MB39C011APFT-G-BND-ERE1 Datasheet, PDF (38/53 Pages) Cypress Semiconductor – 2 ch DC/DC Converter IC with Synchronous Rectification

MB39C011A
19.3 Fly-back Diode Selection
Select a Schottky barrier diode (SBD) that has a small forward voltage drop.
The peak current flowing through the Fly-back diode needs to be found in order to determine whether the current flowing through
the Fly-back diode is within the rated value. When the DC/DC converter IC is used with asynchronous rectification, the maximum
current through the Fly-back diode is given by the following formula.
I_f ≥ I_OMAX
IL
2
I_f
I_OMAX
IL
: Forward current [A]
: Maximum load current [A]
: Coil ripple current peak to peak value [A]
Furthermore, the power dissipation of the Fly-back diode needs to be found in order to determine whether the power dissipation
of the Fly-back diode is within the rated value. The power dissipation of the Fly-back diode is given by the following formula.
P_SBD  I_OMAX (1 
V_O
V_IN
) Vf
P_SBD
I_OMAX
V_IN
V_O
Vf
: Fly-back diode power dissipation [W]
: Maximum load current [A]
: Switching system power supply voltage [V]
: Output voltage [V]
:Forward voltage [V]
When the DC/DC converter is used with synchronous rectification, the length of time that the current flows through the Fly-back
diode is limited to the synchronous rectification period (dead time). For example, at an oscillating frequency of 500 kHz, the
proportion of time that current flows is less than 5. Therefore, select that Fly-back diode current does not exceed the peak forward
surge current (IFSM) rated value. The peak forward surge current value of the SBD is given by the following formula.
I_FSM ≥ I_OMAX 
IL
2
I_FSM
I_OMAX
IL
: Peak forward surge current value of Fly-back diode [A]
: Maximum load current [A]
: Coil ripple current peak to peak value [A]
Document Number: 002-08369 Rev. *A
Page 38 of 53