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AN1440
APPLICATION NOTE
80W POWER-FACTOR-CORRECTED AC-DC ADAPTER
WITH STANDBY USING THE L6561 AND THE L5991A
by C. Adragna
This note describes an 80 W, wide-range mains, power-factor-corrected AC-DC adapter. Its electrical
specification is tailored on a typical hi-end portable computer power adapter. The peculiarity of this de-
sign is its extremely low no-load input consumption (<1 W).
The architecture is based on a two-stage approach: a front-end PFC pre-regulator based on the L6561
TM PFC controller and a back-end DC-DC converter in flyback topology that makes use of the L5991A
PWM controller. The Standby function of the L5991A, which reduces the switching frequency of the DC-
DC converter upon recognition of a light load, is also used to turn off the PFC stage to make it possible
meeting the severe no-load consumption requirement.
Design Specification
The design of an 80W power-factor-corrected AC-DC adapter suitable for hi-end portable computer and the
evaluation results of a prototype are here described.
Table 1 shows the electrical specification of the application, table 2 provides the BOM and tables 3 and 4 list
magnetics' spec. The electrical schematic is illustrated in figure 1 and the PCB layout in figure 2.
Table 1. 80W AC-DC adapter with PFC and Standby: electrical specification
Input Voltage Range (V
in
)
Mains Frequency (f
L
)
Holdup time
Maximum Output Power (P
outmax
)
Output
90 to 265 Vac
50/60 Hz
20 ms
80 W
V
out
= 18 Vdc
±
2%
I
out
= 0 to 4.5 A
V
ripple
= 1%
< 1%
65 kHz
22 kHz
35 kHz
400 Vdc
±
5%
<20 V pk-pk
440 Vdc
η
> 75%
<1W
EN 61000-3-2, class D compliant
EN 55022, class B compliant
1/14
Line and Load regulation
Switching Frequency (Flyback, @ P
out
= 80 W)
Switching Frequency (Flyback, @ P
out
= 0 W)
PFC Minimum Switching Frequency (@ P
out
= 80 W)
PFC Output Voltage
PFC Output Voltage ripple (@f
L
= 50 Hz, full load)
PFC Output Overvoltage threshold
Overall Efficiency (@ P
out
= 80 W, V
in
= 90÷265 V
ac
)
Maximum No-load Input Power
Low-frequency harmonic contents
Conducted EMI
December 2001
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AN1440 APPLICATION NOTE
To meet the requirement on low-frequency emission, active power factor correction will be used, resulting in a
two-stage architecture: a front-end PFC pre-regulator, using boost topology, followed by a cascaded DC-DC
converter. As to the PFC stage, the power rating suggests the use of TM operation, and then the L6561 [1] will
be used as the controller. The cascaded DC-DC converter will use flyback topology: the high input voltage
(400V, output of the PFC stage) and the relatively high output voltage make this topology the most attractive for
this application.
A special requirement concerns the no-load consumption from the mains: less than 1 W. Especially in a two-
stage system, this is a tough job. Special design care needs to be taken, from both the system and the selection
of the PWM controller point of view.
Figure 1. 80W AC-DC adapter with PFC and Standby: electrical schematic
GND_OUT
C27
C26
4.7nF
ZR2
+18V 4.5A
R31
2.2K
L1 2.2µ
C24
680µF
25V
C6
100nF
R33
13K
C22
680µF
25V
C25
330nF
R29
348
TR2
BC547
R9
150K
R10
100K
C9
100nF
C10
3.3nF
C11
10nF
C13
15nF
R36
10K
C15
220pF
D01IN1307mod
C28
100µF
450V
C21
680µF
25V
R32
4.3K
R6A
6.34K
R7
998K
D1 STTA106
C20
680µF
25V
1
BYW51-200
C23
D11
C19
T2B 680µF
25V
2
R30
1.2K
Q1
STP9NB50
R5 A,B
0.28
R4 33
R39
10M
R27
4
1
7
R34 510K
4
STP7
NB80
T2A
R35 2.2K
2
8
D7
MUR1100
3x
1.5KE68
D12 ABC
L6561
R24 1K
R23 22
IC1
C29
10µF
25V
DC-LIM
T2C
D9 BAV19 R28 3.3
R2
68K
PGND
5
3
OUT
Isen
R25 A,B
0.28
C8 1µF
6
T1A
R38 47
C7
10nF
T1B
COMP
3
PC817A
OC1
V
C
D10 1N4148
C1
0.22µF
630V
R1
10K
C18
100µF
25V
R3
1.36M
R26 22
L5991A
SS
SGND
V
CC
IC2
D3
D5
Q3
STD1NB50
C3
0.68µF
630V
C4
4.7nF
R18 330K
TR4
R20 100K BC547
R21
10M+
10M
VREF
R15 10K
R16 22K
D6
Z18
RCT
D2
D4
C5
4.7nF
LF2-A
ZR1
SI0300
LF2-B
R19
C12 100nF 100K
DIS
TR3
BC557
R11
27K
C14
4.7µF
R22
10K+
10K
LF1-A
C2
0.47µF
LF1-B
R17 47K
R13
6.8K
D8
1N4148
DCC
ST-BY
NTC1 10
F1 T4A
2/14
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TR1
BC547
R14
6.8K
R12
47K
R8
150K
TR5
BC557
C17
47µF
25V
VFB
R37
27K
C16
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AN1440 APPLICATION NOTE
Table 2. 80W AC-DC adapter with PFC and Standby: Bill Of Material
Symbol
R1, R15, R36
R2
R3A, R3B
R4
R5A, R5B, R25A, R25B
R6A
R7A, R7B
R8, R9
R10, R19, R20
R11, R37
R12, R17
R13, R14
R16
R18
R21A, R21B
R22A, R22B
R23, R26
R24
R27
R28
R29
R30
R31, R35
R32
R33
R34
R38
R39
C1
C2
C3
C4, C5, C26
C6
C7, C11
C8
C9, C12
C10
Value
10 kΩ
68 kΩ
680 kΩ
33
Ω
0.56
Ω
6.34 kΩ
499 kΩ
150 kΩ
100 kΩ
27 kΩ
47 kΩ
6.8 kΩ
20 kΩ
330 kΩ
10 MΩ
10 kΩ
22
Ω
1 kΩ
---
3.3
Ω
348
Ω
1.2 kΩ
2.2 kΩ
4.3 kΩ
13 kΩ
510 kΩ
47
Ω
10 MΩ
0.22 µF
0.47 µF
0.68 µF
4.7 nF
0.1 µF
10 nF
1 µF
100 nF
3.3 nF
Ceramic
5%
½ W, VR37
630V, polyester
275 AVC, X2
630V, polyester
Ceramic, Y
Polyester
Not assembled
½W
1W, metal film
Only R6A assembled. R6B is for Fine-Tuning
Note
3/14
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AN1440 APPLICATION NOTE
Table 2. 80W AC-DC adapter with PFC and Standby: Bill Of Material
(continued)
Symbol
C13
C14
C15
C16, C23, C27
C17
C18
C19, C20, C21, C22, C24
C25
C28
C29
D1
D2, D3, D4, D5
D6
D7
D8, D10
D9
D11
D12A, D12B, D12C
L1
LF1
LF2
TR1, TR2, TR4
TR3, TR5
Q1
Q2
Q3
IC1
IC2
OC1
VR1
NTC1
T1
T2
ZR1
ZR2
F1
Notes:
Value
15 nF
4.7 µF
220 pF
---
47 µF
100 µF
680 µF
330 nF
100 µF
10 µF
STTA106
KBP208M
1N5248B
MUR1100E
1N4148
BAV19
BYW51-200
1.5KE68
ELC08D2R2E
B82732
B82734
BC547
BC557
STP9NB50
STP7NB80
STD1NB50
L6561
L5991A
PC817A
TL431C
S236/10M
473201A8
RDT13560
S14K300
---
T4A
Not assembled
25 V, electrolytic
25 V, electrolytic
25V Rubycon, ZL series
16 V, electrolytic
Note
450 V, electrolytic, EPCOS B43502
25 V, electrolytic
600 V / 1 A, Turboswitch, ST
800 V / 2 A Bridge recitifier, or equivalent
18V, ½ W Zener, or equivalent
1100 V / 1 A, Ultrafast
75 V / 0.3 A p-n diode, or equivalent
100 V / 0.25 A p-n diode, or equivalent
200 V / 2x10 A Ultrafast, ST
1.5 kW / 68 V Transil, ST
2.2 µH / 7.2A inductor, Panasonic, or equivalent
15 mH / 1.1 A EPCOS
47 mH / 1.3 A, EPCOS
Small-signal NPN
Small-signal PNP
500 V / 9A MOSFET, ST
800 V / 7A MOSFET, ST
500V / 1A MOSFET, ST
PFC TM controller, ST
PWM controller, ST
Optocoupler, SHARP
Programmable shunt regulator, ST
10
Ω
NTC
PFC inductor (see table 3), OREGA
Flyback transformer (see table 4), RD Elettronica
MOV, EPCOS, or equivalent
Not assembled
250V / 4A, ELU or equivalent
if not otherwise specified: all resistors are 1%, ¼ W, all capacitors may be plastic film or ceramic, 20% tolerance
Q1 is provided with a 25 °C/W heatsink, Q2 and D11 are provided with a 9.5°C/W heatsink
4/14
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AN1440 APPLICATION NOTE
Table 3. 80W AC-DC adapter with PFC and Standby: PFC inductor spec (p.n. 473201A8)
Core
Bobbin
Air gap
Windings
Spec & Build
Pin Start/End
2/7
12/17
B1ET2910A, B1 Material from THOMSON
Vertical mounting, 18 pins, slotted
≈
1.25 mm on center leg for an inductance 2-7 of 430 µH
Winding
Pri
Aux
Wire
10 x AWG32
AWG32
Turns
90
7
Notes
Table 4. 80W AC-DC adapter with PFC and Standby: Flyback transformer spec (p.n. RDT13560)
Core
Bobbin
Air gap
Leakage inductance
Windings
Spec & Build
E32/16/9, N67 Material or 3C85 or equivalent
Horizontal mounting, 14 pins
≈
1 mm on center leg for an inductance 10-9 of 430 µH
< 10 µH (@ 65 kHz) measured between pins 10-9 with 3,5,12,13 shorted
Pin Start/End
10/1
3/5
1/9
12/13
Winding
Pri1
Sec
Pri2
Aux
Wire
AWG26
4xAWG22
AWG26
AWG32
Turns
28
10
28
8
Notes
Innermost winding
Separated from the primary windings
by a 3-layer polyester isolation
Pin 1 will be cut for safety
Evenly spaced, 2-layer isolation
As to the PWM controller, the choice is the L5991A [2]: above all else, its Standby function makes this device
particularly suitable for building a "highly-efficient" converter under no-load conditions.
From the overall system point of view, a fundamental point is:
s
Under no-load conditions the PFC pre-regulator must be shut down.
Then the optimization effort for low light-load losses will be concentrated on the flyback converter. Based on the
advice given in [3], the following design choices have been made:
s
s
Use of an active start-up circuit.
Use of a Transil clamp to handle the leakage inductance spikes.
The critical point where maximum design effort needs to be put to optimize the performance is the design and
the construction of the transformer. In particular the points to look at are:
s
The primary to-secondary leakage inductance, which must be as low as possible, to minimize the en-
ergy dissipated in the clamp circuit so as to make it possible the use of a Transil clamp.
The intrawinding capacitance of the primary winding, which must be as low as possible, to minimize the
capacitive losses of the MOSFET.
The coupling between the secondary and the auxiliary winding, which must be as good as possible, to
minimize the drop of the auxiliary voltage (used for supplying the controllers) as the converter's load
goes to zero. This is very important, since a stable self-supply circuit avoids the use of dummy loads
that would increase no-load consumption.
s
s
5/14
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