Multi-Output Flyback Off-Line
Power Supply
Basic Concept
• Add additional secondary windings, using the same
turns/volt as the original secondary.
Vout 1
Vin
1
n
Load (R1)
Vout 1 =
nD
Vin
D'
Vout 2 =
mD
Vin
D'
Vout 2
m
•
•
•
Outputs can be positive or negative, depending on which side of
the output (top or bottom) is grounded.
Either output can be the “master” by connecting it to the feedback
sensing circuit
Formulas are not exact, due to the diode drops not being
proportional to the number of turns!
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Load (R2)
Example of Adding a Negative Output
• There is no theoretical limit to the number of outputs.
Vin
Vout 1
1
n
Load (R1)
Vout 1 =
nD
Vin
D'
Vout 2 =
mD
Vin
D'
Vout 3 =
pD
Vin
D'
Vout 2
m
Load (R2)
Vout 3
p
Load (R3)
• In this case, the negative output drawn like the positive
ones, with the diode reversed and the polarity of the winding
as shown.
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Two Outputs with Feedback Regulation
•
Typical regulated flyback converter
– One output is the master (output 2 in this case)
– Second output (output 1, in this case) is the “slave” (quasi-regulated).
– For output voltages less than 2.5 V, a TLV431 (1.25 V) or other can
be used.
– Why do we need R3?
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Improvement #1 – Stacked Windings
•
Regulation of second output is improved, because only part of it is “alone.”
– Only the “n” portion is unregulated. (Leakage inductance of n is less.)
•
Again, one output is the master (output 2 in this case)
– Second output (output 1, in this case) will vary with the load on the main output,
due to its current flowing through the winding of output 2.
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Improvement #2 Stacked Outputs
Vout 1
n
Vin
Load (R1) Vout 1 =
m+n)D
Vin
D'
1
Vout 2
PWM
Controller
m
Load (R2) Vout 2 =
mD
Vin
D'
Optocoupler
R3
R4
TL431
2.5 V ref. amplifier
R5
• Now, output 1 current flows through output #2’s diode.
– Output 1 is less dependent on output 2’s load, because the
bottom of its output doesn’t move.
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Improvement #3 No-Load Clamp
12 V
n
Vin
Load (R1) Vout 1 =
m+n)D
Vin
D'
1
9 V Zener
5V
PWM
Controller
Vout 1
m
Vout 2
Load (R2) Vout 2 =
mD
Vin
D'
Optocoupler
R3
R4
TL431
2.5 V ref. amplifier
R5
• When output 1 is unloaded, its stray output current
flows down through the Zener and into the 5 V output.
• In this case, output 1 would be clamped at 14 V.
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Improvement #4 – Combined Feedback
Vout 1
n
Vin
Load (R1) Vout 1 =
m+n)D
Vin
D'
1
Vout 2
PWM
Controller
m
Load (R2) Vout 2 =
mD
Vin
D'
Optocoupler
R6
R3
R4
TL431
2.5 V ref. amplifier
•
R5
Now, both outputs are sensed, and the regulator controls the
combination of outputs.
– Remember: There’s only one feedback point. Neither output will be as
tightly regulated as the main one when it had the feedback to itself!
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Weighting the Feedback
Vout 2
Vout 1
Optocoupler
TL431
2.5 V ref.
amplifier
i2 = W2 • i0
R2
i0
R0
R1
i1 = W1 • i0
Vref
i0 = i1 + i2 = W1 • i0 + W2 • i0 = i0 (W1 + W2)
Therefore, W1 + W2 = 1
Wn is the “weight” of the feedback from output n.
•
If W1 = 0.9 and W2 = 0.1, then output 1 is nine times as important as
output 2.
– (W1 has a weight of 90%, and W2 has a weight of 10%)
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Designing the Feedback
( i1 + i2 = i0 )
Vout1 − Vref = i1 R1
R1 =
R2 =
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Vout1 − Vref
i1
Vout 2 − Vref
i2
=
=
Vout1 − Vref
W1i0
Vout 2 − Vref
W2i0
Example
Procedure:
– Given: Vout 1 = 5, Vout 2 = 12, Vref = 2.5
– Choose i0 = 1 mA
– Choose W1 = 0.7 and W2 = 0.3
Calculating the values:
Vref
2.5
R0 =
=
= 2.5 kΩ
i0 1 mA
R1 =
R2 =
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Vout1 − Vref
W1i0
Vout 2 − Vref
W2i0
5 − 2.5
=
= 3.57 kΩ
0.7 ⋅1 mA
=
12 − 2.5
= 31.7 kΩ
0.3 ⋅1 mA
More Outputs? No Problem
Vout 2
Vout 1
Vout n
Optocoupler
TL431
2.5 V ref.
amplifier
•
•
i2 = W2 • i0
R2
i0
R0
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i1 = W1 • i0
Rn
Vref
Feedback can be from any number of outputs.
Provided that: W1 + W2 + ……..+Wn = 1
Rn =
12
R1
Vout n − Vref
Wn ⋅ i0
in = Wn • i0
The “Magic” Capacitor
With cap: Clean pulse; improved
regulation at low-current load
Vout 1
Vout 1 =
n
nD
Vin
D'
Low-current load (R1 = large)
Vin
1
Vout 2
m=n
PWM
Controller
Vout 2 = Vout 1 =
Load (R2)
Optocoupler
R3
R4
TL431
2.5 V ref. amplifier
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R5
nD
Vin
D'
Another Version of the “Magic” Capacitor
n
Load (R1) Vout 1 =
Vin
D'
m
Vin
2m+n)D
Example: 12 V
1
Vout 2
PWM
Controller
mD
Vin
D'
Example: 5 V
Load (R2) Vout 2 =
m
Optocoupler
R3
R4
TL431
2.5 V ref. amplifier
•
•
Here, since the bottom of upper secondary is tied to Vout 2 (which is dc),
waveforms at each end of the capacitor are identical.
Overshoot & ringing at light load on Vout 1 is reduced by 5/7, since 5 of
the 7 added turns are tightly coupled via the capacitor. (m = 5, n = 2,
m+n = 7).
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R5
Adding an Output to a Buck Converter
•
•
•
During the “off” time of the switch, the output voltage across the
inductor is coupled to a new output via an added winding!
No free lunch. There must be enough energy stored in the choke
to feed the new output.
Ampere-turns are preserved, so current drawn from the new
output causes discontinuous current in the main output.
– Ripple current in the main output capacitor increases.
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Design Example, Built and Tested
65 Watt, 8 Output
Set Top Box
Power Supply
Frank Cathell,
Senior Applications Engineer
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General Specifications
• Input: 90 to 135 Vac, 47 – 63 Hz
• Inrush current: 30 A cold start; 60 A warm start
• Efficiency: > 80% at nominal loading
• Output Voltages/Regulation/Ripple:
Channel
1
2
3
4
5
6
7
8
Vout
2.6 V
3.3 V
5V
6.2 V
9V
12 V
30 V
-5 V
Output type
Buck reg.
Buck reg.
Main output
Quasi-reg.
3-T reg.
Main output
Quasi-reg.
3-T reg.
Regulation
+/-1%
+/-1%
+/-2%
+/-6%
+/-1%
+/-2%
+/-8%
+/-1%
Max Ripple
40 mVp/p
40 mVp/p
50 mVp/p
50 mVp/p
30 mVp/p
50 mVp/p
100 mVp/p
30 mVp/p
Current
3A
4A
3A
1.5 A
100 mA
1A
20 mA
30 mA
Surge
4A
5A
4A
2A
200 mA
3A
40 mA
60 mA
• Output overshoot: 5% max; typically <1%
• Overcurrent/short circuit protection: Protected against accidental overloads via reduced duty
cycle, burst mode operation
• No load: Output voltages are controlled and stable under no load conditions
• Hold-up time/power fail detection: Output will hold up for 20 ms following drop out at 100 V ac
and nominal load; power fail warning following holdup period with 5 ms minimum delay to output
voltage dropout.
• Temperature: Operation from 0 to 50O C (no over temp protection included)
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17
Circuit Features
• Critical conduction mode flyback converter
¾NCP1207
• 2.6 V and 3.3 V outputs derived from 12 V output
¾NCP1580 synchronous buck controllers
• Low current outputs on -5 V and +9 V allowed use of conventional 3-T
regulators
• Control loop closed via sum of 5 V & 12 V outputs; all other outputs quasiregulated
• Transformer main secondary made from foil winding for low leakage
inductance
• “Stacked” secondary windings utilized for improved cross-regulation
• Simple but effective power fail detection circuit utilizing TL431 and 2N2222
• Overcurrent protection implemented by initiating burst mode of NCP1207A
• 2-wire ac input with dual common mode EMI filter inductors
• Single-sided printed circuit board
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Set-Top Box Test Results
Regulation Data (120VAC input)
Parameter
2.6V
3.3V
5V
Outputs
6V
Output type
Buck
Buck
Main
Quasi-reg
3-T reg
Main
Quasi-reg
3-T reg
Vout setpoint at
typical loads
2.53V
3.4V
4.89V
6.27V
8.94V
12.54V
31.0V
4.96V
Vout setpoint at
minimum loads
2.55V
3.42V
4.96V
6.38V
8.94V
12.33V
32.70V
4.98V
Vout setpoint at
maximum loads
2.54V
3.34V
4.90V
6.29V
8.94V
12.53V
30.10V
4.95V
Vout setpoint at
no output loading
2.56V
3.43V
5.02V
6.54V
8.93V
12.13V
29.60V
4.97V
Note: Vout setpoints measured at PC board
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19
9V
12V
30V
neg 5V
More Test Results
E ffic ie n c y M e a s u re m e n ts (1 2 0 V A C in p u t)
P a ra m e te r
2.6V
3.3V
5V
O u tp u ts
6V
9V
12V
30V
n e g 5V
O u tp u t V o lta g e
2 .5 4
3.42
4.91
6.31
8.94
12.48
30.06
4.96
O u tp u t C u rre n t
3 .8 A
2.9A
1.56A
1.3A
91m A
1.0A
30m A
73m A
O u tp u t P o w e r (W )
9 .6 5
9.92
7.66
8.2
0.81
12.48
0.9
0.36
9V
12V
30V
neg 5V
(4 9 . 9 8 W t o t a l)
T o ta l P o u t = 4 9 . 9 8 W
P in a t 1 2 0 V A C = 6 1 . 4 W
E ffic ie n c y = 8 1 .4 %
Parameter
2.6V
3.3V
5V
Outputs
6V
Output Ripple
(@ max loads)
27mV
45mV
50mV
50mV
40mV
30mV
100mV
20mV
Output Overshoot
(turn-on)
none
none
none
none
none
none
none
none
Holdup Time (prior to PF warning) at 100 Vac in, maximum output loads: 25ms
Power Fail warning time (Vout decay to 90%): 15ms
Line Regulation: Minimal on all outputs; +/- 20mV max
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20
(10:1 scope probe)
Schematic
R40 10
T1
8
560pf 1KV
C5
FIGURE 1: Schematic
30V
D13
16
1
Stacked windings
MUR120
C11
330/50V
R21
30K
C13
0.1uf
R2
P1
D4
1N5406
L1
15
AC INPUT
Q1
IRF740
1
C2
0.22/250V
BU10-1311R6B
R1
1M .5W
MBR1645
+
+
C3
470/250V
BU16-4021R5B
D2
t
3
C1
0.22/250V
TH1
1
2
12V-BUCKS
D12
1N5406
L2
3A
1
2
Before diode
After diode
6
2
D1
F1
15,
1W
C14
680/16V
560pf 1KV
C6
12V
1
L3
1N5406
R5
4.7
1N5406
R4
4.7K
R3
0.33 1W
+
+
C15
680/16V
C16
680/16V
C17
0.1uf
U3 MC78M09
3
I
O
1
9V
G
1
D11
R19
10K
+
C18
270/25V
2
C4
Y -CAP
L4
1N5820
4.7uH
6V
1
+
+
C20
680/16V
+
C21
680/16V
MBR1635
C23
C22
680/16V
L5
0.1uf
4.7uH
5V
1
D10
NCP1207A
1
2
3
2
2
4
4
3
C28
270/25V
9
JUMPER
U5
R9 1K
4
C8
22/25V
D9 MUR110
+
C10
470 pf
+
C51
270/25V
I
O
3
3
2
H11A817A
D8
R11 270
COM
C30
1K
1N4148
0.1uf
R10
1
R17
10K
Combined, weighted
feedback
C29
0.1uf
-5V
1
10
R14 30K
3
R13
C27
0.1uf
1
U4 MC79L05
1
R12
1K
C9
Not installed
C26
1200/6.3V
1
JP3
1
+
C25
1200/6.3V
+
R8 22K
1
C24
1200/6.3V
14
13
3
G
7
R7 100
D6 1N4148
6
6
+
1
8
+
8
5
C7
1nf
1
U1
2
2
1
R18
1K
10
NC 7
1
C19
0.1uf
12
11
Not installed
5
1
4.7uH
D3
10 Ohm 4A
R6
3.6K
1
+
1
U6
TL-431
R25
47K
12V-BUCKS
C31
10nf
U7
TL-431
1
R28
4.7K
1
R16
3.6K
C32
0.1uf
3
R27
1K
PF
Q2
PN2222A
D7
1
1
1N5226B
R24
4.7K
R23
4.7K
R22
1K
2
2
R26
6.2K
R15
6.8K
JP5
2
JUMPER
1200/6.3V
C34 +
2
JUMPERD15
1N5818
Q3
NTD60N02R
8
5
C39 1nf
7
C40 10nf
4
6
4
C42 1nf
7
R30 33K
R36
33K
6
68
C41
C38
0.1uf
10nf
C44 10nf
R37
3
33K
8
R33 10.5K
R34 10K
8
R35 22K
JP1 2
L6
4
D17
1N5818
10uH
2.6V
1
JUMPER
R38
4.7
6
C45
0.1uf
Q5
NTD60N02R
1
7
68
C37
1nf
R32
2
C43 10nf
6
R31
1
5
5
5
7
1
+
C35
680/16V
1
8
R29
4.7
1
NCP1580
2
3
JP2
1
2
JUMPER
U9
1
1
+
2
1
U8
NCP1580
2
2
3
C33
0.1uf
12V-BUCKS
JP4
4
+
C48
680/16V
C49
1200/6.3V
C50
0.1uf
1
C46
1nf
Q6
NTD60N02R
3
1
0.1uf
1
3
10uH
2
1
L7
C47
3
3
1
3-3V
0.1uf
2
C36
D16 MUR110
NTD60N02R
Q4
3
2
D14 MUR110
Title
Schematic - 60W set-top box
Size
Date:
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21
Document Number
<Doc>
Tuesday , May 31, 2005
Rev
C
Sheet
1
of
1
Conclusion
• Multiple output switched-mode power supplies save space,
save cost, and can have high performance.
– The “tricks” you’ve seen here can make them even better!
• Flybacks are popular, because there is only one magnetic
component.
• They work best where the load ranges of the outputs are wellknown.
– This allows the designer to tailor the regulation characteristics to the
load regulation requirements, favoring certain loads when necessary.
• For good cross-regulation, construction of the transformer is
important.
– Beware of changing vendors during production!
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For More Information
•
View the extensive portfolio of power management products from ON
Semiconductor at www.onsemi.com
•
View reference designs, design notes, and other material supporting
the design of highly efficient power supplies at
www.onsemi.com/powersupplies
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