VIV0104THJ

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

8.0 INPUT AND OUTPUT FILTER DESIGN

A major advantage of a SAC systems versus 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 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 VTM dynamic response, the

impedance presented to its input terminals must be low

from DC to approximately 5 MHz. The connection of the

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

2.Further reduce input and/or output voltage ripple without

sacrificing dynamic response:

Given the wide bandwidth of the VTM, 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 multiplied by its

K factor.

3.Protect the module from overvoltage transients imposed

by the system that would exceed maximum ratings and

cause failures:

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.

Owing to the wide bandwidth and low output impedance of

the VTM, low frequency bypass capacitance and significant

energy storage may be more densely and efficiently provided

by adding capacitance at the input of the VTM.