Power Electronics

Adaptive Ballast Control IC for Fluorescent Lamps

International Rectifier's (IR) IR2520D is a compact fluorescent lamp (CFL) ballast control IC that adapts to changing supply voltage, frequency and lamp conditions to increase lamp life and reliability. All necessary ballast features are integrated into a single 8-pin DIP or SOIC, reducing parts count, simplifying circuits and increasing reliability.

The IR2520D is a complete ballast controller and 600-V half-bridge driver for CFLs. The new device includes a novel zero voltage switching (ZVS) circuit that maintains soft switching, regardless of supply voltage, frequency and lamp conditions, and includes internal crest factor overcurrent protection and an integrated bootstrap diode. At the heart of this IC is a voltage-controlled oscillator (VCO) with an externally programmable minimum frequency.

Other common compact fluorescent lamp ballasts that use the self-oscillating bipolar transistor solution are simple, but are not self-starting, and require additional components and circuitry such as DIACs and free-wheeling diodes. In addition, unreliable, “always hot” positive-temperature-coefficient (PTC) thermistors are used for preheat, and they do not have protection against lamp non-strike or open filament conditions. These can give a high susceptibility to component and load tolerances and/or catastrophic failure of ballast output stage components, which can result in poor performance, poor quality and failure.

IR2520D is based upon IR’s high-voltage junction-isolation (HVJI) IC technology. This proprietary technology allows high-voltage circuits to be isolated from low-voltage circuitry, so that both high- and low-side drive functions can be housed on a single, compact chip for controlling various switching converter topologies. Because lighting ballasts require both high- and low-voltage circuits to function, components made with IR’s HVJI technology are particularly well-suited to the application.

A key feature of the IR2520D is its adaptive zero voltage switching control. During each half-bridge switching cycle, half-bridge voltage slews to the opposite rail during deadtime. If the voltage has not completely slewed to the opposite rail so that there is zero voltage across the appropriate switch before turn-on, then the operating frequency is too close to resonance and the frequency increases. The built-in adaptive zero voltage switching (ZVS) and zero current switching (ZCS) closed loop control matches operating frequency as close as possible to the half-bridge output stage resonance. Operating a ballast in ZVS/ZCS mode minimizes switching losses in the half-bridge MOSFETs, ensuring maximum efficiency and lifetime, regardless of component and lamp tolerances as well as line voltage variations.

The IR2520D features advanced fault and protection circuitry. An open filament lamp fault will cause hard-switching at the half-bridge, and the non-ZVS circuit or the crest factor circuit will detect this condition and will enter fault mode, and both gate driver outputs will be latched low, turning the lamp off.

If a lamp non-strike condition occurs when the filaments are intact but the lamp does not ignite, the lamp voltage and output stage current will increase during the ignition ramp until excessive currents occur or the resonant inductor saturates.

Key specifications for the IR2520D include output drive capability of 250/400 mA, 15.6-V Zener diode clamp on Vcc, micropower startup current and built-in 30-kHz to 125-kHz oscillator.

For more information, visit www.irf.com.

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