Better ballasts for HID lighting

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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