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Posted on 17 July 2019

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Simplified design of electronic ballasts for HID lamps

While offering benefits of efficiency, lifetime and good colour rendering high intensity discharge lamps (HIDs) present designers with the challenge of ballast implementation. Electronic ballasts are smaller, lighter and more efficient than electromagnetic ballasts but, to date, have been more complex to implement.

By Peter Bredermeier, International Rectifier, Germany

 

Now, however, the latest semiconductor developments are enabling multi-mode buck control circuits that simplify ballast design and reduce component count.

A typical electronic ballast for a high intensity discharge (HID) lamp comprises three key functional blocks: a boost PFC stage to maintain sinusoidal input current and generate a stable DC bus voltage; a buck converter responsible for optimising lamp current and power; and an output stage to drive the lamp.

As Figure 1 shows, the output stage also includes an ignition circuit capable of striking the lamp whether hot or cold.

Topology and control blocks for traditional electronic HID ballast

Figure 2 shows the typical start-up profile for HID lamps.

Start-up waveforms for HID lamp

The ignition voltage for cold lamps may be up to 4kV, but a hot lamp can require more than 20kV. Before ignition the lamp behaves like an open circuit. After ignition the lamp displays a low resistance characteristic that causes the voltage to drop quickly from the open circuit value to around 20V. The ballast has to restrict the current during this phase so as not to exceed the lamp manufacturer’s specified limit. During lamp warm-up the current decreases as the voltage and power increase. After some warm-up time the lamp voltage stabilises to around 100V. The ballast must then regulate the power to the correct level.

HID Ballast Design

Referring to Figure 1, the boost PFC stage shapes the line current into a sinusoidal waveform to meet the requirements of high power factor and low current harmonics. This power stage is also responsible for providing a stable, constant DC bus voltage - typically 400V DC - for the lamp power control stage. The boost converter operates at a free-running frequency, with approximately constant ON time and with OFF time varying according to changes in the peak current demand.

The buck converter controls the voltage and current delivered to the load during warming up and running. Typically the buck would be managed using a separate controller IC and a high-voltage level-shift IC to boost the gate drive signal up to the buck switch’s potential. Controlling the buck presents the most complex challenge to ballast designers. During the ignition phase the buck must regulate its output voltage to the minimum take-over voltage to ensure reliable ignition. The takeover voltage for ceramic lamps is 250V and for quartz lamps is 280V. Immediately after the lamp ignites, the buck must supply sufficient current to keep the lamp from extinguishing as the resistance of the lamp falls quickly. On the other hand, the buck must limit the current to prevent the inductor from becoming saturated. While the lamp is running the buck’s ON time is adjusted to keep the lamp power constant.

The output stage comprises a full-bridge circuit to supply the lamp with a low-frequency square wave voltage and an ignition circuit for striking the lamp. The top of the fullbridge circuit connects to the buck output voltage and the two half-bridge midpoints oscillate 180 degrees out of phase to produce the necessary AC voltage. The driver for the bridge can be implemented using a self-oscillating full-bridge driver IC such as International Rectifier’s IRS2453D. This device combines high-voltage IC (HVIC) and latch-immune CMOS technologies to integrate a high-voltage full-bridge gate driver with a front-end oscillator. Using the IC’s shutdown pin external protection circuitry is able to turn off the driver to protect against damage that can be caused by lamp failure shutdown pin.

The ignition circuit comprises a diac (DIGN), transformer (TIGN), capacitor (CIGN), resis- tor (RIGN) and switch (MIGN). An ignition control signal is required to turn on switch MIGN, which causes capacitor CIGN to discharge through resistor RIGN. When the voltage across the diac reaches the diac threshold voltage, the diac turns on and a current pulse flows from the buck output, through the primary winding of the ignition transformer (TIGN) and into capacitor CIGN. This arrangement generates a high-voltage pulse on the secondary to ignite the lamp. The capacitor CIGN charges up until the diac turns off, and CIGN then discharges down through resistor RIGN 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 must be able to turn switch MIGN off after the lamp has ignited to disable the pulses.

Historically, control of the main blocks has been achieved using multiple ICs. For ballasts built using a combination of discrete controller and driver ICs, specific protection circuits are also necessary to enable the ballast to be safely shut down or reset in the event of lamp faults. These may include failure to ignite or warm up, open-circuit or short-circuit faults, or instability indicating end-of-life.

Integrated HID Ballast Control

Clearly implementing the ballast with as few components and in as short as time as possible is particularly important. It is with this in mind that International Rectifier has developed a new ballast control IC dedicated to HID applications. The IRS2573D implements multi-mode buck control, ignitor-circuit control and fault-protection capabilities in a single device, minimising component count and improving ballast performance.

The integrated controller is able to operate the buck in critical-conduction mode or continuous- conduction mode, to optimise the voltage level and current delivery for the various operating stages of the lamp. Figure 3 illustrates the architecture of the buck-control circuit, showing the off-time control, on-time control, current sense, zero-crossing detection and feedback blocks that allow this device to operate the buck in either mode.

Buck-control scheme of integrated ballast controller

During steady state the IC regulates the lamp power to a constant level. To achieve this, the IC measures the lamp voltage at its VSENSE input and the lamp current at the ISENSE input, and multiplies the voltage and current together using an internal multiplier circuit to calculate power. It then increases or decreases the buck ON time to regulate the output of the multiplier circuit to a constant reference voltage. For example, if the lamp power is too low then the output of the multiplier will be below the internal reference voltage. An internal Operational Trans-conductance Amplifier (OTA) will output a sourcing current to an external pin (PCOMP) that charges up an external capacitor to a higher voltage. This increases the ON time of the buck and increases the output current to the lamp so as to increase the output power. Conversely, if the power is too high, the OTA will decrease the buck ON time and decrease the output current to the lamp. The speed of the constant-power control loop is set by the value of an external capacitor at the PCOMP pin. This determines how fast the loop will react to adjust the buck ON time over the changing load conditions. The timing diagram of figure 4 shows the buck operating in critical-conduction mode or continuous- conduction mode.

Critical-conduction and continuous-conduction modes

During warm-up, the lamp voltage can be low and the constant-power loop will attempt to increase the buck current to several Amps to maintain constant power. This high current can exceed the maximum current of the HID lamp. The controller’s current-sense block enables current limiting to prevent this condition occurring.

Built-in Protection

The IC includes an overvoltage fault counter at the VSENSE pin, which counts the time during which an overvoltage condition at the output of the buck exists. This allows the controller to detect an open-circuit condition, lamp removal or end-of-life, or turn-off of the lamp. If the voltage at the VSENSE pin remains above 0.4-times the overvoltage threshold (OV), and the overvoltage fault counter times out (typically 1180 seconds), then the IC will enter Fault mode and shutdown. If the voltage at the VSENSE pin decreases below 0.4xOV before the overvoltage fault counter times out, then the lamp has successfully ignited and the IC will enter General mode. The ignition gate-driver output, presented at the IGN pin, will remain ‘high’ until the ignition timer has timed out.

There is also an undervoltage fault counter at the VSENSE pin. Once the lamp has ignited, the lamp voltage will decrease sharply to around 20V but will subsequently rise slowly to a nominal 100V running voltage as the lamp warms up. If the lamp voltage does not rise as expected, the lamp may be faulty and the ballast must shut down. To detect this, the VSENSE pin includes an undervoltage threshold of 0.133xOV. If the voltage at the VSENSE pin remains below this level and the undervoltage fault counter times out (typically 295 seconds), the IC assumes that a fault is preventing the lamp voltage from rising and shuts down. If the voltage at the VSENSE pin increases above 0.133xOV before the undervoltage counter times out, this indicates successful warm up and the IC will remain in General mode.

Summary

HID lamps are known to offer high lighting efficiency, good colour rendering and long lifetime, but their negative impedance presents a complex lamp-control challenge to designers of electronic ballasts. Consolidating the control functions for the ballast’s buck converter and driver/ignition circuit in a single IC, however, significantly simplifies the ballast design challenge to make the size, weight, power-factor, efficiency and stability advantages of electronic ballasts accessible to a wider range of markets.

 

 

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