QM48T45 Family DC-DC Converter Data Sheet
36-75 VDC Input; 1.0-3.3 VDC @ 45 A Output
horizontal converter mountings. Ambient temperature
was varied between 25 °C and 85 °C, with airflow
rates from 30 to 500 LFM (0.15 to 2.5 m/s).
Characterization
General Information
For each set of conditions, the maximum load current
was defined as the lowest of:
The converter has been characterized for many
operational aspects, to include thermal derating
(maximum load current as a function of ambient
temperature and airflow) for vertical and horizontal
mountings, efficiency, startup and shutdown
parameters, output ripple and noise, transient
response to load step-change, overload, and short
circuit.
(i) The output current at which any FET junction
temperature does not exceed a maximum specified
temperature
(120°C)
as
indicated
by
the
thermographic image, or
(ii) The nominal rating of the converter (45 A on
3.3-1.0 V).
The figures are numbered as Fig. x.y, where x
indicates the different output voltages, and y
associates with specific plots (y = 1 for the vertical
thermal derating, …). For example, Fig. x.1 will refer
to the vertical thermal derating for all the output
voltages in general.
During normal operation, derating curves with
maximum FET temperature less or equal to 120 °C
should not be exceeded. Temperature on the PCB at
thermocouple location shown in Fig. E should not
exceed 118 °C in order to operate inside the derating
curves.
The following pages contain specific plots or
waveforms associated with the converter. Additional
comments for specific data are provided below.
Efficiency
Fig. x.3 shows the efficiency vs. load current plot for
ambient temperature of 25 ºC, airflow rate of 300 LFM
(1.5 m/s) with vertical mounting and input voltages of
36 V, 48 V and 72 V. Also, a plot of efficiency vs. load
current, as a function of ambient temperature with
Vin = 48 V, airflow rate of 200 LFM (1 m/s) with
vertical mounting is shown in Fig. x.4.
Test Conditions
All data presented were taken with the converter
soldered to a test board, specifically a 0.060” thick
printed wiring board (PWB) with four layers. The top
and bottom layers were not metalized. The two inner
layers, comprised of two-ounce copper, were used to
provide traces for connectivity to the converter.
Power Dissipation
The lack of metalization on the outer layers as well
as the limited thermal connection ensured that heat
transfer from the converter to the PWB was
minimized. This provides a worst-case but consistent
scenario for thermal derating purposes.
Fig. x.5 shows the power dissipation vs. load current
plot for Ta = 25 ºC, airflow rate of 300 LFM (1.5 m/s)
with vertical mounting and input voltages of 36 V,
48 V and 72 V. Also, a plot of power dissipation vs.
load current, as a function of ambient temperature
with Vin = 48 V, airflow rate of 200 LFM (1 m/s) with
vertical mounting is shown in Fig. x.6.
All measurements requiring airflow were made in the
vertical and horizontal wind tunnel using Infrared (IR)
thermography and thermocouples for thermometry.
Startup
Ensuring components on the converter do not
exceed their ratings is important to maintaining high
reliability. If one anticipates operating the converter
at or close to the maximum loads specified in the
derating curves, it is prudent to check actual
Output voltage waveforms, during the turn-on
transient using the ON/OFF pin for full rated load
currents (resistive load) are shown without and with
external load capacitance in Fig. x.7 and Fig. x.8,
respectively.
operating
temperatures
in
the
application.
Thermographic imaging is preferable; if this capability
is not available, then thermocouples may be used.
The use of AWG #40 gauge thermocouples is
recommended to ensure measurement accuracy.
Careful routing of the thermocouple leads will further
minimize measurement error. Refer to Fig. E for the
optimum measuring thermocouple location.
Ripple and Noise
Fig. x.10 shows the output voltage ripple waveform,
measured at full rated load current with a 10 µF
tantalum and 1 µF ceramic capacitor across the
output. Note that all output voltage waveforms are
measured across a 1 µF ceramic capacitor.
The input reflected ripple current waveforms are
obtained using the test setup shown in Fig x.11. The
corresponding waveforms are shown in Fig. x.12 and
Fig. x.13.
Thermal Derating
Load current vs. ambient temperature and airflow
rates are given in Fig. x.1 and Fig x.2 for vertical and
MCD10073 Rev. 1.0
Page 7 of 36
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