1
dc1739afb
DEMO MANUAL DC1739A-B
DESCRIPTION
LTC3765/LTC3766
150W High Efficiency Active Clamp
Forward Converter
Demonstration circuit 1739A-B is a 150W isolated forward
converter with synchronous rectification featuring the
LTC3765/LTC3766.
This circuit demonstrates a high level of performance, ef-
ficiency, and small solution size attainable using these parts
in an active-clamp-reset forward converter power supply.
It operates at 240kHz and produces a regulated 12V, 12.5A
output from an input voltage range of 18V to 72V, making
it well suited for telecom, industrial, and other applica-
tions. It has an eighth-brick footprint size of 0.90"
×
2.3".
L
, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation and Direct Flux Limit is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
PERFORMANCE SUMMARY
OPERATING PRINCIPLES
Synchronous rectification helps to attain efficiency ex-
ceeding 94%. Secondary-side control eliminates complex
opto-coupler feedback, providing fast transient response
with minimum output capacitance. The proprietary Direct
Flux Limit
™
scheme guarantees no transformer saturation.
For other output requirements, see the LTC3766 data sheet
or contact the LTC factory.
Design files for this circuit board are available at
http://www.linear.com/demo
(T
A
= 25°C)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
IN
Input Supply Range
18
72
V
V
OUT
Output Voltage
12.0
V
I
OUT
Output Current Range, Continuous
200LFM
0
11
A
I
OUT
Output Current , Peak
200LFM
12.5
A
f
SW
Switching (Clock) Frequency
240
kHz
V
OUT P-P
Output Ripple
V
IN
= 48V, I
OUT
= 12A (20MHz BW)
60
mV
P–P
I
REG
Output Regulation
Line and Load (18-72V
IN
, 0-12A
OUT
)
±0.08
%
P
OUT
/P
IN
Efficiency (see Figure 3)
V
IN
= 48V, I
OUT
= 10A
93.6
%
Isolation
Basic
1500
VDC
Approximate Size
Component Area
×
Top Component Height
2.3
×
0.9
×
0.47
Inches
The LTC3765 active clamp forward controller and gate
driver is used on the primary and provides start-up,
gate drive, and protection functions. Once start-up is ac-
complished, the LTC3766 high efficiency, secondary-side
synchronous forward controller takes over, and provides
the LTC3765 with timing information and bias power
through a small pulse transformer.
When input voltage is applied, the LTC3765 commences
soft-start of the output voltage. When the secondary bias
source reaches the undervoltage threshold, the LTC3766
comes alive and takes control by sending encoded PWM
gate pulses to the LTC3765 through T3. These pulses also
provide primary bias power efficiently over a wide input
voltage range.
