®
AN1060
APPLICATION NOTE
FLYBACK CONVERTERS WITH THE L6561 PFC
CONTROLLER
by C. Adragna & G. Gattavari
The L6561, controller specifically designed for Power Factor Correction (PFC) circuits, may be suc-
cessfully used in flyback converters as well.
The excellent performance of the device, along with its characteristics in terms of low current con-
sumption, makes L6561-based flyback converters really attractive in medium-low power applications.
There are basically three different configurations that an L6561-based flyback converter can assume,
each of them with its own characteristics, benefits and peculiarities. This paper describes these con-
figurations and highlights advantages/drawbacks with the aim of identifying the most suitable appli-
cations they can fit.
INTRODUCTION
Common practice bounds their use in conventional boost PFC stages, yet Transition Mode (TM) Power
Factor Corrector IC’s can be used in applications different from those they are primarily intended for.
This is particularly true for the L6561, PFC controller for medium-low power applications, because of its
peculiar characteristics.
Reference [2] presents a special example showing how to extend the use of this device to Mag Amp ap-
plications.
Figure 1 - L6561 Internal Block Diagram
COMP
2
INV
1
2.5V
-
+
MULT
3
4
40K
CS
MULTIPLIER
VOLTAGE
REGULATOR
OVER-VOLTAGE
DETECTION
+
-
5pF
V
CC
8
V
CC
INTERNAL
SUPPLY 7V
R1
+
UVLO
R
S
DRIVER
Q
7
20V
GD
R2
V
REF2
2.1V
1.6V
ZERO CURRENT
DETECTOR
STARTER
6
GND
The outperforming L6561 offers a number of unique advantages that make the device an interesting al-
ternative to the traditionally used PWM controllers where quite a good performance is required at low
cost:
January 2003
-
+
-
DISABLE
5
ZCD
D97IN547D
1/11
AN1060 APPLICATION NOTE
•
•
•
•
•
•
disable function for power management and/or protection schemes;
true micropower start-up current, 50µA typ., for cost-effective start-up circuits;
very low quiescent current, 3mA typ., for high efficiency at light load;
two-level (static and dynamic) overvoltage protection (OVP);
on-chip RC filter on current sense pin for improved noise immunity;
pulse-by-pulse current limiting. In conjunction with TM operation, this ensures a safe operation under
short circuit conditions.
Refer to [1] for a detailed explanation of the internal architecture (shown in fig. 1) and the functionality of
the device.
L6561-based flyback converters can be realised as schematically illustrated in fig. 2a, 2b, 2c, and which
will be referred to as "TM", "Synchronised" and "High-PF" respectively.
Figure 2. L6561-based flyback converter configurations
Vout
Vac
C
BULK
SYNCH
Vac
C
BULK
Vout
DISABLE
DISABLE
ZCD
VCC
ZCD
VCC
L6561
GD
OPTO
+
TL431
L6561
GD
OPTO
+
TL431
a) TM Flyback
b) Synchronised Flyback
Vout
Vac
C
IN
DISABLE
ZCD MULT VCC
L6561
COMP
INV
GD
OPTO
+
TL431
(B
W
<100 Hz)
c) High-PF Flyback
Each of them has its own peculiarities but they all share some key points:
•
low parts count, which helps reduce total cost and space;
•
high efficiency at very light load: an L6561-based flyback can be easily compliant with Blue Angel
standards;
•
standby function: the internal start-up timer may be used to make the system work at a (fixed) low
frequency under light load conditions, so as to minimise losses;
•
disable function: pin ZCD, if grounded, turns off the L6561 and reduces its consumption at a couple
of mA; this can be used either for power management or protection.
2/11
AN1060 APPLICATION NOTE
In the following, the three basic configurations will be taken into consideration and their advantages,
benefits and drawbacks will be highlighted so as to identify their most appropriate field of application.
This will be made easier by some application examples.
TM Flyback
This configuration, very similar to a free-running flyback, always works on (actually, very close to) the
boundary between Continuous and Discontinuous Mode (i.e. Transition Mode, or TM), therefore at a fre-
quency dependent on the input voltage and on the output current.
This type of operation requires a low inductance and therefore a small-size magnetics but on the other
hand, involves high peak current. Therefore it can be reasonably used for power levels up to 50-60 W in
110 V or wide-range mains applications, and up to 100 W with 220/240 V mains.
At high input voltage and especially at light load, the switch ON-time becomes very short and the switch-
ing frequency tends to become quite high. There is, however, a minimum ON-time (0.4-0.5µs) below
which it is not possible to go. This is due to the internal delay of the L6561 as well as the turn-off delay of
the MOSFET.
When this minimum is reached, TM operation can no longer be kept. The energy drawn each cycle ex-
ceeds the short-term demand from the load and the control loop delays MOSFET’s turn-on so as to
maintain the long-term energy balance. Switching becomes asynchronous, and this can be seen as a
"ghosting" of the waveform on the scope.
If the load is decreased further on, so many cycles need to be skipped that the amplitude of the drain
voltage ringing becomes very small, and the ZCD can no longer be triggered. In this case the internal
starter of the IC will start a new switching cycles sequence. Under this condition, the system will operate
in "burst" mode: there will be short periods of switching spaced out by long intervals where L6561’s OVP
keeps the switch in OFF state.
Fig. 3 shows a 7W power supply, realised in TM flyback. It is intended as an auxiliary power supply suit-
able for systems provided with power management, such as monitor displays, printers, servers, photo-
copiers, fax machines, etc.
According to an approach that is becoming more and more popular, when the system is requested to go
into some low-consumption mode, a
µP
switches off the main SMPS. A small auxiliary supply, optimised
for a low power level, keeps alive the
µP
itself and the circuits needed for waking up the system again.
This approach allows to minimise the power consumption from the mains, in compliance with regulations
coming into force (such as Blue Angel and others).
Figure 3. 7W, Wide-range, Auxiliary Power Supply.
Vin=90 to 400 Vdc
BZW04-154
N1
to L4990A or L5991A
UC3843A/B or UC3845A/B
or L4981A
470 kΩ
STTA106
N2
2x330
µF
STPS360B
1
3
5 Vdc / 1A
L4955V5.1
2
100
µF
1N4148
47 µF
Input bulk capacitor
of the main SMPS
100 nF
3
8
22
Ω
1N4148
47 µF
N3
47 kΩ
4.7 nF
33 kΩ
2
1
L6561
6
4
5
7
22
Ω
STD1NB60
7.5 kΩ
2
Ω
TRANSFORMER SPECS:
CORE: E20x10x6, 3C85 material or equivalent
≈0.5
mm air gap for a primary inductance of 1.7 mH
N1: 2 series windings 66 T each, AWG32
(∅ 0.24 mm)
N2: 11 T, AWG24
(∅ 0.57 mm)
N3: 21 T, AWG32
3/11
AN1060 APPLICATION NOTE
The converter is powered by the high-voltage DC bus, ranging from 90 to 400 VDC, generated by the
front-end AC-DC stage (bridge rectifier + input capacitor) shared with the main SMPS (power factor cor-
rected or not).
The output is post-regulated in order to provide a better regulation and supplies the
µP
as well as the
logic circuit needed to wake up the system.
The auxiliary winding will be properly designed so as to supply the controller(s) of the main SMPS be-
sides powering the L6561. To minimise component count, a primary sensing feedback technique is
used.
The auxiliary winding is used also by the ZCD circuit for detecting transformer’s full demagnetisation and
turning on the MOSFET to start a new switching cycle (TM operation). The resistor driving the ZCD pin
is in the ten kΩ but can be optimised so as to achieve a "quasi zero-voltage turn-on" as described in Ref.
[1]. The optimum value depends mainly on the inductance of transformer’s primary winding and on the
C
oss
of the power MOSFET, thus it can be found empirically after bench tests.
With the component values shown in fig. 3 the wake-up time of the converter, that is the time the system
takes to start operating after being powered, does not exceed 3 s at 90 V
DC
supply and 1 s at 400 V
DC
.
In fig. 4, the circuit of fig. 3 is proposed with a different power rating: 15W output power so as to be able
to support USB function in computer equipment. The modifications concern the MOSFET, the trans-
former and the sense resistor on the primary side, the catch diode and the filter capacitors on the secon-
dary side. They all have been increased in size.
Figure 4. 15W, Wide-range, Auxiliary Power Supply supporting USB function
Vin=90 to 400 Vdc
BZW04-154
N1
to L4990A or L5991A
UC3843A/B or UC3845A/B
or L4981A
470 k
Ω
STTA106
N2
2x1000
µF
STPS560B
1
3
5 Vdc / 3A
L4955V5.1
2
220
µF
1N4148
47 µF
Input bulk capacitor
of the main SMPS
100 nF
3
8
22
Ω
1N4148
47 µF
N3
47 kΩ
4.7 nF
33 kΩ
2
1
L6561
6
4
5
7
22
Ω
STP3NB60FP
7.5 kΩ
1
Ω
TRANSFORMER SPECS:
CORE: E20x10x6, 3C85 material or equivalent
≈0.5
mm air gap for a primary inductance of 0.8 mH
N1: 2 series windings 48 T each, AWG30
(∅ 0.30 mm)
N2: 8 T, 2xAWG22
(∅ 0.71 mm)
N3: 15 T, AWG32
(∅ 0.24 mm)
Fig. 5 shows another example of low-power TM flyback application, an AC-DC adapter for battery
charger of cellular phones. The system looks very simple and very few parts are required.
The feedback uses a popular arrangement making use of a TL431 as secondary reference/error ampli-
fier and of an optocoupler for transferring the control signal to the primary side. This provides very good
regulation of the output voltage and galvanic isolation from the primary side at the same time.
The self-supply winding both powers the L6561and provides transformer’s demagnetisation signal to the
ZCD pin. The start-up circuit arrangement and its component values ensures that the wake-up time of
the converter does not exceed 3 s at 90 VAC supply (it will be less than 1 s at 270 V
AC
).
In fig. 6 an example of multi-output SMPS for inkjet printer is presented. The converter accepts input
voltages from 85 to 270 Vac and is rated for 40W output power. The 28V output is used for motors, the
12V output for the printhead and the 5V bus supplies the logic circuitry.
4/11
AN1060 APPLICATION NOTE
Figure 5. 7.5 W, Wide-range Mains AC-DC Adapter for cellular phones.
2200pF 2KV
2A fuse
4x1N4007
85 to 270 Vac
10
µF
400V
BZW04-154
110 kΩ
BYW98-100
9V / 0.85A
2 x 470µF
16V
N1
STTA106
47 kΩ
N2
GND
1N4148
33µF
N3
1 kΩ
5
8
10
Ω
STD1NB60
4N35
0.022µF
4.7 kΩ
7
L6561
4
2
3
1
6
2Ω
1/2 W
TL431
1.8 kΩ
100 kΩ
4N35
3.3 nF
10 kΩ
1 kΩ
TRANSFORMER SPECS:
CORE: E19x8x5, 3C85 material or equivalent
≈0.6
mm air gap for a primary inductance of
1.8mH
N1:170 T, AWG34 (∅ 0.20 mm)
N2: 15 T, 3xAWG34
N3: 19 T, AWG34
The isolated feedback is realised with the configuration TL431 + optocoupler. Output cross-regulation is
improved by multiple sensing technique.
The system works in TM but can be forced to work at fixed frequency (that of L6561’s internal timer) for
minimum consumption at light load by the STANDBY signal (see fig. 6). This signal can be generated by
either the
µP
or a current sense circuit that enables low (fixed) frequency operation when the load cur-
rent falls below a defined threshold. To achieve this functionality, the ZCD pin is connected to ground
through a 4.3 kΩ resistor.
Figure 6. 40W, Wide-range Mains SMPS for Inkjet Printer.
4700pF 4KV
4700pF 4KV
2A fuse
4.7MΩ
KBU4G
85 to 270 Vac
100
µF
400V
110 kΩ
56 kΩ
2W
4.7MΩ
BYW100-200
28V / 0.7A
22 nF
250V
N1
N2
BYW98-100
2 x
1000µF
16V
2 x 470µF
35V
12V / 1.5A
STTA106
43 kΩ
N3
GND
BYW100-50
4.3 kΩ
5V / 0.5A
47µF
1N4148
N5
N4
470µF
16V
47 kΩ
STANDBY
5
8
10
Ω
STP4NA60FP
220
Ω
7
47 kΩ
L6561
4
2
3
1
6
0.39
Ω
1/2 W
DISABLE
4N35
100 nF
3.9 kΩ
270 kΩ
100 kΩ
5.1 kΩ
TL431
3.3 nF
10 kΩ
1 kΩ
4N35
2.7 kΩ
TRANSFORMER SPECS:
CORE: ETD29x16x10, 3C85 material or equivalent
≈1
mm air gap for a primary inductance of 530 µH
N1: 69 T, AWG25
(∅ 0.51 mm)
N2: 11 T, AWG25
N3: 9 T, AWG20
(∅ 0.89 mm)
N4: 4 T, AWG25
N5: 11 T, AWG32
(∅ 0.24 mm)
5/11
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