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www.onsemi.com2
Basic Concept
• 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!
• Add additional secondary windings, using the same turns/volt as the original secondary.
Load (R1)
VinVout 1
n1
Load (R2)
Vout 2
m
Vout 1 = VinnDD'
Vout 2 = VinmDD'
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Example of Adding a Negative Output
• In this case, the negative output drawn like the positive ones, with the diode reversed and the polarity of the winding as shown.
Load (R1)
Vin Vout 1
n1
Load (R2)
Vout 2m
Vout 1 = VinnDD'
Vout 2 = VinmDD'
Load (R3)p Vout 3 = VinpDD'
Vout 3
• There is no theoretical limit to the number of outputs.
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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
• 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.
Load (R1)Vin
Vout 1
n
1
Load (R2)
Vout 2
m
Vout 1 = Vinm+n)D
D'
Vout 2 = VinmDD'
PWMController
Optocoupler
TL4312.5 V ref. amplifier
R3R4
R5
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Improvement #3 No-Load Clamp
• 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.
Load (R1)Vin
Vout 1
n
1
Load (R2)
Vout 2
m
Vout 1 = Vinm+n)D
D'
Vout 2 = VinmDD'
PWMController
Optocoupler
TL4312.5 V ref. amplifier
R3R4
R5
12 V
5 V
9 V Zener
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Improvement #4 – Combined Feedback
• 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!
Load (R1)Vin
Vout 1
n
1
Load (R2)
Vout 2
m
Vout 1 = Vinm+n)D
D'
Vout 2 = VinmDD'
PWMController
Optocoupler
TL4312.5 V ref. amplifier
R3R4
R5
R6
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Weighting the Feedback
• 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%)
Optocoupler
TL4312.5 V ref. amplifier
R2
R0
R1
Vout 1Vout 2
Vref
i2 = W2 • i0
i0
i1 = W1 • i0
i0 = i1 + i2 = W1 • i0 + W2 • i0 = i0 (W1 + W2)
Therefore, W1 + W2 = 1Wn is the “weight” of the feedback from output n.
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Designing the Feedback
02
2
2
22
01
1
1
11
111
iWVV
iVV
R
iWVV
iVV
R
RiVV
refoutrefout
refoutrefout
refout
−=
−=
−=
−=
=−
( i1 + i2 = i0 )
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Example
Calculating the values:
Procedure:– Given: Vout 1 = 5, Vout 2 = 12, Vref = 2.5– Choose i0 = 1 mA– Choose W1 = 0.7 and W2 = 0.3
Ω=⋅−
=−
=
Ω=⋅−
=−
=
Ω===
kmAiW
VVR
kmAiW
VVR
kmAi
VR
refout
refout
ref
7.3113.0
5.212
57.317.0
5.25
5.21
5.2
02
22
01
11
00
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More Outputs? No Problem
• Feedback can be from any number of outputs.• Provided that: W1 + W2 + ……..+Wn = 1
Optocoupler
TL4312.5 V ref. amplifier
R2
R0
R1
Vout 1Vout 2
Vref
i2 = W2 • i0
i0
i1 = W1 • i0 Rn
Vout n
in = Wn • i0
0iWVV
Rn
refnoutn ⋅
−=
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The “Magic” Capacitor
Low-current load (R1 = large)Vin
Vout 1 n
1
Load (R2)
Vout 2
m = n
Vout 1 = VinnDD'
Vout 2 = Vout 1 = VinnDD'
PWMController
Optocoupler
TL4312.5 V ref. amplifier
R3R4
R5
With cap: Clean pulse; improved regulation at low-current load
www.onsemi.com14
Another Version of the “Magic” Capacitor
• 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).
Load (R1)
Vin
n
1
Load (R2)
Vout 2
mVout 1 = Vin
2m+n)DD'
Vout 2 = VinmDD'
PWMController
Optocoupler
TL4312.5 V ref. amplifier
R3R4
R5
m
Example: 5 V
Example: 12 V
www.onsemi.com15
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.
www.onsemi.com16
Design Example, Built and Tested
65 Watt, 8 OutputSet Top Box
Power Supply
Frank Cathell,Senior Applications Engineer
www.onsemi.com17
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 Vout Output type Regulation Max Ripple Current Surge1 2.6 V Buck reg. +/-1% 40 mVp/p 3 A 4 A2 3.3 V Buck reg. +/-1% 40 mVp/p 4 A 5 A3 5 V Main output +/-2% 50 mVp/p 3 A 4 A4 6.2 V Quasi-reg. +/-6% 50 mVp/p 1.5 A 2 A5 9 V 3-T reg. +/-1% 30 mVp/p 100 mA 200 mA6 12 V Main output +/-2% 50 mVp/p 1 A 3 A7 30 V Quasi-reg. +/-8% 100 mVp/p 20 mA 40 mA8 -5 V 3-T reg. +/-1% 30 mVp/p 30 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)
www.onsemi.com18
Circuit Features• Critical conduction mode flyback converter
NCP1207• 2.6 V and 3.3 V outputs derived from 12 V output
NCP1580 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 quasi-
regulated• 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
www.onsemi.com19
Set-Top Box Test Results
Note: Vout setpoints measured at PC board
Regulation Data (120VAC input)Outputs
Parameter 2.6V 3.3V 5V 6V 9V 12V 30V neg 5V
Output type Buck Buck Main Quasi-reg 3-T reg Main Quasi-reg 3-T reg
Vout setpoint attypical loads 2.53V 3.4V 4.89V 6.27V 8.94V 12.54V 31.0V 4.96V
Vout setpoint atminimum loads 2.55V 3.42V 4.96V 6.38V 8.94V 12.33V 32.70V 4.98V
Vout setpoint atmaximum 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
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More Test Results
OutputsParameter 2.6V 3.3V 5V 6V 9V 12V 30V neg 5V
Output Ripple(@ max loads) 27mV 45mV 50mV 50mV 40mV 30mV 100mV 20mV (10:1 scope probe)
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
E ffic ien cy M easu rem en ts (120V AC in p u t)O u tp u ts
P a ra m e te r 2.6V 3.3V 5V 6V 9V 12V 30V n e g 5V
O u tp u t V o lta g e 2.54 3.42 4.91 6.31 8.94 12.48 30.06 4.96
O u tp u t Cu rre n t 3.8A 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 ) 9.65 9.92 7.66 8.2 0.81 12.48 0.9 0.36 (49.98W total)
T o ta l P o u t = 49.98W
P in a t 120V AC = 61.4W
E ffic ien cy = 81 .4%
www.onsemi.com21
+C25
1200/6.3V
R2130K
JP4
JUMPER
1 2
R54.7
C9Not installed
R156.8K
D13
MUR120
12V-BUCKS
2.6V1
1
C39 1nf
D171N5818
C30 0.1uf
<Doc> C
Schematic - 60W set-top box
1 1Tuesday , May 31, 2005
Title
Size Document Number Rev
Date: Sheet of
R14 30K
Q3NTD60N02R
2
1
3
FIGURE 1: Schematic
R1710K
tTH1
10 Ohm 4A
1 2
R9 1K
C40 10nf
U6TL-431
2
1
3
JP2
JUMPER
1 2
C3110nf
R10 10
+C14
680/16V
D9 MUR110
D151N5818
R11M .5W
R294.7
P1
AC INPUT
12
C270.1uf
NC
12V-BUCKS
R44.7K
Q5NTD60N02R
2
1
3
30V1
1
Q1IRF740
2
1
3
C461nf
Q6NTD60N02R
2
1
3
C320.1uf
L5 4.7uH
C36 0.1uf
+C3
470/250V
R163.6K
+C24
1200/6.3V
D12
MBR1645
Q4NTD60N02R
2
1
3
+C22
680/16V
R37 68
JP3
JUMPER
1 2
L6 10uH
C41 10nf
12V1
1
R11 270
D16 MUR110
C4Y-CAP
R271K
+C21
680/16V
C371nf
9V1
1
C290.1uf
C500.1uf
+C20
680/16V
+C11
330/50V
D7
1N5226B
-5V1
1
C380.1uf
JP1
JUMPER
1 2
D10
MBR1635
C130.1uf
L2
BU16-4021R5B
C20.22/250V
C10.22/250V
F1
3A
C330.1uf
+C15
680/16V
R30 33K
D3
1N5406
12V-BUCKS
T18
2
3
6
16
15
1112
10
13
9
14
R2
R63.6K
C10470 pf
Not installed
R1910K
R244.7K
+C48
680/16V
C5560pf 1KV
R13 1K
L3 4.7uH
L4 4.7uH
C170.1uf
R284.7K
R8 22K
L1
BU10-1311R6B
R40 10
+C18
270/25V
D6 1N4148
D11 1N5820
MC79L05U4
32
1
OI
G
D4
1N5406
C44 10nf
NCP1580
U9
1 2
3
4
5
6
78
1 2
3 4
5
6
78
R181K
C6560pf 1KV
5V1
1
R266.2K
C42 1nf
C190.1uf
3-3V11
COM1
1
D2
1N5406
JP5
JUMPER
12
D1
1N5406
C450.1uf
R31 68
D8 1N4148
C47 0.1uf
Q2PN2222A
R234.7K
R32 33K
+C341200/6.3V
L7 10uH+
C491200/6.3V
R221K
+C35
680/16V
NCP1207A
U1
1
2 3
4
5 6
7
8
1
2 34
5 6
7
8
R30.33 1W
C71nf
+C822/25V
R34 10K
C23
0.1uf
PF1
1
MC78M09U331
2
OI
G
NCP1580U8
12
3
4
5
6
78
12
34
5
6
78
U7TL-431
2
1
3
R384.7
15,1W
+C28
270/25V
R36 33K
+C16
680/16V
R2547K
+C26
1200/6.3V
R35 22K
R7 100
D14 MUR110
6V1
1
C43 10nf
R33 10.5K
+C51
270/25V
R121K
U5
H11A817A
1
2
4
3
Schematic Stacked windings
Before diode After diode
Combined, weightedfeedback
www.onsemi.com22
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 well-known.– 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!