ST AN2755 Application note

AN2755

Application note

400 W FOT-controlled PFC pre-regulator with the L6562A

Introduction

This application note describes an demonstration board based on the transition-mode PFC controller L6562A and presents the results of its bench demonstration. The board implements a 400 W, wide-range mains input PFC pre-conditioner suitable for ATX PSU, flat screen displays, etc. In order to allow the use of a low-cost device like the L6562A at this power level, usually prohibitive for this device, the chip is operated with fixed-off-time control. This allows continuous conduction mode operation, normally achievable with more expensive control chips and more complex control architectures.

Figure 1. Demonstration board (EVL6562A-400W)

July 2008 Rev 1 1/24

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Contents AN2755

Contents

1 Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . . 5

2 Test results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Harmonic content measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Inductor current in FOT and L6562A THD optimizer . . . . . . . . . . . . . . . . 10

2.3 Overvoltage protection and disable function . . . . . . . . . . . . . . . . . . . . . . 11

3 Layout hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Audible noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Thermal measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Conducted emission pre-compliance test . . . . . . . . . . . . . . . . . . . . . . 16

7 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8 PFC coil specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8.1 General description and characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8.2 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8.3 Mechanical aspect and pin numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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AN2755 List of tables

List of tables

Table 1. Measured temperature table at 115 Vac and 230 Vac - full load . . . . . . . . . . . . . . . . . . . . 15

Table 2. Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Table 3. Winding characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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List of figures AN2755

List of figures

Figure 1. Demonstration board (EVL6562A-400W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. EVL6562A-400W demonstration board electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. EVL6562A-400W compliance to EN61000-3-2 standard at full load . . . . . . . . . . . . . . . . . 7

Figure 4. EVL6562A-400W compliance to JEIDA-MITI standard at full load . . . . . . . . . . . . . . . . . . . . 7

Figure 5. EVL6562A-400W compliance to EN61000-3-2 standard at 70 W load . . . . . . . . . . . . . . . 7

Figure 6. EVL6562A-400W compliance to JEIDA-MITI standard at 70 W load . . . . . . . . . . . . . . . . . . 7

Figure 7. EVL6562A-400W input current waveform at 100 V - 50 Hz - 400 W load . . . . . . . . . . . . . . 8

Figure 8. EVL6562A-400W input current waveform at 230 V - 50 Hz - 400 W load . . . . . . . . . . . . . . 8

Figure 9. EVL6562A-400W input current waveform at 100 V - 50 Hz - 200 W load . . . . . . . . . . . . . . 8

Figure 10. EVL6562A-400W input current waveform at 230 V - 50 Hz - 200 W load . . . . . . . . . . . . . . 8

Figure 11. EVL6562A-400W input current waveform at 100 V - 50 Hz - 70 W load . . . . . . . . . . . . . . . 8

Figure 12. EVL6562A-400W input current waveform at 230 V - 50 Hz - 70 W load . . . . . . . . . . . . . . . 8

Figure 13. Power factor vs. Vin and load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 14. THD vs. Vin and load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 15. Efficiency vs. Vin and load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 16. Static Vout regulation vs. Vin and load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 17. EVL6562A-400W inductor current ripple envelope at 115 Vac - 60 Hz - full load . . . . . . . 10

Figure 18. EVL6562A-400W inductor current ripple (detail) at 115 Vac - 60 Hz - full load . . . . . . . . . 10

Figure 19. EVL6562A-400W inductor current ripple envelope at 230 Vac - 50 Hz - full load . . . . . . . 11

Figure 20. EVL6562A-400W Inductor current ripple (detail) at 230 Vac-50 Hz -full load. . . . . . . . . . . 11

Figure 21. EVL6562A-400W PCB layout (not scaled 1:1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 22. Thermal map at 115 Vac - 60 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 23. Thermal map at 230 Vac - 50 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 24. 115 Vac and full load - phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 25. 115 Vac and full load - neutral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 26. 230 Vac and full load - phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 27. 230 Vac and full load - neutral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 28. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 29. Pin side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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AN2755 Main characteristics and circuit description

1 Main characteristics and circuit description

The main characteristics of the SMPS are listed here below:

● Line voltage range: 90 to 265 Vac

● Minimum line frequency (f

● Regulated output voltage: 400 V

● Rated output power: 400 W

● Maximum 2f

● Hold-up time: 22 ms (V

● Maximum switching frequency: 85 kHz (V

● Minimum estimated efficiency: 90 % (V

● Maximum ambient temperature: 50 °C

● EMI: In accordance with EN55022 class-B

● PCB type and size: Single side, 70 µm, CEM-1, 148.5 x 132 mm

● Low profile design: 35 mm component maximum height

output voltage ripple: 10 V pk-pk

The demonstration board implements a power factor correction (PFC) pre-regulator delivering 400 W continuous power on a regulated 400 V rail from a wide-range mains voltage and provides for the reduction of the mains harmonics, which allows meeting the European norm EN61000-3-2 or the Japanese norm JEIDA-MITI. This rail is the input for the cascaded isolated DC-DC converter that provides the output rails required by the load.

): 47 Hz

after hold-up time: 300 V)

DROP

= 90 Vac, P

= 400 W)

out

= 400 W)

out

The board is equipped with enough heat sinking to allow full-load operation in still air. With an appropriate airflow and without any change in the circuit, the demonstration board can easily deliver up to 450 W.

The controller is the L6562A (U1), integrating all the functions needed to control the PFC stage.

The L6562A controller chip is designed for transition-mode (TM) operation, where the boost inductor works next to the boundary between continuous (CCM) and discontinuous conduction mode (DCM). However, with a slightly different usage, the chip can operate so that the boost inductor works in CCM, hence surpassing the limitations of TM operation in terms of power handling capability. The gate-drive capability of the L6562A is also adequate to drive the MOSFETs used at higher power levels. This approach, which couples the simplicity and cost-effectiveness of TM operation with the high-current capability of CCM operation, is the Fixed-OFF-time (FOT) control. The control modulates the ON-time of the power switch, while its OFF-time is kept constant. More precisely, it uses the line-modulated FOT (LM-FOT) where the OFF-time of the power switch is not rigorously constant but is modulated by the instantaneous mains voltage. Please refer to [2] for a detailed description of this technique.

The power stage of the PFC is a conventional boost converter, connected to the output of the rectifier bridge D2. It includes the coil L4, the diode D3 and the capacitors C6 and C7. The boost switch is represented by the power mosfets Q1 and Q2. The NTC R2 limits the inrush current at switch-on. It has been connected on the DC rail, in series to the output electrolytic capacitor, in order to improve the efficiency during low-line operation. Additionally, the splitting in two of output capacitors (C6 and C7) allows managing the AC current mainly by the film capacitor C7 so that the electrolytic can be cheaper as it just has to bear the DC part only.

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Main characteristics and circuit description AN2755

At startup the L6562A is powered by the Vcc capacitor (C12) that is charged via the resistors R3 and R4. Then the L4 secondary winding (pins 8-11) and the charge pump circuit (R5, C10, D5 and D4) generate the Vcc voltage powering the L6562A during normal operation.

The divider R32, R33 and R34 provides the L6562A multiplier with the information of the instantaneous voltage that is used to modulate the boost current. The divider R9, R10, R11, R12 & 13 is dedicated to sense the output. The Line-Modulated FOT is obtained by the timing generator components D6, C15, R15, C16, R16, R31, Q3.

The board is equipped with an input EMI filter designed for a 2-wire input mains plug. It is composed of two stages, a common mode pi-filter connected at the input (C1, L1, C2, C3) and a differential mode pi-filter after the input bridge (C4, L3, C5). It also offers the possibility to easily connect a downstream converter.

Figure 2. EVL6562A-400W demonstration board electrical schematic



1 2

90 - 265Vac

8A/250V

R32

620k

C3 680nF-X2

INV

COMP

MULT

R10

510k

R12 47K

L6562A

Q3 BC857C

D15X B60

-~~

R102 0R0

R11 510k

R13

12k

VCC

GND

ZCDCS

+400Vdc

CM-1.5mH-5A

470nF-X2

1M5

C14

2.2uF

R33

620k

C2 470nF

510k

C13 22 0nF

R14 47k

C21

R34

10nF

10k

L3 DM-51uH-6A

C4 470n F-630 V

R3 100K

R4 100K

C11 470nF/50V

R31 3k

C16 120p F

R101 0R0

R16 30k

C12 100uF/50V

C15 68pF

C5 470nF-63 0V

R15 1k8

811

D4 LL4148

LL41 48 D6

5-6

T PQ40- 500uH

R5 47R

C10 22N

D5 BZX8 5-C 18

C20 330pF

1N54 06

1-2

STTH8R06

470nF-630V

NTC 2R5-S237

330uF-450 V

+400Vdc

RTN

+400Vout

D7 LL4148

D8 LL4148

R20 0R47-1 W

STP1 2NM50FP

R21

0R4 7-1W

Q2 STP1 2NM50FP

R22

R23

0R47-1W

0R4 7-1W

R36 3R9

R17 6R8

R35 3R9

R18 6R8

R19

1K0

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AN2755 Test results and significant waveforms

2 Test results and significant waveforms

2.1 Harmonic content measurement

One of the main purposes of a PFC pre-conditioner is the correction of input current distortion, decreasing the harmonic contents below the limits of the actual regulations. Therefore, the board has been tested according to the European rule EN61000-3-2 class-D and Japanese rule JEIDA-MITI class-D, at full load and 70 W output power, at both the nominal input voltage mains.

As shown in the following figures of this page, the circuit is capable of reducing the harmonics well below the limits of both regulations from full load down to light load. 70 W of output power has been chosen because it approaches the lower power limit at which the harmonics have to be limited according to the mentioned rules.

Figure 3. EVL6562A-400W compliance to



EN61000-3-2 standard at full load

Measurements @ 230Vac Full load EN61000-3-2 class D limits

Figure 4. EVL6562A-400W compliance to

JEIDA-MITI standard at full load

Measurements @ 100Vac Full load JEIDA-MITI class D limits

0.1

0.01

Ha rm on i c c u rr en t ( A)

0.001

0.0001 1 3 5 7 9 111315171921232527293133353739

Harmonic Order (n)

Figure 5. EVL6562A-400W compliance to



0.1

0.01

Harmonic current (A)

0. 00 1

0.0001

EN61000-3-2 standard at 70 W load

Measurements @ 230Vac 70W EN61000-3-2 class D limits

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Harmonic Order (n)

0.1

0.01

Harmonic current (A)

0.001

0.0001 1 3 5 7 9111315171921232527293133353739

Harmonic Order (n)

Figure 6. EVL6562A-400W compliance to

JEIDA-MITI standard at 70 W load



Harmonic current (A)

0.001

0.0001

Measurements @ 100Vac 70W JEIDA-MITI cla ss D lim its

0.1

0. 01

1 3 5 7 9 1113151719 212325272931 33 3537 39

Harmonic Order (n)

For user reference, waveforms of the input current and voltage at the nominal input voltage mains and different load conditions are shown in the following figures.

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Test results and significant waveforms AN2755

Figure 7. EVL6562A-400W input current

waveform at 100 V - 50 Hz - 400 W load

Figure 9. EVL6562A-400W input current

waveform at 100 V - 50 Hz - 200 W load

Figure 8. EVL6562A-400W input current

waveform at 230 V - 50 Hz - 400 W load

Figure 10. EVL6562A-400W input current

waveform at 230 V - 50 Hz - 200 W load

Figure 11. EVL6562A-400W input current

waveform

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at 100 V - 50 Hz - 70 W load

Figure 12. EVL6562A-400W input current

waveform at 230 V - 50 Hz - 70 W load

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