QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617
ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK
LT3781 and LTC1698
DESCRIPTION
Demonstration circuit 617 is an isolated synchronous
forward converter featuring the LT3781 and LTC1698
controllers. DC617 is designed to be a board level
replacement for "quarter-brick" DC/DC converters. The
design can provide an isolated 2.5V at 20A from a
48V (36V to 72V) input. Isolation voltage is 1500VDC.
The circuit features low input capacitance, over tem-
perature protection, soft start with input undervoltage
and overvoltage lockout. Cycling short circuit protec-
tion minimizes thermal stress. The output overvoltage
circuit provides protection for the load should a fault
occur on the sense lines. The standard footprint al-
lows 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
24
1000
15
40
86.1
50
100
5
-5
2.70
0.6
100
200
6
-4
60
2.40
V
IN
= 36V, Full Load
V
IN
= 72V
100
2.50
2.60
0.1
0.2
20
CONDITION
MIN
36
TYP
48
1.6
0.2
MAX
72
UNITS
V
A
As
mA
P-P
V
%
%
A
A
ms
mV
RMS
mV
P-P
%
mV
µs
%
%
V
V
V
mA
mA
ms
ms
2
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617
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
VDC
MΩ
pF
OPERATING PRINCIPLES
CIRCUIT OVERVIEW
This two-transistor forward converter operates at a
nominal switching frequency of 240kHz. Pulse width
modulation control is done by U3, the LT3781 syn-
chronous forward controller. Galvanic isolation is met
with transformers T1, T2 and optocoupler ISO1. C10
is used as a local bypass to reduce common mode
currents.
The main switching power path is comprised of C2
and C3 feeding the primary winding of T1, 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. D1 and
D2 recover energy stored in the leakage inductance of
T1 during the off cycle. The input inductor L1 was
chosen for it’s relatively low parallel impedance,
which helps to damp the input filter. C1 bypasses the
input terminals. For large values of input inductance,
an external 12µF 100V low ESR aluminum electrolytic
capacitor will damp the input filter and provide ade-
quate stability. See Linear Technology’s application
note AN19 for a discussion on input filter stability
analysis.
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
transient to no load can cause the pulse width modu-
lation 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.
Secondary side bias supply power is provided by T3,
a small inexpensive gate drive transformer. This steps
up the voltage across the output inductor L3 produc-
ing about 7V. Feedback control of the output voltage
and synchronous drive is done using U2, the
LTC1698. The LTC1698 synchronizes with the LT3781
via T2, a small pulse transformer. The LTC1698 in-
cludes an error amplifier and optocoupler drive
buffer, eliminating the output feed-forward path asso-
ciated with ’431 type references. U2 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
dissipation to a minimum. The cycling rate is nomi-
nally 1Hz with 48V input. When the short is removed,
the converter returns to normal operation.
The demo board uses all surface mount devices and
will deliver 15A at room temperature with no airflow.
To prevent thermal shutdown, utilize the curve in Fig-
ure 11. The thermal shutdown at RT1 is set for
100°C. To run at 20A out with 50°C ambient will re-
quire at least 200LFM of airflow. Figure 6 through
Figure 10 show other component temperature rise.
Building the circuit on a multi-layer system board can
provide improved thermal performance. Typical effi-
ciency is shown in Figure 5.
For –48V inputs requiring hot swap capability, the
LT4250H negative voltage hot swap controller pro-
vides 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
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617
ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK
threshold and hysteresis are set internally by U3. R8
and R9 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 ab-
normally 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 voltage on the
transformer bias winding collapses. Restart time is
determined by C12 and R15, and is set to approxi-
mately 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, re-
sulting in a turn on delay of approximately 750ms at
48V in.
OPTIONAL DIFFERENTIAL SENSE
The LT1783 operational amplifier U1 provides true
differential remote sense. If this feature is not re-
quired the circuit can be removed. To maintain volt-
age 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 cy-
cle is limited to 50% with the two-transistor forward.
This topology is used quite extensively in many
modular designs. 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 out
put capacitor. Worst case ripple current for the input
capacitor occurs at 50% duty cycle. Two 0.82µF ce-
ramic capacitors, C2 and C3 are used for the input
filter. An aluminum electrolytic type can be substi-
tuted 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 relationships are shown below.
Basic Duty Cycle Equation:
V
OUT
=
V
IN
•
DC
•
N
S
N
P
Input Capacitor RMS Current:
I
RMS
=
I
OUT
•
N
S
•
DC
−
DC
N
P
2
Output Capacitor RMS Current:
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 require-
ments of UL 60950, 3
rd
edition for basic insulation in
secondary circuits. The input is considered to be a
TNV-2 circuit, and the output is SELV. The optocou-
pler 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 insula-
tion requirement, with an isolation voltage of
1500VDC. The core is considered to be part of the
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617
ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK
secondary circuit. As currently built, the transformer
uses a class A material insulating system.
CONDUCTED EMI
Tests for conducted emissions were performed for
the demo board. A small external PI filter using a
12µF aluminum electrolytic capacitor, 15µH inductor
and 10µF film capacitor allows the converter to meet
the CISPR 22 class B limit. See Figure 12 and Figure
13 for test results. No tests for radiated RFI were per-
formed because the radiation is application specific.
Proper grounding and layout technique must be ob-
served to minimize radiation. See Figure 4 for test
setup.
RELIABILITY
Reliability prediction for the circuit has been calcu-
lated using the Telcordia (formerly Bellcore) SR-332.
The black box technique was used. The calculation
was made assuming a ground, fixed, controlled envi-
ronment 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 617 is easy to set up to evalu-
ate the performance of the LT3781 and LTC1698. Re-
fer to Figure 1 for proper measurement equipment
setup and follow the procedure below:
NOTE:
When measuring the input or output voltage
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
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the input or out-
put 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 perform-
ance.
Shorting this pin to –Vin will turn off the converter.
2.
Connect -Sense to –Vout and +Sense to +Vout. The
Trim pin should be left floating.
Figure 1. Proper Measurement Equipment Setup
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617
ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK
Figure 2. Measuring Input or Output Ripple
+
+
V
IN
N
P
L
+
+
•
•
N
S
V
OUT
–
–
Figure 3. Basic Two-transistor Forward Converter
DC617
Figure 4. EMI Test Setup
5
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