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

FARM, Filter/Autoranging Rectifier Module

Rev. 4.7

Page 8 of 12

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PRELIMINARY

*

Operating Power (W)

0

5

10

15

20

25

30

35

40

2200µF

1600µF

1300µF

1000

750

500

250

1100µF

820µF

680µF

*

*

(FARM2XXX)

(FARM1XXX)

Figure 14—Hold-up time vs. operating power and total bus

capacitance, series combination of C1, C2 (see Fig. 9)

Operating Power (W)

0

5

10

15

20

25

30

2200µF

1600µF

1300µF

1000

750

500

250

*

1100µF

820µF

680µF

*

*

(FARM1XXX)

(FARM2XXX)

Figure 16—Ripple voltage vs. operating power and bus

capacitance, series combination of C1, C2 (see Fig. 9)

Operating Power (W)

0

10

20

30

40

50

60

70

80

90

100

90Vac

115Vac

1000

750

500

250

Total

capacitance

820µF

Figure 15—Ride-through time vs. operating power

Figure 13—Hold-up time

π – θθ

205V

185V

Power

Fail

Power Fail

Warning

B OK

Converter

Shutdown

Hold-up Time

254V

Ride-Thru Time

Ripple (V p-p)

The power fail warning time (

∆t) is defined as the interval

between B OK and converter shutdown (EN) as illustrated

in Fig. 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:

C = 2P

∆t / (205

C = 2P

∆t / (7,800)

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

see Fig. 9) 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 Fig. 17). Equation 3 is again used to determine the

instantaneous values of bus voltage at the peaks and valleys

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

hold up 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:

θ = Cos

-1

(6)

Application Note (continued)