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AN4350

Circuit description and design guidelines

34

lower switching losses on the flyback Power MOSFET) and the absolute voltage on the

MOSFET's drain). Typical range of this parameter is between 70 V and 140 V.

Assuming a desiderated reflected voltage of 100 V the ratio becomes:

Equation 5

guarantee the supply voltage for the IC when the output voltage is regulated (during the

startup the IC supply voltage is generated by the internal high voltage startup).

Equation 6

where typically the IC voltage is designed in the range of 17 V - 18 V (typ.).

In this design the IC voltage has been selected 15 V resulting in a higher secondary-

auxiliary transformer turns ratio:

Equation 7

Putting the value of Equation 7 into Equation 3 and resolving the Equation 2, the maximum

transformer magnetization inductance can be estimated:

Equation 8

The leakage inductor of the transformer should be minimized to reduce the voltage spike on

the drain node when the MOSFET is turned OFF - the maximum leakage inductor value is

-------

+

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

100V

25V 0.8V

+

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

3.87

==

=

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

+



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

=

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

25V 0.8V

+

15V

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

1.72

==

200V

1

200V

100V

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

+





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



1

100kHz

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

1

310mA

2 3.87





1

100V

0.85 200V



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

+





2







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



1.73mH

==