DC968A-D - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
LT1952
DESCRIPTION
Demonstration circuit 968A-D is isolated input to high
current output 1/8th Brick footprint converter featuring 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 operates 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 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-C 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 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 evaluate the performance of LT1952 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-D
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 = 28V, +/-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 (330uF) 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-D
ISOLATED DC/DC POWER CONVERTER
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 circuits need to be implemented.
To get higher than 28V 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.
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 selecting appropriate MOSFET. The active reset circuit
does not always help with the efficiency and is disabled on DC968-A, -B and -C demo boards.
LT1952 DC968-D 28V @4A Output Efficiency
96%
95%
94%
93%
92%
91%
90%
1
2 Iout [A]
3
4
5
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 capacitance is present (6.8uF ceramic and 330uF 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-D
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-D
ISOLATED DC/DC POWER CONVERTER
+Vb
R1
+Vin
1.5mH
Vu1
2.2R
Cin1
3x2.2uF
D3
BAS516
+Vr2
R11
82k
-Vin
D5
PDZ9.1B
U2
LTC4440-5
1
C3
1u
2
C21
220p
3
Vcc
2
Q12
BC857BF
R15
115k 3
R42
0R 4
5
6
7
13.3k
8
R32
383k
R46
2.2k
442k
Out
R22
R28
C10
0.1u
+Vr2 +Vb
D12
BAS516
+5V
R4710k
Comp
FB=1.23V
Vin
Rosc
Out
Sync
PGND
Delay
Vr=2.5V
OC
SD
Isense
GND
Blank
R33
22K
158k
+5V
R44
11.3k
A1 Vcc
B1 RC1
R1 C1
-Q1 Q1
Q2 -Q2
C2 R2
RC2 B2
GND A2
16
15
14
13
12
11
10
9
16
D10
C9
12
10
R23
R9
47R
D6
BAS21H
Cu1
4.7uF
R20
43.2k
100p
11
1.5k
R48
150R
R16
56k
R18
10k
1
2
R21
10k
Vaux
CG
3
4
C14
1u
Vu1
3
C30
100p
Boost
GND
TG
Inp
TS
Sync
CS- Timer
CG
GND
Vcc
FG
8
Sout
7
6
C8
100p
5
R17
560R
T2
PE-68386
C12
470p
R19
38.3K
C29
0.1u
5
4
BAS516
Vaux
R31
10R
C16
.22u, 10V
C25
47n
C23
1uF
U7
6
U6
PS2801-1
Sw0
Vu1
R43
1k
Vcc
CS+
C7
220p
D11
R26
220R
Vfb
1
FG
U5
LTC3900
D9
PDZ9.1B
C13
.47u
D13
PDZ5.1B
R8
2.2R
C5
0.22u
C11
opt.
9
C28
C27 0.1u
0.1u
GND
C4
680pF
Vu1
2
+5V
CG
Out
Sout
14
13
Csys
330uF, 35V
+
5
D8
BAS21H
15
C26
220p
Co1
R24
82K
R29
U3
HC123
1
2
3
4
5
6
7
8
Sout
MaxDC
Q3
HAT2174
D1
BAS516
BAT760
1
Vfb
Q2
HAT2174
4
TS
U1
LT1952-1
R14
33K
10
6
R3 41.2
R13
22K
6
+Vo1
+Vout
5
TG
Inp
7
FG
Sw0
Q1
PHM21NQ15
Boost
GND
1
Rcs1
0.010R
Vu1
PA2050.393
2
+Vin
R5
10R
L1
PA0856.003
6.8uF, X7R
+Vin
T1
D2
BAS516
L2
Q10
PZTA42
Q13
IRF6217
R45
10k
1
2
3
R30
1.2k
U4
LT4430
R27
270R
Vcc
Opto
GND
Comp
SS
C19
1uF, X5R
C15
2.2nF, 2kV
LTC4440
D14
BAS516
5
FB
C17
15n
R34
16.9k
6
5
R35
280k +Vo1
4
R38
6.19k
C18
opt.
C24
opt.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
ISOLATED DC/DC POWER CONVERTER
6
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