QUICK. CONTROL FOR HIGH INTENSITY

HID lamps
may 2010
QUICK. CONTROL
FOR HIGH INTENSITY
High-Intensity Discharge (HID) lamps are becoming more and more
popular due their very high brightness, good colour rendering and
long lifetime. Tom Ribarich guides us through the development of a
ballast circuit for a HID lamp using a new control Ie
to
n lighting applications such as retail and
accent lighting, HID lamps are replacing
older halogen technology due to their
equivalent colour temperature and
brightness with four times less power
consumption. 150 W halogen lamps, for
example, are now replaceable with 35 W
HID lamps that produce the same light
output. These low-power HID lamps require
an electronic ballast to ignite, warm-up and
maintain a constant power through them
during steady state. This article describes
an off-line 35 W HID electronic ballast
circuit designed around the new IRS2573D
HID Control IC.
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HID lamp requirements
HID lamps are available in the form of
metal halide, mercury or sodium vapour.
These lamps are popular because they are
efficient and have a high-brightness output.
HID metal halide lamps are typically five
times more efficient than incandesents and
last 20 times longer. HID lamps produce
light using a technique similar to that used
in fluorescent lamps where a low-pressure
mercury vapour produces ultraviolet light
that excites a phosphor coating on the
tube. In the case of HID lamps, it is a highpressure 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 typical, > 20 KV if the
lamp is hot), current limitation during
warm-up, and constant power control
during running. It is important to have a
tight regulation of lamp power to minimise
lamp-to-Iamp colour and brightness
variations. Also, HID lamps are driven with a
low-frequency AC voltage «200 Hz typical)
to avoid mercury migration and to prevent
damage of the lamp due to acoustic
resonance. A typical Metal Halide 35 W
HID lamp has the following requirements:
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Warm-up
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Current
Limitation
Nominal Wattage (W):
Nominal Voltage (Vrms):
Nominal Current (Arms):
Warm-up Time (min):
Ignition Voltage (Vpk):
Constant Power
35
100
0.35
2.0
4000
Figure 1 shows the typical start-up profile
for HID lamps. Before ignition, the lamp is
open circuit. After the lamp ignites, the lamp
voltage drops quickly from the open-circuit
voltage to a very low value (20 V typical) due
Rectff~r
BoostPFC
to the low resistance of the lamp. This
causes the lamp current to increase to a
very high value and should therefore be
limited to a safe maximum level. As the lamp
warms up, the current decreases as the
voltage and power increase. Eventually the
lamp voltage reaches its nominal value
(100 V typical) and the power is regulated
to the correct level.
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continues on page 26 ....
Buck
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25
HID lamps
may 2010
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In order to satisfy the lamp requirements
and different operating modes, an electronic
ballast topology is needed that efficiently
converts the AC mains voltage to the
desired AC lamp voltage, ignites the lamp
and regulates lamp power.
HID ballast topology
A typical HID ballast block diagram [Figure 2)
includes electromagnetic EMI filtering to
block ballast generated noise, a bridge
rectifier to convert the AC mains voltage to
a full-wave rectified voltage, a boost PFC
stage for power factor correction and a
constant DC bus voltage, a step-down buck
converter for controlling the lamp current, a
full-bridge output stage for AC operation of
the lamp, and an ignition circuit for striking
the lamp. Control ICs are then used to
control the boost PFC stage and the
buck/full-bridge stages. This is presently one
of the standard approaches to powering HID
lamps with a low-frequency AC voltage.
The boost PFC stage runs in criticalconduction mode. During this mode, the
boost stage operates with a constant ontime and variable off-time resulting in a freerunning frequency across each rectified halfwave of the AC line cycle. The frequency
range is typically from 200 kHz near the
valley of the half-wave to 50 kHz at the peak.
The on-time is used to regulate the DC bus
to a constant level and the off-time is the
time it takes for the inductor current to
reach zero each switching cycle. The
triangular shaped inductor current is filtered
by the EMI filter to produce a sir ;oidal input
current at the AC mains input tor 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. The buck stage is
necessary to step-down the constant DC bus
voltage from the boost stage to the lower
lamp voltage across the full-bridge stage.
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26
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This particular buck circuit has the ability to
run in continuous or critical-conduction
operating modes, depending on the condition
of the load. The lamp voltage and current
are measured and multiplied together to
produce a lamp power measurement, which
is fed back to control the buck on-time.
During the lamp warm-up period [after
ignition) when the lamp voltage is very low
and the lamp current is very high, the lamp
current feedback will determine the buck ontime to limit the maximum lamp current.
During lamp steady state running, the power
feedback will then determine the buck ontime to control the lamp power. The
continuous-conduction mode allows the buck
circuit to supply more current to the lamp
during the warm-up without saturating the
buck inductor.
The full-bridge stage is necessary to
produce an AC lamp current and voltage
during running. The full-bridge typically
operates at 200 Hz with a 50% duty-cycle.
The full-bridge also contains a pulse
transformer circuit for producing 4 KV
pulses across the lamp necessary for
ignition.
The HID Control IC includes a complete
state diagram (Figure 3) to ignite and run
the lamp, as well as shutdown when fault
conditions occur. The IC initially starts in
under-voltage lock-out [UVLo) mode when
the supply voltage to the IC is below the turnon threshold. When VCC increases high
enough, the IC exits UVLo mode and enters
ignition mode, and the on/off ignition timer
is activated to deliver high voltage pulses to
the lamp for ignition. If the lamp ignites
successfully, the IC transitions into run mode
and the lamp is regulated to a constant
power level. If fault conditions occur such as
open/short circuit or the lamp fails to ignite
or warm-up, then the IC will enter fault mode
and shutdown safely before any damage
occurs to the ballast.
The complete buck and full-bridge control
circuit schematic is shown in Figure 4. The
circuit is designed around the IRS2573D
HID Control IC from International Rectifier.
The IRS2573D includes control for the buck
stage, the full-bridge, lamp current and
voltage sensing, and feedback loops for
controlling lamp current and lamp power.
The IC includes an integrated high-side driver
for the buck gate drive [SUCK pin) and highside buck cycle-by-cycle over-current
protection (CS pin). The on-time ofthe buck
switch is controlled by the lamp power
control loop [PCOMP pin) or lamp current
limitation loop [ICOMP pin). The off-time of
the buck switch is controlled by the inductor
current zero-crossing detection input (ZX
pin) during critical-conduction mode, or, by
the off-time timing input [TOFF pin) for
continuous-conduction mode. The IC also
includes a fully-integrated 600 V high- and
low-side full-bridge driver. The operating
frequency of the full-bridge is con~rolled with
an external timing pin [CT pin). The IC
provides lamp power control by sensing the
lamp voltage and current [VSENSE and
ISENSE pins) and then multiplying them
together internally to generate the lamp
power measurement. The ignition control is
performed using an ignition timing output
(IGN pin) that drives an external ignition
MOSFET [MIGN) on and off to enable the
ignition circuit of the lamp [DIGN, CIGN,
TIGN). The ignition timer is programmed
externally (TIGN pin) to set the ignition circuit
on and off times. Finally, the IC includes a
programmable fault timer [TCLK pin) for
programming the allowable fault duration
times before shutting the IC off safely. Such
fault conditions include failure of the lamp to
ignite, failure of the lamp to warm-up, lamp
end-of-life, and open/short circuit of the
output.
may 2010
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Experimental results
(150 Hz] AC lamp current and voltage.
The experimental results are shown in
Figure 5. Figure 5A shows the buck
inductor current (ILBUCK, upper trace] and
the voltage across the buck diode (VDBUCK,
lower trace] during steady state running
conditions. The buck is working in criticalconduction mode at a switching frequency
of about 50 kHz. The on-time is controlled
by the constant power feedback loop.
Figure 5B shows each half-bridge
switching node (VS1, VS2, lower trace) and
lamp current (upper trace) during steady
state running conditions. The full-bridge
produces the necessary low-frequency
HID lighting is a growing market with many
applications. Retail lighting is especially
attractive due to the long life-time, highbrightness and energy savings that these
lamps offer. The lamp requirements are
critical and the ballast requirements are
challenging, making the design of the
electronic ballast a difficult task. The design
presented in this article is a low-risk
approach due to the standard 3-stage
topology used and, it contains a highlyintegrated control IC to greatly simplify the
Conclusions
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circuit. This solution also allows for
scalability of design so that the same basic
circuit can be used as a platform to realise
a family of electronic ballasts for many lamp
types and power levels. The new IRS2573D
control IC contains the complete HID
system-in-a-chip, including lamp control.
lamp ignition, and all fault protections,
making this solution very reliable and ideal
for designers to accelerate their products
into the marketplace.
TOM RIBARICH is Director, Lighting
Systems, International Rectifier