QM48 Series - 45A
Data Sheet
Thermal Derating
Characterization
Load current vs. ambient temperature and airflow rates are
given in Figs. x.1 and x.2 for vertical and horizontal converter
mounting. 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).
General Information
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 mounting, efficiency, start-up and shutdown
parameters, output ripple and noise, transient response to
load step-change, overload and short circuit.
For each set of conditions, the maximum load current was
defined as the lowest of:
(i) The output current at which either any FET junction tem-
perature did not exceed a maximum specified temperature
(120°C) as indicated by the thermographic image, or
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.
(ii) The nominal rating of the converter (45 A on 3.3 – 1.0 V).
During normal operation, derating curves with maximum FET
temperature less or equal to 120°C should not be exceeded.
Temperature on the PCB at the thermocouple location
shown in Fig. H should not exceed 118°C in order to operate
inside the derating curves.
The following pages contain specific plots or waveforms as-
sociated with the converter. Additional comments for specific
data are provided below.
Efficiency
Test Conditions
Fig. x.3 shows the efficiency vs. load current plot for ambient
temperature of 25ºC, 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.
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, comprising two-ounce cop-
per, were used to provide traces for connectivity to the con-
verter.
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 pur-
poses.
Fig. x.5 shows the power dissipation vs. load current plot for
Ta = 25ºC, 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 di/dt’s ver-
tical and horizontal wind tunnel facilities using Infrared (IR)
thermography and thermocouples for thermometry.
Start-up
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.
Ensuring components on the converter do not exceed their
ratings is important to maintaining high reliability. If one an-
ticipates operating the converter at or close to the maximum
loads specified in the derating curves, it is prudent to check
actual operating temperatures in the application. Thermo-
graphic imaging is preferable; if this capability is not avail-
able, then thermocouples may be used. di/dt recommends
the use of AWG #40 gauge thermocouples to ensure meas-
urement accuracy. Careful routing of the thermocouple leads
will further minimize measurement error. Refer to Figure H
for optimum measuring thermocouple location.
Ripple and Noise
Fig. x.10 shows the output voltage ripple waveform, meas-
ured 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 ca-
pacitor.
QM48T45 Family DS Ver 3 03-27-03
USA Toll Free 866 WOW-didt
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