VIB0002TFJ
PRELIMINARY DATASHEET
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 BCM shall not
exceed the specified maximum. Owing to the wide bandwidth
and low output impedance of the BCM, low frequency bypass
capacitance and significant energy storage may be more
densely and efficiently provided by adding capacitance at the
input of the BCM. At frequencies <500 kHz the BCM 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:
CIN
K2
COUT
=
Eq. 6
1.Guarantee low source impedance:
To take full advantage of the BCM dynamic response, the
impedance presented to its input terminals must be low
from DC to approximately 5 MHz. The connection of the
This enables a reduction in the size and number of capacitors
used in a typical system.
•
V I Chip 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.
2.Further reduce input and/or output voltage ripple without
sacrificing dynamic response:
Given the wide bandwidth of the BCM, 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 BCM 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 V I Chip 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.
Rev. 1.3
9/2009
Page 14 of 16
V•I CHIP INC. (A VICOR COMPANY) 25 FRONTAGE RD. ANDOVER, MA 01810 800-735-6200
vicorpower.com
VICOR [ VICOR CORPORATION ]
