Flyback Controller Improves Cross Regulation for Multiple
Output Applications – Design Note 344
Tom Hack
Also, outputs remain uncoupled when the rectifiers
are off, which contributes to poor cross regulation.
Introduction
Flyback converters are often used in power supplies
requiring low to medium output power at several output
voltages. With a flyback, multiple outputs incur little additional cost or complexity—each additional output requires
only another transformer winding, rectifier and output
filter capacitor. Power over Ethernet (PoE) applications
may require several low to medium power outputs
and are good candidates for multiple output flyback
converters.
The LTC ®3806 is a new flyback controller that improves
load and cross regulation for multiple output flyback
converters. Multi-output systems that previously used
post regulators, multiple switching supplies or other
methods to provide tight output voltage tolerances can
now use lower cost flyback converters. The LTC3806
uses forced continuous conduction operation improving
cross regulation by keeping the rectifiers on and the
outputs coupled (via transformer action) for a larger
fraction of each power conversion cycle. The LTC3806
also uses synchronous rectification to reduce rectifier
voltage drops, improving converter efficiency and load
regulation.
While multiple output flyback converters are simple
and inexpensive, they often suffer from poor cross
regulation. Usually, one output is tightly regulated
via feedback, while the other outputs are controlled
via less accurate transformer action. Schottky diode
voltage drops, transformer leakage inductances and
transformer winding resistances degrade regulation.
L1
3.3μH
VIN
25V TO 60V
+
D1
1N4148W
C1
10μF
63V
C2
220nF
R3
232k
D2
1A 60V
B260A
T1
XFMR EFD20
C3
2.2μF
100V
R4
47k
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respective owners.
12
11
R1
(OPT)
VOUT1
12V
400mA
C4
10μF
FB
7
VOUT2
5V
400mA
6
R2
(OPT)
10
8
1
2
C5
47μF
FB
9
4
+
5
3
C6
470μF
4V POSCAP
VOUT3
3.3V
3A
C7
100μF
Q1
Si4490
R6
12.4k
+
20V
R7
33k
C13
100μF
35V
TANT
C9 C8
100pF 1nF
C14
220nF
LTC3806
12
1
RUN SENSE
C12
11
2
220nF
ITH
NC
10
3
SS
FB
9
4
G1
NC
8
5
G2
VIN
6
INTVCC
GND
C15
4.7μF
10V
+
R8
0.033Ω
Q2
Si7806DN
Q3
Si7806DN
C10
470μF
4V POSCAP
VOUT4
2.5V
2A
R5
(OPT)
C11
100μF
FB
Q4
Si7806DN
13
R10
20.5k
DN344 F01
FB
R9
12.4k
Figure 1. Multiple Output Synchronous Flyback Converter
08/04/344_conv
Load regulation measurements were taken with one
output swept from no load to maximum output current
while all other outputs delivered maximum output current. Table 1 shows how load regulation is improved in
the synchronous design. Load regulation for output 1
and output 4 in the nonsynchronous design are given
only from 20% to 100% of maximum output current
because load regulation rapidly deteriorates below 20%
of maximum output current.
Cross regulation measurements were taken under the
same conditions as the load regulation measurements.
Table 2 shows the superior cross regulation performance
of the synchronous design.
Table 1. Load Regulation (% VOUT)
OUTPUT
1
Synchronous
7.86%
Nonsynchronous
OUTPUT OUTPUT
2
3
rectification does not. To get reasonable regulation in a
nonsynchronous design, all outputs require preloading
of several percent of maximum IOUT.
Efficiency
Peak efficiency for this design is 87.6% at 60% of
maximum output power (Figure 2). Such high efficiency
is possible because of the significant reduction in conduction loss afforded by the synchronous rectification,
particularly with low voltage outputs. At full load (21.4W
total output power), supply dissipation is only 3.38W. For
the nonsynchronous design, peak efficiency is 81.9%
and power dissipation is 5.31W. If light load regulation is
required, this degrades to 78.9% due to preloading.
90
EFFICIENCY (%)
Improved Load and Cross Regulation
Figure 1 shows a multiple output flyback converter that
takes full advantage of the features of the LTC3806.
Performance of this design was compared against a
nonsynchronous version created by replacing Q2 with
an SL13 Schottky rectifier, and Q3 and Q4 with B540C
Schottky rectifiers.
85
80
OUTPUT
4
6.31%
1.39%
9.47%
6.42%
8.88%
(20%-100%
of IMAX)
2.43%
10.32%
(20%-100%
of IMAX)
Table 2. Cross Regulation (% VOUT)
(Nonsynchronous Results in Parenthesis)
OUTPUT 1
OUTPUT 2
OUTPUT 3
OUTPUT 4
Swept*
0.87%
(2.02%)
0.22%
(0.42%)
0.29%
(0.43%)
0.31%
(0.94%)
Swept
0.16%
(0.23%)
0.17%
(0.30%)
–5.49%
(–8.52%)
–4.88%
(–8.20%)
Swept
–4.43%
(–8.61%)
–0.63%
(–0.36%)
–0.12%
(0.13%)
0.16%
(0.39%)
Swept*
*20% to 100% of IMAX for nonsynchronous
The case for using synchronous rectification is even
stronger when load and cross regulation measurements are taken with one output swept, and all other
outputs unloaded. Synchronous rectification provides
acceptable load and cross regulation while Schottky
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75
10
20 30 40 50 60 70 80 90 100
% OF MAXIMUM OUTPUT POWER
DN344 F02
Figure 2. Efficiency vs Output Power
Composite Feedback Provides Additional Design
Flexibility
In Figure 1, only output 3 is controlled via feedback while
the remaining outputs are set via transformer action. To
further improve load and cross regulation on these other
outputs (though at the expense of degraded load and
cross regulation on output 3), composite feedback may
be used by adding optional resistors R1, R2, and R5.
Conclusion
The LTC3806 makes cost effective multiple output flyback converters attractive for low and medium power
applications such as Power over Ethernet devices.
Improved load and cross regulation eliminates the need
for additional switching supplies or post regulators.
Synchronous rectification improves efficiency and
lowers power dissipation.
For applications help,
call (408) 432-1900
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