AN993

Description of the demonstration application

Doc ID 5656 Rev 10

17/27

4.1

Power factor section

Even if the PFC stage is not strictly necessary for electronic ballast applications, in this

design it has been introduced for the following reasons.

The PFC stage is necessary if the ballast input power is higher than 25 W. This provides

many benefits.

The front-end stage of conventional off-line converters, typically made up of a full wave

rectifier bridge with a capacitor filter, gets an unregulated DC bus from the AC mains.

Therefore, the instantaneous line voltage will be below the voltage on the capacitor most of

the time, which means that the rectifiers will only conduct for a small portion of each line’s

half-cycle. The current drawn from the mains will then be a series of narrow pulses whose

amplitude will be 5-10 times higher than the resulting DC value.

Lots of drawbacks result from this: much higher peak and RMS current drawn from the line,

distortion of the AC line voltage, overcurrents in the neutral line of the 3-phase systems, all

contributing to a poor utilization of the power system's energy capability.

This energy capability can be measured in terms of either total harmonic distortion (THD),

as norms provides for, or power factor (PF), intended as the ratio between the real power

(the one transferred to the output) and the apparent power (RMS line voltage times RMS

line current) drawn from the mains, which is more immediate. A traditional input stage with a

capacitive filter has a low PF (0.5-0.7) and a high THD (> 10%).

The new European norms and the international standard requirements have spurred the

design of high-power factor ballasts and are starting to impose a limit on the input current

harmonic content. For these reasons, power factor correctors (PFC) are now being widely

diffused in consumer and industrial lighting. With a high power factor switching pre-

regulator, interposed between the input rectifier bridge and the bulk filter capacitor, the

power factor is improved (by up to 0.99). The current capability is increased, while the bulk

capacitor peak current and the harmonic disturbances are reduced.

The L6561/2 is an IC intended to control PFC pre-regulators by using the transition mode

technique and is optimized for lamp ballast applications.

The operation is summarized below (for more information, see AN966). The AC mains

voltage, that can range from 85 V to 265 V, is rectified by a diode bridge and delivered to the

boost converter.

The boost converter consists of a boost inductor (T1), a controlled power switch (Q1), a

catch diode (D2), an output capacitor (C6) and, obviously, control circuitry (see Figure 3).

The PFC section has been designed to supply a 400 V DC and a power of 60 W.

4.2

Ballast section

The regulated voltage is delivered to the ballast section. The ballast is based on the high-

performance L6574, an offline half bridge driver designed using 600-V BCD technology. It

adds to the fully-integrated half-bridge driver topology a built-in voltage controlled oscillator

(VCO), a preheating start-up procedure and an operational amplifier dedicated to the

feedback loop. To avoid cross-conduction of the power MOSFETs or IGBTs, the internal

logic ensures a minimum dead time.