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Shedding Light on

HID Ballast ControlAided by a new full-bridge controller, a popularelectronic HID ballast topology manages lampoperation, while enhancing lamp safety.

By Tom Ribarich, Director, Lighting IC Design Center,International Rectier, El Segundo, Calif.

Electronic ballasts for uorescent lamps have already overtaken magnetic ballasts in both volume andvalue. The same trend is now taking place in thehigh-intensity discharge (HID) lamp ballast mar-ket. HID lamps deliver a high-brightness output

and typically serve indoor applications such as retail accentor ceiling lighting, and outdoor applications such as streetlighting. New applications including automotive headlamps,front projection for meeting rooms and rear projection (DLPTVs) are also now using HID ballasts.

HID lamps have unique electrical characteristics and re-quire a careful and specic control method. There are basicHID lamp requirements the designer must consider, as wellas key protection requirements necessary for safety and toprevent destruction of the lamp or ballast. Let us look at thevarious methods of controlling each ballast subcircuit, withemphasis on the full-bridge output stage. A fundamentalunderstanding of these concepts will help the designer gainfurther insight to the nature of HID lamps and the circuitsthat control them.

HID Lamp Requirements

HID lamps are available in the form of metal halide,mercury or sodium vapor. These lamps are popular becausethey are efcient and have a high-brightness output. HIDmetal-halide lamps are typically ve times as efcient asincandescent lamps and last 20 times longer. In the case of sodium vapor, they are twice as efcient as normal uo-rescent bulbs. HID lamps produce light using a techniquesimilar to that in uorescent lamps in which a low-pressuremercury vapor produces ultraviolet light that excites a phos-phor coating on the tube. In the case of HID lamps, the gasis under high pressure, the distance between the electrodesis 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

lamps also require current limitation during warmup andconstant power control while running. It is important totightly regulate lamp power with respect to lamp voltageto 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 lessthan 200 Hz, to prevent lamp damage or explosion due to

acoustic resonance. A typical metal-halide 70-W HID lamphas the following requirements: a nominal wattage of 70 W;a warmup time of 1 min to 2 min; and a cold-start ignitionvoltage of 4000 VPK.

Fig. 1 shows the typical startup prole for HID lamps.Before ignition, the lamp is an open circuit. After the lampignites, the lamp voltage drops quickly from the open-cir-cuit voltage to a very low valuetypically 20 Vdue to thelow resistance of the lamp. If otherwise unimpeded, thischaracteristic causes the lamp current to increase to a highvalue; therefore, the ballast must limit the lamp current toa safe maximum level. As the lamp warms up, the current

decreases as the voltage and power increase. Eventually, thelamp voltage reaches its nominal value, typically 100 V, andthe ballast regulates the power to the correct level.

Fig. 1.HID lamp ignition, warmup and running modes have distinct electrical characteristics that can be monitored by the ballast circuit.

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To satisfy the lamp requirements and different operat-ing modes, an electronic-ballast topology must efciently convert the ac mains voltage to the appropriate ac lampvoltage, ignite the lamp and regulate power.

HID Ballast TopologyA typical HID ballast ( Fig. 2) performs eight basic func-tions. An electromagnetic interference (EMI) lter blocksballast-generated noise. A full-wave rectier provides thehigh-voltage bus power. A power-factor-correction (PFC)block ensures sinusoidal input current. A buck convertercontrols the lamp current. A full-bridge output stage providesthe ac lamp drive. An ignition circuit strikes the lamp. Controlcircuitry manages each stage. Finally, protection circuitry safely deactivates the ballast in the event of a lamp- or bal-last-fault condition. Currently, this is one of the most popularapproaches to powering HID lamps with a low-frequency

ac voltage.The PFC stage is a boost converter that operates in criti-cal-conduction mode with a free-running frequency. This

is a standard topology that many power-supply and ballastapplications use for power levels below 100 W. The PFCstage maintains a sinusoidal current that is in phase withthe ac line input (to attain a high power factor and low totalharmonic distortion) and regulates the dc bus output to aconstant level, typically 400 Vdc. When the PFC switch (M1)turns on the current, the boost inductor (L BOOST) ramps uplinearly to a peak value. Switch M1 then turns off and theinductor current discharges back down to zero. When thecurrent reaches zero, M1 turns on again and the cycle repeatsitself. The amount of current necessary to keep the dc busregulated at a constant level for a given load power deter-mines the on-time. Since the input voltage to the PFC stageis sinusoidal, the resulting current will be triangular withineach switching cycle, with the peaks following a sinusoidalenvelope ( Fig. 3).

The on-time will be approximately constant and the off-time will vary depending on how high the peak is for eachswitching cycle, resulting in a free-running frequency system.When the EMI lter at the input smoothes these triangular-shaped currents, the result is a sinusoidal current that is inphase 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 run-ning. Immediately after the lamp ignites, the lamp resistancedrops and the lamp passes a large current. The buck control-ler should supply adequate current to keep the lamp fromextinguishing, 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 thebucks on-time to keep the lamp power constant. Currentows from the dc bus through the buck inductor to the loadwhen the buck switch (M2) turns on. During the on-time,the current in the buck inductor (L BUCK) increases linearly as it supplies load current.

When the on-time ends, the buck switch turns off andload current continues to ow in the buck diode (D BUCK) andthe buck inductor. The current through the buck inductor

ig. 2.This block diagram shows the eight functions performed by a typical HID ballast.

ig. 3.The PFC stage of the HID ballast circuit denes a sinusoidal peak urrent envelope (solid line) that contains the triangular PFC inductor urrent and smoothed sinusoidal line input current (dashed line) over ne-half cycle of the line input voltage.

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specic methods a ballast design implements to detect eachfault, the protection circuits should be robust and reliable toensure proper safety in the ballast application and to preventcatastrophic eld failures should fault conditions occur.

New applications and lamp types are continuously emerging in the marketplace and each includes its own

unique design challenges. International Rectier and othermanufacturers in the industry will continue to improve andsimplify control methods and ICs in the eld of HID light-ing. Designers will need to stay on top of the rapid changesthat are sure to take place in the coming years. PETech

This arrangement generates a high-voltage pulse on thesecondary to ignite the lamp. The capacitor C IGN chargesup until the diac turns off, and C IGN then discharges downthrough resistor R IGN until the diac voltage again reaches thedevices threshold and another ignition pulse occurs. Whenthe lamp ignites, the buck output voltage decreases quickly to

the lamp voltage as the converter provides the lamp current.The ignition controller disables the pulses after the lamp hasignited by turning switch M IGN off.

An IRS2453D full-bridge-control IC manages thelamp-drive bridge. This high-voltage IC contains all of thenecessary circuitry for the full-bridgeoscillator and high- and low-side gatedrivers. The IC also contains a non-latched and latched shutdown pin aswell as integrated bootstrap diodesfor the high-side driver supplies.The timing diagram shows the CToscillator timing pin, the gate-driveroutputs, and the resulting midpointand lamp voltages ( Fig. 5). The IC alsoincludes an internal 1.5-s dead timebetween the low-side (LO) and high-side (HO) gate-drive outputs. Thisdead time prevents external MOSFETshoot-through and allows for eachhalf-bridge voltage to self-commutatefor zero-voltage switching.

Protection RequirementsThe HID ballast should include

specic protection circuits to detectvarious lamp- and ballast-fault condi-tions and safely shutdown or reset theballast. These fault conditions includeac-mains interrupt or brownout, lampignition failure, lamp warmup failure,lamp open circuit, lamp short circuitand lamp end-of-life. A summary of these conditions appear in the tablealong with the proper ballast response

to each fault and the possible outcomeif the ballast does not protect againstthe fault.

Voltage and current signals withinthe various stages can serve as detec-tion points to realize the protectioncircuitry. The ac line or dc bus volt-ages can reset the ballast if a brownoutcondition occurs. Timers are typically necessary to deactivate the ballast aftera predetermined time period shouldthe lamp fail to ignite or warmup. A

lamp voltage or power monitor candetect if the lamp is unstable or isreaching end of life. Regardless of the

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