English
Language : 

V048X060Y040A Datasheet, PDF (13/18 Pages) Vicor Corporation – High Efficiency, Sine Amplitude Converter
Not Recommended for New Designs
V048x060y040A
12.0 INPUT AND OUTPUT FILTER DESIGN
A major advantage of a SAC system versus a conventional
PWM converter is that the former does 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 achieving
high 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 VTM module dynamic
response, the impedance presented to its input terminals
must be low from DC to approximately 5 MHz. Input
capacitance may be added to improve transient
performance or compensate for high source impedance.
2.Further reduce input and /or output voltage ripple without
sacrificing dynamic response.
Given the wide bandwidth of the VTM 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 VTM module multiplied
by its K factor.
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 must
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.
13.0 CAPACITIVE FILTERING CONSIDERATIONS
FOR A SINE AMPLITUDE CONVERTER™
It is important to consider the impact of adding input and
output capacitance to a Sine Amplitude Converter on the
system as a whole. Both the capacitance value and the
effective impedance of the capacitor must be considered.
A Sine Amplitude Converter has a DC ROUT value which has
already been discussed in section 11. The AC ROUT of the
SAC contains several terms:
• Resonant tank impedance
• Input lead inductance and internal capacitance
• Output lead inductance and internal capacitance
The values of these terms are shown in the behavioral model in
section 11. It is important to note on which side of the
transformer these impedances appear and how they reflect
across the transformer given the K factor.
The overall AC impedance varies from model to model. For
most models it is dominated by DC ROUT value from DC to
beyond 500 KHz. The behavioral model in section 11 should be
used to approximate the AC impedance of the specific model.
Any capacitors placed at the output of the VTM module reflect
back to the input of the module by the square of the K factor
(Eq. 9) with the impedance of the module appearing in series.
It is very important to keep this in mind when using a PRM®
regulator to power the VTM module. Most PRM modules have
a limit on the maximum amount of capacitance that can be
applied to the output. This capacitance includes both the PRM
output capacitance and the VTM module output capacitance
reflected back to the input. In PRM module remote sense
applications, it is important to consider the reflected value of
VTM module output capacitance when designing and
compensating the PRM module control loop.
Capacitance placed at the input of the VTM module appear to
the load reflected by the K factor with the impedance of the
VTM module in series. In step-down ratios, the effective
capacitance is increased by the K factor. The effective ESR of
the capacitor is decreased by the square of the K factor, but
the impedance of the module appears in series. Still, in most
step-down VTM modules an electrolytic capacitor placed at the
input of the module will have a lower effective impedance
compared to an electrolytic capacitor placed at the output. This
is important to consider when placing capacitors at the output
of the module. Even though the capacitor may be placed at
the output, the majority of the AC current will be sourced from
the lower impedance, which in most cases will be the module.
This should be studied carefully in any system design using a
module. In most cases, it should be clear that electrolytic
output capacitors are not necessary to design a stable,
well-bypassed system.
VTM® Current Multiplier
Page 13 of 18
Rev 1.0
01/2014
vicorpower.com
800 927.9474