BCM4414xD1E13A2yzz
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 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.
ΦINT
+ TC_BOT
s
PDISS
s
Figure 23 — Single-sided cooling VIA thermal model
n 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.
+
ΦINT_TOP
TC_TOP
–
ΦHOU
s
–
TC_BOT
ΦINT_BOT
Current Sharing
+
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, ΦINT_TOP
and ΦINT_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 notice 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 ΦHOU). This feature enables two main options regarding
thermal designs:
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.
n Single 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:
n Dedicate common copper planes/wires within the PCB/Chassis
to deliver and return the current to the modules.
n Provide as symmetric a PCB/Wiring layout as possible
among modules
In this case, ΦINT can be derived as follows:
For further details see AN:016 Using BCM Bus Converters
in High Power Arrays.
(ΦINT_TOP + ΦHOU) • ΦINT_BOT
ΦINT
=
(14)
ΦINT_TOP + ΦHOU + ΦINT_BOT
BCM® in a VIA Package
Page 21 of 42
Rev 1.5
10/2016
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