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AN1262 APPLICATION NOTE

then the total power dissipation and the hot spot temperature rise will be respectively:

Finally, some suggestions on the transformer construction techniques. When building a transformer, the general

rule is to minimize parasitics, basically leakage inductance and winding capacitance.

In order for a transformer to meet isolation and safety norms, primary and secondary windings must be sepa-

rated by isolation layers, thus their coupling cannot be intimate. Moreover, in a margin wound construction the

entire window breadth cannot be used (2.5 to 3 mm margin on each side must be considered to achieve suffi-

cient creepage distance) thus the winding becomes shorter and thicker, which hurts coupling. This is why triple

insulation construction is recommended.

Figure 4. Interleaved winding technique

As a result, it is not possible to reduce leakage inductance below a certain extent. Practically, for a well assem-

bled transformer, leakage inductance will be about 1 to 3% of the primary inductance.

Interleaved windings technique (putting on half the primary turns first, then the secondary and finally the other

half of the primary, see fig. 4) may considerably reduce leakage inductance (theoretically almost four times).

The two primary halves must be series connected, never paralleled. Other tricks, such as spacing windings

evenly across a layer (when they do not completely fill it), or using multiple strands of wire, or keeping isolation

between windings to a minimum are also effective. Besides, the use of split bobbins is not recommended.

Primary winding capacitance is the major component of the Cdrain capacitance earlier mentioned. Besides con-

tributing to internal MOSFET power losses, it causes ringing and noise problems that may force the use of ad-

ditional damping networks to comply with EMC requirements.

To achieve a low capacitance, always wind first the primary winding and, in particular, the half whose end is to

be connected to the drain of the MOSFET. In this way the second half primary has a shielding effect that reduces

the capacitive coupling. In case of multiple layer windings, which exhibit higher capacitance, it is useful to embed

one layer of isolation between two adjacent winding layers. This, however, tends to increase leakage inductance

and therefore should be done with care.

tot

Rp Ip

RMS

Rs Is

RMS

⋅

∆

tot

⋅

1/2 primary turns

secondary turns

air gap on

centre leg

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