QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
LT3781 and LTC1698
DESCRIPTION
Demonstration circuit 479 is an isolated synchronous
forward converter featuring the LT3781 and LTC1698
controllers. DC479 is designed to be a board level re-
placement for "quarter-brick" DC/DC converters. The de-
sign can provide 3.3V at 15A from an isolated 48V (36V
to 72V) input. Isolation voltage is 1500V DC. The circuit
features low input capacitance, over temperature protec-
tion, soft start with input undervoltage and overvoltage
lockout. Cycling short circuit protection minimizes ther-
mal stress. The output overvoltage circuit provides pro-
tection for the load should a fault occur on the sense
lines. The standard footprint allows for immediate on
board evaluation by plugging directly into the modules’
socket.
Design files for this circuit board are available. Call
the LTC factory.
Table 1. Performance Summary. T
A
=25°C, V
IN
=48V, full load, ON/OFF and TRIM pins open, +SENSE shorted to +VOUT, –SENSE shorted
to –VOUT, unless otherwise specified.
PARAMETER
Input Voltage Range
Maximum Input Current
Inrush Transient
Reflected Ripple Current
Output Voltage
Output Regulation
Line
Load
Output Current
Output Current Limit
Output Short Circuit
Output Ripple and Noise
Cycling, Auto-restart
RMS
Peak-to-peak (5Hz to 20MHZ)
Efficiency
Dynamic Response
Load Step 50% to 100%
Output Voltage Trim
Peak Deviation
Settling Time (to within 10mV of set point)
V
TRIM
= 3.3V
V
TRIM
= 0V
Output Overvoltage
On/Off Control
Logic Low Voltage: Off
Logic High Voltage: On
Logic Low Current : Off
Quiescent Current: Off
Start-up Inhibit Period
Turn on Time
0
1.0
0.2
1.4
7.5
10
15
4
–6
18
1000
15
40
88.5
50
100
5
–5
3.65
0.6
100
200
6
–4
60
3.24
V
IN
= 36V, Full Load
V
IN
= 72V
100
3.30
3.36
0.1
0.2
15
CONDITION
MIN
36
TYP
48
1.6
0.2
MAX
72
UNITS
V
A
A
2
s
mA
P–P
V
%
%
A
A
ms
mV
RMS
mV
P–P
%
mV
µs
%
%
V
V
V
mA
mA
ms
ms
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
PARAMETER
Thermal Shutdown
Isolation Voltage
Isolation Resistance
Isolation Capacitance
10
2200
CONDITION
At RT1
MIN
TYP
100
1500
MAX
UNITS
°C
V DC
MΩ
pF
OPERATING PRINCIPLES
CIRCUIT OVERVIEW
This two-transistor forward converter operates at a
nominal switching frequency of 240 kHz. Pulse width
modulation control is done by U3, the LT3781 synchro-
nous forward controller. Galvanic isolation is met with
transformer T1 and optocoupler ISO1. C10 is used as a
local bypass to reduce common mode induced current.
The main switching power path through T1 is comprised
of L1, C2 and C3 as the input filter, with Q1 and Q3 as
the primary switches. MOSFETs Q4, 5, 6, and 7 are the
secondary synchronous rectifiers. L3 and C4-7 are the
secondary output filter. Power is transferred during the
on cycle of Q1 and Q3, and integrated by the output fil-
ter, just as in a buck regulator. D1 and D2 recover energy
stored in the leakage inductance of T1 during the off cy-
cle. The input filter component values for L1, C2 and C3
are optimal and should not be changed without careful
evaluation. C1 bypasses the input terminals. For large
values of input inductance, an external aluminum elec-
trolytic capacitor will damp the input filter and provide
adequate stability. See Linear Technology’s application
note AN19 for a discussion on input filter stability analy-
sis.
When the primary switches turn off, the transformer
voltage reverses, with D1 and D2 conducting to reset the
transformer during normal operation. A startup or tran-
sient to no load can cause the pulse width modulation to
narrow, with insufficient energy to force the reset diodes
into conduction. When this occurs, the charge on C20
gets depleted and the top gate drive shuts off. This will
result in the converter cycling on and off. To overcome
this, Q10 provides a return path to refresh the top gate
boost capacitor C20.
Feedback control of the output voltage and synchronous
drive is done using U1, the LTC1698. The LTC1698 syn-
chronizes with the LT3781 via T2, a small pulse trans-
former. The LTC1698 includes an error amplifier and op-
tocoupler drive buffer, eliminating the output feed-
forward path associated with ’431 type references. U1
also provides output overvoltage protection. The margin
pin allows the output voltage to be adjusted
±5%.
During an output short circuit condition, the primary
bias supply at Vcc collapses. This results in the con-
verter harmlessly cycling on and off, keeping power dis-
sipation to a minimum. The cycling rate is nominally 1Hz
with 48V input. When the short is removed, the con-
verter returns to normal operation.
The demo board uses all surface mount devices and will
deliver the full rated current at room temperature. With
elevated temperature operation, airflow is required for
full rated load. The demo board features thermal over-
load protection.
For –48V inputs requiring hot swap capability, the
LT4250H negative voltage hot swap controller provides a
seamless interface.
OPTIONAL FAST START CIRCUIT
When power is first applied, Vcc must rise to 15V for the
LT3781 to turn on. The bias supply turn on threshold
and hysteresis are set internally by U3. R8 and 9 charge
the 100µF capacitor C25, and are gated by Q9. With
200Ω resistance, the charge time is 7.5ms at 48V in.
The values for R8 and R9 can be adjusted in order to
change the turn on delay. Values lower than 100Ω for
each resistor will result in abnormally high peak power,
and possible component failure. Once the LT3781 turns
on, the 5Vref charges C12 causing Q11 to turn off Q9.
Bias supply power is delivered through L2 by a winding
on T1. In the event of an output short circuit, the volt-
age on the transformer bias winding collapses. Restart
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
time is determined by C12 and R15, and is set to ap-
proximately 1 second.
The optional fast start circuit can be removed, and a
20kΩ resistor installed for R25. The peak bias supply
voltage is self limiting by an internal 18V clamp on the
LT3781 Vcc pin. R25 will trickle charge C25, resulting in
a turn on delay of approximately 750ms at 48V in.
OPTIONAL DIFFERENTIAL SENSE
The LT1783 operational amplifier U1 provides true dif-
ferential remote sense. If this feature is not required the
circuit can be removed. To maintain voltage regulation, a
zero ohm resistor must be installed for R28.
FORWARD CONVERTER DESIGN EQUATIONS
The two-transistor forward converter is a good choice
for 48V telecom applications. The maximum duty cycle
is limited to 50% with the two-transistor forward. This
topology is used quite extensively in many modular de-
signs. Unlike the flyback, energy is not intentionally
stored in the power transformer. This allows for a much
smaller transformer design.
The forward converter has pulsating current in the input
capacitor, and continuous current in the output capaci-
tor. Worst case ripple current for the input capacitor
occurs at 50% duty cycle. Two 0.82µF ceramic capaci-
tors, C2 and C3 are used for the input filter. An alumi-
num electrolytic type can be substituted as long as it is
rated for at least 1.9A RMS. The basic two-transistor
synchronous forward converter diagram is shown in
Figure 3. The idealized equations for duty cycle relation-
ships are shown below.
Basic Duty Cycle Equation:
V
OUT
=
V
IN
•
DC
•
N
S
N
P
I
RMS
=
I
L
(pk
−
pk)
12
Inductor Ripple Current:
I
L
(pk
−
pk)
=
(V
OUT
+
V
D
)
•
(1
−
DC)
•
f
SW
L
Primary RMS Current:
I
RMS
=
I
OUT
•
N
S
•
DC
N
P
Secondary RMS Current:
I
RMS
=
I
OUT
•
DC
SAFETY AND ISOLATION
The demo board is designed to meet the requirements of
rd
UL 60950, 3 edition for basic insulation in secondary
circuits. The input is considered to be a TNV-2 circuit,
and the output is SELV. The optocoupler and bridging
capacitor both have agency file numbers. A 3A fast blow
type fuse must be placed in series with the ungrounded
(hot) input line.
The transformer is designed to meet the basic insulation
requirement, with an isolation voltage of 1500VDC. The
core is considered to be part of the secondary circuit.
As currently built, the transformer uses a class A mate-
rial insulating system.
CONDUCTED EMI
Tests for conducted emissions were performed for the
demo board. A small external PI filter using a 12µF alu-
minum electrolytic capacitor, 15µH inductor and 10µF
film capacitor allows the converter to meet the CISPR 22
class B limit. No tests for radiated RFI were performed
because the radiation is application specific. Proper
grounding and layout technique must be observed to
minimize radiation. See Figure 4 for test setup.
Input Capacitor RMS Current:
I
RMS
=
I
OUT
•
N
S
•
DC
−
DC
N
P
2
Output Capacitor RMS Current:
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
RELIABILITY
Reliability prediction for the circuit has been calculated
using the Telcordia (formerly Bellcore) SR-332. The
black box technique was used. The calculation was made
assuming a ground, fixed, controlled environment and
quality level II. A 50% electrical stress at 40°C yields an
MTBF (mean time between failures) of 1.5 million hours.
QUICK START PROCEDURE
Demonstration circuit 479 is easy to set up to evaluate
the performance of the LT3781 and LTC1698. Refer to
Figure 1 for proper measurement equipment setup and
follow the procedure below:
NOTE:
When measuring the input or output voltage rip-
2.
Connect –Sense to –Vout and +Sense to +Vout. The
Trim pin should be left floating.
3.
Connect the power supply and meters to the Vin pins.
4.
Connect the load and meters to the Vout pins.
5.
After all connections are made, turn on the input
ple, care must be taken to avoid a long ground lead on
the oscilloscope probe. Measure the input or output
voltage ripple by touching the probe tip directly across
the Vin or Vout and GND terminals. See Figure 2 for
proper scope probe technique.
1.
For normal operation, leave the On/Off pin open.
power and verify the output voltage, regulation, ripple
voltage, efficiency and other parameters.
See Figure 5 to Figure 13 for expected performance.
Shorting this pin to –Vin will turn off the converter.
+
+
–
+
–
–
+
LOAD
–
+
+
–
–
VIN
GND
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
+
+
V
IN
N
P
L
+
+
•
•
N
S
V
OUT
–
–
Figure 3. Basic Two-transistor Forward Converter
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
SPECTRUM
ANALYZER
+
POWER
SUPPLY
LISN
50µH
50
10µF
IN
+
–
+
OUT
LOAD
DC479
–
–
Figure 4. EMI Test Setup
95
90
85
Vin = 36V
80
Vin = 48V
Vin = 72V
75
70
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 5. Typical Efficiency
5
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