BAT54WS View Datasheet(PDF) - Intersil
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BAT54WS Datasheet PDF : 10 Pages
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Application Note 1612
The RMS current through the MOSFET can be calculated from:
Irms, FET = Ipk ⋅
d--
3
(EQ. 9)
= 1.06 ⋅ 0----.--3---5--= 0.362A
3
Selecting the conduction loss in the MOSFET to 1% of total output
power, 0.03W. The required MOSFET’s rDS(ON) to achieve the
required conduction loss is shown in Equation 10.
rDS(ON)
=
P-----F---E----T---,---c---o---n----d---–---l-o----s---s-
IF2 ET, rms
(EQ. 10)
=
----0---.--0---3-----
0.3622
=
0.229 Ω
Vishay’s SI4436DY is selected in this design.
Output Diode Selection
Schottky diodes are recommended for the output diode due to
their low forward voltage drop. The voltage stress across the
output diode can calculated by:
VDiode = n × VIN, MAX + VOUT
= 1 × 26.4 + 15= 41.4V
(EQ. 11)
Diodes Inc’s B180 are employed in this design.
Output Filter
The output capacitance needs to meet the ripple and noise
requirements, and also be able to handle the ripple current.
Assuming ceramic capacitors are used as the output filter, the
voltage ripple from the capacitor’s ESR is negligible. The
minimum capacitance required to meet specifications can be
approximately calculated from Equation 12.
CO
U
T
>
-Δ----V----P----P--
2
⋅
-(--1-----–----d----2---)----⋅---T----S----W---
IOUT
> 5----0----×---1---0---–---3- ⋅ -------(--1-----–-----0---.--5----)------
2
0.1 ⋅ 300×103
> 0.42μF
(EQ. 12)
10µF ceramic capacitors are selected for each output. Design
margin has been provided to account for noise spikes.
Snubber Circuit
When the MOSFET switches off, it interrupts the current that
flows through the transformer leakage inductance. An RCD
snubber circuit is typically used in flyback converters to clamp
voltage spikes on the MOSFET.
Assuming that the transformer leakage inductance is 2% of the
magnitizing inductance, the energy stored in the leakage
inductance during MOSFET’s on-time is:
WL
=
1--
2
⋅
LL
⋅
IL2M
=
1--
2
⋅
0.02
⋅
23.8 ×10–6
⋅
( 1.06 ) 2 =
267.4 n J
(EQ. 13)
Average power transferred to the snubber circuit is:
PL = WL ⋅ FSW
= 267.4×10–9 ⋅ 300×103= 0.08W
(EQ. 14)
To limit peak voltage spikes across the MOSFET to 50V, the
snubber voltage is set to:
VS = peakVMOSFET – VIN, MIN
= 50 – 21.6 = 28.4V
(EQ. 15)
The average power transferred to the snubber circuit in
Equation 14 is dissipated by the snuuber resistor, so RS is
determined by:
RS
=
V-----S2--
Pl
=
-2---8---.--4---2--
0.08
=
10.08 k Ω
(EQ. 16)
So RS = 10kΩ is selected. Cs is selected such that the RSCS time
constant is substantially longer than the switching period to keep
low ripple voltage on the snubber circuit. A time constant of 10
times the switching period is used for calculation:
CS
≈
10
⋅
T----S----W----
RS
=
10
⋅
3----.--3---3----×---1---0---–---6-
10 ×103
=
3.33 n F
(EQ. 17)
CS = 3.33nF is used in the design.
Feedback Network
The feedback is being tapped off of the primary auxiliary
winding. This is one of the advantages of selecting the flyback
topology, since the auxiliary winding voltage follows the output.
This scheme was fully exploited, since the load fluctuation is
minimal, and that load regulation does not suffer much at these
power levels. For tighter regulation requirements, an
opto-coupled solution would need to be used, which leads to
additional cost.
Referring to the schematic on page 8, the output voltage can be
set by:
R-----2---2-
R23
=
V-----O----U----T-----+-----V----F--
Vref
–
1
=
1----5-----+-----0---.--6-
2.514
–
1=
5.2
(EQ. 18)
R23 = 1kΩ and R22 = 5.23kΩ are selected.
The control-to-output transfer function of the DCM flyback
converter is [1]:
Gvc = K ⋅
-R----E-----⋅---L----M-----⋅----F---S----W----
2
⋅
(---1-----+-1----s+----⋅-s--0---⋅.--5-E----⋅S---R--R---E---⋅--⋅-C--C----E----)
(EQ. 19)
Where:
RE = Equivalent load resistor reflected to the auxiliary output.
3
AN1612.1
November 28, 2011
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