ST AN2521 Application note

AN2521

Application note

19 V - 75 W laptop adapter with tracking boost PFC

pre-regulator, using the L6563 and L6668

Introduction

This application note describes the characteristics and features of a 75 W wide range input mains and power-factor-corrected ac-dc adapter evaluation board. Its electrical specification is tailored to a typical high-end portable computer power adapter. The distinctive attributes of this design are the very low standby input consumption (< 0.3 W at 265 V), the excellent global efficiency (> 85%) for a two stage architecture and the low cost.

Figure 1. L6668 and L6563-75W adapter evaluation board (EVAL6668-75W)

October 2007 Rev 1 1/33

www.st.com

Contents AN2521

Contents

1 Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . . 4

2 Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Efficiency measurements at full load, tracking boost option (TBO) . . . . . . 8

2.2 Harmonic content measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1 Normal operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Standby and no-load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Over current and short circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.4 Overvoltage and open loop protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 EVAL6668-75W: thermal map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2 Thermal map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5 Conducted emission pre-compliance test . . . . . . . . . . . . . . . . . . . . . . 22

6 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7 PFC coil specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.1 General description and characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.2 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.3 Electrical schematic and winding characteristics . . . . . . . . . . . . . . . . . . . 28

7.4 Mechanical aspect and pin numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

8 Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8.1 General description and characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8.2 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8.3 Electrical diagram and winding characteristics . . . . . . . . . . . . . . . . . . . . . 30

8.4 Mechanical aspect and pin numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2/33

AN2521 List of figures

List of figures

Figure 1. L6668 and L6563-75W adapter evaluation board (EVAL6668-75W) . . . . . . . . . . . . . . . . . . 1

Figure 2. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 3. EVAL6668-75W global efficiency measurements at full load . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 4. L6563 tracking boost and voltage feed-forward blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 5. EVAL6668-75W PFC output voltage vs. ac input voltage . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6. PFC efficiency with and without TBO function at full load . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 7. Flyback converter efficiency with and without TBO function at full load . . . . . . . . . . . . . . . 10

Figure 8. Comparison between the global efficiency with and without TBO . . . . . . . . . . . . . . . . . . . 11

Figure 9. EVAL6668-75W compliance to EN61000-3-2 standard @230 V, 50 Hz - full load . . . . . . 11

Figure 10. EVAL6668-75W compliance to JEIDA-MITI standard @100 V, 60 Hz - full load . . . . . . . . 11

Figure 11. EVAL6668-75W compliance to EN61000-3-2 standard @230 V, 50 Hz - half load . . . . . 12

Figure 12. EVAL6668-75W compliance to JEIDA- MITI standard @100 V, 60 Hz - half load . . . . . . . 12

Figure 13. EVAL6668-75W input current waveform @100 V, 60 Hz - full load . . . . . . . . . . . . . . . . . . 12

Figure 14. EVAL6668-75W input current waveform @230 V, 50 Hz - full load . . . . . . . . . . . . . . . . . . 12

Figure 15. EVAL6668-75W flyback stage waveforms @115 V, 60 Hz-full load. . . . . . . . . . . . . . . . . . 13

Figure 16. EVAL6668-75W flyback stage waveforms @230 V, 50 Hz-full load. . . . . . . . . . . . . . . . . . 13

Figure 17. Adapter circuit primary side waveforms 265 V, 50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 18. EVAL6668-75 W no-load operation waveforms @90 V, 60 Hz . . . . . . . . . . . . . . . . . . . . . 14

Figure 19. EVAL6668-75 W no-load operation waveforms @265 V, 50 Hz . . . . . . . . . . . . . . . . . . . . 14

Figure 20. EVAL6668-75 W transition full load-to-no load at 265 V, 50 Hz . . . . . . . . . . . . . . . . . . . . . 15

Figure 21. EVAL6668-75 W transition no load-to-full load at 265 V, 50 Hz . . . . . . . . . . . . . . . . . . . . . 15

Figure 22. EVAL6668-75 W short circuit at full load & 230 Vac-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 23. EVAL6668-75 W short circuit removal at full load & 230 Vac-50 Hz . . . . . . . . . . . . . . . . . 17

Figure 24. EVAL6668-75 W short circuit at no-load & 230 Vac-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 25. EVAL6668-75 W short circuit removal at no-load & 230 Vac-50 Hz. . . . . . . . . . . . . . . . . . 18

Figure 26. EVAL6668-75W Open loop at 115 Vac-60 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 27. Thermal map at 115 Vac-60 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 28. Thermal map at 230 Vac-50 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 29. CE peak measure at 100 Vac and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 30. CE peak measure at 230 Vac and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 31. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 32. Mechanical aspect and pin numbering of PFC coil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 33. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 34. Winding position on coil former. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 35. Mechanical aspect and pin numbering of flyback transformer . . . . . . . . . . . . . . . . . . . . . . 32

3/33

Main characteristics and circuit description AN2521

1 Main characteristics and circuit description

The main characteristics of the SMPS are listed here below:

● Universal input mains range: 90 - 264 Vac, 45 − 65 Hz

● Output voltages: 19 V @ 4 A continuous operation

● Mains harmonics: in accordance with EN61000-3-2 class-D

● Standby mains consumption: less than 0.3 W @ 265 Vac

● Overall efficiency: greater than 85%

● EMI: in accordance with EN55022-class B

● Safety: in accordance with EN60950

● PCB single layer: single side, 70 µm, CEM-1, 78 x 174 mm, mixed PTH/SMT

The circuit is made up of two stages: a front-end PFC using the L6563 and a flyback converter based on the L6668. The electrical schematic is shown in Figure 2.

The flyback stage works as the master stage and therefore is dedicated to controlling circuit operation, including standby and protection functions. Additionally, it switches the PFC stage on and off the by means of a dedicated pin on the control IC, thus helping to achieve good efficiency even at light load. The input EMI filter is a classic Pi-filter, 1-cell for differential and common mode noise. An NTC in series with the PFC output capacitor limits the inrush current produced by the charging of the capacitor at plug-in.

The purpose of the PFC stage is to reduce the harmonic content of the input current to be within the limits imposed by European norm EN61000-3-2. Additionally, it provides a regulated dc bus used by the downstream converter.

The PFC controller is the L6563 (U1), working in transition mode. It integrates all functions needed to control the PFC as well as an interface to the master converter. Its power stage topology is a conventional boost converter, connected to the output of the rectifier bridge. It includes the coil L2, the diode D3, the capacitor C6 and the power switch Q2, a power MOSFET.

The secondary winding of L2 (pins 8-3) provides the L6563 with information about the core demagnetization of the PFC coil, needed by the controller for TM (transition mode) operation. The divider R7, R12 and R18 provides the L6563 with the instantaneous input voltage information that is used to modulate the boost current, and to derive additional information such as the average value of the ac line, which is used by the V

(voltage feed-

forward) function. The divider R2, R6, R8, R9 is dedicated to sensing the output voltage and feeds the information to the error amplifier, while the divider R3, R5, R11, R19, directly connected to the output voltage, is dedicated to protecting the circuit in case of voltage loop failure. To maximize overall efficiency, the PFC makes use of the so-called "tracking boost option" (TBO). With this function implemented the dc output voltage of the PFC changes proportionally with the mains voltage. The L6563 achieves this functionality by adding a resistor (R30) connected to the dedicated TBO pin (#6).

The PFC is switched on and off by a switch (Q1) on the V

pin of the L6563, which is

activated by the PFC-STOP pin of the L6668. The PFC-STOP pin is intended to stop the PFC controller at light load by cutting its supply. This happens when the COMP pin on the L6668 controller goes below 2.2V.

The downstream converter, acting as the master stage, is managed by the L6668 IC (U2), a current mode controller. The 65 kHz nominal switching frequency has been chosen to

4/33

AN2521 Main characteristics and circuit description

achieve a compromise between the transformer size and the harmonics of the switching frequency, thereby optimizing the input filter size and the total solution cost. The power MOSFET is a standard, inexpensive 800 V component housed in a TO-220FP package, requiring a small heat sink. The transformer is the layer type, using the standard ferrite core EER35. The transformer is manufactured by TDK and designed in accordance with EN60950. The reflected voltage is ~130 V, providing sufficient room for the leakage inductance voltage spike while maintaining a margin for the reliability of the power MOSFET. The rectifier D8 and the Transil D4 clamp the peak of the leakage inductance voltage spike at turn-off of the power MOSFET.

The controller L6668 offers maximum flexibility by integrating all the functionality needed for high performance SMPS control with a minimum component count. A new feature embedded in the device is a high voltage current source used at start-up which draws current directly from the dc bus and charges capacitor C33. After the voltage on C33 has reached the L6668 turn-on threshold and the circuit starts to operate, the controller is powered by the transformer via the auxiliary winding and diode D11. After start-up, the HV current source is deactivated, saving power during normal operation and allowing very good circuit efficiency during standby.

The L6668 utilizes a Current Mode control system, so the current flowing through the primary winding is sensed by R52 and R53 and is then fed into pin #12 (ISEN). Resistor R41 connected between pin #12 (ISEN) and pin #15 (S_COMP) provides the correct slope compensation to the current signal, necessary for correct loop stability in CCM mode at duty cycles greater than 50%. The circuit connected to pin #7 (DIS) provides over-voltage protection in case of feedback network failure, while the thermistor R58 provides for a thermal protection of the power MOSFET (Q5). This pin is also connected to the PWM_LATCH pin of the L6563 which is dedicated to stopping activity of the flyback converter in case of PFC loop failure that could be damaging to the circuit. To definitively latch this state, the internal circuitry of the L6668 monitors the V

and periodically

reactivates the HV current source to supply the IC. After OVP detection and L6668 Disable intervention, circuit operation can be resumed only after disconnection of the mains plug. The switching frequency is programmed by the RC connected to pin #16 (RCT) and in case of reduced load operation the controller can decrease the operating frequency via pin #13 (STBY) and resistor R42, proportionate with the load consumption. The resistor divider R60 and R61 connected to pin #9 (SKIPADJ) allows setting of the initial L6668 threshold to Burst Mode functionality when the power supply is lightly loaded. Additional functions embedded in the L6668 are the programmable soft-start and a 5 V reference, available externally.

Circuit regulation is achieved by modulating the voltage on the COMP pin (#10), by means of the optocoupler U3. Also connected to the COMP pin is the Q6, Q8, R44, R62, C42 and D13 network, which is dedicated to driving ISEN over its hiccup mode threshold in case of overload or short condition. In this case the device will be shut down and its consumption will decrease almost to pre-start-up level. The device will resume operation as soon as the V

voltage has dropped below the VCC restart level. Thus a reliable hiccup mode is

invoked until the short is removed. A short on-time and long off-time of the hiccup mode are obtained allowing the average current flowing in the secondary side components to be kept at a safe level, avoiding consequent catastrophic failures due to their overheating.

Output regulation is done by means of two loops, a voltage and a current loop working alternately. A dedicated control IC, the TSM1014, has been used. It integrates two operational amplifiers and a precise voltage reference. The output signal of the error amplifiers drives optocoupler SFH617A-4 to transfer the information to the primary side and achieve the required insulation of the secondary side. The output rectifier D7 is a dual common-cathode Schottky diode. The output rectifier has been selected according to the

5/33

Main characteristics and circuit description AN2521

calculated maximum reverse voltage, forward voltage drop and power dissipation. The snubber, made up of R14, R66 and C8, damps the oscillation produced by the diode D7. A small LC filter has been added on the output in order to filter the high frequency ripple.

6/33

AN2521 Main characteristics and circuit description

Figure 2. Electrical diagram

R55

22R

C36

100N

C43

12J2

19V@4A

CON2-IN

C17

100N

100uF-25V YXF

C13

TSL0706 - 1R5-4R3

R20

20K

R22

R015-1W - MSR1

1000uF-25V ZL

R66

3R9

1000uF-25V ZL

C16

R14

3R9

2N2 - Y2

2M2R52M2

1M0-1%

100uF-450V

NTC 10R-S236

STTH2L06

1N4005

SRW25CQ-T03H102

6 5

GBU4J

HF2826-253Y1 R2-T01

FUSE 4A

INPUT CONN.

68K

470N-400V

470NF-X2

2N2

470NF-X2

123

R11

2M2

1M0-1%

C12

1N0-200V

STPS20H100CFP

10-11

15-16

2-3

1.5KE250A

STTH10 8A

C10

RES

R13

RES

BZV55-B30

R19

36K

R16

10K

BC857C

C11

RES

C14

220N

L6563

100N

R15

RES

R12

3M3

R21

RES

VCC

INV1COMP2MULT3CS4VFF5TBO6PFC-OK

R17

C15

R10

RES

75K-1%

3M3

C24

2N2 - Y2

SRW32EC-T01H114

5-6

R35

D11

R72

0R0

R32

RES

R31

4K7

C22

2u2-25V

R28

2K2

BZV55-C8V2

STP9NK50ZF P

R27

0R33

R29

RES

R24

100K

R25

470R

R23

27R

C20

10N

62K

1uF

ZCD

RUN

GND

PWM-STOP

C19

2N2

C21

470N

R18

51K

D10

PWM-LATCH

C26

22N

R30

22K

C25

220PF

C23

RES

R26

120K

R69

1K0

R67

6K2-1%

R68

120K-1%

R39

56K-1%

R40

RES

C29

RES

R45

2K2

R36

1K8

SFH617A-4

C28

RES

R38

2R7

BAV103

LL4148

RES

C27

47uF-50V

R73

62K

R71

RES

D15

RES

R43

RES

R33

10K

R34

270K

4R7

D12

LL4148

R37

10K-1%

2N2-5%

C30

RCT

L6668

HV1HVS2GND3OUT4VCC5N.C.6DIS

VCC

CC_OUT

V_REF

R49

24K-1%

R48

4K7-1%

C31

RES

STP10NK80ZFP

R53

0R39

R52

0R39

R47

100K

R46

47R

R51

2K2

C34

100PF

R42

8K2

R41

10K

ISEN

STBY

S_COMP

PFC_STOP

C33

22uF-50V

C32

100N

R50

1K0

2N2

GND

CV_OUT

TSM1014

CV-

CC+3CC-

R65

22K

C44

100N

C35

270N

R54

47K

TS3431IZ-RES

D14

LL4148

R64

43K-1%

BC857C

R44

47K

BC847C

R62

3K3

C42

10uF-50V

D13

LL4148

R57

100R

C37

82N

C39

4N7

COMP

C41

10N

SKIP_ADJ

R61

33K

R60

56K

VREF

C40

100N

R58

M57703

R59

24K

C38

470PF

OTP PROT

R56

4K7-1%

JP7

RES

1 2

90-264Vac

7/33

Test results AN2521

2 Test results

2.1 Efficiency measurements at full load, tracking boost option (TBO)

The following table and diagrams show the single converter and overall efficiency measured at different input voltages. These measurements are performed with nominal load (4 A).

Table 1. Efficiency measurements at full load using the TBO function

Vin

Efficiency

PFC dc-dc Global

90 [V] 93.63% 89.83% 84.11%

115 [V] 95.62% 89.07% 85.17%

230 [V] 97.84% 89.81% 87.87%

265 [V] 97.53% 89.06% 86.86%

1. Compliant to CEC, EU-COC, regulation. In Table 1 and Figure 3 the single converter efficiency

measurement is shown. Thanks to the very good efficiency of any single block the overall efficiency is very high too, especially if we compare this data with similar converters using a double stage and a flyback topology as downstream converter.

Figure 3. EVAL6668-75W global efficiency measurements at full load

90% 89% 88% 87% 86% 85% 84% 83% 82%

OVERALL EFFICIENCY

81% 80%

WITH TBO

90 115 230 265

Vin [Vrms]

(1)

Table 2. ENERGY STAR compliance

Vin

115 [V] 85.26% 86.32% 86.28% 85.17% 85.75%

230 [V] 83.4% 85.2% 86.74% 87.87% 85.8%

1 A 2 A 3 A 4 A Average

In Ta bl e 2 the ENERGY STAR efficiency measurements are shown. The average of the two mains voltage inputs in four different load conditions is compliant with the target requirement (better than 84%).

8/33

ENERGY STAR efficiency

AN2521 Test results

To achieve optimal efficiency the PFC stage implements the tracking boost function. It consists of a PFC output voltage that follows the input voltage. Typically, in traditional PFC stages, the dc output voltage is regulated at a fixed value (typically 400 volts) but in some applications, such as this one using a flyback as the downstream converter, it could be advantageous to regulate the PFC output voltage with the tracking boost or "follower boost" approach. In this way the circuit with the TBO function provides improved efficiency and, thanks to the lower differential voltage across the boost inductor, the value of L2 can be reduced as compared to the same circuit without the TBO function. In the present case a 400 µH inductor has been used, while with a fixed output voltage PFC working at a similar operating frequency, a 700 µH inductor is required.

To achieve the TBO function on the L6563, a dedicated input of the multiplier is available on TBO pin #6. This function can be implemented by simply connecting a resistor (RT) between the TBO pin and ground.

Figure 4. L6563 tracking boost and voltage feed-forward blocks

Vout

COM

INV 1

9.5V

TBO

2.5V

E/A

CURRENT

1:1

L6563

L6563A

ITBO

current

reference

MULTIPLIER

TBO

1/V

"ideal"

diode

9.5V

VFF

Rectified mains

MUL

The TBO pin presents a dc level equal to the peak of the MULT pin voltage and is then representative of the mains RMS voltage. The resistor defines the current, equal to V(TBO)/RT, which is internally mirrored 1:1 and sunk from the INV pin (pin 1) input of the error amplifier. In this way, when the mains voltage increases, the voltage at the TBO pin will increase as well, and so will the current flowing through the resistor connected between TBO and GND. A larger current will then be sunk by the INV pin and the output voltage of the PFC pre-regulator will be forced higher. Obviously, the output voltage will move in the opposite direction if the input voltage decreases.

To avoid an unwanted rise in output voltage should the mains voltage exceed the maximum specified value, the voltage at the TBO pin is clamped at 3 V. By properly selecting the multiplier bias it is possible to set the maximum input voltage above which input-to-output tracking ends and the output voltage becomes constant. If this function is not used, the pin should be left open; the device will regulate at a fixed output voltage.

9/33

Test results AN2521

Figure 5. EVAL6668-75W PFC output voltage vs. ac input voltage

417 384 351 318 285 252 219

PFC OUTPUT VOLTAGE [V]

186

80 130 180 230 280

242

218

Vin [Vrms]

351

384

In Figure 5 we can see that the PFC output voltage variation vs. the ac input voltage (i.e. the input voltage for the flyback stage) is dependent on the input mains voltage, but its range is narrower than a wide range input. Thus the design of the flyback converter is not completely optimized as with a standard PFC delivering a stable 400 V output, but its design is much simpler than that of a wide range flyback. Additionally, the PFC converter using the TBO, with its lower differential voltage across the inductor and lower current ripple, will have lower RMS current and therefore better efficiency at low mains, where normally the efficiency of typical PFCs is lower. The result is a global efficiency of the circuit that will be higher than that of a fixed output voltage one circuit, especially at lower mains. Most of the power dissipation will not be concentrated on the PFC only but will be shared with the flyback. Therefore, there will not be thermal hotspots and the reliability of the circuit will be improved.

This is confirmed in the diagram in Figure 6, where the efficiency of the PFC has been measured both with the active TBO function and without it. As shown, at low input mains the circuit has an efficiency improvement better than 2 percent. As the input mains voltage increases the switching losses become more significant and the fixed output voltage PFC appears more efficient.

Figure 6. PFC efficiency with and without

TBO function at full load

100%

PFC STAGE EFFICIENC

99% 98% 97% 96% 95% 94% 93% 92% 91% 90%

WITHOUT TBO WITH TBO

90 115 230 265

Vin [Vrms]

Using the TBO function even the flyback converter efficiency is very good, as shown in

Figure 7 where it is compared with the efficiency of the same converter powered by a fixed

Figure 7. Flyback converter efficiency with

and without TBO function at full load

95% 94% 93% 92% 91% 90% 89% 88% 87% 86% 85%

FLYBACK STAGE EFFICIENC

400 Vdc FIXED I/P VOLTAGE WITH TBO

90 115 230 265

Vin [Vrms]

10/33

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