BCM400x500y1K8A3z
Input and Output Filter Design
Thermal Considerations
A major advantage of SAC™ systems versus conventional PWM
converters is that the transformer based SAC does not require external
filtering to function properly. The resonant LC tank, operated at
extreme high frequency, is amplitude modulated as a function of input
voltage and output current and efficiently transfers charge through the
isolation transformer. A small amount of capacitance embedded in the
primary and secondary stages of the module is sufficient for full
functionality and is key to achieving power density.
The ChiP package provides a high degree of flexibility in that it presents
three pathways to remove heat from internal power dissipating
components. Heat may be removed from the top surface, the bottom
surface and the leads. The extent to which these three surfaces are
cooled is a key component for determining the maximum power that is
available from a ChiP, as can be seen from Figure 1.
Since the ChiP has a maximum internal temperature rating, it is
necessary to estimate this internal temperature based on a real thermal
solution. Given that there are three pathways to remove heat from the
ChiP, 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
19 shows the “thermal circuit” for a VI Chip® BCM module 6123 in an
application where the top, bottom, and leads are cooled. In this case,
the BCM power dissipation is PDTOTAL and the three surface
temperatures are represented as TCASE_TOP, TCASE_BOTTOM, and TLEADS. This
thermal system can now be very easily analyzed using a SPICE
simulator with simple resistors, voltage sources, and a current source.
The results of the simulation would provide an estimate of heat flow
through the various pathways as well as internal temperature.
This paradigm shiꢀ requires system design to carefully evaluate
external filters in order to:
nGuarantee low source impedance:
To take full advantage of the BCM module’s dynamic
response, the impedance presented to its input terminals
must be low from DC to approximately 5 MHz. The
connection of the bus converter module to its power
source should be implemented with minimal distribution
inductance. If the interconnect inductance exceeds
100 nH, the input should be bypassed with a RC damper
to retain low source impedance and stable operation. With
an interconnect inductance of 200 nH, the RC damper
may be as high as 1 μF in series with 0.3 Ω. A single
electrolytic or equivalent low-Q capacitor may be used in
place of the series RC bypass.
Thermal Resistance Top
MAX INTERNAL TEMP
1.33°C / W
nFurther reduce input and/or output voltage ripple without
Thermal Resistance Bottom
Thermal Resistance Leads
1.29°C / W
5.64°C / W
sacrificing dynamic response:
+
–
+
–
+
–
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 output of the module multiplied by its
K factor.
TCASE_BOTTOM(°C)
TLEADS(°C)
TCASE_TOP(°C)
Power Dissipation (W)
Figure 19 — Top case, Bottom case and leads thermal model
nProtect the module from overvoltage transients imposed
by the system that would exceed maximum ratings and
induce stresses:
Alternatively, equations can be written around this circuit and
analyzed algebraically:
The module primary/secondary voltage ranges shall not be
exceeded. An internal overvoltage lockout function
prevents operation outside of the normal operating input
range. Even when disabled, the powertrain is exposed
to the applied voltage and power MOSFETs must
withstand it.
TINT – PD1 • 1.24 = TCASE_TOP
TINT – PD2 • 1.24 = TCASE_BOTTOM
TINT – PD3 • 7 = TLEADS
PDTOTAL = PD1+ PD2+ PD3
Where TINT represents the internal temperature and PD1, PD2, and PD3
represent the heat flow through the top side, bottom side, and leads
respectively.
Total load capacitance at the output of the BCM module shall not
exceed the specified maximum. Owing to the wide bandwidth and low
output impedance of the module, low-frequency bypass capacitance
and significant energy storage may be more densely and efficiently
provided by adding capacitance at the input of the module. At
frequencies <500 kHz the module appears as an impedance of RSEC
between the source and load.
Thermal Resistance Top
MAX INTERNAL TEMP
1.33°C / W
Thermal Resistance Bottom
Thermal Resistance Leads
Within this frequency range, capacitance at the input appears as
effective capacitance on the output per the relationship
defined in Eq. (13).
1.29°C / W
5.64°C / W
+
–
+
–
TCASE_BOTTOM(°C)
TLEADS(°C)
TCASE_TOP(°C)
Power Dissipation (W)
CPRI_EXT
(13)
CSEC_EXT
=
K2
Figure 20 — Top case and leads thermal model
This enables a reduction in the size and number of capacitors used in a
typical system.
BCM® Bus Converter
Page 21 of 25
Rev 1.4
07/2015
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