BCM4414xD1E13A3yzz
Thermal Considerations
The VIA package provides effective conduction cooling from
either of the two module surfaces. Heat may be removed from
the pin-side surface, the non-pin-side 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 BCM,
as can be seen from the specified thermal operating area in
Figure 1. Since the BCM 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 BCM in a VIA package.
θINT
TC_NON_
+
PIN_SIDE
s
PDISS
s
Figure 23 — Single-sided cooling 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 heat sinks with
independent airflows or a combination of chassis/air cooling.
+
θINT_PIN_SIDE
TC_PIN_SIDE
–
θHOU
s
–
Current Sharing
TC_NON_
θINT_NON_
PIN_SIDE
PIN_SIDE
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.
+
PDISS
s
Figure 22 — Double-sided cooling 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_PIN_SIDE and
θ
INT_NON_PIN_SIDE are the thermal resistance characteristics of the
BCM and the pin-side and non-pin-side surface temperatures are
represented as TC_PIN_SIDE and TC_NON_PIN_SIDE. It is interesting to
note that the package itself provides a high degree of thermal
coupling between the pin-side and non-pin-side case surfaces
(represented in the model by the resistor θHOU). 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.
nꢀSingle-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 non-pin-side cooling only is shown in
Figure 23.
Some general recommendations to achieve matched array
impedances include:
nꢀDedicate common copper planes/wires within the PCB/Chassis to
deliver and return the current to the modules.
nꢀProvide as symmetric a PCB/Wiring layout as possible
among modules
In this case, θINT can be derived as follows:
For further details see AN:016 Using BCM Bus Converters
in High Power Arrays.
θ
INT_PIN_SIDE + θHOU • θ
)
(
INT_NON_PIN_SIDE
θINT
=
(13)
θINT_PIN_SIDE + θHOU + θINT_NON_PIN_SIDE
BCM® in a VIA™ Package
Page 21 of 43
Rev 1.3
08/2020
VICOR [ VICOR CORPORATION ]
