AN2994 datasheet(13/38 Pages) STMICROELECTRONICS

13 / 38 page

AN2994

Test results and significant waveforms

Doc ID 15796 Rev 2

13/38

On both the drain voltage traces reported in Figure 18 and Figure 20, close to the zero

crossing points of the sine wave it is possible to note the action of the THD optimizer

embedded in the L6563S. This optimizer is a circuit that minimizes the conduction’s dead-

angle occurring at the AC’s input current near the zero-crossings of the line voltage

(crossover distortion). In this way, the THD of the current is considerably reduced. A major

cause of this distortion is the inability of the system to transfer energy effectively when the

instantaneous line voltage is very low. This effect is magnified by the high-frequency filter

capacitor placed after the bridge rectifier, which retains some residual voltage that causes

the diodes of the bridge rectifier to be reverse-biased and the input current flow to

temporarily stop. To overcome this issue, the device forces the PFC pre-regulator to process

more energy near the line voltage’s zero-crossing, as compared to that commanded by the

control loop.

This results in minimizing the time interval where energy transfer is lacking and fully

discharging the high-frequency filter capacitor after the bridge. Essentially, the circuit

artificially increases the ON time of the power switch with a positive offset added to the

output of the multiplier in the proximity of the line voltage zero crossing. This offset is

reduced as the instantaneous line voltage increases, so that it becomes negligible as the

line voltage moves towards the top of the sinusoid, and it is modulated by the voltage on the

VFF pin so as to have little offset at low lines, where the transfer of energy at zero crossings

is typically quite good, and a larger offset at high lines, where the energy transfer gets

worse.

To get the maximum benefit from the THD optimizer circuit, the high-frequency filter

capacitors after the bridge rectifier should be minimized, compatibly with EMI filtering needs.

A large capacitance, in fact, introduces a conduction dead-angle of the AC input current in

itself, thus reducing the effectiveness of the optimizer circuit.

Figure 19.

EVL6563S-400W inductor current

ripple envelope at 230 Vac, 50 Hz,

full load

Figure 20.

EVL6563S-400W inductor current

ripple (detail) at 230 Vac, 50 Hz,

full load

CH1: Q1/Q2 drain voltage

CH2: MULT voltage - pin #3

CH4: L4 inductor current ripple envelope

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