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Electronic Design

Fluorescent Ballast ICs Demonstrate Clever Dimming Design

International Rectifier has two new control ICs for dimming compact and tube-type fluorescents. Most designers won’t ever see them, though, because they go into the bases of compact bulbs or into lighting fixtures. Still, they’re interesting to study as examples of energy-efficient design and for the elegant way they provide dimming, including dimming in legacy systems that use triac controls.

The IRS2530D DIM8 squeezes a complete dimming ballast control IC with a 600-V half-bridge driver into an eight-pin package. Outside of the input bridge and the half-bridge on the output, only a small number of external passives is required. The catch, however, is that this chip won’t dim down below 10% of full brightness.

For that, you need the IRS2158D. It has a few more pins— 16—and a slightly richer feature set. The eight-pin device is intended for tube ballasts as well as use in the bases of screw-in compact fluorescent lamps (CFLs). It additionally includes internal non-ZVS (zero-voltage switching) protection and internal crest-factor protection to prevent lamp failure from damaging the ballast.

The extra features in the 16-pin IC comprise overcurrent protection, programmable pre-heat and run frequencies, programmable preheat time and dead time, closed-loop halfbridge ignition, current regulation, programmable end-of-life protection, and brownout protection, as well as a low-input offset operational amplifier for dimming, current, or power control. In addition, the device features an end-of-life window comparator pin and an internal 60-event current-sense up/ down fault counter to accommodate T5 lamp and multi-lamp ballasts.

The big challenge for IR’s engineers was figuring out how to add the dimming feature to a ballast controller and keep the pin count down to eight. Essentially, there was only one spare pin, so the company gave that pin the ability to sense both current and voltage.

When the lamp ignites, the IC enters the dimming function of its state machine. In this state, the ballast output stage functions as a serial-L, parallel-RC circuit. The ac lamp-current flows through the current-sensing resistor RCS. The resulting ac voltage across RCS is coupled to the DIM pin on the IC through RFB and CFB.

The voltage that the DIM pin sees is a combination of the dc offset voltage derived from the user’s dimmer setting and the ac from the lamp-current sensing resistor. The control loop regulates the ac-superimposed-on-a-dc voltage at the DIM pin to keep the valley of the ac voltage at COM.

When the dc level goes down, the valleys of the ac voltage are pushed below COM. The dimming control circuit increases the frequency to decrease the ac lamp current until the valleys are at COM again, and vice versa.

Capacitor CVCO programs the speed of the dimming loop. RVCO and CPH form an additional compensation network to maintain smooth and stable dimming. Capacitor CDIM filters high-frequency switching noise

Pricing for the IRS2530D begins at $1.09 in 10,000-unit quantities. Pricing for the IRS2158D begins at $1.29 each. Reference designs are also available.

International Rectifier

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