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1N4004

OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY

STMicroelectronics 

AN1262 APPLICATION NOTE

When the switch turns off, the primary circuit is open and the energy stored in the primary is transferred to the

secondary by magnetic coupling. The catch diode is forward-biased, and the stored energy is delivered to the

output capacitor and the load. The output voltage Vout is reflected back to the primary through the turns ratio n

(VR, reflected voltage) and adds up to the input voltage Vin, giving origin to a much higher voltage on the drain

of the MOSFET.

Flyback is operated in DCM (Discontinuous Conduction Mode) when the input -or primary - current starts from

zero at the beginning of each switching cycle. This happens because the secondary of the transformer has dis-

charged all the energy stored in the previous period. If this energy transfer is not complete, the primary current

will start from a value greater than zero at the beginning of each cycle. Then flyback is said to be operated in

CCM (Continuous Conduction Mode). DCM is characterized by currents shaped in a triangular fashion, whereas

CCM features trapezoidal currents.

The boundary between these two types of operation depends on several parameters. For a given converter,

that is, as the switching frequency, inductance of the primary winding, transformer turns ratio and regulated out-

put voltage are defined, it depends on the input voltage and the output load.

At design time, whether the converter will be operated in CCM or in DCM and where the boundary will be located is

up to the designer. Usually CCM is selected with the objective of maximizing converter's power capability or minimiz-

ing primary RMS current. However, in CCM operation the system's dynamic behavior is considerably worse.

Usually, the converters based on the L6590 family devices are able to deliver the desired output power even

with DCM operation, thus CCM will not be considered.

Table 1. Converter specification data and pre-design choices

Converter Electrical Specification

VACmin

Minimum mains voltage

VACmax

Maximum mains voltage

fL

Mains frequency (@ min. mains)

NH

Number of holdup cycles

Vout

∆Vout%

Regulated output voltage

Percent output voltage tolerance (±)

Vr%

Percent output voltage ripple

Poutmax

Maximum output power

η

Expected converter efficiency

Tamb

Maximum ambient temperature

Pre-design Choices

VR

Reflected voltage

ηT

Vspike

Transformer efficiency

Leakage inductance overvoltage

Vcc

IC supply voltage

VF

Secondary diode forward drop

VBF

Bridge Rectifier + EMI filter voltage drop

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