AN-9741
APPLICATION NOTE
[STEP-7] Determine the Output Filter Stage
The peak to peak ripple of capacitor current is given as:
∆
I
CO
=
N
P
PK
×
I
DS
N
S
N
2
voltage does not change significantly during one switching
cycle. The snubber capacitor should be ceramic or a
material that offers low ESR. Electrolytic or tantalum
capacitors are unacceptable for these reasons.
The snubber capacitor voltage at full-load condition (V
SN
) is
given as:
(40)
The voltage ripple on the output is given by:
∆
V
O
=
∆
I
CO
×
t
DIS
∆
I
CO
−
I
O
×
∆
I
2
×
C
O
CO
+ ∆
I
CO
×
R
C
(41)
V
SN
=
V
RO
+
V
OS
(42)
The power dissipated in the snubber network is obtained as:
Sometimes it is impossible to meet the ripple specification
with a single output capacitor (C
O
) due to the high ESR (R
C
)
of the electrolytic capacitor. Additional LC filter stages
(post filter) can be used. When using post filters, do not to
place the corner frequency too low as this may make the
system unstable or limit the control bandwidth. It is typical
to set the corner frequency of the post filter at around 1/10 ~
1/5 of the switching frequency.
V
V
SN
1
PK
P
SN
=
SN
= ×
L
lk
×
(
I
DS
)
2
×
×
f
S
R
SN
2
V
SN
−
V
OS
2
(43)
where I
DSPK
is peak-drain current at full load, L
lk
is the
leakage inductance, V
SN
is the snubber capacitor voltage
at full load, and R
SN
is the snubber resistor.
The leakage inductance is measured at the switching
frequency on the primary winding with all other windings
shorted. Then, the snubber resistor with proper rated
wattage should be chosen based on the power loss. The
maximum ripple of the snubber capacitor voltage is
obtained as:
[STEP-8] Design the RCD Snubber in the
Primary Side
When the power MOSFET is turned off, there is a high-
voltage spike on the drain due to the transformer leakage
inductance. This excessive voltage on the MOSFET may
lead to an avalanche breakdown and, eventually, failure of
the device. Therefore, it is necessary to use an additional
network to clamp the voltage. The RCD snubber circuit and
MOSFET drain-voltage waveform are shown in Figure 6.
The RCD snubber network absorbs the current in the
leakage inductance by turning on the snubber diode (D
SN
)
once the MOSFET drain voltage exceeds the voltage of
cathode of D
SN
. In the analysis of snubber network, it is
assumed that the snubber capacitor is large enough that its
∆
V
SN
=
V
SN
C
SN
×
R
SN
×
f
S
(44)
In general, 5~20% ripple of the selected capacitor voltage is
reasonable.
In the snubber design in this section, neither the lossy
discharge of the inductor nor stray capacitance is
considered. In the actual converter, the loss in the snubber
network is less than the designed value due to this effect.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 6/27/11
www.fairchildsemi.com
7
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