DN1000 - Secondary Side Dual Output Controller Tightly Regulates Two Isolated Outputs

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Secondary Side Dual Output Controller Tightly Regulates Two
Isolated Outputs – Design Note 1000
Charlie Zhao and Wei Chen
Introduction
Many systems with dual output isolated power supplies
require tight regulation on both outputs. The LT®3804
makes it possible to satisfy this requirement in a space
saving and efficient circuit.
The LT3804 is a secondary side dual output controller. It
contains an error amplifier and an optocoupler driver to
regulate the first output. It also includes a complete PWM
controller and dual N-channel MOSFET drivers for the
second output. The power to the second output is transferred directly from the transformer secondary winding
without going through the first output stage, thus minimizing the size of the output inductor and capacitor of the
first output. Accurate output regulation for both outputs is
assured—even if the power supply’s outputs are far away
from the actual loads—due to true differential remote
sensing.
Dual Output Isolated Power Supply
Figure 1 shows a dual output high efficiency isolated
DC/DC power supply with 36V to 72V input range and two
outputs of 3.3V/15A and 1.8V/15A. The basic power stage
topology is a 2-switch forward converter with synchronous rectification. The primary side is controlled by the
LT3781, a current mode 2-switch forward controller with
built-in MOSFET drivers.
On the secondary side, the LT3804 provides voltage
feedback for the 3.3V output and regulation for the 1.8V
output. The 3.3V output is monitored by the LT3804 and
fed to the LT3781 on the primary side via an optocoupler
(MOC207) and the LT3804’s built-in optocoupler driver.
An LTC1693-1 high speed dual N-channel MOSFET driver
12/03/1000
provides the gate drive for the synchronous MOSFETs at
the 3.3V output stage.
The 1.8V output stage is a special synchronous buck
converter whose input is a pulsed waveform instead of a
DC voltage. The LT3804 precisely regulates the 1.8V
output by further reducing and controlling the duty cycle
of the switching waveform from the power transformer
(T1) secondary winding.
The circuit provides 1500V input to output isolation and a
switching frequency of 230kHz. The complete circuit fits
easily on a standard half brick size PC board. With a 48V
input, the overall efficiency is about 87% at full loads for
both outputs.
LT3804 Versatility
The LT3804 can be used to regulate the outputs of any
buck derived single-ended or dual-ended isolated topologies, including forward, push-pull, half-bridge and fullbridge converters. The device’s leading-edge modulation
allows it to operate with either current mode or voltage
mode primary controllers.
Conclusion
The LT3804 is a high efficiency secondary side switching
controller ideal for dual output isolated power supplies.
Unlike a traditional auxiliary supply from the secondary
circuits, an LT3804-based design can precisely regulate
both outputs. Its true differential remote sensing and
synchronous rectification features make it suitable for
high current applications where the output voltage can be
as low as 0.6V.
, LTC, and LT are registered trademarks of Linear Technology Corporation.
VI
P1
+VIN
Q1
Si7456DP
L1
1.2µH
DO1813P-122HC
T1
PULSE
PAO191
SEC
L3
CEP125-1R8
•
1
VO1
•
7
C3
1.5µF
100V
×2
36V TO
72V
D2
B2100
D1
B2100
5
2
4
L2 1mH
COILCRAFT
DO1608C-105
VIN
R7 10k
VCC
D20
B0540W
R8
10Ω
R26
73.2k
1%
R24
270k
0.25W
C20
0.1µF
25V
D13
BAS21TA
13
VCC
2
OVLD
1
SHDN
5VREF
D17
MMBD4148
P9
OFF ON
C25
4.7µF
16V
R43
10k
C7
1µF
C28
1000pF
19
VBST
R36
1k C19
0.1µF
TG
18
15
11
BSTREF BG SENSE PGND
SG
VFB
THERM
5 5VREF 6
SYNC SS
R38
1k C17
0.1µF
9
SGND VC
8
7
3
12
R37
10 680Ω
5VREF
4
C26 6.8nF
R45
1.24k
1%
C31
1µF
RT1
100k
C32
82pF
R59
52.3k
1%
R42
2.43k
1%
C29
0.01µF
C30
0.015µF
R46
330Ω
7
6
ISO1
MOC207
4 3 1
5
R33
6.8k
D10
MMBZ5240B
10V
C21
1µF
16V
D9
BAT54
LTC1693-1
8
VCC2
V
1 CC1
IN1
OUT1
3
IN2
OUT2
2
GND1 GND2
R31
470Ω
•
C16
0.01µF
C13
4.7µF
25V
R32
6.8k
D8
BAT54
6
7
5
4
OPTO
R35
1k
2
FROM TRANSFORMER
SECONDARY WINDING
VCCS
C45
10pF
VCCS
OPTO
R5
100k
P3
PGOOD
23
19
16
C37 680pF
7
1
2
R13 1.5k
C24
R19 10nF
1k
R65
10Ω
VO1
P4
VO1S+
3.3V OUTPUT
REMOTE SENSE
22
C50
1000pF 18
C51 15nF
P5
VO1S–
•
C15
4.7µF
16V
VCCS
•
C33
0.01µF
R16
2k
0.25W
R18
100Ω
C22
4700pF
8
SEC
•
R34
470Ω
T2
P2033
PULSE ENG
R29
1k
14
LT3781
FSET
T3
P2033
PULSE ENG
R30
1k
C1
0.01µF
20
VCCS
D6
B340A
Q12
FZT690B
VCC
C14
330pF
VOUT
C6
4.7µF
25V
D12
BAT54S
VIN
VCC
R6
3.3Ω
D4
BAT21TA
D7
BAT54
Q7
FZT853
D18
B0540W
C10
2200pF
250VAC
D3
BAT21TA
VCC
Q10
ZVN3310F
D19
5241B
11V
3
+
P6
VO1
3.3V
15A
P7
VO
RTN
•
R9
0.015Ω
1% 1/2W
P2
–VIN
Q4
Si7892DP
×2
•
Q3
Si7456DP
R25
20k
C7
470µF
4V
POSCAP
×3
Q6
Si7892DP
×2
13
10
28
R14
10k
1%
R15
10k
1%
R66
2.21k
1%
R67
2.21k
1%
R57
10k
C47
1µF
D14
25 16V 4 CMDSH-3
8
SYNC
VCC
OPTO
BOOST
GBAIS
LT3804
PGOOD
TGATE
BGS
SW
BGATE
CLIP
CL2P
CLIN
CL2N
VAOUT2
VFB2
GNDS1
GNDS2
PGIN1
PGIN2
17
R64
10Ω
26
Q14
Si7892DP
×2
14
12
11
3
SS2
9
C41
+
680µF
2.5V
POSCAP
×3
P10
+VO2
1.8V
15A
(2R5TPD680M ×3)
15
ILCOMP2
24
L4
CEP125-1R8
21
VFB1
PGND
Q13
Si7892DP
5
20
VAOUT1
SS1
D15
CMDSH-3
27
6
CSET
ILCOMP1
C36 C44
4.7µF 0.1µF
16V 16V
C39
330pF
R50
3.3k
C38
0.033µF
R53
220Ω
C42
4700pF
R54
3.01k
1%
R62
3.01k
1%
R60
1.5k
1%
R61
1.5k
1%
R63
10Ω
P6
VO2S+
1.8V OUTPUT
REMOTE SENSE
P11
VO2S–
C43
0.1µF
DN1000 F01
R68
10Ω
Figure 1. 36V-72V DC to 3.3V/15A and 1.8V/15A Dual Output Isolated Power Supply
Data Sheet Download
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