QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-B
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
LTC3725 / LTC3726
DESCRIPTION
Demonstration circuit 1031A-B is a 36V-72Vin, syn-
chronous forward converter featuring the
LTC3725/LTC3726. This circuit was designed spe-
cifically to attain a high current, low ripple, synchro-
nously rectified forward converter to efficiently
power 3.3V loads at up to 20A from a typical telecom
input voltage range. This circuit features secondary-
side control of the supply eliminating the need for an
optocoupler, self-starting architecture, input under-
voltage lockout, and output overvoltage protection.
Design files for this circuit board are available.
Call the LTC factory.
,
LTC and LT are registered trademarks of Linear Technology Corporation.
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Maximum Output Current
Typical Output Ripple V
OUT
Size
Load Transient Response
Nominal Switching Frequency
Efficiency
V
IN
= 48V, I
OUT
= 20A
V
IN
= 36V to 72V, I
OUT
= 0A to 20A
200LFM Airflow
V
IN
= 72V, I
OUT
= 20A
Component Area x Top Component Height
Peak Deviation with Load Step of 10A to 20A (10A/us)
Settling Time
CONDITION
VALUE
36V
72V
3.3V
20A
100mV
P–P
2.3” x 0.9” x 0.394”
±200mV
40us
200kHz
91.5% Typical
OPERATING PRINCIPLES
The LTC3726 controller is used on the secondary and
the LTC3725 driver with self-starting capability is
used on the primary. When an input voltage is ap-
plied, the LTC3725 begins a controlled soft-start of
the output voltage. As this voltage begins to rise, the
LTC3726 secondary controller is quickly powered up
via T1, D25, and Q27. The LTC3726 then assumes
control of the output voltage by sending encoded
PWM gate pulses to the LTC3725 primary driver via
the small signal transformer, T2. The LTC3725 then
operates as a simple driver receiving both input sig-
nals and bias power through T2.
The transition from primary to secondary control oc-
curs seamlessly at a fraction of the output voltage.
From that point on, operation and design simplifies to
that of a simple buck converter. Secondary sensing
eliminates delays, tames large-signal overshoot and
reduces output capacitance while utilizing off-the-
shelf magnetics and attaining high efficiency.
For large values of input inductance, a 100V, 47uF elec-
trolytic capacitor can be added across the input termi-
nals to damp the input filter and provide adequate stabil-
ity. See Linear Technology Application Note AN19 for a
discussion on input filter stability analysis. A recom-
mended part is the Sanyo 100MV39AX.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-B
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
QUICK START PROCEDURE
Demonstration circuit 1031A-B 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:
measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the output (or
input) voltage ripple by touching the probe tip and
probe ground directly across the input or output ca-
pacitor. See Figure 2 for proper scope probe tech-
nique.
1.
NOTE:
When
3.
Turn on the power at the input.
NOTE:
Make
sure that the input voltage never ex-
ceeds 72V.
4.
5.
6.
Check for the proper output voltage of 3.3V
Turn off the power at the input.
Once the proper output voltages are established,
connect a variable load capable of sinking 20A at
3.3V to the output terminals +Vout and –Vout. Set
the current for 0A.
a.
Set an input power supply that is capable of 36V to
72V at a current of at least 2.5A to a voltage of 36V.
Then, turn off the supply.
With power off, connect the supply to the input
terminals +Vin and –Vin.
a.
2.
If efficiency measurements are desired, an am-
meter or a resistor current shunt that is capable
of handling at least 20Adc can be put in series
with the output load in order to measure the
DC1031A-B’s output current.
A voltmeter with a capability of measuring at
least 3.3V can be placed across the output ter-
minals in order to get an accurate output voltage
measurement.
b.
Input voltages lower than 36V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LTC3725/LTC3726.
If efficiency measurements are desired, an am-
meter capable of measuring 2.5Adc can be put
in series with the input supply in order to meas-
ure the DC1031A-B’s input current.
A voltmeter with a capability of measuring at
least 72V can be placed across the input termi-
nals in order to get an accurate input voltage
measurement.
7.
b.
Turn on the power at the input.
there is no output, temporarily disconnect
the load to make sure that the load is not set too
high.
NOTE:
If
c.
8.
Once the proper output voltage is established, ad-
just the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other desired parameters.
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-B
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-B
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
MEASURED DATA
Figures 3 through 11 are measured data for a typical DC1031A-B. Figures 12 through 21 are schematics, bill of mate-
rials and layout.
94
92
EFFICIENCY (%)
90
88
36V
48V
72V
86
6
8
10
12
14
16
18
20
LOAD CURRENT (A)
Figure 3. Efficiency (200lfm airflow)
50mV/div
2us/div
Figure 4. Output Ripple Voltage (72Vin, 20Aout)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-B
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
Vout
200mV/DIV
Iout
10A/div
20us/DIV
Figure 5. Output Voltage Transient Response (48Vin, 10A to 20A step)
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