ST AN2432 Application note

AN2432

Application note

EVALSTSR30-60W: 60W AC-DC Adapter with synchronous rectification using L6668 and STSR30

Introduction

This document describes a 60W adapter application using the L6668 fixed frequency current mode PWM controller and the STSR30 smart driver for flyback synchronous rectification.

This chipset guarantees low no-load consumption and high efficiency, making it easy to comply with world-wide mandatory and voluntary energy saving requirements.

EVALSTSR30-60W demo board

October 2006

Rev 2

1/27

www.st.com

AN2432	Contents

Contents

1	Adapter features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
	1.1	Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.2	Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Electrical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
3	Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
	3.1	Start-up behavior at full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	3.2	Wake-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	3.3	Power-down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	3.4	Short-circuit tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	3.5	Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
4	Conducted noise measurements (pre-compliance test) . . . . . . . . . . .		17
5	Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
6	Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		20
7	PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		23
8	Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		24
	8.1	Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
	8.2	Mechanical aspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		26

2/27

List of figures	AN2432

List of figures

Figure 1. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 2. VIN = 115VRMS - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 3. VIN = 230VRMS - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. VIN = 115VRMS - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. VIN = 230VRMS - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 6. VIN = 115VAC - CCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 7. CCM - Anticipation detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. VIN = 230VAC - DCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 9. DCM - INHIBIT synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 10. Burst mode operation at 230VAC and no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 11. Start-up at 88VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 12. Start-up at 264VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 13. Wake-up at 115VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 14. Wake-up at 230VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 15. Power-down at 115VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 16. Power-down at 230VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 17. Short circuit at 88VAC - 60Hz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 18. Short circuit at 264VAC - 50Hz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 19. OVP at 115VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 20. OVP at 230VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 21. OVP at 115VAC - no load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 22. OVP at 230VAC - no load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 23. CE peak measure at 115VAC and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 24. CE peak measure at 230VAC and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 25. VIN = 115VAC - full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 26. VIN = 230VAC - full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 27. Silk screen - top side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 28. Silk screen - bottom side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 29. Copper tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 30. Transformer electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 31. Windings position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3/10

AN2432	List of tables

List of tables

Table 1. Line and load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 2. Efficiency at 115VRMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 3. Efficiency at 230VRMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 4. No load consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 5. Power consumption with 0.5W output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 6. Mandatory energy saving requirements (from 1 January 2008) . . . . . . . . . . . . . . . . . . . . . 12 Table 7. Voluntary energy saving requirements (from 1 January 2008). . . . . . . . . . . . . . . . . . . . . . 12 Table 8. Comparison between standard and SR flyback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 9. Key components temperature at 115VAC - full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 10. Key components temperature at 230VAC - full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 11. Part list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 12. Winding characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 13. Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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Adapter features	AN2432

1 Adapter features

1.1Main characteristics

The 60W AC-DC adapter board described in this application note has the following main characteristics:

●Input:

–VIN: 88 ~ 264 VRMS

–f: 45 ~ 66 Hz

●Output:

–12VDC ± 2% - 5A

●No - Load:

–Pin below 0.3W

●Short circuit: protected with Auto-Restart feature

●PCB type and size:

–FR4

–Single side: 70 µm

–120 x 75 mm

●Safety: according to EN60065

●EMI: Compliance with EN55022 - Class B specifications

1.2Circuit description

This circuit implements a flyback transformer which is a very popular topology for this kind of application and power level, thanks to its simplicity and good trade-off between cost and performance. To improve the converter's efficiency, the EVALSTSR30 demo board uses synchronous rectification.

The converter works in both Continuous and Discontinuous conduction mode depending on the input voltage (the circuit has a wide input voltage range) and the output load. The 68-kHz switching frequency provides a good compromise between the transformer size and the harmonics of the switching frequency, optimizing input filter.

The input section includes protection elements (varistor, fuse and NTC for inrush current limiting), a standard Pi-filter for EMC suppression, a bridge and an electrolytic bulk capacitor as the front-end AC-DC converter. The transformer is a layer type, uses a standard ETD34 ferrite core and is designed to have a reflected voltage of 95V. The power MOS is a 700V - 1Ω and a transil clamp network is used for leakage inductance demagnetization.

On the primary side, the ST L6668 PWM controller integrates all the functions needed in a SMPS (switch mode power supply) and enables building a complete system with a low amount of external components. It includes a high voltage start-up generator, an overvoltage protection input, frequency foldback for better efficiency at light loads, programmable burst mode operation and soft start circuit.

5/27

J2

		1
88 to 264 Vac		2

Mains input

R25

15K

R27

20k

C12			C11
100nF			1nF

6/27

				NTC1
F1				5R		2
T2A						D7	HV_bus
						W06G
	4	3
RV1	C14	L1	C15	4	-	+	1
S14K275	100nF	2x27mH	100nF

1

2

				+ C8
3					100uF

HV_bus

T1

1

13

12

D8

R16

C5

1.5KE200

2.7Meg

1nF

C13

R19

D5 STTH1L06U

470pF

33k

3

10

D11 BAV103

L3

100uH

6

9

7

C16

D6

R28

100nF

R17

BAV103

R23

3.9k

2.7Meg

+

N.M.

C6

3

2

47uF

Q2

BC807

C18

1

2.2nF

15

R18

8

7

5

1

U3

9.1k

VREF

COMP-S

DIS

VCC

HV

R33

C23

9.1k

220pF

R11

22

Q3

13

4

STP9NK70ZFP

STBY

OUT

14

L6668

D10

R29

PFC_STOP

LL4148

10

12

16

ISEN

N.C.

HVS

SKIPADJ

SS

COMP

GND

RCT

R12

1k

C20

220pF

6

2

9

11

10

3

R10

R9

0.56

0.56

C10

4

U5B

R26

68nF

PC817

82k

R24

C9

C17

47K

1nF

2.2nF

3

AN2432

.1 Figure

diagram Electrical

L2

D1

2.2uH

J1

STPS2H100

+ C1

+

C19

+

1

1000uF

1000uF

C21

C26

2

12V / 5A

100uF

N.M.

Output Connector

Q1

STP75NF75L

R20

2.7k

D3

R3

+5V

LL4148

22

C3

U2

L78L05

1

IN

3

OUT

+5V

100nF

GND

2

4

5

C25

C2

R1

100nF

U1

2

OUTgate

PWRGND

Vcc

100nF

1k

D2

R4

R22

C22

LL4148

33k

SGLGND

3

470nF

8

CK

1.5k

DISABLE

SETANT

7

1

R21

R37

1

R2

20k

330k

INHIBIT

1k

R5

C29

U5A

33k

STSR30

6

100nF

PC817

R13

2

N.M.

8

D9

U4

R32

1N4148WS

D4

C27

3

Vcc

1

0

Vref

CV Out

LL4148

100pF

C28

R38

R6

1nF

33k

C7

100k

470nF

5

Cc+

2

Cv -

Adapter

4

R8

+5V

Cc-

Gnd

33k

6

CC Out

R14

7

2.2k

TSM1015

R15

68k

C4

R40

C24

4.7nF

1Meg

220pF

features

Adapter features	AN2432

The self supply circuit (Q2, R33, C23, L3, D6 and C6) ensures:

●a constant VCC voltage with respect to load variations

●enough energy during no-load periods

●a poor (under UVLO) supply voltage during short-circuit failures

A separate rectifying circuit (D11, R19, R28 and C13) derives a voltage level that best matches the output voltage for accurate overvoltage protection.

As seen, the primary side is quite standard. The most interesting part of this demo board lies in the secondary side. Here we can find the STSR30, a smart driver for flyback synchronous rectification (SR). The flyback output diode is substituted with a power MOSFET (a 75V - 10mΩ) that dramatically reduces the conduction losses. A small Schottky diode (D1) is mounted in parallel to the MOSFET body diode to keep low the voltage drop during dead times (while the SR MOS is off and current is circulating in the secondary).

The STSR30 can work in both Continuous and Discontinuous conduction mode and uses 2 pins to synchronize the SR MOSFET with the flyback. The SR MOSFET drain provides the synchronization information; when the primary side MOSFET is turned off, the drain voltage of the SR MOSFET falls from VOUT + VIN/n (where n is the transformer turns ratio n1/n2) down to zero. This falling edge is sensed by the CK pin and the IC turns on the SR MOSFET. Behavior varies according to the flyback transformer operating mode:

●Continuous conduction mode (CCM): the STSR30 uses an internal digital counter to predict when it has to turn off the SR MOSFET.

●Discontinuous conduction mode (DCM): the STSR30 senses the voltage on the INHIBIT pin (that is, Rdson x Isec) and turns off the SR MOSFET when it reaches the -25mV threshold (i.e. the current is approaching zero).

During CCM operation, a certain amount of anticipation is used to prevent cross-conduction of Q3 and Q1. This anticipation can be selected among three values by biasing the SETANT pin. In the demo board, the SETANT voltage is 2.5V so the anticipation is 225ns.

The STSR30 works at 5V so it is necessary to obtain such voltage from the output. A low cost linear regulator (L78L05) is used. For the same reason the gate drive of the IC has a high value of 5V so a low threshold (logic level) MOSFET has to be used.

Another interesting feature of the STSR30 is its disable input. This is useful at low loads to turn off the IC and reduce its power consumption. In this condition, the Schottky diode D1 works like in a standard flyback. The information on the load level is obtained by averaging the voltage on the CK pin using R6, R15 and C4. The CK pin is low (~ 0V) only when the current in the secondary winding is flowing (SR MOSFET on). Otherwise, the pin is pulled up at 5V. As the load decreases, the average voltage on CK pin becomes higher and higher. This voltage level is monitored by the last IC used, the TSM1015, a CV/CC controller that includes a voltage reference and two op-amps. The reference and the CV op-amp are used for the voltage control loop of the converter. The CC op-amp is not used for the current control loop but it acts as a comparator to sense the average voltage of the CK pin. At light loads, the CK voltage exceeds the threshold (VREF) and the TSM1015 turns off the STSR30. By adding a little hysteresis (using R40), the DISABLE pin of the STSR30 is driven digitally with a good noise rejection.

The next two pictures show some waveforms during normal operation at full load. It is possible to see that the converter operates in CCM at 115 VRMS and in DCM at 230 VRMS.

7/27

AN2432	Adapter features
Figure 2. VIN = 115VRMS - 60Hz	Figure 3. VIN = 230VRMS - 50Hz

Ch1: Q3 drain voltage	Ch1: Q3 drain voltage
M1: ISEN pin voltage	M1: ISEN pin voltage

Figure 4 and Figure 5 show some of most important signals of the L6668 while operating at full load. The oscillator signal is stable and clean in all conditions.

Figure 4. VIN = 115VRMS - 60Hz

Figure 5. VIN = 230VRMS - 50Hz

CH1: Out	CH1: Out
CH2: S-COMP	CH2: S-COMP
CH3: COMP	CH3: COMP
CH4: RCT	CH4: RCT

On the secondary side, in CCM operation (full load with VIN = 115VAC), the gate drive of the STSR30 is synchronized with the CK pin (copy of SR MOSFET drain voltage clamped at 5V) as shown in Figure 6.

In Figure 7, the turn-off detail is zoomed and it is possible to see the anticipation amount (225ns) and the jitter due to the digital counter inside the IC. In fact, most times the anticipation has its typical value but sometimes the counter vary its value of ±1 cycle (approximately ±70ns using the 14-MHz internal oscillator). In any case, cross-conduction is always avoided.

8/27

Adapter features	AN2432
Figure 6. VIN = 115VAC - CCM	Figure 7. CCM - Anticipation detail

CH1: L6668 gate drive CH2: STSR30 gate drive CH3: STSR30 CK pin

CH1: L6668 gate drive CH2: STSR30 gate drive CH3: STSR30 CK pin

In DCM operation, the gate-drive turn-on is still triggered by the falling edge of the CK pin voltage, while turn-off is determined by the INHIBIT pin voltage crossing the -25mV internal threshold. Figure 8 and Figure 9 show this mechanism at full load and VIN = 230VAC conditions.

Figure 8. VIN = 230VAC - DCM

Figure 9. DCM - INHIBIT synchronization

CH1: L6668 gate drive	CH3: STSR30 CK pin
CH2: STSR30 gate drive	CH4: STSR30 gate drive
CH3: STSR30 CK pin

9/27