Last, but not least significant, the technology. A good technology device offers lower gate charge and

Transformer.

The design and the assembly of the transformer plays a significant role in the process of power losses

minimization. The most annoying parasitic is the so-called "leakage inductance", that represents the

stray primary magnetic flux, modeled as an inductor in series with the primary and not coupled to the

secondary. The energy stored in the leakage inductance produces an overvoltage spike on the drain of

the MOSFET at turn-off. An external circuit will be necessary to clamp this spike so that the voltage rat-

ing of the MOSFET is never exceeded.

Therefore, when designing and building a transformer with the aim of optimizing the efficiency of the

converter at light load, the priorities are basically three:

a) make the leakage inductance as low as possible. In terms of efficiency, there is a double noxious ef-

fect due to the leakage inductance. It not only dumps its own energy into the clamp circuit but also

delays energy transfer from primary to secondary, after MOSFET turn-off, until it has run out of en-

ergy. The result is that the energy stored in the mutual inductance is not completely transferred to the

secondary and is partly diverted into the clamp circuit and partly dissipated in the resistance of the

primary winding. This inefficiency is worsened by a light load and a high input voltage: both reduce

the primary peak current and also the voltage across the leakage inductance (the leakage inductance

spike) that resets the inductance itself. The lower this voltage is, the more energy transfer is delayed

and the less energy is brought to the secondary.

In practice, besides improving the energy transfer, a low leakage inductance will allow to lighten the

action of the external clamp and/or to select a lower voltage rating MOSFET. This will be beneficial to

efficiency at heavy load as well.

In order for a transformer to meet isolation and safety regulations, primary and secondary windings

must be separated by isolation layers, thus their coupling cannot be intimate. As a result, it is not pos-

sible to reduce leakage inductance below a certain extent. Practically, for a well assembled trans-

former, leakage inductance will be about 1

÷3% of the primary inductance.

Interleaved windings technique (putting on half the primary turns first, then all the secondaries and fi-

nally the other half of the primary; see fig. 11) can reduce leakage inductance by 50%. The two pri-

mary halves must be series connected, never paralleled.

In general, multifilar winding technique (twisting the wire of two or more windings together) gives

maximum coupling between windings. In off-line converters, however, this technique is usually appli-

cable only to secondary windings to get good cross-regulation, in case of multiple output. When multi-

filar winding technique is not practicable because of very different turns number (or wire size), the

secondary winding with the highest output power should be wound closest to the primary; for the

same power the lowest voltage should be given priority.

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

1/2 primary turns

1/2 primary turns

secondary turns

air gap on

centre leg

Figure 11. Interleaved winding technique

AN1049 APPLICATION NOTE

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