Shedding Light on HID Ballast Control

Shedding Light on
HID Ballast Control
By Tom Ribarich, Director, Lighting IC Design Center,
International Rectifier, El Segundo, Calif.
Aided by a new full-bridge controller, a popular
electronic HID ballast topology manages lamp
operation, while enhancing lamp safety.
E
lectronic ballasts for fluorescent lamps have already
overtaken magnetic ballasts in both volume and
value. The same trend is now taking place in the
high-intensity discharge (HID) lamp ballast market. HID lamps deliver a high-brightness output
and typically serve indoor applications such as retail accent
or ceiling lighting, and outdoor applications such as street
lighting. New applications including automotive headlamps,
front projection for meeting rooms and rear projection (DLP
TVs) are also now using HID ballasts.
HID lamps have unique electrical characteristics and require a careful and specific control method. There are basic
HID lamp requirements the designer must consider, as well
as key protection requirements necessary for safety and to
prevent destruction of the lamp or ballast. Let us look at the
various methods of controlling each ballast subcircuit, with
emphasis on the full-bridge output stage. A fundamental
understanding of these concepts will help the designer gain
further insight to the nature of HID lamps and the circuits
that control them.
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Fig. 1. HID lamp ignition, warmup and running modes have distinct
electrical characteristics that can be monitored by the ballast circuit.
lamps also require current limitation during warmup and
constant power control while running. It is important to
tightly regulate lamp power with respect to lamp voltage
to minimize lamp-to-lamp color and brightness variations.
Also, HID lamps use an ac-voltage drive to avoid mercury
migration. They operate at a low frequency, typically less
than 200 Hz, to prevent lamp damage or explosion due to
acoustic resonance. A typical metal-halide 70-W HID lamp
has the following requirements: a nominal wattage of 70 W;
a warmup time of 1 min to 2 min; and a cold-start ignition
voltage of 4000 VPK.
Fig. 1 shows the typical startup profile for HID lamps.
Before ignition, the lamp is an open circuit. After the lamp
ignites, the lamp voltage drops quickly from the open-circuit voltage to a very low value—typically 20 V—due to the
low resistance of the lamp. If otherwise unimpeded, this
characteristic causes the lamp current to increase to a high
value; therefore, the ballast must limit the lamp current 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, typically 100 V, and
the ballast regulates the power to the correct level.
HID Lamp Requirements
HID lamps are available in the form of metal halide,
mercury or sodium vapor. These lamps are popular because
they are efficient and have a high-brightness output. HID
metal-halide lamps are typically five times as efficient as
incandescent lamps and last 20 times longer. In the case of
sodium vapor, they are twice as efficient as normal fluorescent bulbs. HID lamps produce light using a technique
similar to that in fluorescent lamps in which a low-pressure
mercury vapor produces ultraviolet light that excites a phosphor coating on the tube. In the case of HID lamps, the gas
is under high pressure, the distance between the electrodes
is short and the lamp produces light directly without the
need for a phosphor.
HID lamps require a high voltage for ignition, typically
3 kV to 4 kV, but more than 20 kV if the lamp is hot. The
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Fig. 2. This block diagram shows the eight functions performed by a typical HID ballast.
is a standard topology that many power-supply and ballast
applications use for power levels below 100 W. The PFC
stage maintains a sinusoidal current that is in phase with
the ac line input (to attain a high power factor and low total
harmonic distortion) and regulates the dc bus output to a
constant level, typically 400 Vdc. When the PFC switch (M1)
turns on the current, the boost inductor (LBOOST) ramps up
linearly to a peak value. Switch M1 then turns off and the
inductor current discharges back down to zero. When the
current reaches zero, M1 turns on again and the cycle repeats
itself. The amount of current necessary to keep the dc bus
regulated at a constant level for a given load power determines the on-time. Since the input voltage to the PFC stage
is sinusoidal, the resulting current will be triangular within
each switching cycle, with the peaks following a sinusoidal
envelope (Fig. 3).
The on-time will be approximately constant and the offtime will vary depending on how high the peak is for each
switching cycle, resulting in a free-running frequency system.
When the EMI filter at the input smoothes these triangularshaped currents, the result is a sinusoidal current that is in
phase with the ac input voltage (the dashed line in Fig. 3).
The buck stage controls the amount of current that the
ballast delivers to the lamp load while warming up and running. Immediately after the lamp ignites, the lamp resistance
drops and the lamp passes a large current. The buck controller should supply adequate current to keep the lamp from
extinguishing, but the current limiter must prevent the buck
inductor from saturating while the lamp is warming up.
While the lamp is running, the controller manages the
buck’s on-time to keep the lamp power constant. Current
flows from the dc bus through the buck inductor to the load
when the buck switch (M2) turns on. During the on-time,
the current in the buck inductor (LBUCK) increases linearly
as it supplies load current.
When the on-time ends, the buck switch turns off and
load current continues to flow in the buck diode (DBUCK) and
the buck inductor. The current through the buck inductor
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Fig. 3. The PFC stage of the HID ballast circuit defines a sinusoidal peak
current envelope (solid line) that contains the triangular PFC inductor
current and smoothed sinusoidal line input current (dashed line) over
one-half cycle of the line input voltage.
To satisfy the lamp requirements and different operating modes, an electronic-ballast topology must efficiently
convert the ac mains voltage to the appropriate ac lamp
voltage, ignite the lamp and regulate power.
HID Ballast Topology
A typical HID ballast (Fig. 2) performs eight basic functions. An electromagnetic interference (EMI) filter blocks
ballast-generated noise. A full-wave rectifier provides the
high-voltage bus power. A power-factor-correction (PFC)
block ensures sinusoidal input current. A buck converter
controls the lamp current. A full-bridge output stage provides
the ac lamp drive. An ignition circuit strikes the lamp. Control
circuitry manages each stage. Finally, protection circuitry
safely deactivates the ballast in the event of a lamp- or ballast-fault condition. Currently, this is one of the most popular
approaches to powering HID lamps with a low-frequency
ac voltage.
The PFC stage is a boost converter that operates in critical-conduction mode with a free-running frequency. This
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Power Electronics Technology October 2006
HID BALLASTS
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Fig. 4. The HID ignition circuit produces high-voltage pulses that are
discontinued once the ballast circuit detects the lamp has ignited.
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Fig. 5. The IRS2453D full-bridge IC inserts dead time to protect the
switching devices in the full-bridge circuit that drives the HID lamp.
decreases linearly for the duration of the cycle. The controller adjusts the on-time depending on how much current
the load needs to regulate the power. The time it takes for
the buck inductor current to discharge to zero determines
the off-time. A standard PWM circuit can control the buck
stage and a high-voltage level-shift IC (such as the IR2117)
boosts the gate-drive signal up to the buck switch’s gate-tosource potential.
The output stage includes a full-bridge circuit for driving
the lamp with a low-frequency square-wave voltage and an
ignition circuit for striking the lamp. The top of the fullbridge circuit connects to the buck output voltage and the
two half-bridge midpoints oscillate 180 degrees out of phase
from each other to produce the necessary ac voltage.
During the ignition phase, the lamp is an open circuit and
the buck output voltage is limited to a maximum value. The
ignition circuit comprises a diac (DIGN), transformer (TIGN),
capacitor (CIGN), resistor (RIGN) and switch (MIGN). When
the ignition controller turns on switch MIGN, capacitor CIGN
discharges through resistor RIGN.
When the voltage across the diac reaches the diac threshold voltage (Fig. 4), the diac turns on and a current pulse
flows from the buck output, through the primary winding
of the ignition transformer (TIGN) and into capacitor CIGN.
Fault condition
Ballast action
Unprotected outcome
AC mains interrupt or brownout
Reset ballast and restrike lamp
Lamp can extinguish and remain off
Lamp does not ignite
Deactivate ballast if lamp does not
ignite after a maximum time period
High-voltage safety hazard at output
terminals
Lamp does not warm up
Deactivate ballast if lamp does not
reach nominal power after a maximum
time period
High current stress on buck and full-bridge
stages can cause component failures.
Lamp is not connected or has a broken
connection
Deactivate ballast after a maximum time Ballast will try to ignite lamp
period
High-voltage safety hazard at output
terminals
Short circuit at ballast output terminals
Deactivate ballast after a maximum time High current stress on buck and full-bridge
period
stages can cause component failures
Unstable lamp or end of life
Deactivate ballast after maximum time
period
Lamp can extinguish, flicker, conduct in one
direction, or encounter color or brightness
shifting
Unstable lamp can cause disruption or
failure of ballast circuits
Table. Summary of fault condtions for ballast and lamp.
Power Electronics Technology October 2006
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HID BALLASTS
This arrangement generates a high-voltage pulse on the
specific methods a ballast design implements to detect each
secondary to ignite the lamp. The capacitor CIGN charges
fault, the protection circuits should be robust and reliable to
up until the diac turns off, and CIGN then discharges down
ensure proper safety in the ballast application and to prevent
through resistor RIGN until the diac voltage again reaches the
catastrophic field failures should fault conditions occur.
device’s threshold and another ignition pulse occurs. When
New applications and lamp types are continuously
the lamp ignites, the buck output voltage decreases quickly to
emerging in the marketplace and each includes its own
the lamp voltage as the converter provides the lamp current.
unique design challenges. International Rectifier and other
The ignition controller disables the pulses after the lamp has
manufacturers in the industry will continue to improve and
ignited by turning switch MIGN off.
simplify control methods and ICs in the field of HID lightAn IRS2453D full-bridge-control IC manages the
ing. Designers will need to stay on top of the rapid changes
lamp-drive bridge. This high-voltage IC contains all of the
that are sure to take place in the coming years.
PETech
necessary circuitry for the full-bridge
oscillator and high- and low-side gate
drivers. The IC also contains a nonlatched and latched shutdown pin as
well as integrated bootstrap diodes
for the high-side driver supplies.
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The timing diagram shows the CT
oscillator timing pin, the gate-driver
outputs, and the resulting midpoint
and lamp voltages (Fig. 5). The IC also
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includes an internal 1.5-µs dead time
between the low-side (LO) and highside (HO) gate-drive outputs. This
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Protection Requirements
The HID ballast should include
specific protection circuits to detect
various lamp- and ballast-fault conditions and safely shutdown or reset the
ballast. These fault conditions include
ac-mains interrupt or brownout, lamp
ignition failure, lamp warmup failure,
lamp open circuit, lamp short circuit
and lamp end-of-life. A summary of
these conditions appear in the table
along with the proper ballast response
to each fault and the possible outcome
if the ballast does not protect against
the fault.
Voltage and current signals within
the various stages can serve as detection points to realize the protection
circuitry. The ac line or dc bus voltages can reset the ballast if a brownout
condition occurs. Timers are typically
necessary to deactivate the ballast after
a predetermined time period should
the lamp fail to ignite or warmup. A
lamp voltage or power monitor can
detect if the lamp is unstable or is
reaching end of life. Regardless of the
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