Flyback stage design

AN2528

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2

Flyback stage design

Well known to all SMPS designers, the voltage stress on the device (power switch) is given

by:

Equation 1

collector due caused by leakage inductance. This over-voltage is not limited by any

clamping network in order to minimize as much as possible the solution cost using also the

primary side while Ns is the number of turns on the main output secondary winding.

Now, taking into account a 300 V margin, the maximum flyback voltage that can be chosen

is:

Equation 2

After the calculation of the flyback voltage, we can proceed with the next step in the

converter design. The turns ratio between primary and secondary side is calculated with the

following formula:

Equation 3

As a first approximation, since the turn-on of the device occurs immediately after the energy

stored on the primary side, inductance is completely transferred to the secondary side:

Equation 4

and

Equation 5

Equation 6

The next step is to calculate the peak current. The output power is set to 6 W and the

desired transformer efficiency must be set by the designer (at least 80% in this case).

Excluding the energy losses on the input diode bridge, on the power switch and on the

secondary side rectifier, the following approximate formula can be used:

V

off

V

inmax

V

fl

V

spike

–

–

=

V

fl

BV

V

inmax

–

V

spike

–

V

min

arg

1700

850

–

200

–

300

350V

=

–

=

–

=

N

p

N

s

-------

V

fl

V

out

V

F diode

,

+

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

350

14

1

+

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

23.3

=

=

=

V

dcmin

T

onmax

•

V

fl

T

reset

•

=

T

onmax

T

reset

–

T

S

=

T

onmax

V

fl

T

S

•

V

dcmin

V

fl

+

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

14

µs

≅

=