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

4/10

Adapter features AN2432

1 Adapter features

1.1 Main characteristics

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

● Input:

–V
– f: 45 ~ 66 Hz

● Output:

– 12V

● 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

: 88 ~ 264 V

IN

± 2% - 5A

DC

RMS

1.2 Circuit 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

AN2432 Adapter features

Figure 1. Electrical diagram

12V / 5A

J1

1

2

Output Connec tor

C26

N.M.

+

C21

100uF

L2

2.2uH

U2 L78L05

C19

1000uF

+

C1

1000uF

+

D1

STPS2H100

13

T1

C8

100uF

+

HV_bus

1

+

D7

W06G

2

NTC1

5R

F1

T2A

3

-

4

C15

100nF

L1

2x27mH

2 3

1 4

C14

100nF

RV1

S14K275

1

HV_bus

10129

3

D5 STTH1L06U

D8

1.5KE200

C5

1nF

R16

2.7Meg

R19

33k

C13

470pF

Q1

6

L3 100uH

D11 BAV103

+5V

C3

R3

22

R20

2.7k

D3

STP75NF75L

LL4148

7

Q2

R23

N.M.

3 2

D6

BAV103

C6

47uF

+

R17

2.7Meg

C16

100nF

R28

3.9k

C22

470nF

C25

100nF

R22

3

IN

OUT

1

100nF

+5V

C18

2.2nF

BC807

1

R18

1.5k

GND

2

R1

1k

C2

100nF

3

Vcc

5

SGLGND

PWRGND

4

OUTgate

2

CK8INHIBIT

U1

R4

33k

D2

LL4148

Q3

STP9NK70ZFP

C23

220pF

R29

R11

22

R33

9.1k

1

5

7

15

8

U3

D10

9.1k

4

OUT

HV

VCC

DIS

L6668

S-COMP

VREF

STBY

13

14

R37

R21

7

10

LL4148

PFC_STOP

330k

20k

12

R2

SETANT

1

U5A

C29

1k

DISABLE

R5

12

ISEN

RCT

16

R32

PC817

8

100nF

U4

R13

N.M.

6

STSR30

D9

33k

R12

1k

C20

220pF

GND

COMP

SS

SKIPADJ

HVS

N.C.

0

C7

470nF

1

2

Cv-

CV Out

Vcc

Vref3Cc+5Cc-

R38

33k

C28

1nF

C27

100pF

D4

LL4148

1N4148WS

R6

100k

R9

0.56

R10

0.56

3

10

11

9

2

6

+5V

R8

33k

U5B

PC817

43

C10

68nF

R26

R14

2.2k

7

CC Out

Gnd

4

TSM1015

6

C24

220pF

R40

1Meg

C4

4.7nF

R15

68k

C9

1nF

R24

47K

C17

2.2nF

82k

R25

1

2

J2

Mains input

15K

R27

20k

C11

1nF

C12

100nF

88 to 264 Vac

6/27

Adapter features AN2432

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

● a constant V

● enough energy during no-load periods

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

voltage with respect to load variations

CC

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 V

+ VIN/n (where n is the transformer turns ratio n1/n2)

OUT

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 (V

) and the TSM1015 turns off the

REF

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 V

and in DCM at 230 V

RMS

RMS

.

7/27

AN2432 Adapter features

Figure 2. V

IN

= 115V

- 60Hz Figure 3. VIN = 230V

RMS

Ch1: Q3 drain 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 = 115V

- 60Hz Figure 5. VIN = 230V

RMS

Ch1: Q3 drain voltage
M1: ISEN pin voltage

RMS

RMS

- 50Hz

- 50Hz

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

CH1: Out
CH2: S-COMP
CH3: COMP
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 V

= 230VAC

IN

conditions.

Figure 8. VIN = 230VAC - DCM Figure 9. DCM - INHIBIT synchronization

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

9/27

CH3: STSR30 CK pin
CH4: STSR30 gate drive