QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-C
36V-72VIN, SYNCHRONOUS FORWARD CONVERTER
1
LTC3725 / LTC3726
DESCRIPTION
Demonstration circuit 1031A-C 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 to efficiently power 5.0V
loads at up to 20A from a typical telecom input volt-
age range. This circuit features secondary-side con-
trol of the supply eliminating the need for an opto-
coupler, 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 (T
A
= 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 20A
5.0V
Maximum Output Current
200LFM Airflow
20A
Typical Output Ripple V
OUT
V
IN
= 72V, I
OUT
= 20A
100mV
P–P
Size
Component Area x Top Component Height
2.3” x 0.9” x 0.394”
Peak Deviation with Load Step of 10A to 20A (10A/us)
±200mV
Load Transient Response
Settling Time
40us
Nominal Switching Frequency
300kHz
Efficiency
V
IN
= 48V, I
OUT
= 20A
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, D1, 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.
