PRELIMINARY
Application Note
Parallel Operation
The BCM will inherently current share when operated in an array. Arrays
may be used for higher power or redundancy in an application.
Current sharing accuracy is maximized when the source and load
impedance presented to each BCM within an array are equal.
The recommended method to achieve matched impedances is to
dedicate common copper planes within the PCB to deliver and return the
current to the array, rather than rely upon traces of varying lengths. In
typical applications the current being delivered to the load is larger than
that sourced from the input, allowing traces to be utilized on the input
side if necessary. The use of dedicated power planes is, however,
preferable.
The BCM power train and control architecture allow bi-directional power
transfer, including reverse power processing from the BCM output to its
input. Reverse power transfer is enabled if the BCM input is within its
operating range and the BCM is otherwise enabled. The BCM’s ability to
process power in reverse improves the BCM transient response to an
output load dump.
Thermal Management
The high efficiency of the V•I Chip results in relatively low power
dissipation and correspondingly low generation of heat. The heat
generated within internal semiconductor junctions is coupled with low
effective thermal resistances, Rθ
JC
and Rθ
JB
, to the V•I Chip case and the
PCB allowing thermal management flexibility to adapt to specific
application requirements (Figure 19).
CASE 1 Convection via heatsink to air.
The total Junction-to-Ambient thermal resistance, Rθ
JA
, of a surface
mounted V•I Chip with a 0.25"heatsink is 5°C/W in 300 LFM air flow
(Figure 21). At full rated output power of 240 W, the heat generated by
the BCM is approximately 11 W (Figure 6). Therefore, the junction
temperature rise to ambient is approximately 53°C. Given a maximum
junction temperature of 125°C, a temperature rise of 53°C allows the
V•I Chip to operate at rated output power at up to 72°C ambient
temperature. At 100 W of output power, operating ambient
temperature extends to 103°C.
Output Power (W)
V•I Chip Bus Converter Module
CASE 2—Conduction to the PCB
The low thermal resistance Junction-to-board, Rθ
JB
, allows use of the
PCB to exchange heat from the V•I Chip, including convection from the
PCB to the ambient or conduction to a cold plate.
For example, with a V•I Chip surface mounted on a 2" x 2" area of a
multi-layer PCB, with an aggregate 8 oz of effective copper weight, the
total Junction-to-Ambient thermal resistance, Rθ
JA
, is 6.5°C/W in 300
LFM air flow (see Thermal section, page 8). Given a maximum junction
temperature of 125°C and 11 W dissipation at 240 W of output power,
a temperature rise of 72°C allows the V•I Chip to operate at rated
output power at up to 53°C ambient temperature.
240
0
-40
-20
0
20
40
60
80
100
120
140
Operating Junction Temperature (°C)
Figure 20—
Thermal derating curve
BCM with 0.25'' Heatsink
10
9
8
Tja
θ
JC
=
1.1°C/W
θ
JB
=
2.1°C/W
7
6
5
4
3
0
100
200
300
400
500
600
Airflow (LFM)
Figure 19—Thermal
resistance
Figure 21—Junction-to-ambient
thermal resistance of BCM with 0.25"
Heatsink
vicorpower.com
800-735-6200
V•I Chip Bus Converter Module
B352F110T24
Rev. 1.1
Page 9 of 13
VICOR [ VICOR CORPORATION ]
