26
www
.fair
childsemi.com/fps
FPS
™
Design Guide
1
2
Drain
1
GND
2
Vcc
3
Vfb
4
L&S
5
7
4
1
10
6
5
8
3
1
2
NC
1
C
2
E
3
NC
4
GND
5
COMP
6
FB
7
LED
8
R22 (NS)
390R
0.6W
C2
100nF
250V
R1
30K
Q10
FDS7088N7 or
FDZ5047N
D1
SB380
T1
C11
330uF
16V
+
R21
1K
C3
1nF
25V
U1
FS6X1220RT
R15
K6
1W
R17
K6
1W
R2
120K
R10
10K 1%
C7
4.7nF
25V
R8
330R
C13 (NS)
330pF
C12
47uF
100V
+
C9
330uF
16V
+
D2
MMBD914
C4
22uF
50V
+
D3
ES2B
R5
22R
0.6W
C6
4.7nF
25V
C8
330uF
16V
+
J1
Vout 3.3V/4A
R14
K6
1W
R9
3k 31%
C8
100nF
250V
U5
FOD2741A
C5
47nF
50V
30V-57V
DC Input
3.3V/4A
Output
This solution is a interesting development of the 24W power supply concept. Here, synchronous rectification maximizes the
efficiency of the power supply by using a MOSFET instead of a diode. Instead of having a loss of around 0.5V to 1.0V
across a diode, the drop is now around 0.1V to 0.15V. This lead is to a 4% point increase in efficiency. An auxiliary winding
drives the MOSFET.
Also shown in Fairchild’s FOD2741A which combines the optocoupler and reference functionality into one chip. All relevant nodes
are accessible: LED drive, reference drive and the reference voltage itself.
•
Implementation of Power over Ether
net with low component count
– Cost effective solution
• High efficiency further improved by synchronous rectification
– Four percentage points more efficiency lowers overall system power load
•
FOD2741A combines optocoupler and r
eference in one chip
– Lower component count and manufacturing costs
Fairchild Devices Description
FS6X1220R
T Fairchild Power Switch (3.2A/300kHz)
FOD2741A
Optocoupler + Refer
ence
FDZ5047N BGA MOSFET (30V/2.9m
Ω)
SB380
Schottky Diode (3A/80V)
ES2B General Purpose Diode (2A/100V)
MMBD914
General Purpose Diode (200mA/100V)
Secondary Side Synchronous Rectification in a Power Over Ethernet Application
Typical Application – Power Over Ethernet, 48V DC Input Industrial Equipment
Examples of Typical Application Circuits
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