LT3710
U
W U U
APPLICATIO S I FOR ATIO
forward converter with synchronous rectification. The
primary side controller uses an LT3781, a current mode
2-transistor forward controller with built-in MOSFET driv-
ers. On the secondary side, an LTC1698 is used to provide
thevoltagefeedbackforthe3.3Voutput,aswellasthegate
drive for the synchronous MOSFETs. The error amplifier
output is fed into the optocoupler and then relayed to
LT3781 on the primary side to complete the 3.3V regula-
tion. The 1.8V output is generated by the LT3710 circuit.
AplanartransformerPA0191builtbyPulseEngineeringis
employed as the power transformer in this design. This
transformerisconstructedonaPQ20corewithanineturn
primary winding, two turn secondary winding and seven
turnauxiliarywindingfortheLT3781biassupply.Because
the maximum secondary voltage VSP is about 16V, 30V
MOSFETs are chosen with the consideration that the
secondary voltage overshoot is typically 20% to 30% of
VSP. In this particular design, Si7440DP is selected due to
its low RSD(ON), 30V VDSS rating and its compact and
thermally enhanced PowerPak SO-8 package.
The switching frequency of the circuit is about 230kHz.
1500V input to output isolation is provided. Additional
features of this design include primary side on/off control,
±5% secondary side trimming on the 3.3V output, input
overvoltage protection and undervoltage lockout. The
completedesignwillmountwithinastandardhalfbrickPC
board with about half inch height.
0.1µF
16V
4.7µF
16V
10pF
1
Si7440DP
CMDSH-3
10k
BOOST
5
7
16
2
1.8µH
SUMIDA
CEP125-IR8
SEC
SYNC
SS
GBIAS
TGATE
SW
10Ω
0.006Ω
1%
0.01µF
V
13
4
3
OUT2
1.8V/10A
V
CC
LT3710
V
15
9
CCS
680µF
+
+
680µF
4V
POSCAP
CSET
BGATE
BGS
CMDSH-3
Si7440DP
C37 680pF
4V
14
6
1µF
PGND
ILCOMP
PGND
POSCAP
B340A
11
12
+
CL
10k
17
–
CL
180pF
VA
OUT
V
FB
0.01µF
10
8
4700pF
0.033µF
3.01k
1%
3.3k
220Ω
2.32k
1%
330pF
3710 F03b
Figure 3b. 36V to 72V DC to 3.3V/10A and 1.8V/10A Dual Output Isolated Power Supply (Part 2 of 2, See Previous Page)
3710f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
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