Linear Technology DC776 Manuel utilisateur

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776

36V-72VIN, FORWARD CONVERTER

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

= 36V to 72V, I

OUT

= 0A to 45A, 400LFM 1.5V ± 1%

Maximum Output Current 400LFM

200LFM

45A

40A

Typical Output Ripple V

OUT

= 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

= 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

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.

Set an input power supply that is capable of 36V to

72V to a voltage of 36V. Then turn off the supply.

With power off, connect the supply to the input

terminals +Vin and –Vin.

Input voltages lower than 36V can keep the con-

verter from turning on due to the undervoltage

lockout feature of the LTC3705.

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.

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.

Turn on the power at the input.

NOTE:

Make sure that the input voltage never ex-

ceeds 72V.

Check for the proper output voltage of 1.5V ± 1%.

Turn off the power at the input.

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.

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.

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.

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.

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

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

MEASURED DATA

Figures 3 through 13 are measured data for a typical DC776A.

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

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

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

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

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

Figure 13.

Control Loop Bode Plot (48Vin and 20A)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776

36V-72VIN, FORWARD CONVERTER

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