ICE3BR0365 Datasheet, PDF (11/16 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
STEP 10:
Output Rectifier Diode Selection:
(Equation 14)
(Equation 15)
Chose the output diode so that its VRRM (maximum reverse voltage) is at least 30% higher than VRVdiode
and IF (ave forward current) is at least 50% higher than the IsecRMS. Use schottky diode on the main secondary
output for lower conduction losses. For DCM mode Flyback peak current is high, and keeping the forward voltage
low may require using what seems like a relatively high current diode, depending on the efficiency target.
STEP 11:
Output Capacitor Selection: For Flyback converter the proper choice of output capacitor is
extremely important . This is because the Flyback mode converters have no inductive energy storage between the
rectifier and ouput capacitor. The output capacitor needs to be selected to meet these 3 important parameters:
capacitance, ESR (equivalent series resistance) and RMS current rating.
Determining the minimum output capacitance is a function of the allowable maximum peak to peak ouput ripple
voltage:
(Equation 16)
Where: Ncp is the number of internal clock cycles needed by the control loop to reduce the duty cycle from
maximum to minimum value. This usually takes around 10-20 switching periods. Iout is the maximum output current
(
).
The minimum capacitor RMS current rating of the chosen capacitor is:
(Equation 17)
Given the high switching frequency, the high secondary peak current for a Flyback will produce a corresponding
ripple voltage across the output capacitor's equivalent series resistance (ESR). The capacitor must be chosen not
to exceed ESRmax or the permissible ripple current capability of the capacitor, which is a thermal limitation for the
capacitor. The final selection may more reflect the required voltage rating and ripple current capability, depending
on the actual ratio of output voltage and current.
(Equation 18)
Make sure to use the ESR specification from the data sheet at a frequency greater than 1kHz; this will usually be 10
kHz or 100 kHz.
Note: Using a single capacitor with lower ESR value could meet the desired output ripple voltage. A small LC
filter is also advisable for higher peak currents specially when operating in DCM mode in order to hit a consistent
output ripple voltage performance.
STEP 12:
Other Design Considerations
A.
Input Diode Bridge Voltage and Current Rating:
(Equation 19)
Where PF is the power factor of the power supply, use 0.5 if there is no better reference data available. Select the
bridge rectifier rating such that the forward current twice than that of IACRMS. A 600V part is commonly use for
maximum input voltage of ~400V
B.
Current Sense Resistor, Rsense: The sense resistor Rsense is used to defined the maximum output
power. Vcsth can be found on the controller's data sheet while Ipmax is the primary peak current while also
considering the short term peaks in output power.
(Equation 20)
C. VCC Capacitor: The capacitance value is important for proper startup time, a value of 22uF -47uF is usually in
the right range for most applications. If the capacitance is too low, the under voltage lockout of the IC may trigger
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