QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
1
LT3825
DESCRIPTION
Demonstration circuit 894C-B is a 36V-72Vin,
synchronous flyback converter featuring the
LT3825. This circuit was designed specifically
to attain a high current, low ripple, synchro-
nously rectified flyback to efficiently power 5.0V
loads at up to 8A from a typical telecom input
voltage range. This circuit features synchro-
nous rectifier drive outputs, output voltage reg-
ulation without the need of an optocoupler,
self-starting architecture and input undervolt-
age lockout.
Design files for this circuit board are avail-
able. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Cor-
poration.
Table 1.
Performance Summary (TA = 25°C)
PARAMETER CONDITION VALUE
Minimum Input Voltage 36V
Maximum Input Voltage 72V
Output Voltage VOUT V
IN = 36V to 72V, IOUT = 0A to 8A 5.0V
Maximum Output Current 8A
Output Ripple VOUT V
IN = 48V, IOUT = 8A < 20mVP–P (typical)
Output Regulation Over All Input Voltages and Output Currents ±1.0% (typical)
Load Transient Response Peak Deviation with Load Step of 6A to 8A (10A/us) ±100mV (< ± 2%)
Settling T ime 100us
Nominal Switching Fr equency 200kHz
Efficiency VIN = 48V, IOUT = 8A 92% (typical)
OPERATING PRINCIPLES
The LT3825 controller exhibits a self-starting
capability. When an input voltage is applied, a
trickle charge resistor, R8, charges C10 (See
Figure 9) to power Vcc. Then, the IC begins a
controlled soft-start of the output voltage. As
this voltage begins to rise, Vcc power is quickly
taken over by T1, D2, and R7. When the soft-
start period is over, the LT3825 then regulates
output voltage by observing the pulses across
the auxiliary winding of T1 during the flyback
time. The Primary Gate drive (PG) and Syn-
chronous Gate (SG) drive is then Pulse Width
Modulated (PWM) in order to keep the output
voltage constant. The synchronous gate drive
signal is transmitted to the secondary via the
small signal transformer, T2. The output of T2
then drives a discreet gate drive buffer, R26,
Q12, and Q13 in order to achieve fast gate
transition times, hence a higher efficiency.
The two-stage input filter, C25, L1, and C30 and
output filter, C1, C2, C5, L2, and C29 are the
reasons that this flyback has exceptionally low
conducted emissions.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
2
QUICK START PROCEDURE
Demonstration circuit 894C-B is easy to set up
to evaluate the performance of the LT3825. Re-
fer to Figure 1 for proper measurement equip-
ment setup and follow the procedure below:
NOTE:
When measuring the input or output volt-
age ripple, care must be taken to avoid a long
ground lead on the oscilloscope probe. Meas-
ure the output (or input) voltage ripple by touch-
ing the probe tip and probe ground directly
across the +Vout and –Vout (or +Vin and –Vin)
terminals. See Figure 2 for proper scope probe
technique.
1.
Set an input power supply that is capable of
36V to 72V at a current of at least 2A to a
voltage of 36V. Then, turn off the supply.
2.
With power off, connect the supply to the in-
put terminals +Vin and –Vin.
a.
Input voltages lower than 36V can keep the
converter from turning on due to the un-
dervoltage lockout feature of the LT3825.
b.
If efficiency measurements are desired, an
ammeter capable of measuring 2Adc can
be put in series with the input supply in or-
der to measure the DC894C-B’s input cur-
rent.
c.
A voltmeter with a capability of measuring
at least 72V can be placed across the input
terminals in order to get an accurate input
voltage measurement.
3.
Turn on the power at the input.
NOTE:
Make sure that the input voltage never
exceeds 72V.
4.
Check for the proper output voltage of 5.0V
5.
Turn off the power at the input.
6.
Once the proper output voltages are estab-
lished, connect a variable load capable of
sinking 8A at 5.0V to the output terminals
+Vout and –Vout. Set the current for 0A.
a.
If efficiency measurements are desired, an
ammeter or a resistor current shunt that is
capable of handling at least 8Adc can be
put in series with the output load in order to
measure the DC894C-B’s output current.
b.
A voltmeter with a capability of measuring
at least 5.0V can be placed across the out-
put 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 dis-
connect the load to make sure that the load is
not set too high.
8.
Once the proper output voltage is estab-
lished, adjust the load within the operating
range and observe the output voltage regula-
tion, ripple voltage, efficiency and other de-
sired parameters.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
3
Figure 1. Proper Measurement Equipment Setup
VIN
GND
Figure 2. Measuring Input or Output Ripple
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
4
MEASURED DATA
Figures 3 through 10 are measured data for a typical DC894C-B. Figures 11 through 13 are schematics
and bill of materials.
76
78
80
82
84
86
88
90
92
12345678
36Vin
48Vin
72Vin
Efficiencyvs.LoadCurrent
Efficiency(%)
LoadCurrent(A)
Figure 3. Efficiency (no airflow)
Figure 4. Regulation (n o airflow)
O ut put Voltage vs . Loa d Curre nt
4.90
4.95
5.00
5.05
5.10
12345678910
Load Current (A)
Outpu t Voltag e (V )
36Vin
48Vin
72Vin
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
5
Figure 5. Output Voltage Ripple (48Vin 8A)
Figure 6. Load Transient Response (10A/us)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
6
Figure 7. Temp Data (48Vin, 8A, 100LFM airflow – front)
Figure 8. Temp Data (48Vin, 8A, 100LFM airflow – back)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
7
Figure 9. Temp Data (48Vin, 8A, no airflow – front)
Figure 10. Temp Data (48Vin, 8A, no airflow – back)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
8
R13
10K
-VIN
C23
2.2n
R22
2k
T1
D7
D8
BAT760
Q12
FMMT718 FMMT618
C4
100pF
R4
330
+
C10
47uF
20V
Q3
Q1
BSC320N20NS
C29
SANYO
C2
6.3V
47uF
0.018
R9
0.5W
R19
100k
0.1uF
C11
47K
R8
0.25W
L2
SG
C21
1uF
D5
BAT54
+VOUT
-VOUT
L1
4.7uH
Q13
+
C30
22uF
100V
C8
0.1uF
R11
150
0.25W
R2
4.7
0.25W
R27
0.125W
E2
E1
R16
169k
1% 0.1uF
C13
R26
15
C1
6.3V
47uF C5
6.3V
47uF
C25
2.2uF
100V
C19
220p
5.0V @ 8A
R15
3.01k
1%
R3
1%
C20
2.2nF
T2
PA0184
R7
20
0.25W
D2
BAS21LT1
29.4k
SG
PA1735NL
R20
12k
B0540W
BSC011N03LSI
C3
2.2nF
C3225X5R0J476M
R6
150
0.25W
C7
4.7nF
250V
FP2S-100-R
47
47pF
C17
15k
R14
1%
R18
1.47k
1%
6TPE470M
412K
R5
1%
U1
LT3825EFE
SENP 12
SFST
6
ROCMP
14
CCMP
13
OSCAP
7
VC 9
FB
8
PGDLY
15
UVLO
10
SGND/PGND
17
SYNC
5
ENDLY
4
TON
3
VCC 2
PG 16
SG 1
SENN 11
36V- 72V
+VIN
F i g u r e 1 1 . Simplified Schematic
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
9
R13
10K
-VIN
OPT
OPT
C23
2.2n
R22
2K
T1
5
6 9
1
11
3
10
8
7
12
2
4
D7
Q12
FMMT718 FMMT618
C4
100pF
R4
330
+
C10
47uF
20V
Q3
4
5
23
678
1
Q1
BSC320N20NS
4
5
2 3
6 7 8
1
C29
SANYO
C2
6.3V
47uF C5
6.3V
47uF
0.018
R9
0.5W
Q4
4
5
23
678
1
C1
6.3V
R19
100k
0.1uF
C11
47K
R8
0.25W
L2
SG
C21
1uF
D5
BAT54
R10
OPT
+VOUT
-VOUT
L1
4.7uH
Q13
R6
150
0.25W
+
C30
22uF
100V
C8
0.1uF
R11
150
0.25W
R2
4.7
0.25W
R27
0.125W
C6
OPT
R28
E3
E2
E1
E4
R16
169k
1% 0.1uF
C13
R26
15
0R1
C25
2.2uF
100V
C19
220p
5.0V @ 8A
D8
BAT760
R15
3.01k
1%
R3
1%
C20
2.2nF
T2
PA0184
8
1
5
4
R7
20
0.25W
D2
BAS21LT1
29.4k
SG
PA1735NL
R20
12k
B0540W
47 OPT
BSC011N03LSI
C3
2.2nF
C1, C 2, C5: C3225X5R0J476M
C7
4.7nF
250V
OPT
C12
FP2S-100-R
47pF
C17
10nF
C18
15k
R14
1%
47uF
R18
1.47k
1%
6TPE470M
412K
R5
1%
U1
LT3825EFE
SENP 12
SFST
6
ROCMP
14
CCMP
13
OSCAP
7
VC 9
FB
8
PGDLY
15
UVLO
10
SGND/PGND
17
SYNC
5
ENDLY
4
TON
3VCC 2
PG 16
SG 1
SENN 11
36V- 72V
+VIN
F i g u r e 1 2 . Full Board Schematic
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
10
Item Qty Ref-Des Des cription Manufacturer's Part Num ber
REQUIRED CIRCUIT COMPONENTS1
1 1 C3 CAP, 1206 2.2nF 20% 100V X7R AVX 12061C222MAT
2 1 C4 CAP, 1206 100pF 10% 630V COG TDK C3216COG2J101K
3 3 C5,C2,C1 CAP, 1210 47uF 20% 6.3V X5R TDK C3225X5R0J476MT
4 1 C7 CAP, 4.7nF 10% 250V X7R MURATA GA343DR7GD472KW01L
5 3 C8,C11,C13 CAP, 0603 0.1uF 10% 50V X7R TDK C1608X7R1H104K
6 1 C10 CAP, 6032 47uF 20% 20V TANT AVX TAJC476M020R
7 0 C12 CAP, 0603 470pF 10% 25V NPO AVX 06033A471KAT2A OPTION
8 1 C17 CAP, 0603 47pF 10% 25V NPO AVX 06033A470KAT2A
9 1 C19 CAP, 0603 220pF 5% 50V COG AVX 06035A221JAT
10 1 C20 CAP, 0603 2.2nF 5% 50V X7R AVX 06035C222JAT
11 1 C21 CAP, 1206 1uF 20% 25V X7R AVX 12063C105MAT2A
12 1 C23 CAP, 0603 2.2nF 5% 50V X7R AVX 06035C222JAT
13 1 C25 CAP, 1812 2.2uF 10% 10 0V X7R TDK C4532X7R2A225K
14 1 C29 CAP, 7343 470uF 20% 6.3V POSCAP SANYO 6TPE470M
15 1 D2 DIODE, BAS21-7 DIODES INC. BAS21-7-F
16 1 D5 DIODE, SCHOTTKY DIODES INC. BAT54-7
17 1 D7 DIODE, SCHOTTKY DIODES INC. B0540W-7-F
18 1 D8 DIODE, SCHOTTKY DIODES INC. BAT760-7
19 1 L2 IND, 0.1uH COILTRONICS FP2S-100-R
20 1 Q1 MOSFET, N-CH 2 00V 36A TDSON-8 INFINEON, BSC320N20NS3 G
21 1 Q3 MOSFET, N-CH 3 0V 37A TDSON-8 INFINEON, BSC011N03LSI
22 1 Q12 XSTR, PNP ZETEX FMMT718TA
23 1 Q13 XSTR, NPN ZETEX FMMT618TA
24 1 R2 RES, 1206 4.7 OHMS 5% 1/4W VISHAY, CRCW12064K70JNEA
25 1 R3 RES, 0603 29.4K OHMS 1% 1/10W VISHAY, CRCW060329K4FKEA
26 1 R4 RES, 0603 330 OHMS 5% 1/10W VISHAY, CRCW0603330RJNEA
27 1 R5 RES, 0603 412K OHMS 1% 1/10W VISHAY, CRCW0603412KFKEA
28 2 R6,R11 RES, 1206 150 OHMS 5% 1/4W VISHAY, CRCW1206150RJNEA
29 1 R7 RES, 1206 20 OHMS 5% 1/4W VISHAY, CRCW120620R0JNEA
30 1 R8 RES, 1206 47K OHMS 5% 1/4W VISHAY, CRCW120647K0JNEA
31 1 R9 RES, 1206 0.018 OHMS 1 % 0.5W SUSUMU, RL1632T-R018-G
32 1 R13 RES, 0603 10K OHMS 5% 1/10W VISHAY, CRCW060310K0JNEA
33 1 R14 RES, 0603 15K OHMS 1% 1/10W VISHAY, CRCW060315K0FKEA
34 1 R15 RES, 0603 3.01K OHMS 1% 1/10W VISHAY, CRCW06033K01FKEA
35 1 R16 RES, 0603 169K OHMS 1% 1/10W VISHAY, CRCW0603169KFKEA
36 1 R18 RES, 0603 1.47K OHMS 1% 1/10W VISHAY, CRCW06031K47FKEA
37 1 R19 RES, 0603 100K OHMS 5% 1/10W VISHAY, CRCW0603100KJNEA
38 1 R20 RES, 0603 12K OHMS 5% 1/10W VISHAY, CRCW060312K0JNEA
39 1 R22 RES, 0603 2K OHMS 5% 1/10W VISHAY, CRCW06032K00JNEA
40 1 R26 RES, 0603 15 OHMS 5% 1/10W VISHAY, CRCW060315 R 0JNEA
41 1 R27 RES, 0805 47 OHMS 5% 1/8W VISHAY, CRCW080547R0JNEA
42 1 T1 XFMR, PA1735NL PULSE PA1735NL
43 1 T2 XFMR, 1.4mH MIN, 50KHz PULSE PA0184NL
44 1 U1 IC, LT3825EFE LINEAR TECH LT3825EFE
ADDITIONAL DEMO BOARD CIRCUI T COMPONENTS2
45 0 C6 CAP, 1210 47uF 20% 6.3V X5R OPTION TDK C3225X5R0J476MT OPTION
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B
36V-72VIN, SYNCHRONOUS FLYBACK
11
46 0 C18 CAP, 0603 10nF 10% 25V X7R OPTION AVX 06033C103KAT OPTION
47 1 C30 CAP, 22uF 100V SUN ELECTRONIC 100ME22AX
48 1 L1 IND, 4.7uH VISHAY IHPL2525CZER4R7M01
49 0 Q4 MOSFET N- CHANNEL OPTION INFINEON, BSC011N03LSI OPTION
50 1 R1 RES, 0603 0 OHM JUMPER VISHAY, CRCW06030000Z0EA
51 0 R10 RES, 1206 OPTIONAL OPTION
52 0 R28 RES, 0805 OPTION OPTION
Notes:
1. Required Circuit Components are those parts that are required to implement the circuit function
2. Additional Demo Board Circuit Components are those parts that provide added functionality for the demo
board but are or may not be required in the actual circuit.
Figure 13. Bill of Materials
