FUJITSU MB39A104 - Manual (Page 29)

MB39A104
29
• Flyback diode
The flyback diode is generally used as a Shottky barrier diode (SBD) when the reverse voltage to the diode is
less than 40V. The SBD has the characteristics of higher speed in terms of faster reverse recovery time, and
lower forward voltage, and is ideal for achieving high efficiency. As long as the DC reverse voltage is sufficiently
higher than the input voltage, the average current flowing through the diode is within the average output current
level, and peak current is within peak surge current limits, there is no problem. In this application the Rohm
RB053L-30 is used. The diode average current and diode peak current can be calculated by the following
formulas.
Diode mean current : I
Di
Diode peak current : I
Dip
Example: Using the Rohm RB053L-30
VR (DC reverse voltage)
= 30 V, average output voltage = 3.0 A, peak surge current = 70 A,
VF (forward voltage) = 0.42 V, IF = 3.0 A
CH1
CH2
CH1
CH2
I
Di
≥ I
O
× (1 −
V
O
)
V
IN
I
Dip
≥ (I
O
+
V
O
toff)
2L
I
D
i ≥ I
O
× (1 −
V
O
)
V
IN
≥ 3 × (1 − 0.263)
≥ 2.21 A
I
D
i
≥ I
O
× (1 −
V
O
)
V
IN
≥ 3 × (1 − 0.174)
≥ 2.48 A
I
D
ip
≥ (I
O
+
V
O
toff)
2L
≥ 3.24 A
I
D
ip ≥ (I
O
+
V
O
toff)
2L
≥ 3.18 A