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PRELIMINARY

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FARM, Filter/Autoranging Rectifier Module

Rev. 4.7

Page 9 of 12

Output Voltage

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Figure 17—Converter ripple rejection vs. output voltage

Determining Ride-through Time. Figure 15 illustrates ride-

through time as a function of line voltage and output power,

and shows that at a nominal line of 90Vac, ride-through would

be 68 ms. Ride-through time is a function of line voltage.

Determining Ripple Voltage on the Hold-up Capacitors.

Figure 16 is used to determine ripple voltage as a function of

operating power and bus capacitance, and shows that the

ripple voltage across the hold-up capacitors will be 12V p-p.

Determining the Ripple on the Output of the

DC-DC Converter. Figure 17 is used to determine the ripple

rejection of the DC-DC converter and indicates a ripple

rejection of approximately 60 dB for a 12V output. Since the

ripple on the bus voltage is 12Vac and the ripple rejection of

the converter is 60 dB, the output ripple of the converter due

to ripple on its input (primarily 120 Hz) will be 12mV p-p.

Note that 2nd Generation converters have greater ripple

rejection then either VI-200s or VI-J00s.

For more information about designing an autoranging

AC input power supply using the FARM and Vicor DC-DC

converter modules, contact Vicor Applications Engineering at

the nearest Vicor Technical Support Center, or send E-mail to

apps@vicr.com.

• • •

Another consideration in hold-up capacitor selection is their

ripple current rating. The capacitors’ rating must be higher

than the maximum operating ripple current. The approximate

operating ripple current (rms) is given by:

I rms = 2P/Vac

(7)

Where: P = total output power

Vac = operating line voltage

Calculated values of bus capacitance for various hold-up

time, ride-through time, and ripple voltage requirements are

given as a function of operating power level in Figures 14, 15,

and 16, respectively.

Example

In this example, the output required from the DC-DC converter

at the point of load is 12Vdc at 320W. Therefore, the output

power from the FARM would be 375W (assuming a converter

efficiency of 85%). The desired hold-up time is 9 ms over an

input range of 90 to 264Vac.

Determining Required Capacitance for Power Fail

Warning. Figure 14 is used to determine capacitance for a

given power fail warning time and power level, and shows

that the total bus capacitance must be at least 820 µF. Since

two capacitors are used in series, each capacitor must be at

least 1,640 µF. Note that warning time is not dependent on

line voltage. A hold-up capacitor calculator is available on the

Vicor website, at vicorpower.com.

Application Note (continued)