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.

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 (T_IGN) and into capacitor C_IGN. This arrangement generates a high-voltage pulse on the secondary to ignite the lamp. The capacitor C_IGN charges up until the diac turns off, and C_IGN then discharges down through resistor R_IGN until the diac voltage again reaches the device's threshold and another ignition pulse occurs. When the 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 has ignited by turning switch M_IGN off.

An IRS2453D full-bridge-control IC manages the lamp-drive bridge. This high-voltage IC contains all of the 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. 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 includes an internal 1.5-µs dead time between the low-side (LO) and high-side (HO) gate-drive outputs. This dead time prevents external MOSFET shoot-through and allows for each half-bridge voltage to self-commutate for zero-voltage switching.

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 specific methods a ballast design implements to detect each fault, the protection circuits should be robust and reliable to ensure proper safety in the ballast application and to prevent catastrophic field 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 Rectifier and other manufacturers in the industry will continue to improve and simplify control methods and ICs in the field of HID lighting. Designers will need to stay on top of the rapid changes that are sure to take place in the coming years.

More on Buck Converters

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• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

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