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