Linear Technology DC776 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
1
LTC3705 and LTC3706
DESCRIPTION
Demonstration circuit 776 is a 36V-72Vin, forward
converter featuring the LTC3705 and LTC3706. This
circuit was designed specifically to attain a high step-
down ratio power supply in one stage in order to ef-
ficiently power 1.5V loads from a typical telecom in-
put voltage range. Isolation voltage is 1500VDC.
This circuit features secondary-side control of the
supply without the need of an optocoupler, self-
starting architecture, input undervoltage 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 (TA= 25°C)
PARAMETER CONDITION VALUE
Minimum Input Voltage 36V
Maximum Input Voltage 72V
Output Voltage V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 45A, 400LFM 1.5V ± 1%
Maximum Output Current 400LFM
200LFM
45A
40A
Typical Output Ripple V
OUT
V
IN
= 48V, I
OUT
= 30A (20MHz BW) 30mV
P–P
Line ±0.01%
Output Regulation
Load ±0.18%
Dynamic Response
Peak Deviation
Load Step of 20A to 40A (1A/us min)
Settling Time (to within 15mV of set point)
65mV
50us
Nominal Switching Frequency 215kHz
Efficiency V
IN
= 48V, I
OUT
= 45A, 400LFM 84.75% Typical
Logic Low Voltage-Off 1V MAX
On/Off Control
Logic High Voltage-On 2.5V MIN
Isolation Voltage Basic Insulation 1500VDC
OPERATING PRINCIPLES
The LTC3706 controller is used on the secondary and
the LTC3705 driver with self-starting capability is
used on the primary. When an input voltage is ap-
plied, the LTC3705 begins a controlled soft-start of
the output voltage. As this voltage begins to rise, the
LTC3706 secondary controller is quickly powered up
via T1, D3, and Q13. The LTC3706 then assumes
control of the output voltage by sending encoded

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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PWM gate pulses to the LTC3705 primary driver via
the small signal transformer, T3. The LTC3705 then
operates as a simple driver receiving both input sig-
nals and bias power through T3.
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. This, 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.
QUICK START PROCEDURE
Demonstration circuit 776 is easy to set up to evalu-
ate the performance of the LTC3705 and LTC3706.
Refer to Figure 1 for proper measurement equipment
setup and follow the procedure below:
NOTE: When 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 +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 to a voltage of 36V. Then turn off the supply.
2. With power off, connect the supply to the input
terminals +Vin and –Vin.
a. Input voltages lower than 36V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LTC3705.
b. If efficiency measurements are desired, an am-
meter capable of measuring 3Adc can be put in
series with the input supply in order to measure
the DC776A’s input current.
c. 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.
3. Turn on the power at the input.
NOTE: Make sure that the input voltage never ex-
ceeds 72V.
4. Check for the proper output voltage of 1.5V ± 1%.
Turn off the power at the input.
5. Once the proper output voltages are established,
connect a variable load capable of sinking 50A at
1.5V 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 50Adc can be put in series with the output
load in order to measure the DC776A’s output
current.
b. A voltmeter with a capability of measuring at
least 2V can be placed across the output termi-
nals in order to get an accurate output voltage
measurement.
6. 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.
7. Once the proper output voltage is again estab-
lished, adjust the load within the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parameters.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 1. Proper Measurement Equipment Setup
VIN GND
Figure 2. Measuring Input or Output Ripple

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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MEASURED DATA
Figures 3 through 13 are measured data for a typical DC776A.
70
72
74
76
78
80
82
84
86
88
90
5 10 15 20 25 30 35 40 45 50
Current (A)
Efficiency (%)
36VIN
48VIN
72VIN
Figure 3. Efficiency (400LFM)
Figure 4. Regulation (400LFM)
1.425
1.445
1.465
1.485
1.505
1.525
1.545
1.565
5 10 15 20 25 30 35 40 45 50
Current (A)
Regulation (V)
36VIN
48VIN
72VIN

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 5. Output Voltage Ripple (72Vin and 50A)
Figure 6. Load Transient Response (>1A/us)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 7. Temp Data (48Vin, 45A, 400LFM – front)
Figure 8. Temp Data (48Vin, 45A, 400LFM – back)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 9. Temp Data (48Vin, 40A, 200LFM – front)
Figure 10. Temp Data (48Vin, 40A, 200LFM – back)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 11. Turn-on Waveforms (72Vin and 50A)
Figure 12. Turn-off Waveforms (72Vin and 50A)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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Figure 13. Control Loop Bode Plot (48Vin and 20A)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
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DC776A