DC968A-C - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
LT1952
DESCRIPTION
Demonstration circuit 968A-C is isolated input to high
current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968-C
converts isolated 36V to 75V input to 12V output and
provides over 12A of output current. The converter
operates at 300kHz with efficiency up to 95%. With
proper amount of airflow, the DC968-C converter can
generate up to 15A of output current. The DC968 can
be easily modified to generate output voltages in the
range from 0.6V to 28V. The output currents are limited by total output power of up to 200W. The other
available versions of DC968A are:
DC968A-A 36-75Vin to [email protected]
DC968A-B 18-36Vin to [email protected]
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 recovering from overload condition.
The DC968 has precise over-current protection circuit
that allows for continuous operation under short circuit 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.
DC968A-D 36-75Vin to [email protected]
Also, the DC968 can be modified for other input voltages like 8V-36V, 18V-75V, 44V-52V, and so on. The
wider input voltage range will decrease the converter
LT is a trademark of Linear Technology Corporation
Table 1. Performance Summary
PARAMETER
CONDITION
VALUE
Minimum Input Voltage
IOUT = 0A to 12A
36V
Maximum Input Voltage
IOUT = 0A to 12A
72V
VOUT
VIN = 36V to 72V, IOUT = 0A to 12A
12V ±3%
Typical Output Ripple VOUT
VIN = 36V to 72V, IOUT = 0A to 12A
100mVP–P
Nominal Switching Frequency
300kHz
QUICK START PROCEDURE
Demonstration circuit 968 is easy to set up to evaluate the performance of LT1952-1 circuit. Refer to Figure 1 for proper measurement equipment setup and
follow the procedure below:
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.
NOTE: When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
1. With power off, connect the input power supply to
Vin and GND. Make sure that the input power sup-
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
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 = 12V, +/-3%.
4. Once the proper output voltage is established, ad-
just the load within the operating range and observe the output voltage regulation, ripple voltage,
efficiency and other parameters.
5. The DC968 is equipped with an output capacitor
CSYS (150uF) that approximates typical system
rail capacitance. If system board already has capacitance of similar value CSYS can be removed.
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To get higher than 12V output voltages transformer
with higher turns ratio may be required. Also, output 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 important. If U2 is disabled, the DELAY pin of LT1952
should be readjusted to optimize the efficiency.
Please contact LT factory for assistance.
OPTIONAL RESET CIRCUIT
The DC968 is equipped with an optional active reset
circuit that is disabled on DC968A-C. In most applications the circuit is not required because there is
no benefit from active reset. However, if the active
reset is required, adding few resistors can enable
the circuit. Please consult LT factory for assistance.
LT1952 DC968-C 12V @12A output
96%
95%
Efficiency
To set the output voltage lower than 12V, change
the bottom voltage divider resistor connected to
LT4430 FB pin (see the simplified schematic on
page 5). For example, to get 9V output, change R38
resistor value to 24.9k. 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 circuits need
to be implemented.
94%
93%
92%
91%
90%
0
2
4
6
8
10
12
14
Iout [A]
Figure 3. High efficiency of DC968A-C allows the
board to be used in thermally critical applications
with outputs up to 15A.
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-C is very fast
even though relatively small amount of output capacitance is present (33uF ceramic and 150uF electrolytic). This is thanks to fast error amplifier of
LT4430, optimal amount of current slope compensation 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-C
ISOLATED DC/DC POWER CONVERTER
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.
Figure 4. Fast transient response of DC968 is superior 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.
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 current 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 delay from turn on of FG and gate of Q1 is usually in
the range from -100ns to +100ns. By slowly increasing the +Vin from 0V to 48V the output voltage
and input current can be monitored. The input current should not exceed 100mA without the output
load. If one of the MOSFETs is damaged, the input
current will exceed 100mA.
Figure 5. The LT4430 opto coupler driver produces monotonic output voltage rise at startup
without output voltage overshoot.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
+Vb
R1
+Vin
L2
Vu1 DO1607B-155ML-C
Q10
PA1393.302
7
R11
82k
Cin1
Cin2
Cin3
2.2uF, 100V
6
D3
BAS516
D5
PDZ10B
Q2
HAT2244
10
Vu1
1
C3
1u
2
3
C21
220p
1
33K
2
R15
133k 3
4
C10
0.1u
5
6
R22
7
+Vr2
442k
8
R28
442k
13.3k
R32
BC857BF
Q12
R14
+Vb
TG
Inp
TS
5
L3
DO1608C-684ML-C
C2
820p
5
4
Comp
Sout
FB=1.23V
Vin
Rosc
Out
Sync
PGND
MaxDC
Delay
Vr=2.5V
OC
SD
Isense
GND
Blank
16
Sout
14
Rcs1
0.010R
C9
12
11
10
R23
Vu1
R9
47R
100p
R20
68.1k
2k
C11
2.2n
9
R26
220R
C13
1uF
4
R30
1.2k
C15
CG
GND
Vcc
FG
Vaux
R17
560R
C7
220p
8
Sout
7
C8
100p
6
5
T2A
PE-68386
FG
R19
C14
1uF
C12
470p
38.3K
D11
BAS516
U6
PS2801-1
Vfb
Sync
CS- Timer
3
CG
U5
LTC3900
CS+
2
R31
10R
C16
.22uF
C23
1uF
1
R21
10k
D9
PDZ7.5B
R24
82K
158K
R18
10k
R10
1k
Vu1
R8
2.2R
R16
47k
Q9
BCX55 Vaux
Cu1
4.7u
D10
D7
BAS521
D6
BAS521
C5
0.22u, 25V
15
13
CG
GND
BAS516
R33
R29
22K
GND
BAT760
R13
22K
Vfb
Boost
6
Csys
150uF, 16V, APXE
+
2
Q1
PHM21NQ15
R3
57
D1
U1
LT1952-1
Vcc
Q3
HAT2244
PA0423
+Vin
U2
LTC4440
L1
T1
FG
R5
10R
+Vo1
+Vout
BAS516
+Vr2
-Vin
1
PZTA42
2.2R
36V to 72V
D2
Co1
33uF, 16V
+Vin
C19
1uF
1
2
3
U4
LT4430
R27
270R
Vcc
Opto
GND
Comp
SS
FB
R38
18.2K
2.2nF, 2kV
5
C17
3.3n
R34
8.45k
6
5
R35
348K
4
C18
10p
+Vo1
C24
10p
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
6
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