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HIGH-FREQUENCY POWER TRANSFORMERS Design a high-frequency power transformer based on flyback topology DINESH KUMAR
E
ssentially, switched-mode power supplies, or SMPS, act as AC-to-DC converters. These rectify the AC input voltage (85V-265V AC) to convert it into DC. Depending upon the design considerations, these chop the rectified voltage (DC) at very high frequencies. SMPS find use in computer power supplies, TV sets, CD players, battery chargers, adaptors, etc. Their major advantages are ligher weight, smaller size, higher efficiency and lesser cost. Let’s now consider the merits of SMPS individually. Lighter weight and smaller size are due to operation at a significantly higher frequency range and use of smaller inductive elements. Rapid switching of the power transistor between saturation and cut-off regions of its operation results in very little energy dissipation and hence reduced heat-sink requirements. Costs are reduced owing to the absence of large bulky power transformers, a huge reduc-
0 to 150 watts, fo ers at 50 to 500 w at 100 to 1000 w bridge usually o Full-bridge and pologies with full aries have the high efficiency becaus the windings are Let’s assume 12V DC output a for your stereo from 220V AC, 5 output powe 12V×2A=24W, ogy for this desig To fully unde power supply de to review the th topology and the of a switched-mo as conti ply such Sign up to vote on this title continuous oper Useful Not useful a high-switching f former design. Th former design is t
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DESIGN
TABLE I Core Size Selection on the Basis of Power Handling Capacity Output power level
Recommended core types
0-10W
EFD15 SEE16 EF16 EPC17 EE19 EF(D)20 EPC25 EF(D)25 EE19 EPC19 EF(D)20 EE or EI22 EF(D)25 EPC25 EI25 EF(D)25 EPC25 EPC30 EF(D)30 ETD29 EER28(L) EI28 EER28(L) ETD29 EF(D)30 EER35 EER28L ETD34 EER35 ETD39 ETD34 EER35 ETD39 EER40 E21
10-20W
20-30W
30-50W
50-70W
70-100W
Fig. 2: Primary and secondary currents in (a) discontinuous and (b) continu
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the fewest components. A and there is also a dead time between the levels, the total compone instant the secondary reaches Unlock full access current with a free trial. zero than with other techniques and the start of the next cycle. tween 75 and 150W, incr In the continuous mode there is still Download With Free Trial and current stresses caus some energy left in the secondary at the ponent cost to increase s beginning of the next cycle. It is possible these power levels, topolog for flyback to operate in both modes, but it voltage- and current-stress has different characteristics. The discon the forward converter) a tinuous mode has higher peak currents Sign up to vote on this title effective, even with high and therefore higher output voltage spikes Useful Not useful hand, it counts. during the turn-off. On the other To design a flyback tra has faster load transient response and lower need to go through the follo primary inductance, and therefore the trans-
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TABLE II EE and EF Core Specifications Type
Bmax=
Dimensions (mm) A
B
C
EE10
10.2 ±0.3
5.5 ±0.2
EE10A
10.2 ±0.3
5.5 ±0.2
EE13
13.0 ±0.4
EE13A
D
E min
F
5.0 +0 –0.3 9.8 +0.3 –0.1
2.5±0.2
7.5
4.3±0.2
2.5±0.2
7.6
4.3±0.2
5.9 ±0.3
2.8 +0 –0.4 3.6 ±0.2
9.8
13.0 ±0.4
6.0 +0.2 –0.1 6.5 ±0.2
4.6 +0.3 –0.1 4.6 ±0.2
EE12.6 EE16A
12.7 ±0.4 16.0 ±0.4
6.4 ±0.2 7.3 ±0.3
3.3 ±0.2 5.1 +0 –0.4
3.7 ±0.2 4.0 ±0.2
8.8 11.7
EE16B
16.0 ±0.4
12.4 ±0.3
EF16
16.0 +0.7 –0.5 19.0 ±0.4
8.2 +0 –0.3
5.1 +0 4.0 ±0.2 12.4 –0.4 4.7 +0 4.7 +0 11.3 –0.3 –0.4 5.9 +0.1 5.0 +0 13.8 –0.5 –0.5 5.1 +0 5.1 +0 14.0 –0.5 –0.5 5.9 +0.4 5.9 +0.4 14.1 –0.5 –0 You're Reading a Preview
EE19A EE19B
19.0 ±0.4
8.0 ±0.3 13.6 ±0.3
9.8 +0.3 –0.1
8.5
4.65 ±0.3 5.2 +0.2 –0 10.4 ±0.2 5.7 +0.4 –0
9.3 +0 –0.4
EE20 EE20A EE20/20 EE23
20.5 ±0.5 20.3 ±0.5 20.1 ±0.4 22.7 ±0.3
10.7 ±0.3 8.4 ±0.3 10.15 ±0.25 11.0 ±0.3
7.0 ±0.3 5.0 ±0.3 14.7 Unlock full a free trial. 4.8 ±0.2 4.8 access ±0.2 with 15.3 9.2 ±0.25 4.5 ±0.2 15.2 10.2 ±0.2 7.5 +0 16.4 +0.5 Download With Free Trial –0.3 –0
7.0 ±0.3 6.2 ±0.2 7.9 ±0.2 7.5 +0 –0.3
EE25A EE25B EE25C
25.0 ±0.5 25.0 ±0.5 25.4 ±0.6
9.9 ±0.3 9.8 ±0.2 9.6 +0.3 –0.1
6.35 ±0.25 6.1 ±0.3 6.75 ±0.2
6.9 ±0.3 6.8 ±0.2 6.5 +0.3 –0.1
Po η×VDC min ….(2)
6.35 ±0.2 5.8 ±0.2 6.35 ±0.25
18.6 18.6 18.6
6.1 +0.4 –0
Ae
µr =
11.3 ±0.3
20.0 ±0.6
Np×Ip×A
The calculated B 0.2 to 0.3 tesla. If y density more than 0. to turns per volt (T crease Te to get high and Np and a lower still you get Bmax tesla, again increase the process until you than 0.3 tesla. Now calculate th gap, which means yo calculate the relative the ungapped core ( culated from core par fective cross-sectiona Le (effective magnet in cm2) and AL (ind in nH/turn2) as follow
5.6 +0.4 –0.1
EF20
Average primary current IAV =
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A 0.4
The gap length (L calculated. The ga ground only in the c core. If the gap is pu legs, it will need to b culated here. The min Lg is 0.051 mm, an lated from the follow
(
Lg=
0.4π×N2p×Ae
Lp Sign up to vote on this title Step 5. Selection Useful Not useful Now let us define turns per volt (Te) for primary and secondary to decide Np and Ns. If some conditions For primary and secon (explained later) are not satisfied, we’ll wire that doesn’t generate
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DESIGN
TABLE IV Wire Paramet
TABLE III EE and EF Core Specifications Type
EE10 EE10A EE13 EE13A EE12.6 EE16A EE16B EF16 EE19A EE19B EF20 EE20 EE20A EE20/20 EE23 EE25A EE25B EE25B/20 EE25C EF25 EE25D EE25E EE26 EE28A EE28B EE28C EE28Q EE30 EE30/30 EF32A EF32B EE33 EE35A EE35B EE40A EE40B EE41
AL(nH/N 2)±25%
SWG diam (mm)
Ae
Le
Ve
(mm -1)
(mm2)
(mm)
(mm2)
SK
2.1 1.1 1.7 0.8 2.4 1.92 2.8 1.87 1.68 2.65 1.2 1.21 1.9 1.2 0.75 1.25 1.3 1.3 1.15 1.340 1.20 1.2 0.6 0.95 0.57 0.82 0.8 0.5 1.1 0.92 0.89 0.57 0.804 0.83 0.6 0.50 0 50
12.7 23.1 17.2 37.4 12.4 18.4 19.6 20.1 23.3 23.4 35.6 39 22 41.1 70.0 39.6 36.9 36.9 43.4 33.5 40.7 49.2 75.8 71.6 99.8 87.4 83.1 108.6 59.4 70.1 83 124.0 86.2 88.3 127 153 152 0
26.1 26.4 30.0 29.6 29.7 35.5 55.2 37.6 39.2 62.1 42.2 47.1 42.8 49.7 49.6 49.5 49.7 49.7 48.2 44.9 49.2 57.9 46 63.0 48.1 73.1 64.4 57.7 65.5 61.3 74 66.1 69.3 69.4 77.0 76.5 77 1
330.7 611.6 513.8 1107.6 369.0 655 1080 750 914 1450 1505 18.50 942.8 2042 3469 1963 1823 1823 2091 1500 2004 2850 3488.1 4508 4801 6391 6350.5 6269.7 3879 4295 6140 8192 3973 6121 9922 11710 11722
OOOO 10.16 810 810 1.8 OOO 9.45 810 1000 1.5 OO 8.84 1000 1000 3.6 O 8.23 1000 1500 3.5 1 7.62 900 800 2.0 2 7.01 1100 1100 3.3 3 6.40 750 750 5.3 4 5.89 1000 800 800 4.0 5 5.38 1050 1200 1200 4.6 6 4.88 1000 840 840 7.5 7 4.47 1800 1550 1450 7.8 8 4.06 2100 1900 1800 9.7 9 3.66 1100 1900 1800 3 10 3.25 2100 2000 1800 5 11 2.95 2300 2300 17.5 12 2.64 ≥1600 1900 1900 10.2 13 2.34 1600 1600 9.0 ≥1400 14 2.03 1600 1600 9.0 ≥1400 15 1.83 ≥1600 1800 1800 10.0 16 1.63 ≥1630 1900 1900 14.8 17 1.42 ≥1600 1800 1800 11 18 1.22 1850 1850 13.5 ≥1600 19 1.02 1850 10 ≥1600 You're Reading a1700 Preview 20 0.92 3400 3060 2850 35 21 0.81 4500 4000 4000 23.5 Unlock full access with a free trial. 22 0.71 3200 2806 2650 33.5 23 0.61 3200 2806 3500 26 24 0.56 Download Free Trial 2200 1800 With 1900 20 25 0.51 2200 1900 1900 22 26 0.46 2500 2500 21.5 27 0.41 2400 2400 30 28 0.38 4500 4000 4000 40.6 0.35 29 2990 2600 2600 Sign 31 up to vote on this title 30 0.31 3680 3200 3000 31.5 Useful Not useful 31 0.29 55 4250 3700 3700 4600 4000 4000 60 flyback transformers beca 4704 4200 4200 61
≥700 ≥700 ≥860 ≥860 ≥650 ≥950 ≥680
SP3
Weight
Wire gauge size
Ct
SP4
(gm)
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Flyback Transformer Application in An SMPS
CD players need 3V to 12V DC to play music and mobile phones need around 4.5V DC to recharge their battery. But we plug the CD cellphone’s battery charger into 220V AC, 50Hz through an adaptor (which generally comes with CD player or the cellphone). Now how this 220V AC is converted into 3V or 4.5V DC? Or what is there inside the CD player adaptor or cellphone charger to do this job? Devices like CD players, cellphone battery chargers, fax machine power supplies, TV power supplies, stereo power supp supplies, electronic toy power supplies and PDA adaptors contain nothing but the SMPS shown in Fig. 3. The figure shows the a flyback transformer in an SMPS. The equipment take 220V, 50Hz AC power from the wall-mounted power socket in our home or office. This 220V AC is rectifi into DC voltage without an input isolation transformer. The rectified and filtered DC voltage (in the range of 260V DC to 360V DC) flyback transformer, where the rectified high-voltage DC is stepped down to 3.3V, 4.5V DC or some other value to feed the devic
Fig. 3: SMPS block diagram
with the help of a power-switch control pulse-width modulation (PWM) circuit. When you connect the device to the output DC vo flyback transformer after output rectifier, this voltage may vary from the nominal value because of fluctuations in the input overloading. To maintain the output voltage at the desired level, a sampling network is required. The sampling network gives a sample You're Reading a Preview voltage to check with the reference voltage. For any difference between the sampled voltage and the reference voltage, the PW action to maintain the output voltage constant. Unlock full access with a free trial. In the block diagram of the SMPS, the power transformer need not be a flyback transformer only. It could be a forward transfo full-bridge transformer. The selection of the topology (type of the transformer) depends on the output power level.
(d) Bias voltage Vb=18V (generally, 16V to 20V) (e) AC mains frequency f L=50 Hz (f) Minimum AC mains voltage VACmin=85V and maximum AC mains voltage VACmax=265V (g) Maximum duty cycle Dm=0.5 (h) Estimated power supply efficiency η=0.85
Download Trial CalculationWith of theFree number of turns Step 3.
Table II shows that the in primary and secondary windings: sectional area for the selecte Ns=Te×Vo is 39.6 mm2. Now calculate Taking Te= 1 turn/volt, we get flux density as follows: Ns=11×12=12 turns Sign up to vote on this title Np×Ip×A Now primary turns (Np) are calculated useful Useful NotBmax= as follows: Ae Np=Ns×
VAC min
×
Dmax
=
90×1.128×1. 39.6×10
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DESIGN
low, which means the core is underutilised. Let’s calculate once more with Te =1.35. We get: Ns=17 Np=128 ALG=7.324×10–8 H/turn2 Bmax= 0.26 tesla This value is within the acceptable limits. The relative permeability is calculated as follows (if it is not specified in the core datasheet by the core vendor): µr = =
AL×Le
1900×10–9×49.5×10–3 0.4π×39.6×10–6
(
0.4π×N2p×Ae Lp
require the fewest components. At lower power levels, the total component cost is less
×10–3 mm =(679.4–26.20)×10–3=0.653 mm
Air-gap length (Lg)=
(
The flyback topology is used extensively because flyback power supplies
than with other techniques
0.4π×Ae
=1.889×10–3
=
=
–
Le µr
)×10
–3
mm
0.4π×1282×39.6×10–6 49.5×10–3 – = 1.2×10–3 1.889×10–3
)
This gap will be in the centre of the core. The minimum limit for Lg is 0.051 mm. Step 5. Selection of wire area for primary and secondary windings: Area of primary winding conductor (Ap): Input rms current (Irms) =You're Reading a Preview mm2 J
0.436 =0.0970 m 4.5
The rms value of the p (Irms) is given by:
√
Ipp×
As=
2 =0.444 m 4.5
After calculating area ( mary and secondary, select from Table IV. The SWG for primary c and the SWG for secondar 21 or 20. The design results can as follows: 1. Core type: EE25A 2. Primary number of t 28 SWG insulated copper w 3. Secondary number with 21 SWG insulated cop 4. Air-gap in the EE cor
The author is a hardw
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