ICE3BR0365 Datasheet, PDF (6/16 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
III. DCM Flyback Design Equations and Sequential Decision Requirements
STEP 1:
Define and determine system requirements: Every SMPS design starts in determining the system
requirements and specifications. The following parameters need to be defined and determined.
Parameters
VACmax
VACmin
fsw
Eff
Pout
Vout
fline
∆Vout
Standby Power, Light load
efficiency, Various Protections,
EMI
Descriptions
Maximum AC input voltage
Minimum AC input Voltage
Switching Frequency
Efficiency
Output Power (maximum)
Main Output voltage
AC Line frequency
Output ripple voltage
Other Requirements
Table 2: Input specifications and system requirements
Efficiency is required to calculate the maximum input power. If no previous requirement is set, use 75%-
80% as a reasonable target for low cost Flyback. Choosing Fsw is usually a tradeoff between the
transformer size and switching loss, as well as EMI concerns and keeping the first fundamental below 150 kHz.
Common practice is between 50kHz-100kHz. Additionally, if there is more than one output voltage involved, Pout
max should be the sum of each individual output.
Note that while traditionally the controller and power MOSFET switch have been separate components, the wide
popularity of this converter type has led to the development of high performance integrated components combining
the MOSEFET switch and controller in one package, such as the Infineon CoolSET™ products, which use custom
CoolMOS™ transistors that include an integrated depletion mode start up FET for the controller power supply. The
CoolSET™ controllers are optimized for DCM applications, and the standalone controller (ICE3BS03LJG) includes a
startup cell.
STEP 2:
Choose the right controller considering the output power requirement:
Table 3: Infineon FF Flyback Controller
Choosing a Flyback controller usually depends on specific application and other design consideration such as cost,
design form factor and ease of design. Other requirements such as standby power and protection features are
easily be met by choosing the right controller. Table 3 shows a selection guide using Infineon’s solution for fixed
frequency Flyback with regards to its maximum output power. An integrated solution offers low parts count and
easier implementation while a separate controller and MOSFET approach has more flexibility, especially on
operating frequency and thermal design
Determining Input Capacitor Cin and the DC input voltage range: The capacitor Cin is also known as the DC
link capacitor, depending on the input voltage and input power, the rule of the thumb for choosing Cin is shown
below.
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