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 JEIDAMITI 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 VFF (voltage feedforward) 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 VCC 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 VCC 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 VCC 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

Figure

FUSE 4A

1N4005

INPUT CONN.

HF2826-253Y1R2-T01

GBU4J

SRW25CQ-T03H102

NTC 10R-S236

2N2

STTH2L06

470NF-X2

470N-400V

100uF-450V

2N2

90-264Vac

2M2

Electrical

470NF-X2

1M0-1%

68K

2M2

1M0-1%

2N2 - Y2

R11

2M2

diagram

75K-1%

R14

R10

1N0-200V

3R9

3M3

RES

C10

R13

RES

1.5KE250A

R66

100N

3R9

TSL0706 - 1R5-4R3

R12

BZV55-B30

2-3

15-16

3M3

R15

RES

L6563

C14

C11

BC857C

R16

STPS20H100CFP

220N

RES

10K

C12

C16

R20

C13

C17

R19

20K

100N

36K

1000uF25V-

1000uF25VZL-

100uFYXF25V-

INV

VCC

R21

STTH108A

CON2-IN

C15

R17

10-11

RES

1uF

62K

BZV55-C8V2

R22

19V@4A

COMP

5-6

R015-1W - MSR1

STP9NK50ZFP

MULT

GND

R23

R18

C19

27R

R24

51K

2N2

100K

ZCD

R25

R28

470R

2K2

VFF

RUN

C20

R29

R27

C24

C21

10N

RES

0R33

C22

2N2 - Y2

470N

TBO

PWM-STOP

2u2-25V

C23

R30

PFC-OK PWM-LATCH

R31

R32

RES

22K

4K7

RES

Main

SRW32EC-T01H114

R26

R72

120K

C25

C26

0R0

220PF

22N

D10

D11

R35

characteristics

RES

LL4148

BAV103

2R7

R34

R33

RES

R71

R73

C27

270K

10K

RES

62K

47uF-50V

RES

C28

RES

R40

R67

R36

RES

6K2-1%

D15

1K8

R39

R37

RES

R38

56K-1%

R69

L6668

10K-1%

RES

1K0

C29

RES

R68

120K-1%

RCT

C30

D12

R43

SFH617A-4

R45

2N2-5%

LL4148

4R7

3 4

2 1

2K2

HVS

S_COMP

R55

R42

STP10NK80ZFP

R48

R49

22R

GND

PFC_STOP

R41

8K2

4K7-1%

24K-1%

10K

R46

R47

47R

100K

OUT

STBY

R50

C31

and

1K0

C32

VCC

ISEN

RES

C36

100N

C34

R51

R52

R53

100N

C33

100PF

2K2

0R39

V_REF

VCC

22uF-50V

N.C.

C37

C44

CC-

CC_OUT

DIS

COMP

82N

R54

C35

100N

circuit

R57

47K

270N

R56

100R

CC+

GND

4K7-1%

C38

VREF

SKIP_ADJ

470PF

R58

CV-

CV_OUT

M57703

D13

TSM1014

C39

LL4148

R62

4N7

3K3

BC847C

OTP PROT

R60

BC857C

R65

C43

56K

22K

2N2

R59

C40

24K

100N

R61

C41

TS3431IZ-RES

description

33K

10N

C42

10uF-50V

R44

47K

R64

D14

43K-1%

LL4148

7/33

JP7

RES

Test results	AN2521

2 Test results

2.1Efficiency 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
	Vinac		Efficiency

		PFC	dc-dc	Global

	90 [V]	93.63%	89.83%	84.11%

	115 [V]	95.62%	89.07%	85.17% (1)
	230 [V]	97.84%	89.81%	87.87% (1)
	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%
EFFICIENCY	89%	WITH TBO
		WITH TBO
	88%
	87%
	86%
	85%
OVERALL	85%
	84%
	83%
	82%
	81%
	80%
	90	115	230	265

Vin [Vrms]

Table 2.	ENERGY STAR compliance
			ENERGY STAR efficiency

Vinac	1 A	2 A	3 A	4 A	Average
115 [V]	85.26%	86.32%	86.28%	85.17%	85.75%

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

In Table 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

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	Rectified mains
Vout

				2	current
IR	R1				reference
	R1		2.5V
			2.5V
	INV	1	+	E/A	MULTIPLIER	2	R5
			-		1/V		R5
			-		1/V
		9.5V				"ideal"
				1:1	-	"ideal"
		ITBO			-	diode	3
		ITBO			+	diode	3
				CURRENT	+
IR	R2				3V		MUL
	R2				3V	9.5V	MUL
						9.5V
							R6

	6	5
	TBO	VFF
ITBO	RT	CF
		RF

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
[V]	384				384
[V]
VOLTAGE
	351			351
				351
	318

OUTPUT	285
	252
		242
PFC	219	218
		218

	186
	80	130	180	230	280

Vin [Vrms]

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

	100%					95%
	99%	WITHOUT TBO			FLYBACK STAGE EFFICIENCY	94%	400 Vdc FIXED I/P VOLTAGE
	99%
PFC STAGE EFFICIENCY	98%	WITH TBO				93%	WITH TBO
	98%					93%
	97%					92%
	97%
	96%					91%
	96%
	95%					90%
	95%
	94%					89%
	94%
	93%					88%
	93%
	92%					87%
	92%
	91%					86%
	91%
	90%					85%
	90%
		90	115	230	265	90	115	230	265
		90	115	230	265
				Vin [Vrms]				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

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