Creating an HID Ballast with Constant Lamp Power Control

Posted on Feb 7, 2014

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.

Creating an HID Ballast with Constant Lamp Power Control
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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.

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