Electronic ballasts are used as a replacement for
magnetic ballasts to supply power to fluorescent
tubes (including compact fluorescent lamps) and
discharge lamps. They also provide the “starter”
function and do not need any compensation
capacitor. They were first introduced in the
middle of the 1980s.
Principle and characteristics
The principle of the electronic ballast (see fig. 9 )
consists of supplying the lamp arc via an
electronic device that generates a rectangular
form AC voltage.
A distinction is made between low-frequency or
hybrid devices, with a frequency between 50 and
500 Hz, and high-frequency devices with a
frequency between 20 and 60 kHz.
Supplying the arc with a high-frequency voltage
can totally eliminate the flicker phenomenon and
strobe effects. The electronic ballast is totally
silent.
During the preheating period of a discharge
lamp, this ballast supplies the lamp with
increasing voltage, imposing an almost constant
current. In steady state, it regulates the voltage
applied to the lamp independently of any
fluctuations in the line voltage.
Since the arc is supplied in optimum voltage
conditions, this results in energy savings of 5 to
10% and increased lamp service life. Moreover,
: electronic ballast.
the efficiency of the electronic ballast can
exceed 93%, whereas the average efficiency of
a magnetic device is only 85%. The power factor
is high (> 0.9).
The electronic ballast is also used to provide the
light dimming function. Varying the frequency in
fact varies the current magnitude in the arc and
hence the luminous intensity.
Layout
An electronic ballast essentially consists of a
rectifier stage (with Power Factor Correction -
PFC - if necessary), a smoothing capacitor for
: simplified schematic of a lamp supplied by an electronic ballast.
the rectified voltage, and a half-bridge inverter
stage . It can also be supplied with
direct current
constrains
The main constraint that electronic ballasts bring
to line supplies is the high inrush current on
switch-on linked to the initial load of the
smoothing capacitors .In reality, due to the wiring impedances, the
inrush current for an assembly of lamps is much
lower than these values, in the order of 5 to 10 In
for less than 5 ms.
Unlike magnetic ballasts, this inrush current is
not accompanied by an overvoltage.
Harmonic currents
For ballasts associated with high-power
discharge lamps, the current drawn from the line
supply has a low total harmonic distortion
(< 20% in general and < 10% for the most
sophisticated devices). Conversely, devices
associated with low-power lamps, in particular
compact fluorescent lamps, draw a very distorted
current . The total harmonic
distortion can be as high as 150%. In these
conditions, the rms current drawn from the line
supply equals 1.8 times the current
corresponding to the lamp active power, which
corresponds to a power factor of 0.55.
In order to balance the load between the
different phases, lighting circuits are usually
connected between phases and neutral in a
balanced way. In these conditions, the high level
of third harmonic and harmonics that are
multiples of 3 can cause an overload of the
neutral conductor. The least favorable situation
leads to a neutral current which may reach
etimes the current in each phase. For more
information, read Cahier Technique no. 202
“The singularities of the third harmonic”.
Harmonic emission limits for lighting systems are
set by standard IEC 61000-3-2. For example, for
power devices above 25 W, the percentage of
third harmonic should be less than 30% of the
fundamental current.
Leakage currents
Electronic ballasts usually have capacitors
placed between the power supply conductors
and the earth. These interference-suppressing
capacitors are responsible for the circulation of a
permanent leakage current in the order of 0.5 to
1 mA per ballast. This therefore results in a limit
being placed on the number of ballasts that can
be supplied by a Residual Current Differential
Safety Device (RCD) (See Cahier Technique
no. 114).
At switch-on, the initial load of these capacitors
can also cause the circulation of a current peak
whose magnitude can reach several amps for
10 μs. This current peak may cause unwanted
tripping of unsuitable devices.
High-frequency emissions
Electronic ballasts are responsible for highfrequency
conducted and radiated emissions.
The very steep rising edges applied to the
ballast output conductors cause current pulses
circulating in the stray capacities to earth . As a result, stray currents circulate in
the earth conductor and the power supply
conductors. Due to the high frequency of these
currents, there is also electromagnetic radiation.
To limit these HF emissions, the lamp should be
placed in the immediate proximity of the ballast,
thus reducing the length of the most strongly
radiating conductors.
To avoid these conducted and radiated
emissions disturbing certain sensitive systems
(power line or radio wave communication
devices), interference-suppressing filters are
incorporated in the ballasts.
Conformity with standard EN55015 requires
emission limits in the 9 kHz - 30 MHz band.
Light dimmers for electronic ballasts
The use of electronic ballasts makes it possible
to vary the brightness of fluorescent tubes.
There are a number of possibilities, depending
on the ballast technology:
Ballast powered by a dimmer switch that varies
the voltage by phase angle. The current supplied
to the tube is a function of the voltage applied to
the ballast entry point.
signal. The ballast then supplies the tube with a
variable-frequency voltage which can vary the
current and hence the level of brightness
produced. This is the most commonly used
solution .
Ballast controlled by a digital control signal.
The use of dimmers can also save energy by
reducing the amount of light at certain times,
depending on occupation of the premises.
Electronic ballasts are incompatible with timers
with a switch-off warning.
Comment: If the electronic ballast is powered by
an electronic switch, there is a risk of intermittent
ignition of the fluorescent tubes. In fact, a
capacitor (0.1 to 0.2 μF) is usually placed in
parallel on the switch to protect it from transient
overvoltages. As a result, a leakage current
occurs which can trigger ignition unintentionally.
The use of a pre-charging circuit, which can shift
the leakage current, is compulsory.
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