Creating an HID Ballast with Constant Lamp Power Control
With all of the excitement surrounding LEDs these days, high-intensity discharge (HID) lamps are still popular due to their high brightness, excellent color temperature and long lifetime. Metal halide and sodium vapor HID lamps, for example, produce 120 lumens/watt, deliver a full level of illumination beyond 20,000 hours, have good light penetration at long distances (>15ft.), and are a fraction of the cost of LEDs. Electronic ballasts are necessary to control these lamps and perform such functions as ignition, warm-up and lamp power control. This technical blog describes a 250W electronic HID ballast circuit with constant output power control.
The HID ballast circuit includes (Figure 1) a buck stage for controlling lamp current and power, a full-bridge output stage for driving the lamp with a low-frequency (200Hz), AC square-wave voltage and current, and an ignition circuit for producing 4KV peak voltage pulses to ignite the lamp. The input to the buck stage is a constant 400VDC voltage that is typically supplied by a standard PFC boost circuit (not shown). The buck, full-bridge, ignition, and lamp power control circuits are all designed using the IRS2573D HID Control IC from International Rectifier. This IC includes an integrated 600V high-side driver for the buck gate drive (BUCK pin) and a 600V full-bridge driver (LO1, HO1, LO2, HO2 pins). 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 maximum off-time timing input (TOFF pin) for continuous-conduction mode.
The operating frequency of the full-bridge is controlled with an external timing pin (CT pin). The IC provides lamp power control by sensing the lamp voltage and current (VSENSE, ISENSE pins), multiplying them together internally to generate the lamp power measurement, and then steering the buck on-time to keep the lamp power constant. The ignition control is performed using an ignition timing circuit (IGN pin) that drives the ignition circuit of the lamp (MIGN, DIGN, CIGN, TIGN) on and off at a set interval (TIGN pin). Finally, the IC includes a programmable fault timer (TCLK pin) for programming the allowable fault duration times before shutting the IC off safely. Such fault conditions include failure of the lamp to ignite, failure of the lamp to warm-up, lamp end-of-life, arc instability, and open/short circuit of the output.
Figure 1, HID ballast schematic.
This design is a low-risk standard solution that contains a highly-integrated control IC to greatly simplify the circuit. This solution also allows for ease of scalability for controlling all lamp types and power levels. The new IRS2573D control IC contains the complete HID system-in-a-chip, making this solution very reliable and ideal for designers to accelerate their products into the marketplace. For additional datasheet or reference design information, please visit IR’s website at: www.irf.com.