Datasheet L6562 (STMicroelectronics) - 9

L6562
When the load of a PFC pre-regulator is very low, the output voltage tends to stay steadily above the nom- inal value, which cannot be handled by the Dynamic OVP. If this occurs, however, the error amplifier out- put will saturate low; hence, when this is detected, the external power transistor is switched off and the IC put in an idle state (Static OVP). Normal operation is resumed as the error amplifier goes back into its lin- ear region. As a result, the L6562 will work in burst-mode, with a repetition rate that can be very low. When either OVP is activated the quiescent consumption of the IC is reduced to minimize the discharge of the Vcc capacitor and increase the hold-up capability of the IC supply system.
4.2 THD optimizer circuit
The L6562 is equipped with a special circuit that reduces the conduction dead-angle occurring to the AC input current near the zero-crossings of the line voltage (crossover distortion). In this way the THD (Total Harmonic Distortion) of the current is considerably reduced. A major cause of this distortion is the inability of the system to transfer energy effectively when the instan- taneous 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.
Figure 22. THD optimization: standard TM PFC controller (left side) and L6562 (right side)
Input current Input current Rectified mains voltage Rectified mains voltage Imains Input current Imains Input current MOSFET's Vd drai ra n v in oltage MOSFET's Vd drain ra voiln tage To overcome this issue the circuit embedded in the L6562 forces the PFC pre-regulator to process more energy near the line voltage zero-crossings as compared to that commanded by the control loop. This will result in both minimizing the time interval where energy transfer is lacking and fully discharging the high- frequency filter capacitor after the bridge. The effect of the circuit is shown in figure 23, where the key waveforms of a standard TM PFC controller are compared to those of the L6562. Essentially, the circuit artificially increases the ON-time of the power switch with a positive offset added to 9/16 Document Outline Figure 1. Packages Table 1. Order Codes 1 Features 1.1 APPLICATIONS 2 Description Figure 2. Block Diagram Table 2. Absolute Maximum Ratings Figure 3. Pin Connection (Top view) Table 3. Thermal Data Table 4. Pin Description Table 5. Electrical Characteristics (Tj = -25 to 125˚C, VCC = 12, CO = 1 nF; unless otherwise specified) 3 Typical Electrical Characteristics Figure 4. Supply current vs. Supply voltage Figure 5. Start-up & UVLO vs. Tj Figure 6. IC consumption vs. Tj Figure 7. Vcc Zener voltage vs. Tj Figure 8. Feedback reference vs. Tj Figure 9. OVP current vs. Tj Figure 10. E/A output clamp levels vs. Tj Figure 11. Delay-to-output vs. Tj Figure 12. Multiplier characteristic Figure 13. Multiplier gain vs. Tj Figure 14. Vcs clamp vs. Tj Figure 15. Start-up timer vs. Tj Figure 16. ZCD clamp levels vs. Tj Figure 17. ZCD source capability vs. Tj Figure 18. Gate-drive output low saturation Figure 19. Gate-drive output high saturation Figure 20. Gate-drive clamp vs. Tj Figure 21. UVLO saturation vs. Tj 4 Application Information 4.1 Overvoltage protection 4.2 THD optimizer circuit Figure 22. THD optimization: standard TM PFC controller (left side) and L6562 (right side) Figure 23. Typical application circuit (250W, Wide-range mains) Figure 24. Demo board (EVAL6562-80W, Wide-range mains): Electrical schematic Figure 25. EVAL6562-80W: PCB and component layout (Top view, real size: 57 x 108 mm) Table 6. EVAL6562N: Evaluation results at full load Table 7. EVAL6562N: Evaluation results at half load Table 8. EVAL6562N: No-load measurements Figure 26. Line filter (not tested for EMI compliance) used for EVAL6562N evaluation 5 Package Information Figure 27. DIP-8 Mechanical Data & Package Dimensions Figure 28. SO-8 Mechanical Data & Package Dimensions 6 Revision History Table 9. Revision History