DC479A - Demo Manual

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 replacement for "quarter-brick" DC/DC converters. The design 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 protection, soft start with input undervoltage and overvoltage
lockout. Cycling short circuit protection minimizes thermal stress. The output overvoltage circuit provides protection 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. TA =25°C, VIN =48V, full load, ON/OFF and TRIM pins open, +SENSE shorted to +VOUT, –SENSE shorted
to –VOUT, unless otherwise specified.
PARAMETER
CONDITION
Input Voltage Range
Maximum Input Current
VIN = 36V, Full Load
Inrush Transient
VIN = 72V
MIN
TYP
MAX
36
48
72
A
0.2
100
Output Voltage
Output Regulation
V
1.6
Reflected Ripple Current
3.24
3.30
UNITS
A2s
mAP–P
3.36
V
Line
0.1
%
Load
0.2
%
15
A
Output Current
Output Current Limit
18
Output Short Circuit
Cycling, Auto-restart
Output Ripple and Noise
RMS
15
Peak-to-peak (5Hz to 20MHZ)
40
A
1000
Efficiency
ms
mVRMS
60
88.5
mVP–P
%
Dynamic Response
Peak Deviation
50
100
mV
Load Step 50% to 100%
Settling Time (to within 10mV of set point)
100
200
µs
Output Voltage Trim
VTRIM = 3.3V
4
5
6
%
VTRIM = 0V
–6
–5
–4
%
Output Overvoltage
On/Off Control
3.65
Logic Low Voltage: Off
0
Logic High Voltage: On
1.0
V
0.6
V
V
Logic Low Current : Off
0.2
mA
Quiescent Current: Off
1.4
mA
Start-up Inhibit Period
7.5
ms
Turn on Time
10
15
ms
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
PARAMETER
CONDITION
Thermal Shutdown
At RT1
MIN
TYP
Isolation Capacitance
1500
10
UNITS
°C
100
Isolation Voltage
Isolation Resistance
MAX
V DC
MΩ
2200
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 synchronous 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 filter, just as in a buck regulator. D1 and D2 recover energy
stored in the leakage inductance of T1 during the off cycle. 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 electrolytic capacitor will damp the input filter and provide
adequate 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 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 transformer. The LTC1698 includes an error amplifier and optocoupler drive buffer, eliminating the output feedforward 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 converter harmlessly cycling on and off, keeping power dissipation to a minimum. The cycling rate is nominally 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 the full rated current at room temperature. With
elevated temperature operation, airflow is required for
full rated load. The demo board features thermal overload 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 voltage 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 approximately 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 differential 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 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 output capacitor. Worst case ripple current for the input capacitor
occurs at 50% duty cycle. Two 0.82µF ceramic capacitors, C2 and C3 are used for the input filter. An aluminum 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 relationships are shown below.
Basic Duty Cycle Equation:
VOUT = VIN • DC • NS
NP
Input Capacitor RMS Current:
IRMS = IOUT • NS • DC − DC
NP
2
IRMS =
IL(pk −pk)
12
Inductor Ripple Current:
IL(pk − pk) =
(VOUT+VD)•(1−DC)•fSW
L
Primary RMS Current:
IRMS = IOUT • NS • DC
NP
Secondary RMS Current:
IRMS = IOUT • DC
SAFETY AND ISOLATION
The demo board is designed to meet the requirements of
UL 60950, 3rd 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 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. 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.
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-
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.
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
power and verify the output voltage, regulation, ripple
voltage, efficiency and other parameters.
See Figure 5 to Figure 13 for expected performance.
1. For normal operation, leave the On/Off pin open.
Shorting this pin to –Vin will turn off the converter.
–
+
+
+
+
+
–
–
–
–
+
LOAD
–
Figure 2. Measuring Input or Output Ripple
L
+
+
VIN
•
NP
GND
VIN
Figure 1. Proper Measurement Equipment Setup
+
+
•
NS
VOUT
–
–
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
–
–
+
DC479
OUT
LOAD
–
Figure 4. EMI Test Setup
95
90
85
Vin = 36V
Vin = 48V
80
Vin = 72V
75
70
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 5. Typical Efficiency
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
80
T1 TRA NSFORM ER
70
Q3 M OSFET
L3 INDUCTOR
60
RT1 THERM ISTOR
50
40
30
20
10
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 6. Temperature Rise at VIN = 36V, No Airflow
90
80
70
60
50
40
30
T1 TRA NSFORM ER
20
Q3 M OSFET
L3 INDUCTOR
10
RT1 THERM ISTOR
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 7. Temperature Rise at VIN = 72V, No Airflow
6
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
90
NO A IRFLOW
80
100 LFM
70
200 LFM
300 LFM
60
400 LFM
50
40
30
20
10
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 8. Q3 Temperature Rise at VIN = 48V (Hottest PCB Spot)
90
NO A IRFLOW
80
100 LFM
70
200 LFM
300 LFM
60
400 LFM
50
40
30
20
10
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 9. T1 Temperature Rise at VIN = 48V
7
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
80
NO A IRFLOW
70
100LFM
200 LFM
60
300 LFM
400 LFM
50
40
30
20
10
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 10. L3 Temperature Rise at VIN = 48V
70
NO A IRFLOW
60
100 LFM
200 LFM
50
300 LFM
400 LFM
40
30
20
10
0
0
3
6
9
12
15
LOA D CURRENT (A )
Figure 11. RT1 Temperature Rise at VIN = 48V
8
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER—QUARTER BRICK
CISP R A "A VG"
CISP R B "A VG"
CISP R 22 setup (uses
10uF capacito r)
100
80
60
40
20
0
1.E+05
1.E+06
1.E+07
1.E+08
FREQUENCY (Hz)
Figure 12. Conducted Emissions at VIN = 48V
CISP R A "A VG"
CISP R B "A VG"
With 12uF, 15uH and
10uF external P I filter
100
80
60
40
20
0
1.E+05
1.E+06
1.E+07
1.E+08
FREQUENCY (Hz)
Figure 13. Conducted Emissions at VIN = 48V with External PI Filter
9
VIN
DO1608C-332
COILCRAFT
R1
10
Q1
Si7456DP
Q2
MMBT3906
C3
0.82uF
100V
D2
MURS120
D1
MURS120
2
3
Q4
Q5
Si7892DP Si7892DP
R5
0.030
C10
2200pF
P3
-VIN
VCC
-VIN
D3
BAS21
L2
1.0mH
VIN
D5
MBR0540
OPT
D7
BAT54
R16
2K
1/4W
D10
MMBZ5240B
10V
-VIN
C24
0.1uF
50V
+
C25
100uF
20V
C28
R43
1000pF 10K
R45
1.24K
1%
R39
52.3K
R42
1%
2.43K
C32
C31
1%
82pF
1uF
25V
SG 12
SENSE 11
9V
4
8
5
50V
R32
470
PGND 14
R27
1K
R49
OPT
C16
1000pF
C21
0.1uF
50V
R31
C19
0.022uF 1K
0805
15 SYNC
C29
0.01uF
10 VC
SS
SGND
4
SYNC
7
R40
10K
VFB 9
R37
3K
ISO1
MOC207
43
5VREF
7
5
R46
1K
R35
1K
C22
4700pF
6
R41 C26
10 3300pF
C30
4700pF
1
8
+
3
-
4
5 OPTODRV
R21
3.01K
1%
-SENSE
R22
100
1/4W
OPTIONAL DIFFERENTIAL SENSE
R28
OPT
R34
3.01K
1%
-VOUT
R33
2.43K
1%
VFB 8
OVPIN 9
P6
MARGIN 7
2
C33
0.1uF
50V
P5
1%
VOUT
U2
LTC1698EGN
14 VAUX
R14
3.01K
1%
R20
3.01K
D11
BAT54
OPT
-VIN
8
BG 15
3
THERM
NC 16
NC 17
RT1
100K
D14
BAT54
1
P7
+SENSE
1%
GND
5VREF
C15
4.7uF
R19
3.01K
10 PWRGGD
C18
3300pF
TG 19
FSET
5
ON/OFF
U3
LT3781CG
6
1 SHDN
BSTREF 18
2 OVLO
5VREF
P2
VBST 20
13 VCC
D15
MMBD914
1/4W
Q12
FZT690B
PULSE ENG.
T2
C17
220pF PA0184
D12
BAT54
-VIN
100V
1
R18
100
R23
1K
C27
0.1uF
50V
C20
0.1uF
D13
BAS21
VCC
U1
LT1783CS5
16V
VCC
R26
73.2K
1%
C7
330uF
6.3V
KEMET
+
9V
5
C13
0.22uF
50V
VIN
R25
OPT
C6
330uF
6.3V
KEMET
+
R12
100
1/4W
CG 2
VCOMP 6
OPTIONAL FAST START
R24
270K
1/4W
C11
0 Ohm
D6
MBR0540
R17
10K
C5
330uF
6.3V
KEMET
VOUT
2
C14
330pF
+
-VOUT
R10
4.7
R11
62K
1/4W
D8
Q11
MMBT3904 MMBZ5248B
18V
C12
4.7uF
16V
R4
10
1/4W
C4
330uF
6.3V
KEMET
+
P4
D4
BAS21
Q10
ZVN3310F
R15
47K
R3
10
1/4W
Q6
Q7
Si7892DP Si7892DP
-VOUT
VCC
5VREF
D9
MMBD914
P8 [email protected]
+VOUT
250V
4
Q9
FQT7N10L
C9
1000pF
100V
R6
3.3
ISNSGND 11
FG 16
R9
100
1/4W
C8
1000pF
100V
Q3
Si7456DP
Q8
MMBT3906
VOUT
5
4
R2
10
R8
100
1/4W
L3
1.5uH
6
PGND
C2
0.82uF
100V
3
C1
0.82uF
100V
T1
1
VDD 1
ISNS 12
L1
3.3uH
P1
+VIN
R38
1K
ICOMP 13
R47
1.78K
1%
VOUT
C23
0.22uF
TRIM
R48
1.24K
1%
NOTES: UNLESS OTHERWISE SPECIFIED
1. TO DISABLE OPTIONAL FAST START:
REMOVE C12, D8, D9, R8, R9, R11, R15, Q9, Q11; ADD R25 = 20K.
2. TO DISABLE OPTIONAL DIFFERENTIAL SENSE:
REMOVE R12, R14, R19-22, U1; ADD R28 = 0 OHM.
3. ALL RES. ARE IN OHMS.
ALL DIODES ARE MMBD914BLT1.
ALL NPNs ARE MMBT3904LT1.
ALL PNPs ARE MMBT3906LT1.
10
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
11
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
12
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
13
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
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14
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
IDT
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15
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
SCHURTER
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16
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
17
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
18
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
19
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479
ISOLATED SYNCHRONOUS FORWARD CONVERTER – QUARTER BRICK
20
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