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# Effect of winding arrangement on cross-regulation of flyback converters with multi-outputs

Effect of winding arrangement on cross-regulation of flyback converters with multi-outputs

Typical circuit diagram for a multioutput flyback converter
T1 D2 + RS CS

N1

N2
D4

C2 R2

DS
VIN

N4
Q1 D3

C4 R4

N3
Feedback

C3

R3

Cross regulation problem
The auxiliary output increases at light load when the regulated output is at full load; The auxiliary output drops at full load when the regulated output is at light load.

Objectives:

1. Study the effect of winding arrangement of the transformer and the air gap location on cross regulation of multi-output flyback converter
2. Give the general design guide lines for flyback transformer windings

Content
1. 2. 3. 4. Effect of transformer winding arrangements Effect of air gap location Effect of loop leakage inductance Effect of the clamp voltage

1. Effect of transformer winding arrangements

Transformer models
k12
T1 D2 +

Winding arrangement
C2
R2

RS

CS DS

N1

N2
D4

VIN

N4
Q1 D3

C4

R4

N3

C3

R3

Calculating coupling coefficient using Ansoft Field Simulator (k12,k13,k14,k23,k24,k34…)

This model can be used for transformers with any amount of windings But how to obtain mutual inductances using Ansoft Field Simulator?

Method to obtain mutual inductances
1 2

L?2 ?L1?L2 ? 2M12 1

Series connecting the two terminals with opposite polarity
1 2

L?2 ?L1?L2 ? 2M12 1

Series connecting the two terminals with the same polarity

Coupling coefficient k12 ?
?

M 12 [ L?12 ? L1 ? L2 ] / 2 ? L1 ? L2 L1 ? L2

or L? can be obtained using L1 2 12 Ansoft Field Simulator easily

Application of our approach in the prediction of cross-regulation for SMP-43EP power supply
+12V Transformer +5V R21 4.7k

Vo1: +5V/0.5A~11A
Vo2: +12V/0.02A~3A Vo3: -12V/0.02~0.5A

R23 24k R22 2.6k

11 1 L1 3 5 L5 6 L2 Vo1 10 9 L3 Vo2 12

Single feedback

Weighted feedback

L1 Turns Wire gauge 46 f0.5

L2 4 T18* 0.003

L3 4+5 f0.5* 2

L4 9 f0.5

L5 9 f0.3

7
L4 Vo3 8

Proposed different winding arrangements for evaluation
Case 1 L1 Case 2 Case 3 Case 4 Case5

L2
L3 L4 L5

Coupling coefficient
L1 : Primary

k12
0.993 0.994

k13
0.987 0.981

k14
0.98 0.985

k23
0.996 0.989

k24
0.987 0.993

k34
0.993 0.993

Case 1 Case 2

L2 : 5V/7A
L3 : 12V3A L4 : -12V/0.5A L5 : IC power

Case 3
Case 4 Case 5

0.988
0.9822 0.981

0.994
0.9876 0.9926

0.978
0.9918 0.9861

0.995
0.994 0.9882

0.993
0.9874 0.9923

0.986
0.9936 0.9934

L3(12V/3A) is the auxi. output we studied

Case 6

Case 7

Case 8

Case 9

Case 10

9 Ts independent

Coupling coefficient Case 6 Case 7 Case8 Case 9

k12

k13

k14

k23 0.99

k24

k34

5 Ts in series with L2 4 Ts

0.9952 0.9938 0.9926 0.994

0.9939 0.9943

0.9949 0.9936 0.9926 0.9951 0.9891 0.9943 0.9939 0.9932 0.9953 0.9933 0.9866 0.976 0.9937 0.9937 0.9839 0.9856 0.9923

The air gap is at the middle of the center leg

?Case1 case2: main output close to primary, auxi. Vo2 close to main output ?Case3 case4 case5: auxi. Vo2 close to primary, main output away from primary

?Case6 case case8: sandwich structure (primary_secondary_primary)
?Case9 : sandwich structure (secondary_primary_secondary)

The coupling coefficients can tell the difference of the above arrangements

Study of the effect of winding arrangement on cross-regulation based on experiment

3.1 Experiment result for auxiliary output Vo2 (12V/3A)
17 16 15
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8

Auxiliary output Vo2 vs. its load @Io1=7A Io3=0.02A

Vo2 (V)

14 13 12 11 10 0.02 0.52 1.02 1.52 2.02 2.52

Io2 (A)

Main Vo1: heavy load Auxiliary Vo3: light load Auxiliary Vo2 increases sharply when decreasing load current

Auxiliary output Vo2 vs.main load @ Io2=0.02A Io3=0.02A
20 18
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8

Vo2 (V)

16 14 12 10 1 3 5 7 9 11

Main load Io1 (A)

Auxiliary Vo2 and Vo3 : very light load

Auxiliary Vo2 increases when increasing load current of main output Vo1

12 11.5 11 10.5 10 9.5 9 8.5

Auxiliary output Vo2 vs.main load @Io2=3A Io3=0.5A
case case case case case case case case 1 2 3 4 5 6 7 8

Vo2 (V)

0.5

1.5

2.5

3.5

4.5

5.5

6.5

Main load Io1 (A)

When the auxiliary outputs(Vo2 and Vo3) are at heavy load, Vo2 decreases sharply when main load is very light for case1 and case2

Experiment result for auxiliary output Vo2 (12V/3A)
CASE Vo2max Vo2min DVo2 Vo2ave DVo2%

Case 1
Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Case 9 Case 10

13.926
13.443 18.494 17.042 18.762 16.462 15.129 15.938 18.554 13.568

9.291
8.627 10.575 10.543 10.558 9.987 10.082 10.422 10.04 10.23

4.635
4.816 7.919 6.499 8.204 6.475 5.047 5.516 8.514 3.338

11.608
11.035 14.534 13.792 14.66 13.224 12.605 13.18 14.297 11.899

-22.57% ~+12.02%
-28.11% ~+12.02% -11.87% ~+54.12% -12.02% ~+42.02% -12.02% ~+56.35% -16.77% ~+37.18% -15.98% ~+26.07% -13.15% ~+32.82% -16.33% ~+54.62% -14.75% ~+13.07%

DVo2 = Vo2max – Vo2min

Vin = 220V ~

Vo2max at Io1=11A, Io2=0.02A, Io3=0.02A Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

The auxiliary output voltages for different cases
auxiliary output voltage (V)

Vo2 vs. arrangements

20 18 16 14 12 10 8
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8 case 9

Vo2max Vo2min Vo2ave

case

Vo2ave = (Vo2max + Vo2min )/2 Vo2max at Io1=7A, Io2=0.5A, Io3=0.5A Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

The difference DVo2 of auxiliary output voltage for different cases
auxiliary output voltage difference (V)
9 8 7 6 5 4
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8 case 9 4.635 4.816 7.919 8.204 8.514

6.499

6.475 5.516 5.047

case

DVo2 = Vo2max – Vo2min
Vo2max at Io1=11A, Io2=0.02A, Io3=0.02A Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

The cross regulation of auxiliary output for different cases
60.00%

14.8% 54.1% 56.4% 54.6%

50.00%

-DVo2%
42.0%

+DVo2%
37.2% 32.8%

Delta Vo2 (V)%

40.00%

30.00%

28.1% 22.6%

26.1%

20.00%

16.8% 16.0% 12.0% 12.0% 11.9% 12.0% 12.0% 13.2%

16.3% 13.1%

10.00%
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8 case 9 case 10

case

Summary
1. 2. Coupling coefficients can model the multi-winding transformer Winding arrangement has great effect on cross-regulation and the auxiliary output variation with load Maintaining good coupling between primary and auxiliary windings while sacrificing coupling between the primary and main output windings will generally result in very high auxiliary output at light load and poor cross regulation Sandwich structure (primary-secondary-primary) makes good coupling among all the windings, but good coupling between auxiliary winding and primary winding cause high auxiliary output and poor cross regulation

3.

4.

5.

Sandwich structure (secondary-primary-secondary) make good coupling between primary and main output, but bad coupling between auxiliary and main output and good coupling between primary and auxiliary output results in extremely high output at light load and the worst cross regulation

6. To achieve a good cross-regulation: ? ? ? The main output winding should be as close as possible to the primary winding the auxiliary output winding should as close as possible to the main output winding and the better the coupling between auxiliary and main output, the good the cross regulation The auxiliary output winding should keep away from the primary winding

Experimental results of the cross regulation of the converter

Variation of the winding inductance with frequency measure

A

fs (kHz) L1 (uH) L2 (uH) L3 (uH) L4 (uH) L5 (uH) 0.1 537.2 4.21 8.05 20.34 21.07 1 520.8 4.053 7.27 20.24 20.48 10 489 3.857 6.148 19.66 19.23 100 485.6 3.789 6.042 19.27 19

B

fs (kHz) L1 (uH) L2 (uH) L3 (uH) L4 (uH) L5 (uH) 0.1 522.2 4.24 6 24.9 21.25 1 509.6 4.07 5.95 22.9 20.64 10 484.9 3.847 5.92 19.75 19.32 100 480.1 3.781 5.851 19.23 18.98

L4 closed winding

Comparison of the cross regulation
EIR core
Vo2min Vo2max DVo2 Vo2ave DVo2%

A
B C

10.43
10.384 10.413 Vo3min

13.861
13.854 13.912 Vo3max 13.355 13.337 13.55

3.431
3.47 3.499 DVo3 2.637 2.565 2.88 k14

12.145
12.119 12.162 Vo3ave 12.036 12.054 12.11 k24 0.9773

28.25
28.63 28.77 DVo3% 21.91 21.28 23.78

C

A B C

10.718 10.772 10.67

L4 : space winding
L4 closed winding 0.9684

L4 space winding

0.9787

0.9901

Comparison of the cross regulation
EER core
Vo2min A 10.402 10.372 Vo3min A C 10.716 10.713 Vo2max 14.024 14.052 Vo3max 13.477 13.71 DVo2 3.622 3.68 DVo3 2.761 2.997 Vo2ave 12.213 12.212 Vo3ave 12.096 12.211 DVo2% 29.66 30.13 DVo3% 22.83 24.54

A

C

C
Vin = 220V ~

DVo2 = Vo2max – Vo2min Vo2max at Io1=11A, Io2=0.02A, Io3=0.02A

Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

L4 space winding causes higher Vo3 than close winding when its load is light

Coupling coefficient and cross regulation of different arrangements
d1 Vo2min D1 10.132 Vo2max 15.811 DVo2 5.679 Vo2ave 12.972 DVo2% 43.78 d2

D2

10.411
Vo3min

15.994
Vo3max 16.964

5.583
DVo3 5.917

13.202
Vo3ave 14.005

42.29
DVo3% 42.25

D1

11.047

D2

10.352

16.554

6.202

13.453

46.1

Winding away from the gap

High voltage in the whole load range

simulati on
EIR_d1

Gap (mm)
0.7

L1 (uH)
467.3

L2 (uH)
3.642

L3 (uH)
5.132

L4 (uH)
20.35

k13
0.9637

k23
0.9487

k14
0.9606

k24
0.9486

EIR_d2

0.7

474.3

3.696

6.312

16.75

0.9654

0.9532

0.9591

0.9446

Larger inductances when away from air gap cause lager turn ratio of secondary and primary, finally higher output voltage than that near the air gap

Coupling coefficient and cross regulation of case f1 (without shield)
f1 f2

Vo2min
f2 f1 10.499 10.581 Vo3min f2 f1
Gap (mm) EIR_f1 EIR_f2 0.7 0.7 L1 (uH) 471.4 468.7

Vo2max
14.971 15.06 Vo3max 16.89 16.77
L3 (uH) 5.95 5.44

DVo2
4.472 4.479 DVo3 5.768 6.018
L4 (uH) 17.66 19.27

Vo2ave
12.735 12.82 Vo3ave 13.996 13.761

DVo2%
35.12 34.94 DVo3% 41.21 43.73

11.102 10.752
L2 (uH) 3.682 3.654

k13 0.9781 0.9782

k23 0.9818 0.9824

k14 0.9758 0.9755

k24 0.9779 0.9783

The results of case f1 and f2 is the same with d1 and d2.
But why the voltage at heavy load rise more than that at light load ?

Ip = 1A Is2 = -5.111A

-4.2029e-008

Ip = 1A

Is3 = -5.111A

-1.2693e-007

Winding away from air gap

larger leakage inductance in this winding

Decreasing voltage at light load

Coupling coefficient and cross regulation of case e1 and e2 (with shield)
e1 e2

Vo2min
e2 e1 10.655 10.664 Vo3min e2 e1 11.053 11.073

Vo2max
15.178 15.146 Vo3max 16.626 17.140

DVo2
4.523 4.482 DVo3 5.573

Vo2ave
12.916 12.905 Vo3ave 13.84 14.106

DVo2%
35.02 34.73 DVo3% 40.27 43.01

?

6.067

Simulat ion EIR_e1 EIR_e2

Gap (mm) 0.7 0.7

L1 (uH) 449.35 448.3

L2 (uH) 3.493 3.482

L3 (uH) 5.461 5.373

L4 (uH) 17.45 17.66

k13 0.9847 0.9847

k23 0.9888 0.9886

k14 0.9840 0.9840

k24 0.9873 0.9874

g1

g2

Vo2: L3 & Vo3: L4 f0.5 9 turns independent
Vo2min Io2max= Io3max = 0.5A Vo2max Vo3min Vo3max

g1
g2

10.675
10.727

16.084
16.384

10.674
10.641

17.142
16.6

Winding near from gap

Higher voltage at light load

Why a shield makes different results?

L3=17.72u H

L4=17.45u H

Ip = 1A

Is2 = -5.111A

-4.2393e-008

Ip = 1A -5.6176e-008

Is3 = -5.111A

Adding a shield Near gap

Inductances L3 & L4 become almost equal

Lower leakage inductance

Only higher voltage at light load

3. Effect of leakage inductance of the loop
Case f1

Effect of main loop leakage inductance

Lk1=37.7nH Rac1=1mohm

Lk2=22nH Rac2=1.1mohm

Effect of main loop leakage inductance
Io1=11A Io2=0.02A Io3=0.02A With Lk1 With Lk2

Vo3=18.24V

Vo3=17.8V

Vo1=4.838V
Vo2=15.85V

Vo1=4.838V
Vo2=15.49V

Without L: Vo1=4.839V Vo2=14.93V Vo3=16.64V

The lager the main loop leakage inductance, the higher the auxiliary voltage, and the worse the cross regulation

Effect of main loop leakage inductance
Io1=0.5A Io2=3A Io3=0.5A

With Lk1 Vo1 (V) Vo2 (V) Vo3 (V) 4.843 10.55 10.81

With Lk2 4.843 10.54 10.8

Without Lk 4.843 10.53 10.74

The main loop leakage inductance has little effect on auxiliary outputs when the main load is light and auxiliary loads are heavy

Effect of auxiliary leakage inductance
Io1=0.5A Io2=3A Io3=0.5A

With L2 and L3 Io1=11A Io2=0.02A Io2=0.02A
Io1=0.5A Io2=3A Io2=0.5A

Only with L3 4.839
14.97 16.64 4.843 10.56 10.75

Without L2 and L3 4.839
14.98 16.72 4.843 10.56 10.75

Vo1 (V)
Vo2 (V) Vo3 (V Vo1 (V) Vo2 (V) Vo3 (V)

4.839
14.92 16.64 4.843 10.53 10.74

The auxiliary loop leakage inductance decreases the auxiliary output voltage

4. Effect of clamp voltage on cross-regulation

Vc

Clamp voltage(RCD snubber of primary side)

RS VIN DS Q1

N1

Clamp Vc vs.main load @Io2=3A Io3=0.5A
310 260
case 1 case 2 case 3 case 4 case 5 case 6 case 7 case 8

Total leakage inductance: measured from primary side with all secondary windings shorted
Lk1 = 11.53uH Lk2 = 13.92uH

Vc (V)

210 160 110 60 10 0.5 2.5 4.5 6.5

Lk3 = 9.112uH
Lk4 = 9.8uH Lk5 = 8.606uH Lk6 = 5.04uH

Main Load Io1 (A)

The clamp voltages of case 1 and case 2 are the highest, so there is a trade-off between cross regulation and efficiency

Lk7 = 5.618uH
Lk8 = 5.065uH

? Usually it is considered the lower the clamp voltage is, the better the cross regulation is. But the experimental results show that case1 and case2 have the highest clamp voltage although their cross regulations are the best. ? It can be seen that the lager the total leakage inductance is, the higher the clamp voltage is.

Effect of R and C on clamp voltage and cross regulation

Simulation circuit of Pspice

Comparison of the clamp voltage VC with variational capacitance

C: 4000pF~6000pF~8000pF~1000pF~12000pF R:47kohm

4000 pF 500 pF

C: 500pF~4000pF

R:47kohm

C: 500pF~4000pF R:470kohm

Comparison of the clamp voltage VC with variational resistance

R:6k~26k~46k~66kohm

C:4700pF

R:6k~66kohm

C:47000pF

Effect of clamp resistance on clamp voltage (case 1) experiment
Clamp voltage Vc vs. main output load @Io2=3A Io3=0.5A 275 R=47kohm 225 R=8.38kohm

Vc (V)

175 125 75 0.5 1.5 2.5 3.5 4.5 5.5 6.5

Mian output Io1 (A)

Clamp voltage Vc vs. main output load @Io2=0.02A Io3=0.02A 250 200 R=47kohm R=8.38kohm

Vc (V)

150 100 50 0 1

3

5

7

9

11

Mian output Io1 (A)

Cross regulation of winding arrangement case 1 with different clamp resistances
Rclamp(ohm) 47k 8.38k Rclamp(ohm) 47k Vo2min 9.214 9.388 Vo3min 9.543 Vo2max 13.731 13.528 Vo3max 13.708 DVo2 4.517 4.14 DVo3 4.165 Vo2ave 11.472 11.458 Vo3ave 11.625 DVo2% 39.37 36.13 DVo3% 35.83

8.38k
Vin = 220V ~

9.719

13.573

3.854

11.646

33.09

DVo2 = Vo2max – Vo2min Vo2max at Io1=11A, Io2=0.02A, Io3=0.02A Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

Lower clamp resistance causes better cross regulation

R = 47K

R = 8.38K

clamp loads R (ohm) Io1=7A Io2=3A Io3=0.5A 47k 8.38k C (pF) 6060 6060

Vp-p (V) 15 33.2

Vmean (V) 194.7

DVp (V)

Psnubber (W) 0.806

DPsnubber (W)

-63.24 122.36 1.787

+0.981

Better cross regulation is achieved by decreasing R is at the cost of efficiency

Effect of clamp capacitance on clamp voltage experiment
Peak clamp voltage Vc vs. main output load @Io2=3A Io3=0.5A

270

Vc (V)

220 170 120 0.5 1.5 2.5 3.5 4.5 5.5 C=4700pF C=2200pF C=940pF 6.5

Mian output Io1 (A)

Peak clamp voltage Vc vs. main output load @Io2=0.02A Io3=0.02A 260 230 200 170 140 110 80 50 1 3 5 7

Vc (V)

C=4700pF C=2200pF C=940pF 9 11

Mian output Io1 (A)

Clamp voltage waveforms with different capacitance
C = 4700pF C = 940pF

Vcpeak = 241V Vcave = 225.96V

Vcpeak = 278V Vcave= 205.4V

Load: Io1=7A Io2=3A Io3=0.5A

Cross regulation of winding arrangement case 1 with different clamp capacitance

Cclamp(pF) 4700 2200

Vo2min 9.207 9.191

Vo2max 13.758 13.759

Vo3min 9.539 9.52

Vo3max 13.747 13.744

940

9.17

13.748

9.497

13.719

DVo2 = Vo2max – Vo2min Vin = 220V ~ Vo2max at Io1=11A, Io2=0.02A, Io3=0.02A

Vo2min at Io1=0.5A, Io2=3A, Io3=0.5A

The clamp capacitance has no effect on cross regulation

The end ! Thanks !

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