Shedding Light on HID Ballast Control

Oct 1, 2006 12:00 PM
By Tom Ribarich, Director, Lighting IC Design Center, International Rectifier, El Segundo, Calif.

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Electronic 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.

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 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 V_PK.

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.

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.

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