DC968A-B - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B
ISOLATED DC/DC POWER CONVERTER
LT1952
DESCRIPTION
Demonstration circuit 968A-B is isolated input to high
current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968
converts isolated 18V to 36V input to 5V output and
provides over 20A of output current. The converter
operates at 300kHz with efficiency greater than 93%.
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]
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.
DC968A-C 36-75Vin to [email protected]
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
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 20A
18V
Maximum Input Voltage
IOUT = 0A to 20A
36V
VOUT
VIN = 18V to 36V, IOUT = 0A to 20A
5V ±3%
Typical Output Ripple VOUT
VIN = 18V to 36V, IOUT = 0A to 20A
50mVP–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:
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 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.
1. With power off, connect the input power supply to
Vin and GND. Make sure that the input power supply has sufficient current rating at minimum input
voltage for the required output load.
2. Turn on the power at the input.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B
ISOLATED DC/DC POWER CONVERTER
NOTE: Make sure that the input voltage does not
exceed 40V.
3. Check for the proper output voltage.
Vout = 5V.
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.
5. The DC968 is equipped with an output capacitor
CSYS (470uF) that approximates typical system
rail capacitance. If system board already has capacitance of similar value CSYS can be removed.
4. Once the proper output voltage is established, ad-
just the load within the operating range and ob-
Figure 1. Proper Measurement Equipment Setup
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B
ISOLATED DC/DC POWER CONVERTER
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To get higher than 5V 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.
LT1952 DC968A-B 24V in to 5V out
94%
93%
Efficiency
To set the output voltage lower than 5V, change the
bottom voltage divider resistor connected to
LT4430 FB pin (see the simplified schematic on
page 5). For example, to get 2.5V output, change
R38 resistor value to 43.2k. However, keep in mind
that changing the transformer as well may increase
the efficiency.
92%
91%
90%
89%
88%
0
5
10
15
20
25
Iout [A]
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-B. 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.
Figure 3. High efficiency of DC968A-B allows the
board to be used in thermally critical applications
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-B is very fast
even though relatively small amount of output capacitance is present (200uF ceramic and 470uF
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-B
ISOLATED DC/DC POWER CONVERTER
DEBUGGING AND TESTING
The DC968 can easily be tested and debugged by
powering the bias circuit separately from the main
power circuit. 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) diode OR a 10V, 100mA power source
into collector of Q9.
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 delay from
turn on of FG and gate of Q1 should be in the range
from -100ns to +100ns. By slowly increasing the
+Vin from 0V to 48V 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-B
ISOLATED DC/DC POWER CONVERTER
Vu1
2.2R
Input
Cin1
Cin2
6.8uF, 50V
-Vin
1
D3
BAS516
+Vr2
R11
33k
1.5mH
6
D5
PDZ9.1B
Q1
HAT2244
Vu1
R5
10R
1
C3
1u
2
3
Vcc
Boost
GND
TG
Inp
TS
R14
33K
Vfb
Q12
BC857BF
2
R15
115k 3
R42
0R 4
5
6
R22
7
13.3k
8
R28 196k
R32 332k
C10
0.1u
+Vr2 +Vb
R8
2.2R
PA0872.004
Sout
FB=1.23V
Vin
Rosc
Out
Sync
PGND
MaxDC
Delay
Vr=2.5V
OC
SD
Isense
GND
Blank
R29
R33
22.1K
158k
16
Sout
14
13
12
11
10
D10
C9
100p
R23 910
C11
10n
R24
82k
C13
.47u
R9
47R
Vu1
R26
220R
CG
3
4
U5
LTC3900
CS+
7 C12
CS- Timer
CG
GND
Vcc
FG
C7
220p
8
Sync
C14
1u
Sout
470p
6
5
FG
R17
560R
R19
C8
100p
T2A
PE-68386
38.3K
D9
PDZ7.5B
U6
PS2801-1
Vfb
Vaux
R31
10R
C16
.22u, 10V
C23
1uF
1
2
R21
10k
R10
1k
Cu1
4.7u
R20
66.5k
9
R18
10k
Q9
BCX55 Vaux
Vu1
15
R16
10k
D6
B0540W
Rcs1
0.008R
C5
0.22u
Comp
Csys
470uF, 6.3V
+
CG
T1
U1
LT1952-1
1
Q3
HAT2165
GND
C2
1.5n, 200V
BAT760
R13
22K
10
FG
5
U2
LTC4440
Q2
HAT2165
4, 5
6
4
+Vout
2, 3
+Vin
+Vo1
CDEP147NP-1R5MC-95
8
100uF, X7R
+Vb
R1
L1
Co1
+Vin
+Vin
D2
BAS516
L2
Q10
PZTA42
R30
1.2k
D11
BAS516
1
2
3
C15
U4
LT4430
R27
270R
Vcc
Opto
GND
Comp
SS
FB
C19
1uF, X7R
5
R34
4.42k
6
5
R35
137k
4
R38
18.2K
2.2nF, 2kV
C17
4.7n
C18
10p
+Vo1
C24
47p
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B
ISOLATED DC/DC POWER CONVERTER
6
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