BCM352 x 125 y 300A00
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.
Total load capacitance at the output of the bus converter 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. 5.
This paradigm shift requires system design to carefully evaluate
external filters in order to:
1.Guarantee low source impedance:
CIN
K2
COUT
=
Eq. 6
To take full advantage of the BCM® bus conveter 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
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.
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 bus converter, 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 VI Chip® 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 16 of 18
Rev 1.7
vicorpower.com
800 735.6200
1/2013
VICOR [ VICOR CORPORATION ]
