AN827 datasheet(7/20 Pages) STMICROELECTRONICS

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AN827 APPLICATION NOTE

The minimum boost inductor value can be calculated as follows:

The Table shown below relates the current ripple to the input voltage.

The number of turns, N, can be calculated according to the following formula:

Where:

L is the calculated inductance value to limit the ripple current,

∆IL.

ILpk is the worst case inductor current occurring at low-line input voltage (88V)

Aeff is the effective cross-sectional area of the core

Bmax is the maximum allowable flux density of the core

The air gap is determined by referring to the magnetic core manufacturer’s AL vs. air gap curves. The air gap

needed for the specified inductance, turns and core type is found to be 2.8mm in the center post. To approxi-

mate the minimum core size needed for the conversion, the following equation may be used:

Where K is the specific energy constant that depends on the ratio of the gap length (lgap) and the effective length

(leff) of the core set and the maximum ∆B swing. Practically, K can be estimated as follows:

Thus, we have the following calculation for the minimum core set volume in cm3:

The core chosen for this design is an ETD geometry ferrite core set with the following characteristics:

Vin (rms)

Vin(peak)

IL(rms) Iin (rms)

IL(peak)

Current Ripple

124

6.31

8.92

2.13

0.119

120

170

4.63

6.55

2.44

0.186

141

199

3.94

5.57

2.50

0.224

180

255

3.09

4.37

2.31

0.264

200

283

2.78

3.93

2.07

0.263

220

311

2.53

3.58

1.73

0.242

240

339

2.31

3.27

1.29

0.197

264

373

2.10

2.97

0.63

0.106

∆I

Lpk

⋅

------------------

Lpk

Lrm s

⋅

inrms

--------------------

⋅

;

min

out

∆I

Lm ax

⋅⋅

---------------------------------------

400

480kHz2.50

⋅⋅

------------------------------------------

0.5mH

Lpk

⋅

eff

max

⋅

----------------------------

0.5mH 8.92m A

⋅

211 10

0.36T

⋅⋅

----------------------------------------------------

59 Turns

Volume

K L I

Lpk

∆I

()

⋅

[]

⋅

≥

K11.5

eff

gap

----------

11.5

114

2.8

----------

468

⋅

Volume

468 0.5 10

–

8.92

2.5

()

⋅

[] 23.8cm

⋅⋅

≥

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