BCM48BH120120B00 Datasheet, PDF (14/16 Pages) Vicor Corporation – Fixed Ratio DC-DC Converter

BCM48BH120T120B00
8.0 INPUT AND OUTPUT FILTER DESIGN
A major advantage of SAC™ systems versus conventional
PWM converters is that the transformers do not require large
functional filters. 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 input and output stages of the
module, is sufficient for full functionality and is key to achieve
power density.
This paradigm shift requires system design to carefully evaluate
external filters in order to:
1.Guarantee low source impedance:
To take full advantage of the BCM^® module dynamic
response, the impedance presented to its input terminals
must be low from DC to approximately 5 MHz. The
connection of the 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
47 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.
Total load capacitance at the output of the 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 ROUT between the source and load.
Within this frequency range capacitance at the input appears
as effective capacitance on the output per the relationship
defined in Eq. 6.
C_OUT = ^CIN
K²
Eq. 6
This enables a reduction in the size and number of capacitors
used in a typical system.
2.Further reduce input and/or output voltage ripple without
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. This is illustrated in Figures 11 and 12.
3.Protect the module from overvoltage transients imposed
by the system that would exceed maximum ratings and
cause failures:
The BCM module input/output voltage ranges shall not
be exceeded. An internal overvoltage lockout function
prevents operation outside of the normal operating input
range. Even during this condition, the powertrain is exposed
to the applied voltage and power MOSFETs must withstand
it. A criterion for protection is the maximum amount of
energy that the input or output switches can tolerate if
avalanched.
BCM^® Bus Converter
Page 14 of 16
Rev 1.2
07/2015
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