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.

HID Ballast Topology

A typical HID ballast (Fig. 2) performs eight basic functions. An electromagnetic interference (EMI) filter blocks ballast-generated noise. A full-wave rectifier provides the high-voltage bus power. A power-factor-correction (PFC) block ensures sinusoidal input current. A buck converter controls the lamp current. A full-bridge output stage provides the ac lamp drive. An ignition circuit strikes the lamp. Control circuitry manages each stage. Finally, protection circuitry safely deactivates the ballast in the event of a lamp- or ballast-fault condition. Currently, this is one of the most popular approaches to powering HID lamps with a low-frequency ac voltage.

The PFC stage is a boost converter that operates in critical-conduction mode with a free-running frequency. This is a standard topology that many power-supply and ballast applications use for power levels below 100 W. The PFC stage maintains a sinusoidal current that is in phase with the ac line input (to attain a high power factor and low total harmonic distortion) and regulates the dc bus output to a constant level, typically 400 Vdc. When the PFC switch (M1) turns on the current, the boost inductor (L_BOOST) ramps up linearly to a peak value. Switch M1 then turns off and the inductor current discharges back down to zero. When the current reaches zero, M1 turns on again and the cycle repeats itself. The amount of current necessary to keep the dc bus regulated at a constant level for a given load power determines the on-time. Since the input voltage to the PFC stage is sinusoidal, the resulting current will be triangular within each switching cycle, with the peaks following a sinusoidal envelope (Fig. 3).

The on-time will be approximately constant and the off-time will vary depending on how high the peak is for each switching cycle, resulting in a free-running frequency system. When the EMI filter at the input smoothes these triangular-shaped currents, the result is a sinusoidal current that is in phase with the ac input voltage (the dashed line in Fig. 3).

The buck stage controls the amount of current that the ballast delivers to the lamp load while warming up and running. Immediately after the lamp ignites, the lamp resistance drops and the lamp passes a large current. The buck controller should supply adequate current to keep the lamp from extinguishing, but the current limiter must prevent the buck inductor from saturating while the lamp is warming up.

While the lamp is running, the controller manages the buck's on-time to keep the lamp power constant. Current flows from the dc bus through the buck inductor to the load when the buck switch (M2) turns on. During the on-time, the current in the buck inductor (L_BUCK) increases linearly as it supplies load current.

When the on-time ends, the buck switch turns off and load current continues to flow in the buck diode (D_BUCK) and the buck inductor. The current through the buck inductor decreases linearly for the duration of the cycle. The controller adjusts the on-time depending on how much current the load needs to regulate the power. The time it takes for the buck inductor current to discharge to zero determines the off-time. A standard PWM circuit can control the buck stage and a high-voltage level-shift IC (such as the IR2117) boosts the gate-drive signal up to the buck switch's gate-to-source potential.

The output stage includes a full-bridge circuit for driving the lamp with a low-frequency square-wave voltage and an ignition circuit for striking the lamp. The top of the full-bridge circuit connects to the buck output voltage and the two half-bridge midpoints oscillate 180 degrees out of phase from each other to produce the necessary ac voltage.

During the ignition phase, the lamp is an open circuit and the buck output voltage is limited to a maximum value. The ignition circuit comprises a diac (D_IGN), transformer (T_IGN), capacitor (C_IGN), resistor (R_IGN) and switch (M_IGN). When the ignition controller turns on switch M_IGN, capacitor C_IGN discharges through resistor R_IGN.

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