DC1174A-C - Demo Manual

DEMO CIRCUIT 1174A-C
LTC3725
/ LTC3726
QUICK
START
GUIDE
LTC3725 / LTC3726
50W Isolated Forward Converter
with Synchronous Rectification
DESCRIPTION
Demonstration circuit 1174A-C is a 50W Isolated
Forward Converter with Synchronous Rectification
featuring the LTC3725 / LTC3726.
tion helps to attain efficiency exceeding 90%. Isolation
is 1500VDC basic.
This circuit was designed to demonstrate the high levels of performance, efficiency, and small solution
size attainable using this part in a Resonant-Reset
Forward Converter power supply. It operates at
250kHz and produces a regulated 15V, 3.3A output
from an input voltage range of 9 to 36V: suitable for
automotive, industrial, and other applications. It has
an eighth-brick footprint area. Synchronous rectifica-
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
VIN
VOUT
IOUT
FSW
VOUT P-P
IREG
POUT/PIN
PARAMETER
Input Supply Range
Output Voltage
Output Current Range
Switching (Clock) Frequency
Output Ripple
Output Regulation
Efficiency (see Figure 3)
Isolation
Size
CONDITIONS
MIN
9*
VIN = 9 –36V
VIN = 18V, IOUT = 4.2A (20MHz BW)
Line and Load (9-36V, 0-3.3A)
VIN =18V, IOUT = 3A
Basic
Component Area x Top Component Height
TYP
MAX
36
UNITS
V
15.0
V
0
3.3
A
250
kHz
100
mVP–P
±0.04
%
89
%
1500
Vdc
2.3 x 0.9 x 0.35
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 accomplished, the LTC3726 Secondary-Side Synchronous
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 commences soft-start of the output voltage. When the
secondary bias source reaches the undervoltage threshold, 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.
1
LTC3725 / LTC3726
The transition from primary to secondary control occurs 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 stability. 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 providing damping for the filter elements provided on the
DC1174A. For bench testing, adding an electrolytic capacitor such as a Sanyo 50ME470AX to the input terminals
will provide suitable damping and ripple current capability. 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 1174 is easy to set up to evaluate the performance of the LTC3725 / LTC3726. Refer 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 sustained 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 DC1174A’s input current.
c. A voltmeter with a capability of measuring at
least 36V can be placed across the input terminals in order to get an accurate input voltage
measurement.
NOTE. Make sure that the input voltage never exceeds 36V.
5. Check for the proper output voltage of 15V. Turn
off the power at the input.
6. Once the proper output voltages are established,
connect a variable load capable of sinking 3.3A at
15V to the output terminals +Vout and –Vout. Set
the current for 0A.
a. If efficiency measurements are desired, an am-
meter or a resistor shunt that is capable of handling 3.3Adc can be put in series with the output load in order to measure the DC1174A’s
output current.
b. A voltmeter with a capability of measuring at
least 15V can be placed across the output terminals in order to get an accurate output voltage 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 established, adjust the load within the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parameters.
4. Turn on the power at the input.
2
LTC3725 / LTC3726
Figure 1. Proper Measurement Equipment Setup
Figure 2. Proper Noise Measurement Setup
3
Efficiency (%)
LTC3725 / LTC3726
92
90
88
86
84
82
80
78
76
74
72
70
68
66
64
62
60
9V IN
18V IN
36V IN
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Curre nt in Amps
Figure 3. Efficiency
Figure 4. Output Ripple at 18Vin and 4.2Aout (50mV, 5us / div, 25MHz)
4
LTC3725 / LTC3726
Figure 5. Transient Response Waveform at 18Vin and 2.1 - 4.2Aout (1A, 100mV, 200us / div)
5
LTC3725 / LTC3726
Figure 6. Thermal Map, Frontside at 18Vin and 3.3Aout (Ta = 25 degrees C)
Figure 7. Thermal Map, Backside at 18Vin and 3.3Aout (Ta = 25 degrees C)
6
D4
4.7V
MMSZ5230BS
E2
D3
22V
MMSZ5251B
28.7K
R22
C55
470pF
-VOUT
D2
1N4148WS
1
C3
3.3uF
50V
1210
2
Q28
FMMT38C
R18
147K
C2
3.3uF
50V
1210
3
R89
511
C82
75pF
2
C27
2.2nF
R3
100
R49
0
1
R93
147
1%
VIN
5
4
VCC
FS/IN-
FB/IN+
R13
0
1206
VSB
Q27C
R63
90.9K
R61
100
C73
470pF
C72
0.1uF
R58
5.1K
R48
.010
1W
C66
2.2nF
100V
7
8
9
10
11
D6
4.3V
MMSZ4687T1
C9
0.22uF
C7
(Opt.)
C6
(Opt.)
Q11
(Opt.)
Q8
Si7852DP
U1
LTC3725EMSE
Q37
MMBT2907A
R94
147
1%
1
7
R87
0
R86
261K
SSFLT
ULVO
VCC
R88
47pF
2
1
8
10K
R6
10K
VIN
D29
PT1N4148WS
SG
C29
33nF
1
Ra
Rb
162K
1
CUT
C81
10pF
C5
3.3uF
50V
1210
Q6
MMBFJ201
3
C4
3.3uF
50V
1210
Q34
2N7002
VSW
C24
4.7uF
2
1
2
-Vin
1
2
10
IS
9V-36V
1
2
5
6
4
3
C8
470pF
R91
10.0K
R90
7.50K
-VOUT
C30
2.2nF
250V
-VOUT
R7
47K
8
1
R2
196
1/4W
R1
196
1/4W
R51
196
1/4W
1/4W
R52
196
T2
PA0297
2
3
4
1
5
6
R92
47K
5
14
15
PT-
PT+
R84
0
SS
VA
VSW
-VOUT
2
C70
1nF
C78
4.7nF
SS
3
1
Q12
Si7450DP
3
D1
(Opt.)
Q32
FMMT718
3
2
R66
100K
R55
100
1%
R85
(Opt.)
R54
100
1%
R50
0.012
1W
1
D25
(Opt.)
C75
47pF
R69
113K
R83
(Opt.)
R56
100
FB/PH
4
Q27
FCX491
1
R76
680
1206
C76
(Opt.)
C77
10uF
E3
R46
619
E4
47uF
20V
+ C80
+VOUT
R41
14.7K
VCC
-VOUT
C31
(Opt.)
1210
+VOUT
R9
(Opt.)
C10
(Opt.)
-Vout
15V/3.3A
+Vout
LINEAR TECHNOLOGY CORPORATION
C79
470pF
R68
27.4K
SG
VSB
Q27C
C33
22uF
16V
D28
MMSZ5236BS
7.5V
R4
10
1/4W
1630 McCARTHY BLVD.
MILPITAS, CA. 95035
408-432-1900
Linear Technology Has Made A Best Ef f ort To Design A
408-434-0507 FAX
Circuit That Meets Customer-Supplied Specif ications;
Howev er, It Remains The Customer's Responsibility To
Verif y Proper And Reliable Operation In The Actual
Title
Application. Component Substitution And Printed
LTC3725EMSE, LTC3726EGN 9V-36Vin Forward Converter
Circuit Board Lay out May Signif icantly Af f ect Circuit
Document Number
Rev
Perf ormance Or Reliability . Contact Linear Technology Size
Applications Engineering For Assistance.
Demo Circuit 1174A-C
A
This Circuit Is Proprietary To Linear Technology And
Wednesday, August 12, 2009
1
1
Date:
Sheet
Supplied For Use With Linear Technology Parts.
of
Customer Notice
C67
4.7uF
25V
VCC
Q26
FMMT619
2
3
L2
12uH
VA
LTC CONFIDENTIAL - FOR CUSTOMER USE ONLY
Q36
MBT3946DW1T1
-VOUT
U2
LTC3726EGN
PT-
C71
1uF
C69
100pF
200V
C1
390pF
200V
Q14
Si7456DP
R79
3.3K
10
E1
3
2
2
FG
12
SW
3
NDRV
VSLMT
9
GND
8
GATE
PGND
6
GND
11
PGND
13
ISRUN/SS
6
11
IS+
L1
0.68uH
4
3
SLP
7
1
FS/SYNC
9
1
SG
T1
PA0810
1
+Vin
2
3
1
2
6
1
ITH
5
2
3
16
VCC
3
MODE
2
1
L3
(Opt.)
LTC3725 / LTC3726
7
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