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Posted on 05 July 2019

Driving an HID Lamp

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Control Innovations Reveal a Brighter Future

Emerging single-chip controllers for electronic ballasts will maximise the potential of HID lamps in the ultra-competitive lighting marketplace.

By Tom Ribarich, Director, Lighting Systems, International Rectifier

 

HID lamps achieve efficacy and lifetime comparable to fluorescent lamps, while also producing high brightness and excellent colour temperature to satisfy applications such as floodlights, street lighting and vehicle headlamps. However, although electronic ballasts improve HID efficiency compared to traditional magnetic ballasts, they also present complex design challenges leading to high costs. A new generation of controller ICs now allows faster design cycles and lower costs, with the added advantage of scalability to support a number of lamp variants spanning a range of power levels.

Basic Ballast Requirements

HID lamps are driven with a low-frequency AC voltage (<200Hz typical) to avoid mercury migration and to prevent damage of the lamp due to acoustic resonance. A typical metal halide 250W HID lamp requires a nominal voltage and current of 100V and 2.5A respectively, and requires a minimum warm-up time of two seconds. Electronic ballasts for HID lamps must provide a high voltage of around 4kV for ignition (or more than 20kV if hot), should manage current limitation during warm-up and has to maintain constant power while running. Lamp power must also be tightly regulated to minimise lamp-to-lamp colour and brightness variations.

Figure 1 shows the generic start-up profile for an HID lamp. Before ignition, the lamp is open circuit. After the lamp ignites, the lamp voltage drops quickly from the open-circuit voltage to a very low value (20V typical) due to the low resistance of the lamp. The lamp current also increases dramatically and should be limited 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 (100V typical) and the power is regulated to the correct level.

HID Operating Phases

Functional Analysis

Figure 2 shows the functional blocks of an electronic ballast comprising EMI filtering to block ballast-generated noise, a bridge rectifier, a boost PFC stage that also produces a constant DC bus voltage, a step-down buck converter for controlling the lamp current, a fullbridge output stage for AC operation of the lamp, and an ignition circuit for striking the lamp. Control ICs manage the boost PFC stage and the buck/full-bridge stages. This is an accepted approach to powering HID lamps with a low-frequency AC voltage.

Functional Blocks of an Electronic HID Ballast

The boost PFC stage runs in critical-conduction mode. During this mode, the boost stage operates with a constant ontime and variable off-time resulting in a free-running frequency across each rectified half-wave of the AC line cycle. The frequency range is typically from 200kHz near the valley of the half-wave to 50kHz at the peak. The on-time is used to regulate the DC bus to a constant level and the off-time is the time for the inductor current to reach zero in each switching cycle. The triangular shaped inductor current is filtered by the EMI filter to produce a sinusoidal input current at the AC mains input for high power factor and low harmonic distortion.

The buck control circuit is the main control circuit of the ballast and is used to control the lamp current. The buck stage steps down the constant DC bus voltage from the boost stage to the lower lamp voltage across the full-bridge stage. The buck circuit shown can run in continuous- or critical-conduction modes, depending on the condition of the load. The lamp voltage and current are measured and multiplied together to produce a lamp power measurement, which is fed back to control the buck on-time. During the lamp warm-up period (after ignition) when the lamp voltage is very low and the lamp current is very high, the lamp current feedback will determine the buck on-time to limit the maximum lamp current. When the lamp is running in a steady state, power feedback determines the buck on-time to control the lamp power. Operating in continuous-conduction mode allows the buck circuit to supply more current to the lamp during the warm-up without saturating the buck inductor.

The full-bridge stage produces the AC lamp current and voltage, typically at 200Hz with a 50% duty-cycle, to maintain normal running. There is also a pulse transformer circuit for producing the 4kV ignition pulses.

Single-Chip Ballast Control

The HID control IC manages ignition and running of the lamp. In the IRS2573D controller this is achieved using a state machine, as shown in figure 3. Initially starting in Under-Voltage Lock-Out (UVLO) mode when the IC supply voltage is below the turn-on threshold, the device enters ignition mode when VCC exceeds the threshold. The on/off ignition timer is then activated to deliver high-voltage pulses to the lamp for ignition. If the lamp ignites successfully, the IC transitions into run mode and the lamp is regulated to a constant power level. The IC also integrates safety features to shut the lamp down and protect the ballast if fault conditions - such as open/short circuit, failure to ignite or warm up, arc instability, or lamp End-Of-Life (EOL) – occur.

State Machine for Single-Chip Ballast Control

Figure 4 shows the complete buck and full-bridge control circuit schematic. The IRS2573D includes control for the buck stage, the full-bridge, lamp current and voltage sensing, and feedback loops for controlling lamp current and lamp power. The IC includes an integrated high-side driver for the buck gate drive (BUCK pin) and high-side buck cycle-by-cycle over-current protection (CS pin). 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 off-time timing input (TOFF pin) for continuous-conduction mode.

Ballast Schematic Based on IRS2573D

The IC also includes a fully integrated 600V high- and low-side fullbridge driver. 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 and ISENSE pins) and then multiplying them together internally to generate the lamp power measurement. The ignition control is performed using an ignition timing output (IGN pin) that drives an external ignition MOSFET (MIGN) on and off to enable the ignition circuit of the lamp (DIGN, CIGN, TIGN). The ignition timer is programmed externally (TIGN pin) to set the ignition circuit on and off times. A programmable fault timer (TCLK pin) determines the allowable fault duration times before shutting the IC off safely.

Ballast Operation

Figure 5a shows the buck switching node voltage (upper trace) and buck current (lower trace) during lamp warm-up. The buck on-time during this mode is controlled by the buck current limitation feedback loop. Figure 5b shows the buck switching node voltage (upper trace) and buck current during steady-state running conditions. The buck is working in critical-conduction mode during running conditions and the on-time is controlled by the constant power feedback loop. Figure 5c shows each half-bridge output voltage (upper and middle traces) and AC lamp current (lower trace) during normal running conditions.

5a,b,c - Buck, Full-Bridge and Lamp Waveforms

Conclusion

HID lamps have exacting requirements for successful ignition and driving, which complicate the design of electronic ballasts. A highly integrated ballast control IC provides a low-risk, standardised approach that simplifies design and also allows for scalability so that the same basic design can be used as a platform to realise a family of electronic ballasts for many lamp types and power levels. Consolidating all the necessary functions for lamp ignition, lamp control and fault protection in a single chip such as the IRS2573D also delivers a highly reliable solution.

 

 

 

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