DF06M View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'DF06M' PDF : 42 Pages View PDF
AN1262 APPLICATION NOTE
then the total power dissipation and the hot spot temperature rise will be respectively:
Ptot = PFe + Rp ⋅ IpR2 MS + Rs ⋅ IsR2 MS
∆T = Ptot ⋅ Rth
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
secondary turns
1/2 primary turns
1/2 primary turns
air gap on
centre leg
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.
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