DN299 - Multiple Output Isolated Power Supply Achieves High Efficiency with Secondary Side Synchronous Post Regulator

Multiple Output Isolated Power Supply Achieves High Efficiency with Secondary Side Synchronous Post Regulator
Design Note 299
Charlie Zhao and Wei Chen
Introduction
The newly released LT®3710, a secondary side synchronous post regulator controller, provides tight regulation
for all outputs of multiple output isolated power supplies. Solutions that use the LT3710 are efficient, use
minimal space and provide fast transient response.
controller uses the LT3781; a current mode 2-switch
forward controller with built-in MOSFET drivers. On
the secondary side a synchronous rectifier controller, the LTC1698, provides the voltage feedback for
the main 3.3V output as well as the gate drive for the
synchronous MOSFETs. The LT3710 circuit precisely
regulates the 2.5V output. Total load and line regulation for the 2.5V output is better than 0.2%. From a
48V input, the overall efficiency is about 87% with full
load on both outputs.
The LT3710 is a voltage mode controller with leading
edge modulation, programmable current limit and
dual MOSFET drivers. It generates a tightly regulated
second output directly from the transformer secondary
winding, thus minimizing the size of the output inductor
and capacitor of the main output stage. The use of synchronous MOSFETs significantly improves efficiency,
making it suitable for low output voltage applications.
Conclusion
The LT3710 is a high efficiency secondary side synchronous post regulator controller that is designed
to generate a tightly regulated secondary output for
multiple output isolated power supplies. It can be used
with any buck derived single-ended or dual-ended
isolated topologies, such as forward, push-pull, halfbridge and full-bridge converters.
Design Example
Figures 1a and 1b show a dual output high efficiency
isolated DC/DC power supply with 36V to 72V input
range and two outputs of 3.3V/10A and 2.5V/10A. The
basic power stage topology is a 2-switch forward converter with synchronous rectification. The primary side
FROM
TRANSFORMER
SECONDARY
WINDING
SEE FIGURE 1b
C45
10pF
R58
10k
5
7
C34 0.01μF
VCCS
C47
1μF
13
4
C37 680pF
14
6
C35
180pF
R57
10k
1
SYNC
SS
GBIAS
TGATE
VCC
CSET
C44
0.1μF
16V
C36
4.7μF
16V
BOOST
LT3710
SW
BGATE
PGND
BGS
ILCOMP
CL+
VFB
10
C38
0.033μF R50
3.3k
C39
330pF
D14
CMDSH-3
16
2
Q13
Si7892DP
R56
10Ω
3
15
D15
Q14
CMDSH-3 Si7892DP
9
11
12
CL–
VAOUT
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
L4
1.8μH
R49
SUMIDA 0.006Ω
CEP125-IR8 1%
+
B340A
C43
0.01μF
C40
+
680μF
4V
POSCAP
C42
4700pF
8
R51
3.01k
1%
R53
220Ω
R52
1.4k
1%
DN299 F01a
Figure 1a. 36V to 72V DC to 3.3V/10A and 2.5V/10A Dual Output Isolated Power Supply
(Part 1 of 2: Post Regulator Circuit)
11/02/299_conv
VOUT2
2.5V/10A
C41
680μF
4V
POSCAP
Data Sheet Download
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Linear Technology Corporation
FAX: (408) 434-0507 ● www.linear.com
OFF ON
FZT
853
4.7μF
0.1μF
10k
1N4148
270k
1.24k
73.2k
1
2
13
20
1nF
1μF
5VREF
FSET
0.1F
5
6
52.3k
1%
SHDN 5VREF
OVLO
VCC VBST
BAS21
DO1608C-105
1.5μF
100V
1.5μF
100V
19
18
15
11
82pF
3
7
0.1μF
4
8
1k
3.3nF
3k
10Ω
10
9
PGND 12
SG
14
BAS21
THERM SYNC SGND SS VC VFB
LT3781
TG BSTREF BG SENSE
10k
BAS21
BAT54
ZVN3310F
BAT54
300pF
B2100
5
7
6
5VREF
8
MOC207
4 3
2
1
t8
7
T2
PULSE
P2033
4
220pF
1t
BAT54
0.020Ω
1/2W
Q2
Si7456DP
4
2
3
1μF
1
VCCS
12
4.7μF
FZT690B
11
2.2nF
250VAC
Si7892DP
×2
5
16 2
6
VOUT1
0.01μF
14
VAUX
0.1μF
3
4
10
PGND GND PWRGD ICOMP
MARGIN
1μF
13
7
+
B0540W
Si7892DP
470μF
4V
POSCAP
2.5μH
SUMIDA CEP125-2R5
+
1.24k
1%
2.43k
1%
1.78k
1%
3.01k
1%
UNLESS OTHERWISE SPECIFIED:
ALL CAPS 25V
ALL RESISTORS 0.1W, 5%
0.22μF
B0540W
22nF
470Ω
1nF
CMPZ5240B
10V
10Ω
1nF
100V
2k
1nF
100V
10Ω
TO POST REGULATOR CIRCUIT
SEE FIGURE 1a
VDD ISNS ISNSGND FG CG VCOMP
15
8
SYNC
VFB
1k
5
9
OPTODRV
OVPIN
LTC1698
4.7nF
470Ω
B2100
T1
PULSE
Q1
Si7456DP PA0191
1
7
t
Figure 1b. 36V to 72V DC to 3.3V/10A and 2.5V/10A Dual Output Isolated Power Supply (Part 2 of 2: Main Converter)
11V
MMSZ5241B
20k
B0540W
VIN–
36V–72V
VIN+
1.2μH
COILCRAFT
D01813P-122HC
t
●
t
(408) 432-1900
For applications help,
call (408) 432-1900
dn299f_conv LT/TP 1102 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
© LINEAR TECHNOLOGY CORPORATION 2002
470μF
4V
POSCAP
DN299 F01b
VOUT1
TRIM
VOUT
RTN
VOUT1+
3.3V
AT 10A
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