PRELIMINARY
V•I Chip Bus Converter Module
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.
CASE 2 – Conduction to the PCB
The low thermal resistance junction-to-board, RθJB, 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.
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.
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, RθJA, is 6.5°C/W in 300
LFM air flow (see Thermal section, page 5). Given a maximum junction
temperature of 125°C and 12 W dissipation at 240 W of output power,
a temperature rise of 78°C allows the V•I Chip to operate at rated
output power at up to 47°C ambient temperature.
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.
The thermal resistance of the PCB to the surrounding environment in
proximity to V•I Chips may be reduced by low profile heat sinks surface
mounted to the PCB.The PCB may also be coupled to a cold plate by low
thermal resistance standoff elements as a means of achieving effective
cooling for an array of V•I Chips, without a direct interface to their case.
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, RθJC and RθJB, to the V•I Chip case and the
PCB allowing thermal management flexibility to adapt to specific
application requirements (Figure 19).
CASE 3 – Combined direct convection to the air and conduction to the
PCB.
Parallel use of the V•I Chip internal thermal resistances (including
junction-to-case and junction-to-board) in series with external thermal
resistances provides an efficient thermal management strategy as it
reduces total thermal resistance. This may be readily estimated as the
parallel network of two pairs of series configured resistors.
CASE 1 – Convection via heat sink to air.
The total junction-to-ambient thermal resistance, RθJA, of a surface
mounted V•I Chip with a 0.25"heat sink is 5°C/W in 300 LFM air flow
(Figure 20). At full rated output power of 240 W, the heat generated by
the BCM is approximately 12 W (Figure 6). Therefore, the junction
temperature rise to ambient is approximately 58°C. Given a maximum
junction temperature of 125°C, a temperature rise of 58°C allows the
V•I Chip to operate at rated output power at up to 67°C ambient
temperature. At 100 W of output power, operating ambient
temperature extends to 97°C.
BCM with 0.25'' heat sink
10
9
8
7
6
5
4
3
0
100
200
300
400
500
600
Airflow (LFM)
θJC = 1.1°C/W
Figure 19 —Thermal resistance
θJB = 2.1°C/W
Figure 20 — Junction-to-ambient thermal resistance of BCM with 0.25"
heat sink
vicorpower.com
800-735-6200
V•I Chip Bus Converter Module
B384F120T24
Rev. 1.2
Page 9 of 13
VICOR [ VICOR CORPORATION ]
