Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715

ARM, Autoranging Rectifier Module

Rev. 2.4

Page 9 of 12

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Application Note, continued

Output Voltage

40

45

50

55

60

65

70

75

80

50

30

15

5

2

Figure 16—Converter ripple rejection vs. output voltage

Determining Ripple Voltage on the Hold-up Capacitors.

Fig. 15 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 12Vac.

Determining the Ripple on the Output of the

DC-DC Converter. Fig. 16 is used to determine the ripple

rejection of the DC-DC converter and indicates a ripple

rejection of approximately 60 dB for a 12 Volt output. If 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 120Hz) will be 12 mV 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 ARM and Vicor DC-DC

converter modules, contact Vicor Applications Engineering at

the nearest Vicor Technical Support Center (see back cover),

or send an 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 = operating power level

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 13, 14, and 15,

respectively.

Example

In this example, the output required at the point of load is 12Vdc

at 320 Watts. Therefore, the output power from the ARM would

be 375 Watts (assuming a converter efficiency of 85%). The

desired hold-up time is 9 ms over an input range of 90 to 264Vac.

Determining Required Hold-up Capacitance. Fig. 13 is used

to determine hold-up capacitance for a given hold-up time and

power level, and shows that the total bus capacitance must be

at least 820µF. Since two capacitors are used in series, each

capacitor must be at least 1,640µF. Note that hold-up time is

not dependent on line voltage.

Determining Ride-through Time. Figure 14 illustrates ride-

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

and shows that at a nominal line of 115Vac, ride-through would

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