AN2783 datasheet(23/47 Pages) STMICROELECTRONICS

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AN2783

Power converter

23/47

Power converter

The PM8800 reference board implements a flyback converter operating in DCM

(discontinuous mode) at low output power and in CCM (continuous mode) for medium to

high output power.

The output secondary rectifier can be a classic diode for low to medium output power or a

synchronous rectifier for high output current.

Flyback configuration is the standard choice for a low-power isolated converter. It is the

simplest isolated converter, using the lowest number of power components.

CCM has been selected in order to reduce the stress on the power components, especially

on the secondary side.

CCM is assured only at medium to full output power, while in low output power the converter

works in DCM which allows reducing the size of the power transformer.

It is out of the scope of this document to show the whole flyback converter theory, which can

be found in every basic power supply handbook. In this application note we focus only on

the aspects directly related to the use of the PM8800 in a flyback converter.

10.1

Flyback continuous conduction mode

The flyback converter is in DCM when the energy stored during the ON phase has been

completely transferred to the secondary side during the OFF phase. This means that a

small period of time still remains during which no current is flowing on either side of the

power transformer.

When this period of time does not exist, i.e. when the energy stored has not been

completely transferred during the ON phase, the flyback is said to work in CCM.

Compared to CCM, DCM presents higher peak and rms current values on the primary

switch and on the output rectifier. This implies higher output ripple and require bigger input

and output filters.

CCM presents an RHP zero, which slightly complicates the control loop compensation. A

flyback converter designed to work in CCM is also stable in DCM.

The transfer function of a flyback in CCM is:

Equation 5

The above equation can be written as:

Equation 6

Vin

Vds

–

() Ton

⋅

Vout

() NTsw Ton

–

()

⋅⋅

Vout

()

Vin

Vds

–

()

--------------------------------

–

()

⋅

---------------------------

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