DC1143A - Demo Manual

DEMO C IR C U IT 1 1 4 3
Q U IC K S TA R T GLT3825
U IDE
LT3 8 2 5
Is o 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
DESCRIPTION
Demonstration circuit 1143 is a 20 W att Isolated Flyback Converter w ith Synchronous R ectification and
Primary-Side R egulation featuring the LT3825.
cation helps to attain efficiency exceeding 88%. Isolation voltage is 1500VDC.
This circuit w as designed to demonstrate the high levels of performance, efficiency, and small solution size
attainable using this part in a flyback pow er supply. It
operates at 250kH z and produces a regulated 3.3V, 6A
output from an input voltage range of 36 to 72V:suitable for telecom and other applications. It has a footprint area that is 1.0 inch square. Synchronous rectifi-
Design files for this circuit board are available. Call
the LTC factory.
P ER F O R M A N C E S U M M A R Y
SYM BO L
VIN
VOUT
IOUT
FSW
VOUT P-P
IREG
POUT/PIN
PAR AM ETER
Input Supply R ange
O utput Voltage
O utput Current R ange
Sw itching (Clock) Frequency
O utput R ipple
O utput R egulation
Efficiency (see Figure 2)
, LTC, LTM , LT, Burst M ode, O PTI-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 , N o Latency Delta-Sigma, N o R SEN SE, O perational Filter, PanelProtect,
Pow erPath, Pow erSOT, SmartStart, SoftSpan, Stage Shedding, Sw itcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. O ther product names
may be trademarks of the companies that manufacture the products.
Specifications are at TA = 25°C
CO N DITIO N S
M IN
36
VIN = 36 –72V
0
TYP
M AX
72
3.3
VIN = 48V, IOUT = 6A (20M H z BW )
Line and Load (36-72V, 0-6A)
VIN =48V, IOUT = 6A
6
250
20
±1.2
88.5
U N ITS
V
V
A
kH z
mVP–P
%
%
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 O SFET through a pulse transformer to provide a
synchronous rectified output.
W hen an input voltage is applied, an undervoltage circuit 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 ‘burpmode’, cutting off w hen Vcc declines to 10V, maintaining 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.
R egulation 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 G ate
drive (PG ) and Synchronous G ate drive (SG ). The
1
LT3825
LT3825 is programmed to compensate for circuit resistance that is outside of the control loop.
O ptional LC filter stages on the input and output facilitate 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 evaluate the performance of the LT3825. R efer 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 converter from turning on due to the undervoltage
lockout feature of the LT3825.
b. If
c.
4. Check for the proper output voltage of 3.3V. Turn
off the pow er at the input.
5. O nce 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 ammeter or a resistor shunt that is capable of handling 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 terminals in order to get an accurate output voltage measurement.
efficiency measurements are desired, an ammeter 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 current.
6. Turn on the pow er at the input.
A voltmeter w ith a capability of measuring at
least 72V can be placed across the input terminals in order to get an accurate input voltage
measurement.
7. O nce the proper output voltage is again established, adjust the load w ithin the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parameters.
3. Turn on the pow er at the input.
2
NOTE. M ake sure that the input voltage never exceeds 72V.
NOTE. If there is no output, temporarily disconnect the load to make
sure that the load is not set too high.
LT3825
Figure 1. Proper M easurem ent Equipm ent Setup
3
LT3825
DC1143A Lt3825 3.3V-6A Efficiency
90
Efficiency in %
86
82
Vin=36V
78
Vin=48V
Vin=72V
74
70
66
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Output Curre nt in Am pe re s
Figure 2. Efficiency – N ote 88% efficiency over a w ide current range.
3.45
Output Voltage (V)
3.40
3.35
36Vin
3.30
48Vin
72Vin
3.25
3.20
3.15
0
0.6
1.2
1.8
2.4
3
3.6
4.2
4.8
5.4
Load Current (A)
Figure 3. Regulation – N ote regulation band of 1.5%
4
6
LT3825
Figure 4. Output Ripple at 48Vin and 6Aout (20M H z) - 2uS and 20m V /div
Figure 5. Transient R esponse W aveform at 48Vin and 5 - 6Aout – 200uS, 5A, and 200m V /div
5
LT3825
6
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