BCM4414xG0F4440yzz
Thermal Considerations
The VIA package provides effective conduction cooling from either
of the two module surfaces. Heat may be removed from the top
surface, the bottom surface or both. The extent to which these
two surfaces are cooled is a key component for determining the
maximum power that can be processed by a VIA, as can be seen
from the specified thermal operating area in Figure 1. Since the
VIA has a maximum internal temperature rating, it is necessary to
estimate this temperature based on a system-level thermal solution.
For this purpose, it is helpful to simplify the thermal solution into
a roughly equivalent circuit where power dissipation is modeled as
a current source, isothermal surface temperatures are represented
as voltage sources and the thermal resistances are represented as
resistors. Figure 22 shows the “thermal circuit” for the VIA module.
θINT
+ TC_BOT
s
PDISS
s
Figure 23 — Single-sided cooling VIA thermal model
ꢀ Double side cooling: while this option might bring limited
advantage to the module internal components (given the
surface-to-surface coupling provided), it might be appealing
in cases where the external thermal system requires allocating
power to two different elements, such as heatsinks with
independent airflows or a combination of chassis/air cooling.
+
θINT_TOP
TC_TOP
–
θHOU
s
–
Current Sharing
TC_BOT
θINT_BOT
+
PDISS
The performance of the BCM is based on efficient transfer
of energy through a transformer without the need of closed
loop control. For this reason, the transfer characteristic can be
approximated by an ideal transformer with a positive temperature
coefficient series resistance.
s
Figure 22 — Double-sided cooling VIA thermal model
This type of characteristic is close to the impedance characteristic
of a DC power distribution system both in dynamic (AC) behavior
and for steady state (DC) operation.
In this case, the internal power dissipation is PDISS, θINT_TOP and
θ
INT_BOT are the thermal resistance characteristics of the VIA module
and the top and bottom surface temperatures are represented as
TC_TOP and TC_BOT. It is interesting to note that the package itself
provides a high degree of thermal coupling between the top and
bottom case surfaces (represented in the model by the resistor
When multiple BCM modules of a given part number are
connected in an array, they will inherently share the load current
according to the equivalent impedance divider that the system
implements from the power source to the point of load. Ensuring
equal current sharing among modules requires that BCM array
impedances be matched.
θHOU). This feature enables two main options regarding
thermal designs:
ꢀnSingle side cooling: the model of Figure 22 can be simplified by
calculating the parallel resistor network and using one simple
thermal resistance number and the internal power dissipation
curves; an example for bottom side cooling only is shown in
Figure 23.
Some general recommendations to achieve matched array
impedances include:
ꢀnDedicate common copper planes/wires within the PCB/Chassis
to deliver and return the current to the modules.
ꢀnProvide as symmetric a PCB/Wiring layout as possible
In this case, θINT can be derived as follows:
among modules
For further details see AN:016 Using BCM Bus Converters
in High Power Arrays.
(θINT_TOP + θHOU) • θINT_BOT
θINT_TOP + θHOU + θINT_BOT
θINT
=
(13)
BCM® in a VIA™ Package
Page 21 of 43
Rev 1.2
01/2018
VICOR [ VICOR CORPORATION ]
