ICE3BR0365 Datasheet, PDF (7/16 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
Table 4: Recommended Cin per Watt of Input
Power
For wide range operation use a DC link capacitor more than 2uF per watt of input power so as to get a better quality
of DC input voltage. With the input capacitor chosen the minimum DC input voltage (DC link capacitor voltage) is
obtained by:
(1)
Where: dcharge is the DC link capacitor duty ratio, typically around
0.2.
Figure 3 shows the DC link capacitor voltage. The minimum DC
input voltage occur at maximum output power and minim AC input
voltage while the maximum DCinput voltage occurs at minimum
input power (no load) and maximum AC input voltage. The
maximum DC input voltage can be found during no load condition
when the capacitor peak charge to the peak of the AC input voltage
and is given by;
(2)
Figure 3: DC link capacitor max and min voltage
STEP 3:
Decide on the Flyback reflected voltage (VR) and the maximum VDS MOSFET voltage stress: The
reflected voltage VR is the volatage across the primary winding when the switch Q1 is turned off. This also affects
the maximum VDS rating of Q1.The maximum drain to source voltage is given by:
(3)
Where: Vspike is the voltage spike caused by the leakage inductance of the transformer. For a starting point
assume Vspike is 30% of VDSmax. The table below list a recommended reflected voltage given a 650V and 800V
rated MOSFET. As a starting point limit VR below 100V for a wide range input voltage.
Table 5: Recommended VR for 650V and 850V MOSFET
Choosing the VR is a compromise between the primary MOSFET and the secondary rectifier voltage stress.
Setting it too high, by means of higher turns ratio, would mean higher VDSmax but lower voltage stress on the
secondary diode. While setting it too low, by lower turn ratio, would lower VDSmax but would increase secondary
diode stress, considering the required adjustments to the transformer turns ratio. Apart from cost, a higher primary
VDSmax will mean lower diode stress and lower primary side current stress.
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