AN1262 APPLICATION NOTE

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and maximum output power) does not exceed 62-64%. The other limitation is that the sum of the max-

imum input voltage, reflected voltage and overvoltage spike - due to the leakage inductance - must be

below the breakdown of the internal MOSFET (700 V min.). Some margin needs also to be considered:

at least 50V is recommended to take the forward recovery of the diode of the clamp circuit and param-

eter spread into account. Figure 2 illustrates schematically how the drain voltage is apportioned. The

leakage inductance demagnetization (see below). The maximum duty cycle will be about 60% in

Figure 2. Drain voltage composition.

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primary side is not completely transferred to the secondary, after MOSFET turn-off, until the leakage

inductance is demagnetized. This delays and makes inefficient the energy transfer from primary to sec-

ondary. To minimize this noxious effect the voltage across the leakage inductance (the leakage induct-

ance spike) that resets the inductance itself should be as high as possible. Obviously, this is limited by

the maximum allowable drain voltage. With the reflected voltage selected as previously discussed, it is

applications (see fig. 2). This will affect the design of the clamp circuit.

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the power entering the primary. The secondary power includes the converter output power and the one

dissipated in the secondary rectifier. Besides the secondary one, the primary power includes the one

dissipated inside the transformer and that not transferred to the secondary side and dissipated on the

leakage inductance. For typical transformers used in converters based on the L6590 family IC's, typical

values of efficiency ranges between 88% and 95%, depending on the power level and on the construc-

tion technique. Efficiency increases with the power level and by using winding interleaving construction

technique. For consistency, check that the input power of the transformer be less than the converter

input power.

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is envisaged to accommodate the variation that the voltage generated by the self-supply winding may

Vin

Clamp Diode

forward recovery

700 V

374 V

187 V

504 V

317 V

≤ 650 V

margin

ON

OFF

Leak. Inductance

demagnetization

Current flows at the

secondary side

Prim. Inductance resonates

with drain capacitance

Leak. Inductance resonates

with drain capacitance

Transformer

demagnetised