AN1262 Datasheet, PDF (4/42 Pages) STMicroelectronics – APPLICATION NOTE

AN1262 APPLICATION NOTE
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
suggested value of V_R is 130 V: it leads to a maximum drain voltage slightly exceeding 500 V in 220V_AC
or WRM applications, and about 320 V in 110 V_AC application, thus leaving enough room for an efficient
leakage inductance demagnetization (see below). The maximum duty cycle will be about 60% in
110V_AC and WRM applications, and close to 36% in 220 V_AC applications.
Figure 2. Drain voltage composition.
700 V
≤ 650 V
504 V
317 V
Leak. Inductance resonates
with drain capacitance
374 V
187 V
Clamp Diode
forward recovery
Leak. Inductance
demagnetization
margin
Vspike
Current flows at the
secondary side
Transformer
demagnetised
VR
Prim. Inductance resonates
with drain capacitance
Vin
ON
OFF
s Leakage inductance overvoltage. The energy stored in the mutual inductance of the transformer at the
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
possible to allow about 140 V extra voltage in 220 V_AC or WRM applications and much more in 110 V_AC
applications (see fig. 2). This will affect the design of the clamp circuit.
s Transformer efficiency. By definition, it is the ratio of the power delivered by the secondary winding to
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.
s Device supply voltage. The supply voltage range of the IC spans from 7 to 16.5 V. Such a wide range
is envisaged to accommodate the variation that the voltage generated by the self-supply winding may
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