DC968A-A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
LT1952
DESCRIPTION
Demonstration circuit 968A-A is isolated input to
high current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968 converts isolated 36V to 75V
input to 3.3V output and provides over 30A of
output current. The converter operates at
300kHz with efficiency greater than 91%. 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-B 18-36Vin to [email protected]
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
Table 1. Performance
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 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.
LT is a trademark of Linear Technology Corporation
Summary
PARAMETER
CONDITION
VALUE
Minimum Input Voltage
IOUT = 0A to 30A
36V
Maximum Input Voltage
IOUT = 0A to 30A
75V
VOUT
VIN = 36V to 75V, IOUT = 0A to 30A
3.3V ±3%
Typical Output Ripple VOUT
VIN = 36V to 75V, IOUT = 0A to 30A
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
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
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.
4.
Once the proper output voltage is established, adjust the load within the operating
range and observe the output voltage regulation, ripple voltage, efficiency and other parameters.
2.
Turn on the power at the input.
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.
NOTE: Make sure that the input voltage does
not exceed 75V.
3.
Check for the proper output voltage.
Vout = 3.3V.
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
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To get higher than 3.3V output voltages transformer with higher turns ratio will be required.
Also, output MOSFETs with higher voltage ratings may be required. Please contact LTC factory for details.
DC968-A 3.3V @30A Output Efficiency
Efficiency
To set the output voltage lower than 3.3V,
change the bottom voltage divider resistor
connected to LT4430 FB pin (see the schematic on page 5). For example, to get 2.5V
output, change R38 resistor value to 26.1k.
92%
91%
90%
89%
88%
87%
86%
85%
84%
5
15
25
35
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 DC968AA. 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-A allows the board to be used in thermally
critical applications
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-A 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.
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
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 exceeded 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.
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
R1
+Vin
1.5mH
Vu1
2.2R
D3
BAS516
+Vr2
36V to 72V
R11
82k
Cin1
Cin2
Cin3
2.2uF, 100V
-Vin
L1
PA1671.650
1
7
6
10
+Vout
Q2
HAT2165
Q1
PHM21NQ15
U2
LTC4440
Vcc
2
GND
TG
Inp
TS
4
1
Vfb
2
Q12
BC857BF
R15
115k 3
R42
0R 4
SS
C10
0.1u
5
6
R22
7
13.3k
8
R32
332k
R28
442k
+Vr2 +Vb
C2
1n, 200V
D7
B0540W
D8
B0540W
Comp
Sout
FB=1.23V
Vin
Rosc
Out
Sync
PGND
MaxDC
Delay
Vr=2.5V
SD
GND
OC
Isense
Blank
16
14
13
12
11
R33
158k
Vu1
10
D10
C9
D6
B0540W
Cu1
1uF
R20
72.8K
100p
R18
10k
C5
0.22u, 25V
R23 910
1
2
R21
10k
Q9
BCX55 Vaux
3
CG
4
R10
1k
R19
38.3K
U5
LTC3900
CS+
Sync
CS-
Timer
CG
GND
Vcc
FG
C7
220p
8
Sout
C12
470p
7
6
5
C8
100p
FG
R17
560R
T2A
PE-68386
C14
1u, 10V
D9
PDZ7.5B
9
SS
R29
22.1K
Sout
15
Vaux
R16
33k
C6
0.1u
U1
LT1952-1
R14
33K
R8
2.2R
C4
0.1u
5
BAT760
R13
22K
GND
6
Rcs1
3
Boost
CG
5
0.015R
1
C3
1u
Csys
470uF, 6.3V
+
2
+Vin
Vu1
Q3
HAT2165
D5
PDZ10B
FG
R5
10R
+Vo1
100uF, X7R
+Vb
PA0861.004
Co1
Co2
+Vin
T1
D2
BAS516
L2
Q10
PZTA42
C13
.47u
R24
39K
D11
R26
220R
U6
PS2801-1
Vfb
BAS516
Vaux
R31
10R
Vu1
C16
.22u, 10V
C23
1uF
1
2
3
R30
1.2k
R27
270R
Vcc
Opto
GND
Comp
SS
C19
1uF, X5R
C15
2.2nF, 2kV
U4
LT4430
FB
C17
15n
R34
1.96k
6
5
R35
82.5k
4
+Vo1
C18
10p
C24
47p
R38
18.2K
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
6
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