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

ENMods System: FARM3 and MiniHAM

Rev. 1.3

Page 10 of 12

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PRELIMINARY

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 ENMods would be 375W (assuming a converter

efficiency of 85%). The desired hold-up time is 9ms 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 should be at least 820µF. Since two

capacitors are used in series, each capacitor should be at least

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

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

Vicor website, at vicorpower.com.

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 68ms. 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.

A variety of hold-up capacitor assemblies (HUBs) are

available. Please visit the Vicor website @ vicorpower.com.

For more information about designing an autoranging

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

converter modules, contact Vicor Applications Engineering at

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

apps@vicorpower.com.

• • •

The power fail warning time (

∆t) is defined as the interval

between BOK and converter shutdown (EN) as illustrated in

Figure 13. The Bus-OK and Enable thresholds are 205V and

185V, respectively. A simplified relationship between

hold-up time, operating power, and bus capacitance is

obtained by inserting these constants in equation (3):

C = 2P

C = 2P

∆t / (7,800)

It should be noted that the series combination (C1, C2,

see Figure 1a) requires each capacitor to be twice the

calculated value, but the required voltage rating of each

capacitor is reduced to 200V.

Allowable ripple voltage on the bus (or ripple current in the

capacitors) may define the capacitance requirement.

Consideration should be given to converter ripple rejection

and resulting output ripple voltage. The ripple rejection (R) of

Vicor converters is specified as a function of the input/output

voltage ratio:

R = 30 + 20log(Vin / Vout)

(4)

For example, a converter whose output is 15V and nominal

input is 300V will provide 56dB ripple rejection, i.e., 10V p-p

of input ripple will produce 15mV p-p of output ripple

(see Figure 17). Equation 3 is again used to determine the

instantaneous values of bus voltage at the peaks and valleys

(see Figure 13) of the ripple, respectively. The capacitors

must holdup the bus voltage for the time interval (

∆t) between

peaks of the rectified line as given by:

∆t = (π – θ) / 2πf

(5)

Where:

f = line frequency

θ = rectifier conduction angle

The approximate conduction angle is given by:

(6)

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

Application Note (continued)