BCM3814x60E10A5yzz
Filter Design
Thermal Considerations
A major advantage of BCM systems versus conventional PWM
converters is that the transformer based BCM does not require
external filtering to function properly. The resonant LC tank,
operated at extreme high frequency, is amplitude modulated as
a function of HI side voltage and LO side current and efficiently
transfers charge through the isolation transformer. A small amount
of capacitance embedded in the high voltage side and low voltage
side stages of the module is sufficient for full functionality and is
key to achieving power density.
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.
This paradigm shift requires system design to carefully evaluate
external filters in order to:
n Guarantee low source impedance:
To take full advantage of the BCM module’s dynamic
response, the impedance presented to its HI side terminals
must be low from DC to approximately 5MHz. The
connection of the bus converter module to its power
source should be implemented with minimal distribution
inductance. If the interconnect inductance exceeds
100nH, the HI side should be bypassed with a RC damper
to retain low source impedance and stable operation. With
an interconnect inductance of 200nH, the RC damper
may be as high as 1µF in series with 0.3Ω. A single
electrolytic or equivalent low-Q capacitor may be used in
place of the series RC bypass.
+
RJC_TOP
TC_TOP
–
RHOU
s
–
TC_BOT
RJC_BOT
+
PDISS
s
n Further reduce HI side and/or LO side voltage ripple without
sacrificing dynamic response:
Figure 22 — Double sided cooling VIA thermal model
Given the wide bandwidth of the module, the source
response is generally the limiting factor in the overall
system response. Anomalies in the response of the source
will appear at the LO side of the module multiplied by its
K factor.
In this case, the internal power dissipation is PDISS, RJC_TOP and
RJC_BOT are 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 RHOU). This
feature enables two main options regarding thermal designs:
n Protect the module from overvoltage transients imposed
by the system that would exceed maximum ratings and
induce stresses:
The module high side/low side voltage ranges shall not be
exceeded. An internal overvoltage lockout function
prevents operation outside of the normal operating HI side
range. Even when disabled, the powertrain is exposed
to the applied voltage and power MOSFETs must
withstand it.
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, RJC can be derived as following:
Total load capacitance at the LO side of the BCM module shall not
exceed the specified maximum. Owing to the wide bandwidth
and small LO side impedance of the module, low-frequency bypass
capacitance and significant energy storage may be more densely
and efficiently provided by adding capacitance at the HI side of
the module. At frequencies <500kHz the module appears as an
impedance of RLO between the source and load.
(RJC_TOP + RHOU) • RJC_BOT
R
=
(14)
JC
RJC_TOP + RHOU + RJC_BOT
Within this frequency range, capacitance at the HI side appears as
effective capacitance on the LO side per the relationship
defined in Eq. (13).
CHI_EXT
K2
(13)
CLO_EXT
=
This enables a reduction in the size and number of capacitors used
in a typical system.
BCM® in a VIA Package
Page 20 of 39
Rev 1.4
09/2016
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