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Better ballasts for HID lighting

Better ballasts for HID lighting

New integrated circuits help control high-intensity discharge lamps that deliver super-efficient illumination.

With all of the excitement surrounding LEDs these days, one might say high-intensity discharge (HID) lamps are out of the limelight. But though LEDs get most of the headlines, HID sources are still efficient in their own right. They are widely used for outdoor lighting because of their high brightness, excellent color temperature, and long lifetime. The output these lamps deliver, along with the enormous energy savings of their electronic ballasts, make them useful sources of illumination. Metal halide and sodium vapor HID lamps, for example, produce 120 lumens/W, deliver a full level of light beyond 20,000 hrs, have good light penetration at long distances (past 15 ft.), and are a fraction of the cost of LEDs. Thus HID lamps are likely to be widely applied for some time to come.

One obstacle when designing with HID lamps is that they are difficult to control and the design of the electronic ballast to drive them is complex. The electronic HID ballast must perform functions that include ignition, warm-up, constant power control, power factor correction, and protection against all lamp and ballast fault conditions.

Consequently, IC makers have devised chips to simplify the chore of ballast design. One device in this category is the new IRS2573D HID Control IC which can serve as the basis for an electronic ballast circuit handling a typical 250-W HID lamp.

HID lamps are available in the form of metal halide, mercury or sodium vapor. These lamps produce light using a technique similar to that in fluorescent lamps where a lowpressure mercury vapor produces ultraviolet light that excites a phosphor coating on the tube. In the case of HID lamps, the lamp contains a high-pressure gas and metal salts, the distance between the electrodes is short, and visible light is produced directly without the need for the phosphor.

Light from an HID lamp comes from an electric arc struck between the tungsten electrodes inside its translucent or transparent fusedquartz or fused-alumina arc tube. The gas in the tube facilitates the initial arc strike. Once the arc starts, it heats and evaporates the metal salts in the tube gas to form a plasma, which boosts the intensity of light from the arc and reduces lamp power consumption.

HID lamps have higher luminous efficacy than fluorescent and incandescent lamps because more of their radiation goes into visible light instead of heat. They also give a greater amount of light output per watt of electricity than either of the other two lamps.

HID lamps need a high voltage for ignition (3 to 4 kV typical, more than 20 kV if the lamp is hot) to strike the arc. Because the arc constitutes an extremely low resistance, there must be a mechanism to limit current during warm-up and a way to control power while the lamp is on. It is important to tightly regulate lamp power to minimize lamp-to-lamp color and brightness variations.

Also, HID lamps are driven with a low-frequency ac voltage (less than 200 Hz typical) to avoid mercury migration and to prevent acoustic resonances from damaging the lamp. All in all, a typical metal halide 250-W HID lamp has a nominal voltage (V rms) of 100 V, a nominal Current (A rms) of 2.5 A, takes at least two minutes to warm up, and has a 4 kV (V peak) ignition voltage.

The accompanying figure shows the typical start-up profile for HID lamps. Before ignition, the lamp is open circuit. After the lamp ignites, lamp voltage drops quickly from the open-circuit voltage to a low value (typically 20 V) because of the lamp’s low resistance. This makes the lamp current rise to a high value. As the lamp warms up, the current drops as the voltage and power rise. Eventually the lamp voltage reaches its nominal value (100 V typical) and the ballast regulates power to the correct level.

A typical HID ballast circuit starts with EMI filtering to block ballast-generated noise from the ac line. It feeds into a bridge rectifier to convert the ac mains voltage to a full-wave-rectified voltage. Next comes a power factor correction stage. It is typically an active PFC circuit able to provide a 0.99 power factor. The resulting constant dc bus voltage output goes to a step-down buck converter which produces dc voltage levels compatible with the HID lamp. Next is a full-bridge output stage to operate the lamp and an ignition circuit for striking the lamp. A control IC typically handles the buck and full-bridge stages and manages the different lamp modes. Today this is one of the most standard circuit topologies used to power HID lamps.

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The buck controller is the ballast’s main control circuit and manages the lamp current and power. The buck stage steps-down the constant dc bus voltage to the level of lamp voltage across the fullbridge stage. The buck circuit operates in continuous-conduction or critical-conduction mode, depending on the condition of the load.

The controller chip multiplies the lamp voltage and current, ILAMP and VLAMP, together to produce a measurement of lamp power which feeds back to control the on-time of the buck converter. During the lamp warm-up period (after ignition) the lamp voltage is low and the lamp current is high. In this condition, the lamp current feedback will determine the buck on-time to limit the maximum lamp current. When the lamp is running steady state, the power feedback will determine the buck on-time to control the amount of power the lamp dissipates. Operation in continuous-conduction mode allows the buck circuit to supply more current to the lamp during the warm-up without saturating the buck inductor.

To determine a value for the buck inductor, assume there is a constant input voltage to the buck circuit of 400 Vdc (from the output of the PFC boost stage). Also assume a buck nominal operating frequency of 70 kHz and that the 250-W lamp nominal electrical data given earlier is operative. Then the buck inductor value, LBUCK, is calculated as:

The full-bridge stage serves to produce an ac lamp current and voltage while the lamp is 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 the 4-kV pulses that the lamp needs for ignition. The ignition circuit includes a diac circuit to produce the required ignition pulses. The ignition circuit is activated by turning on the FET MIGN, causing the lower leg of the diac, DIGN, to discharge with a time constant determined by the RC circuit RIGN and CIGN. When the voltage across the diac reaches the diac threshold, VDIAC, the diac breaks down and generates a voltage pulse across the primary winding of the ignition transformer, TIGN. This produces the higher 4-kV pulse across the secondary winding of TIGN and across the lamp for ignition.

The buck on-time while the lamp warms up is controlled by the buck current limitation feedback loop. Once the lamp is running in its steady state condition, the buck works in critical-conduction mode. On-time is controlled by the constant power feedback loop.

The standard 3-stage topology and highly-integrated control IC allows for design scalability so the same basic circuit can serve as a platform for many lamp types and power levels.

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Inside an HID lamp controller

HID lamp controller

The complete buck and full-bridge control 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 600-V high-side driver for the buck gate drive (BUCK pin) with cycle-by-cycle over-current protection (CS pin).

The on-time of the 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 this bridge is controlled 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 uses 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 amount of time that elapses before the IC shuts off safely in the event of various fault conditions. Such fault conditions include failure of the lamp to ignite, failure of the lamp to warm-up, lamp end-of-life, arc instabilities, and open/short circuit of the output.


Information

International Rectifier Web site, www.irf.com

Wikipedia page on power factor control, en.wikipedia.org/wiki/Power_factor

Wikipedia page on buck converters, en.wikipedia.org/wiki/Buck_converter

Wikipedia page on HID lamps, en.wikipedia.org/wiki/Gasdischarge_lamp

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