AN993 Datasheet, PDF (17/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

AN993
Description of the demonstration application
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.
Doc ID 5656 Rev 10
17/27