LT38
25
1
DESCRIPTION
Demonstration circuit 1143 is a 20 W att Isolated Fly-
back Converter w ith Synchronous Rectification and
Primary-Side Regulation featuring the LT3825.
This circuit w as designed to demonstrate the high lev-
els of performance, efficiency, and small solution size
attainable using this part in a flyback pow er supply. It
operates at 250kHz and produces a regulated 3.3V, 6A
output from an input voltage range of 36 to 72V: suit-
able for telecom and other applications. It has a foot-
print area that is 1.0 inch square. Synchronous rectifi-
cation helps to attain efficiency exceeding 88%. Isola-
tion voltage is 1500VDC.
Design files for this circuit board are available. Call
the LTC factory.
, LTC, LTM , LT, Burst M ode, OPTI-LOO P, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Pow er, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Sw ap, LinearView , µM odule, M icropow er Sw itcherCAD, M ultimode
Dimming, No Latency , No Latency Delta-Sigma, No R
SENSE
, Operational Filter, PanelProtect,
Pow erPath, Pow erSOT, SmartStart, SoftSpan, Stage Shedding, Sw itcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
may be trademarks of the companies that manufacture the products.
P ER F O R M A N C E S U M M A R Y
Specifications are at TA = 25°C
SYM BO L PARAM ETER CONDITIONS M IN TYP M AX UNITS
V
IN
Input Supply Range
36 72 V
V
OUT
Output Voltage 3.3 V
I
OUT
Output Current Range V
IN
= 36 –72V
0 6 A
F
SW
Sw itching (Clock) Frequency
250 kHz
V
OUT P-P
Output Ripple V
IN
= 48V, I
OUT
= 6A (20M Hz BW ) 20 mV
P–P
I
REG
Output Regulation Line and Load (36-72V, 0-6A)
±1.2 %
P
OUT
/P
IN
Efficiency (see Figure 2) V
IN
=48V, I
OUT
= 6A
88.5 %
O P ER A T IN G P R IN C IP L ES
The LT3825 Synchronous Flyback PW M Controller is
used on the primary and drives a secondary-side
M OSFET through a pulse transformer to provide a
synchronous rectified output.
W hen an input voltage is applied, an undervoltage cir-
cuit keeps the LT3825 in its quiescent state w hile a
resistor charges Cvcc (C10) to 15V. The controller is
then enabled, and start-up commences. The primary
circuit operates from the charge stored in Cvcc until
the housekeeping w inding of T1 starts to support Vcc.
W hen a heavy overload or short-circuit prevents T1
supporting Vcc, the converter operates in ‘burp-
mode’, cutting off w hen Vcc declines to 10V, main-
taining low pow er dissipation in the circuit. The
LT3825 provides a synchronous rectifier gate drive
signal w hich is passed to the secondary through T2
and subsequently buffered.
Regulation is attained by observing the voltage on the
housekeeping w inding of T1 during the Flyback time,
and Pulse W idth M odulating (PW M ) the Primary Gate
drive (PG) and Synchronous G ate drive (SG). The
DEMO
C IR C U IT
Q U IC K S TA R T G U IDE
L T 3 8 2 5
Iso la te d F ly b a c k C o n v e rte r w ith
S y n c h ro n o u s R e c tific a tio n
LT38
25
2
LT3825 is programmed to compensate for circuit re-
sistance that is outside of the control loop.
Optional LC filter stages on the input and output facili-
tate low noise.
Q U IC K S T A R T P R O C ED U R E
Demonstration circuit 1143 is easy to set up to evalu-
ate the performance of the LT3825. Refer to Figure 1
for proper measurement equipment setup and follow
the procedure below :
NOTE.
W hen measuring the output voltage ripple, care must be taken to
avoid a long ground lead on the oscilloscope probe. M easure the output
voltage ripple by touching the probe tip and ground ring directly across
the last output capacitor (C29) as show n in Figure 1.
1.
Set an input pow er supply that is capable of 36V to
72V to 48V. Then turn off the supply.
2.
W ith pow er off, connect the supply to the input
terminals +Vin and –Vin.
a.
Input voltages low er than 36V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LT3825.
b.
If efficiency measurements are desired, an am-
meter capable of measuring 1Adc or a resistor
shunt can be put in series w ith the input supply
in order to measure the DC1143A’s input cur-
rent.
c.
A voltmeter w ith a capability of measuring at
least 72V can be placed across the input termi-
nals in order to get an accurate input voltage
measurement.
3.
Turn on the pow er at the input.
NOTE.
M ake sure that the input voltage never exceeds 72V.
4.
Check for the proper output voltage of 3.3V. Turn
off the pow er at the input.
5.
Once the proper output voltages are established,
connect a variable load capable of sinking 6A at
3.3V 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 han-
dling 6Adc can be put in series w ith the output
load in order to measure the DC1143A’s output
current.
b.
A voltmeter w ith a capability of measuring at
least 3.3V can be placed across the output ter-
minals in order to get an accurate output volt-
age measurement.
6.
Turn on the pow er at the input.
NOTE.
If there is no output, temporarily disconnect the load to make
sure that the load is not set too high.
7.
Once the proper output voltage is again estab-
lished, adjust the load w ithin the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parame
ters.
LT38
25
3
Figure 1.
Proper M easurem ent Equipm ent Setup
LT38
25
4
DC1143A Lt3825 3.3V-6A Efficiency
66
70
74
78
82
86
90
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Output Curre nt in Am pe re s
Efficiency in %
V in= 36V
V in= 48V
V in= 72V
Figure 2.
Efficiency – Note 88% efficiency over a w ide current range.
3.15
3.20
3.25
3.30
3.35
3.40
3.45
0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6
Load Current (A)
Output Voltage (V)
36Vin
48Vin
72Vin
Figure 3.
Regulation – Note regulation band of 1.5%
LT38
25
5
Figure 4.
Output Ripple at 48Vin and 6Aout (20M H z) - 2uS and 20m V / div
Figure 5.
Transient Response W aveform at 48Vin and 5 - 6Aout – 200uS, 5A, and 200m V / div
LT38
25
6