ICE3BR0365 Datasheet, PDF (8/16 Pages) Infineon Technologies AG – Design Guide for Off-line Fixed Frequency DCM Flyback Converter

DCM Flyback
Design Note DN 2013-01
V1.0 January 2013
Determine Dmax based on Vreflected and Vinmin: The maximum duty cycle will appear during VDCmin, at this
condition we will design the transformer to be at the boundary of DCM and CCM. Duty cycle here is given by;
(Equation 4)
STEP 4:
Calculate primary inductance and primary peak current: The primary peak current can be found by
using Equation 5 and 6.
(Equation 5)
The primary inductance should then be design within the limit of maximum duty cycle;
(Equation 6)
(Equation 7)
In order to ensure that the Flyback would not enter into CCM operation at any loading condition make sure to
consider the maximum power in calculating Poutmax in Equation 5. Increasing inductance beyond the calculated
Lprimax can also push the converter towards CCM mode.
DCM Flyback Transformer Design: Steps 7- Step 12
STEP 5:
Choosing the proper core type and size: Choosing the core type and geometry for the first time is
quite difficult and usually involve a lot of factors and variables to consider. Among these variables to consider are
the core geometry (e.g. EE core/RM core/PQ core etc.) , core size (eg. EE19, RM8 PQ20 etc.) and core material
(eg.3C96. TP4, 3F3 etc.).
If there is no previous reference on choosing the right core size, a good way to start is to refer to the manufacturer's
core selection guide. Below are some commonly used core size for 65kHz DCM Flyback with respect to the output
power.
Even if you choose to work with a transformer vendor such as Wurth/Midcom for the ultimate design and
construction to applicable safety standards, working through a design example is a good basis for submittal to a
transformer vendor as you will want to establish turns ratio and other parameters for your supplier.
Table 6: Recommended core size for DCM Flyback (Fsw=65kHz)
After selecting core size, the right bobbin can also be chosen on the corresponding core's data sheet. Choose
the bobbin considering the number of pins, through-hole or surface mount and horizontal or vertical orientation. Core
materials are chosen considering the frequency of operation, magnetic flux density, and considering core losses.
Core material name varies depending on the core manufacturer, a suitable core materials to start with are 3F3,
3C96 or TP4A.
Determining minimum primary turns: The minimum number of turns on the primary is a function of the magnetic
core area and the allowed operating flux density for the chosen material.
(Equation 8)
Where Bmax is the operating maximum flux density, Lp is the primary inductance, Ip Is the primary peak current and
Ae is the cross sectional area of the chosen core type.
8