BCM4414XD1E5135YZZ Datasheet, PDF (20/41 Pages) Vicor Corporation – Isolated Fixed-Ratio DC-DC Converter

BCM4414xD1E5135yzz
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 internal temperature based on a system-level thermal solution.
To 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.
R_JC
+ T_{C_BOT}
s
P_DISS
s
Figure 23 — Single-sided cooling VIA thermal model
R_{JC_TOP}
R_HOU
P_DISS
R_{JC_BOT}
s
+
T_{C_TOP}
–
s
–
T_{C_BOT}
+
Figure 22 — Double sided cooling VIA thermal model
In this case, the internal power dissipation is P_DISS, R_{JC_TOP} and
R_{JC_BOT} are thermal resistance characteristics of the VIA module and
the top and bottom surface temperatures are represented as T_{C_TOP},
and T_{C_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 R_HOU). This
feature enables two main options regarding thermal designs:
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.
In this case, R_JC can be derived as following:
RJC =
(R_{JC_TOP} + R_HOU) • R_{JC_BOT}
R_{JC_TOP} + R_HOU + R_{JC_BOT}
(14)
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, like for example heatsinks with
independent airflows or a combination of chassis/air cooling.
Current Sharing
The performance of the BCM in a VIA package 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.
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.
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.
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 VIA modules.
n Provide as symmetric a PCB/Wiring layout as possible among
VIA™ modules
For further details see AN:016 Using BCM Bus Converters
in High Power Arrays.
BCM^® in a VIA Package
Page 20 of 41
Rev 1.4
09/2016
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