AN2994 Application note 400 W FOT-controlled PFC pre-regulator with the L6563S Introduction This application note describes a demonstration board based on the transition-mode PFC controller L6563S and presents the results of its bench evaluation. The board implements a 400 W, wide-range mains input, PFC pre-conditioner suitable for ATX PSU, flat screen displays, etc.
Contents AN2994 Contents 1 Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . . 5 2 Test results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1 Harmonic content measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Inductor current in FOT and L6563S THD optimizer . . . . . . . . . . . . . . . . 12 2.3 Voltage feed-forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AN2994 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 33. Figure 34. Figure 35. Figure 36. Figure 37. Figure 38. Figure 39. Figure 40. Figure 41. Figure 42. Figure 43.
List of tables AN2994 List of tables Table 1. Table 2. Table 3. Table 4. 4/38 Measured temperature at 115 Vac and 230 Vac - full load . . . . . . . . . . . . . . . . . . . . . . . . 29 Bill of materials for the EVL6563S-400W demonstration board . . . . . . . . . . . . . . . . . . . . . 31 Winding characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AN2994 1 Main characteristics and circuit description Main characteristics and circuit description The EVL6563S-400W demonstration board has the following characteristics.
Main characteristics and circuit description AN2994 At start-up the L6563S is powered by the Vcc capacitor (C12) that is charged via the resistors R3 and R4, and then the L4 secondary winding (pins 8-11), and the charge pump circuit (R5, C10, D5 and D4) generates the Vcc voltage that powers the L6563S during normal operations. The dividers R32, R33 and R34 provide the L6563S multiplier with information relating to the instantaneous voltage used to modulate the boost current.
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Test results and significant waveforms AN2994 2 Test results and significant waveforms 2.1 Harmonic content measurement One of the main purposes of a PFC pre-conditioner is to correct the input current distortion, decreasing the harmonic contents below the limits of the relevant regulations. Therefore, the board has been tested according to the European rule EN61000-3-2 Class-D and Japanese rule JEITA-MITI Class-D, at full load and 70 W output power, at both the nominal input voltage mains.
AN2994 Test results and significant waveforms For reference, the following waveforms show the input current and voltage at the nominal input voltage mains and at different load conditions. Figure 7. EVL6563S-400W input current waveform at 100 V, 60 Hz, 400 W load Figure 8. EVL6563S-400W input current waveform at 230 V, 50 Hz, 400 W load CH1: Rectified mains voltage CH4: Input current CH1: Rectified mains voltage CH4: Input current Figure 9. Figure 10.
Test results and significant waveforms AN2994 Figure 11. EVL6563S-400W input current Figure 12. EVL6563S-400W input current waveform at 100 V, 60 Hz, 70 W load waveform at 230 V, 50 Hz, 70 W load CH1: Rectified mains voltage CH4: Input current CH1: Rectified mains voltage CH4: Input current The power factor (PF) and the total harmonic distortion (THD) have also been measured and the results are reported in Figure 13 and Figure 14.
AN2994 Test results and significant waveforms Figure 15. Efficiency vs. Vin and load Figure 16. Static Vout regulation vs. Vin and load Vout [Vdc ] Eff. [%] 98% 408.0 96% 407.5 93% 407.0 91% 406.5 400 W 200 W 70W 400W 88% 15W 406.0 200W 405.5 70W 86% 405.
Test results and significant waveforms 2.2 AN2994 Inductor current in FOT and L6563S THD optimizer The following figures show the waveforms of the inductor current at different voltage mains: as shown in Figure 17 and Figure 19, the inductor current waveform over the half-period of a line is very similar to that of a CCM PFC.
AN2994 Test results and significant waveforms 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 deadangle 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.
Test results and significant waveforms 2.3 AN2994 Voltage feed-forward The power stage gain of PFC pre-regulators varies with the square of the RMS input voltage. So does the crossover frequency (fc) of the overall open-loop gain because the gain has a single pole characteristic. This leads to large trade-offs in the design. For example, setting the gain of the error amplifier to get an fc of 20 Hz at 264 Vac means having an fc of 4 Hz at 88 Vac, resulting in sluggish control dynamics.
AN2994 Test results and significant waveforms Figure 21. EVL6563S-400W input current shape at 100 Vac, 60 Hz, CFF = 470 nF, RFF = 390 kΩ Figure 22. EVL6563S-400W input current shape at 100 Vac, 60 Hz, CFF = 1.5 µF, RFF = 390 kΩ THD [%]: 3.39% - 3RD harmonic: 0.126 A CH3: VFF voltage - pin #5 CH4: Input current THD [%]: 2.75% - 3RD harmonic: 0.
Test results and significant waveforms AN2994 Figure 23. EVL6563S-400W input mains surge Figure 24.
AN2994 Test results and significant waveforms Figure 25. EVL6563S-400W input mains dip from 140 to 90 Vac, full load, CFF = 1 µF Figure 26.
Test results and significant waveforms 2.4 AN2994 Start-up and RUN pin Figure 27 and Figure 28 show the waveforms during the start-up of the circuit, at mains plug-in. The Vcc voltage rises up to the turn-on threshold, and the L6563S starts the operation. For a short time the energy is supplied by the Vcc capacitor, then the auxiliary winding and the charge pump circuit take over.
AN2994 2.5 Test results and significant waveforms Brownout function A dangerous event for any PFC is the operation during a mains undervoltage. This condition may cause overheating of the power section due to an excess of RMS current. To protect the PFC from this abnormal operation, a brownout protection is needed. It is basically a nonlatched shutdown function that has to be activated when a mains undervoltage condition is detected.
Test results and significant waveforms AN2994 Figure 30. EVL6563S-400W start-up with slow Figure 31.
AN2994 2.6 Test results and significant waveforms Start-up at light loads Figure 32. EVL6563S-400W start-up at 265 V, 50 Hz, 30 mA load Figure 33. EVL6563S-400W start-up at 265 V, 50 Hz, no load and with external Vcc CH1: Q1/Q2 drain voltage CH2: Output voltage CH3: Pin #13 - gate drive CH4: Pin #14 - Vcc voltage CH1: Q1/Q2 drain voltage CH2: Output voltage CH3: Pin #13 - gate drive CH4: Pin #14 - Vcc voltage The board, as it is, is able to properly handle an output load as low as 12 W.
Test results and significant waveforms 2.7 AN2994 Overvoltage and open-loop protection Normally, the voltage-control loop keeps the output voltage Vo of the PFC pre-regulator close to its nominal value, set by the ratio of the resistors of the output divider (R9, R10, R11 and R12 in parallel to R13). The device’s PFC_OK pin (#7) is dedicated to monitoring the output voltage with a separate resistor divider (R6, R7, R8 and R24). This divider has been selected so that the voltage at the pin reaches 2.
AN2994 Test results and significant waveforms Figure 34.
Test results and significant waveforms 2.8 AN2994 Power management/housekeeping functions A special feature of the L6563S is that it facilitates the implementation of the "housekeeping" circuitry needed to coordinate the operation of the PFC stage with that of the cascaded DCDC converter. The functions realized by the housekeeping circuitry ensure that transient conditions such as power-up or power-down sequencing or failures of either power stage are properly handled.
AN2994 Test results and significant waveforms Figure 36. Interface circuits that let the L6563S switch a PWM controller on or off /. 25.
Layout hints 3 AN2994 Layout hints The layout of any converter is a very important phase in the design process that sometimes does not get the required attention by engineers. Even if the layout phase is sometimes time-consuming, a good layout saves time during the functional debugging and qualification phases. Additionally, a power supply circuit with a correct layout needs smaller EMI filters or less filter stages and therefore contributes to money saving.
AN2994 Layout hints Figure 37.
Audible noise 4 AN2994 Audible noise Differential mode currents in a circuit with high- and low-frequency components (like in a PFC) may produce audible noise due to inter-modulation between the operating frequency and the mains line frequency. This phenomenon occurs because of the mechanical vibration of reactive components like capacitors and inductors. Current flowing in the winding can cause the vibration of wires or ferrite, and as such produce buzzing noise.
AN2994 5 Thermal measures Thermal measures To verify the reliability of the design, a thermal mapping by means of an IR camera has been done. Figure 38 and Figure 39 show the thermal measurements on the board component side at nominal input voltages and full load. Pointers (visible on the images) placed across the key components show the relevant temperature. Table 1 provides the correlation between the measured points and the components, for both thermal maps.
Conducted emissions pre-compliance tests peak detection 6 AN2994 Conducted emissions pre-compliance tests peak detection The following figures show the peak measurement of the conducted noise at full load and nominal mains voltages. The limits shown on the diagrams are relevant to the EN55022 Class-B, the most popular norm for domestic equipment using a two-wire mains connection. As shown, a good margin of the measures with respect to the limits is achieved in all test conditions. Figure 40.
AN2994 Bill of materials 7 Bill of materials Table 2. Bill of materials for the EVL6563S-400W demonstration board Ref. Case/ Des. Part type/part value package C1 470 nF - X2 C10 Description Supplier DWG X2 film capacitor R46-I 3470--M1- Arcotronics 18N 1206 100 V SMD cercap - general-purpose AVX C11 470 nF/50 V 1206 50 V SMD cercap - general-purpose AVX C12 68 uF/50 V DIA6.
Bill of materials Table 2. AN2994 Bill of materials for the EVL6563S-400W demonstration board (continued) Ref. Part type/ Case/ Des. Part value package JP102 JUMPER L1 1.
AN2994 Table 2. Bill of materials Bill of materials for the EVL6563S-400W demonstration board (continued) Ref. Part type/ Case/ Des.
PFC coil specification AN2994 8 PFC coil specification 8.1 General description and characteristics 8.
AN2994 8.3 PFC coil specification Mechanical aspects and pin numbering ● Maximum height from PCB: 31 mm ● Ferrite: two symmetrical half cores, PQ40-30 ● Material grade: PC44 or equivalent ● Central leg air gap: to be defined, in order to get the required inductance value ● Coil former type: vertical, 6 + 6 pins ● Pin distance: 5 mm ● Row distance: 45.5 mm ● Cut pins: 9 - 12 ● External copper shield: not insulated (for EMI reasons), connected to pin 11 (GND) Figure 45.
References 9 36/38 AN2994 References 1. "L6563S enhanced transition-mode PFC controller", datasheet. 2. "Design of fixed-off-time controlled PFC pre-regulators with the L6562", AN1792. 3. "How to design a transition mode PFC pre-regulator with the new L6562A", AN2761. 4. "Design of 400 W fixed-off-time controlled PFC pre-regulator with the new L6562A", AN2782.
AN2994 10 Revision history Revision history Table 4. Document revision history Date Revision Changes 20-Apr-2010 1 Initial release. 30-Nov-2010 2 Updated: Section 2.
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