Multi-Output Flyback Off-Line Power Supply

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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

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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

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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 OutputSet 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 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)

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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

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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!

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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