QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
1
LT1952
DESCRIPTION
Demonstration circuit 830 is an isolated input to high
current output ¼ Brick footprint converter featuring
the LT®1952 switching controller. The DC830 con-
verts isolated 36V to 75V input to 3.3V output and
provides over 40A of output current. The converter
operates at 300kHz with efficiency greater than 92%.
The DC830 can easily be 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.
Demonstration circuit 830 can generate output cur-
rents as high as 55A. The output voltage can also be
changed to any value between 0.6V and 28V by
changing the MOSFETs, power transformer, output
inductor and output capacitors. Please consult LTC
factory for details.
The DC830 circuit features soft-start which prevents
output voltage overshoot on startup or when recover-
ing from an overload condition.
The DC830 has a precise over-current protection cir-
cuit 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 the 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 36V
Maximum Input Voltage 75V
V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 40A 3.3V ±3%
Typical Output Ripple V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 40A 50mV
P–P
Nominal Switching Frequency 300kHz
QUICK START PROCEDURE
Demonstration circuit 830 is easy to set up to evalu-
ate the performance of the LT1952. 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 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-
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 ex-
ceed 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.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
2
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 DC830 is equipped with an output capacitor
CSYS (470uF) 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
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
3
CHANGING THE OUTPUT VOLTAGE
To set the output voltage lower than 3.3V, change the
bottom voltage divider resistor connected to LT4430
FB pin (R38). For example, to get 2.5V output, change
R38 resistor value to 26.1k.
To get higher than 5V output voltages a transformer
with higher turns ratio will be required. Also, output
MOSFETs with higher voltage ratings may be re-
quired. Please contact LTC factory for details.
OPTIONAL PRIMARY MOSFET DRIVER LTC4440
The DC830 has an optional LTC4440 MOSFET driver
which can be enabled in place of the LTC4441 driver.
The LTC4440 has lower output drive capability than
the LTC4441 but it may be adequate in some applica-
tions. To enable the LTC4440, first disable the
LTC4441 by removing resistors R36 and R37. Then
install 1 ohm resistors R6 and R7. Measure the effi-
ciency before and after the change to see if swapping
drivers made a significant difference.
USING OUTPUT MOSFET BUFFERS
The optional MOSFET buffers Q13 and Q14 can im-
prove the efficiency if MOSFETs with high Qg are
used. By default, the MOSFET buffers are disabled. To
test the performance with the buffers perform the
following steps.
1.
Turn the power off.
2.
Change the base-emitter resistors R12 and R25 to
10ohm.
3.
Turn the power on.
4.
Measure the efficiency.
88%
89%
90%
91%
92%
93%
10 15 20 25 30 35 40 45 50 55 60
Iout [A]
Efficiency
Figure 3. High efficiency of DC830 allows the board
to be used in thermally critical applications
OUTPUT LOAD STEP RESPONSE
The load step response of DC830 is very fast even
though a relatively small amount of output capaci-
tance is present (300uF ceramic and 470uF electro-
lytic). This is due to the fast error amplifier of the
LT4430, the optimal amount of current slope com-
pensation of the 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 desired levels.
The load step transients are shown in Figure 4.
Figure 4. Fast transient response of DC830 is
superior to many competing power modules without
the additional output capacitors
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
4
SOFT START FUNCTION
DC830 features the LT4430 opto coupler driver that
has a soft start function which produces the mono-
tonic startup ramp shown in Figure 5. The rise time of
the output voltage is controlled by capacitor C19
which is connected to the OC (overshoot control) pin
of the LT4430.
Figure 5. The LT4430 opto coupler driver produces
monotonic output voltage rise at startup without
output voltage overshoot.
DEBUGGING AND TESTING
The DC830 can easily be tested and debugged by
powering the bias circuit separately from the main
power circuit. To place DC830 into debug mode re-
move resistor R1 and connect a 48V, 100mA power
source to the +Vb node (right side of R1). By doing
this, the primary PWM controller LT1952 can be acti-
vated without the main primary power being applied
to +Vin.
To activate the secondary side MOSFET driver
LTC3900 connect a 10V, 100mA power source to the
collector of Q9 via a 1A, 50V diode. The 1A diode will
prevent current from flowing back to the 10V supply
once the converter starts running.
Once the primary and secondary controllers are run-
ning 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 G1 should be 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 MOS-
FETs is damaged, the input current will exceed
100mA.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 830
48VIN TO 3.3V, 40A ISOLATED "QUARTER BRICK" TELECOM DC/DC CONVERTER
6
