AN4130 datasheet(6/41 Pages) STMICROELECTRONICS

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Theory of operation

AN4130

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Doc ID 023315 Rev 1

Theory of operation

2.1

Transition-mode flyback

Flyback power converters operate by storing energy from the primary side in an inductor's

air gap, and discharging the energy into a load on the secondary side. The converter can

run in two modes:

Discontinuous conduction, where there is a deadtime between discharge and charge

cycles.

Continuous conduction, where the discharge cycle is ended by starting the charge

cycle before all the stored energy is delivered to the load.

Neither mode fully utilizes the magnetic structure of the inductor. However, if the recharge

cycle is started just after the discharge cycle ends, the natural ringing of the inductor and

stray capacitance can be used to reduce turn-on voltage stress on the switch. Transition

mode converters can be very efficient as a result, having greatly reduced turn-on loss - the

switch does not have to discharge its own and stray capacitance from a high voltage.

Figure 3.

FET drain voltage waveforms

Operating frequency is a function of source and load voltages, and load current. If the

source voltage varies, the operating frequency varies. This makes the transition mode

converter very popular in low-cost commercial applications, where the varying frequency,

due to input voltage ripple, spreads noise over a wide spectrum, reducing the noise at any

one frequency. Conducted EMI tests can be easier to pass.

AN4130 Datasheet(PDF) 6 Page - STMicroelectronics