the total current to the current rerouted 
in this way is indeed very precise. This fourth bypass terminal is then 
connected to a resistive charge in series with the integrator. 
The integrator 6 may be a simple RC lowpass filter. It may also, as shown 
in FIG. 2, make use of an operational amplifier mounted as an integrator 
with one input receiving the current rerouted by the charge 5 and one 
input connected to a reference ref6 which conventionally depends on the 
voltage excursion on the first input, as is well known to those skilled in 
the art. Finally, a capacitor is parallel-connected to this first input 
and the output of the amplifier. 
A comparator 7 receives the output of the integrator as well as a current 
reference value Iref, computed according to the nominal power of the lamp 
and as a function of the current rerouting charge. In one example, for a 
7-watt lamp, the current reference value may be in the range of a hundred 
milliamperes according to the rerouting charge. The output of the current 
comparator 7 gives the current detection signal s2 which is used to 
control the gate of the power transistor T.sub.a. 
The signals s1 and s2 are applied to a logic circuit that prepares the 
voltage command of the gate. We have seen that it is necessary to turn the 
transistor off when an integral of the current through the transistor 
exceeds a current reference value, this information being given by the 
signal s2, and to turn the transistor on upon the detection of a null 
voltage at the terminals of the diode (hence of the transistor), this 
information being given by the signal s1. 
In the example, the logic circuit comprises an RS type bistable flip-flop 
circuit with a one-setting input S, controlled by the signal for the 
detection of a voltage s1 and a zero-setting input controlled by the 
signal for the detection of current s2. In the preferred example of a 
MOSFET type N channel power transistor, it is necessary to have a voltage 
of about 15 volts on the gate to turn it on and about zero volts to turn 
it off. Thus, if a null voltage is detected, a logic voltage of the order 
of 15 volts is activated on the gate g.sub.a of the power transistor, and 
if a sufficient level of current is detected, a logic voltage of the order 
of zero volts is activated on the gate of the power transistor. 
As a protective measure, it is preferable to be able to turn the transistor 
off if there should be a positive voltage at the terminal B1.sub.a while 
the current detection output is no longer at one (namely, when the current 
flows into the other switch). For this purpose, it is provided that the 
logic circuit will furthermore comprise a logic gate 9, which is an AND 
type logic gate in the example, to force in this case the gate g of the 
transistor to zero volts, irrespectively of the current detection output. 
In the example, the logic gate receives the output Q of the bistable 
flip-flop circuit at input and the reverse voltage detection signal /s1. 
The logic gate may, for example, have an inverter input or an inverter may 
be provided, series-connected between the output of the comparator and the 
input of this logic gate 9. 
Finally, the logic circuit preferably has a starting circuit comprising a 
logic gate 10 and a random pulse generator 1. The logic gate 10 is, in the 
example, an OR gate receiving at input the detection signal pertaining to 
a voltage s1 and the output of the random pulse generator 11. This 
generator 11 delivers a pulse at the end of a certain random time after 
the voltage is turned on. The logic gate 10 delivers, at output, a signal 
s1' to control the one-setting input of the bistable flip-flop circuit 8. 
The purpose of this starting circuit is to be capable of enforcing the 
detection of voltage to the level corresponding to the detection of a null 
voltage when the pulse is sent, to oblige the switching control circuit to 
perform its control in the closed state (transistor on). The value, for a 
device using two circuits Com.sub.a and Com.sub.b according to the 
invention, is that one of the two is enforced into the closed state, the 
one for which the pulse will be first generated, to make the device start. 
If not, the voltage gets distributed between the two circuits Com.sub.a 
and Com.sub.b and the system does not start. 
However, when a generator has sent its pulse first, the other one has to be 
off. According to the invention, this is done simply by sending the output 
Q of the flip-flop to a zero-setting input R of the generator. Since a 
switching half-period is of the order of 10 microseconds, it is provided 
that the pulse will occur within a greater period of time. 
The random pulse generator may, for example, use a leakage current in a 
semiconductor junction, for which it is known that it varies from one 
integrated circuit to another, owing to the variations that are intrinsic 
to the manufacturing process and cannot be controlled, to charge a 
capacitor that delivers the random pulse. The leakage current may, for 
example, vary from one nanoampere to one milliampere. The occurrence of 
the pulse may thus take place randomly after 0.1 to 100 milliseconds 
following the turning on of the voltage. 
Finally, a logic supply device 12 is designed to generate a logic voltage 
V1 of the order of 15 volts to activate the different logic circuits 
(integrator, comparator, etc.) and give the gate voltage of the order of 
15 volts needed to turn the power transistor on. 
In one embodiment, the diode which is parallel-mounted on the power 
transistor and reverse-mounted is a parasitic diode of the transistor, 
hence one that is internal to its structure, and the control circuit 
therefore controls, firstly, the voltage at the terminals of the diode 
(hence of the transistor) and the current flowing in the transistor. 
FIG. 3 shows the voltage and current curves as a function of time for an 
alternation, for the control device of FIG. 1 using the circuits Com.sub.a 
and Com.sub.b shown in detail in FIG. 2. 
The operation starts from an open state of the circuit Com.sub.a (with the 
power transistor T.sub.a OFF): all the high voltage, and no current, is 
retrieved at the terminals of the transistor. Then, corresponding to the 
time when the other circuit Com.sub.b will go into the open state, the 
current of the lamp which must continue to flow somewhere, goes into the 
diode D.sub.a which is parallel-mounted on the transistor T.sub.a which is 
then reverse-biased: this corresponds to the negative part of the curve of 
the current in FIG. 3. The passage of the current into the diode causes 
the voltage at its terminals B1.sub.a and B2.sub.a to drop to a voltage 
close to zero (threshold voltage of the diode). This drop in voltage is 
detected by the detection circuit of the voltage which activates the ON 
state of the transistor: the circuit Com.sub.a goes to the closed state. 
At the same time, the other circuit Com.sub.b being in the open state, the 
oscillating circuit gets demagnetized: the negative current rises again 
gradually to zero. The current then changes its direction to become 
positive in the circuit Com.sub.a which is conducted by the power 
transistor T.sub.a. This current is measured by the current measuring 
circuit. When current has passed to a sufficient extent, corresponding to 
the nominal power of the lamp, the circuit for the measuring of the 
current detects this occurrence (s2) and activates the OFF state of the 
transistor: the circuit Com.sub.a goes to the open state and all (or 
almost all) the high voltage is retrieved at its terminals (except for the 
threshold voltage of the diode of the other circuit Com.sub.b which is 
negligible). 
According to the illustrative embodiment of the invention, all the voltage 
therefore is recovered at the terminals of either circuit Com.sub.a or 
Com.sub.b alternately and the alternations are controlled by the 
integration of the current through the transistor. 
Preferably, to integrate only the positive part of the current, the 
transistor is activated into the ON state only after a short known time 
that corresponds to the demagnetizing of the resonant circuit of the lamp. 
For this purpose, it is possible to use a monostable circuit at output of 
the comparator (not shown). And it is provided that the voltage detection 
signal also controls the start of the integration (not shown). 
In another embodiment of the present invention, the circuit comprising the 
power transistor and its switching control circuit according to the 
invention advantageously take the form of a small three-pin integrated 
circuit. Two of them are needed for a control device according to this 
embodiment of the invention. They are not differentiated in the device for 
they are completely independent and insulated from one another. Internally 
they perceive only the current that flows in the transistor and the 
voltage at the terminals. 
The only particular feature relates to the definition of the reference 
current which varies according to the nominal power of the low-pressure 
fluorescent lamp to be controlled (7, 12 or 18 watts, for example). 
Having thus described at least one illustrative embodiment of the 
invention, various alterations, modifications and improvements will 
readily occur to those skilled in the art. Such alterations, modifications 
and improvements are intended to be within the spirit and scope of the 
invention. Accordingly, the foregoing description is by way of example 
only and is not intended as limiting. The invention is limited only as 
defined in the following claims and the equivalents thereto.