L
T
C3725 /
1
DESCRIPTION
Demonstration circuit 1300A-A is a 100W Isolated
Forward Converter with Synchronous Rectification
featuring the LTC3725 / LTC3726.
This circuit was designed to demonstrate the high le-
vels of performance, efficiency, and small solution
size attainable using this part in a Resonant-Reset
Forward Converter power supply. It operates at
200kHz and produces a regulated 5.0V, 20A output
from an input voltage range of 9 to 36V: suitable for
automotive, industrial, and other applications. It has a
quarter-brick footprint area. Synchronous rectification
helps to attain efficiency exceeding 90%. Secondary-
side control eliminates complex optocoupler feedback,
providing fast transient response with minimum out-
put capacitance. For other output requirements, see
DC1300A-B/C (12V@8.4A / 15V@6.7A) or DC1174A-
A/B/C (5V@10A /12V@4.2A / 15V@3.3A). For telecom
input requirements, see DC1031A-A/B/C
(2.5V/3.3V/5V@20A), or DC1032A-A (12V@12A), or
DC888A-A/B/C (3.3V@50A / 5V@35A /12V@20A).
Design files for this circuit board are available. Call
the LTC factory.
, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Swap, LinearView, μModule, Micropower SwitcherCAD, Multimode
Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect,
PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
may be trademarks of the companies that manufacture the products.
PERFORMANCE SUMMARY Specifications are at TA = 25°C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Input Supply Range 9* 36 V
VOUT Output Voltage 5.0 V
IOUT Output Current Range 200LFM 0 20 A
FSW Switching (Clock) Frequency 200 kHz
VOUT P-P Output Ripple VIN = 18V, IOUT = 20A (20MHz BW) 60 mVP
–
P
IREG Output Regulation Line and Load (9-36V, 0-20A) ±0.2 %
POUT/PIN Efficiency (see Figure 3) VIN =18V, IOUT = 18A 90 %
Isolation Basic 1500 Vdc
Approximate Size Component Area x Top Component Height 2.3 x 1.45 x 0.40 Inches
*Typical minimum startup is 9.3V
OPERATING PRINCIPLES
The LTC3725 Single-Switch Forward Controller is
used on the primary and provides start-up, gate drive,
and protection functions. Once start-up is accom-
plished, the LTC3726 Secondary-Side Synchronous
DEMO CIRCUI
T
1300A-A
QUICK START GUIDE
LTC3725 / LTC3726
100W Isolated Forward
Converter with Synchronous
Rectification
L
T
C3725 /
2
Forward Controller takes over, and provides the
LTC3725 with timing information and bias power
through a small pulse transformer.
When input voltage is applied, the LTC3725 com-
mences soft-start of the output voltage. When the
secondary bias source reaches the undervoltage thre-
shold, the LTC3726 comes alive and takes control by
sending encoded PWM gate pulses to the LTC3725
through T2. These pulses also provide primary bias
power efficiently over a wide input voltage range.
The transition from primary to secondary control oc-
curs at a fraction of the nominal output voltage. From
then on, operation and design is simplified to that of a
simple buck converter. Secondary control eliminates
delays, tames large-signal overshoot, and reduces
output capacitance needed to meet transient response
requirements.
An optional LC filter stage on the input lowers rms input
current. The filter must have output impedance that is
less than the converter input impedance to assure stabili-
ty. This may require a damping impedance. (See Linear
Technology Application Note AN19 for a discussion of
input filter stability.) A source with a 50mOhm or higher
ESR at the filter resonant frequency is one way of provid-
ing damping for the filter elements provided on the
DC1300A. For bench testing, adding an electrolytic capa-
citor such as a Sanyo 50ME470AX to the input terminals
will provide suitable damping and ripple current capabili-
ty. The values selected have a filter resonant frequency
that is below the converter switching frequency, thus
avoiding high circulating currents in the filter.
QUICK START PROCEDURE
Demonstration circuit 1300 is easy to set up to eva-
luate the performance of the LTC3725 / LTC3726. Re-
fer to Figure 1 for proper measurement equipment
setup and follow the procedure below:
NOTE.
When measuring the output voltage ripple, care must be taken to
avoid a long ground lead on the oscilloscope probe. Measure the output
voltage ripple by touching the probe tip and ground ring directly across
the last output capacitor as shown in Figure 12.
1.
Set an input power supply that is capable of 9V to
36V to 18V. Then turn off the supply.
2.
Direct an airflow of 200lfm across the unit for sus-
tained operation at full load.
3.
With power off, connect the supply to the input
terminals +Vin and –Vin.
a.
Input voltages lower than 9V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LTC3725 / LTC3726.
b.
If efficiency measurements are desired, an am-
meter capable of measuring 7Adc or a resistor
shunt can be put in series with the input supply
in order to measure the DC1300A’s input cur-
rent.
c.
A voltmeter with a capability of measuring at
least 36V can be placed across the input ter-
minals in order to get an accurate input voltage
measurement.
4.
Turn on the power at the input.
NOTE.
Make sure that the input voltage never exceeds 36V.
5.
Check for the proper output voltage of 5V. Turn off
the power at the input.
6.
Once the proper output voltages are established,
connect a variable load capable of sinking 20A at
5V to the output terminals +Vout and –Vout. Set
the current for 0A.
L
T
C3725 /
3
a.
If efficiency measurements are desired, an am-
meter or a resistor shunt that is capable of han-
dling 20Adc can be put in series with the output
load in order to measure the DC1300A’s output
current.
b.
A voltmeter with a capability of measuring at
least 5V can be placed across the output ter-
minals in order to get an accurate output vol-
tage measurement.
7.
Turn on the power at the input.
NOTE.
If there is no output, temporarily disconnect the load to make
sure that the load is not set too high.
8.
Once the proper output voltage is again estab-
lished, adjust the load within the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parameters.
Figure 1.
Proper Measurement Equipment Setup
LTC3725 / LTC3726
4
Figure 2.
Proper Noise Measurement Setup
Efficiency vs. Load Cur rent
70%
72%
74%
76%
78%
80%
82%
84%
86%
88%
90%
92%
2 4 6 8 10 12 14 16 18 20
Load Curre nt (A)
Efficien cy (%)
9VIN
18VIN
36VIN
Figure 3.
Efficiency
LTC3725 / LTC3726
5
Figure 4.
Output Ripple at 18Vin and 20Aout (25MHz) (20mV, 5us / div, 25MHz)
Figure 5.
Transient Response Waveform at 18Vin and 10 - 20Aout (10A, 1000mV, 100us / div)
LTC3725 / LTC3726
6
Figure 6.
Thermal Map, Frontside at 18Vin and 20Aout (Ta = 25 degrees C, 200 LFM)
Figure 7.
Thermal Map, Backside at 18Vin and 20Aout (Ta = 25 degrees C, 200 LFM)
Thermotekni
x
TVS-700 8:20:51 AM 2/19/2008 e : 0.95 Bg : 28.8°C
-22.0
153.0
40.0
90.0
Celsius
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
Thermotekni
x
TVS-700 9:00:28 AM 2/19/2008 e : 0.95 Bg : 29.6°C
-22.0
153.0
40.0
90.0
Celsius
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
LTC3725 / LTC3726
7
VCC
VCp
1210
R49
(Opt.)
T3
PA1005.100
87
13
4 56
2
T2
PA0297
8
1
6
3
4
5
Q37
MMBT2907A
1
23
R92 47K
VSW
C29
33nF
C69
2.2nF
200V
Q11
Si7852DP VA
VCp
C27 2.2nF
R76
470
1206
Q1
FMMT619
1
2 3
9V - 36V
D7
BAS21
3 1
+VOUT
R4
(Opt.)
R93
147
1%
Q36
MBT3946DW1T1
3
2
4
5
61
Supplied For U s e Wit h Linear Tec hnology Parts.
R83
(Opt.)
C78
4.7nF
R23
16.2
1/4W
VSB
R13
(Opt.)
CUT
C81
10pF
1210
LTC3725EMSE
U1
3
8
9
10
7
2
4
5
1
6
11
NDRV
VCC
VSLMT
IS
GATE
SSFLT
FB/IN+
FS/IN-
ULVO
PGND
GND
VCC
R2
31.6
C55
1nF R61
100
L1
0.47uH
C77
22uF
PA1494.242
L2
2.4uH
-VOUTS
Q39
Si7852DP
1206
VSB
C30
2.2nF
250V
-Vin
R95
0
L3
470uH
C67
4.7uF
25V
C6
100pF
C1
3.3nF
100V
R52
31.6
R1
31.6
R6 10K
T1
PA0901.005
7 2
4
11
5
3
10
9
8
1
6
-VOUTS
C10
1.0nF
1812
+Vin
R90
7.50K
R79
1.0K
D28
MMSZ5236BS
7.5V
21
SG
R91
10.0K
SG
Performance Or Reliability . C ontac t Linear Technology
C8
470pF
R7
47K
-VOUTP
D3
MMSZ5242BS
12V
2 1
R12
1.0
1812
Q38
Si7852DP
-VOUTP1
R11
1.0 E4
Applic at i on. C om ponent Subst itution And Print ed
D4
MMSZ5230BS
4.7V
2 1
Q3
Si7852DP
PT-
VIN
Demo Ci rcu i t 1300A-A
L T C3725E MSE, LTC 3726EGN 9V-36Vin Forward Converter
LINEAR TECHNOLOGY CORPORATION
1630 M cCARTHY BLVD.
MILPITAS, CA. 95035
408-432-1900
408-434-0507 FAX
13Friday, June 05, 2009
Title
Size Document Number Rev
Date: Sheet of
C70
2.2nF
SS
R85
(Opt.)
R22
28.7K
C71
1uF
Ci rc uit Boa rd Layout May Signif icantly Af f e c t C irc uit
R98
22
+Vout
VIN
E2
-VOUTP
R18
147K
1206
-VOUTS
D6
MMSZ4687T1
4.3V
2 1
+
C83
220uF
6.3V
C31
100uF
6.3V
Q28
FMMT38C 1
2 3
1206
Applic at i ons Engineer ing F or As si s t anc e.
Q27C
R58 5.1K
D29
1N4148WS
2 1
VSW
C89
0.22uF
R46
619
R97 2.2K
C5
6.8uF
50V
R84
0
R3
100
-VOUTP
Q32
FMMT718
1
23
R54
100
1%
E1
R50
0.002
1W
PT-
Q2
FMMT718
1
23
D1
BAS21
3 1
D2
1N4148WS
21
Q27
FCX491A
1
23
R69
0
Q27C
C4
6.8uF
50V
Q12
Si7852DP
VA
+
C80
220uF
6.3V
D25
BAS21
31
Q34
2N7002
1
2 3
R66
100K
R94 147
1%
C82
75pF
C20
1.5nF
200V
R9
100
Q14
HAT2169H
R63
90.9K
Ci rc ui t That Meets Cu s tomer -Suppl ied S pecif i c ati ons ;
R51
31.6
E3
1210
Rb
162K
R24
16.2
1/4W
U2
LTC3726EGN
5
2
12
4
8
11
10
1
16
15
14
7
9
6
3
13
ITH
FG
SW
FB/PH
GND
IS+
IS-
SG
VCC
PT+
PT-
SLP
FS/SYNC
RUN/SS
MODE
PGND
C66
5.6nF
100V
Q8
Si7852DP
Q26
FMMT619
1
2 3
C9
0.22uF
C7
100pF
Q15
(Opt.)
C79
2.2nF
5V/20A
R89
604
-Vout
VCC
R41
4.53K
C76
(Opt.)
Q6
MMBFJ201
3
2 1
C2
6.8uF
50V
C73
470pF
R56
100
R68
6.81K
Ra 10K
Linear Tec hnology Has Made A Bes t Ef f o rt To Des ig n A
Verify Proper And Reliable Operation In The Actual
-VOUTS
Cu stomer Notice
C3
6.8uF
50V
R87
0
-VOUTP -VOUTS
C33
100uF
6.3V
VIN
R86
261K
C88
47pF
R55
100
1%
However, It Remains The Customer's Responsibility To
C24
4.7uF
This C irc ui t I s Propr iet ary To Linear Tec hnology An d
+VOUT
-VOUTS
C72
0.1uF
SS
C75
68pF
LTC CONFIDENTIAL - FOR CUSTOMER USE ONLY
