seperate damping resistor.
鈥?/div>
-In
K
Ro
-Out
Figure 16鈥擵TM
test circuit
Application Note
Parallel Operation
In applications requiring higher current or redundancy, VTMs can be
operated in parallel without adding control circuitry or signal lines. To
maximize current sharing accuracy, it is imperative that the source and
load impedance on each VTM in a parallel array be equal. If VTMs are
being fed by an upstream PRM, the VC nodes of all VTMs must be
connected to the PRM VC.
To achieve matched impedances, dedicated power planes within the PC
board should be used for the output and output return paths to the
array of paralleled VTMs. This technique is preferable to using traces of
varying size and length.
The VTM power train and control architecture allow bi-directional
power transfer when the VTM is operating within its specified ranges.
Bi-directional power processing improves transient response in the
event of an output load dump. The VTM may operate in reverse,
returning output power back to the input source. It does so efficiently.
Thermal Management
The high efficiency of the VTM results in low power dissipation
minimizing temperature rise, even at full output current. The heat
generated within the internal semiconductor junctions is coupled
through very low thermal resistances, R胃
JC
and R胃
JB
(see Figure 17),
to the PC board allowing flexible thermal management.
CASE 1 Convection via optional Pin Fins to air (Pin Fins available as a
separate item.)
In an environment with forced convection over the surface of a PCB
with 0.4" of headroom, a VTM with Pin Fins offers a simple thermal
management option. The total Junction to Ambient thermal resistance
of a surface mounted V048K030T070 with pin fins attached is 4.8 潞C/W
in 300 LFM airflow, (see Figure 18).
At 3 Vout and full rated current (70A), the VTM dissipates
approximately 13 W per Figure 4. This results in a temperature rise of
approximately 62 潞C, allowing operation in an air temperature of
63 潞C without exceeding the 125 潞C max junction temperature.
CASE 2 Conduction via the PC board to air
The low Junction to BGA thermal resistance allows the use
of the PC board as a means of removing heat from the VTM.
Convection from the PC board to ambient, or conduction to a cold
plate, enable flexible thermal management options.
With a VTM mounted on a 2.0 in
2
area of a multi-layer PC board with
appropriate power planes resulting in 8 oz of effective copper weight,
the Junction-to-BGA thermal resistance, R胃
JA
, is 6.5 潞C/W in 300 LFM
of air. With a maximum junction temperature of 125 潞C and 13 W of
dissipation at full current of 70 A, the resulting temperature rise of
85 潞C allows the VTM to operate at full rated current up to a 40 潞C
ambient temperature. See thermal resistances on Page 9 for additional
details on this thermal management option.
Adding low-profile heat sinks to the PC board can lower the thermal
resistance of the PC board surrounding the VTM. Additional cooling
may be added by coupling a cold plate to the PC board with low
thermal resistance stand offs.
CASE 3 Combined direct convection to the air and conduction to the
PC board.
A combination of cooling techniques that utilize the power planes and
dissipation to the air will also reduce the total thermal impedance. This
is the most effective cooling method. To estimate the total effect of the
combination, treat each cooling branch as one leg of a parallel resistor
network.
vicorpower.com
800-735-6200
V鈥 Chip Voltage Transformation Module
V048K030T070
Rev. 1.0
Page 10 of 15
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