Linear Technology 968A-D User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
1
LT1952
DESCRIPTION
Demonstration circuit 968A-D is isolated input to high
current output 1/8th Brick footprint converter featur-
ing the LT®1952 switching controller. The DC968-D
converts isolated 36V to 75V input to 28V output and
provides over 4A of output current. The converter op-
erates at 300kHz with efficiency up to 95%. With
proper amount of airflow, the DC968-D converter can
generate up to 5A of output current. The DC968 can
be easily modified to generate output voltages in the
range from 0.6V to 48V. The output currents are lim-
ited by total output power of up to 200W. The other
available versions of DC968A are:
DC968A-A 36-75Vin to 3.3V@30A
DC968A-B 18-36Vin to 5V@20A
DC968A-C 36-75Vin to 12V@12A
Also, the DC968 can be modified for other input volt-
ages like 8V-36V, 18V-75V, 44V-52V, and so on. The
wider input voltage range will decrease the converter
efficiency. Therefore, narrow input voltage range will
be more desirable.
The DC968 circuit features soft-start which prevents
output voltage overshoot on startup or when recover-
ing from overload condition.
The DC968 has precise over-current protection circuit
that allows for continuous operation under short cir-
cuit conditions. The low power dissipation under
short circuit conditions insures high reliability even
during short circuits.
The LT1952 can be synchronized to an external clock
of up to 400kHz. Please refer to LT1952 data sheet for
design details and applications information.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
Table 1. Performance Summary
PARAMETER CONDITION VALUE
Minimum Input Voltage IOUT = 0A to 4A 36V
Maximum Input Voltage IOUT = 0A to 4A 72V
VOUT VIN = 36V to 72V, IOUT = 0A to 4A 28V ±3%
Typical Output Ripple VOUT VIN = 36V to 72V, IOUT = 0A to 4A 100mVP–P
Nominal Switching Frequency 300kHz
QUICK START PROCEDURE
Demonstration circuit 968 is easy to set up to evalu-
ate the performance of LT1952 circuit. Refer to Fig-
ure 1 for proper measurement equipment setup and
follow the procedure below:
NOTE: When measuring the input or output voltage
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. With power off, connect the input power supply to
Vin and GND. Make sure that the input power sup-
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
2
ply has sufficient current rating at minimum input
voltage for the required output load.
2. Turn on the power at the input.
NOTE: Make sure that the input voltage does not
exceed 75V.
3. Check for the proper output voltage.
Vout = 28V, +/-3%.
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
4. Once the proper output voltage is established, ad-
just the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.
5. The DC968 is equipped with an output capacitor
CSYS (330uF) that approximates typical system
rail capacitance. If system board already has ca-
pacitance of similar value CSYS can be removed.
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
3
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To set the output voltage lower than 28V, change
the bottom voltage divider resistor connected to
LT4430 FB pin (see the simplified schematic on
page 5). For example, to get 24V output, change
R38 resistor value to 7.15k. However, keep in mind
that changing the transformer as well may increase
the efficiency. The schematic on page 6 shows all of
the components on the PCB. The schematic on
page 6 can be used as a reference if optional cir-
cuits need to be implemented.
To get higher than 28V output voltages transformer
with higher turns ratio may be required. Also, out-
put MOSFETs with higher voltage ratings may be
required. Please contact LTC factory for details.
PRIMARY MOSFET DRIVER LTC4440
The DC968 has an LTC4440 MOSFET driver U2. The
LTC4440 can be disabled if the efficiency is not im-
portant. If U2 is disabled, the DELAY pin of LT1952
should be readjusted to optimize the efficiency.
Please contact LT factory for assistance.
RESET CIRCUIT
The DC968 is equipped with an active reset circuit.
The active reset circuit is used in order to improve
the efficiency by 1% to 2%.
If the efficiency is not important, active reset circuit
can be disabled. In that case, MOSFET Q2 will need
to be replaced with a MOSFET with higher voltage
rating. Please contact LTC factory for help in select-
ing appropriate MOSFET. The active reset circuit
does not always help with the efficiency and is dis-
abled on DC968-A, -B and -C demo boards.
LT1952 DC968-D 28V @4A Output Efficiency
90%
91%
92%
93%
94%
95%
96%
12345
Iout [A]
Figure 3. High efficiency of DC968A-D allows the
board to be used in thermally critical applications
with outputs up to 5A.
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-D is very fast
even though relatively small amount of output ca-
pacitance is present (6.8uF ceramic and 330uF elec-
trolytic). This is thanks to fast error amplifier of
LT4430, optimal amount of current slope compen-
sation of LT1952, fast opto coupler and fast error
amplifier of LT1952. If higher load steps need to be
handled more output capacitance can be added in
order to keep the voltage transients at the desired
level. The load step transients are shown in Figure
4.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
4
Figure 4. Fast transient response of DC968 is su-
perior to many competing power modules without
the additional output capacitors.
SOFT START FUNCTION
The DC968 features LT4430 opto coupler driver that
has soft start function which produces monotonic
startup ramp shown if Figure 5. The rise time of
output voltage is controlled by capacitor C19 that is
connected to OC (overshoot control) pin of LT4430.
Figure 5. The LT4430 opto coupler driver pro-
duces monotonic output voltage rise at startup
without output voltage overshoot.
DEBUGGING AND TESTING
The DC968 can easily be tested and debugged by
powering the bias circuit with a separate power
source. To place DC968 into debug mode remove
the resistor R1 and connect 48V, 100mA power
source to +Vb node (right side of R1). By doing
this, the primary PWM controller LT1952 can be
activated without the main primary power being
applied to +Vin.
To activate the secondary side MOSFET driver
(LTC3900) connect a 10V, 100mA power source to
collector of Q9 via 1A, 40V diode. That way the cur-
rent coming from DC968 bias supply will not be
able to flow back to 10V supply once the converter
is running.
Once the primary and secondary controllers are
running the MOSFET gate timing can be checked.
If the MOSFET gate timing is correct the main
power input can be applied to +Vin. The correct de-
lay from turn on of FG and gate of Q1 is usually in
the range from -100ns to +100ns. By slowly in-
creasing the +Vin from 0V to 48V the output voltage
and input current can be monitored. The input cur-
rent should not exceed 100mA without the output
load. If one of the MOSFETs is damaged, the input
current will exceed 100mA.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
5
Rcs1
0.010R
Cin1
3x2.2uF
+Vin
R9 47R
R29
22K
Cu1
4.7uF
R23 1.5k
R24
82K
Vu1
R15 115k
R20
43.2k
C10
0.1u
Vu1
Q1
D10
BAT760
R3 41.2
C15
2.2nF, 2kV
C11
opt.
R28
442k
R22
13.3k
PHM21NQ15
2 5
PA0856.003
GND
8
SD
7
MaxDC
5
FB=1.23V
2
Comp
1
Rosc
3
Vr=2.5V
6
Blank 9 Delay 12
OC 11
Sync
4
Isense 10
PGND 13
Out 14 Vin 15
Sout 16
U1
LT1952-1
Sout
R33
158k C13
.47u
R26
220R
R30
1.2k
R14
33K
C23
1uF
Vfb
Vfb
R13
22K
R42 0R
Co1
6.8uF, X7R
+Vo1
R17
560R PE-68386
Vaux
C14
1u
C12
470p
R19
38.3K
CS-
2
CS+
1
CG
3
Vcc
4FG 5
GND 6
Timer 7
Sync 8
U5
LTC3900
R21
10k
R27
270R
R38
6.19k
R35
280k
R34
16.9k
C19
1uF, X5R
C16
.22u, 10V
C17
15n
+Vo1
Vaux
R31
10R
SS
3
Vcc
1
FB 4
Opto 6
GND
2Comp 5
U4
LT4430
C18
opt.
R18
10k
+Vin
D5 PDZ9.1B
Vu1
D2 BAS516
R11
82k
1 6
-Vin +Vin
R1 2.2R
R32
383k
L1
PA2050.393
Q2 HAT2174
Q3 HAT2174
7 10
R16
56k
C5 0.22u
D6
BAS21H
D9 PDZ9.1B
R8 2.2R
FG
FG
CG
CG
C3 1u
R5 10R
Q12
BC857BF
D3
BAS516
Q10
PZTA42
+Vb
+Vb
L2
1.5mH
U6
PS2801-1
T1
Inp
3
Vcc
1
TS 4
Boost 6
GND
2TG 5
U2
LTC4440-5
C21
220p
D1 BAS516
Vu1
Out
D11
BAS516
GND
+Vout
C7 220p
Sout
C8 100p
C4 680pF
D8
BAS21H
+
Csys
330uF, 35V
C24
opt.
C9 100p
C25
47n
R43
1k
D12
BAS516
R4710k
C26
220p
R46
2.2k
R44
11.3k
Q13
IRF6217
Inp
3
Vcc
1
TS 4
Boost 6
GND
2TG 5
U7
LTC4440
C28
0.1u C29
0.1u
Vu1
D14
BAS516
R45
10k
B1
2A1
1
-Q1
4
Q2
5
C2
6
RC2
7
Vcc 16
GND
8
R1
3
B2 10
A2 9
-Q2 12
Q1 13
C1 14
RC1 15
R2 11
U3
HC123
+5V
+5V
Out
D13
PDZ5.1B
C27
0.1u
R48
150R
C30
100p
+Vr2
+Vr2
T2
+5V Sw0
Sw0
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
6
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