The present invention relates to an electronic circuit for the gradual start-up of electric loads, such as halogen lamps.  More particularly, but not exclusively, the invention relates to an electronic circuit comprising a power device having an output terminal connected to the electric load and at least one control terminal receiving a predetermined driving current value. 
In the following description reference will be made to an electric load represented by a halogen lamp without limitation to other similar types of loads.  For almost all the applications involving electric loads there is the need for reducing the stress applied to the driving circuit during the switching phases of the power device. 
The start-up phase is considered a stress condition for halogen lamp driving.  Indeed, the start-up phase is comparable to a short circuit, with the current in the driving circuit and in the load reaching high values.  This condition ends when the lamp impedance changes because of the internal filament heating as this causes a reduction of the load current to the nominal value. 
The starting condition in which the load current is considerably high deteriorates the lamp filament and adversely influences lamp life.  It is therefore desired to limit the number of high-current oscillations that the circuit and the load sustain without excessively varying the lamp trigger time.  The lower the current flowing in the load, the slower the load heats, and, consequently, the slower the system ends the start-up phase. 
An excessive current reduction in the start-up phase would result in a lamp start-up delay perceivable by the human eye.  For example, a time period of 300-400 ms is generally considered as an optimum time for the lamp to reach the steady state condition. 
FIG.  1 is a schematic diagram of the structure of an AC/AC converter used for halogen lamp driving in accordance with the prior art.  Differently from what happens with fluorescent lamps, the circuit 1 of FIG.  1 is powered by an external AC voltage source network, rectified at double half-wave.  A diac 2 enables the converter circuit during each supply cycle. 
The circuit 1 comprises a power device 3 in each portion of a half-bridge structure including a pair of driving elements.  More particularly, a high side driver component 4 and a low side driver component 5 are connected in series between a high supply voltage reference and a ground GND.  The interconnection node X between the components 4 and 5 is connected to a halogen lamp 6.  A first winding 7 is provided between the node X and the high side component 4, while a second winding 8 is provided between the node X and the second low side component 5. 
The current Iload flowing in the lamp 6 is alternately switched, preferably at a frequency of 30 to 50 KHz, by the half-bridge branches.  The high supply voltage is derived from the alternating current (AC) external supply through the diac 2.  Several RC circuits are provided between the high supply voltage and ground to obtain voltage values to be applied to the low side component 5 or to the high side component 4. 
For these applications a circuit 9 shown in FIG.  2 is typically used.  This circuit serves to implement a gradual start-up, generally called a xe2x80x9csoft start-upxe2x80x9d.  The circuit 9 has a first terminal connected to the voltage supply Valim, produced inside circuit 1, and a second terminal connected to the node X.
This circuit 9 comprises a power bipolar transistor Q1 having collector and emitter terminals coupled to the second terminal and to ground respectively.  A sensing resistor Rsense is provided between the emitter and ground for measuring the current Ie flowing through the conduction terminals. 
The base terminal B1 of the transistor Q1 is coupled to the first supply terminal by a diac D and a resistance R3.  A second bipolar transistor Q2 has its conduction terminals, that is, the collector and emitter terminals, connected respectively to the transistor base B1 by the diac D and to ground.  A capacitor Cd is connected in parallel between the driving terminal and conduction terminal of the transistor Q2. 
A resistance R2 is provided between the base B2 of the second transistor Q2 and ground.  An electrolytic capacitance C1 is included in a first circuit portion comprising the resistance R2 and an additional resistance R1 having a terminal connected to the base B2.  The capacitance C1 is inserted also in a second circuit portion comprising the resistor Rsense and a diode D1. 
The electrolytic capacitance C1 is charged when the voltage drop Rsense*Ie is higher than the voltage sum Vbed1+VC1 and drives the transistor Q1.  The time constant generated by the capacitance C1 and the resistance R1 has a high value and ensures that the transistor Q1 is kept in the on state for several half waves of the supply voltage waveform Valim. 
The transistor Q1 performs the function of draining part of the current which would flow, though the resistance R3, on the capacitor Cd, thus slowing the corresponding charge and delaying the start of the diac 2.  This causes a shift of the instant in which, in the half wave of the supply voltage, the circuit 9 starts oscillating.  Because of the gradual impedance variation inside the lamp, the currents become lower and lower and the transistor Q1 will have less base current available if the capacitance C1 is charged at a lower value. 
Consequently, the diac 2 will be delayed by a lower time than the previous half wave.  Therefore, the circuit 9 will keep on operating, but with a decreasing impact, until the current switched in the lower branch reaches the steady state value. 
In view of the foregoing background, it is therefore an object of the present invention to provide an electronic circuit for the gradual start-up of electric loads, in particular halogen lamps, having relatively simple structural and functional characteristics and allowing the start-up phase to be driven in a rapid, but also gradual way.  This avoids or reduces any stress on the halogen lamp filament and on the driving circuitry, overcoming the shortcomings of prior art approaches. 
The present invention reduces the current supplied alternately by the two branches of the power device at an appropriate value.  Such an appropriate value should generally be twice the nominal value, for example. 
By checking the maximum value of the load current, besides protecting the lamp filament, it is also possible to influence the corresponding heating dynamics.  The circuit according to the invention drives the start-up phase duration in a manner directly depending on the selected limitation value. 
One embodiment of the invention relates to an electronic circuit for the gradual start-up of electric loads, particularly halogen lamps.  The electronic circuit may comprise a power device having an output terminal connected to the electric load and having at least one control terminal receiving a predetermined driving current value, and a comparator having a first input terminal coupled to the power device output and a second input terminal kept at a reference potential.  In addition, the circuit may include a controlled switch driven by the output of the comparator and inserted between the control terminal of the power device and a voltage reference to be driven according to the value of the reference potential, thus adjusting the start-up phase of the power device.