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AN4105

Design Considerations for Switched Mode Power Supplies Using A Fairchild Power Switch (FPS) in a Flyback Converter

Fairchild Semiconductor 

AN4105

APPLICATION NOTE

Table 1. KU, KP, and K

CCM Buck, Boost Inductor

DCM Buck, Boost Inductor

CCM Flyback Transformer

DCM Flyback Transformer

KU

KP

K = KU KP

0.7

1.0

0.7

0.7

1.0

0.7

0.4

0.5

0.2

0.4

0.5

0.2

6.2 Determining The Number of Turns

Although the equation for the minimum number of flyback

transformer turns can be determined using applied voltage

and maximum turn on time, the equations used here are

derived from the relationship between L and IP.

For an AP determined from equation (a), above, calculate

NMIN using the following equations:

NMIN

=

--------L---I--P--------- 104

BMAXAC

L = Inductance of Transformer

Ip = Operating peak current

Bmax = Maximum operating flux density

Ae = Effective cross-sectional area of core

For an AP determined from equation (b), above, calculate

NMIN using the following equations:

NMIN

=

----L---∆----I--m------ 104

∆BmAC

L = Inductance of Transformer

Ip = Operating peak current

Bmax = Maximum operating flux density

Ae = Effective cross-sectional area of core

6.3 The Windings

The cross sectional area of the winding must be obtained

from the calculated effective current and the current density

(from the appropriate equation above), factoring in the

number of turns (as determined from a calculation of

NMIN). If eddy current loss is not a serious problem simply

divide the effective current by the current density, thereby

determining the coil cross sectional area.

Be aware, however, that as the coil becomes thicker, the

problem of eddy current loss will arise. Using twisted thin

coil strands (Litz wire), instead of a single heavy wire, can

reduce the eddy current loss, but KU will become smaller.

6.4 Determining The Gap

It is not easy to precisely calculate the required gap.

However, the gap can be calculated from the following

equation. It is based on the fringing effect of the surrounding

flux. Note that the calculated value of L is usually larger than

required.

Ig

=

-u---O----u----r--N----2---A----C--

L

10–2

[

c

m

]

Uo = Permeability of free space

Ur = Relative permeability

Therefore, the gap must be changed to obtain the required

value of L.

References

1. Transformer and Inductor Design Handbook. 2nd ed.

Col. Wm. T. McLyman. Marcel Dekker, Inc., 1988.

2. Switch Mode Power Supply Handbook. Keith H.

Billings. McGraw-Hill, Inc., 1989.

KA1H0280RB, KA1M0280RB, KA1H0380RB, KA1M0380RB,

KA1L0380RB, KA1L0380B, KA1H0680B, KA1M0680B,

KA1H0680RFB, KA1M0680RB, KA1M0880B, KA1L0880B,

KA1M0880BF, KA1M0880D, KA2S0680B, KA2S0880B,

KA3S0680RB, KA3S0680RFB, KA3S0880RB, KA3S0880RFB,

KA1H0165RN, KA1H0165R, KA1H0265R, KA1M0265R,

KA1H0365R, KA1M0365R, KA1L0365R, KA1H0565R,

KA1M0565R, KA1M0765RC, KA1M0765R, KA1M0965R,

KA2S0765, KA2S0965, KA2S09655, KA2S1265, KA3S0765R,

KA3S0765RF, KA3S0965R, KA3S0965RF, KA3S1265R,

KA3S1265RF, KA5H0280R, KA5M0280R, KA5H0380R,

KA5M0380R, KA5L0380R, KA5P0680C, KA5H0165R,

KA5M0165R, KA5L0165R, KA5H0165RV, KA5M0165RV,

KA5L0165RV, KA5M0165RI, KA5L0165RI, KA5H0165RN,

KA5M0165RN, KA5L0165RN, KA5H0165RVN,

KA5M0165RVN, KA5L0165RVN, KA5H02659RN,

KA5M02659RN, KA5H0265RC, KA5M0265R, KA5L0265R,

KA5M0365RN, KA5L0365RN, KA5H0365R, KA5M0365R,

KA5L0365R, KA5M0765RQC, KA5M0965Q, KA5S0765C,

KA5S09654QT, KA5S0965, KA5S12656, KA5S1265,

KA5Q0740RT, KA5Q0945RT, KA5Q0765RT, KA5Q12656RT,

KA5Q1265RF, KA5Q1565RF, FSDH0165, FSDH565,

FS6M07652RTC, FS6M12653RTC, FS6S0765RCB,

FS6S0965RT, FS6S0965RC, FS6S1265RB, FS6S1565RB,

FS7M0880, FS8S0765RC

20

©2002 Fairchild Semiconductor Corporation

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