Example of Adding a Negative Output

(1)

Multi-Output Flyback Off-Line

Power Supply

(2)

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)

Vin Vout 1

n 1

Load (R2) Vout 2

m

Vout 1 = nD Vin D'

Vout 2 = mD Vin D'

(3)

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

n 1

Load (R2) Vout 2

m

Vout 1 = nD Vin D'

Vout 2 = mD Vin D'

Load (R3)

p Vout 3 = pD Vin

D'

Vout 3

• There is no theoretical limit to the number of outputs.

(4)

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?

(5)

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.

(6)

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 = m+n)D Vin D'

Vout 2 = mD Vin D'

PWM Controller

Optocoupler

TL431 2.5 V ref. amplifier

R3

R4

R5

(7)

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 = m+n)D Vin D'

Vout 2 = mD Vin D'

Optocoupler

R3

R4

R5 12 V

5 V

9 V Zener

(8)

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 = m+n)D Vin D'

Vout 2 = mD Vin D'

Optocoupler

R3

R4

R5

R6

(9)

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

TL431 2.5 V ref.

amplifier

R²

R⁰

R¹ V^{out 1} V^{out 2}

V^ref i²= W² • i⁰

i⁰

i¹= W¹ • i⁰

i⁰ = i¹ + i² = W¹ • i⁰+ W² • i⁰= i⁰ (W¹+ W²) Therefore, W¹+ W² = 1

Wⁿ is the “weight” of the feedback from output n.

(10)

Designing the Feedback

2 2

2

0 1 1 1

1 1

1 1 1

i W

V V

i V R V

i W

V V

i V R V

R i V

V

ref out

= −

=

−

( i

¹

+ i

²

= i

⁰

)

(11)

Example

Calculating the values:

Procedure:

– Given: V^{out 1} = 5, V^{out 2} = 12, V^ref = 2.5 – Choose i⁰= 1 mA

– Choose W¹ = 0.7 and W² = 0.3

Ω

⋅ =

= −

Ω

⋅ =

= −

Ω

=

mA k i

W V R V

mA k i

W V R V

mA k i

R V

ref out

ref

7 . 1 31

3 . 0

5 . 2 12

57 . 1 3

7 . 0

5 . 2 5

5 . 1 2

5 . 2

2 2

0 1 1 1

0 0

(12)

More Outputs? No Problem

• Feedback can be from any number of outputs.

• Provided that: W1 + W2 + ……..+Wn = 1

Optocoupler

TL431 2.5 V ref.

amplifier

R²

R⁰

R¹

V^{out 1} V^{out 2}

V^ref i²= W² • i⁰

i⁰

i¹= W¹ • i⁰ Rⁿ

V^{out n}

iⁿ= Wⁿ • i⁰

i

0

W

V R V

n

ref n

out

n

⋅

= −

(13)

The “Magic” Capacitor

Low-current load (R1 = large) Vin

Vout 1

n

1

Load (R2) Vout 2

m = n

Vout 1 = nD Vin D'

Vout 2 = Vout 1 = nD Vin D'

Optocoupler

R3

R4

R5

With cap: Clean pulse; improved regulation at low-current load

(14)

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,

Load (R1) Vin

n

1

Load (R2) Vout 2

m

Vout 1 = 2m+n)D Vin D'

Vout 2 = mD Vin D'

Optocoupler

R3

R4

R5

m

Example: 5 V Example: 12 V

(15)

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.

(16)

Design Example, Built and Tested

65 Watt, 8 Output Set Top Box

Power Supply

Frank Cathell,

Senior Applications Engineer

(17)

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 Surge

1 2.6 V Buck reg. +/-1% 40 mVp/p 3 A 4 A

2 3.3 V Buck reg. +/-1% 40 mVp/p 4 A 5 A

3 5 V Main output +/-2% 50 mVp/p 3 A 4 A 4 6.2 V Quasi-reg. +/-6% 50 mVp/p 1.5 A 2 A 5 9 V 3-T reg. +/-1% 30 mVp/p 100 mA 200 mA 6 12 V Main output +/-2% 50 mVp/p 1 A 3 A 7 30 V Quasi-reg. +/-8% 100 mVp/p 20 mA 40 mA 8 -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.

(18)

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 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

(19)

Set-Top Box Test Results

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 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

(20)

More Test Results

Outputs

Parameter 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

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)

O u tp u ts

P a ra m e te r 2 . 6 V 3 . 3 V 5 V 6 V 9 V 1 2 V 3 0 V n e g 5 V

O u tp u t V o l ta g e 2 . 5 4 3 . 4 2 4 . 9 1 6 . 3 1 8 . 9 4 1 2 . 4 8 3 0 . 0 6 4 . 9 6

O u tp u t C u rre n t 3 . 8 A 2 . 9 A 1 . 5 6 A 1 . 3 A 9 1 m A 1 . 0 A 3 0 m A 7 3 m A

O u tp u t P o w e r (W ) 9 . 6 5 9 . 9 2 7 . 6 6 8 . 2 0 . 8 1 1 2 . 4 8 0 . 9 0 . 3 6 (4 9 . 9 8 W t o t a l)

T o ta l P o u t = 4 9 . 9 8 W P i n a t 1 2 0 V A C = 6 1 . 4 W

E ffic ie n c y = 8 1 .4 %

(21)

+ C25 1200/6.3V

R21 30K

JP4

JUMPER

1 2

R5 4.7

C9 Not installed

R15 6.8K D13

MUR120

12V-BUCKS

2.6V 1 1 C39 1nf

D17 1N5818

C30 0.1uf

Schematic - 60W set-top box Title

R14 30K

Q3 NTD60N02R

2

1

3

FIGURE 1: Schematic

R17 10K

t

TH1

10 Ohm 4A

1 2

R9 1K

C40 10nf

U6 TL-431

2

1

3

JP2 JUMPER

1 2

C31 10nf R10 10

+ C14 680/16V

D9 MUR110

D15 1N5818

R1 1M .5W

R29 4.7 P1

AC INPUT 1 2

C27 0.1uf

NC

12V-BUCKS

R4 4.7K

Q5 NTD60N02R

2

1

3

30V 1 1

Q1 IRF740

2

1

3

C46 1nf

Q6 NTD60N02R

2

1

3

C32 0.1uf

L5 4.7uH

C36 0.1uf

+ C3 470/250V

R16 3.6K +

C24 1200/6.3V

D12

MBR1645

Q4 NTD60N02R

2

1

3

+ C22 680/16V

R37 68

JP3

JUMPER

1 2

L6 10uH

C41 10nf

12V 1 1

R11 270

D16 MUR110 C4

Y -CAP

R27 1K

+ C21 680/16V

C37 1nf

9V 1 1

C29 0.1uf

C50 0.1uf C20 +

680/16V + C11 330/50V

D7

1N5226B

-5V 1 1

C38 0.1uf

JP1 JUMPER

1 2

D10 MBR1635

C13 0.1uf

L2

BU16-4021R5B C2 0.22/250V C1

0.22/250V F1

3A

C33 0.1uf

+ C15 680/16V

R30 33K D3

1N5406

12V-BUCKS T1

8

2 3 6

16

15

11 12

10

13

9 14 R2

R6 3.6K

C10 470 pf

Not installed

R19 10K

R24 4.7K

+ C48 680/16V C5

560pf 1KV

R13 1K

L3 4.7uH

L4 4.7uH

C17 0.1uf

R28 4.7K R8 22K

L1

BU10-1311R6B

R40 10

+ C18 270/25V

D6 1N4148

D11 1N5820

MC79L05 U4

3 2

1

O I

G

D4 1N5406

C44 10nf

NCP1580 U9

1 2

3

4

5

6 7

8

1 2

3 4

5

6 7

8

R18 1K C6

560pf 1KV

5V 1 1

R26 6.2K

C42 1nf

C19 0.1uf

3-3V 1 1

COM 1 1 D2

1N5406

JP5 JUMPER

12

D1 1N5406

C45 0.1uf R31 68

D8 1N4148

C47 0.1uf

Q2 PN2222A

R23 4.7K

R32 33K +

C34 1200/6.3V

L7 10uH

+ C49 1200/6.3V

R22 1K

+ C35 680/16V

NCP1207A U1

1

2 3

4

5 6

7

8

1

2 4 3

5 6

7

8

R3 0.33 1W

C7 1nf

C8 + 22/25V

R34 10K

C23 0.1uf

PF 1 1 MC78M09

U3 3 1

2

O I

G

NCP1580 U8

1 2

3

4

5

6 7 8

1 2

34

5

6 7 8

U7 TL-431

2

1

3

R38 4.7 15,

1W

+ C28 270/25V

R36 33K

+ C16 680/16V

R25 47K

+ C26 1200/6.3V

R35 22K R7 100

D14 MUR110

6V 1 1

C43 10nf

R33 10.5K

+ C51 270/25V

R12 1K U5

H11A817A 1

2 4

3

Schematic

Stacked windings Before diode After diode

Combined, weighted feedback

(22)

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!

(23)

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