DUAL-MODE BUCK
AND FULL-BRIDGE IC
CONTROLS HID LAMPS
H
igh-inlensily discharge (HID)
lamps boost high efficacy,good
color rendering, and a long
lifetime. Electronic ballasts are
used 10 control HID lamps, bullypically
they're complex due 10 the extensive lamp
requirements thl?,' must fulfill.
Designers, howl?ler,can lake advantage of a novel dual-mode buck and
full-bridge control CIa control HID lamps,
which have unique electrical characteristics
and require a careful and specific control
sokJlion.A fundamental understanding of
these components will help the designer
gain further insighllo the nature of HID
lamps and the circuits used 10 control them.
Available in the form of metal halide, mercury, or sodium I,{Jpor, HID lamps ore popular
because they ore efficient and h<Jv'e a tigh
brightness output. HID melol-lDlide lamps ore
generally ti'R limes more efficient than incondescenllomps and 105120 limes longer.
HID lamps produce light using a
technique similar to that used in nuorescent
lamps, where a low-pressure mercury vapor
produces ultraviolet light that excites a
phosphor coating on the tube. In the case
of HID lamps, it's a high-pressure gas, the
distance between the electrodes is very
short, and the light is produced directly
without the need for the phosphor.
HID lamps require a high voltage for ignition (3 to 4 kV Iypical,greater than 20 kV
if the lamp is hot),currentlimitation during
warm-up,and constant power control during running. Tight regulation of lamp power
is important for miniTlizing klmp-to-Iamp
color and brightness variations. Also, HID
lamps are driven with a low-frequency ac
voltage (less than 200 Hz typical) to avoid
mercury migratbn and to prevent lamp
damage due to acoush:: resonance.
Atypical metal halide 70W HID lamp
features a nominal v,{IIIage 0NJ of 70 W,
a v,{Irm-up time of 1 to 2 minutes, and a
cold-start ignition voltage (Vpk) of 4 kV.
Before ignition, the lamp is open-circuit (Fig.
I). After the lamp ignites, the lamp voltage
drops quickly from the open<:ircuit voltage
to a very low value (20 Vtypical) due to the
low resistance of the lamp.
This drop causes the lamp current to
increase to a very high value, so it should
be limited toa safe maximum 1~e1.As the
lamp warms up, the current decreases as
the voltage and power increase. The lamp
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1. During a t}p1cal lflD lamp startup, the lamp
Is op!Il·drcult b(fore Ignition AIl(!' the lamp
Ignites, \"oltage drops quickly but e\I?Iltually
reac~ Its nominal value.
voltage eventually reaches ns nominal
value (100 Vtypical),and the power is
regulata::lto the correctll?'el.
To satisfy the lamp reqLirements and
different operating modes, the electronic ballasttopdogy must efficiently convert the ac
mains voRo;je to the desired 0::: lanp voltage,
ignne the lamp, and regulate lamp p:JWer.
HID BallastTopology
Atypical HID ballast includes electromagnetic interference (EMI) fiRering to
block ballast-generata::l noise, a bridge
rectifier to convert the ac mains voltage to
a full-wave rectified voltage, a boost powerfactor correction (PFC) stage for PFC and a
constant dc bus voltage, a step-down buck
converter for controlling the lamp current, a
full-bridge output stage for ac opellltion of
the lamp,and an ignition circuit for striking
the lamp (Fig. 2).This is one of the standard
approaches for powering HID lamps with a
low-frequency ac voltage.
The boost PFC stage typically runs
in crih::al-conduction mode, but can be
controlled with a continuous-conduction
mode for higher powers (greater than 200
W) when the peak boost inductor currents
using critical<:onduction mode become
too high. During critical-conduction mode,
the boost stage operates with a constant
on-time and variable off-time, resulting in
a free-running frequenGY across each halfWfNe of the ac line cycle.
The frequenGY range is typically from
200 kHz near the ac line zero-crossings
to 50 kHz at the peak of the ac line. The
on-time is used to regulate the dc bus to
a constant level. The off-time is the time it
takes for the inductor current to reach zero
each switching cycle. The EMI filter then
2. Atypcal HID ballast Include; EMI fJlterlng, a
bridge M:tifier, a boost PFe &age, a Step-doY,l1
buck oon~, a full-bridge outPJt stage, and
an Ignition circuit
filters the triangular inductor current to
produce a sinusoidal input current at the ac
mains input for high power factor and low
harmonic distortion.
The buck control circuit is the main
control circuit of the ballast, as it is used to
control the lamp current (Fig. 3). The bLK:k
stage is needed to step-<lo'Wflthe constant
dc bus valtage from the boost stage to the
lower lamp valtage at the fUll-bridge stage.
This particular circuit can run in continuous or critical<:onduction operating
modes,based on the condition of the load.
The lamp output current is measured
in the full-bridge stage and fed back to the
buck circuit to control the buck on-time. The
lamp voRage and current are multiplied together to produce a lamp power measurement, which is also fed back to control the
buck on-time.
During the lamp warm-up perbd (after
ignition) when the lamp voltage is very
low and the lamp current is very high, the
lamp current feedback will determine the
buck on-time to limit the maximum lamp
current. During lamp steady-state running,
the power feedback will then determine the
buck on-time to control the lamp power.
The off-time is determined by the zerocrossing of the buck inductor current during
crih::al-conduction mode or by a maximum
off-time limit during continuous<:onduction
mode.The continuous-conduction mode