QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 888A-B
36V-72VIN, ISOLATED SYNCHRONOUS FORWARD
1
LTC3725/LTC3706
DESCRIPTION
Demonstration circuit 888A-B is a high power iso-
lated synchronous forward converter featuring the
LTC3725 and LTC3706. When powered from a 36-
72V input, a single DC888A-B provides an isolated
5V at 40A in a quarter-brick footprint. If higher out-
put current is required, multiple DC888A boards may
be stacked together using on-board connectors for a
complete PolyPhase current sharing solution. The
converter operates at 250kHz and achieves efficiency
up to 93.5% with synchronous output rectifiers.
Secondary-side control eliminates complex optocou-
pler feedback, providing fast transient response with
a minimum amount of output capacitance. Additional
DC888A versions include DC888A-A (3.3V at 50A)
and DC888A-C (12V at 20A). The simple architecture
can be easily modified to meet different input and
output voltage requirements.
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 40A
5V
Maximum Output Current
200LFM
40A
Typical Output Ripple V
OUT
V
IN
= 48V, I
OUT
= 40A, 250kHz
< 60mV
P–P
Output Regulation
Over All Input Voltages and Output Currents
±1% (Reference)
Peak Deviation with 20A to 40A Load Step (10A/us)
±250mV
Load Transient Response
Settling Time
•50us
Nominal Switching Frequency
250kHz
Efficiency
V
IN
= 36V, I
OUT
= 28A
93.5% Typical
Isolation
BASIC
1500VDC
Size
Component Area x Top x Bottom Component Height
2.3” x 1.45” x 0.4” x 0.075”
OPERATING PRINCIPLES – SINGLE PHASE
The LTC3706 secondary side controller is used on the
secondary and the LTC3725 smart driver with self-
starting capability is used on the primary. When an
input voltage is applied, the LTC3725 (U1 in Figure
15), which is powered through R29 and Q28, begins
a controlled soft-start of the output voltage by switch-
ing MOSFETs Q9 and Q11. As the output voltage be-
gins to rise, the LTC3706 secondary controller is
quickly powered up via D24, Q29, C67, and Q27. The
LTC3706 then assumes control of the output voltage
by sending encoded PWM gate pulses to the
LTC3725 primary driver via signal transformer, T2.
The LTC3725 then operates as a simple driver receiv-
ing both input signals 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. The LTC3706 regu-