Rev 1.0

vicorpower.com

Page 13 of 18

01/2014

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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.

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.

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

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.

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.