Electronic Design

Switch Tricks Electronic Ballast ICs Into Working On Low Voltages

Ballast-control ICs from International Rectifier and other similar half-bridge gate drivers feature an undervoltage lockout (UVLO) on the high-side output. This function protects the driven MOSFETs or insulated gate bipolar transistors (IGBTs) in the event that VB drops to potentially dangerous low levels.

When designing an electronic ballast for operation off a 12-V dc (10.5 V to 13.6 V) battery, however, the UVLO creates a challenge because these devices require a minimum VCC of 12 V. When a nominally 12-V battery becomes partially discharged on load, the UVLO prevents the IC from functioning. For practical purposes, running off a 12-V dc source is marginal to say the least.

International Rectifier recommends using a dc-dc boost converter to lift the supply voltage, preferably to just below the VCC clamp limit, typically 15.7 V. The circuit shown in the figure offers a different solution that eliminates the need for a dc-dc converter.

This unconventional but simple solution employs a capacitor (C2) and a double-pole double-throw (DPDT) switch (S1a, S1b). When the circuit is in the OFF condition, the switch’s normally closed (N/C) contacts charge C2 to the battery potential. When the DPDT switch is thrown to the ON position, C2 gets connected in series with the battery through the other set of switch contacts. Now the potential at point “A” is almost double, which guarantees a smooth startup. Once the oscillation starts, an auxiliary winding in coil LRES takes over and supplies the operating current to VCC, which causes the internal Zener clamp to regulate.

For the rest of the components, designers should use the ballastdesigner software available as a free download at www.irf.com.

See associated figure

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