DocID10240 Rev 2

7/17

AN1897

Application description and design

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For safety requirements, a leakage inductance value is 1 to 3% of the open circuit primary

inductance. A high efficiency transformer should have low inter-winding capacitance to

decrease the switching losses. Energy stored in the parasitic capacitance of the transformer

is absorbed by the VIPer22A-E cycle-by-cycle during the turn-on transition. Excess

capacitance also rings with stray inductance during switch transitions, causing noise

problems. Capacitance effects are usually the most important in the primary winding, where

the operating voltage (and consequent energy storage) is high. The primary winding should

be the first winding on the transformer. This allows the primary winding to have a short

length per turn, reducing the internal capacitance. The driven end of the primary winding

(the end connected to the drain pin) should be the beginning of the winding rather than the

end. This takes advantage of the shielding effect of the second half of the primary winding

and reduces capacitive coupling to adjacent windings. A layer of insulation between

adjacent primary windings can cut the internal capacitance of the primary winding by a four

factor, with consequent reduction of losses. A common technique for winding multiple

secondaries with the same polarity sharing a common return, is to stack the secondaries

(see Figure 6). This arrangement improves the load regulation, and reduces the total

number of secondary turns. Commonly a clamper based on an RCD network or a diode with

a Zener to clamp the rise of the drain voltage is used.

Figure 6. Multiple output winding

Figure 4. RCD clamp topology

Figure 5. Zener clamp topology